Dynesys Dynamic Stabilization System Research Articles

Index-level fusion and adjacent segment disease following dynamic stabilization for lumbar degenerative disc disease: illustrative case.

Dynesys dynamic stabilization (DDS) is an alternative to surgical fusion for the operative management of degenerative lumbar spondylosis. Compared to rigid instrumentation and fusion, DDS is purported to preserve a higher degree of spinal range of motion and reduce the risk of developing adjacent segment disease (ASD). A 60-year-old female presented with severe back pain and bilateral leg pain, which had progressed over the prior 4 years. Nine years earlier, she had undergone DDS system implantation at L5-S1 for lumbar stenosis and spondylosis. Repeat imaging revealed an unintended fusion at the index level (L5-S1) and ASD causing severe lateral recess stenosis at L4-5. She underwent DDS system removal, decompression at L4-5, and extension of the fusion to L4. Although DDS has been marketed as a motion-preserving system that avoids fusion and reduces the risk of ASD, unintended index-level fusion and ASD can still occur after DDS system surgery. These potential complications should be assessed when determining the optimal primary surgical treatment for patients with lumbar degenerative disc disease. https://thejns.org/doi/10.3171/CASE24179.

Complications associated with the Dynesys dynamic stabilization system: a comprehensive review of the literature

The Dynesys dynamic stabilization system is an alternative to rigid instrumentation and fusion for the treatment of lumbar degenerative disease. Although many outcomes studies have shown good results, currently lacking is a comprehensive report on complications associated with this system, especially in terms of how it compares with reported complication rates of fusion. For the present study, the authors reviewed the literature to find all studies involving the Dynesys dynamic stabilization system that reported complications or adverse events. Twenty-one studies were included for a total of 1166 patients with a mean age of 55.5 years (range 39-71 years) and a mean follow-up period of 33.7 months (range 12.0-81.6 months). Analysis of these studies demonstrated a surgical-site infection rate of 4.3%, pedicle screw loosening rate of 11.7%, pedicle screw fracture rate of 1.6%, and adjacent-segment disease (ASD) rate of 7.0%. Of studies reporting revision surgeries, 11.3% of patients underwent a reoperation. Of patients who developed ASD, 40.6% underwent a reoperation for treatment. The Dynesys dynamic stabilization system appears to have a fairly similar complication-rate profile compared with published literature on lumbar fusion, and is associated with a slightly lower incidence of ASD.

Is Dynesys a Good Device for Motion Preservation and Prevention of Idiopathic Unstable after Decompression Surgery

Introduction The Dynesys dynamic stabilization system was first implanted in patients in 1994 in an attempt to overcome the drawbacks of fusion. It is supposed to preserve motion at the treat levels, while avoiding hypermobility and thus spondylosis at the adjacent levels. The aim of this retrospective study is to investigate the clinical outcome and the effect of preservation of ROM and developed instability after decompression surgery. Patients and Methods A total of 30 patients were treated with Dynesys between August 2008 to March 2014. The diagnosis included degenerative spinal stenosis, severe spinal stenosis with HIVD, huge intervertebral herniated disc, or recurrent HIVD. Back pain and leg pain were evaluated using visual analog scale (VAS). The Oswestry Disability Index (ODI) was used to evaluate the patient's function. The range of motion at the operative level and whole lumbar spine were measured on preoperative and postoperative radiographs. Results All the patients were followed up for 3 to 96 months (mean, 26.33 ± 25.3 months). Overall, 87 segments were applied in 30 patients. The mean age of the patients was 49.5 ± 10.4 years (range, 22–67 years). There were 19 male and 11 female patients, with slightly male predominant. VAS of leg pain, lower back pain, and the ODI were significantly improved postoperatively. There was a significant difference in the range of motion at the operative levels and the whole spine. The mean range of motion of whole spine was 28.89 ± 15.3 degrees preoperatively and 16.29 ± 8.39 degrees postoperatively, with 43.61% loss. The range of motion of operative levels were 12.01 ± 7.73 degrees preoperatively and 4.03 ± 3.19 degrees postoperatively, with 66.44% loss. There were four broken screws and no screws loosening were noted. No developed unstable conditions occurred and no cases need revision surgery in our study. Conclusion The preliminary results of Dynesys for lumbar degenerative stenosis with/without HIVD in our series are similar to the published results of the previous literature. It can significantly improve the clinic symptoms and signs. And the Dynesys device is a semirigid fixation system for the stabilization of lumbar degenerative disease. We think the role of Dynesys is a device to delay the nature decade of lumbar spine and prevent the possibility of idiopathic unstable after decompression surgery. So, the indication of the Dynesys should seriously restriction of the age. It should be used in the patients with no or mild unstable condition, and it is better in the younger patients. But the true effect should depend on longer follow-up.

