Posterior Stabilization Devices Research Articles

Toward the definition of a new worst-case paradigm for the preclinical evaluation of posterior spine stabilization devices.

Mechanical reliability tests on posterior spine stabilization devices are based on standard F1717 by the American Society for Testing and Materials, which describes how to assemble the implant with vertebrae-like test blocks in a corpectomy model. A recent study proposed to revise the standard to describe the anatomical worst-case scenario, instead of the average one currently implemented, and introduce the unsupported screw length as a mechanical parameter. This article investigates the implications of such revisions on the endurance properties of an implant already on the market. Experimental fatigue tests demonstrate that the revision of F1717 standard leads to a reduction of 3.2 million cycles in the fatigue strength of the tested implant: this amount is comparable to the run-out number of cycles (5 million cycles) currently recommended. The numerical analysis, validated with static tests and strain gauges, supports the experimental findings and demonstrates that the stress on the implant may increase upon revision up to a 50% on the screw (most recurrent failure mode), with the unsupported screw length contributing alone up to 40%. The revision of ASTM F1717 standard would guarantee higher safety for the implant to test, potentially covering for a wider population of patients.

In vitro measurement of load-sharing in spinal implants

Efficient load-sharing in the spinal column relies on the proper working of the components of the Functional Spinal Unit (FSU). Due to various reasons such as trauma, ageing and diseases, these components or even the entire FSU can get degenerated or injured during a person’s lifetime. Spinal column reconstruction surgerieswere created with the aimto restore the functioning of the diseased or injured spine. Several spinal implants are available today for the surgeon to aid in this process. There are three major categories of these devices: anterior stabilization devices, posterior stabilization devices and motion preservation devices. This review highlights thein vitro researchdone on these devices,over the past five decades, to evaluate their ability to effectively share loads at the operated level of the spinal column. Some conclusions have been drawn based on this research.Dynamic anterior cervical plates are more successful at maintaining load-sharing after graft subsidence and anterior stabilization devices can be used to provide support to posterior stabilization devices during severe anterior column injuries. Motion preservation devices, specifically cervical disc prostheses and facet replacement systems,show great potential in maintaining physiologicalloads in the spinal column. Further clinical investigation of all these implants would help to identify the contributing factors for their success or failure post-surgery. It would also aid in determining the requirements of an ideal spine stabilization device which perfectly mimics the physiological load-sharing properties of the FSU and its components.

ISO 12189 standard for the preclinical evaluation of posterior spinal stabilization devices--I: Assembly procedure and validation.

The International Standardization Organization introduced standard 12189 for the preclinical evaluation of the mechanical reliability of posterior stabilization devices. The well-known vertebrectomy model formalized in standard F1717 by the American Society for Testing and Materials was modified with the introduction of a modular anterior support made up of three calibrated springs, which allows to describe a more realistic scenario, closer to the effective clinical use, as well to test even very flexible and dynamic posterior stabilization implants. Despite these important improvements, ISO 12189 received very little attention in the literature. The aim of the work is to provide a systematic procedure for the assembly and validation of a finite element model capable of describing the experimental test according to ISO 12189. The validated finite element model is able to catch very well the effective stiffness of the unassembled and assembled constructs (percentage differences <2% and <10%, respectively). As concern the assembled construct, the experimentally measured and predicted strains were found in a good agreement (R2 > 0.75, root mean square error < 30%), but the procedure without precompression lead to much better results (R2 > 0.96, root mean square error < 10%). Given the prediction errors of the assembled construct fall within the experimental range of repeatability, the finite element model can be systematically implemented to support the mechanical design of a variety of spinal implants, to quantitatively investigate the load-sharing mechanism, as well as to investigate the loading conditions set by ISO 12189 standard.

Minimally-invasive posterior lumbar stabilization for degenerative low back pain and sciatica. A review

The most diffused surgical techniques for stabilization of the painful degenerated and instable lumbar spine, represented by transpedicular screws and rods instrumentation with or without interbody cages or disk replacements, require widely open and/or difficult and poorly anatomical accesses. However, such surgical techniques and approaches, although still considered “standard of care”, are burdened by high costs, long recovery times and several potential complications. Hence the effort to open new minimally-invasive surgical approaches to eliminate painful abnormal motion. The surgical and radiological communities are exploring, since more than a decade, alternative, minimally-invasive or even percutaneous techniques to fuse and lock an instable lumbar segment. Another promising line of research is represented by the so-called dynamic stabilization (non-fusion or motion preservation back surgery), which aims to provide stabilization to the lumbar spinal units (SUs), while maintaining their mobility and function. Risk of potential complications of traditional fusion methods (infection, CSF leaks, harvest site pain, instrumentation failure) are reduced, particularly transitional disease (i.e., the biomechanical stresses imposed on the adjacent segments, resulting in delayed degenerative changes in adjacent facet joints and discs). Dynamic stabilization modifies the distribution of loads within the SU, moving them away from sensitive (painful) areas of the SU. Basic biomechanics of the SU will be discussed, to clarify the mode of action of the different posterior stabilization devices. Most devices are minimally invasive or percutaneous, thus accessible to radiologists’ interventional practice. Devices will be described, together with indications for patient selection, surgical approaches and possible complications.

