3. Background and Significance
Complex Regional Pain Syndrome (CRPS) is the current diagnostic label for the syndrome historically
referred to as Reflex Sympathetic Dystrophy (RSD), Causalgia, and a variety of other terms 21. It is a chronic
neuropathic pain disorder distinguished by significant involvement of the autonomic nervous system, and
typically develops in an extremity following acute tissue trauma. In addition to classic neuropathic pain
characteristics (intense burning pain, hyperalgesia, allodynia), CRPS is associated with local edema and
changes suggestive of autonomic involvement (altered sweating, skin color, and skin temperature in the
affected region). Trophic changes to skin, hair, and nails, and altered motor function (loss of strength,
decreased active range of motion, tremor) may also occur. CRPS is subdivided into CRPS-I (Reflex
Sympathetic Dystrophy) and CRPS-II (Causalgia), reflecting respectively the absence or presence of
documented nerve injury118. Despite this traditional diagnostic distinction, signs and symptoms of the two
CRPS subtypes are identical and there is no evidence that they differ in terms of pathophysiological
mechanisms or treatment responsiveness75,76.
Results of two epidemiologic studies in the general population48,136 indicate that at least 50,000 new
cases of CRPS-I occur annually in the United States alone22. It is more common in women and with increasing
age48,136. While CRPS can develop following virtually any (even minimal) injury, the most common initiating
events are surgery, fractures, crush injuries, and sprains76. CRPS patients experience not only significant
pain, but often describe significant functional impairments and psychological distress29,67-69,96. In clinical
settings outside of specialty pain clinics, CRPS may often be under-recognized130.
CRPS is one of the more challenging chronic pain conditions to treat successfully33. There is no
definitive medical treatment, and clinical trials have failed to support the efficacy of many commonly employed
interventions44,99,126. Due to the absence of other effective medical treatments, invasive and expensive
palliative interventions are often used, such as spinal cord stimulators and implanted drug pumps (costing at
least $30,000 each), contributing to the high costs of managing CRPS. Lack of adequate treatments for
CRPS has resulted in part from the incomplete understanding of its pathophysiological mechanisms74. Indeed,
an NIH State-of-the-Science meeting on CRPS held in 2001 concluded that existing research on mechanisms
of human CRPS is inadequate, and that it has failed to adequately capture the complex nature of the condition
observed in clinical patients3. Key mechanisms hypothesized to contribute to CRPS are now overviewed.
Pathophysiological Mechanisms of CRPS
While attempts have been made to reduce CRPS to a single pathophysiological mechanism (e.g.,
sympatho-afferent coupling), it has become increasingly accepted that there are multiple mechanisms
involved. Only in the last few years has it been recognized that CRPS is not simply a sympatheticallymediated peripheral pain condition, but rather is a disease of the central nervous system as well88. Evidence
for this comes from the fact that CRPS patients display changes in somatosensory systems processing
thermal, tactile, and noxious stimuli, that bilateral sympathetic nervous system (SNS) changes are observed
even in patients with unilateral symptoms, and that the somatomotor system may also be affected88. There is
some evidence that subtypes of CRPS may exist, reflecting differing relative contributions of multiple
underlying mechanisms27. To date, there is a dearth of research with designs that adequately reflect the likely
multifactorial nature of CRPS – virtually no studies have examined multiple mechanisms in the same patients
simultaneously, and these have not used prospective designs that would facilitate making causal links. The
proposed project will focus on four hypothesized mechanisms that prior human research suggests may
contribute to CRPS. These mechanisms are described below.
Altered Sympathetic Nervous System (SNS) Function
Historically, it was assumed that common autonomic features of CRPS such as a cool, bluish limb were
the result of vasoconstriction reflecting excessive SNS outflow, and that the pain in CRPS was sympatheticallymaintained131. This latter phenomenon can be seen clinically in the findings that forehead cooling which elicits
sympathetic activation leads to increased clinical pain intensity in CRPS patients52. The presumed role of
excessive SNS outflow in key CRPS characteristics was the traditional rationale for clinical use of selective
sympatholytic blocks (e.g., stellate ganglion) for pain and symptom relief in CRPS patients. Possible reasons
for links between CRPS pain and SNS activity have been suggested. Animal studies indicate that following
nerve trauma, catecholamine receptors are expressed on nociceptive fibers, providing one mechanism by
which SNS outflow might directly trigger nociceptive signals50,87. Given that even in CRPS-I, some type of
nerve trauma is likely involved in onset of CRPS11,125, expression of catecholamine receptors on injured
nociceptive fibers might help explain the impact of SNS outflow on CRPS pain. Interestingly, a recently
developed animal model (chronic post-ischemic pain) does not appear to result in clear nerve injury and thus
may better reflect CRPS-I, yet this model also indicates enhanced nociceptive firing in response to presence of
norepinephrine, the primary neurotransmitter mediating SNS activity165. This finding suggests that pain could
be directly induced by SNS activity even in CRPS-I patients.
While the findings above suggest that CRPS pain and other symptoms may in some cases be linked to
SNS activity, they do not necessarily imply that excessive SNS outflow is responsible. Indeed, the only
prospective human data on the issue of SNS function in CRPS fails to support this common clinical
assumption. Schurmann et al.142 assessed SNS function (peripheral vasoconstrictor responses induced by
contralateral limb cooling) in unilateral fracture patients shortly after injury. Development of CRPS 12 weeks
later was predicted by early impairments in SNS function (reduced vasoconstrictor response). Impaired SNS
function was observed prior to onset of CRPS on both the affected and the unaffected side, suggesting
systemic alterations in SNS regulation shortly after injury. Cross-sectional studies in patients with acute CRPS
confirm these findings of impaired SNS function relative to pain patients without CRPS9,71. Reduced SNS
function (and therefore excessive vasodilation) in early acute CRPS would help account for the observation
that acute CRPS is most often associated with a warm, red extremity rather than the cool, bluish presentation
often noted in chronic CRPS10,142.
Other work indicates that whole body cooling and warming produces symmetrical vasoconstriction and
vasodilation in healthy controls and non-CRPS pain patients, but demonstrates dysfunctional SNS
thermoregulatory activity in CRPS patients161. Vasoconstriction to cold challenge in this study was absent in
patients with acute CRPS (“warm CRPS”), while exaggerated vasoconstriction was noted in patients with
chronic CRPS (“cold CRPS”). Although such exaggerated vasoconstriction to cold challenge on the affected
side in chronic CRPS patients is common52,160,161, CRPS patients nonetheless exhibit lower norepinephrine
(NE) levels on the affected side compared to the unaffected side79,159,161, consistent with diminished local SNS
outflow. Taken together, these findings suggest that exaggerated vasoconstrictive responses in chronic CRPS
patients may occur even in the presence of reduced SNS outflow, most likely as a result of local adrenergic
receptor upregulation. That is, the decreased SNS activity noted above in acute CRPS would be expected to
lead to compensatory upregulation of peripheral catecholaminergic receptors79,102. The resulting
supersensitivity to circulating catecholamines may then lead to exaggerated sweating and vasoconstriction
upon exposure to circulating catecholamines, and thus the characteristic cool, blue, sweaty extremity typically
seen in chronic CRPS patients35. As noted previously, circulating catecholamines in CRPS patients may also
trigger firing of nociceptive fibers via adrenoceptors sprouting on these fibers following tissue injury50,87,165.
Inflammatory Factors
Findings in clinical trials that corticosteroids significantly improved symptoms in some patients with
acute CRPS suggested the possibility that inflammatory mechanisms might contribute to CRPS, at least in the
acute phase16,43. Recent work supports this hypothesis. Inflammation contributing to CRPS can arise from two
sources. Classic inflammatory mechanisms can contribute through actions of immune cells such as
lymphocytes and mast cells that excrete pro-inflammatory cytokines following tissue trauma (IL-1 beta, IL-2, IL6, and TNF-alpha)43. One effect of these cytokines is to increase plasma extravasation in tissue, thereby
producing localized edema like that observed in CRPS. Cytokines with anti-inflammatory actions are also
secreted, such as IL-1011.
Neurogenic inflammation may also occur, mediated by release of proinflammatory cytokines and
neuropeptides directly from nociceptive fibers in response to various triggers, including nerve injury8.
Neuropeptide mediators involved in neurogenic inflammation include substance P and calcitonin gene related
peptide (CGRP). These neuropeptides both increase plasma extravasation and produce vasodilation, and
thus can produce a warm, red, edematous extremity as is most often observed in acute CRPS11,72,99,122.
Substance P has been shown to contribute to allodynia in a post-fracture animal model of CRPS-I 72,99. In
addition, substance P and TNF-alpha also both activate osteoclasts, which could contribute to the patchy
osteoporosis frequently noted radiographically in CRPS patients, and CGRP can increase hair growth and
increase sweating responses, both features sometimes noted in CRPS patients11,141. The proinflammatory
cytokines and neuropeptides above also produce peripheral sensitization leading to increased nociceptive
responsiveness.
A number of studies have specifically examined associations between CRPS and pro- and antiinflammatory cytokines and neuropeptides. Several studies indicate that compared to pain-free controls and
non-CRPS pain patients, CRPS patients display significant elevations in proinflammatory cytokines (TNF-
alpha, IL-1 beta, IL-2, IL-6) in both plasma and CSF1,113,152,162,163. CRPS patients also appear to have reduced
systemic levels of anti-inflammatory cytokines (IL-10) compared to controls, potentially contributing to elevated
inflammation in the condition152. Increased TNF-alpha levels appear to impact on sensory CRPS symptoms.
CRPS-I patients with hyperalgesia had significantly higher plasma levels of soluble TNF-alpha receptor type I
than CRPS patients without hyperalgesia113, and neuropathic pain patients with allodynia display higher
plasma TNF-alpha levels than similar patients without allodynia109. Animal work also supports this conclusion,
indicating that TNF-alpha levels in a post-fracture model of CRPS-I contribute to hyperalgesia135. TNF-alpha is
a key cytokine of interest in the proposed project because not only does it have direct pronociceptive actions,
but it also induces production of other cytokines involved in inflammation, including IL-1b and IL-6145.
Other work supports an association between CRPS and proinflammatory neuropeptides. Several
studies have reported increased systemic CGRP in CRPS patients compared to healthy controls12,13,140.
CGRP can produce vasodilatation, edema, and increased sweating, all features associated with acute
CRPS12,122,141. Successful treatment of CRPS in one study was associated with reduced CGRP levels and
decreased clinical signs of inflammation12. Other work indicates that plasma levels of substance P are
significantly higher in acute CRPS patients than in healthy controls139,140. Moreover, intradermal application of
substance P on either the affected or unaffected limb in CRPS patients has been shown to induce protein
extravasation in that limb, whereas it does not do so in healthy controls104. These authors suggested that
capacity to inactivate substance P was impaired in CRPS patients. Animal work also supports a role for
substance P in CRPS, revealing that elevated substance P levels in a post-fracture model of CRPS-I contribute
to warmth, edema, and allodynia like that observed in human CRPS72,100.