In Vitro Comparison of Dynesys, PEEK, and Titanium Constructs in the Lumbar Spine.

Introduction. Pedicle based posterior dynamic stabilization systems aim to stabilize the pathologic spine while also allowing sufficient motion to mitigate adjacent level effects. Two flexible constructs that have been proposed to act in such a manner, the Dynesys Dynamic Stabilization System and PEEK rod, have yet to be directly compared in vitro to a rigid Titanium rod. Methods. Human lumbar specimens were tested in flexion extension, lateral bending, and axial torsion to evaluate the following conditions at L4-L5: Intact, Dynesys, PEEK rod, Titanium rod, and Destabilized. Intervertebral range of motion, interpedicular travel, and interpedicular displacement metrics were evaluated from 3rd-cycle data using an optoelectric tracking system. Results. Statistically significant decreases in ROM compared to Intact and Destabilized conditions were detected for the instrumented conditions during flexion extension and lateral bending. AT ROM was significantly less than Destabilized but not the Intact condition. Similar trends were found for interpedicular displacement in all modes of loading; however, interpedicular travel trends were less consistent. More importantly, no metrics under any mode of loading revealed significant differences between Dynesys, PEEK, and Titanium. Conclusion. The results of this study support previous findings that Dynesys and PEEK constructs behave similarly to a Titanium rod in vitro.

Dynesys dynamic stabilization system for the lumbar degenerative disease: a preliminary report from China

Background Dynesys dynamic stabilization system was first implanted in patients in 1994, and introduced to China in 2007. Therefore, it was a new technique for Chinese orthopedics and hence necessary to collect clinical data about Dynesys in China. The objective of this study was to report the preliminary results of Dynesys for the lumbar degenerative disease in China. Methods Twenty-seven patients were treated with the Dynesys between July 2007 and January 2009. The diagnosis included degenerative spondylolisthesis (12 cases), degenerative spinal stenosis (nine cases), and lumbar intervertebral disc herniation (six cases). Back pain and leg pain were evaluated using 100-mm visual analog scales (VAS). The Oswestry Disability Index (ODI) was used to evaluate the patients' function. The intervertebral disc height and range of motion at the operative level were taken on radiographs. Results All the patients were followed-up, with an average of (22.40±4.23) months (range 15-32 months). VAS of back pain and leg pain were improved significantly (P <0.05) at follow-up. The ODI scores were reduced from (62.58±12.01)% preoperatively to (15.01±5.71)% at follow-up (P <0.05). The preoperative mean height of the intervertebral disc was (11.21±1.58) mm (range 8.5-13.8 mm) and mean was (10.10±1.78) mm (range 7.0-13.4 mm) at follow-up (P <0.05). The mean range of motion of the implanted segment was (6.00±1.79)° (range 2.5-9.3°) preoperatively and (5.47±1.27)° (range 2.9-7.8°) at follow-up (P=0.11). Conclusions The preliminary results of Dynesys for the lumbar degenerative disease in China are similar to the published results of other countries. It can significantly improve the clinic symptoms and preserved motion at the level of implantation. However, the long-term follow-up data need to be collected.

