Posterior Dynamic Stabilization Devices Research Articles

Biomechanical analysis of lumbar nonfusion dynamic stabilization using a pedicle screw-based dynamic stabilizer or an interspinous process spacer.

This study aimed to investigate and compare the effects of two widely used nonfusion posterior dynamic stabilization (NPDS) devices, pedicle screw-based dynamic stabilizer (PSDS) and interspinous process spacer (IPS), on biomechanics of the implanted lumbar spine under static and vibration loadings. The finite element model of healthy human lumbosacral segment was modified to incorporate NPDS device insertion at L4-L5 segment. Bioflex and DIAM were used as PSDS-based and IPS-based NPDS devices, respectively. As a comparison, lumbar interbody fusion with rigid stabilization was also simulated at L4-L5. For static loading, segmental range of motion (ROM) of the models under moments of four physiological motions was computed using hybrid testing protocol. For vibration loading, resonant modes and dynamic stress of the models under vertical excitation were extracted through random response analysis. The results showed that compared with the rigid fusion model, ROM of the nonfusion models was higher at L4-L5 level but lower at adjacent levels (L1- L2, L2-L3, L3-L4, L5-S1). Compared with the Bioflex model, the DIAM model produced higher ROM at L4-L5 level but lower ROM at adjacent levels, especially under lateral bending and axial rotation; resonant frequency of the DIAM model was slightly lower; dynamic response of nucleus stress at L4-L5 level was slightly higher for the DIAM model, and the dynamic stress at adjacent levels was no obvious difference between the nonfusion models. This study reveals biomechanical differences between the Bioflex and DIAM systems, which may provide references for selecting surgical approaches in clinical practice.

Preliminary Results of a Prospective Clinical and Radiological Study with Roentgen Stereophotogrammetric Analysis (RSA) after Implantation of a Posterior Dynamic Stabilization Device in the Lumbar Spine

Introduction: Posterior dynamic stabilization (PDS) can be based on interspinous distraction devices (IDD). The goals of these implants are maintaining or restoring intervertebral range of motion (ROM) in a controlled fashion and avoiding a complete restriction of mobility. Clinical and radiological data with the Wallis® spacer as one type of IDD have been rarely reported. The goal of this study was to present clinical and radiological data including roentgen stereophotogrammetric analysis (RSA) after a short- to mid-term follow-up period.
 Patients and Methods: 10 patients were included in this prospective monocentric study and had PDS of the lumbar spine with an IDD (Wallis® spacer). Before and soon after operation and 3, 6, and 12 months later clinical and radiological evaluations were performed. Pain and disability were analyzed by use of visual analog scale for back and leg pain, Oswestry Disability Index, Roland-Morris Disability Questionnaire and Short-Form-36 Health Survey. The ROM of the operated levels and the total lumbar spine was determined by use of lateral functional x-ray images with calculation of the differences of the segmental and total lumbar spine angles in flexion and extension. Furthermore, RSA was used to measure the segmental ROM.
 Results: After a follow-up of 12 months, the results of the pain intensity and the disability and health related quality of life scores showed statistical significant improvement. The segmental angles of the operated levels demonstrated statistical significant reduction in ROM during the different follow-up examinations. The discrepancy of the conventionally determined segmental angles and the data measured by RSA were low with a mean of 1.77°. The mean total lumbar spine angles did not change statistically significantly during the postoperative controls.
 Conclusions: According to the radiological results of this study, the used implant leads to a posterior dynamic stabilization. The clinical findings are promising, but they are to be interpreted with caution because of the small number of patients and the lack of a control group.

Current concepts of spondylosis and posterior spinal motion preservation for radiologists.

Spinal fusion is performed to eliminate motion at a degenerated or unstable segment. However, this is associated with loss of motion at the fused levels and increased stress on adjacent levels. Motion-preserving implants have been designed in effort to mitigate the limitations of fusion. This review will focus on posterior spinal motion-preserving technologies. In the cervical spine, laminoplasty is a posterior motion-preserving procedure used in the management of myelopathy/cord compression. In the lumbar spine, motion-sparing systems include interspinous process devices (also referred to as interspinous process spacers or distraction devices), posterior dynamic stabilization devices (also referred to as pedicle screw/rod fixation-based systems), and posterior element replacement systems (also referred to as total facet replacement devices). Knowledge of the intended physiologic purpose, hardware utilized, and complications is important in the assessment of imaging in those who have undergone posterior motion preservation procedures.