In sum, inflammatory factors can account for a number of the cardinal features of CRPS, particularly in
the acute “warm” phase. Findings in clinical research that edema is less likely with increasing CRPS duration
also are consistent with a greater role for inflammatory mechanisms in the acute phase76. Despite the crosssectional studies and animal studies suggesting a role for inflammatory factors in the onset of CRPS, this
possibility has not previously been tested in prospective human studies.
Brain Plasticity
A recent review of the neuroimaging literature119 concluded that there is little support for a distinct “pain
network” associated with neuropathic pain nor is there a consistent brain activation pattern associated with
allodynia (a key clinical characteristic of CRPS). However, several neuroimaging studies in CRPS patients
suggest at least one consistent and specific brain alteration associated with the condition: a reorganization of
somatotopic maps. Specifically, there is a reduction in size of the representation of the CRPS affected limb in
the somatosensory cortex compared to the unaffected side92,111,112,127,128. Two studies indicate that these
alterations return to normal after successful CRPS treatment112,128, suggesting that they may reflect brain
plasticity occurring as part of CRPS development rather than reflecting premorbid brain differences. However,
it is not yet known at what point in development of CRPS this reorganization in somatotopic maps occurs. That
these brain changes have meaningful clinical effects is supported by several findings. The degree of
somatotopic reorganization correlates significantly with CRPS pain intensity and degree of hyperalgesia111.
Moreover, CRPS patients exhibiting such reorganization demonstrate impaired two-point tactile
discrimination127 and impaired ability to localize tactile stimuli, including perceiving sensations outside of the
nerve distribution stimulated114. This latter finding could help explain the non-dermatomal distribution of pain
and sensory symptoms often noted in CRPS patients (e.g., stocking or glove pattern17). Previous findings that
sensory deficits to touch and pinprick in CRPS patients are often displayed throughout the affected body
quadrant or the entire ipsilateral side of the body may also be accounted for in part by the somatotopic
reorganization described above134. Although the origin of somatotopic reorganization in CRPS is not known,
work in other pain conditions indicates that similar reorganization occurs when afferent input from an extremity
is substantially reduced or absent (i.e., phantom limb pain60). Studies in non-human primates are consistent
with this view. Partial loss of sensory inputs as a consequence of peripheral nerve damage61 or partial spinal
cord lesions86 lead to extensive reorganization of multiple brain areas, including subregions of S1, with
expansion of the somatotopic representations of adjacent non-deafferented areas into those cortical areas
whose inputs have been lost. This reorganization can lead to blurring of the four distinct somatotopically
organized areas (areas 1, 2, 3a, 3b) of S1. Although the significance of these latter findings is as yet unclear,
recent reports of differential activation of these subregions of S1 in response to noxious versus non-noxious
levels of the same somatosensory stimulus37 suggest that these findings might represent the neural correlates
of aberrant early processing of non-noxious sensory stimuli that could have relevance to characteristic signs of
CRPS (e.g., allodynia).
Psychological Factors
Given the other pathophysiological mechanisms described above, any psychological factor associated
with elevated catecholamine release could potentially exacerbate vasomotor signs of CRPS (via upregulated
adrenergic receptors), directly increase CRPS pain intensity (via catecholamine receptors sprouting on injured
nociceptive fibers), and by exacerbating pain, could indirectly help maintain the central sensitization associated
with CRPS. Psychological factors such as emotional distress (e.g., anxiety, anger, depression) can be
associated with increased catecholaminergic activity34,80,106, and thus, could in theory interact with the
pathophysiological mechanisms above. Consistent with this hypothesis, results of a diary study indicate that
increased depression levels are a predictor of greater subsequent CRPS pain intensity58, and our past work
suggests that the pain exacerbating effects of emotional distress are significantly greater in CRPS patients
than in non-CRPS pain patients23,29. Although these studies did not assess circulating catecholamines, other
work indicates that greater depression80 and stress94 levels in CRPS patients are associated with significantly
higher circulating levels of epinephrine and norepinephrine, in line with hypotheses.
More recent work suggests that interactions between psychological and immune factors may also be
important to consider. For example, laboratory work in healthy individuals revealed that greater pain-related
catastrophizing is associated with increased pro-inflammatory cytokine activity in response to painful stimuli169.
Moreover, in CRPS patients, psychological stress has been shown to be associated with alterations in immune
function that impact on inflammatory cytokines hypothesized to contribute to CRPS95.
Our review of the existing research literature indicates that most studies assessing the role of
psychological factors in CRPS have been limited to case series descriptions or cross-sectional psychological
comparisons between CRPS patients and non-CRPS chronic pain patients17. Several studies suggest that
CRPS patients may be more emotionally distressed than patients with non-CRPS chronic pain
conditions29,45,68,81, although similar studies with negative findings have also been reported47,73. This leaves
open the possibility that the positive findings simply reflect bias due to clinic referral patterns. Regardless,
cross-sectional studies cannot address causation. Prospective studies are required, and to date, very few
CRPS studies of this type have been published. One prospective study indicated that among 88 consecutive
patients assessed shortly after acute distal radius fracture, 14 had significantly elevated life stress but did not
develop CRPS, and the one patient who did develop CRPS had no apparent psychological risk factors (no
major life stressors and average emotional distress levels)51. However, our previous prospective work indicated
that higher levels of anxiety (p<.05) and depression (p<.10 nonsignificant trend) prior to undergoing total knee
arthroplasty (TKA) were associated with greater likelihood of a CRPS diagnosis at one month post-surgery77.
For the purposes of this proposal, this latter dataset was re-examined, and a continuous CRPS score like that
proposed for use in the current project (see below) was derived to provide an index of CRPS signs and
symptoms at 6 and 12 months post-TKA. Greater increases in both depression (BDI) and anxiety (STAI) from
pre-TKA baseline to 4 weeks post-TKA predicted significantly higher CRPS scores at 6 months post-TKA
(p’s<.05), with these acute post-TKA increases in depression also predicting significantly higher CRPS scores
at 12 month follow-up (p<.05).
In sum, although theoretical links and these latter prospective findings suggest that psychological
factors could potentially impact on CRPS development, empirical tests of this hypothesis to date have been
inadequate. Additional prospective tests of hypothesized psychological CRPS mechanisms are required.
Other Potential Mechanisms
The current proposal will focus solely on testing the key mechanisms described above. However, it is
important to note that other mechanisms may also contribute to CRPS. Persistent or intense noxious input
resulting from tissue damage or nerve injury triggers increased excitability of nociceptive neurons in the spinal
cord, a phenomenon termed central sensitization90. Central sensitization is mediated by nociception-induced
release of neuropeptides such as substance P and bradykinin, and the excitatory amino acid glutamate acting
at spinal NMDA receptors90,157. Central sensitization results in exaggerated responses to nociceptive stimuli
(hyperalgesia) and permits normally non-painful stimuli such as light touch or cold to activate nociceptive
pathways (allodynia)90. An objective indicator of central sensitization is wind-up, reflecting an increase in the
excitability of spinal cord neurons that is evoked by repeated brief mechanical or thermal stimulation occurring
at a frequency similar to the natural firing rate of nociceptive fibers83. CRPS patients display significantly
greater wind-up to repeated stimuli applied to the affected limb than on the contralateral or other limbs56,144.
While persistent nociceptive input following tissue injury triggers central sensitization processes in the
spinal cord and brain, the initial tissue trauma itself also elicits local peripheral sensitization42. Following tissue
trauma, primary afferent fibers in the injured area release several pronociceptive neuropeptides (including
substance P) that increase background firing of nociceptors, increase firing in response to nociceptive stimuli,
and decrease the firing threshold for thermal and mechanical stimuli42,46. These latter two effects contribute
respectively to the hyperalgesia and allodynia that are key diagnostic features of CRPS28. Local hyperalgesia
resulting in part from peripheral sensitization can be seen in findings of significantly reduced acute pain
thresholds in the affected extremity of chronic CRPS patients compared to their unaffected extremity155. Given
that peripheral sensitization is triggered by the initial tissue trauma leading to persistent pain, it is likely that it is
present in CRPS patients very early in the development of the condition. In sum, both central and peripheral
sensitization may play a role in the development of CRPS. Because focusing on central and peripheral
sensitization mechanisms in the proposed study would produce a serious statistical confound, with
hyperalgesia and allodynia serving both as predictors (independent variables) and diagnostic indicators of
CRPS (dependent variables), these mechanisms will not be examined in the proposed study.
A possible role for genetic factors in determining susceptibility to CRPS has also been suggested, but
few studies have empirically evaluated this possibility. One report examining familial occurrence patterns of
CRPS supports possible heritability of the disorder49. Of the CRPS studies obtaining genetic data, one focus
has been on genes of the major histocompatibility complex (MHC) which encodes human leukocyte antigen
(HLA) molecules. One genetic study found a significantly higher frequency of several MHC-related alleles in a
group of CRPS patients compared to healthy controls153. A TNF-alpha promoter gene polymorphism at
position -308 was also investigated for associations with CRPS when compared to a healthy population154.
The TNF2 allele was significantly more likely to be present in warm CRPS patients than in controls. The
functional effect of this allele is production of higher amounts of TNF-alpha, which could help contribute to an
exaggerated inflammatory response in these CRPS patients154. Other inflammation-related work focuses on
the fact that angiotensin converting enzyme (ACE) helps degrade pronociceptive neuropeptides such as
bradykinin84. One small study in CRPS-I patients (n=14) found a significantly greater likelihood of a
Deletion/Deletion genotype for the Insertion/Deletion polymorphism at intron 16 of the ACE gene compared to
the general population98. However, an attempt to replicate this study by other investigators failed to reveal any
genetic association between CRPS and this ACE gene polymorphism84. At present these are the only
available genetic studies in CRPS patients, and none has the large sample needed for adequate evaluation of
genetic factors. Given the lack of adequate prior genetic data in CRPS on which to base hypotheses, genetic
mechanisms will not be examined in the proposed project.
A Speculative Model of Interacting Pathophysiological Mechanisms in CRPS
Although interactions between the mechanisms above have not been empirically tested, there are
numerous ways in which such interactions could occur in theory21. Tissue injury to an extremity may result in
nerve trauma that elicits local release of proinflammatory cytokines and neuropeptides, producing signs of
inflammation and locally increased nociceptive responsiveness (peripheral sensitization). This inflammatory
response may be exaggerated in those who are genetically or emotionally susceptible. Following the initiating
injury, nociceptive fibers in the area begin to express catecholamine receptors, after which circulating
catecholamines (elicited in part by elevated catastrophizing and emotional distress) can directly trigger
nociceptive firing. Reduced SNS outflow in the region following the initiating trauma produces signs of
vasodilation and impaired thermoregulatory responsiveness. Diminished SNS outflow also results in
upregulated sensitivity of local catecholamine receptors, contributing to exaggerated vasoconstrictive
responsiveness in the affected region in the presence of catecholamines. The resulting reductions in regional
blood flow may facilitate regional accumulation of pronociceptive substances, and contribute to local nutritive
deficits that may lead to trophic changes (e.g., skin, nails) associated with CRPS. The ongoing nociceptive
input resulting from the interacting mechanisms above activates central sensitization processes in the brain
and spine, which further heighten nociceptive responsiveness and result in allodynia and hyperalgesia. Altered
afferent input from the extremity following the injury contributes to altered somatotopy in the brain, specifically
a reduced somatosensory representation of the affected region, and these changes are associated with
impaired tactile sensation and nondermatomal sensory symptoms. While speculative, the proposed model is
consistent with known mechanisms. Prospective studies are needed to test these hypothesized mechanisms
as contributors to CRPS development in human clinical patients following acute tissue trauma.