Intervertebral Disc Rehydration after Lumbar Dynamic Stabilization: Magnetic Resonance Image Evaluation with a Mean Followup of Four Years

Objective. To compare the clinical and radiographic outcomes in patients of different ages who underwent the Dynesys stabilization. Methods. This retrospective study included 72 patients (mean age 61.4 years) with one- or two-level lumbar spinal stenosis who underwent laminectomy and the Dynesys (Zimmer Spine, Minneapolis) dynamic stabilization system. Thirty-seven patients were younger than 65-year old while the other 35 were older. Mean followup was 46.7 months. Pre- and postoperative radiographic and clinical evaluations were analyzed. Results. The mean calibrated disc signal (CDS) at the index level was significantly improved from 60.2 ± 25.2 preoperatively to 66.9 ± 26.0 postoperatively (P > 0.001). Screw loosening occurred in 22.2% of patients and 5.1% of screws. The improvement in CDS at index level was seen to be significant in younger patients but not in older patients. Overall, the mean visual analogue scale (VAS) of back pain, VAS of leg pain, and the Oswestry disability index (ODI) scores improved significantly after operation. There were no significant differences in pre- and postoperative VAS and ODI and screw loosening rates between the younger and older patients. Conclusions. There is significant clinical improvement after laminectomy and dynamic stabilization for symptomatic lumbar spinal stenosis. Intervertebral disc rehydration was seen in younger patients.

Effect of Spacer Diameter of the Dynesys Dynamic Stabilization System on the Biomechanics of the Lumbar Spine

A finite element analysis to simulate the behavior of lumbar spines implanted with a posterior dynamic neutralization system, Dynesys, under displacement-controlled loading. To investigate whether Dynesys spacers with different diameters would alter the distribution of range of motion, disk stress, and facet contact force at the Dynesys bridging level and the cranial adjacent level. The Dynesys system is designed to preserve intersegmental motion and reduce loading at adjacent levels, but clinical reports do not support these claims. This system has been shown to be almost as stiff as rigid fixation, which acts to hinder intersegmental motion. Few studies have investigated methods of reducing this stiffness. In the finite element study, a previously validated lumbar spine model was used. Five Dynesys constructs with different spacer diameters (0.8, 0.9, 1.0, 1.1, and 1.2 times the original standard size) were implanted into the spine model and bore 4 displacement-controlled loading cases: flexion, extension, torsion, and lateral bending. Resultant range of motions (ROMs), disk stress, and facet contact forces at the bridged level and the cranial adjacent level were compared with the results of a spine model without Dynesys implantation. The results of ROMs, disk stress, and facet contact forces at the bridged levels were all less than those in the intact spine, except for contact forces at the left facet under lateral bending, facet contact forces at the right facet under torsion, and disk stress under torsion. The results of ROMs, disk stress, and facet contact forces at the cranial adjacent levels were all higher than those in the intact spine. The results of the present study show that changing the diameter of the spacers will alter the stiffness of the Dynesys construct. Dynesys constructs with larger diameters behave stiffer under flexion but behave softer under extension, torsion, and lateral bending. Changing the diameter of the Dynesys spacers does not significantly influence the load distribution at adjacent levels.

Effect of Graded Facetectomy on Biomechanics of Dynesys Dynamic Stabilization System

Finite element (FE) method was used to compare the biomechanics of L3-S1 lumbar spine with graded facetectomy before and after placement of Dynesys. To evaluate the biomechanics of Dynesys as a function of graded bilateral facetectomies. Spinal fusion or posterior dynamic stabilization systems are used to restore stability after facetectomies. The intact FE spine was modified to simulate decompression at L4-L5 with 50% and 75% and total facetectomy with/without dynamic stabilization with Dynesys. Biomechanics of the implanted level was investigated under different physiological loadings. Total facetectomy increased the motion in extension (8.7° vs. 2.7° for intact) and axial rotation (8.4° vs. 2.4° for intact). However the decrease in motion in the Dynesys model ranged from 65% in axial rotation to 80% in flexion for all facetectomies, except in the total facetectomy axial rotation case (motion higher than intact). The center of rotation of dynamic stabilized segment moved inferior/posterior in partial facetectomy and superior/posterior in total facetectomy with respect to the intact and destabilized cases. The Dynesys screws observed peak stresses up to 28% higher than those of a rigid fixation system in certain loadings, such as lateral bending and extension. The critical loosening torque applied to the screws in total facetectomy case was 6 times the partial facetectomy case in axial rotation. Partial facetectomy had a minimal effect on range of motion on the Dynesys-implanted segment. However, in the case of total facetectomy the motion increased by almost 40% in flexion and by 200% in axial rotation. The higher stresses applied to the screws in Dynesys in specific loadings may lead to higher risk of screw failure in Dynesys than in a generic rigid fixation construct.

