Posterior Stabilization System Research Articles

Adjacent Segment Disease

Spinal fusion is gold-standard treatment for degenerative disc disease (DDD) both at cervical and lumbar spine, and it is time tested. Perhaps it has its bundle of complications. Elimination of motion results in accentuated degeneration of adjacent segments leading to adjacent segment degeneration radiographically and, if symptomatic, leads to adjacent segment disease. There is still a debate on whether there is such an entity or whether it is a manifestation of natural history or an iatrogenic phenomenon. Motion preservation surgeries were developed to address the same issue, which includes total disc replacement, nucleus replacement, interspinous implants, and dynamic posterior stabilization systems. The primary goal of motion preservation surgery is to maintain normal or near-normal motion in an attempt to prevent adverse outcomes, which are commonly seen with conventional spinal fusion, most notably the development of adjacent-level DDD. A search was conducted in PubMed using the terms (“adjacent segment”) AND (“disease” OR “degeneration” or “pathology”). Then the articles were shortlisted based on time of publication (2005 onward), publication in English and inclusion of human subjects. This resulted in 253 articles. Another search for ((“Motion preservation”) AND (“Spine”)) OR (“Adjacent segment disease”) OR (“Adjacent segment pathology”) OR (“Adjacent segment degeneration”) yielded 76 articles. This narrative review discusses various issues pertaining to the current evidence regarding adjacent segment disease (ASD), including the controversy on whether ASD is actually an entity, its etiopathogenesis, clinical features, as well as the role of motion preservation technologies to reduce its incidence. There is still enthusiasm and concerns regarding the benefits of motion preservation surgery since it is still an area of ongoing research.

An Unusual Complication of Intramedullary Pin Migration during Total Knee Arthroplasty - A Case Report.

Total knee arthroplasty (TKA) is one of the most effective operations to relieve pain and improve function in the end stage of osteoarthritis (when bone on bone contact occurs). The intraoperative complications of TKA include fracture, tendon or ligament injury, and nerve or vascular complications. We herein describe an unusual complication of intramedullary pin migration within the femoral canal during TKA. A 72-year-old male patient underwent TKA with a posterior stabilization system with sacrifice of the posterior cruciate ligament. The distal femur was sectioned and balance was checked in extension. Then to make anterior, posterior, chamfer and notch cuts, the five-in-one anteroposterior (AP) cutting block was placed on the distal femur and the cuts were initiated. As there was a medial overhang of the cutting block, it was shifted laterally. While doing so, the pins had to be shifted too, and one of them was inadvertently hammered into the previously-created medullary canal opening of the femur. As usual orthopedic instruments, like the long straight artery forceps and pituitary rongeurs, failed to remove the migrated pin, an extralong laparoscopic grasper was used under fluoroscopy control to locate, grasp, and remove the migrated pin.

Biomechanical stability analysis of transpedicular screws combined with sublaminar hook-rod system using the finite element method

AimTo develop and test a new posterior stabilization system by augmenting the posterior hook-rod system with screws and rods.MethodsA biomechanical analysis was performed using the finite element method. The anatomical structures were modeled based on computed tomography data. Instrumentation (hooks, rods, and screws) was modeled based on the data obtained by 3D scanning. The discretized model was verified by converging solutions and validated against data from a previously published experiment. A Th12-L1 spinal segment was modeled and modified by removing the body of the L1 vertebra (corpectomy) and the entire L1 vertebra (spondylectomy). The model was additionally modified by incorporating stabilization systems: i) posterior stabilization (transpedicular screws and rods); ii) combined posterior stabilization with sublaminar hooks; and iii) combined anterior (titanium cage) and posterior (sublaminar hooks) stabilization. The rotation angles in each group, and the strains on each part of the three stabilization constructs, were analyzed separately.ResultsThe combined anterior and posterior stabilization system was the stiffest, except in the case of lateral bending, where combined posterior stabilization was superior. Stress analysis showed that the posterior stabilization system was significantly unloaded when augmented with a hook-rod system. A significant strain concentration was calculated in the cranially placed hooks.ConclusionStiffness analysis showed comparable stiffness between the tested and proposed stabilization construct. Stress analysis showed luxation tendency of the cranially placed hooks, which would most likely lead to system failure.

Biomechanical analysis of lumbar interbody fusion supplemented with various posterior stabilization systems.

