Abstract
Background During lumbar total joint replacement (LTJR), component misalignment during implantation may affect the bearing surface interaction. In this study, validated computational models of the lumbar spine were used to investigate a range of clinically relevant misalignment scenarios.
Methods A finite element model (FEM) of the LTJR, exposed to mode I (normal wear) and mode IV (impingement) wear boundary conditions, was previously validated following the ASME V&V 40 standard. The LTJR FEM was virtually implanted into a previously validated FEM of the lumbar spine (L3–L5) at L4 to L5. The model included vertebrae, major spinal ligaments, erector muscle forces, and intervertebral discs. Misalignment was introduced by adjusting the bilateral implant axial plane convergence angle (20°–40°), anterior-posterior offset (0–4 mm), and coronal plane tilt (±20°). Analyses were conducted using LS-DYNA3D (ANSYS) under boundary conditions simulating bending at the waist. Contact pressures and von Mises stresses were evaluated for each misalignment scenario and compared with those developed during mode I and mode IV impingement scenarios.
Results Axial plane convergence angle had minimal impact on contact stress and von Mises stress magnitude and distribution. Increasing anterior-posterior offset led to higher stresses on the anteriorly shifted component but did not significantly alter the overall stress pattern. Coronal tilt had the most substantial effect on both stress magnitude and distribution.
Conclusion Overall, polyethylene stresses in all misalignment scenarios remained below mode IV impingement levels. Contact areas remained within the intended spherical bearing surfaces without signs of impingement. LTJR contact stresses were found to be reasonably insensitive to misalignment under boundary conditions representing bending at the waist.
Clinical Relevance This work assesses the impact of clinically relevant implant misalignment scenarios on the polyethylene stresses associated with damage and wear for a novel LTJR and offers best practice guidelines for surgeons.
Level of Evidence 5.
- lumbar spine
- total joint replacement
- finite element analysis
- polyethylene wear
- surface damage
- contact stress
Introduction
Lumbar total joint replacement (LTJR) is an investigational procedure for lumbar degeneration that preserves motion and is implanted via a traditional posterior approach.1–3 Unlike lumbar total disc replacement (LTDR), which requires an anterior approach with higher revision risks4,5 and treats only the anterior spinal column, LTJR replaces both the disc and facet joints, addressing all 3 spinal columns. The LTJR design (MOTUS; 3Spine, Chattanooga, TN, USA) incorporates vitamin E-stabilized, highly crosslinked polyethylene bearings that articulate against CoCr alloy surfaces.1,6 Currently in US clinical trials, early 1-year results show promise compared with lumbar fusion.1
Preclinical testing of LTJR, part of the requirements for establishing the reasonable expectations for safety and efficacy for the device before clinical trials, has included wear testing under standardized, baseline loading, also known as mode I conditions.6 Additional wear tests were performed in which the device components were malpositioned in the sagittal plane to ensure posterior impingement, referred to as mode IV conditions. These tests were conducted according to ISO 18192–1 7 and ASTM F32958 standards, both recognized by the US Food and Drug Administration (FDA) for applicability to intervertebral prostheses. Mode I testing involved 10 million cycles of a compound motion profile (including flexion-extension, lateral bending, and axial rotation) under an axial compressive load of 600 to 2000 N. Mode IV testing conditions included 5 million cycles of a motion profile including flexion-extension and axial rotation under a 1200 N axial compressive load. Both mode I and mode IV tests showed that the LTJR exhibited wear behavior consistent with previous generations of anterior LTDRs.6
LTJR is implanted using manual instruments tailored for the procedure and implant components.3 Because the device consists of separate bilateral components, implant placement is both a matter of position within the intervertebral space and also how the components are positioned with respect to one another. As part of an investigational device exemption (IDE) study, it is important to understand the sensitivity of the procedure to clinically relevant device misalignment. Limited information is available on how potential misalignment may affect polyethylene wear and surface damage.
As the first step to addressing questions regarding misalignment, we previously developed, verified, and validated a finite element model (FEM) of the LTJR, which was exposed to mode I and mode IV in vitro boundary conditions (ie, simulating the simulator).9 The model was then used to explore the sensitivity of the LTJR to misalignment during a standardized mode I duty cycle. In these previous misalignment simulations, the polyethylene stresses were less severe than those observed during simulated mode IV wear testing, suggesting that the LTJR was relatively insensitive to misalignment under conditions simulating in vitro bench wear testing.9 Testing these misalignment scenarios under conditions that more accurately reflect in vivo loading in the spine is an important next step for better understanding how implant position may impact wear.