Total Knee Arthroplasty (TKA) as an Acute Trauma Model for Prospective Studies of CRPS
It is believed that even in CRPS-I, some form of initial nerve trauma (to small distal nerve branches) is
an important trigger for the cascade of events leading to CRPS11,125. It would therefore not be surprising that
the significant tissue trauma inherent in many surgical procedures might trigger CRPS. Prior work indicates
that surgery is a common inciting event for CRPS (e.g., 24% of a large sample of CRPS patients reported
surgery as the inciting event76). With regard to lower extremity CRPS, surgery was the inciting event in 41% of
a series of 36 CRPS patients, with TKA the single most common trigger93.
TKA is used primarily to treat osteoarthritis of the knee that is not adequately controlled by nonsurgical
interventions85,121. Knee pain in osteoarthritis is mechanically-based, resulting from degenerative structural
alterations to the knee joint85. Osteoarthritic pain occurs primarily during activities loading the knee joint
(walking, stairs), but may be absent during activities not loading the joint59. TKA replaces the dysfunctional
knee structure that is the generator of pain, and therefore, once acute post-surgical healing has occurred, is
expected to significantly reduce pain. As of 2003, approximately 300,000 TKA procedures were performed
annually in the United States (U.S)121, with statistical projections indicating that by 2030 more than 3 million
TKAs may be performed annually in the U.S. alone101. Approximately 15% of TKA patients experience
unsatisfactory pain outcomes2,15,121,124, indicating that at least 45,000 patients each year are affected. The
proposed project will attempt to better characterize the mechanisms leading to CRPS and other negative painrelated outcomes in this growing number of TKA patients. It is notable that TKA is more common in women
and older patients, both of which are groups that appear to be at increased risk of developing CRPS48,136.
Natural History and Predictors of Post-TKA Pain
There are a limited number of prospective studies examining pain-related outcomes and predictors of
outcomes following TKA, and only one such study has addressed CRPS specifically77. Existing prospective
studies of TKA have generally examined only a very limited set of outcome predictors, primarily related to
demographics and medical comorbidity. Overall results of such studies have been largely negative, leaving
unclear the reasons for variability in pain-related outcomes post-TKA.
Pain data regarding post-TKA pain at 2-3 year follow-up indicate a mean pain intensity of 20/100,
although there is substantial variability in these responses107,124. For example, in a prospective sample of 90
TKA patients, 10% reported no improvements in pain at 3 year follow-up124. Long-term follow-up studies
indicate that any improvements in pain occur almost exclusively during the first 6 months post-TKA64,107.
Multiple studies have tested age, gender, and number of medical comorbidities as predictors of pain outcomes
ranging from 4 months to 3 years post-TKA. Most have failed to support any association between these
predictors and pain outcomes15,65,91,97,107,124. Significant predictors of worse post-TKA pain outcomes in
previous work include greater body mass index (BMI)124, presence of other musculoskeletal chronic pains124,
higher presurgical pain intensity15,64,91,107, and elevated depression and anxiety pre-TKA15,107. This latter finding
is consistent with prospective findings by Edwards and colleagues54 indicating that elevated depression and
anxiety scores predict smaller subsequent improvements in back pain following both surgical and nonsurgical
treatments for sciatica.
Recent prospective studies further reinforce the potential importance of anxiety, depression, and painrelated catastrophizing in determining post-TKA outcomes. For example, higher pain catastrophizing levels
pre-TKA were found to predict greater pain intensity at 6 week post-TKA follow-up. Similarly, elevated
catastrophizing and depression levels were significant predictors of worse pain outcomes 12 months following
TKA53. Other prospective work indicates even longer term associations, with higher pre-surgical pain
catastrophizing levels significantly predicting persistent pain elevations at 24 months post-TKA63.
While outcome data indicate that up to 20% of TKA patients may have unsatisfactory pain
outcomes2,15,121,124, prior work exploring mechanisms that may underlie these negative outcomes has been
limited. Physicians not specializing in chronic pain are often only passingly familiar with CRPS, and therefore
may not recognize its presence130. However, results of one study suggest that a high proportion of TKA
patients with poor pain outcomes may be experiencing signs and symptoms of CRPS77.
CRPS Following TKA
We have conducted the only previous prospective study of CRPS following TKA77. A sample of 116
predominately osteoarthritis patients (95%) was followed from pre-TKA baseline to 6 months post-TKA. At 6
months post-TKA, 68% of patients reported highly successful pain outcomes (pain improved by at least 50%).
It was notable that at one month post-TKA, 50% of patients continued to report at least moderate pain (≥
35/100), despite radiographic verification of successful TKA. Pain trajectories of these latter patients were
variable, with 48% reporting significant pain improvements (at least 50% relief) from baseline by 6-month
follow-up, but with 35% of those initially reporting poor pain outcomes continuing to report at least moderate
pain at 6-month follow-up. For the overall study sample, 13% met diagnostic criteria for CRPS at 6-month
follow-up (persistent pain without clear pathology, allodynia and/or hyperalgesia, edema, temperature and
color changes). This CRPS incidence rate post-TKA is similar to that found following distal radial fracture in
the only other available prospective CRPS studies (11%-15%)142,143. Patients displaying CRPS at 6-month
follow-up reported significantly higher baseline pre-TKA pain intensity (0-100 VAS scale; mean of 71.8 ± 6.95
vs. 45.9 ± 3.32 for those without CRPS; p<.01), as well as significantly higher pain intensity at 6-month followup (mean of 40.6 ± 5.82 vs. 20.9 ± 3.10 for those without CRPS; p<.05). Greater anxiety levels pre-TKA
significantly predicted presence of CRPS at one-month follow-up, but not at 6-month follow-up77.
Summary
Results of prior prospective studies of post-TKA pain outcomes indicate that approximately 15% of
patients fail to obtain adequate pain relief despite surgical correction of the original pain generator
(dysfunctional knee structure). The limited set of variables tested as pre-surgical predictors of outcomes in
these prior studies generally failed to indicate significant effects, with the most consistent finding being an
association between greater pre-TKA pain intensity and less improvements in pain at follow-up. TKA may
provide a reasonable human model of acute tissue trauma for prospective studies of CRPS. One prior
prospective study reported that while TKA resulted in successful pain relief for the majority of patients,
approximately 1 in 8 develop CRPS associated with significant ongoing pain at 6-month follow-up. Multiple
pathophysiological mechanisms might theoretically contribute to development of CRPS, although none of
these has been adequately tested in prospective studies. Based on the review above, we propose a 12-month
prospective study to test four potential mechanisms that may contribute to development of CRPS following
TKA: reduced SNS function, elevated proinflammatory cytokines and neuropeptides, altered brain somatotopy
(reduced somatosensory representation of the operated limb), and elevated psychological distress.
4. Preliminary Studies/Progress Report
The PI has substantial prior experience with CRPS research, as well as experience with many of the
methods proposed to assess CRPS mechanisms in the current project. In terms of CRPS specifically, the PI
has published work related to prospective incidence of CRPS after TKA77, CRPS diagnosis28,66,75,76, CRPS
mechanisms and subtypes30,27,80, and psychological factors influencing CRPS20,23,29. Regarding the methods
proposed to evaluate CRPS mechanisms, the PI has prior experience assessing hemodynamic responses to
cold challenge30,31,32, neuropeptide levels quantified via enzyme immunoassay techniques25, and functional
imaging19. To enhance expertise on the research team, co-investigators have been included with knowledge
and experience in the areas of functional imaging (Dr. Avison, Dr. Chen), immunology (Dr. Rock), CRPS
assessment (Dr. Chung), and statistical analysis (Dr. Schildcrout). Dr. Wiley has been added as a collaborator
to assist with study aspects relating to inflammatory mechanisms. The study consultant, Dr. Birklein, has
substantial prior CRPS research experience using several key methods proposed to evaluate CRPS
mechanisms in the proposed study, including assessment of proinflammatory cytokines and neuropeptides,
assessment of altered SNS function via thermoregulatory challenge, and functional imaging to assess changes
in somatotopic maps. Dr. Drummond, another study consultant, has also been added to the team. He has
expertise regarding CRPS pathophysiology, in particular, sympathetic and catecholaminergic mechanisms. He
has significant experience in assessment of sympathetic function via thermoregulatory challenge as is
proposed in the current project. Relevant research experience of the PI is overviewed below by topic.
Prospective Evaluation of TKA Outcomes
The PI has conducted prior prospective research regarding the natural history of post-TKA pain15, as
well as the only prior prospective study of CRPS following TKA77. This project examined 116 TKA patients with
assessment at pre-TKA baseline, and 1, 3, 6, and 12 months post-TKA. Acquisition of 12 month follow-up data
was hampered by lack of personnel resources (the study was unfunded; n=70 for 12-month follow-up).
Measures obtained prospectively included signs and symptoms of CRPS, pain intensity, symptoms of anxiety
and depression, and functional outcomes. Published results of this project have been detailed above. For
purposes of the current proposal, this prospective dataset was also re-examined to derive CRPS scores like
those proposed for use in the current project (described below). These CRPS scores provided a continuous
measure of CRPS signs and symptoms at 6 and 12 months post-TKA. Increases in levels of both depression
(BDI) and anxiety (STAI) from pre-TKA baseline to 4 weeks post-TKA significantly predicted CRPS scores as 6
months post-TKA (p’s<.05), with post-TKA increases in depression also predicting higher CRPS scores at 12
months follow-up (p<.05).
Other than psychological factors, potential pathophysiological mechanisms of CRPS were not assessed
in this project, although the results support the feasibility of the proposed study. Moreover, results of this prior
work help inform estimates in the current proposal for rates of CRPS symptomatology following TKA
(described below). More generally, involvement by the PI and the co-investigator Dr. Chung in this prior TKA
project helps provide the team with realistic expectations regarding the challenges of prospective clinical
studies of this type, and will thereby facilitate optimal planning and conduct of the proposed project.