Biomechanical comparison between rigid and non-rigid posterior stabilization systems following lumbar interbody fusion has been conducted in several studies. However, most of these previous studies mainly focused on investigating biomechanics of adjacent spinal segments or spine stability. The objective of the present study was to compare biomechanical responses of the fusion devices when using different posterior instrumentations. Finite-element model of the intact human lumbar spine (L1-sacrum) was modified to simulate implantation of the fusion cage at L4-L5 level supplemented with different posterior stabilization systems including (i) pedicle screw-based fixation using rigid connecting rods (titanium rods), (ii) pedicle screw-based fixation using flexible connecting rods (PEEK rods) and (iii) dynamic interspinous spacer (DIAM). Stress responses were compared among these various models under bending moments. The highest and lowest stresses in endplate, fusion cage and bone graft were found at the fused L4-L5 level with DIAM and titanium rod stabilization systems, respectively. When using PEEK rod for the pedicle screw fixation, peak stress in the pedicle screw was lower but the ratio of peak stress in the rods to yield stress of the rod material was higher than using titanium rod. Compared with conventional rigid posterior stabilization system, the use of non-rigid stabilization system (i.e., the PEEK rod system and DIAM system) following lumbar interbody fusion might increase the risks of cage subsidence and cage damage, but promote bony fusion due to higher stress in the bone graft. For the pedicle screw-based rod stabilization system, using PEEK rod might reduce the risk of screw breakage but increased breakage risk of the rod itself.

Evaluation of Efficacy and Safety of All Pedicle Screw Posterior-Only Surgery in Patients with Dystrophic Neurofibromatosis Scoliosis

Background Data: Neurofibromatosis type 1 (NF-1) patients with dystrophic changes of the spine develop severe spinal deformity and/or instability and always need early surgical treatment. Combined anterior-posterior fusion was the treatment of choice of such curves because of the risk of progression and pseudoarthrosis even in the presence of solid posterior fusion mass. Recent 3rd-generation posterior stabilization systems using pedicle screw fixation allow better 3-column purchase and 3D control and correction of deformity and stable posterior fixation of the spine preventing progression of the curves. Posterior-only surgery has been recently used in dystrophic NF-1 scoliosis due to use of pedicle screws fixation and development of different types of posterior release/osteotomies which lead to successful and sustained correction of these curves.Study Design: A retrospective clinical cohort study.Purpose: To study the results of posterior stabilization alone using all pedicle screws fixation in NF-1 patients with dystrophic scoliosis.Patients and Methods: 16 patients with NF-1 having dystrophic scoliosis were treated by posterior instrumentation using all pedicle screws. There were 5 female and 11 male patients and the average age was 13 years (range, 9–16 years). The study was done in the period between 2009 and 2016. Preoperative clinical examination included examination for neurofibromatosis, assessment of curve flexibility, and thorough neurological examination. Radiological examination included standard plain X-rays, anterior-posterior (A-P) and lateral and flexibility views, and MRI. Screw density of the whole construct and of the apical region (two vertebrae above and below the apex) was calculated. Functional outcomes were assessed by the ODI and SRS-30 scores at the last follow-up.Results: The mean follow-up was 5.15±1.78 years (range, 3–8). The mean Cobb angle improved in the coronal plane from 67.13±11.88 (range, 45–90◦) to 20±8.9 (range, 6–30◦) showing 70.1±11.2%. The mean preoperative sagittal plane deformity significantly improved from 61.82±20.28 (range, 35–90◦) to 25.91±7.68 (range, 10–35◦) postoperatively yielding 53.15±21.43 (range, 25–83.33%) mean correction with insignificant loss of correction at last follow-up. Loss of correction was 2±1.8% and 1.8±2.1% in the coronal and sagittal planes, respectively. At last follow-up the average SRS-30 score was 121±17.7 and the

A new lumbar fixation device alternative to pedicle-based stabilization for lumbar spine: In vitro cadaver investigation

