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Patient-Specific Templating of Lumbar
Total Disk Replacement to Restore Normal
Anatomy and Function
Jill A. Fattor, MS, PA; Justin F. M. Hollenbeck, MS; Peter J. Laz, PhD; Paul J. Rullkoetter, PhD;
Evalina L. Burger, MD; Vikas V. Patel, MD; Christopher M. J. Cain, MD
abstract
The purpose of this study was to develop a tool to determine optimal placement and size for total disk replacements (TDRs) to improve patient outcomes of pain and function. The authors developed a statistical shape model
to determine the anatomical variables that influence the placement, function, and outcome of lumbar TDR. A patient-specific finite element analysis
model has been developed that is now used prospectively to identify patients
suitable for TDR and to create a surgical template to facilitate implant placement to optimize range of motion and clinical outcomes. Patient factors and
surgical techniques that determine success regarding function and pain are
discussed in this article. [Orthopedics. 2016; 39(2):97-102.]
O
ver the years, there has been an
increased focus on restoration of
both anatomy and function in orthopedic surgery. This is evidenced by newer techniques in anterior cruciate ligament
repair and hip replacement that attempt
to better restore native mechanics.1,2 For
lumbar spinal degenerative disk disease,
conventional posterior fusion and instrumentation techniques have been implicated
in accelerated degeneration of adjacent
segments,3,4 and with the introduction of
stable “stand-alone” anterior fusion devices, there has been a resurgence of anterior
fusion techniques to stabilize the segment,
restore disk height, and optimize lumbar
MARCH/APRIL 2016 | Volume 39 • Number 2
lordosis. Fusion by any approach, however,
does not restore or preserve motion. Lumbar total disk replacement (TDR) has the
potential to restore disk height and lordosis
and maintain or restore motion. Clinical
trials have delivered favorable outcomes of
TDR when compared with anteroposterior
“360°” and anterior-only fusion; however,
these implants have also failed to achieve
global acceptance and use.
Degenerative disk disease affects a
large proportion of the population and is
associated with significant direct medical
and indirect economic costs. Total disk
replacement is indicated when degeneration is localized to 1 or 2 levels in patients
without segmental instability or associated facet degeneration and dysfunction.
The theoretical advantage of TDR over fuThe authors are from the Department of Orthopedics (JAF, ELB, VVP, CMJC), University of
Colorado, Anschutz Medical Campus, Aurora,
and the Department of Mechanical and Materials Engineering (JFMH, PJL, PJR), University of
Denver, Denver, Colorado.
Ms Fattor, Mr Hollenbeck, and Dr Rullkoetter
have no relevant financial relationships to disclose.
Dr Laz has received grants from DePuy Synthes.
Dr Burger is a paid consultant for DSM, Paradigm Spine, and Signus and has received grants
from OMeGA, Globus, Aesculap, SI-Bone, Vertiflex, MEDICREA Group, Medtronic, Orthofix,
Integra Life Sciences Corporation, Pfizer, Spinal
Kinetics, Medtronic Sofamor-Danek, and Synthes.
Dr Patel has received grants from Medtronic,
MEDICREA Group, Aesculap, Pfizer, SI-Bone,
Globus, Orthofix, and Vertiflex; is a paid consultant
for Stryker; and receives royalties from Biomet. Dr
Cain receives royalties from DePuy Synthes and
has received grants from Medtronic, MEDICREA
Group, Aesculap, Pfizer, and SI-Bone and his institution receives consulting fees from DePuy Synthes
and DSM.
This research was supported by a grant from
the Anschutz Foundation.
Correspondence should be addressed to:
Christopher M. J. Cain, MD, Department of Orthopedics, University of Colorado, Anschutz Medical
Campus, 12631 E 17th Ave, Mail Stop B202, Aurora, CO 80045 (christopher.cain@ucdenver.edu).
doi: 10.3928/01477447-20160304-06
97
n Trending in Orthopedics
B
A
C
Figure 1: Patient computed tomography files (A) were segmented and used to generate patient-specific
finite element analysis models of the motion segment (B) for analysis with virtual implantation (C) of a
ProDisc-L (DePuy Synthes, Raynham, Massachusetts) total disk replacement.
available implants and to produce a surgical template to indicate the ideal location to place the implants intraoperatively.
This tool creates a patient-specific 3dimensional (3D) model from a computed
tomography (CT) scan and uses finite element analysis (FEA) to “virtually” implant the various implants available and
determine which is most likely to deliver
the best results. The custom software tool
uses a CT scan with appropriate resolution and computer-aided design files of
the implants to create an FEA model to
assess the segmental motion.
