[Show abstract][Hide abstract] ABSTRACT: Background context
Alternatives to autologous bone graft (ABG) with osteoconductive, osteoinductive and osteogenic potential continue to prove elusive. Demineralized bone matrix (DBM) however with its osteoconductive and osteoinductive potential remains a viable option to ABG in posterolateral spine fusion.
To compare the efficacy of a new formulation of DBM putty to that of ABG in a rabbit posterolateral spinal fusion model.
Efficacy of a new formulation of demineralised bone matrix (DBM) was studied in an experimental animal posterolateral spinal fusion model.
Twenty four (24) male New Zealand White (NZW) rabbits, underwent bilateral posterolateral spine arthrodesis of the L5- L6 intertransverse processes, using either ABG (control group, n= 12) or DBM (DBM made from rabbit bone) putty (test group, n =12). The animals were killed 12 weeks after surgery and the lumbar spines were excised. Fusion success was evaluated by manual palpation, high-resolution x-rays, micro-computed tomography (micro-CT) imaging, biomechanical 4-point bending tests and histology.
Two animals were lost due to anesthetic related issues. Manual palpation to assess fusion success in the explanted lumbar spines showed no statistical significant difference in successful fusion in 81.8% (9/11) of the DBM group and 72.7% (8/11) of the ABG group (p=0.99). Reliability of these assessments was measured between three independent observers and found near perfect agreement (ICC: 0.92 and 0.94, respectively). Fusion using high resolution xrays was solid in 10 of the DBM group and 9 of the ABG group(p=0.59). Biomechanical testing showed no significant difference in stiffness between the control and test groups on flexion, extension, left lateral and right lateral bend, with p values accounting for 0.79, 0.42, 0.75 and 0.52, respectively. The bone volume/total volume was greater than 85% in the DBM treated fusion masses. Histological evaluation revealed endochondral ossification in both groups, but the fusion masses were more mature in the DBM group.
The DBM putty achieved comparable fusion rates to ABG in the rabbit posterolateral spinal fusion model.
No preview · Article · Sep 2014 · The spine journal: official journal of the North American Spine Society
[Show abstract][Hide abstract] ABSTRACT: Introduction: The use of bone graft or bone graft substitute in spinal arthrodesis surgery is performed more than 325,000 times each year in the United States to treat various spinal disorders. Autologous iliac crest bone graft (ICBG) remains the gold standard bone grafting substrate available in spinal fusions, because of its osteoconductive, osteogenic and osteoinductive properties. Significant morbidity however is associated with the harvesting of ICBG and this has stimulated substantial interest in bone graft substitutes. PRP harvesting techniques differ vastly and previous results of PRP in spinal fusion applications are therefore limited and inconsistent. The objective of this study is to review the clinical results of a novel preparation that retains the fibrin and does not require exogenous thrombin to create a platelet rich fibrin matrix to be combined with an osteoconductive scaffold for posterolateral fusion.
Methods: We retrospectively reviewed all patients undergoing a standard posterolateral lumbar fusion (PLF) using local autograft, in combination with platelet rich fibrin matrix (PRFM) and bone marrow aspirate, by a single surgeon in a single institution. The PRFM is a fibrin matrix formed by the activation of the fibrin clotting cascade by the addition of calcium chloride and a novel second centrifugation step. This flexible PRFM membrane was then combined with the bone marrow aspirate preparation, which consisted of a porous calcium phosphate bone graft substitute soaked in an autologous bone marrow aspiration. This biological amalgamation was then added to local autograft and placed bilaterally in the posterolateral fusion beds. Postoperatively, each patient was assessed in the outpatients, by clinical examination and by the completion of the Oswestry Disability Index (ODI). Radiologically, each patient was evaluated by an independent radiologist using plain radiographs and computed tomography (CT).
Results: A total of 62 patients underwent standard PLF using local bone, PRFM and the bone marrow aspirate preparation. The male to female ratio was 29 male: 33 female. The mean age was 63.6 years (range: 21-85 years). The mean BMI was 28.75 (range: 17-44). Fifty-three (85%) patients underwent PLF for lumbar spinal stenosis. The remaining patients had either degenerative disc disease (6) or degenerative spondylolisthesis (3) respectively. Thirty-four (55%) patients underwent a single level fusion, twenty-five (40%) patients underwent a two level PLF, while the remaining three (5%) patients underwent a three level fusion. Postoperatively, there were no cases of dural lacerations, neurological deficits or infections. At minimum clinical follow up at one year (range 1-6 years), forty (64%) patients had a statistically significant decrease of 89% (>0.05) in their mean Oswestry Disability Index (ODI) score. Plain radiograph and computed tomograms confirmed satisfactory fusion in all (n=62) patients at twelve months (range: 12-72 months).