Context: To evaluate the stability provided by a new bilateral fixation technique using an in vitro investigation for posterior lumbar segmental instrumentation.Design: Experimental cadaver study. In this study, we propose an alternative technique for a posterior lumbar fixation technique called “inferior-oblique transdiscal fixation” (IOTF).Setting: Study performed at Engineering Center for Orthopedic Research Exellence (ECORE) in Toledo University-Ohio.Participants: Six human lumbar cadaveric specimen used in this study.Interventions: In this study, we propose an alternative technique for a posterior lumbar fixation technique called “inferior-oblique transdiscal fixation” (IOTF). As a novel contribution to the classical technique, the entry point of the screw is the supero-lateral point of the intersecting line drawn between the corpus and the pedicle of the upper vertebra. This approach enables the fixation of two adjacent vertebrae using a single screw on each side without utilizing connecting rods.Outcome Measures: Flexion (Flex), extension (Ext), right and left lateral bending (LB & RB), and right and left axial rotation (LR & RR), and the position data were captured at each load step using the Optotrak motion measurement system and compared for IOTF and posterior transpedicular stabilization.Results: The Posterior stabilization system (PSS) and IOTF significantly reduced the ROM of L4-L5 segment compared to intact segment’s ROM. During axial rotation (AR) IOTF fused index segment more than PSS. Besides this, addition of transforaminal lumbar interbody fusion (TLIF) cage improved the stabilization of IOTF system during flexion, extension and lateral bending. Whereas, PSS yielded better fusion results during extension compared to IOTF with and without interbody fusion cages.Conclusions: We hypothesized that the new posterior bilateral system would significantly decrease motion compared to the intact spine. This cadaver study showed that the proposed new posterior fusion technique IOTF fused the index segment in a similar fashion to the classical pedicle screw fusion technique.

Spinal motion preservation surgery: indications and applications.

Fusion is one of the most commonly performed spinal procedures, indicated for a wide range of spinal problems. Elimination of motion though results in accelerated degeneration of the adjacent level, known as adjacent level disease. Motion preservation surgical methods were developed in order to overcome this complication. These methods include total disc replacement, laminoplasty, interspinous implants and dynamic posterior stabilization systems. The initial enthusiasm about these methods was followed by certain concerns about their clinical usefulness and their results. The main indications for total disc replacement are degenerative disc disease, but the numerous contraindications for this method make it difficult to find the right candidate. Application of interspinous implants has shown good results in patients with spinal stenosis, but a more precise definition is needed regarding the severity of spinal stenosis up to which these implants can be used. Laminoplasty has several advantages and less complications compared to fusion and laminectomy in patients with cervical myelopathy/radiculopathy. Dynamic posterior stabilization could replace conventional fusion in certain cases, but also in this case the results are successful only in mild to moderate cases.

Outcomes of Decompression and Stabilization with a Pedicle-Based Semirigid Posterior Stabilization System in the Lumbar Spine

Introduction Patients with lumbar spinal stenosis who do not respond satisfactorily to nonoperative measures may require decompression. Laminectomies/decompression without instrumentation may cause instability and associated symptoms. Fusion often involves multiple segments potentially increasing the risk of proximal junctional kyphosis (PJK), adjacent segment disease (ASD), and surgical morbidity of lateral exposure to prepare fusion bed. We have therefore been using the TRANSITION Stabilization System (Globus Medical, Inc., Audubon, Pennsylvania, United States) to provide stability without fusion at the operated levels. Following insertion of pedicle screws, to which the rod is attached, a semirigid rod is custom built along a central cord using a combination of rigid screw head attachments, flexible spacers, and a compressible bumper. Biomechanical studies have indicated superior performance to other similar systems. We report our clinical results. Patients and Methods A retrospective analysis of prospectively collected data was performed for patients who underwent surgery between September 2011 and December 2013. Outcome measures included Oswestry Disability Index (ODI), Visual Analog Scale (VAS) back/leg pain, Hospital Anxiety and Depression Scale, and complications. Statistical analysis was performed with paired and unpaired tests. Results Demographics: Data were available for 24 patients; 8 females and 16 males. Mean age at surgery was 66.8 years. Mean body mass index was 30.4 kg/m2. Mean length of follow-up was 44 (range, 10–109) weeks. Mean postoperative length of stay was 7.2 days, and when those patients requiring reoperation for complications were excluded it was 4.4 days. Surgery: A total of 10 L3–S1, 4 L4–S1, 1 L4–L5, 3 L2–L5, 4 L3–L5, 1 L1–L5, and 1 L2–S1 stabilizations were performed with a variety of combined central and foraminal decompressions. Mean blood loss was 648 mL and surgical time 176 minutes. Eight patients sustained intraoperative dural tears that were sealed with fibrin sealant. Outcomes: Mean ODI improved from 45.1 to 31.3 ( p = 0.001), VAS back from 6.7 to 3.7 ( p ≤ 0.01), VAS legs from 6.7 to 2 ( p ≤ 0.01), anxiety from 6.1 to 5.4 ( p = 0.37), and depression from 7.3 to 3.7 ( p = 0.033). There was no statistical correlation between pre- and postoperative ODI, VAS back or legs, depression/anxiety scores, and demographics. Reoperations: There were three early reoperations: one patient for discharging wound with positive microbiology and for revision of a screw, one patient for discharging wound with negative microbiology, and one patient for revision of a screw. There were no implant failures or revisions for PJK or ASD to date. Conclusion Our results using the TRANSITION Stabilization System suggests that pedicle-based semirigid posterior stabilization systems warrant further consideration as a potential tool in providing satisfactory early postoperative results while potentially reducing the long-term morbidity associated with fusions for lumbar stenosis. We would encourage further research to assess longer term results and radiographic outcomes.