Materials and Methods
A
B
Figure 2: Instances representing composite images and size range at L4-5 of the 3 smallest (A) and the
3 largest (B) patients evaluated.
sion is that, although both can restore disk
height and lumbar lordosis, only TDR has
the potential to restore or maintain motion
without disrupting or interfering with the
function of posterior musculature. Additionally, the incidence of adjacent level
degeneration has been shown to be significantly lower among patients undergoing TDR compared with fusion.4-7 However, when the range of motion (ROM)
achieved with TDR is less than 5°, the incidences of adjacent segment disease and
poor patient outcomes increase.8-10
Facet-mediated pain at the operative
level is also a factor that may compromise
the clinical outcome of patients undergoing TDR. Approximately one-fourth to
one-third of patients report facet-mediated
pain in the postoperative period.11,12
The authors believe that these issues
are related to a mismatch between patient anatomy, specifically vertebral size
98
and facet orientation, and the mechanics of the implant selected. In the United
States, limited implants are available for
implantation, having a limited range of
dimensions and a fixed or limited axis of
rotation; however, considerable variability exists in the size and shape of patients
requiring treatment for degenerative disk
disease. Also, selecting an appropriate implant but placing it in a “nonideal” location can compromise the clinical outcome.
The authors hypothesize that by
matching the anatomy and mechanics of
patients to the dimensions and mechanics
of implants, surgeons will maximize the
ROM achieved, deliver the best possible
clinical outcome in terms of improvement
in both pain and function, and limit adjacent level disease progression.
The authors have developed a novel
patient-specific tool to identify patients
whose anatomy is compatible with the
Statistical Shape Modeling
Approval for this study was obtained
from the Colorado Multiple Institutional
Review Board. Patients who had undergone either a single or a 2-level lumbar
fusion or disk replacement at the University of Colorado Hospital since 2008 and
who had a preoperative CT scan or CT
diskogram with sufficient resolution (pixel
size, 0.31 mm; slice thinness, 1 mm) were
identified. The lumbar spine from L1 to
S1 was segmented and 3D patient-specific FEA models were developed (Figure
1). Fifty-two patients with a mean age 35
years (range, 20-58 years) were part of
the analysis. Anatomical variations were
identified. Variables were identified that
influenced the potential ROM that could be
achieved following the virtual implantation
of a ProDisc-L (DePuy Synthes, Raynham,
Massachusetts) TDR.
To characterize variability in the lumbar
spine (L1-S1), a statistical shape modeling
approach was employed. After establishing
correspondence, statistical shape modeling
describes the variation in the population
as a series of models. The first 4 modes of
variation described independent changes in
scaling, disk height, disk angle (lordosis),
and facet height and captured 60.9% and
63.7% of the variation for the L4-5 and the
L5-S1 segments, respectively. The greatest
variability between patients was related to
the overall size of the vertebra (Figure 2).
Copyright © SLACK Incorporated
n Trending in Orthopedics
Figure 4: Representation of the ProDisc-L (DePuy Synthes, Raynham, Massachusetts) implant in flexion
(left), neutral alignment (center), and extension (right) showing the anterior and posterior translation of
the implant around the axis of rotation on flexion and extension, respectively.
Figure 3: Graphic representation of the facet orientation (top) and the distance from the center of the
vertebral body to the anterior portion of the superior facet of L5 (bottom).
Other measures assessed included facet
orientation, vertical slope and sagittal vs
coronal orientation of the articular surface,
and the distance of the facet joint to the center of the vertebral body (Figure 3). These
variables impact the normal rotations that
can occur through the specified intervertebral segment and should be matched by the
geometry and mechanics of the implant to
reproduce normal motion.
Determination of the Ideal Implant Size
and Placement
The native disk has a variable axis of
rotation in flexion and extension as a result of the anatomical constraints and the
flexible nature of the nucleus. The annular
fibers provide stability and, with the facet
orientation, the limits of the ROM of the
functional spinal unit. The ProDisc-L implant is a ball and socket articulation that
rotates around a single, fixed axis of rotation located in the inferior vertebra of the
motion segment. Two footprint sizes are
currently available in the United States
(medium, 27 × 34.5 mm; large, 30 × 39
mm). The superior endplate of the implant
effectively translates forward in flexion
and backward in extension as it rotates
MARCH/APRIL 2016 | Volume 39 • Number 2
over the polyethylene ball of the implant.
Thus, unless the anatomical arrangement
and orientation of the facet joints perfectly matches the mechanics of the implant
and its axis of rotation, implant rotation
can result in closure or compression of the
facets in flexion and distraction or opening in extension (Figure 4).
The ProDisc-L computer-aided design
files were provided by the manufacturer
and used to virtually implant the artificial disk into the patient-specific 3D FEA
model of the functional spinal unit. The
implant was placed in the midline of the
disk space so that its posterior margin was
level with the posterior aspect of the vertebral body, so as not to encroach on the
spinal canal. The ROM of the functional
spinal unit was then determined in this
location, with extension usually blocked
by impingement of the components of
the implant and flexion usually blocked
by the facet joints. The implant was then
moved in 1-mm increments toward the
front of the vertebra and an analysis of the
ROM was repeated to identify the location
where the maximum range of both flexion
and extension was achieved.