Discussion and Conclusion: The results of the retrospective analysis suggest that the combination of platelet rich fibrin matrix and bone marrow aspirate preparation enhances spinal fusion success. The implantation of the platelet membrane made intraoperatively with the patient's peripheral blood may be responsible for the successful fusion mass seen radiologically. Despite the previous controversies in the usage of platelet rich formulations in orthopedic surgery, our data suggests that this unique formulation may induce bone growth in spinal applications.
[Show abstract][Hide abstract] ABSTRACT: Study Design. In vitro Biomechanical Study.Objective. To test the hypotheses: (1) an anchored-spacer device would decrease motion similarly to a plate-spacer construct, and (2) the anchored-spacer would achieve a similar reduction in motion when placed adjacent to a previously fused segment.Summary of Background Data. An anchored-spacer device has been shown to perform similar to the plate-spacer construct in previous biomechanical evaluation. The prevalence of adjacent segment disease after fusion is well established in the literature. There is currently no evidence supporting the use of an anchored interbody spacer device adjacent to a previous fusion.Methods. Eight human cervical spines (age: 45.1±13.1) were tested in moment-control (±1.5Nm) in flexion-extension, lateral bending, and axial rotation without preload. Flexion-extension was then retested under 150N preload. Spines were tested intact and after ACDF at C4-C5 and C6-C7 with either a plate-spacer or anchored-spacer construct (randomized). The specimens were tested finally with an ACDF at the floating C5-C6 segment using the anchored-spacer device adjacent to the previous fusions.Results. Both the plate-spacer and anchored-spacer significantly reduced motion from the intact spine in flexion-extension, lateral bending and axial rotation (p<0.005). There was no statistically significant difference between the two fusion constructs in their abilities to reduce motions (p = 1.0). ACDF using the anchored-spacer at the floating C5-C6 level (in between the plate-spacer and anchored-spacer constructs) resulted in significant motion reductions in all modes of testing (p<0.05). These motion reductions did not significantly differ from those of a single-level anchored-spacer construct or a single level plated ACDF.Conclusion. The anchored-spacer provided significant motion reductions, similar to a plated ACDF, when used as a single-level fusion construct or placed adjacent to a previously plated segment.
[Show abstract][Hide abstract] ABSTRACT: Background Context: Interest in lumbar disc arthroplasty as an alternative to fusion surgery continues to grow. The goal of disc arthroplasty is to replace the diseased disc while preserving and/or restoring motion at the operated spinal level. Different paradigms exist in the design of total disc arthroplasty devices. Purpose: The purpose of this study was to compare the in vitro biomechanics of a more constrained ball-and-socket design (Prodisc-L, Synthes Spine and Maverick, Medtronic) and a less constrained mobile-bearing design (Charité, DePuy). The biomechanical performance of the disc prostheses was compared to harvested and fused spine conditions. The fused model was simulated using single-level posterior pedicle screw fixation instrumentation. Study Design/Setting: In vitro test to compare the biomechanical properties of three different lumbar disc replacement devices in a human cadaveric model. Methods: Twenty human cadaveric lumbar spines (L1-sacrum) were tested in flexion, extension, lateral bending, and axial rotation under displacement control to a target bending moment of 8Nm. The spine conditions tested were: harvested spine (n=19); L5-S lumbar disc replacement using Prodisc-L (n=13); Maverick (n=7); Charité (n=6); and L5-S pedicle screw fixation (PSF) (n=19). The first 12 spines were split into 2 groups: 6 were instrumented with the Charité and 6 with the ProDisc-L. The next 7 spines tested were split into 2 groups: 4 instrumented with ProDisc-L and 3 with the