Time-dependent clinical results of rotating-platform total knee arthroplasty according to mechanical axis deviation.

PurposeWe hypothesized that the low contact stress (LCS) posterior stabilization system in knees with ≤3° deviation of coronal alignment would provide more favorable clinical outcomes and survival rate over the course of time.Materials and MethodsA retrospective study was performed on 253 consecutive cases of primary total knee arthroplasty (TKA). Patients were classified according to the degree of deviation of coronal alignment on the initial postoperative radiograph as Group 1 (≤3° deviation) and Group 2 (>3° deviation). The clinical assessments were performed using the Knee Society score and Hospital for Special Surgery systems and Western Ontario and McMaster Universities index.ResultsThe survival rate was 97.4% in Group 1 and 96.8% in Group 2. No statistically significant intergroup difference was observed in the clinical scores before surgery and since 1 year after surgery (p>0.05). However, a significant intergroup difference was noted between 6 months to 1 year after surgery (p<0.001). Less than 2 mm radiolucent lines were found more frequently in Group 2. Time-dependent improvement was noted within one year after TKA in both groups.ConclusionsMost of the expected improvements were achieved at 6 months after surgery in Group 1 and at 1 year after surgery in Group 2. The present study suggests that the LCS system yields time-dependent improvement regardless of coronal alignment deviation.

A Computational Biomechanical Investigation of Posterior Dynamic Instrumentation: Combination of Dynamic Rod and Hinged (Dynamic) Screw

Currently, rigid fixation systems are the gold standard for degenerative disk disease treatment. Dynamic fixation systems have been proposed as alternatives for the treatment of a variety of spinal disorders. These systems address the main drawbacks of traditional rigid fixation systems, such as adjacent segment degeneration and instrumentation failure. Pedicle-screw-based dynamic stabilization (PDS) is one type of these alternative systems. The aim of this study was to simulate the biomechanical effect of a novel posterior dynamic stabilization system, which is comprised of dynamic (hinged) screws interconnected with a coiled, spring-based dynamic rod (DSDR), and compare it to semirigid (DSRR and RSRR) and rigid stabilization (RSRR) systems. A validated finite element (FE) model of L1-S1 was used to quantify the biomechanical parameters of the spine, such as range of motion, intradiskal pressure, stresses and facet loads after single-level instrumentation with different posterior stabilization systems. The results obtained from in vitro experimental intact and instrumented spines were used to validate the FE model, and the validated model was then used to compare the biomechanical effects of different fixation and stabilization constructs with intact under a hybrid loading protocol. The segmental motion at L4-L5 increased by 9.5% and 16.3% in flexion and left rotation, respectively, in DSDR with respect to the intact spine, whereas it was reduced by 6.4% and 10.9% in extension and left-bending loads, respectively. After instrumentation-induced intradiskal pressure at adjacent segments, L3-L4 and L5-S1 became less than the intact in dynamic rod constructs (DSDR and RSDR) except in the RSDR model in extension where the motion was higher than intact by 9.7% at L3-L4 and 11.3% at L5-S1. The facet loads were insignificant, not exceeding 12N in any of the instrumented cases in flexion. In extension, the facet load in DSDR case was similar to that in intact spine. The dynamic rod constructions (DSDR and RSDR) led to a lesser peak stress at screws compared with rigid rod constructions (DSRR and RSRR) in all loading cases. A dynamic construct consisting of a dynamic rod and a dynamic screw did protect the adjacent level from excessive motion.