In the present study, we used finite element analysis to evaluate the impact of implant misalignment on LTJR polyethylene stresses under clinically relevant biomechanical loading conditions associated with bending over at the waist. Stresses associated with polyethylene wear (ie, contact stress) and deformation (ie, von Mises stress) were specifically assessed. Our hypothesis was that misalignment-induced polyethylene stresses would be less severe than those associated with established mode IV boundary conditions.10
Methods
Development and Validation of the LTJR Spine FEM
The lumbar spine FEM consists of 2 lumbar motion segments (L3–L5) and is based on a previously developed and historically validated FEM.11 The lumbar spine FEM includes the L3, L4, and L5 vertebrae, the L3 to L4 and L4 to L5 discs, and the associated 7 ligaments (ie, the supraspinatus, the ligamentum flavum, the intratransverse, the intraspinous, the facet capsule, the posterior longitudinal, and the anterior longitudinal ligaments). Additionally, the FEM uses stiff cable elements (10,000 N/mm) to represent various erector muscle groups. The erector muscles are modeled in this way such that they are capable of generating the tensile force necessary to mitigate flexion rotation in the presence of vertically directed body weight.
The LTJR FEM from our previous study9 was virtually implanted into the lumbar spine FEM at the level of L4 to L5 (Figure 1). Coupling between the superior and inferior faces of the LTJR CoCr components was modeled as a tied contact and considered ideally fixed (ie, no relative motion). The anterior and lateral portions of the anulus fibrosus were maintained according to typical surgical practice. The posterior longitudinal ligament, the supraspinatus ligament, the intraspinous ligament, the ligamentum flavum, the facet capsular ligaments, and facets were removed as part of the virtual implantation (L4–L5 level only). The facet joints were visually present, but any contact at those surfaces was computationally disabled. The posterior erector muscle between the spinous processes of L4 and L5 was removed. A size 15 long LTJR was virtually implanted based on surgeon input, since it was judged to be the appropriate size for the anatomy of the spine FEM. While each simulation used the same base spine model, separate FEMs were created for each implant misalignment scenario.
Image depicting the implanted lumbar total joint replacement -spine finite element model.
Our computational modeling approach has previously predicted real-world LTDR failures and informed in vitro wear testing.11,12 For the present study, the lumbar spine FEM was formally verified and validated per ASME V&V 40:2018, as required by 2023 FDA guidance.13,14 Few spinal implant FEM validations adhering to ASME V&V 40 and the latest FDA guidance exist in the literature.9,15 Therefore, we provide an overview of the FEM verification and validation activities in the Supplemental Digital Content.
Analysis of Axial and Coronal Plane Misalignment and Polyethylene Outcome Measures
The LTJR components in the FEM were positioned parametrically. Misalignment was introduced by adjusting axial plane convergence angle, anterior-posterior (A/P) offset, and coronal plane tilt (Figure 2). Eight misalignment simulations were conducted, and results were compared with the stresses associated with the mode I baseline wear tests16 from the prior validation study9 (Table, Figures 2–3). As in the prior study, these reasonably worst-case implant positions were based on the criteria for unreasonable misuse outlined in the surgical technique guide.
Definitions of convergence angle, axial anterior-posterior (A/P) offset, and coronal tilt angle. Reproduced from Kurtz et al9 with author’s permission.
Images of 3-dimensional computer-aided design (CAD) models illustrating the component positioning of the baseline scenario and cases 1 through 8. A/P, anterior-posterior.
Summary of axial misalignment conditions for finite element models.
The polyethylene outcome measures for the study were contact stress, also known as contact pressure, and von Mises stress, a measure of distortion in the polyethylene and a generally accepted metric associated with material failure. Furthermore, both stress metrics have historically been associated with wear and surface damage in the literature for polyethylene components in total joint replacement.17–23
Boundary Conditions of Reasonably Misaligned Models
FEM mesh discretization, as well as assignment of material properties and boundary conditions, was all scripted parametrically. Given that the present analysis focused on clinically relevant, worst-case loading scenarios, the boundary conditions for a 50th percentile man bending over were used for all tested cases (Figure 4). The location of upper body weight for forward bending is located 36 mm anterior and 317 mm superior of the center of the L4 vertebrae.24 This location represents the approximate horizontal and vertical location of the line of force from body weight. For forward bending, a load of 343 N (60 N in the z-direction and 338 N in the negative y-direction) was applied to represent upper body weight as determined by a biomechanical analysis using 3DSSPP software (version 7.1.3, University of Michigan) for a man in the 50th percentile.12,25 A counteracting AP force of 9 N in the negative z-direction was also applied. Although not an applied load, the muscle reaction forces at both the spinous and transverse processes reach whatever magnitude is necessary to balance intersegmental flexion rotation.