In vitro oxidative degradation of a spinal posterior dynamic stabilization device.

This study quantified the changes of the frequency-dependant viscoelastic properties of the BDyn (S14 Implants, Pessac, France) spinal posterior dynamic stabilization (PDS) device due to in vitro oxidation. Six polycarbonate urethane (PCU) rings and six silicone cushions were degraded using a 20% hydrogen peroxide/0.1 M cobalt (II) chloride hexahydrate, at 37°C, for 24 days. The viscoelastic properties of the individual components and the components assembled into the BDyn PDS device were determined using Dynamic Mechanical Analysis at frequencies from 0.01 to 30 Hz. Attenuated Total Reflectance Fourier Transform Infra-Red spectra demonstrated chemical structure changes, of the PCU, associated with oxidation while Scanning Electron Microscope images revealed surface pitting. No chemical structure or surface morphology changes were observed for the silicone cushion. The BDyn device storage and loss stiffness ranged between 84.46 N/mm to 99.36 N/mm and 8.13 N/mm to 21.99 N/mm, respectively. The storage and loss stiffness for the components and BDyn device increased logarithmically with respect to frequency. Viscoelastic properties, between normal and degraded components, were significantly different for specific frequencies only. This study demonstrates the importance of analyzing changes of viscoelastic properties of degraded biomaterials and medical devices into which they are incorporated, using a frequency sweep. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1237-1244, 2018.

Stability and Load Sharing Characteristics of a Posterior Dynamic Stabilization Device.

BackgroundLumbar interbody fusion is a common treatment for a variety of spinal pathologies. It has been hypothesized that insufficient mechanical loading of the interbody graft can prevent proper fusion of the joint. The purpose of this study was to evaluate the mechanical stability and anterior column loading sharing characteristics of a posterior dynamic system compared to titanium rods in an anterior lumbar interbody fusion (ALIF) model.MethodsRange of motion, interpedicular kinematics and interbody graft loading were measured in human cadaveric lumbar segments tested under a pure moment flexibility testing protocol.ResultsBoth systems provided significant fixation compared to the intact condition and to an interbody spacer alone in flexion extension and lateral bending. No significant differences in fixation were detected between the devices. A significant decrease in graft loading was detected in flexion for the titanium rod treatment compared to spacer alone. No significant differences in graft loading were detected between the spacer alone and posterior dynamic system or between the posterior dynamic system and the titanium rod.ConclusionsThe results of this study indicate that the posterior dynamic system provides similar fixation compared to that of a titanium rod, however, studies designed to evaluate the efficacy of fixation in a cadaver model may not be sufficiently powered to establish differences in load sharing using the techniques described here.

Pedicle screw‐based posterior dynamic stabilizers for degenerative spine: In vitro biomechanical testing and clinical outcomes

AbstractDynamic stabilization in a degenerate symptomatic spine may be advantageous compared with conventional fusion procedures, as it helps preserve motion and minimizes redistribution of loads at instrumented and adjacent segments. This article presents a systematic review of biomechanical and clinical evidence available on some of the pedicle screw based posterior dynamic stabilization (PDS) devices. Using Medline, Embase, and Scopus online databases, we identified four pedicle‐screw‐PDS devices for which both, biomechanical testing and clinical follow‐up data are available: Graf artificial ligaments, Isobar TTL, Polyetheretherketone rods, and Dynesys. The current state‐of‐the‐art of pedicle‐screw‐PDS devices is far from achieving its desired biomechanical efficacy, which has resulted in a weak support for the posited clinical benefits. Although pedicle‐screw‐PDS devices are useful in salvaging a moderately degenerate functionally suboptimal disc, for severe disc degeneration cases fusion is still the preferred choice. We conclude that a pedicle‐screw‐PDS device should aim at restoring load sharing amongst spinal elements while preserving the qualitative and quantitative nature of spinal motion, especially minimize posterior shift of the helical axis of motion. More precise and objective assessment techniques need to be standardized for in vivo evaluation of intervertebral motion and load sharing amongst spinal elements across different pedicle‐screw‐PDS devices. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 102A: 3324–3340, 2014.