Polysegmentale Dynesys-Instrumentation

The advantages of using the Dynesys System are claimed to be sufficient and polysegmental dorsal stabilization of the spine while preserving a certain amount of motion. In this study 20patients with a polysegmental spinal pathology were treated with polysegmental decompression and using the Dynesys Dynamic Stabilization system. None of the patients had had spinal surgery previously. Bone density was measured in all patients preoperatively. All 20patients were followed up for an average of 11.8months postoperatively. The ability to walk was improved from 581.5m to 1640.0m after surgery. Complications observed were one leakage of the dura and one hypertensive crisis. Implant loosening was observed in one female patient, which was revised and dorso-ventral fusion was carried out. The use of the Dynesys Dynamic Stabilization system is a reasonable treatment in patients with polysegmental pathology of the spine. However, the system does not allow restoration of extensive malalignments or deformities of the spine.

In Vivo Deformation, Surface Damage, and Biostability of Retrieved Dynesys Systems

Retrospective retrieval analysis. To evaluate wear, deformation and biodegradation within retrieved polycarbonate urethane (PCU) components of Dynesys systems. The Dynesys Dynamic Stabilization System (Zimmer Spine) consists of pedicle screws (Ti alloy), polycarbonate urethane (PCU) spacers, and a polyethylene-terephthalate cord. Seventeen retrieved (mean implantation: 2.5 years, range: 0.7-7.0 years) and 2 exemplar implant systems were available. Reasons for revision were persistent pain (16/17), infection (1/17), and/or screw loosening (11/17), with 1/17 case of implant migration. Optical microscopy, microCT, and scanning electron microscopy were conducted to evaluate PCU spacer wear and deformation. Attenuated total reflectance Fourier transform infrared spectroscopy was used to assess spacer surface chemical composition. Retrieved spacer components exhibited permanent bending deformation (mean: 4.3°, range: 0.0°-15.8°). We observed evidence of PCU spacer contact with pedicle screws, cords, and surrounding bony structures (74/75, 69/75, and 51/75 spacers, respectively). Relatively infrequent damage modes included PCU fracture (1/75 spacers) or cracking (2/75 spacers), as well as pedicle screw fracture (3/103 screws). PCU degradation products were identified in 10/75 spacers, which represented retrievals having significantly longer implantation times (mean: 4.3 years, range: 1.0-7.0 years). Of these spacers, 8/10 had degradation peaks identified along the side of the spacer where the material would have been in contact with bodily fluid. PCU spacers from retrieved Dynesys systems exhibited permanent deformation, focal regions of in vivo wear and surface damage. Chemical changes associated with PCU biodegradation were associated with longer-term retrievals. The most frequently observed complication was pedicle screw loosening, with 3 incidences of screw breakage in 2 patients. These retrieval data provide a crucial basis for developing in vitro tests to simulate in vivo damage and degradation of posterior dynamic motion preservation implants. Longer-term retrievals, as well as retrievals that include more recent design features (e.g., HA coating), will be useful to provide a greater context for the clinical implications of our short-term observations.

Influence of Dynesys System Screw Profile on Adjacent Segment and Screw

Displacement-controlled finite element analysis was used to evaluate the mechanical behavior of the lumbar spine after insertion of the Dynesys dynamic stabilization system. This study aimed to investigate whether different depths of screw placement of Dynesys would affect load sharing of screw, range of motion (ROM), annulus stress, and facet contact force. In clinical follow-up, a high rate of screw complications and adjacent segment disease were found after using Dynesys. The pedicle screw in the Dynesys system is not so easy to implant into the standard position and causes the screw to protrude more prominently from the pedicle. Little is known about how the biomechanical effects are influenced by the Dynesys screw profile. The Dynesys was implanted in a 3-dimensional, nonlinear, finite element model of the L1 to L5 lumbar spine. Different depths of screw position were modified in this model by 5 and 10 mm out of the pedicle. The model was loaded to 150 N preload and controlled the same ROMs by 20, 15, 8, and 20 degrees in flexion, extension, torsion, and lateral bending, respectively. Resultant ROM, annulus stress, and facet contact force were analyzed at the surgical and adjacent level. Under flexion, extension, and lateral bending, the Dynesys provided sufficient stability at the surgical level, but increased the ROM at the adjacent level. Under flexion and lateral bending, the Dynesys alleviated annulus stress at the surgical level, but increased annulus stress at the adjacent level. Under extension, the Dynesys decreased facet loading at the surgical level but increased facet loading at the adjacent level. This study found that the Dynesys system was able to restore spinal stability and alleviate loading on disc and facet at the surgical level, but greater ROM, annulus stress, and facet loading were found at the adjacent level. In addition, profile of the screw placement caused only a minor influence on the ROM, annulus stress, and facet loading, but the screw stress was noticeably increased.