Boundary conditions for the L3 to L5 forward bending model. The L5 vertebra is constrained in all directions.
Results
Convergence Angle
Axial plane convergence angle had a minimal effect on both the magnitudes and distributions of contact pressure during simulated bending over (Figure 5). For the baseline model (40° convergence), the peak contact pressures tended to occur on the posterior aspect of both the right and left bearing surfaces as expected, given the applied boundary conditions of bending over. Specifically, the device is exposed to anterior shear from body weight, and therefore, contact pressure at the posterior aspect of the device is required to resist that loading. Peak contact pressures at the baseline condition were 29.9 MPa. This increased slightly (<3%) to 30.7 and 30.3 MPa at the 30° and 20° convergence angle scenarios, respectively. The results for von Mises stress generally followed the same pattern as those observed for contact pressure (Figure 5) and were generally consistent with the baseline. The peak values of von Mises stresses were approximately 22 MPa for all convergence angle scenarios.
Contact pressure (MPa), von Mises stress (MPa) plots after bending over for the baseline, 30°, and 20° convergence angle simulations. The contour plots are viewed on the LTJR’s superior polyethylene components from the bottom looking up. The up direction in the figure corresponds to the anterior direction.
A/P Offset
Varying A/P offset tended to increase the contact pressures on the anteriorly shifted component, but the general pattern of contact pressure was not altered (Figure 6). Specifically, all areas of contact pressure remained on the bearing surface and tended to occur on the posterior aspect. Peak contact pressures at the baseline condition were 29.9 MPa. This increased slightly to 30.7 MPa (2.7% increase) and 33.0 MPa (10.4% increase) at the 2-mm and 4-mm A/P offset scenarios, respectively. The results for von Mises stress generally followed the same pattern as those observed for contact pressure. Specifically, A/P offset tended to result in greater stress and strain on the anteriorly shifted component while reducing the stress and strain on the other component. For von Mises stresses, the peak values increased negligibly from 21.9 MPa at baseline to 22.1 MPa (0.9% increase) and 23.2 MPa (5.9% increase) for the 2 mm and 4 mm A/P offset scenarios, respectively.
Contact pressure (MPa) and von Mises stress (MPa) after bending over for the baseline, 2-mm offset, and 4-mm offset simulations.
Coronal Tilt
Coronal tilt had the greatest impact on the magnitudes and distributions of contact pressure compared with the baseline scenario. In the baseline model (no coronal tilt), peak contact pressures typically occurred on the periphery of the domed bearing surface and toward the posterior side (Figure 7). This pattern of contact pressures during bending over generally occurred regardless of device convergence angle and A/P offset. However, at higher positive (outward rotation) tilt angles, the distribution of contact pressures moved to the medial aspects of the bearing surfaces. This resulted in the highest peak contact pressures for all tested cases. Specifically, peak contact pressure increased from 29.9 MPa at baseline to 31.1 MPa (4.0% increase) and 36.7 MPa (22.7% increase) for 10° and 20° of coronal tilt, respectively. The peak contact pressures increased to 30.0 MPa (0.3% increase) and 32.2 MPa (7.7% increase) for coronal tilts of −10° and −20°, respectively. The results for von Mises stress and effective strain generally followed the same pattern as those observed for contact pressure (Figure 7). Specifically, a positive coronal tilt resulted in the greatest increase in stress (25.1 MPa vs 21.0 MPa at baseline) and strain (16.9% vs 8.4% at baseline). At baseline, the peak von Mises stress was 21.9 MPa and increased to 22.4 MPa (2.3% increase) and 25.1 MPa (14.6% increase) with positive coronal tilt of 10° and 20°, respectively. Conversely, the peak von Mises stress value decreased to 20.6 MPa (5.9% decrease) and 18.7 MPa (14.6% decrease) during coronal tilts of −10° and −20°, respectively.
Contact pressure (MPa) and von Mises stress (MPa) after bending over for the baseline, −20°, −10°, 10°, and 20° coronal tilt simulations.