Maverick. After completing all tests on the second group of 7, the Maverick and ProDisc-L discs were swapped between spines and retested. The Click'X pedicle screw system (Synthes Spine) was used to simulate the fusion model in all spines tested. For axial rotation tests, a 100N compressive load was applied. Measurements included vertebral motions, total spine rotation, and applied loads. The percent contribution of rotation at the instrumented (L5-S) level relative to total rotation (L1-S), as well as at the remaining adjacent levels relative to total rotation, was determined a common load limit (8Nm) and compared using a one-way ANOVA and SNK test (P<0.05). Results: A significant reduction in motion occurred at the operated level of PSF condition compared to the three disc arthroplasty conditions for all loading modes. No differences occurred between the 3 disc conditions for all modes tested, except at the instrumented level of the ProDiscL (93% of H) and Maverick (128% of H) spine conditions during combined flexion+extension. The reduced motion at the operative level of the PSF condition was transferred to the adjacent levels and caused a significant increase in motion during combined flexion+extension at all adjacent levels for the 3 disc arthroplasty conditions, during combined right+left lateral bending at L1-L2 for all disc conditions and at L3-L4 for the Charité, and during combined right+left axial rotation at L3-L4 for all three disc conditions. Conclusions: Issues pertaining to adjacent segment disease (ASD) with pedicle screw fixation were supported by increased motion contributions at multiple sub-adjacent segments. However, disc arthroplasty eliminated any significant increase and may prevent ASD. Compared to pedicle screw fixation, the three differently designed disc prostheses (Prodisc-L, Maverick, and Charité) remained stable and provided improved lumbar mobility. The only notable difference between the disc designs was the increased combined flexion+extension motion at the operative level of the Maverick disc compared to the ProDisc-L device.
Preview · Article · Feb 2014 · The Open Spine Journal
[Show abstract][Hide abstract] ABSTRACT: Study Design. Analysis of data collected prospectively from the US FDA IDE clinical trial for the 1 and 2 Level ProDisc-L arthroplasty patients performed at two study centers.Objective. To determine if there is any clinical or radiographic differences between 1-level ProDisc-L patients and 2-level ProDisc-L patients with a minimum follow-up of 5 years.Summary of Background Data. Recent prospective US FDA clinical trial results have been published showing efficacy of the ProDisc prosthesis in both single and two level surgical procedures. Results of these prospective, randomized multicenter FDA IDE clinical trials showed safety, efficacy and superiority over circumferential fusion.Methods. Patients were part of the FDA clinical trial for the ProDisc-L versus circumferential fusion study at two institutions. We identified 20 patients who received ProDisc-L at 1 level and 21 who received it at 2 levels for a total of 41 patients. Pre- and post-operative pain, clinical function and range of motion (ROM) were measured. An unpaired t-test was done to compare the 2-verus 1-level disc replacement patients.Results. All patients had more than 60 months of clinical follow-up. The mean scores on the VAS were 37.1 and 28.7 respectively (P = 0.33) for one and two level patients. ODI scores were 33 and 29.1 respectively for one and two level patients (P = 0.60); and SF36 physical scores were 43 and 43.9 respectively for one and two level patients (P = 0.81); SF-36 mental scores were 46.2 and 46.6 respectively for one and two level patients (p = 0.923). The average final follow-up post-op ROM for one and two level TDR were 6.0º and 5.4º respectively (p = 0.632).Conclusion. This study was unable to identify statistical differences in all clinical and radiographic outcomes between 1- and 2-level ProDisc arthroplasty patients in a cohort from two centers. Future studies with longer-term follow-up are needed to confirm these results.
