2004-01 Density in Patients With Chronic Low Back Pain
Study Design.
A randomized study. Objectives. To compare muscle strength, cross-sectional area, and density of
the back muscles in two categories of patients with chronic low back pain, randomized to either
lumbar fusion or cognitive intervention and exercises.
Summary of Background Data. In two clinical trials, patients with chronic low back pain plus disc
degeneration and postlaminectomy syndrome, respectively, were randomized to either lumbar fusion
or cognitive intervention and exercises. We have previously reported that results for the primary
outcome were similar at the 1-year follow-up examination.
Methods.
As the treatment alternatives and test procedures were identical, the two trials were merged into
one. A total of 124 patients 25 to 60 years of age were included. Muscle strength, measured by
isokinetic test device and by the Biering-Sørensen Test, was measured in 112 patients, and the crosssectional area and density of the back muscles were measured in 61 patients at the inclusion and at
the 1-year follow-up examination.
Results.
The exercise group performed significantly better in muscle strength than did the lumbar fusion
group, with the mean difference at 184 Nm (95% confidence interval, 64–303 Nm; P
0.003) and for
the Biering- Sørensen Test 21 seconds (95% confidence interval, 6–36 seconds; P
0.006). The
density at L3–L4 decreased in the lumbar fusion group but remained unchanged in the exercise group.
The mean difference was 5.3 HU (95% confidence interval, 1.1–9.5 HU; P
0.01). The crosssectional area was unchanged in both groups.
Conclusions.
Patients with chronic low back pain who followed cognitive intervention and exercise programs
improved significantly in muscle strength compared with patients who underwent lumbar fusion. In
the lumbar fusion group, density decreased significantly at L3–L4 compared with the exercise group.
[Key words: crosssectional area, density, muscle strength, back muscles, lumbar fusion, cognitive
intervention] Spine 2004;29:3–8
Lumbar fusion rates for chronic low back pain have increased markedly over the last 20 years, but
variations between countries do exist.1 One randomized study reported that lumbar fusion mitigated
pain and disability better than usual care within the primary health system.2 Exercise therapy for
chronic low back pain is recommended by several guidelines.3–5 According to recent systematic
reviews, there is strong evidence for exercises and cognitive therapy being more effective than normal
care provided by general practitioners, although evidence in favor of any specific exercise is
lacking.6,7 Studies have reported that patients with chronic low back pain have reduced muscle
strength and greater atrophy of the back muscles compared with healthy persons. 8,9 An increase in
muscle strength and the crosssectional area of the back muscles has been reported after various
exercise programs.10–12 In contrast, reduced muscle strength and atrophy of the back muscles have
been demonstrated after lumbar fusion.13–15 We have previously reported results of two randomized
studies comparing cognitive intervention and exercises with instrumental lumbar fusion. At the 1-year
follow- up examination, no differences between groups were found for disability, the main outcome
variable (Oswestry Disability Index),16,17 or for pain, a secondary outcome variable.18,19 The
purpose of the present study was to investigate the differences in muscle strength, cross-sectional
area, and density of the back muscles in patients with chronic low back pain and disc degeneration
and in patients with postlaminectomy syndrome, randomized to either lumbar fusion or cognitive
intervention and exercises.
Methods
Patients and Study Design.
Patients with either chronic low back and disc degeneration or postlaminectomy syndrome were
enrolled in two randomized studies of lumbar fusion or cognitive intervention and exercises.17,18 The
patients with postlaminectomy syndrome have previously had back surgery for disc herniation with
laminectomy. As the treatment alternatives and tests procedures were identical, the two studies were
merged into one for evaluating muscle strength and crosssectional area. The patients were referred to
the Department of Orthopaedics, Rikshospitalet University Hospital, Oslo from all regions of Norway.