Autonomic Features of CRPS
The PI has previously conducted work in CRPS patients to evaluate signs of autonomic dysfunction
and their association with CRPS pain30. Twenty-two patients with CRPS whose limb pain was sympatheticallymaintained (SMP; pain reduced by at least 50% after SNS block) were compared to 22 non-CRPS limb pain
patients. Computer controlled thermography was conducted to compare the degree of bilateral temperature
asymmetry in the two patient groups both under resting conditions and after cold challenge to the contralateral
extremity. CRPS patients with SMP demonstrated significantly greater vasoconstriction on the affected relative
to the unaffected side at resting baseline compared to the non-CRPS group (i.e., greater temperature
asymmetry). However, cold challenge produced similar vasoconstrictive responses in both groups, regardless
of whether SMP was present. These findings suggest that CRPS pain can be sympathetically-maintained
independent of differences in SNS vasoconstrictive function, possibly via pronociceptive effects of agonist
activity at catecholamine receptors sprouting on nociceptive fibers. With regards to the proposed study, it is
notable that nearly half of the subjects displaying a “warm CRPS” pattern (possibly indicative of sympathetic
hypofunction) reported onset of CRPS following joint replacement surgery.
Our other work in a sample of 123 chronic CRPS patients reported that greater degree of evoked pain
abnormalities in CRPS (allodynia, hyperalgesia) was associated with greater bilateral temperature asymmetry
(i.e., typically affected side cooler)76. These findings might be consistent with hypothesized interactions
between catecholamines (contributing to observed vasoconstriction) and pain in CRPS.
Circulating Catecholamines
We conducted a study to explore whether systemic venous plasma catecholamine levels in CRPS
subjects with sympathetically maintained pain (SMP) differed from those found in healthy volunteers80. We
also explored whether catecholamine levels were correlated with scores on psychometric measures of
depression and anxiety. Venous blood samples from 33 CRPS/SMP patients (from unaffected limbs) and 30
healthy control subjects were assayed for plasma NE and epinephrine (E) concentrations. Plasma NE levels
were significantly higher in the CRPS group (p < .001). Statistical comparisons of E levels across groups
approached significance (p <. 06). Significant positive correlations were found between E levels and scores on
the Beck Depression Inventory (p < .05). The pattern of findings in this preliminary work suggests that elevated
NE and E levels in CRPS/SMP patients could result from the pain of CRPS and/or its consequent affective
distress. Circulating catecholamines will not be assayed in the proposed study due to the difficulty of
interpreting their functional effects in the absence of information regarding receptor sensitivity.
Relationships Between CRPS Characteristics as a Reflection of Common Mechanisms
The PI has been co-director of an international CRPS research consortium since 2005, with study sites
in the U.S., Germany, the Netherlands, and Israel. This collaborative research consortium includes Dr. Frank
Birklein, one of the proposed study consultants. As part of this work78, detailed standardized evaluations
(guided by and recorded on the CRPS database form to be used in the proposed study) were conducted in 126
CRPS patients. Data from this study support relationships between signs of CRPS that are consistent with
hypothesized mechanisms. For example, greater extent of evoked pain abnormalities (allodynia, hyperalgesia)
was associated significantly with greater edema in the affected limb. This would be consistent with the
hypothesized role of inflammatory cytokines and neuropeptides in generating both edema and peripheral
sensitization. Given that such evoked pain abnormalities are also due in part to central changes (central
sensitization in the brain and spinal cord), it is notable that greater degree of evoked pain abnormalities was
associated with significantly greater degree of spreading of CRPS symptoms, a phenomenon that prior
imaging work suggests may be related to brain changes in CRPS62.
Stages and Subtypes of CRPS
We examined a series of 113 patients meeting IASP criteria for CRPS to test for evidence of the three
sequential stages of CRPS often assumed in clinical lore27. K-Means cluster analysis was used to derive three
relatively homogeneous CRPS patient subgroups based on similarity of sign/symptom patterns. The resulting
CRPS subgroups did not differ significantly regarding pain duration as might be expected in a sequential
staging model. However, the derived subgroups were statistically-distinct, and suggested three possible
CRPS subtypes which may reflect differing contributions from multiple mechanisms: 1) a relatively limited
syndrome with vasomotor signs predominating, 2) a relatively limited syndrome with evoked pain abnormalities
predominating, and 3) a florid CRPS syndrome similar to “classic RSD” descriptions. This work underscores
the likely benefits of examining multiple CRPS mechanisms simultaneously.
Diagnosis of CRPS
The PI previously co-organized a multisite U.S. research consortium to explore CRPS diagnostic issues
empirically28,76. This project used the CRPS database form proposed for the current study to guide and record
medical evaluation of CRPS signs and symptoms. Factor analysis of CRPS signs and symptoms in 123 CRPS
patients revealed four distinct factors76. These factors were: 1) Evoked pain and sensory abnormalities
(allodynia, hyperalgesia, hyperesthesia), 2) Vasomotor disturbance (skin color or temperature changes), 3)
Sudomotor changes/edema, and 4) Motor/Trophic changes. Only the first 3 factors are currently used in
CRPS diagnosis according to the IASP diagnostic criteria. In light of the literature reviewed above, results of
this study may have a bearing on understanding the mechanisms of CRPS. For example, Factor 1 may reflect
the effects of elevated proinflammatory cytokines and peptides72,99,135 as well as altered brain somtatotopy114.
Factor 2 clearly reflects the influence of SNS dysfunction161. Finally, the common element in Factor 3 appears
to be that proinflammatory peptides such as CGRP contribute to both local sweating changes and edema11,141.
The CRPS patients from this dataset and a comparison group of non-CRPS neuropathic pain patients
were also used to develop and test a proposed revision of the current IASP diagnostic criteria for CRPS28.
Results suggested that the current IASP criteria have strong diagnostic sensitivity, but poor specificity. The
proposed modified research criteria appeared to substantially improve diagnostic specificity, yet retained
adequate sensitivity. These revised criteria have been adopted (with slight modification, as the “Budapest
Criteria”) by a CRPS taxonomy subcommittee of the IASP. However, absence of a gold standard for
objectively determining optimal CRPS diagnostic criteria (ideally, a clear known pathophysiology) makes any
improvements in diagnosis of the condition difficult. Other recent pilot data relevant to CRPS diagnosis are
detailed in context in the section below regarding CRPS outcomes.
Psychological Aspects of CRPS
The PI has a long-standing interest in the psychological aspects of CRPS17,20,23,29,80. A primary focus of
this work has been the hypothesis that psychological distress may impact more strongly on clinical pain
intensity in CRPS patients than in non-CRPS pain patients due to the prominent role of catecholaminergic pain
mechanisms in CRPS. In one study29, 34 CRPS patients displaying sympathetically-maintained pain were
compared to 165 chronic back pain patients. Associations between depression levels and MPQ-Sensory pain
intensity were significantly (p<.05) larger in CRPS patients (r=0.44) than in back pain patients (r=0.26). A
similar study examined 34 CRPS patients and 50 myofascial limb pain patients23. Results indicated that the
hyperalgesic effects of anger-out were significantly (p<.05) stronger in CRPS patients than in the non-CRPS
limb pain patients. In light of our findings that depression and catecholamine levels are significantly and
positively correlated in CRPS patients80, these studies support the hypothesis that psychological distress has a
greater negative effect on clinical pain intensity in CRPS patients than in non-CRPS pain patients possibly due
to the impact of catecholamine sensitive pain mechanisms. The PI has also published reviews of the literature
regarding psychological factors in CRPS17,20 and was involved in the only previous study of possible
psychological predictors of CRPS following TKA (results already summarized above77).
Cytokine and Neuropeptide Studies
The proposed research team has prior experience with assaying circulating neuropeptide levels (betaendorphin) using enzyme immunoassay techniques through the Vanderbilt Neuroscience Core Lab.
Specifically, we have previously demonstrated significant associations (p<.05) between greater trait anger
expressiveness and lower release of beta-endorphin in response to acute pain in a sample of 14 healthy
controls and 13 chronic back pain patients25. In another study, we obtained resting condition blood samples in
28 pain-free controls and 20 patients with moderate intensity chronic low back pain. In conjunction with the
Vanderbilt Immunology Core Lab, we used standard cytometric bead array techniques to assess plasma
cytokines, as proposed in the current application. This preliminary study129 examined whether negative affectrelated variables were associated with elevations in peripheral cytokine markers of inflammation. Participants
completed questionnaires assessing anger management style (anger-out, anger–in), trait anger, and current
depressive symptoms. We then quantified circulating plasma levels of the pro-inflammatory cytokines
interleukin-1β (IL-1β), interleukin-6 (IL-6), and TNF-alpha) at rest. Significant positive correlations were
observed between anger-out and both IL-1β and TNF-alpha levels (p < .05), and between anger-in and IL-6
levels (p < .01). Follow-up hierarchical regression analyses indicated that anger-out accounted for significant
unique incremental variance in TNF-alpha levels beyond the influence of age, body mass index, participant
type, depressive symptoms, and trait anger levels.
To address the limited previous experience of the PI specifically with proinflammatory cytokines and
peptides, the study team includes a Vanderbilt co-investigator immunologist with experience in studies of proinflammatory cytokines using the proposed cytometric bead array assays. Moreover, an outside consultant
(Dr. Birklein) has been added to the team who has significant expertise regarding assessment of both
proinflammatory cytokines and neuropeptides in CRPS patients11,12. In addition, Dr. Ronald Wiley has been
added as a collaborator. Dr. Wiley has more than 10 years of research experience regarding the nociceptive
effects of inflammatory peptides, including substance P164.
fMRI Methods
The PI has developed a collaborative relationship with the two Vanderbilt Imaging Institute coinvestigators (Dr. Avison and Dr. Chen) over the past two years, and has participated during this time in
development of pain-related protocols for use in fMRI research at Vanderbilt. Initial pilot fMRI data obtained in
seven healthy subjects using these thermal pain stimulus protocols (block design and event-related) have
recently been published19. Results demonstrated significant (p<.05) associations between elevated trait angerout and pain-related brain activations (orbitofrontal cortex, ACC area 32) and deactivations (ACC area 24,
insula, periaqueductal gray). Both Dr. Avison and Dr. Chen have prior experience with fMRI studies examining
the somatosensory cortex, which will be the primary region of interest in the fMRI component of the proposed
study36-41,148,149,156,168. Dr. Chen also has imaging experience specifically focused on pain and the
somatosensory cortex36,37. Drs. Avison, Chen, and colleagues have exploited the increased sensitivity and
functional BOLD contrast available at the ultra-high field of 7T to demonstrate the feasibility, stability and
reproducibility of high resolution single condition somatotopic mapping of digit representations in sub-areas of
S1 and the ability to resolve thalamic and cortical (S1, S2, insula, ACC) pathways of non-noxious and noxious
sensory processing in human volunteers36-41,148,149,156. To further enhance available imaging expertise for the
study, a study consultant (Dr. Birklein) has been added to the team who has prior functional imaging
experience with CRPS patients, including assessment of somatotopic maps as is proposed in this project.
5. Research Design and Methods
Design
The proposed study will use a prospective mixed between/within subjects design, with assessments at
pre-TKA baseline, and again at 6 weeks, 6 months, and 12 months following TKA.
