Effect of the Total Facet Arthroplasty System after complete laminectomy-facetectomy on the biomechanics of implanted and adjacent segments

Advances in Implant Technologies for Spine Surgery

Spine implants are becoming increasingly diversified. Taking inspiration from other industries, three-dimensional modeling of the spinal column has helped meet the custom needs of individual patients as both en bloc replacements and pedicle screw designs. Intraoperative tailoring of devices, a common need in the operating room, has led to expandable versions of cages and interbody spacers.

Surface Modification of Polyetheretherketone (PEEK) Intervertebral Fusion Implant Using Polydopamine Coating for Improved Bioactivity

The occurrence of bone diseases has been increasing rapidly, in line with the aging population. A representative spinal fusion material, polyetheretherketone (PEEK), is advantageous in this regard as it can work in close proximity to the elastic modulus of cancellous bone. However, if it is used without surface modification, the initial osseointegration will be low due to lack of bioactivity, resulting in limitations in surgical treatment. In this study, we aimed to modify the surface of PEEK cages to a hydrophilic surface by coating with polyethylene glycol (PEG), hyaluronic acid (HA), and polydopamine (PDA), and to analyze whether the coated surface exhibits improved bioactivity and changes in mechanical properties for orthopedic applications. Material properties of coated samples were characterized and compared with various PEEK groups, including PEEK, PEEK-PEG, PEEK-HA, and PEEK-PDA. In an in vitro study, cell proliferation was found to be enhanced on PDA-coated PEEK; it was approximately twice as high compared to the control group. In addition, mechanical properties, including static and torsion, were not affected by the presence of the coating. Thus, the results suggest that PEEK-PDA may have the potential for clinical application in fusion surgery for spinal diseases, as it may improve the rate of osseointegration.

Posterior Lumbar Facet Replacement and Arthroplasty

There is an ongoing desire for the development of motion-preserving facet replacement devices as an alternative to rigid fixation in hopes of better preserving the natural kinematics of the lumbar spine. Theoretically, such a construct would simultaneously address pain associated with spinal instability and prevent abnormal load distribution and adjacent segment degeneration. Several such devices have been developed including the Anatomic Facet Replacement System, the Total Facet Arthroplasty System, and the Total Posterior Arthroplasty System. Of these devices, none have yet proven to be more efficacious than rigid fixation for lumbar spinal stenosis, and studies are ongoing.

Intervertebral disc degeneration and regeneration: a motion segment perspective

Back and neck pain have become primary reasons for disability and healthcare spending globally. While the causes of back pain are multifactorial, intervertebral disc degeneration is frequently cited as a primary source of pain. The annulus fibrosus (AF) and nucleus pulposus (NP) subcomponents of the disc are common targets for regenerative therapeutics. However, disc degeneration is also associated with degenerative changes to adjacent spinal tissues, and successful regenerative therapies will likely need to consider and address the pathology of adjacent spinal structures beyond solely the disc subcomponents. This review summarises the current state of knowledge in the field regarding associations between back pain, disc degeneration, and degeneration of the cartilaginous and bony endplates, the AF-vertebral body interface, the facet joints and spinal muscles, in addition to a discussion of regenerative strategies for treating pain and degeneration from a whole motion segment perspective.

Current Concepts of Contemporary Expandable Lumbar Interbody Fusion Cage Designs, Part 1: An Editorial on Their Biomechanical Characteristics

BACKGROUND

Bidirectional expandable designs for lumbar interbody fusion cages are the latest iteration of expandable spacers employed to address some of the common problems inherent to static interbody fusion cages.

OBJECTIVE

To describe the rationales for contemporary bidirectional, multimaterial expandable lumbar interbody fusion cage designs to achieve in situ expansion for maximum anterior column support while decreasing insertion size during minimal-access surgeries.

METHODS

The authors summarize the current concepts behind expandable spinal fusion open architecture cage designs focusing on advanced minimally invasive spinal surgery techniques, such as endoscopy. A cage capable of bidirectional expansion in both height and width to address constrained surgical access problems was of particular interest to the authors while they analyzed the relationship between implant material stiffness and geometric design regarding the risk of subsidence and reduced graft loading.