The criteria for inclusions were as follows: age 25–60 years; reported low back pain for at least 1
year; a score of 30 of 100 points on the Oswestry Disability
Index (ODI)16,17; and degenerative changes at the L4–L5 and/or L5–S1 on plain radiographs or
previously performed surgery for disc herniation with laminectomy. Patients were excluded if they had
widespread myofacial pain, spinal stenosis with reduced walking distance and neurologic signs, disc
herniation or lateral recess stenosis with clinical signs of radiculopathy, inflammatory disease,
previous spinal fracture, the pelvic girdle syndrome, generalized degenerative changes on plain
radiograph examination, serious somatic or psychiatric disease that excluded either one or both
treatment alternatives, registered medical abuse, or reluctance to accept one or both the treatment
regimens of the study. Of a total of 124 patients, we performed both isokinetic trunk muscle strength
test and the Biering-Sørensen Test at the inclusion and at the 1-year follow-up examination in 112
patients. Of these, 60 patients (33 with chronic low back pain and disc degeneration and 27 with
postlaminectomy syndrome) were randomized to lumbar fusion and 52 patients (25 with chronic low
back pain and disc degeneration and 27 with postlaminectomy syndrome) to cognitive intervention
and exercises. Furthermore, in 61 of the 112 patients, measurements by computed tomography (CT)
of back muscle cross-sectional area and density were performed. Such measurements represent an
expensive and capacity demanding procedure, which is the reason why not all patients had this
examination. ODI was used to evaluate condition-specific disability and pain at inclusion and at 1-year
follow-up. Patients answered 10 questions about pain and pain-related disability of activities of daily
life and social participation.19 All patients were given oral and written information about the study and
the two treatment alternatives. They were told that they could withdraw from the study at any time
without any further explanation. The ethics committee for medical research in health region I of
Norway approved the study.
Treatments.
The lumbar fusion consisted of posterolateral autologous bone transplantation and transpedicular
screw fixation of either the segment L4–L5 and/or L5–S1. Exercises were not recommended for the
first 3 months after the operation. Otherwise, postoperative rehabilitation was at the choice of the
surgeon and not according to any study protocol. As a standard procedure, physiotherapists at the
respective departments gave advice on physical activities during the first 3 months after surgery.
Patients had follow-up consultations with the surgeon after 3 and 6 months. The cognitive intervention
and exercises were conducted at the Department of Physiotherapy, Rikshospitalet University Hospital.
The program had been evaluated in a pilot study. The first week a specialist in physical medicine and
rehabilitation gave a lecture. He explained to the patients about the pain receptors in the disc, facet
joints, and muscles; the reflexive interplay between various structures; and the ability to suppress
and reinforce various peripheral stimuli. The patients were given the comprehension that they could
not do any harm to the disc by ordinary activities of daily life. The patients were told to use their back,
to bend it, and not to be particularly cautious.20 This information was reinforced every day by way of
various physical activities and discussions. An attempt was made to give consistent information and to
ensure that any disagreements between supervisors were discussed in order to reach consensus. The
patients allocated to cognitive intervention and exercises were challenged mentally as they were
encouraged to participate in physical activities, which previously had not been recommended for their
cases. This included vacuum cleaning, jumping, lifting, and ball games. On the basis of knowledge
from studies on applied work physiology, the patients were told to bend their back while lifting light
objects and to bend their knees while lifting heavy objects.21 Individual exercise was given for muscle
strength, training for endurance, and coordination. This included a specific exercise intervention that
advocates training the cocontraction of the deep abdominal muscles with lumbar multifidus, performed
according to the principles outlined by O’Sullivan et al.22 In addition, individual goals for the
rehabilitation process were on the basis of the patient’s answers from the comprehensive
questionnaire (thoughts and feelings) and their test results (physical function and behavior). The
duration of the supervised treatment period was 1 week at first followed by 2 weeks at home and then
another treatment period of 2 weeks. The intensity of the physical activities was gradually increased
during the last 2 weeks. The average duration of the rehabilitation program was 25 hours/week. Since
patients were recruited from all over Norway, most patients stayed at a patient hotel and treatments
were conducted in the outpatient clinic during daytime. The group consisted of 4 to 7 patients. Three
daily workouts were performed: aerobics or outdoor activities, water gymnastics, and individual or
group exercises. In addition, individual consultations, group lessons, and discussion were held. One of
the group lessons focused on imaging, and the patient’s radiograph, CT, or MRI scans were
demonstrated. All patients were recommended to increase their physical activity levels and were
offered a training diary. Follow-up consultations and tests were conducted after 3 and 6 months. Only
a few patients managed to fill in the diary from the time when they finished treatment to the 1-year
follow- up examination.