During this analysis, another variable
was identified that has a significant impact on the ROM that could be achieved.
The superior and inferior endplates of the
ProDisc-L implant can be advanced independently, or at least not uniformly, and the
positioning of the patient on the operating
table at the time of TDR surgery may result in a degree of either anterolisthesis or
retrolisthesis of the superior vertebra relative to the inferior vertebra. Thus, when the
polyethylene insert is secured and load is
applied to the spine, the ball of the articulation relocates into the socket. If the superior
endplate is anterior relative to the inferior
endplate, relocation will result in posterior
translation of the superior vertebra (retrolisthesis), leading to opening or separation of
the facet joints. This may lead to a greater
ROM in flexion because facet contact is less
likely to occur in this situation, but may also
lead to facet-mediated pain and an inferior
clinical outcome due to capsular tension
and altered facet mechanics. The reverse
may also occur: posterior placement of the
superior endplate relative to the inferior
endplate. This will lead to facet impingement because the superior vertebra will be
translated anteriorly (anterolisthesis) as the
ball and socket engage under load. The result will be a reduced ROM in flexion due
to early or preexisting facet impingement,
leading to a reduced ROM and possibly
also facet-mediated pain (Figure 5).
Evaluation of the Model
To date, several forms of evaluation of
the model have been undertaken. The first
was to validate the model and to confirm
that the ROM determined through the virtual implantation of the implant was accurate.
Pre- and postoperative images of patients
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Figure 5: Segmental range of motion. The effect of moving the superior endplate of the implant forward or
backward from the “neutral” or “aligned” placement of both components is depicted by the blue column.
Posterior placement of the superior endplate relative to the inferior endplate results in translation of the
vertebra forward, facet impingement, and a reduced total range of motion.
Figure 6: Postoperative standing radiograph (left), 3-dimensional model overlay on radiograph to determine location of implant (center), and implanted finite element analysis model of the operative level used
for analysis (right).
Figure 7: Comparison of actual, predicted, and optimal range of motion for the patient cohort. The dashed
red line indicates 5° total range of motion and was used as the minimum acceptable to consider total disk
replacement (TDR) surgery. Abbreviation: ID, identification.
who had undergone lumbar TDR were analyzed. Patient-specific FEA models were
created for these patients (Figure 6).
Postoperative flexion and extension
radiographs were used to determine the
actual ROM achieved postoperatively.
The implant used was then positioned in
a location identical to that evident on the
imaging and an analysis was undertaken
100
in a blinded fashion to determine the
predicted ROM that could be achieved.
The final component of the analysis was
to determine the ideal implant size and
configuration and to assess what ROM
might have been achieved had a different implant been used or had the same
implant been used in a different location.
Seventeen patients, 12 having undergone
a single-level procedure and 5 having had
a 2-level TDR, were part of the analysis.
The results are presented in Figure 7.
This analysis indicated good correlation between actual and predicted ROM
of the operated on levels, with Pearson’s
r being 0.94 and the average difference
being 11.8% (range, 2.9%-27.1%). The
analysis also indicated that 16 of the 22
operated on levels could have achieved a
greater ROM had a different implant been
used or had the same implant been located in a different position. Seven patients
could have achieved an increased ROM
of more than 1° and 2 patients could have
achieved an increased ROM of more than
3.5°. Only 1 patient was identified whose
anatomical features did not match the mechanics of the implant and who achieved
less than 5° of motion for all implant
configurations and locations. This patient
may have been better off undergoing a fusion rather than a disk replacement.
RESULTS
Several of the patients involved in the
analysis described here were enrolled in
the Food and Drug Administration Investigational Drug Exemption for TDR and
also had preoperative and postoperative
clinical outcome data that could be analyzed. Of the patients who underwent a
ProDisc-L TDR at L5-S1, the patient with
the best clinical outcome had a decrease
in Oswestry Disability Index score of 58
points and achieved a postoperative ROM
of 5.5°. For this patient, trialing other
implants and the same implant in different locations indicated that the appropriate implant had been used and that it was
located close to the ideal position. The
predicted ROM improved by only 0.1°
with the implant positioned slightly more
posteriorly. The patient with the worst
clinical outcome still showed improvement, but the decrease in this patient’s
Oswestry Disability Index score was only
22 points and the actual ROM was 3.75°,
below what is considered the minimum
clinically acceptable ROM of 5.0°.8-10 A
Copyright © SLACK Incorporated
n Trending in Orthopedics
large 6° implant with a 10-mm insert was
used for this patient. However, the current
authors’ analysis indicated that the ideal
implant would be a medium 11° implant
with a 10-mm insert and that locating the
implant a little farther back in the vertebral body would have been ideal. In the
ideal location, this implant would have
nearly doubled the total predicted ROM to
6.4°. Whether the predicted improvement
of ROM would lead to actual improvement in ROM and subsequently pain and
function has yet to be elucidated.