CONCLUSIONS

Biomechanical advantages of new bidirectional, multimaterial expandable interbody fusion cages allow insertion through minimal surgical access and combine the advantages of proven device configurations and advanced material selection. The final construct stiffness is sufficient to provide immediate anterior column support while accommodating reduced sizes required for minimally invasive surgery applications.

LEVEL OF EVIDENCE

7.

Bone-Preserving Decompression Procedures Have a Minor Effect on the Flexibility of the Lumbar Spine

Objective

To mitigate the risk of iatrogenic instability, new posterior decompression techniques able to preserve musculoskeletal structures have been introduced but never extensively investigated from a biomechanical point of view. This study was aimed to investigate the impact on spinal flexibility caused by a unilateral laminotomy for bilateral decompression, in comparison to the intact condition and a laminectomy with preservation of a bony bridge at the vertebral arch. Secondary aims were to investigate the biomechanical effects of two-level decompression and the quantification of the restoration of stability after posterior fixation.

Methods

A universal spine tester was used to measure the flexibility of six L2–L5 human spine specimens in intact conditions and after decompression and fixation surgeries. An incremental damage protocol was applied : 1) unilateral laminotomy for bilateral decompression at L3–L4; 2) on three specimens, the unilateral laminotomy was extended to L4–L5; 3) laminectomy with preservation of a bony bridge at the vertebral arch (at L3–L4 in the first three specimens and at L4–L5 in the rest); and 4) pedicle screw fixation at the involved levels.

Results

Unilateral laminotomy for bilateral decompression had a minor influence on the lumbar flexibility. In flexion-extension, the median range of motion increased by 8%. The bone-preserving laminectomy did not cause major changes in spinal flexibility. Two-level decompression approximately induced a twofold destabilization compared to the single-level treatment, with greater effect on the lower level. Posterior fixation reduced the flexibility to values lower than in the intact conditions in all cases.

Conclusion

In vitro testing of human lumbar specimens revealed that unilateral laminotomy for bilateral decompression and bone-preserving laminectomy induced a minor destabilization at the operated level. In absence of other pathological factors (e.g., clinical instability, spondylolisthesis), both techniques appear to be safe from a biomechanical point of view.

An acellular bioresorbable ultra-purified alginate gel promotes intervertebral disc repair: A preclinical proof-of-concept study

Background

The current surgical procedure of choice for lumbar intervertebral disc (IVD) herniation is discectomy. However, defects within IVD produced upon discectomy may impair tissue healing and predispose patients to subsequent IVD degeneration. This study aimed to investigate whether the use of an acellular bioresorbable ultra-purified alginate (UPAL) gel implantation system is safe and effective as a reparative therapeutic strategy after lumbar discectomy.

Methods

Human IVD cells were cultured in a three-dimensional system in UPAL gel. In addition, lumbar spines of sheep were used for mechanical analysis. Finally, the gel was implanted into IVD after discectomy in rabbits and sheep in vivo.

Findings

The UPAL gel was biocompatible with human IVD cells and promoted extracellular matrix production after discectomy, demonstrating sufficient biomechanical characteristics without material protrusion.

Interpretation

The present results indicate the safety and efficacy of UPAL gels in a large animal model and suggest that these gels represent a novel therapeutic strategy after discectomy in cases of lumbar IVD herniation.

Fund

Grant-in-Aid for the Ministry of Education, Culture, Sports, Science, and Technology of Japan, Japan Agency for Medical Research and Development, and the Mochida Pharmaceutical Co., Ltd.

Facet Joints of the Spine: Structure–Function Relationships, Problems and Treatments, and the Potential for Regeneration

The zygapophysial joint, a diarthrodial joint commonly referred to as the facet joint, plays a pivotal role in back pain, a condition that has been a leading cause of global disability since 1990. Along with the intervertebral disc, the facet joint supports spinal motion and aids in spinal stability. Highly susceptible to early development of osteoarthritis, the facet is responsible for a significant amount of pain in the low-back, mid-back, and neck regions. Current noninvasive treatments cannot offer long-term pain relief, while invasive treatments can relieve pain but fail to preserve joint functionality. This review presents an overview of the facet in terms of its anatomy, functional properties, problems, and current management strategies. Furthermore, this review introduces the potential for regeneration of the facet and particular engineering strategies that could be employed as a long-term treatment.