Pedicle screw-based posterior dynamic stabilizers for degenerative spine: in vitro biomechanical testing and clinical outcomes

Dynamic stabilization in a degenerate symptomatic spine may be advantageous compared with conventional fusion procedures, as it helps preserve motion and minimizes redistribution of loads at instrumented and adjacent segments. This article presents a systematic review of biomechanical and clinical evidence available on some of the pedicle screw based posterior dynamic stabilization (PDS) devices. Using Medline, Embase, and Scopus online databases, we identified four pedicle-screw-PDS devices for which both, biomechanical testing and clinical follow-up data are available: Graf artificial ligaments, Isobar TTL, Polyetheretherketone rods, and Dynesys. The current state-of-the-art of pedicle-screw-PDS devices is far from achieving its desired biomechanical efficacy, which has resulted in a weak support for the posited clinical benefits. Although pedicle-screw-PDS devices are useful in salvaging a moderately degenerate functionally suboptimal disc, for severe disc degeneration cases fusion is still the preferred choice. We conclude that a pedicle-screw-PDS device should aim at restoring load sharing amongst spinal elements while preserving the qualitative and quantitative nature of spinal motion, especially minimize posterior shift of the helical axis of motion. More precise and objective assessment techniques need to be standardized for in vivo evaluation of intervertebral motion and load sharing amongst spinal elements across different pedicle-screw-PDS devices. © 2013 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 102A: 3324–3340, 2014.

Does Semi-Rigid Instrumentation Using Both Flexion and Extension Dampening Spacers Truly Provide an Intermediate Level of Stabilization?

Conventional posterior dynamic stabilization devices demonstrated a tendency towards highly rigid stabilization approximating that of titanium rods in flexion. In extension, they excessively offload the index segment, making the device as the sole load-bearing structure, with concerns of device failure. The goal of this study was to compare the kinematics and intradiscal pressure of monosegmental stabilization utilizing a new device that incorporates both a flexion and extension dampening spacer to that of rigid internal fixation and a conventional posterior dynamic stabilization device. The hypothesis was the new device would minimize the overloading of adjacent levels compared to rigid and conventional devices which can only bend but not stretch. The biomechanics were compared following injury in a human cadaveric lumbosacral spine under simulated physiological loading conditions. The stabilization with the new posterior dynamic stabilization device significantly reduced motion uniformly in all loading directions, but less so than rigid fixation. The evaluation of adjacent level motion and pressure showed some benefit of the new device when compared to rigid fixation. Posterior dynamic stabilization designs which both bend and stretch showed improved kinematic and load-sharing properties when compared to rigid fixation and when indirectly compared to existing conventional devices without a bumper.

Lumbar Spinal Stenosis Minimally Invasive Treatment with Bilateral Transpedicular Facet Augmentation System

The purpose of this study was to evaluate the effectiveness of a new pedicle screw-based posterior dynamic stabilization device PDS Percudyn System™ Anchor and Stabilizer (Interventional Spine Inc., Irvine, CA) as alternative minimally invasive treatment for patients with lumbar spine stenosis. Twenty-four consecutive patients (8 women, 16 men; mean age 61.8 yr) with lumbar spinal stenosis underwent implantation of the minimally invasive pedicle screw-based device for posterior dynamic stabilization. Inclusion criteria were lumbar stenosis without signs of instability, resistant to conservative treatment, and eligible to traditional surgical posterior decompression. Twenty patients (83 %) progressively improved during the 1-year follow-up. Four (17 %) patients did not show any improvement and opted for surgical posterior decompression. For both responder and nonresponder patients, no device-related complications were reported. Minimally invasive PDS Percudyn System™ has effectively improved the clinical setting of 83 % of highly selected patients treated, delaying the need for traditional surgical therapy.