Subjects
Study subjects will include 400 patients undergoing unilateral TKA for treatment of degenerative knee
joint changes related to osteoarthritis. Prior prospective data regarding incidence of CRPS following TKA
(13%)77 and acute fracture (11-15%)142,143 indicate that the proposed sample size would result in approximately
52 patients meeting dichotomous IASP diagnostic criteria for CRPS at 6 months post-TKA. However, as
detailed below, outcome measures, statistical plan, and power analyses will not be dependent on any assumed
rate of dichotomous CRPS diagnosis following TKA. Patients will be recruited from the Vanderbilt Joint
Replacement Center practices of two of the study co-investigators at Vanderbilt University Medical Center
(VUMC). Recruitment will be carried out both via posted flyers in this clinic and in-person introductory
presentations by designated study personnel in accordance with IRB ethical requirements. Electronic medical
records indicate that the two orthopedic surgeon co-investigators carry out more than 300 unilateral TKAs
annually, so it appears feasible to enroll the proposed sample into the study within 4 years. Demographic data
for these practices in the past year indicate that 61% of patients are female, with an average age of 63 years
(racial and ethnic data are not included in these demographic records). Because females are at greater risk
of CRPS48,136, the study will oversample females to achieve a 65% female final sample to maximize
incidence of CRPS symptomatology in the study for statistical power purposes. This gender ratio goal
approximately parallels the natural gender distribution in the study clinic population (61%). VUMC draws
broadly from the Nashville community, so it is expected that the racial and ethnic composition of the proposed
sample will approximately parallel the racial/ethnic composition of the community as a whole based on 2006
U.S. Census Bureau estimates (Non-Hispanic White: 60.9%, African-American: 27.9%, Hispanic: 7.1%).
Potential study patients will not be excluded based on gender, race, or ethnicity.
Inclusion criteria for study participation will be: 1) Intact cognitive status and ability to provide informed
consent, 2) Ability to read and write in English sufficiently to understand and complete study questionnaires, 3)
Age 21 years or older, 4) Undergoing unilateral TKA, and 5) Medical diagnosis of osteoarthritis. Exclusion
criteria will be: 1) Presence of CRPS prior to undergoing TKA (by history or on examination), 2) Undergoing
TKA revision (patients enrolling in the study for primary TKA who later require a revision will continue to be
followed, with a notation of this procedure recorded in the patient’s data file), 3) Presence of lower extremity
vascular disease, 4) Presence of malignancy, 5) Use of beta-blockers (due to the SNS function testing) and 6)
Presence of severe cardiovascular disease or other medical conditions that in the opinion of the orthopedic
surgeon co-investigators would make a patient’s study participation unsafe. All knee prosthesis components
are MRI compatible, however, patients with pacemakers, brain clips, or other conducting metal implants will be
excluded from the fMRI portion of the study to insure patient safety. Due to unknown risks of fMRI scanning to
the fetus, patients who are pregnant will be excluded from this portion of the study. Furthermore, individuals
with a self-reported history of claustrophobia or problematic anxiety in response to prior MRI scans will be
excluded from this portion of the study to minimize risk of anxiety reactions during fMRI scanning.
Surgical Procedure, Anesthesia, and Post-Operative Care
All TKA procedures will use a standard anterior approach with a medial parapatellar incision. All
components used will be Zimmer LPS-flex. All implants will be press-fit and cemented, and patellae will be
resurfaced using a cemented all-PE component. Radiographs will be obtained immediately post-operatively
and again at 6-week follow-up. These radiographs will be scored to confirm proper alignment and fixation
according to the Knee Society roentgenographic scoring system by trained raters57.
A standard anesthesia protocol will be used for all patients. Patients will receive a single spinal
anesthesia injection with preservative-free morphine pre-operatively, and during surgery will receive general
anesthesia with an endotracheal tube. This protocol will be employed in all patients with variations due only to
medical contraindications or patient refusal.
Standard postoperative care protocols will be followed. Discharge pain medications will include
Oxycontin 20mg every 12 hours and oxycodone 5-10mg every 4-5 hours as needed for the first five days postTKA. Hydrocodone 7.5mg every 4 hours as needed will then be provided until the 6 week follow-up.
Prescription data will be extracted from electronic medical records to permit examination of analgesic use as a
secondary pain outcome measure. Patients will all participate in standard physical therapy protocols either in
home (if no transportation is available) or on an outpatient basis. This therapy will address both strength and
range of motion, and will consist of five sessions per week for the first two weeks post-TKA, diminishing to
three days per week until the 6-week follow-up visit. Adherence to physical therapy treatment protocols will be
monitored for use as a potential covariate in analyses via clinic notes and patient diaries (for home therapy).
CRPS Outcomes
Internationally agreed-upon diagnostic criteria for CRPS are published by the International Association
for the Study of Pain (IASP)118 for use in clinical diagnosis. These criteria are: 1) presence of an initiating
noxious event or a cause of immobilization, 2) continuing pain, allodynia, or hyperalgesia with which the pain is
disproportionate to any inciting event, 3) evidence at some time of edema, changes in skin blood flow, or
abnormal sudomotor activity in the region of pain, and 4) no other conditions that would otherwise account for
the degree of pain and dysfunction. Previous attempts to validate these criteria empirically indicate that they
are quite sensitive, but display poor specificity (0.36-0.41)28,78. Poor specificity (high false positive rates) are a
particular problem for research sample selection, and may reduce statistical power by lowering the signal-tonoise ratio. In addition, use of dichotomous CRPS diagnoses as an outcome may be problematic due to
potential for disagreements between diagnosticians. For example, work in a large sample48 reported a kappa
value of 0.66 for CRPS diagnoses across diagnosticians. Finally, given that there is only one prior prospective
study of the incidence of CRPS post-TKA, use of dichotomous CRPS diagnoses as the primary outcome in the
proposed project would result in serious statistical power problems if the rate of CRPS diagnosis were
substantially lower than in prior work77. While ability to generalize study results to the clinical CRPS population
is desirable, the key conceptual aim of the proposed work (Specific Aim 1) is to identify mechanisms
contributing to CRPS phenomena post-TKA rather than those contributing to a specific diagnostic
categorization. Given the problems inherent in use of dichotomous CRPS diagnoses as an outcome measure,
the proposed project will instead use a continuous “CRPS Score” as the key outcome for Specific Aim 1, an
approach similar to that proposed and tested previously by our team and others66,70. Based on signs and
symptoms identified in empirical CRPS diagnostic studies28,75,76, this CRPS score will represent a total count of
CRPS-related signs (observed on physical examination) and symptoms (self-reported) exhibited by each
patient during standardized history and physical examinations (e.g., hyperalgesia, allodynia, asymmetric
edema, temperature asymmetry, skin color changes, altered sweating, weakness, hair or nail changes, etc.).
Data obtained in our recent international CRPS diagnosis project18,78 provide evidence for the validity of the
proposed CRPS score measure. First, there is a relatively large association between CRPS scores and
dichotomous CRPS diagnoses (eta = 0.76), suggesting the former may be a reasonable surrogate for clinical
CRPS diagnoses without the negative statistical features of dichotomous diagnosis measures. The
discriminant validity of CRPS scores is supported by findings that they distinguish between known CRPS and
non-CRPS neuropathic pain patients with very high sensitivity (0.95) and high specificity (0.81). Mean (±SD)
CRPS scores for the CRPS group (n=126; 12.0 ± 2.59) were also significantly higher than in the non-CRPS
neuropathic pain group (n=41; 5.4 ± 3.59; t(164) = 12.84, p<.001), further suggesting a unique association with
CRPS rather than with neuropathic pain generally. Concurrent validity of the proposed CRPS score measure
is provided by SF-36 data obtained in our recent work (see description of this measure below; lower SF-36
subscale scores indicate greater dysfunction) which revealed that higher CRPS scores were associated with
significantly greater pain intensity [r(100) = -0.37, p<.001], greater role limitations due to both physical health
[r(100) = -0.24, p<.05] and emotional problems [r(100) = -0.32, p<.001], greater emotional distress [r(100) = 0.23, p<.05], and greater social impairment [r(100) = -0.30, p<.005]. A sample as in the proposed study
including post-TKA patients with positive outcomes (few or no CRPS features) will have even greater variability
in CRPS scores, thereby enhancing statistical power for examining associations between CRPS mechanisms
and long-term CRPS-related outcomes. Use of the continuous CRPS scores outcome measure will insure that
achieving study aims is not dependent on any assumed incidence of dichotomous CRPS diagnoses post-TKA.
Medical evaluations of CRPS signs and symptoms in this study will be conducted by physician coinvestigators trained in CRPS diagnosis (O.C. and R.G.) who have prior pain management experience.
Evaluation of CRPS signs and symptoms will be guided by a standardized protocol and recorded on a CRPS
database form like that used in our prior published research regarding CRPS diagnosis (see Appendix)27,28,76.
These evaluations will be used to derive a CRPS score for each patient at each assessment period as
described above. Our work78 in a large mixed sample of CRPS and non-CRPS neuropathic pain patients (total
n = 175) with trained physician evaluators using this standardized protocol revealed a high level of internal
consistency reliability (cronbach’s alpha = 0.82) for the sign and symptom items comprising the CRPS Score.
Assessment of temperature asymmetry signs will be determined by infrared (IR) thermometry at the patella
bilaterally, with an asymmetry criterion of 1.5 deg. C based on results of prospective research distinguishing
CRPS from non-CRPS patients following distal radial fracture142. Assessment of asymmetric edema will be
determined based on measurement of leg circumference at the tibial tuberosity, with differences of 2cm or
greater considered asymmetric89. Although three distinct stages of CRPS have historically been considered14,
empirical data do not support such stages27 and therefore no attempt will be made to categorize patients
according to these stages. Given the lack of objective tests that would identify contributory nerve injury with
adequate sensitivity and specificity76, no attempt will be made to systematically distinguish between CRPS-I
and CRPS-II. This appears justified based on prior findings indicating that sign and symptom patterns do not
differ between these two subtypes of CRPS76.
Measures of Hypothesized CRPS Mechanisms
Altered SNS Function
Differences in systemic SNS function can be evaluated by monitoring changes in peripheral blood flow
in an extremity in response to a cold stimulus applied to a different extremity that will elicit systemic SNSmediated vasoconstriction52,142. Assessment of differences in SNS function in this study will permit evaluation
of pre-TKA and early post-TKA changes in SNS function as predictors of longer-term CRPS and post-TKA
chronic pain, similar to work of Schurmann et al. 142 in post-fracture patients.
As in Schurmann et al.142, prior to SNS function assessment, patients will rest for 15 minutes in a
supine position covered by an electrically heated blanket at 40-45○C to maximize vasodilation. During this
adaptation period, digit surface probes from a two-channel Laser Doppler Flowmeter (LDF; Biopac Systems,
Inc., Goleta, CA) will be affixed to the dorsal surface of the great toe on each lower extremity. LDF is a
noninvasive method allowing continuous evaluation of the microcirculation of the skin142. It is based on the
principle of Doppler shift of laser light frequency in response to backscattering of moving blood cells through
the skin. LDF output will be processed by a Biopac MP150 data acquisition system outputting to a Windowsbased computer running AcqKnowledge software (Biopac Systems, Inc., Goleta, CA). Once bilateral stable
LDF baselines are obtained and recorded for 30 seconds, ice packs will be applied simultaneously to both
upper extremities on the bicep for 10 seconds as in prior work142. Data will be recorded from the beginning of
the 30 second baseline through application of the cold stimulus and for 20 seconds following this stimulus. A
sympathetic reflex quotient (SRQ) will be derived using published procedures142: SRQ = Fpre – Fmin / Fpre, where
Fpre is the mean baseline LDF value pre-cooling, and Fmin is the minimum LDF value in the 30 seconds after
onset of the cold stimulus. Lower values will indicate less vasoconstrictive reflex (impaired SNS function).