Unconstrained testing of spine with bi-axial universal testing machine

BACKGROUND

In-vitro biomechanical assessment of the spine reveals significant information on the mechanics of spinal disorders, treatment methods, and surgical implants. Specialized devices for the evaluation of spine biomechanics have thus become popular. However, these devices might not be affordable for all research groups.

PURPOSE

The purpose of this study was to describe an apparatus to be attached to a standard bi-axial universal testing machine that would make unconstrained testing of the spine possible.

STUDY DESIGN/SETTING

A technical note on the definition of a spinal testing fixture with validation.

METHODS

Intact lumbosacral spines (T12-S1) were tested in sagittal and lateral bending and axial rotation. Three-dimensional interlevel rotations at each level (L1-4) were analyzed.

RESULTS

By comparison with the literature, we found that the new fixture was able to successfully produce reasonable relative rotation values for the lumbar spine.

CONCLUSIONS

We demonstrated that the low cost fixture allowed unconstrained (six degree of freedom, 6 DOF) testing of fresh-frozen cadaveric lumbar spine.

Simple facet joint repair with dynamic pedicular system: Technical note and case series

Purpose:

Facet joints are important anatomical structures for the stability of spine. Surgical or degenerative damage to a facet joint may lead to spinal instability and causes clinical problems. This article explains the importance of facet joints, reviews facet replacement systems, and describes a simple and effective method for facet replacement after surgical removal of facet joints.

Materials and Methods:

Ten patients were operated with the diagnosis of unilateral nerve root compression secondary to facet degeneration. The hypertrophic facet joints were removed with microsurgical techniques and the roots were decompressed. Then, a unilateral artificial facet joint was created using two hinged screws and a dynamic rod.

Results:

The clinical outcome of all the patients was determined good or excellent at second and last follow-up (mean 13.3 months) controls using visual analog scale (VAS) and Oswestry Disability Index (ODI) scores. Radiological evaluations also demonstrated no implant-related complications.

Conclusions:

The authors suggest that, if removal of a facet joint is necessary to decompress the nerve roots, the joint can be replaced by a construct composed of two hinged screws connected by a dynamic rod. This simple system mimics the function of a normal facet joint and is an effective technique for unilateral facet joint replacement.

The current testing protocols for biomechanical evaluation of lumbar spinal implants in laboratory setting: a review of the literature

In vitro biomechanical investigations have become a routinely employed technique to explore new lumbar instrumentation. One of the most important advantages of such investigations is the low risk present when compared to clinical trials. However, the best use of any experimental data can be made when standard testing protocols are adopted by investigators, thus allowing comparisons among studies. Experimental variables, such as the length of the specimen, operative level, type of loading (e.g., dynamic versus quasistatic), magnitude, and rate of load applied, are among the most common variables controlled during spinal biomechanical testing. Although important efforts have been made to standardize these protocols, high variability can be found in the current literature. The aim of this investigation was to conduct a systematic review of the literature to identify the current trends in the protocols reported for the evaluation of new lumbar spinal implants under laboratory setting.

Postero-lateral disc prosthesis combined with a unilateral facet replacement device maintains quantity and quality of motion at a single lumbar level

Background

Mechanically replacing one or more pain generating articulations in the functional spinal unit (FSU) may be a motion preservation alternative to arthrodesis at the affected level. Baseline biomechanical data elucidating the quantity and quality of motion in such arthroplasty constructs is non-existent.

Purpose

The purpose of the study was to quantify the motion-preserving effect of a posterior total disc replacement (PDR) combined with a unilateral facet replacement (FR) system at a single lumbar level (L4-L5). We hypothesized that reinforcement of the FSU with unilateral FR to replace the resected, native facet joint following PDR implantation would restore quality and quantity of motion and additionally not change biomechanics at the adjacent levels.

Study Design

In-vitro study using human cadaveric lumbar spines.