Interpedicular Travel in the Evaluation of Spinal Implants

In vitro flexibility testing of the lumbar spine. The goal of this study was to evaluate a motion-preserving posterior dynamic stabilization (PDS) implant based on newly defined parameters describing interpedicular kinematics. PDS implants have been designed as either motion-preserving or adjunct-to-fusion devices to treat various degenerative spinal pathologies. The ambiguity of design and evaluation goals and the inability of traditional biomechanical parameters to appropriately describe the behavior of PDS devices in vitro have served as the impetus to develop kinematic parameters more specific to this class of device. Flexibility testing of 6 fresh-frozen human lumbar spines was conducted before and after destabilization of the index level (L4-L5). Testing under the same protocol was repeated after treatment at the index level with a 1-level PDS device, extension of the device to the adjacent inferior level (L5-S1), and treatment with a hybrid construct consisting of the PDS implant at L4-L5 and rigid fixation at L5-S1. The kinematic response was recorded using an optoelectric tracking system and reported in terms of intervertebral range of motion (ROM) and newly developed parameters describing interpedicular motion. Based on ROM and interpedicular kinematics, the devices implanted at L4-L5 provide significant but not differing stabilization in flexion-extension with implantation after a significant destabilization procedure. Interpedicular kinematic results indicate that the 2-level construct contributes to significantly more motion at L5-S1 compared with rigid fixation. This result was not detected when evaluated by the ROM metric. Those involved in the design and evaluation of PDS devices may benefit from evaluation of interpedicular kinematics. Evaluating intervertebral motion from the perspective of the pedicle screw allows for a direct and intuitive translation between in vitro test results and design parameters. Furthermore, these parameters may provide additional clinical insight into the biomechanics of the healthy and pathological spine. The study presented indicates that this approach may be more sensitive in detecting differences in implant motion between PDS devices.

Rationale of a Test Setup with a Defined COR for Extra-Discal Motion-Preserving Implants with a Low Implant Stiffness

Abstract In the current version of ASTM F2624, the center of rotation (COR) is not specified. Potentially, each device can be tested using a different COR, which subsequently makes a direct design comparison of results difficult. Four posterior dynamic stabilization (PDS) devices (Dynesys, DYN, Zimmer; DSS, Paradigm Spine; and two Aesculap implant concepts) were tested in comparison to a rigid-fixation device and to the native situation of the lumbar spine on fresh-frozen human lumbar spines (L3–L5). The instrumented level was L4–L5. The PDS systems have axial compressive stiffness values ranging from 10 N/mm to 230 N/mm and were all made compatible to connect with the pedicle-screw system. The specimens were loaded in a spinal simulator, applying pure moments for flexion/extension, lateral bending and axial rotation (+/−7.5 Nm) with a defined velocity. The COR was analyzed based on the data measured with a 3-dimensional (3D) motion-analysis system. The effect of the PDS on the location of the COR is most pronounced in the sagittal plane. In general, the higher the implant stiffness, the more the COR shifted in a posterior direction. The DYN had a similar COR to the rigid fixator. However, the PDS systems with low axial compressive stiffness values (range: 10–70 N/mm) showed very similar results on CORs, which are located in the region of the posterior border of the intervertebral disc. In the frontal and transversal plane, the COR was found to be close to the native situation for each system. Therefore, for PDS devices with low implant stiffness, the location of the COR varies only marginally and can be specified for a test setup. An initial proposal that will allow side-by-side comparison for these kinds of PDS systems is given and the feasibility of the new test setup could be proven for all three loading conditions.

Could Junctional Problems at the End of a Long Construct be Addressed by Providing a Graduated Reduction in Stiffness?