Biomechanical Testing of a Lumbar Motion Normalization Device

Introduction Two specific scenarios that may benefit from dynamic lumbar stabilization are single-level moderate instability, where the stabilizing tissues are relatively incompetent, and juxta-level to fusion, where the last instrumented level requires intermediate stiffness (“topping off”) to prevent transfer of high stresses from the stiffer fusion construct to the intact adjacent levels. Both scenarios were evaluated in vitro to determine the biomechanical response of a new pedicle screw-based dynamic stabilizer. Materials and Methods Seven human cadaveric L2-S1 segments were tested (1) intact, (2) after moderate destabilization, (3) after 2-level hybrid posterior fixation, consisting of bilateral dynamic pedicle screws at L4 interconnected with rigid rods to standard pedicle screws at L5 and S1, (4) after 2-level rigid fixation, (5) after 1-level (L4-L5) dynamic fixation, and (6) after 1-level rigid fixation. In each condition, flexion, extension, lateral bending, and axial rotation were induced with pure moments of 7.5 Nm, while angular motion was tracked optoelectronically. Range of motion (ROM) and sagittal instantaneous axis of rotation (IAR) were calculated. Results In 1-level constructs, dynamic hardware allowed 104% of intact ROM, whereas rigid hardware allowed 49% of intact ROM. Relative to the intact IAR location at L4-L5, the IAR was shifted significantly farther posterior by rigid 1-level instrumentation than dynamic 1-level instrumentation. In 2-level constructs, the dynamic level (L4-L5) allowed significantly greater ROM than the rigid level (L5-S1) in all directions but allowed significantly less ROM than the intact level (L3-L4) in all directions except axial rotation. Conclusion Dynamic instrumentation shifted the IAR less than rigid instrumentation, providing more favorable kinematics. This dynamic stabilizer provided 1-level ROM that was close to intact ROM during all loading modes in vitro. In the “topping off” construct, the dynamic segment allowed intermediate ROM to give balanced transitional flexibility. Disclosure of Interest J. Zucherman: Conflict with Spartek Medical, Inc. K. Hsu: Conflict with Spartek Medical, Inc. N. Crawford: Conflict with Spartek Medical, Inc. L. Perez-Orribo: None declared P. Reyes: None declared N. Rodriguez-Martinez: None declared References Bono CM, Kadaba M, Vaccaro AR. Posterior pedicle fixation-based dynamic stabilization devices for the treatment of degenerative diseases of the lumbar spine. J Spinal Disord Tech 2009;22(5):376–383 Bozkuş H, Senoğlu M, Baek S, et al. Dynamic lumbar pedicle screw-rod stabilization: in vitro biomechanical comparison with standard rigid pedicle screw-rod stabilization. J Neurosurg Spine 2010;12(2):183–189 Crawford NR, Brantley AGU, Dickman CA, Koeneman EJ. An apparatus for applying pure nonconstraining moments to spine segments in vitro. Spine 1995;20(19):2097–2100 Crawford NR, Peles JD, Dickman CA. The spinal lax zone and neutral zone: measurement techniques and parameter comparisons. J Spinal Disord 1998;11(5):416–429 Schulte TL, Hurschler C, Haversath M, et al. The effect of dynamic, semi-rigid implants on the range of motion of lumbar motion segments after decompression. Eur Spine J 2008;17(8):1057–1065

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.

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.

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.

Posterior Pedicle Fixation-based Dynamic Stabilization Devices for the Treatment of Degenerative Diseases of the Lumbar Spine

Literature review. To provide a comprehensive review of the available literature reporting clinical results of using pedicle-based posterior dynamic stabilization systems in the lumbar spine. A literature search was performed using a number of key words including: posterior dynamic stabilization, facet replacement, pedicle fixation. There is a wide array of pedicle-based, posterior dynamic stabilization systems available. Purported indications are also varied, spanning from degenerative disk disease to reconstruction after laminectomy for spinal stenosis. Short-term and mid-term clinical results of some of these devices are available. For many others, clinical studies are still underway or have just begun. For the spine community to draw sound conclusions that a posterior dynamic device is better than fusion, results from multiple, similarly designed, independently funded trials must be compiled, compared, and contrasted.

Which axial and bending stiffnesses of posterior implants are required to design a flexible lumbar stabilization system?

Dynamic stabilization devices have been introduced to clinics as an alternative to rigid fixation. The stiffness of these devices varies widely, whereas the optimal stiffness, achieving a predefined stabilization of the spine, is unknown. This study was focused on the determination of stiffness values for posterior stabilization devices achieving a flexible, semi-flexible or rigid connection between two vertebrae. An extensively validated finite element model of a lumbar spinal segment L4-5 with an implanted posterior fixation device was used in this study. The model was exposed to pure moments of 7.5 and 20 Nm around the three principal anatomical directions, simulating flexion, extension, lateral bending and axial rotation. In parametrical studies, the influence of the axial and bending fixator stiffness on the spinal range of motion was investigated. In order to examine the validity of the computed results, an in-vitro study was carried out. In this, the influence of two posterior stabilization devices (DSS™ and rigidly internal fixator) on the segmental stabilization was investigated. The finite element (FE)-model predicted that each load direction caused a pairing of stiffness relations between axial and bending stiffness. In flexion and extension, however, the bending stiffness had a neglectable effect on the segmental stabilization, compared to the axial stiffness. In contrast, lateral bending and axial rotation were influenced by both stiffness parameters. Except in axial rotation, the model predictions were in a good agreement with the determined in-vitro data. In axial rotation, the FE-model predicted a stiffer segmental behavior than it was determined in the in-vitro study. It is usually expected that high stiffness values are required for a posterior stabilization device to stiffen a spinal segment. We found that already small stiffness values were sufficient to cause a stiffening. Using these data, it may possible to develop implants for certain clinical indications.