Discussion
Prior to the introduction of a new spine implant design, such as LTRJ, it is paramount to understand its sensitivity to surgical placement. For the coronal and axial plane misalignment scenarios considered in this study, the LTJR polyethylene stresses remained consistent with baseline conditions. Under axial and coronal plane misalignment, contact between the LTJR components remained confined to the intended spherical bearing surfaces of the design, without evidence of impingement. During elevated coronal misalignment (20° tilt), the location of the bearing contact pressures migrated laterally, resulting in a more concentrated distribution of force and consequent 22.7% increase in contact stress. Nevertheless, the peak stresses were less than those resulting from mode IV boundary conditions.
We previously examined the effect of coronal and axial plane misalignment on the LTJR9 when loaded under the conditions of a spine wear simulator (mode I duty cycle per ISO 18192 7 ). Under both the mode I duty cycle and physiologic bending over, coronal tilt had the greatest impact on stresses. Specifically, under mode I duty cycle conditions, 20° coronal tilt resulted in an increase of 28% in the contact stress (from 32.5–41.6 MPa),9 as compared with a 22.7% increase for the same degree of coronal tilt in the present study. Taken together, both our previous research and the present study hopefully provide clinicians with reassurance of the stability of the LTJR design to variations in surgical placement.
The following technical tips are offered to reduce component misalignment. For context, a radiograph depicting a well-positioned LTJR is provided in Figure 8. Symmetrical, parallel, flat bony resections are required to ensure that superior and inferior endplate surfaces are aligned in the coronal plane. Preoperative evaluation of coronal asymmetry while the patient transitions to and from standing and sitting postures, along with review of CT scans of the native endplates, should be completed with attention to undulations in the cranial vertebral body inferior endplate, where the LTJR devices will be placed. Provided rasps should be used to remove endplate undulations to maintain neutral device alignment between the LTJR implant components to minimize misalignment risk. Bilateral posterior vertebral body osteotomies are made to the superior endplate of the caudal vertebra to create a parallel space for insertion of the bilateral LTJR devices by removing a wedge shape of the vertebra from the posterior column. Care should be taken during posterior vertebral body osteotomies rasping by minimizing internal or external rotation of the rasp with a firm grip and noting the position of hands, wrists, and forearms. The use of multiple intraoperative lateral and A/P fluoroscopic images during rasping prior to LTJR device placement can help identify and correct deviations in coronal plane misalignment. Aligning the A/P fluoroscopic image acquisition parallel to the caudal vertebral body superior endplate can increase the intraoperative visibility of misalignment. The provided radiopaque height trials with A/P and lateral intraoperative fluoroscopy should be used to ensure that the inferior endplate of the cranial vertebral body and the superior endplate of the caudal vertebral body are parallel and co-planar across the midline when the anterior longitudinal ligament and lateral aspects of the annulus are evenly tensioned. If not parallel and co-planar across the midline, any misalignment should be corrected with further rasping. Bony resections and device placement should be symmetric within the pedicle of the caudal vertebra. It is important to obtain true orthogonal intraoperative fluoroscopy views and minimize artifact from off-axis C-arm placement. Once “flat” cuts have been verified for alignment, then vertical keel cuts are made perpendicular to the flat rasp cuts using the provided instrumentation. Much like with an anterior disc replacement, proper implantation of an LTJR device requires placement of the top and bottom device keels simultaneously into prepared cranial and caudal vertebral body endplate cuts. A ball-end probe or nerve hook should be used to verify that all device keels are well placed in their respective keel cuts. If not, one should remove the device, realign the keels with cuts, and reinsert. If needed, iterative rasping can be performed to improve device alignment prior to reinsertion under fluoroscopy.
Flexion and extension radiographs showing proper LTJR placement.
Given the recent introduction of the LTJR, long-term clinical implications of misalignment remain to be established as part of the FDA clinical trial. However, it can be expected that the general risks associated with misalignment may be comparable to what has been observed for total disc arthroplasty. Improper positioning may lead to abnormal load distribution across the vertebral endplates, increasing the risk of implant loosening, subsidence, or migration.26,27 Neural impingement and altered range of motion may also be possible, with the latter possibly contributing to adjacent segment degeneration.27 Biomechanically, misalignment may also result in unintended loading and motion that could alter the expected wear of the implant articulating surfaces.28 Future assessments of misalignment-related complications specific to the LTJR will be examined as outcomes from the FDA clinical trial become available.