[Show abstract][Hide abstract] ABSTRACT: Study Design. Prospective retrieval analysis of failed Prodisc-C CTDRs from 24 explanting surgeons over a 6-year period.Objective. To determine the in vivo mechanical performance and fixation to bone of explanted Prodisc-C CTDRs.Summary of Background Data. The nature and quantity of damage sustained by an implanted device has proven to be important in the prediction of clinical longevity. We hypothesized that retrieval analysis of the Prodisc-C will display characteristic modes of wear consistent with increased posterior angulation and translation of the functional spinal unit following resection of the discoligamentous anatomy.Methods. Thirty CTDRs from 29 patients (mean age 45.1±1.9, range 31-57yrs) after a mean length of implantation of 1.0±0.2 years (range 2 days-3.5 yrs) were studied. Operative level was C4-C5 in 20%(6/30), C5-C6 in 47%(14/30), C6-7 in 20%(6/30), and unknown in 13%(4/30). Polyethylene(PE) and metallic(CoCrMo) components were examined using light stereo-microscopy(6X-31X), scanning electron microscopy(SEM), and energy dispersive x-ray analysis (EDAX).Results. CTDRs were explanted for indications of axial pain(n = 9), radicular symptoms(n = 6), atraumatic loosening(n = 6), trauma(n = 5), metal allergy(n = 1), myelopathy(n = 1), hypermobility(n = 1), and unknown(n = 1). Surface area of ongrowth (mean = 7.2±1.4%) was not associated with operative level(p = .37), surgeon reported axial pain(p = .56) or atraumatic loosening(p = .93). Burnishing consistent with metallic endplate impingement was present in 80%(24/30) of retrieved CTDRs; most commonly in the posterior quadrant(p<.001). There was no association between implant height(p = .19) or depth(p = .17) and posterior impingement. Backside wear was not observed on any of the disassembled implants(0/16). Third-body wear occurred in 23%(7/30) and the donor site was confirmed by SEM/EDAX to be the porous-coated surface of the CTDR.Conclusion. Early clinical failures of Prodisc-C CTDRs display surface damage evidence of metal endplate-endplate impingement, most commonly posteriorly. Backside wear was not evident, however, third-body wear was found. Future studies will determine the clinical impact of these predominant modes of wear on long-term metal-on-PE semi-constrained CTDR performance.
[Show abstract][Hide abstract] ABSTRACT: A retrieval analysis of wear modes and fixation of lumbar total disc replacements (TDRs). Explanted Prodisc-L TDRs were prospectively collected during a 7-year period (2005-2011) and analyzed.
To assess the in vivo modes of wear and fixation of lumbar TDR with the Prodisc-L device.
Inferior clinical outcomes and failure of lumbar TDR may occur because of suboptimal component fixation, wear properties, and impingement in a subset of patients. Posterior component TDR impingement has been demonstrated radiographically; however, despite its widespread use, the in vivo mechanical performance and fixation of the Prodisc-L device remain unknown.
Explanted polyethylene and metallic (CoCrMo) components of Prodisc-L devices were examined by light stereo-microscopy (6X-31X), scanning electron microscopy, and energy-dispersive x-ray analysis from an international retrieval registry, with 13 participating surgeons.
Nineteen ProDisc-L devices from 18 patients (age, 44.7 ± 2.9 yr) following an index TDR at L4-L5 (n = 6), L5-S1 (n = 11), and unknown level (n = 2) were explanted for pain (n = 8), prosthesis subluxation/migration (n = 4), end plate collapse/subsidence (n = 3), polyethylene dislodgement (n = 3), and unknown (n = 2) after a mean length of implantation of 13.0 ± 3.9 months. Surface area of bony ongrowth was 9.6 ± 2.9% (range, 0%-52.5%). TDR burnishing was observed posteriorly consistent with component impingement in extension in 53% (8/15) (P < 0.02), more commonly than anterior 20% (3/15) lateral 20% (n = 3) (3/15) patterns. Circumferential burnishing was not observed. Posterior impingement was associated with 6° lordotic implants (P < 0.05) and 10-mm polyethylene size (P < 0.05). Backside wear occurred in 75% (9 of 12) of the disassembled implants and third-body wear was observed in 33% (5 of 15).
Metallic end plate burnishing was evident in a large percentage of clinically failed Prodisc-L TDR devices, most commonly posteriorly, consistent with impingement in extension. Long-term follow-up studies will evaluate the effects of the observed backside wear, third-body wear, and end plate impingement on clinical outcomes.
[Show abstract][Hide abstract] ABSTRACT: We hypothesized that L5-S1 kinematics will not be affected by the lordosis distribution between the prosthesis endplates.
Twelve cadaveric lumbosacral spines (51.3 ± 9.8 years) were implanted with 6° or 11° prostheses (ProDisc-L) with four combinations of superior/inferior lordosis (6°/0°, 3°/3°, 11°/0°, 3°/8°). Specimens were tested intact and after prostheses implantation with different lordosis distributions. Center of rotation (COR) and range of motion (ROM) were quantified.