Influence of Screw Augmentation in Posterior Dynamic and Rigid Stabilization Systems in Osteoporotic Lumbar Vertebrae

Biomechanical cadaveric study. To determine whether augmentation positively influence screw stability or not. Implantation of pedicle screws is a common procedure in spine surgery to provide an anchorage of posterior internal fixation into vertebrae. Screw performance is highly correlated to bone quality. Therefore, polymeric cement is often injected through specifically designed perforated pedicle screws into osteoporotic bone to potentially enhance screw stability. Caudocephalic dynamic loading was applied as quasi-physiological alternative to classical pull-out tests on 16 screws implanted in osteoporotic lumbar vertebrae and 20 screws in nonosteoporotic specimen. Load was applied using 2 different configurations simulating standard and dynamic posterior stabilization devices. Screw performance was quantified by measurement of screwhead displacement during the loading cycles. To reduce the impact of bone quality and morphology, screw performance was compared for each vertebra and averaged afterward. All screws (with or without cement) implanted in osteoporotic vertebrae showed lower performances than the ones implanted into nonosteoporotic specimen. Augmentation was negligible for screws implanted into nonosteoporotic specimen, whereas in osteoporotic vertebrae pedicle screw stability was significantly increased. For dynamic posterior stabilization system an increase of screwhead displacement was observed in comparison with standard fixation devices in both setups. Augmentation enhances screw performance in patients with poor bone stock, whereas no difference is observed for patients without osteoporosis. Furthermore, dynamic stabilization systems have the possibility to fail when implanted in osteoporotic bone.

Posterior spinal instrumentation: biomechanical study on the role of rods on hardware response to axial load

Posterior spinal instrumentation is frequently used for the treatment of spine disorders. Importantly, different requirements have to be considered for the optimal use of these systems in various clinical scenarios. In this work, we focused on the role of rods diameter on hardware's stiffness. For this purpose, we established an in vitro model and compared the response to axial load of a posterior stabilization system, characterized by rods of different diameter (4, 5, 6 mm), with that of Dynesys®. Intuitively, the higher the stiffness of the hardware, the lower the load is transferred to the disc. However, the 4 hardware tested showed a different trend in the response to the load regimens: when increasing the load, more flexible systems display a progressive reduction in the percentage of load which is transferred to the disc while more rigid system display the opposite trend. Considering that the load which is transferred, and not by-passed by the hardware, influences the healing of a fracture; the integration of a bone graft or a cage; the fusion process, these data have a relevant impact on clinical practice and highlight features that have to be considered in the choice for the optimal posterior spinal instrumentation.

Radiostereometric Analysis of Postoperative Motion After Application of Dynesys Dynamic Posterior Stabilization System for Treatment of Degenerative Spondylolisthesis

Prospective case series This was designed to precisely measure motion after posterior dynamic stabilization using Dynesys instrumentation. The Dynesys posterior dynamic stabilization system, which stabilizes the spinal segment while potentially decreasing the risk of adjacent segment disease, is undergoing evaluation by the US Food and Drug Administration for treatment of degenerative spondylolisthesis without fusion. Evaluation of adjacent segment disease requires precise characterization of motion on the surgical level. Unfortunately, routine clinical radiographic techniques are imprecise and unreliable for full characterization of spinal segment motion. Radiostereometric analysis, which is very precise and reliable for in vivo measurement of motion, was used to examine spinal segment motion after dynamic stabilization with Dynesys. Six patients (age 59+/-7 y) underwent posterior decompression followed by posterior stabilization using Dynesys instrumentation (4 one-level, 2 two-levels). Three to 5 tantalum beads were placed in each vertebral body. Postoperative biplanar radiographs were obtained in flexion, extension, right, and left lateral bending, and 3-dimensional reconstruction was performed using radiostereometric analysis at 3, 6, 12, and 24 months postoperatively. The translations and rotations of the superior vertebral body were measured relative to the inferior vertebral body. Over the 24-month follow-up period, mean flexion, extension, left, and right lateral bending of the motion segments were noted to be 1.0 degrees, 2.4 degrees, 0.6 degrees, and 0.6 degrees or less, respectively. There were no statistically significant changes in the degree of motion. During follow-up, no significant changes in neutral position of the device were noted in any of the 3 planes, and minimal translation was noted in the postoperative period. The Dynesys dynamic instrumentation system seems to stabilize degenerative spondylolisthesis. As expected in the degenerative lumbar spine, the segmental motion of the implanted level in this study was limited and considerably less than normal spinal motion.