Trunk Muscle Strength Test.
Trunk muscle strength was measured on a Cybex Isokinetic Trunk Extension Flexion Device (model
6000; Cybex-Lumex Inc., Ronkonkoma, NY) at the inclusion and at the 1-year follow-up examination.
None of the patients had any previous experience with any strengthmeasurement procedures.
Positioning was standardized according to the manufacturer’s manual. Subjects stood in a vertical
position, and the axis of rotation was set at the interaction of the midaxillary line and L5–S1 in the
vertebral column. Pads and belts stabilized the chest, pelvis, and lower limbs, and exact positions
were noticed. The same experienced physiotherapist gave the instruction. The range of motion was
limited from 10° of extension to 80° of flexion. The test protocol consisted of four repetitions at
60°/second. The isokinetic parameter used was total work. The results of the patients, who attempted
to perform the test but did not manage to generate measurable force, were set to zero. In addition,
muscle strength was evaluated by the Biering- Sørensen Test, which measures how many seconds the
participant is able to keep the unsupported upper part of the body in a horizontal position. In this test
the load is equal to the weight of the upper part of the body, with torque determined by the lever arm
from the pubic symphysis to the upper body center of gravity.23 We used a “Roman Chair” device,
with a padded pelvic support, padded adjustable-height dorsal calf supports, and handlebars to
stabilize subject entry. The subject initially hung flexed 90° with the head down from the waist having
the pelvis supported by the pubic symphysis and anterior iliac prominence and then raised the trunk
to the horizontal position with hands crossed over the chest. The test was continued until the subject
could no longer control the horizontal posture or until he/she reached the limit for fatigue or pain.23
.
Figure 1. A 45-year-old woman with postlaminectomy syndrome randomized to cognitive intervention
and exercises. A: At the inclusion. B: At the 1-year follow-up examination. During the trial, the crosssectional area increased by 12% and density by 16%.
Figure 2. A 51-year-old woman with postlaminectomy syndrome randomized to lumbar fusion. A: At
the inclusion. B: At the 1-year follow-up examination. During the trial, the cross-sectional area was
unchanged and density decreased by 3%.
Cross-sectional Area and Density.
CT (Toshiba XPEED, Tokyo, Japan) was used to measure the cross-sectional area and density of the
back muscles. One 5-mm-thick slice parallel to the disc plane was obtained for each of the disc spaces
at L3–L4 and T12–L1. We also intended to perform CT scans at L4–L5, but it was not possible to get
reliable measurements of the back muscles in all patients who had undergone instrumental lumbar
fusion because of artifacts of the osteosynthesis material. The measurements comprised both the
erector spinae (the longissimus thoracis and iliocostalis lumborum) and the multifidus muscles. An
experienced radiologist traced the border of the back muscle to obtain the cross-sectional area. The
results are presented in cm2. The density was measured in Hounsfield units (HU) and was evaluated
by measuring the mean density of the cross-sectional area in the region of interest (Figures 1 and2).
The results are presented as the mean of the left and right side for density as well as for crosssectional area.
Statistical Methods. The unpaired t test was applied to detect significant differences between
groups. The paired sample t test was applied for testing the mean difference within treatment groups.