Six-degree lordosis prostheses (n = 7) showed no difference in flexion-extension ROM, regardless of design (6°/0° or 3°/3°) (p > 0.05). In lateral bending (LB), both designs reduced ROM (p < 0.05). In axial rotation, only the 3°/3° design reduced ROM (p < 0.05). Eleven-degree lordosis prostheses (n = 5) showed no difference in flexion-extension ROM for either design (p > 0.05). LB ROM decreased with distributed lordosis prostheses (3°/8°) (p < 0.05). Overall, L5-S1 range of motion was not markedly influenced by lordosis distribution among the two prosthesis endplates. The ProDisc-L prosthesis design where all lordosis is concentrated in the superior endplate yielded COR locations that were anterior and caudal to intact controls. The prosthesis with lordosis distributed between the two endplates yielded a COR that tended to be closer to intact.
Further clinical and biomechanical studies are needed to assess the long-term impact of lordosis angle distribution on the fate of the facet joints.
Full-text · Article · Apr 2012 · European Spine Journal
[Show abstract][Hide abstract] ABSTRACT: Containment plates are often placed anteriorly in anterior cervical discectomy and fusion (ACDF) to provide stability and prevent migration of the interbody device or autograft. The main advantage of a bioresorbable plate over the typical metallic plate is that it will resorb after fusion has occurred, thus mitigating any long-term instrumentation-related complications. Furthermore, the plates are radiolucent, allowing complete visualization of the fusion site and eliminating imaging artifact.
To evaluate radiographic fusion, mechanical success rates, and histologic characteristics of a bioresorbable containment plate and screws in a 3-month ovine model of ACDF.
An in vivo prospective analysis of resorbable anterior cervical plates and screws for use in ACDF in an ovine model.
Six sheep underwent C2-C3 and C4-C5 discectomies. Fusions were performed using a polyetheretherketone cage filled with autograft bone. A polymeric plate (70/30 poly-dl-lactic acid), and four screws were placed over an intervertebral disc spacer at each of these two levels. After 3 months, the animals were euthanized and radiographically imaged. Radiographs were analyzed for fusion and instrumentation failures. Functional spinal units were harvested for histologic processing and evaluation.
Radiographic fusion was noted in three of the 12 levels with no evidence of device failure at any of the levels. However, at necropsy, it was observed that six of the 12 specimens had either a broken screw or a cracked plate. These gross observations were confirmed within the histologic sections. Fusion was verified histologically at C2-C3 in three of the six sheep; none of the fusions were successful at C4-C5. Histologic analysis also found that the tissue surrounding the plate and disc spacer consisted of vascularized fibrous tissue with islands of active woven bone. Inflammatory cells were rarely observed.
Although the bioresorbable plates and screws did not elicit an iatrogenic tissue response, a high percentage of them failed mechanically. This phenomenon was difficult to observe radiographically, as the radiolucent markers were not able to convey these instrumentation failures. Additionally, there was only a 25% fusion rate. These findings suggest that resorbable implant materials with the current biomechanical and chemical properties are inadequate for cervical fusion. The results of this study strongly suggest that radiographic outcomes alone may not be adequate and that gross or histologic methods should accompany radiographs in studies of bioresorbable materials in animal models.
No preview · Article · Jul 2011 · The spine journal: official journal of the North American Spine Society
[Show abstract][Hide abstract] ABSTRACT: In vitro biomechanical study.
To characterize cervical total disc replacement (TDR) kinematics above two-level fusion, and to determine the effect of fusion alignment on TDR response.
Cervical TDR may be a promising alternative for a symptomatic adjacent level after prior multilevel cervical fusion. However, little is known about the TDR kinematics in this setting.
Eight human cadaveric cervical spines (C2-T1, age: 59 ± 8.6 years) were tested intact, after simulated two-level fusion (C4-C6) in lordotic alignment and then in straight alignment, and after C3-C4 TDR above the C4-C6 fusion in lordotic and straight alignments. Fusion was simulated using an external fixator apparatus, allowing easy adjustment of C4-C6 fusion alignment, and restoration to intact state upon disassembly. Specimens were tested in flexion-extension using hybrid testing protocols.