Future Directions
During the past 3 years, the authors
have used this 3D templating to evaluate
the complex interaction among individual
patient anatomy, implant geometry, and
intraoperative implant alignment. The ideal implant size and location is determined
and a surgical template is created to assist
the surgeon during the operation in placing the implant in the ideal location. An
example of one of these surgical templates
is shown in Figure 8.
Identifying patients with compatible
anatomy for implant mechanics and the ideal implant size and location are not the only
factors that determine the ROM that can be
achieved with TDR surgery. Another key
factor is ensuring that appropriate mobilization of the segment is achieved without
overdistraction of the segment. The model
enables evaluation of the distraction or mobilization of the disk required to achieve the
maximal ROM (Figure 9). Where adequate
mobilization of the segment is not achieved,
the postoperative ROM that can be achieved
is likely to be limited, even if the ideal implant is placed in the ideal location.
It is also important to evaluate the images and measurements obtained to ensure
that the selected implant will not result in
overdistraction of the segment, specifically the facet joints, as this may also lead to
facet-mediated pain or excessive motion
and instability postoperatively.
One early patient assessed preoperatively using this process was a female of
MARCH/APRIL 2016 | Volume 39 • Number 2
B
A
C
Figure 8: Preoperative template for a patient undergoing lumbar L5-S1 total disk replacement showing
the composite image (A) and detailed representation indicating the location of the implant on L5 (B) and
the sacrum (C).
A
B
Figure 9: The preoperative situation from computed tomography scans and standing plain radiographs
(A) and the amount of distraction required with the template implant to achieve the optimal range of motion (B).
Figure 10: The template range of motion achieved
in a 24-year-old woman of small stature.
small stature enrolled in the military. Her
maximal template ROM was impressive
at 14° with the implant located 1.0 mm
anterior to the posterior margin of the vertebra (Figure 10). However, the majority
Figure 11: Degree of distraction required to implant
a total disk replacement at L4-5 using a 10-mm insert, the lowest available. Note the distraction of
the facet joints, shown here in neutral alignment,
with less than 50% of the articular surface remaining in contact.
of this ROM is achieved in flexion and
such ROM is usually limited by facet impingement. Figure 11 indicates the reason
for this impressive ROM, in that the place-
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n Trending in Orthopedics
ment of the smaller implant with the lowest insert, 10 mm, resulted in the need to
distract the segment by 7 mm. This resulted in overdistraction and near dislocation
of the facet joints that enabled the flexion
achieved in the model. As the flexion was
simulated in the model, no facet impingement occurred, and since the inferior facet
of L4 was elevated as forward translation
of L4 occurred, discussed earlier and illustrated in Figure 4, no impingement
against the superior facet of L5 was evident. In this case, the degree of distraction
needed was considered excessive. The
patient was advised against proceeding
with a TDR and instead underwent an anterior fusion, returning to full active duty
6 months after surgery.
Although this work is still in its early
stages with data collection and analysis
ongoing, it shows great potential to inform TDR surgical planning and implant
design. Continuing activities include preoperative templating of patients undergoing TDR surgery and correlation of the
clinical outcomes with the ROM achieved
and the incidence of adjacent level disease.
Conclusion
Treatment of degenerative disk disease
with TDR has been shown to reduce the
incidence of adjacent level degeneration
in appropriately selected patients and
where anatomical, mechanical, and surgical parameters are aligned. Obtaining
reliable and reproducible results is challenging, but it is clear that the application of this new technology can be further enhanced and that the combination
102
of this sort of analysis with engineering
principles can lead to optimal restoration
of anatomy, normal ROM, and function.
Through this project, it became evident
that, although adequate ROM and function may be achieved with placement of
the implant in several locations, optimal
placement is generally quite localized.
The identification of the optimal position for each patient and placement of the
prosthesis in that position is expected to
improve function and reduce pain.
Patient-specific templating of lumbar
motion segments for patients considered
for TDR surgery has proven to be a valuable clinical tool to identify those patients
whose anatomy is compatible with the
available TDR implants, the most appropriate implant make, size, and geometry,
and where to place the implant to maximize the ROM and clinical outcome that
can be achieved. It is also possible that
no correlation will be identified between
these parameters and patients’ clinical
outcomes, in which case several new
questions will be raised regarding the benefits of this technology over conventional
techniques, namely fusion, to address localized degenerative disorders of the lumbar spine.
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