The Pearson’s correlation analysis was used to test correlation between outcome variables. The
computer package SPSS for Windows, version 11 (SPSS, Inc., Chicago, IL) was applied for the
statistical analyses. The measurements of ODI consist of ordinal categorical data. Such data represent
only a rank order and not a numerical value.24,25 This restricts the application of common
mathematical and statistical methods of analysis, such as calculations based on addition or
subtractions.26 Therefore, to test for significant changes from the inclusion to the 1-year follow-up
examination, we applied the sign test on all questions in the ODI.
Results
The sociodemographic characteristics at inclusion are given in Table 1. The patients who had CT scans
did not differ significantly from those who were not examined by this method, and the patients who
performed the muscle
Table 1. Sociodemographic Characteristics at
Inclusion
All
Patients Patients Patients Patients
Patients Who Had Who Did Who
Who
Enrolled CT Scans Not Have Performed Did Not
in the Performed CT Scans the
Perform
Trial
(n
Performed Muscle
the
61)
(n
(n
Strength Test
124)
63)
Test
(n
12)
(n
112)
strength tests were not different from those who did not (Table 1). At the inclusion and at the 1-year
follow-up, muscle strength, cross-sectional area, and density of back muscles did not differ
significantly between patients with chronic low back pain/disc degeneration and postlaminectomy
syndrome. At the inclusion and at the 1-year follow-up examination, 14 and 21 patients, respectively,
were not able to perform the isokinetic muscle strength test, and 8 and 18 patients, respectively, were
not able to carry out the Biering-Sørensen Test. Moreover, at the inclusion there was no significant
difference in muscle strength, cross-sectional area, and density for patients randomized to either
lumbar fusion or cognitive intervention and exercises. This was also the case for the ODI. At the 1year follow-up examination, both patients randomized to lumbar fusion and cognitive intervention and
exercises improved significantly in disability ratings (both P
0.001), but there was no significant
difference between the groups (Table 2). At the 1-year follow-up examination, muscle strength had
increased significantly in the exercise group but notin the lumbar fusion group, with the mean
difference between the groups of 184 Nm (95% confidence interval [CI], 64–303 Nm; P
0.003). For
the Biering-Sørensen Test, there was a significant decrease in the muscle strength in the lumbar
fusion group with the mean difference between the treatment groups being 21 seconds (95% CI, 6–36
seconds; P
0.006) (Table 2). Limiting the analyses to include only the 61 patients who had CT scans
performed gave similar results: the mean difference between the groups was 193 Nm (95% CI, 32–
353 Nm; P 0.02) for the isokinetic test and 24 seconds (95% CI, 5–43 seconds; P
0.01) for the
Biering-Sørensen Test. There were no significant changes in the crosssectional area from the inclusion
to the 1-year follow-up at either L3–L4 and T12–L1, in the two treatment groups (Table 2). There was
a significant increase in density at level T12–L1 in the exercise group and a nonsignificant increase in
the lumbar fusion group. The mean difference in changes between the groups was 2.4 HU (95%
CI, 4.7 to 9.6 HU, not significant). The density at L3–L4 decreased significantly in the lumbar fusion
group and was unchanged in the exercise group. The mean difference in changes between the groups
was 5.3 HU (95% CI, 1.1–9.5 HU; P
0.01) (Table 2). There were no significant correlations between
changes in muscle performance and density and changes in cross-sectional area, from the inclusion to
the 1-year follow-up examination. The mean number of physiotherapy sessions given after the
hospital stay was 31 in the lumbar fusion group and 4 in the group who followed the exercise
program.