Cytokines and Neuropeptides involved in Inflammation
Several cytokines and neuropeptides relevant to inflammation will be assayed in cerebrospinal fluid
(CSF; at baseline only) and plasma (baseline and each follow-up). During pre-surgical spinal anesthesia
induction, 4-5 mL of CSF will be obtained for analysis of proinflammatory status in the central nervous system.
CSF samples will be placed in additive-free 5-mL polypropylene tubes, and immediately placed on ice for
transport to the lab. Samples will be centrifuged and supernatant will be aspirated and split evenly into two
samples (for cytokine and neuropeptide assays) prior to being frozen at –70C until assayed. Plasma will be
obtained from 5 mL whole blood samples obtained from the antecubital vein at each visit to assess systemic
inflammatory status. Blood will be withdrawn into vacutainer tubes containing EDTA, and will be immediately
placed on ice for transport to the lab. Samples will be centrifuged and plasma will be aspirated and split evenly
into two samples (for cytokine and neuropeptide assays) prior to being frozen at –70C until assayed.
We will examine one prototypic proinflammatory cytokine (TNF-alpha) and one anti-inflammatory
cytokine (IL-10). Evidence from past work indicates elevated TNF-alpha and reduced IL-10 in CRPS patients
compared to controls113,152,162,163. Cytokine assays will be conducted in the Vanderbilt Immunology Core Lab
using standard cytometric bead array (CBA) techniques. CBA beads detecting the cytokines above and
appropriate standards and reagents will be obtained from BD Biosciences (San Jose, CA). All CBA analyses
will be conducted according to the manufacturer’s instructions as in the co-investigator’s prior work133. Twocolor flow cytometric analysis will be performed using a LSRII flow cytometer (BD Biosciences). Cytokine
concentrations will be determined based on standard curves derived from these analyses. The limits of
detection for TNF-alpha and IL-10 are 3.7 pg/mL and 2.0 pg/mL respectively. Corresponding inter-assay
coefficient of variation data are 3% and 5.8%.
Assays of two proinflammatory neuropeptides (Substance P, CGRP) that have been shown to be
elevated in CRPS patients in prior work will be conducted by the Vanderbilt Neuroscience Core Lab using
enzyme immunoassay kits and standards from SPI-Bio (Montigny le Bretonneux, France) following the
manufacturer’s standard procedures. This lab has conducted similar peptide enzyme immunoassays (for betaendorphin) in our prior work25. Neuropeptide levels in the samples will be determined based on standard
curves generated during the assays. Substance P assays can detect this peptide in the range of 3.9 - 500
pg/mL, with zero cross-reactivity with neuropeptide K, neurokinin A and B, somatostatin, and endothelin-1.
CGRP assays can detect this peptide in the range of <5.0 – 1,000 pg/mL, with zero cross-reactivity with
calcitonin, amylin, or substance P.
Brain Plasticity
A subset of patients participating in the study will be offered the opportunity to participate in the brain
imaging (fMRI) study component at pre-TKA baseline and at 6-week and 6-month post-TKA follow-ups. To
permit full prospective data collection yet maximize variability in CRPS scores at long-term follow-up to
optimize statistical power, enriched enrollment based on pre-TKA pain ratings will be employed. This strategy
is based on past findings that higher pre-TKA pain intensity predicted significantly greater likelihood of CRPS
symptomatology at 6 month post-TKA follow-up77. Potential participants for the fMRI study component will be
selected based on ratings of clinical knee pain on the MPQ VAS at the pre-TKA assessment visit. Based on
the 25th and 75th percentiles of pre-TKA pain intensity from past work15,77, patients reporting pre-TKA pain of
37/100 or less (“Low Pain”) or 71/100 or higher (“High Pain”) will be expected to be at low and high risk
respectively for CRPS symptomatology. The estimated base rate for satisfactory post-TKA pain outcomes is
80-85%2,15,121,124, with approximately 15-20% exhibiting continued problematic pain at 6 month follow-up15, with
most of these patients exhibiting signs and symptoms of CRPS77. Therefore we initially plan to enroll 20 Low
Pain subjects in the imaging portion of the study, assuming that 16 patients (80% of 20) will display little pain
and few or no CRPS signs and symptoms at 6-month follow-up. We plan initially on enrolling 55 High Pain
group subjects, assuming that approximately 11 of these patients (20% of 55) will at 6-month follow-up display
relatively intense chronic pain and substantial signs and symptoms of CRPS. As patients enrolled in the
imaging portion of the study complete the 6 month post-TKA assessment period, CRPS scores at that visit will
be reviewed and classified as to whether they meet an operational criterion for “non-CRPS” status: a CRPS
score of 4 or less. This cutoff is based on the upper bound of the 95% confidence interval for CRPS scores in
the non-CRPS group at 6 months post-TKA in our prior prospective TKA work77. Once 16 patients categorized
as “non-CRPS” using this definition have been enrolled in the imaging portion of the study, all remaining
subject recruitment for the imaging study will target High Pain group patients, to insure adequate
representation of patients with significant pain and high CRPS scores at long-term follow-up. The strategy
above of enriching the imaging study sample with patients likely to have more extreme (high and low) CRPS
scores will help maximize statistical power for analyses of associations between CRPS scores and continuous
measures of asymmetry in somatosensory representation of the lower limbs. Recruitment for the fMRI portion
of the study will continue until recruitment targets are met.
Qualifying patients who agree to participate will undergo an initial baseline pre-TKA fMRI evaluation,
with this evaluation repeated at 6-week and 6-month post-TKA follow-up, at which point it is expected that
CRPS will have developed in those susceptible to it. Each fMRI evaluation will consist of bilateral sensory
mapping of the lower extremities using a protocol similar to that routinely used by Dr. Avison, Dr. Chen, and
colleagues for high resolution somatotopic mapping of the hand and digits38-40,148,149,156.
Somatotopic maps of the bilateral lower extremities will be obtained using a research-dedicated wholebody 7 T MRI scanner (Philips Achieva) equipped with a 16-channel parallel array head coil (Nova Medical).
This scanner, located in the Center for Human Imaging at the Vanderbilt University Institute of Imaging Science
(VUIIS), provides significantly increased intrinsic sensitivity as well as increased blood oxygenation level
dependent (BOLD) contrast during functional activation studies. These sensitivity and functional contrast gains
translate to significantly increased (~10-fold) spatial resolution during fMRI mapping studies compared with
standard BOLD fMRI at 3T (1x1x2 mm vs. 2-3 mm isotropic). During a single mapping session on the 7T MRI
scanner, a T1 weighted 3D structural brain image will first be collected for distortion correction and anatomic
coregistration of functional maps, then a series of functional mapping runs will be performed. During
sequential runs, a vibratory stimulus (applied via pulsed air puffs at 8 Hz) will be applied to the large toe,
middle toe, small toe, dorsal heel of foot, and the dorsal calf of the ipsi- and contralateral limb. These BOLD
fMRI data will be collected using a standard block design (three runs each consisting of five blocks of 15 sec
stimulus followed by 15 sec rest applied to the five stimulus targets in randomized order) and single shot
gradient echo BOLD-EPI (Flip angle 80 degrees; TR/TE = 2000/25 ms; 192x192 matrix, 2 mm slice thickness;
192 mm Field Of View), with parallel imaging acceleration factor of 4 to improve brain coverage for a given
temporal resolution and to reduce distortion from macroscopic static magnetic field inhomogeneities.
Psychological Measures
As described previously, elevated psychological distress and catastrophizing could through
associations with catecholamine or inflammatory mechanisms impact on development of CRPS. Prospective
associations between baseline depression and anxiety and chronic pain-related outcomes have been
demonstrated in other pain conditions53,54,55,110,137. Several prospective studies suggest that greater anxiety,
depression and pain-related catastrophizing may predict greater post-TKA pain intensity and possibly risk for
CRPS post-TKA53,63,77,151. Several measures will be used to test the hypothesis that elevated pre-TKA
emotional distress and catastrophizing contributes CRPS development and greater post-TKA pain.
Depression will be assessed using the Beck Depression Inventory (BDI), a widely used standardized
measure of depressive symptoms5. It has demonstrated adequate psychometric properties and validity when
used in chronic pain samples123. Our prior work indicates that greater depression is associated with greater
CRPS pain intensity29,77. Moreover, depression shows a significantly stronger relationship with clinical pain
intensity in CRPS patients than in chronic back pain patients, suggesting the possibility of unique mechanistic
interactions in CRPS29.
Anxiety will be assessed using the trait form of the State Trait Anxiety Inventory (STAI)146, a widely
used and well-validated standardized measure of individuals' tendency to experience anxiety symptoms.
Higher STAI scores at baseline predicted greater likelihood of displaying early CRPS-like phenomena at 4
weeks post-TKA in our prior prospective work77.
Anger expression will be assessed using the Spielberger Anger Expression Inventory (AEI)147. The AEI
is a validated self-report measure of anger management style147, including a subscale assessing anger-out
(managing anger via direct verbal or physical expression). Higher anger-out scores are associated with
greater pain intensity in CRPS patients23. In addition, anger-out has a significantly greater impact on pain
intensity in CRPS than in non-CRPS limb pain, suggesting the possibility of unique mechanistic interactions23.
Pain catastrophizing will be assessed using the 13-item Pain Catastrophizing Scale (PCS)150. Items tap
three core dimensions of pain catastrophizing: rumination (e.g., “I keep thinking about how badly I want the
pain to stop”), helplessness (e.g., “I feel I can’t go on”), and magnification (e.g., “I wonder whether something
serious may happen). The PCS has strong reliability and validity150. It has been used in prior studies that have
demonstrated prospective links between pain catastrophizing and long-term post-TKA pain outcomes63,151.
Each of the psychological measures above will be administered at initial pre-TKA baseline, and again at
each follow-up visit. They will serve both as predictors of CRPS scores and pain at 6-month and 12-month
follow-up, as well as outcome measures tracking post-TKA psychosocial outcomes over time.