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.

Preclinical evaluation of the Dynesys posterior spinal stabilization system: a nonhuman primate model

Background context Posterior dynamic spinal stabilization systems are intended to restore near-normal biomechanical function of the spine without inducing unnatural stresses to the spinal elements or eliciting a histopathological response. These devices must resist loosening within the challenging biomechanical environment of the lumbar spine. Purpose To determine the biomechanical effects of the Dynesys dynamic stabilization system (Zimmer, Inc., Warsaw, IN, USA) in the acute postoperative period and after 6 and 12 months in vivo; to examine the facet joints at the same postoperative intervals for signs of degeneration; and to measure the incidence of screw loosening after in vivo loading. Study design/setting This was an in vitro and in vivo animal survival study. Methods Fourteen baboons were used. Eight animals underwent survival surgery to implant a posterior dynamic stabilization system spanning two lumbar levels. Six animals were sacrificed acutely, and their spines were biomechanically tested in the intact condition and with instrumentation implanted as described above. Six animals in the survival group were sacrificed at 6 months postoperatively and two animals at 12 months postoperatively. Their spines were biomechanically tested with instrumentation in situ and explanted. The facets were then processed using undecalcified technique. Microradiographs of the facets were examined for signs of arthrosis, inflammation, and degenerative changes. Results The range of flexion-extension motion for the acute group of instrumented spines was 27% of the intact condition. After 6 months with instrumentation in situ, flexion-extension was 56% of the intact condition. After 12 months with instrumentation in situ, flexion-extension was 70% of the intact condition. With instrumentation explanted, flexion-extension at 6 and 12 months was not different from the intact condition (p>.05). Similar results were observed for lateral bending. There were no significant differences in axial rotation between any groups at any time point (p>.05). The facet joints at the operative and adjacent levels exhibited normal articular cartilage at both the 6- and 12-month postoperative time points. There was no evidence of facet arthrosis in any animal. At 6 months postoperatively, 0 of 36 screws exhibited radiolucency at the bone-metal interface. At 12 months postoperatively, 3 of 12 screws exhibited radiolucency. Conclusions After 12 months in vivo, spinal motions were stabilized by the dynamic instrumentation system. No facet arthrosis was observed at 6 and 12 months postoperatively. Explantation of the instrumentation restored motion to intact levels. A 25% rate of screw loosening (3 of 12 screws) was observed at the 12-month postoperative time point.

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.

Dynamic simulation of universal spacer in Dynesys dynamic stabilization system for human vertebra

The aim of this study is to analyze the simulated behavior of universal spacer in Dynesys dynamic stabilization system inserted in human vertebra. Dynesys, so-called “Dynamic neutralization system for the spine”, dynamic stabilization system is a new concept in the surgical treatment of lower back pain recently. Universal spacer used as flexible material is to stabilize the spine and the material property of universal spacer is polycarbonate urethane. Universal spacer may apply different kinematic behaviors at implanted level in vertebra. Spinal range of motion(SROM) of inter-vertebra with installed Dynesys dynamic stabilization system was studied using Adams+LifeMOD as simulation software package. The vertebra model was set up to closely resemble the in-vivo conditions. Inter-vertebra rotations were measured by post processor of Adams and compared with the intact values. SROMs of the flexion, extension, lateral bending, and axial rotation of human virtual models were measured, where three spinal fixation systems such as rigid system, Dynesys system, and fused system were installed. As a result, the value of SROM is decreased in flexion-extension and lateral bending when the spinal fixation system is implanted. The movement of Dynesys system is similar to that of intact model by allowing the movement of lumbar. This means that the Dynesys system is proved to be safe and effective in the treatment of unstable spinal condition.