The effect of long, rigid fixation on adjacent level hypermobility was investigated in a human cadaver model with and without a transitional posterior dynamic stabilization (PDS) device placed at the last caudal level. To evaluate if PDS devices are useful in the setting of spinal deformities to restore increased adjacent level motions, which occur in long constructs. The hypothesis is that load-sharing benefits of these devices will be most suitable in long constructs and may reduce thoracolumbar junctional effects. The PDS device evaluated has a compressive spacer and flexion-dampening bumper. Mechanical factors such as excessive mobility, increased disc height due to instrumentation, and abnormal loading are thought to accentuate distal level problems, which occur in extended instrumentation. Specifically adjacent level degeneration and distal junctional kyphosis are known to occur in these cases. Seven cadaver spines were tested from T7 to L3. Long instrumentation was applied in 2 rigid groups, R1: Rigid (T8-L2) and R2: Rigid (T8-L1), and PDS to the last caudal level of each, RP1: Rigid (T8-L1) + PDS (L1-L2), and RP2: Rigid (T8-T12) + PDS (T12-L1). Range of motion was evaluated at surgical and distal adjacent levels after displacement controlled loading in a spine tester. Distal adjacent level motion was increased after 5- and 6-level rigid fixation in flexion-extension, lateral bending, and axial rotation. Most of the increases were seen in axial rotation and lateral bending. Replacing the last caudal instrumented level with the PDS test device was able to alleviate hypermobile conditions of the adjacent noninstrumented level, closer to intact (24%, 12% reduction in RP2, RP1, respectively). Reduction of hypermobility caused by extended arthrodesis may represent a new and ideally suited function for PDS devices. Mechanically, the devices were seen to kinematically restore abnormal distal motion, especially with placement of the PDS at the thoracolumbar junction.

Pedicle Screw-Based Posterior Dynamic Stabilization: Literature Review

Posterior dynamic stabilization (PDS) indicates motion preservation devices that are aimed for surgical treatment of activity related mechanical low back pain. A large number of such devices have been introduced during the last 2 decades, without biomechanical design rationale, or clinical evidence of efficacy to address back pain. Implant failure is the commonest complication, which has resulted in withdrawal of some of the PDS devices from the market. In this paper the authors presented the current understanding of clinical instability of lumbar motions segment, proposed a classification, and described the clinical experience of the pedicle screw-based posterior dynamic stabilization devices.

Can posterior dynamic stabilization reduce the risk of adjacent segment deterioration?

The aim of this study was to systematically review the relevant literature to develop a benchmark for the incidence of adjacent segment degeneration (ASDeg) and adjacent segment disease (ASDis) following the posterior dynamic stabilization (PDS) procedure and to investigate whether conclusions can be made with regard to the isolated PDS procedure in reducing the risk of ASDeg and ASDis compared with fusion, and with regard to the role of additional PDS devices implanted adjacent to fusion in protecting from ASDeg and ASDis caused by the neighboring fusion. We retrieved electronic databases of Medline, Ovid and Cochrane Central Registry of Controlled Trials, combined with a supplemental hand search. Thirty-one articles met our inclusion criteria. The pooled incidence of ASDeg and ASDis following PDS procedure was 16.4% and 5.5% respectively. Data from comparative studies showed a significantly lower incidence of ASDeg and nonsignificantly lower incidence of ASDis following PDS than following fusion surgery. Further, the additional PDS devices implanted adjacent to fusion could significantly reduce the risk of ASDeg and nonsignificantly decrease that of ASDis caused by fusion. These results suggested relative success of the PDS procedure in protecting against ASDeg and ASDis.

The Pedicle-to-Pedicle Distance Translation in Normal Lumbar Spine during Flexion-Extension and its Importance in Dynamic Stabilization

Kinematic effects of pedicle-screw based posterior dynamic stabilization (PDS) devices are closely related to devices’ fatigue failure risk and pain relief function. Conventionally, such effects are assessed mainly by Range of angular Motion (ROM). However, angular ROM was found to be inadequate to explain fatigue failure of some recent PDS devices. The reason behind is normal segmental angular motion may involve changes in the Inter-Pedicular Distance (IPD), whereas PDS device may permit angular motion without permitting changes in the IPD distances.