Other Factors Potentially Influencing Post-TKA Pain Outcomes
Based on earlier prospective studies of post-TKA pain15,124, data will also be obtained on a variety of
clinical and demographic variables that could predict pain-related outcomes following TKA. These data will
include age, gender, body mass index, comorbid presurgical medical conditions, intra-operative complications
(approach requiring osteotomy of the tibial tuberosity, bone defects requiring bone substitutes, severe
intraoperative alteration of patellar tracking, deformity of two axes (varus, valgus) >15 degrees, prolonged
surgical time), and post-surgical complications (infection, wound dehiscence, cellulites, hemarthrosis, deep
vein thrombosis, blood loss requiring transfusion, superficial thrombophlebitis, myocardial infarction,
lymphedema, and pneumonia.). Given that female gender and greater age appear to be risk factors for
CRPS48, data on menopausal status and use of hormone replacement therapy will also be recorded. All of the
data above will be obtained via an initial demographics questionnaire or extracted from electronic medical
records. In addition, presurgical severity of knee osteoarthritis will be assessed as a potential predictor of
outcomes using the Kellgren and Lawrence criteria15,124. Finally, given one study suggesting that pre-surgical
presence of non-knee chronic pain might predict worse pain-relate outcomes124, standardized data on
comorbid pre-surgical chronic pain conditions will be obtained using the Persistent Pain Questionnaire26.
Potential Medication Confounds
Although all patients will receive a standard medication regimen for the first 6-weeks post-surgically, the
possibility is considered that patients with ongoing problematic pain may receive additional interventions for
their pain. For ethical reasons, standard practice interventions in these situations will be permitted, but all such
interventions will be recorded (from electronic medical records) for use as potential covariates in analyses.
Interventions used by the orthopedic co-investigators for persistent post-TKA pain are primarily analgesic
medications as needed. Neither of these co-investigators uses sympatholytic blocks for post-TKA pain, thus
eliminating what otherwise might prove a significant confound to examination of CRPS signs and symptoms.
Primary Measure of Pain Intensity Outcomes
The McGill Pain Questionnaire – Short Form (MPQ)117 will be used to characterize clinical pain intensity
at all time points. It is a widely used and well-validated measure of the sensory and affective components of
pain117. It has been used to assess pain outcomes in our past TKA work77. MPQ instructions will be modified
to refer specifically to pain in the affected knee. For the fMRI portion of the study, the visual analog scale on
the MPQ will be used for purposes of classifying patients as to long-term CRPS risk based on pre-TKA pain
levels as suggested by results of Harden et al.77.
Functional Outcome Measures
In addition to CRPS diagnosis, pain intensity, and the psychological predictors/outcomes described
above, broader functional outcomes will also be assessed at each visit using two instruments. For purposes of
testing study hypotheses, these are considered secondary measures. The most common functional outcome
instrument specific to the TKA literature is the Western Ontario and McMaster Universities Osteoarthritis Index
(WOMAC)6,115. The WOMAC is a well-validated 24 item Likert format (0-4) questionnaire that produces 3
subscales: pain, stiffness, and physical function115. Higher scale scores indicate greater symptoms.
The SF-36 is a more general functional outcomes questionnaire that is widely-used in medical
research116,158. It consists of 36 Likert format items, and produces 8 subscales tapping into the areas of
general health, pain, emotional well-being, and the impact of physical and emotional conditions on physical
and social functioning. Lower scale scores indicate worse health and functioning. Both of the measures above
will be used in part to determine whether patients experiencing CRPS at long-term follow-up were experiencing
pain-related dysfunction prior to surgery to a similar degree as non-CRPS patients.
Project Timeline
The proposed timeline for conduct of the study is summarized below. For simplicity of budgeting,
patients’ initial visits and three follow-up visits have been considered to all occur within a given grant budget
year. In practice, a number of patients enrolled in year 4 will continue to have follow-up information obtained in
budget year 5. We anticipate that all 12-month follow-ups will be completed by mid-year in budget year 5, with
the remainder of the year devoted fully to data analysis, interpretation, and preparing publications.
Project
Year:
1
2
3
4
5
Study
Initiation
XX
Recruit/Train
Personnel
XX
Subject
Recruitment
XX
XX
XX
XX
Primary Subject Goal
(10% oversampling
for attrition)
88
143
143
66
Follow-Up Only
fMRI
Subject Goal
30
15
15
15
Data
Entry
XX
XX
XX
XX
XX
Data
Analysis
Prepare Manuscripts/
Conference Presentations
Interim
XX
XX
XX
XX
XX
Procedures
The figure below presents an overview of the study design and procedures:
Pre-TKA
Baseline
(n=400)
TKA
fMRI study recruitment
based on pre-TKA
pain intensity
Pre-TKA fMRI
Evaluation
n≈20 Low Pain
n≈55 High Pain
TKA
6 Week
Post-TKA
Follow-Up
6 Month
Post-TKA
Follow-Up
12 Month
Post-TKA
Follow-Up
Targeted enriched enrollment based on
CRPS scores at 6 month follow-up
6 Week PostTKA Follow-Up
fMRI
6 Month Post-TKA
Follow-Up fMRI
Low CRPS n≈16
High CRPS n≈11
Primary Outcomes Assessed Prospectively:
Specific Aim 1: Continuous CRPS Score at 6 and 12 Months Post-TKA
Specific Aim 2: Knee Pain Intensity at Each Post-TKA Follow-Up
Hypothesized CRPS Mechanisms Assessed Prospectively:
•Reduced Sympathetic Nervous System Function
•Elevated Proinflammatory Cytokines and Peptides
•Reduced Somatosensory Representation of the Affected Limb
•Elevated Psychological Distress
Pre-TKA Baseline
Potential study patients will be screened in the preoperative evaluation clinic (several days prior to
surgery) to determine eligibility for participation in the study according to the criteria described above. For
those meeting study entrance criteria, study procedures and risks will be described by the study designee, and
written informed consent will be obtained. After instructions are provided, patients will complete a
demographic information questionnaire and the initial baseline packet of questionnaires to describe their
current state (MPQ, BDI, STAI, PCS, AEI, SF-36, WOMAC). Baseline evaluation of CRPS signs and
symptoms will be conducted by a trained pain physician co-investigator (Dr. Gupta and/or Dr. Chung) for use in
calculating CRPS scores. During the course of preoperative evaluation procedures during which blood
samples are normally drawn, an additional sample will be drawn specifically for assay of specified plasma
cytokines and neuropeptides (5 mL).
Psychophysiological testing will then be conducted in a designated temperature-controlled examination
room. Study patients will participate in the SNS function test in a supine position as described above, after
which they will return to a seated upright position in a comfortable chair for a 15-minute rest period to allow
skin temperatures to return to baseline. Patients will receive $50 for their participation in this initial
assessment.
Follow-up Evaluations
At each post-TKA follow-up evaluation (6-week, 6-month, 12-month), patients will again complete the
same packet of questionnaires to describe their current state at that follow-up visit (MPQ, BDI, STAI, PCS, AEI,
SF-36, WOMAC). Medical evaluation of CRPS signs and symptoms will be conducted by the designated
physician co-investigator at each follow-up visit for use in calculating CRPS scores. During each follow-up
visit, blood samples will be drawn from the antecubital vein for assay of specified plasma cytokines and
neuropeptides (5 mL).
At each follow-up visit, psychophysiological testing will again be conducted in a designated
temperature-controlled examination room. Study patients will participate in the SNS function test in a supine
position, after which they will return to a seated upright position in a comfortable chair for a 15-minute rest
period to allow skin temperatures to return to baseline. Patients will receive $50 for their study participation at
each follow-up visit.
fMRI Study
Patients at relatively low and high risk for post-TKA CRPS symptomatology based on pre-TKA knee
pain levels will be identified for the fMRI study component using the enriched enrollment strategy described
above. Patients meeting enrollment criteria for the fMRI study component will be approached during their preTKA assessment visit. Prospective patients will be provided with detailed information on the fMRI study
procedures and risks, and those who qualify and are willing to participate will provide separate written informed
consent. Recruitment for the fMRI portion of the study will continue in the fashion described above until
enrollment is completed as described in the study timeline.
Patients participating in the fMRI portion of the study will be asked to schedule a visit to the Vanderbilt
Imaging Institute for fMRI evaluation as soon after the pre-TKA assessment as is feasible. Detailed
procedures for the fMRI evaluation are provided above. Three fMRI evaluation sessions will be conducted,
one at the pre-TKA time point, a second at the 6-week follow-up, and the last at the 6-month follow-up.
Patients will receive $150 for their participation in each fMRI session.
Data Management Plan
Raw hardcopy psychometric, pain, demographic, and clinical data will be maintained in a single hardcopy
file for each subject, with these raw data files kept in the PI’s locked office files for at least six years. SNS
function testing data will be saved directly to a password protected laptop computer. SNS function data for
each subject will be visually inspected to identify invalid values, which will be recoded as missing data. The
validated SNS function data will be imported into SPSS for data reduction (e.g., calculation of means and SRQ
values). All raw hardcopy data will be double entered into the primary SPSS computer database using SPSS
Data Entry 2.0 to maximize accuracy of data entry. This data entry computer will be password protected, and
stored in a locked office. Visual inspection of the SPSS dataset will be conducted to identify questionable
(e.g., out of expected range) values for key measures, and where identified, these will be recoded as missing
data. After conducting preliminary and descriptive analyses, the final SPSS data file will be exported for use in
the primary analyses described below.
Statistical and Power Analyses
Primary analyses will be performed using the R Programming Language (R Foundation for Statistical
Computing, Vienna, Austria) and specialized packages GEEPACK166 and LME4. A two-tailed probability value
of p<0.05 will be used for statistical significance in all analyses. The goal of Specific Aim 1 is to examine the
relationship between pre-TKA and acute post-TKA risk factors and the CRPS score at six months and one year
post-TKA. The goal of Specific Aim 2 is to study the relationship between the same risk factors and measures
of knee pain at six weeks, six months, and one year post-TKA. We will consider the following risk factor
measures for analyses: SNS function as measured by the sympathetic reflex quotient (SRQ); a proinflammatory cytokine, TNF-alpha; an anti-inflammatory cytokine, IL-10; pro-inflammatory neuropeptides,
substance P and CGRP; psychological measures including the State Trait Anxiety Inventory (STAI), the Beck
Depression Index (BDI), the Pain Catastrophizing Scale (PCS), and the Spielberger Anger Expression
Inventory (SAEI); and the measure of asymmetry in somatosensory representation of the lower extremities
captured via fMRI. With the exception of the fMRI asymmetry measures, risk factors will be collected in 400
patients. The asymmetry measure will be collected in 75 patients.
Hypotheses
Hypotheses to be tested for Specific Aim 1 are that presence of the following risk factors at pre-TKA
baseline and at 6 weeks post-TKA will predict significantly higher CRPS scores at 6 and 12-month post-TKA
follow-up:
1) Lower SNS function as reflected in less vasoconstrictive response to cold challenge (SRQ),
2) Higher CSF (pre-TKA baseline only) and circulating levels of Substance P, CGRP, and TNF-alpha,
and lower levels of IL-10,
3) Higher anxiety (STAI), depression (BDI), catastrophizing (PCS), and anger expression (SAEI), and
4) Greater asymmetry in the somatosensory representation of the operated limb relative to the contralateral
limb (operated limb area of representation < contralateral limb).
Hypotheses to be tested for Specific Aim 2 are that irrespective of CRPS symptomatology, ratings of
knee pain at each post-TKA follow-up will be predicted by risk factor values at the preceding time point (lagged
values) in the directions detailed in Specific Aim 1 above.