Disc Changes in the Bridged and Adjacent Segments After Dynesys Dynamic Stabilization System After Two Years

Prospective case series. To study the radiologic changes in the intervertebral disc after Dynesys dynamic stabilization. Adjacent segment disc degeneration is one of the potential complications of fusion surgery. It has been proposed that nonfusion motion preservation surgery may prevent accelerated adjacent segment degeneration because of the protective effect of persisting segmental motion. Thirty-two patients who underwent Dynesys procedure between November 2002 and June 2004 and have completed 2-year follow-up MRI scans were included in this study. Preoperative and 2 year postoperative lumbar MRI scans were evaluated by 2 independent observers. T2-weighted mid-sagittal images were used and disc degeneration classified according to the Woodend classification of disc degeneration. Anterior and posterior intervertebral disc heights were also measured. Of the 32 patients, 20 patients underwent Dynesys procedure alone and 12 underwent additional fusion at 1 or more levels. A total of 70 levels were operated on, of which 13 levels were fused.There was a statistically significant increase in the mean Woodend score at the operated levels in the Dynesys alone group, a change from 1.95 before surgery to 2.52 after surgery (P < 0.001). The mean Woodend scores changed from 1.27 preoperative to 1.55 postoperative (P = 0.066) at the proximal adjacent levels, and from 1.37 to 1.62 at the distal levels (P = 0.157). There was good interobserver agreement (weighted kappa score of 0.819). The anterior intervertebral disc height reduced by 2 mm from 9.25 to 7.17 (P < 0.001). The posterior disc height increased by 0.14 mm but this change was not significant. Disc degeneration at the bridged and adjacent segment seems to continue despite Dynesys dynamic stabilization. This continuing degeneration could be due to natural disease progression.

11. Twenty-four Month Results from a Prospective Randomized Controlled IDE Study of the DYNESYS Dynamic Stabilization System

11. Twenty-four Month Results from a Prospective Randomized Controlled IDE Study of the DYNESYS Dynamic Stabilization System

The Dynesys Lumbar Spinal Stabilization System

We present the positional magnetic resonance imaging findings of a prospective case series of patients undergoing surgery with the Dynesys spinal stabilization device (Zimmer, Inc., Warsaw, IN). To explore the biomechanical impact of the Dynesys device in vivo. Spinal fusion surgery is widely used for painful degenerative conditions of the lumbar spine that have not responded to conservative measures. It often produces good outcomes but can be associated with adjacent segment hypermobility that may lead to further degeneration and pain. Previous cadaveric biomechanical studies claim that the Dynesys Dynamic Spinal Stabilization System allows some lumbar movement, behaving similar to a normal spine in extension but similar to rigid fixation in flexion. Twenty-four patients with dominant low back pain, with or without leg pain, were treated with the Dynesys. All patients underwent positional magnetic resonance imaging before surgery and 9 months after surgery. Measurements were made to assess the differences at the operated level, adjacent level, and whole lumbar spine. There was a statistically significant reduction in flexion-extension range of movement of both the whole lumbar spine by 13.37 degrees (P = 0.002) and at the instrumented segments by 4.08 degrees (P < 0.001) following surgery. There was an insignificant reduction in range of movement at the level above instrumentation (P = 0.807). Mean anterior disc height at the instrumented level reduced by 0.7 mm following insertion of the Dynesys (P < 0.027). Mean posterior disc height reduced by 0.3 mm (P = 0.453). In a neutral posture, the Dynesys had no significant impact on lordosis or inclination of operated or adjacent levels. Contrary to cadaveric study findings, the Dynesys appears to restrict extension more than flexion with respect to a neutral posture. In vivo, the Dynesys Stabilization System allows movement at the instrumented level, albeit reduced, with no significant increased mobility at the adjacent segments. There was reduction of the anterior disc height without a significant increase of the posterior disc height.

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.