Comparison of Three Posterior Dynamic Stabilization Devices

A biomechanical study using human cadaveric lumbar spinal motion segments and three different posterior stabilization devices. To compare the range of motion, disc height, and foraminal area of a spinal motion segment intact, injured, and fixed with each of three posterior lumbar motion preservation devices. Motion-sparing lumbar posterior dynamic stabilization devices are gaining increasing popularity, particularly for the treatment of degenerative disc disease. The PercuDyn, the X-Stop, and the Isobar posterior stabilization devices were compared using an in vitro cadaveric model. First, pure moments of ±8 Nm were applied in all three planes, then a follower load of 700 N was applied, and finally, sagittal bending tests were repeated. All tests were conducted using an 8-df servohydraulic load frame. Experiments were performed intact, with a simulated injury, and then with each of the three devices for a total of four specimens per device. Foraminal area and disc height (posterolateral and anterior surface) were measured under neutral and peak torques in all three planes and range of motion was recorded for all experimental conditions. Overall, the injury model successfully increased range of motion and decreased disc height and foraminal area. Once treated with one of the three implants, the PercuDyn was most effective at preventing hyperextension, decreasing extension with a follower load by a mean of 52% compared to injured conditions (P = 0.07). The X-Stop stabilized the posterior column, increasing foraminal area under all conditions, particularly extension with a follower load, by 27% compared to injured conditions (P = 0.01). The Isobar, the only device to stabilize the anterior column, increased anterior disc height under flexion with a follower load by 22% (P = 0.03). All three devices functioned as intended by their respective manufacturers, but each appeared to excel in different areas; therefore, each should be used for unique clinical applications.

Intervertebral disc properties: challenges for biodevices

Intervertebral disc biodevices that employ motion-preservation strategies (e.g., nucleus replacement, total disc replacement and posterior stabilization devices) are currently in use or in development. However, their long-term performance is unknown and only a small number of randomized controlled trials have been conducted. In this article, we discuss the following biodevices: interbody cages, nuclear pulposus replacements, total disc replacements and posterior dynamic stabilization devices, as well as future biological treatments. These biodevices restore some function to the motion segment; however, contrary to expectations, the risk of adjacent-level degeneration does not appear to have been reduced. The short-term challenge is to replicate the complex biomechanical function of the motion segment (e.g., biphasic, viscoelastic behavior and nonlinearity) to improve the quality of motion and minimize adjacent level problems, while ensuring biodevice longevity for the younger, more active patient. Biological strategies for regeneration and repair of disc tissue are being developed and these offer exciting opportunities (and challenges) for the longer term. Responsible introduction and rigorous assessment of these new technologies are required. In this article, we will describe the properties of the disc, explore biodevices currently in use for the surgical treatment of low back pain (with an emphasis on lumbar total disc replacement) and discuss future directions for biological treatments. Finally, we will assess the challenges ahead for the next generation of biodevices designed to replace the disc.

The effect of design parameters of dynamic pedicle screw systems on kinematics and load bearing: an in vitro study

As an alternative treatment for chronic back pain due to disc degeneration motion preserving techniques such as posterior dynamic stabilization (PDS) has been clinically introduced, with the intention to alter the load transfer and the kinematics at the affected level to delay degeneration. However, up to the present, it remains unclear when a PDS is clinically indicated and how the ideal PDS mechanism should be designed to achieve this goal. Therefore, the objective of this study was to compare different PDS devices against rigid fixation to investigate the biomechanical impact of PDS design on stabilization and load transfer in the treated and adjacent cranial segment. Six human lumbar spine specimens (L3-L5) were tested in a spine loading apparatus. In vitro flexibility testing was performed by applying pure bending moments of 7.5 Nm without and with additional preload of 400 N in the three principal motion planes. Four PDS devices, "DYN" (Dynesys(®), Zimmer GmbH, Switzerland), "DSS™" (Paradigm Spine, Wurmlingen, Germany), and two prototypes of dynamic rods, "LSC" with a leaf spring, and "STC" with a spring tube (Aesculap AG, Tuttlingen, Germany), were tested in comparison to a rigid fixation device S(4) (Aesculap AG, Tuttlingen, Germany) "RIG", to the native situation "NAT" and to a defect situation "DEF" of the specimens. The instrumented level was L4-L5. The tested PDS devices comprising a stiffness range for axial stiffness of 10 N/mm to 230 N/mm and for bending stiffness of 3 N/mm to 15 N/mm. Range of motion (ROM), neutral zone (NZ), and intradiscal pressure (IDP) were analyzed for all instrumentation steps and load cases of the instrumented and non-instrumented level. In flexion, extension, and lateral bending, all systems, except STC, showed a significant reduction of ROM and NZ compared to the native situation (p < 0.05). Furthermore, we found no significant difference between DYN and RIG (p > 0.1). In axial rotation, only DSS and STC reduced the ROM significantly (p < 0.005) compared to the native situation, whereas DYN and LSC stayed at the level of the native intersegmental rotation (p > 0.05). A correlation was found between axial stiffness and intersegmental stabilization in the sagittal and frontal plane, but not in the transversal plane where intersegmental stabilization is mainly governed by the systems' ability to withstand shear loads. Furthermore, we observed the systems' capacity to reduce IDP in the treated segment. The adjacent segment does not seem to be affected by the stiffness of the fixation device under the described loading conditions.