Discussion
At the 1-year follow-up examination, there was a significant difference in the improvement of muscle
strength between the patients randomized to lumbar fusion and patients who followed the exercise
program, although they improved significantly in disability (Table 2). For the isokinetic muscle
strength test, the increase in the exercise group was about 30%, which is in agreement with previous
studies.10,11 Trunk muscle endurance was measured by the Biering-Sørensen Test, which has the
advantage that it can be performed without expensive test devices. The results of the two methods
showed the same trend: that the exercise group performed better in muscle strength than the lumbar
fusion group. The patients included had had low back pain for 8 to 9 years; about half of them have
had back surgery performed previously, and most patients were physically inactive at the time of
inclusion. The exercise program in the present study did not fulfill the criteria for optimal increase in
muscle strength, that is, a program with repetitions of 60% to 70% of maximum and
progressiveresistance training over an 8-week period27–29 but focused on cognitive intervention,
endurance training, and activities. The cognitive intervention gave the patients the comprehension
that ordinary activities could not harm the disc or back. The main ambition of the exercise program
was to get the patients physically and mentally prepared to resume former activities at home. Muscle
density is an expression of degeneration of the muscles and reflects the number of muscle fibers, the
area of the individual muscle fiber, and the packing of the contractile material,28 whereas the crosssectional area is mainly determined by the total number of muscle fibers and, to a lesser degree, the
size of the fibers.30 Thus, the effect of an exercise program will first lead to a gain in muscle strength
because of neural drive,27–29,31 then to an increase in density because of hypertrophy of the muscle
fibers28,32 and lastly to an increase in the crosssectional area. In the exercise group, the crosssectional area and density remained unchanged at L3–L4, which is in keeping with previous studies
that showed changes within measurement error in patients and healthy persons. 11,33,34 At T12–L1
there was a significant increase in density of 13% while the cross-sectional area was unchanged,
which is in accordance with a previous study that demonstrated an increase in density of about 15%
to 20% as opposed to an increase in cross-sectional area of only 5% to 10% after training.27 The
patients who followed the exercise program managed to achieve an increase in muscle strength and
density at T12–L1, even although the exercise program was not that intensive. The muscle strength
test was performed at the 1-year follow-up examination, about 10 months after the completed
treatment, which suggests that the cognitive intervention and exercise have enabled the patients to
overcome their psychological barriers to pain and to be thereby more physically active. This supports
previous studies, which demonstrated the effect of return to work after similar cognitive interventions
in patients with low back pain.20,35 The patients randomized to lumbar fusion decreased 10% in
density at L3–L4 and they did not manage to increase muscle strength, from the inclusion to the 1year follow-up examination. These findings corresponded well with other studies, which demonstrated
atrophy of the back muscles and reduced muscle strength, brought about most likely because of
denervation and muscle injury after discectomy and lumbar fusion.14,36,37 However, given the fact
that injured muscles can recover with even daily activities28 and that these patients had received
significantly more physiotherapy than patients who followed the exercise program, the decrease in
muscle strength may seem remarkable. A weakness of the present study is that only some of the
patients performed the CT scan of the back muscles. However, the sociodemographic variables and
the performance of the muscle strength test of the patients who performed the CT did not differ from
the patients who were not examined by this method (Table 1). Another weakness is that there were
not equal numbers of scans at the L3–L4 and T12–L1, although this did not result in a skewed
distribution between treatment groups. Furthermore, we do not know anything about the exercise
habits of the patients from the time they completed treatment to
the 1-year follow-up examination, as only a few completed the training diary. However, compliance in
longterm studies investigating the efficacy in lifetime changes is always difficult to monitor.
Conclusion
There was a significant difference in muscle performance between patients randomized to lumbar
fusion and cognitive intervention and exercises. Density at L3–L4 decreased significantly in the lumbar
fusion group but remained unchanged in the exercise group. The difference between the treatments
was significant. The crosssectional area remained unchanged at the two spinal levels in both
treatment groups. There was no correlation between change in muscle strength and muscle
morphology.
Key Points
● Muscle strength, cross-sectional area, and density were investigated in patients with chronic low
back pain, randomized to lumbar fusion or cognitive intervention and exercises, at the inclusion and at
the 1-year follow-up examination.
● Back muscle strength differed significantly between the two treatments group in favor of the
exercise group.
● Density decreased significantly at L3–L4 in the lumbar fusion group.
● The cross-sectional area did not increase from the inclusion to the 1-year follow-up examination.
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