Although our results indicate polyethylene stresses that exceed the nominal compressive yield strength of vitamin E-stabilized, highly crosslinked polyethylene (approximately 13 MPa),29,30 this does not, in itself, indicate a risk for mechanical failure. As established by Bartel et al, many large total joint replacements, specifically total knee replacements, are associated with stresses far exceeding the yield stress of polyethylene.18 In our models, as in Bartel’s seminal research,18 the yield behavior of the polyethylene is governed by a multilinear relationship between the yield stress and plastic strain to capture the stress hardening of the material. Thus, establishing a single numerical limit for acceptable stress and strains based on material yield strength is not appropriate. Instead, designs can be benchmarked against earlier devices by analyzing multiple performance metrics (ie, contact stress, von Mises stress, and von Mises strain).18 While there are no other established LTJR designs that can be used as a benchmark, there are several other LTDRs with similar metal-on-polyethylene designs. In a series of in vitro wear tests, the LTJR exhibited the same or lower wear rates than 2 historical LTDR designs, even under mode I reasonable “worst-case” alignment conditions and mode IV impingement conditions.6 The actual performance of the LTJR during these high-stress conditions was neither unreasonable nor unexpected, with no fracture or wear through over 1 million cycles of testing. These findings support that the stresses and strains associated with mode IV wear testing (ie, 83.3 MPa contact stress, 32.2 MPa von Mises stress, and 42% von Mises strain) represent a reasonable upper bound benchmark for FEM analysis of the LTJR.
Limitations
Our analysis has limitations. First, the spine FEM used in this study reflects the anatomy of a single person and a single set of tissue properties, which may limit the applicability of the results to the broader patient population. This is a typical limitation for FEM studies. However, validated FEMs remain a powerful tool for understanding spinal biomechanics and implant behavior,31 especially in the absence of robust clinical data. Second, the scope for our modeling activities is limited to the analysis of surgical misalignment, not unreasonable misuse or revision scenarios such as subsidence or gross migration. Third, the boundary conditions explored for this analysis were limited to bending over; however, this is one of the most frequently performed and challenging tasks for the lumbar spine. Fourth, the model was limited to replicating the boundary conditions of L4 to L5, corresponding to the most frequently studied lumbar levels examined in the literature, and did not address other levels of the spine, such as L5 to S1. Finally, in the coronal tilt simulation, the interaction between the component endplate and the bone was not necessarily representative of their expected flush contact in vivo. However, because stress at the endplate-bone interface was not the focus of our analysis (and the CoCr endplates and bone were modeled as rigid bodies), the nature of this contact should not affect the stress and strain of the polyethylene, which was the outcome of interest in this study. These limitations do not undermine the credibility of this study or its ability to rigorously address our research question but serve as a caution to readers regarding the potential generalization of our findings to other contexts or research questions without appropriate validation activities. Future observations of implant clinical performance will build on the findings of our finite element analysis presented here.
Conclusion
We developed and validated an FEM of an LTJR implanted in the lumbar spine and used it to investigate the effect of misalignment in the axial and coronal planes. Although the polyethylene stresses associated with implant wear and surface damage showed greater sensitivity to coronal misalignment, overall, the variations were relatively modest and fell far below the stress levels associated with sagittal plane misalignment.9 The present study is also noteworthy for the rigorous verification and validation program set forth in ASME V&V40 standard13 and required by the FDA14,32 to formally establish the credibility of FEM as part of a regulatory submission. We adopted this approach not only to answer the current research question related to misalignment but also to provide a validated foundation for a modeling platform that can potentially address future clinical questions relevant to the introduction of the novel LTJR design.
Supplementary material
Supplementary Material 1.
Footnotes
Funding This research was supported by institutional funding from 3Spine, Inc.
Declaration of Conflicting Interests S.A.R. reports institutional funding rom 3Spine. S.M.K. reports institutional funding as PI from Celanese, Ceramtec, Invibio, Mitsubishi Chemical Advanced Materials, Orthofix, 3Spine, Seqens, Enovis, Orthoplastics, SINTX Technologies, Stryker, and royalties from Elsevier Inc. S.D.H. and R.V.Y. report stock ownership and employment with 3Spine. J.A.G. reports a research grant from 3Spine, royalties/licenses from Globus Medical and Xtant, consulting fees from 3Spine and Globus Medical, support for attending meetings from 3Spine, and participation on an advisory board from 3Spine. H.S. has nothing to report.
Ethics Approval Each author certifies that all investigations were conducted in conformity with ethical principles of research.
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2025 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
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