The external fixator device significantly reduced range of motion (ROM) at C4-C6 to 2.0 ± 0.6°, a reduction of 89 ± 3.0% (P < 0.05). Removal of the fusion construct restored the motion response of the spinal segments to their intact state. The C3-C4 TDR resulted in less motion as compared to the intact segment when the disc prosthesis was implanted either as a stand-alone procedure or above a two-level fusion. The decrease in motion of C3-C4 TDR was significant for both lordotic and straight fusions across C4-C6 (P < 0.05). Flexion and extension moments needed to bring the cervical spine to similar C2 motion endpoints significantly increased for the TDR above a two-level fusion compared to TDR alone (P < 0.05). Lordotic fusion required significantly greater flexion moment, whereas straight fusion required significantly greater extension moment (P < 0.05).
TDR placed adjacent to a two-level fusion is subjected to a more challenging biomechanical environment as compared to a stand-alone TDR. An artificial disc used in such a clinical scenario should be able to accommodate the increased moment loads without causing impingement of its endplates or undue wear during the expected life of the prosthesis.
[Show abstract][Hide abstract] ABSTRACT: Biomechanical study using human cadaveric lumbar spines.
To evaluate effects of total disc replacement (TDR) on spine biomechanics at the treated and adjacent levels.
Previous studies on spine biomechanics after TDR were focused on facet forces and range of motion and report contradictory results. Characterization of contact pressure, peak contact pressure, force, and peak force before and after TDR may lead to a better understanding of facet joint function and may aid in prediction of long-term outcomes after TDR.
Seven fresh-frozen human cadaveric lumbar spines were potted at T12 and L5 and installed in a 6 degrees of freedom displacement-controlled testing system. Displacements of 15° flexion/extension, 10° right/left bending, and 10° right/left axial rotation were applied. Contact pressure, peak contact pressure, force, peak force, and contact area for each facet joint were recorded at L2-L3 and L3-L4 both before and after TDR at L3-L4. The data were analyzed with analysis of variance and t tests.
Axial rotation had the most impact on contact pressure, peak contact pressure, force, peak force, and contact area in intact spines. During lateral bending and axial rotation, TDR resulted in a significant increase in facet forces at the level of treatment and a decrease in contact pressure, peak contact pressure, and peak force at the level superior to the TDR. With flexion/extension, there was a decrease in peak contact pressure and peak contact force at the superior level.
Our study demonstrates that rotation is the most demanding motion for the spine. We also found an increase in facet forces at the treated level after TDR. We are the first to show a decrease in several biomechanical parameters after TDR at the adjacent superior level. In general, our findings suggest there is an increase in loading of the facet joints at the level of disc implantation and an overall unloading effect at the level above.
[Show abstract][Hide abstract] ABSTRACT: A biomechanical study using human lumbar spines.
To test the hypotheses that with increasing implant height (1) the range of motion (ROM) of the implanted segment will decrease, (2) the segmental lordosis will increase, and (3) the size of the neural foramens will increase.
Little is known about the effects of the implant height on the segmental motion and foraminal size at the implanted level.
Seven human lumbar spines (age, 54.4+/-11.4 years; L1-sacrum) were tested intact, and after discectomy at L4-L5 and sequential insertion of ProDisc-L implants (Synthes Spine, Paoli, PA) of increasing heights (10, 12, and 14 mm). The specimens were tested in flexion (8 Nm) and extension (-6 Nm) with a 400 N follower preload as well as in lateral bending (+/-6 Nm) and axial rotation (+/-5 Nm) without preload. Three-dimensional motions were measured at L4-L5. Foraminal sizes at L4-L5 were measured in the specimen's neutral posture under a 400 N preload for the intact spine and after each implantation using finely graded cylindrical probes. Segmental lordosis was measured in the specimen's neutral posture under a 400 N preload by analyzing digital fluoroscopic images. Effects of implant height on the kinematics, foraminal size, and segmental lordosis were assessed using paired comparisons with Bonferroni correction.
Increasing implant height from 10 mm to 14 mm caused a significant decrease (P<0.05) in segmental ROM by up to 37%+/-21% in flexion/extension, 33%+/-18% in lateral bending, and 29%+/-28% in axial rotation. Increasing implant height also produced a significant increase in segmental lordosis (P<0.05): from 9.7 degrees+/-2.9 degrees at 10 mm, to 16.1 degrees+/-5.1 degrees at 14 mm. The increase in foraminal size, while significant, was only 4.6%+/-3.2% when comparing 10 mm to 14 mm implants.
These results suggest that a smaller implant height should be selected to optimize the ROM of the implanted segment and maintain sagittal balance.