165. Preliminary 3-Year Results from a Prospective, Randomized, Controlled IDE Study of the Dynesys Dynamic Stabilization System

BACKGROUND CONTEXT: Background Context: Patients with radicular pathology due to spondylolisthesis or stenosing lesions are typically treated with decompression and spinal fusion. A posterior stabilization system has been developed to stabilize the segment without the need for fusion. Preliminary long-term outcomes from a cohort of patients in an IDE clinical trial examining dynamic stabilization with the Dynesys Dynamic Stabilization System are being reported.

Reviewer’s comment concerning “Anterior cervical pedicle screw and plate fixation using fluoroscope-assisted pedicle axis view imaging: a preliminary report of a new cervical reconstruction technique” by Y. Yukawa et al.: DOI:10.1007/s00586-009-0949-1

Methods for the stabilization of the cervical spine improved enormously in the last two decades as also improved all other spinal fixations. This was possible on one side through the technology and the development of better screw fixation systems having stronger bone purchase and angle stable linkage despite variable insertion angles. On the other side, the recognition of the pedicle as the strongest element for screw anchorage has been demonstrated also in the cervical spine. All these improved implants combined with interbody or reconstruction cages having different size and shape have allowed always more complex cervical spine reconstruction surgeries. In this article the authors report on a new fixation technique of the cervical spine where pedicle screw anchorage is performed through an anterior approach. Screw insertion into the pedicles is performed using fluoroscopy guided technique. Cervical spine stabilization consists in an unilateral plate/screw system. This study demonstrates the technique is feasible when performed by extremely skilled surgeons. Major neurological complications with screw misplacement are evident and could be partially reduced using computer-assisted navigation system. Due to these dangers, the indications for this technique must remain extremely limited to those very rare cases where the traditional anterior or/and posterior stabilization systems cannot be applied and to those spine centers with sufficient experience in complex cervical spine reconstruction surgery. Dante G. Marchesi MD

168. Preliminary Clinical Outcomes from the US FDA IDE Study of a Dynamic Posterior Stabilization System

168. Preliminary Clinical Outcomes from the US FDA IDE Study of a Dynamic Posterior Stabilization System

Biomechanical characterization of the three-dimensional kinematic behaviour of the Dynesys dynamic stabilization system: an in vitro study

The Dynesys, a flexible posterior stabilization system that provides an alternative to fusion, is designed to preserve intersegmental kinematics and alleviate loading at the facet joints. Recent biomechanical evidence suggests that the overall range of motion (ROM) with the Dynesys is less than the intact spine. The purpose of this investigation was to conduct a comprehensive characterization of the three-dimensional kinematic behaviour of the Dynesys and determine if the length of the Dynesys polymer spacer contributes to differences in the kinematic behaviour at the implanted level. Ten cadaveric lumbar spine segments (L2-L5) were tested by applying a pure moment of +/-7.5 Nm in flexion-extension, lateral bending, and axial rotation, with and without a follower preload of 600 N. Test conditions included: (a) intact; (b) injury; (c) injury stabilized with Dynesys at L3-L4 (standard spacer); (d) long spacer (+2 mm); and (e) short spacer (-2 mm). Intervertebral rotations were measured using an optoelectronic camera system. The intersegmental range of motion (ROM), neutral zone (NZ), and three-dimensional helical axis of motion (HAM) were calculated. Statistical significance of changes in ROM, NZ, and HAM was determined using repeated measures analysis of variance (ANOVA) and Student-Newman-Keuls post-hoc analysis with P<0.05. Implantation of the standard length Dynesys significantly reduced ROM compared to the intact and injured specimens, with the least significant changes seen in axial rotation. Injury typically increased the NZ, but implantation of the Dynesys restored the NZ to a magnitude less that that of the intact spine. The Dynesys produced a significant posterior shift in the HAM in flexion-extension and axial rotation. The spacer length had a significant effect on ROM with the long spacer resulting in the largest ROM in all loading directions without a follower preload. The largest differences were in axial rotation. A 4 mm increase in spacer length led to an average intersegmental motion increase of 30% in axial rotation, 23% in extension, 14% in flexion, and 11% in lateral bending. There were no significant changes in NZ with different spacer lengths. Typically, the short spacer caused a greater shift and a greater change in orientation of the HAM than the long spacer. The long spacer resulted in a ROM and a motion pattern, as represented by the HAM, that was closer to that seen in an intact specimen. The results of this study suggest that the length of the Dynesys spacer altered the segmental position and therefore affected kinematic behaviour.