Retrieval analysis of motion preserving spinal devices and periprosthetic tissues

This article reviews certain practical aspects of retrieval analysis for motion preserving spinal implants and periprosthetic tissues as an essential component of the overall revision strategy for these implants. At our institution, we established an international repository for motion-preserving spine implants in 2004. Our repository is currently open to all spine surgeons, and is intended to be inclusive of all cervical and lumbar implant designs such as artificial discs and posterior dynamic stabilization devices. Although a wide range of alternative materials is being investigated for nonfusion spine implants, many of the examples in this review are drawn from our existing repository of metal-on-polyethylene, metal-on-metal lumbar total disc replacements (TDRs), and polyurethane-based dynamic motion preservation devices. These devices are already approved or nearing approval for use in the United States, and hence are the most clinically relevant at the present time. This article summarizes the current literature on the retrieval analysis of these implants and concludes with recommendations for the development of new test methods that are based on the current state of knowledge of in vivo wear and damage mechanisms. Furthermore, the relevance and need to evaluate the surrounding tissue to obtain a complete understanding of the biological reaction to implant component corrosion and wear is reviewed.

Biomechanical Evaluation of Pedicle Screw-Based Dynamic Stabilization Devices for the Lumbar Spine: A Systematic Review

Study Design This study is a systematic review of published biomechanical studies involving pedicle screw-based posterior dynamic stabilization devices (PDS) with a special focus on kinematics and load transmission through the functional spine unit (FSU). Methods A literature search was performed via the PubMed online database from 1990 to 2008 using the following key words: “biomechanics,” “lumbar dynamic stabilization,” “Graf system,” “Dynesys,” and “posterior dynamic implant.” Citations were limited to papers describing biomechanics of pedicle screw-based PDS devices currently available for clinical use. Studies describing clinical experience, radiology, and in vivo testing were excluded from the review. Parameters measured included kinematics of the FSU (range of motion (ROM), neutral zone (NZ), and location of the center of rotation) and load transmission through the disk, facets, and instrumentation. Results A total of 27 publications were found that concerned the biomechanical evaluation of lumbar pedicle screw-based dynamic stabilization instrumentation. Nine in vitro experimental studies and 4 finite element analyses satisfied the inclusion criteria. The Dynesys implant was the most investigated pedicle screw-based PDS system. In vitro cadaveric studies mainly focused on kinematics comparing ROM of intact versus instrumented spines whereas finite element analyses allowed analysis of load transmission at the instrumented and adjacent levels. Conclusion Biomechanical studies demonstrate that pedicle screw-based PDS devices limit intervertebral motion while unloading the intervertebral disk. The implant design and the surgical technique have a significant impact on the biomechanical behavior of the instrumented spinal segment. The posterior placement of such devices results in non-physiologic intervertebral kinematics with a posterior shift of the axis of rotation. Biomechanical studies suggest that the difference at the adjacent level between investigated dynamic devices and rigid stabilization systems may not be as high as reported. Finally, additional investigations of semirigid devices are needed to further evaluate their biomechanical properties compared to soft stabilization PDS systems.