Plan of Analysis: Specific Aim 1
To examine the relationship between risk factors and CRPS outcomes, we will conduct multiple linear
regression analyses. For each of the two endpoints, CRPS score at six months and at one year post-TKA,
three models will be considered. The first model will include risk factors measured at baseline and the second
will consider risk factors measured six weeks post-TKA. To address the related issue of whether changes in
risk factors from pre-TKA to 6 weeks post-TKA impact CRPS outcomes, the third model will use 6 week postTKA measures residualized for pre-TKA baseline values (essentially, baseline adjusted change scores).
Regression analyses will adjust for demographic characteristics including body mass index (BMI),
socioeconomic status (SES), gender, age, and race/ethnicity, and relevant clinical confounds described above.
Because the fMRI asymmetry measure will only be obtained in 75 patients, regression analyses that involve it
will be conducted separately. In addition to the primary analysis, we will conduct secondary analyses that will
include all possible two-way interactions between risk factors. With 400 subjects, we will be able to estimate
all parameters without having to conduct variable selection or shrinkage procedures.
Initial analyses of functional imaging data will be conducted using a standard General Linear Model
(GLM) within the Statistical Parametric Mapping software package (SPM5; Wellcome Department of Cognitive
Neurology, London, UK). Ipsi- and contralateral volumes of activation within S1 will be defined as the
contiguous activated volume at a corrected p<0.05 threshold for subsequent Region of Interest (ROI) analyses.
Mean signal changes will be determined from the appropriate beta weights within the GLM. These data will be
used to generate continuous variables for use in both preliminary and primary analyses of functional imaging
data. Based on the ipsi- and contralateral activation volumes derived above, we will generate the primary fMRI
variable of interest: the asymmetry ratio for these volumes. Preliminary analyses will examine intercorrelations
between the asymmetry ratio measure and remaining pathophysiological measures that may impact on CRPS
development. For example, we anticipate that if deafferentation related to the degree of nerve trauma
contributes to altered somatotopic maps, the fMRI asymmetry ratio may be correlated with other measures that
may be dependent upon degree of nerve injury (proinflammatory cytokines and neuropeptides). Primary
analyses modeled after the regressions described above will then be conducted using the fMRI-derived
asymmetry ratio (at pre-TKA baseline and 6 weeks) as the predictor of 6-month and 12-month CRPS scores.
Power Analysis for Specific Aim 1
In order to examine the feasibility of this study, we conducted simulation based power analyses for one
of the primary analyses. At each of 1000 replications, we generated data for risk factors at baseline and the
six-month CRPS score. We then used multiple linear regression to fit the following model using the simulated
data,
where denotes subject and is unexplained or random error. The estimated power for each risk factor is
equal to the proportion of the replications that the associated parameter was statistically significant at the twosided p<0.05 significance level.
In our search for prior research to inform these analyses, we found that authors studying CRPS
considered it as a binary (yes/no) variable. Since we will examine CRPS as a continuous score, prior research
cannot completely inform our study. However, for the power analysis, we used prior study results to generate
the risk factor data. Using the binary definition, we assume approximately 13% of patients (based on results of
Harden et al.77) will develop CRPS at six months following TKA (52 of the 400 participants). We then
generated risk factor data based on the following results pulled from existing literature: mean (SD) sympathetic
reflex quotient (SRQ) was 0.59 (SD=.16) for healthy patients and 0.19 (SD=0.10) for CRPS patients142;
median [25th, 75th] IL-10 cytokine was 0.1 [0.05, 0.15] for CRPS patients, and 1.1 [1.0, 2.9] for non-CRPS
patients152; median [25th, 75th]; TNF-alpha was 7.8 [5.1, 16.7] in the uninvolved side of CRPS patients and
31.6 [7.7, 90.4] in the involved side120; mean CGRP for CRPS patients was 122.2 (SD=14.6) with a Cohen's D
effect size (compared to healthy controls) equal to 0.8612; and mean substance P was 392 (SD=160.0) for
CRPS patients with Cohen's D effect size = 1.0956. To generate data for the other risk factors, we used our
preliminary study results (from Harden et al.77). Specifically, we assumed the mean BDI and STAI for CRPS
patients was 10.4 (SD=1.67) and 37.6 (SD=5.94), respectively, and for non-CRPS patients, it was 6.9 (0.83)
and 29.9 (1.42). The correlation between the BDI and STAI was approximately equal to 0.81. Though we also
found pain catastrophizing to be highly correlated with STAI (rho=0.63) and BDI (rho=0.58), it is not considered
in this power calculation. For most risk factors, we assumed Gaussian distributions for CRPS and non-CRPS
participants; however, TNF-alpha and IL-10 were generated using gamma distributions with shape and rate
parameters set to values so that median and 75th percentiles were equal to those shown above. Additionally,
we generated the BDI and STAI variables so as to reflect their correlation.
After we generated risk factor data, we used the above model to generate CRPS values for each of the
400 subjects based on a range of effect sizes. Once CRPS data were generated, risk factor and CRPS values
were standardized by dividing by the standard deviation of each variable. The model was fit and statistical
significance was assessed. Power analysis results are shown in Table 1 below, with effect sizes expressed in
terms of standardized beta values ( ). The top half of the table displays results from models where anxiety
and depression were included simultaneously. Due to their high correlation, the power to detect either effect
was low relative to other risk factors. Eighty percent power for these variables was obtained when the =0.2.
All other covariate parameters had 80 percent power as long as was at least 0.14. In the bottom half of the
table, we excluded STAI from the regression model, and the power to detect the effect of depression (BDI) was
at least 97 percent for all values of studied. Because depression, anxiety, and pain catastrophizing are
highly correlated, in the analyses of these data we will run separate regression models for the three, and we
will consider creating a composite negative affect variable. The only available prospective TKA data with
regards to prediction of CRPS features (pilot data described previously) revealed a ranging from 0.25 - 0.36
for predicting CRPS scores 6 months post-TKA using changes in BDI and STAI scores from pre-TKA to 4
weeks post-TKA. Based on these findings and our calculations below, we believe that the sample size for this
study will permit a thorough and well powered examination of the impact that the hypothesized risk factors
have on CRPS symptomatology
Table 1: Simulation-based power calculations for seven primary risk factors on CRPS scores at six months
post-TKA. Effect size is expressed in terms of , the standardized beta which corresponds to standard
deviation change in CRPS score associated with a standard deviation change in the risk factor. Results
displayed in the top half of the table reflect models where all risk factors, including highly correlated BDI and
STAI (rho=0.81) were included in the multiple regression models. Results displayed in the bottom half of the
table reflect results from models where STAI was excluded.
0.11
0.14
0.17
0.20
0.11
0.14
0.17
0.20
SRQ
0.55
0.85
0.98
1.00
0.57
0.86
0.98
1.00
Substance
P
0.66
0.92
0.99
1.00
0.68
0.91
0.99
1.00
CGRP
0.67
0.92
1.00
1.00
0.67
0.92
1.00
1.00
TNF-alpha
0.6
0.87
0.99
1.00
0.61
0.88
0.99
1.00
IL-10
0.66
0.91
1.00
1.00
0.66
0.91
1.00
1.00
BDI
0.19
0.30
0.58
0.88
0.97
1.00
1.00
1.00
STAI
0.21
0.35
0.59
0.86
-----
The study of the relationship between the fMRI asymmetry measure at pre-TKA baseline and 6 weeks
post-TKA and CRPS score at 6 months and 1 year will be conducted on 75 participants who, via enriched
enrollment, should reflect approximately equal numbers of patients with high and low CRPS symptomatology
at 6 months post-TKA. Maihofner et al.111 studied differences between CRPS patients and healthy subjects in
Euclidean distance between upper extremity digits 1 and 5 (D1/D5). According to their figure 5, effect sizes
were very large, with the mean distance for normal patients being greater than seventy percent higher than the
mean distance for CRPS patients. While the measures used in Maihofer et al.111 differ from what we will be
measuring, if we assume effect sizes of the similar magnitudes and further consider that CRPS in our study will
be measured on a continuous rather than a binary scale, we will have very high power to detect an effect of
asymmetry in somatotopic maps on CRPS scores.
Plan of Analysis and Power Analysis: Specific Aim 2
In Specific Aim 2, we will conduct longitudinal data analyses to examine the relationships between risk
factors described above and measures of pain at 6 weeks, 6 months and one year post-TKA. Dependent
variables will include the visual analogue pain (VAP) score (operating on a 0 to 100 scale), and sensory and
affective measures based on the MPQ (operating on 0 to 33 and 0 to 12 scales, respectively). Independent
variables will include the time-fixed demographic and baseline characteristics such as BMI, SES, gender, age
and race/ethnicity and time-varying physiological and psychological risk factors observed at the time-points
preceding the observed response (e.g., the lagged values of the risk factors). For example, ignoring
demographics, the VAP score model is given by
where denotes the time-point with values 6 weeks post-TKA, 6 months post-TKA, and 1 year post-TKA.
When j is equal to 6-weeks post-TKA, the lagged values for the risk factors correspond to those from the preTKA measurement.
Since the
values are likely to be correlated within subjects, we will estimate parameters from linear
mixed effects models103 with restricted maximum likelihood and mean model parameters without assumptions
regarding random effects distributions with generalized estimating equations (GEE105). Unlike the longitudinal
binary data setting167, parameter estimates from fitting mixed models and those based on GEE have the same
interpretations for continuous response data. The choice of the analysis procedure can therefore be based
upon operating characteristics of the estimators which we now detail.
Mixed effects models are appealing because they are insensitive to missing at random dropout and
intermittently missing data108 and they are highly efficient (low variance). However, with mis-specified
longitudinal correlation structures, inferences may be invalid due to biased estimates of uncertainty. In
contrast, GEE estimates tend to be more sensitive to missing data, but they are robust to response correlation
misspecification. Additionally, with proper specification of the working correlation structure their efficiency is
almost as high as maximum likelihood. Our intent is to report results that can be reproduced by other
investigators. So, if exploration of the correlation structure indicates that it is of a complex functional form or if
it appears the normality assumption of the random effects is likely to be violated, we will proceed with GEE. In
contrast, if our realized concerns primarily involve missing data then we will either use mixed effects models or
we will use inverse probability of selection weighted GEE132,138.
By using repeated measures analyses, we have more data to capture risk factor effects than we would
with a single observation per participant. We will therefore be able to detect effect sizes that are smaller than
those from Specific Aim 1. Thus, we believe the sample size will be adequate.
Secondary Analyses
Given that female gender and greater age appear to be risk factors for CRPS48,136, gender, menopausal
status, and use of hormone replacement therapy will also be explored in analyses similar to those above with
regards to their association with CRPS symptomatology and pain outcomes. Interrelationships between these
factors and pathophysiological mechanisms identified as risk factors for CRPS will be explored. Although
existing data do not provide strong evidence that pre-operative demographic and comorbid disease factors,
intra-operative variables, or post-operative complications impact significantly on post-TKA pain outcomes,
these issues will be examined in secondary analyses.