Methods

Six (n = 6) cadaveric lumbar spines (L1-S1) were evaluated using a pure-moment stability testing protocol (±7.5 Nm) in flexion-extension (F/E), lateral bending (LB) and axial rotation (AR). Each specimen was tested in: (1) intact; (2) unilateral FR; and (3) unilateral FR + PDR conditions. Index and adjacent level ROM (using hybrid protocol) were determined opto-electronically. Interpedicular travel (IPT) and instantaneous center of rotation (ICR) at the index level were radiographically determined for each condition. ROM, ICR, and IPT measurements were compared (repeated measures ANOVA) between the three conditions.

Results

Compared to the intact spine, no significant changes in F/E, LB or AR ROM were identified as a result of unilateral FR or unilateral FR + PDR. No significant changes in adjacent L3-L4 or L5-S1 ROM were identified in any loading mode. No significant differences in IPT were identified between the three test conditions in F/E, LB or AR at the L4-L5 level. The ICRs qualitatively were similar for the intact and unilateral FR conditions and appeared to follow placement (along the anterior-posterior (AP) direction) of the PDR in the disc space

Conclusion

Biomechanically, quantity and quality of motion are maintained with combined unilateral FR + PDR at a single lumbar spinal level.

Biomechanical analysis of an interbody cage with three integrated cancellous lag screws in a two-level cervical spine fusion construct: an in vitro study

BACKGROUND CONTEXT

Despite an increase in the clinical use of no-profile anchored interbody cages (AIC) for anterior cervical discectomy and fusion (ACDF) procedures, there is little published biomechanical data describing its stabilizing effect relative to the traditional anterior plating technique over two contiguous levels.

PURPOSE

To biomechanically compare the acute stability conferred by a stand-alone interbody fusion device with three integrated fixation screws ("anchored cage") with a traditional six-hole rigid anterior plate in a two contiguous levels (C4-C5+C5-C6) fusion construct. We hypothesized that the anchored cage would confer comparable segmental rigidity to the cage and anterior plate construct.

STUDY DESIGN

A biomechanical laboratory study using cadaveric human cervical spines.

METHODS

Seven (n=7) cadaveric human cervical spines (C3-C7) were subjected to quasistatic, pure-moment loading (±1.5 Nm) in flexion-extension (flex/ext), right/left lateral bending (RB/LB), and right/left axial rotation (RR/LR) for the following test conditions: intact; after discectomy and insertion of the AIC at C4-C5 and C5-C6 with anchoring screws engaged; after the removal of the integrated anchoring screws and instrumentation of an anterior locking plate (ALP) over both levels; and cage-only (CO) configuration with screws and anterior plate removed. Intervertebral range of motion (ROM) at the instrumented levels was the primary biomechanical outcome.

RESULTS

Flex/ext, RB/LB, and RR/LR ROMs were significantly reduced (p<.001) over both levels by AIC and ALP constructs relative to the CO construct. Significant reduction in flex/ext motion was achieved with the ALP (6.8±3.7) relative to the AIC (10.2°±4.6°) (p=.041) construct. No significant differences were seen in ROM reductions over the two levels between the AIC and APL groups in lateral bending or axial rotation (p>.826).

CONCLUSIONS

The anchored cage fusion construct conferred similar acute biomechanical stability in lateral bending and axial rotation ROMs relative to rigid anterior plating. We identified a statistically significant reduction (Δ=3.4°, combined over two levels) in sagittal plane ROM conferred by the ALP relative to the AIC construct. Our biomechanical findings may support the clinical use of no-profile integrated interbody devices over two contiguous levels in ACDF.

The effect of posterior non-fusion instrumentation on segmental shear loading of the lumbar spine

BACKGROUND

Lumbar stenosis and facet osteoarthritis represent indications for decompression and instrumentation. It is unclear if degenerative spondylolisthesis grade I with a remaining disc height could be an indication for non-fusion instrumentation. The purpose of this study was to determine the influence of a mobile pedicle screw based device on lumbar segmental shear loading, thus simulating the condition of spondylolisthesis.

MATERIALS AND METHODS

Six human cadaver specimens were tested in 3 configurations: intact L4-L5 segment, then facetectomy plus undercutting laminectomy, then instrumentation with lesion. A static axial compression of 400 N was applied to the lumbar segment and anterior displacements of L4 on L5 were measured for posterior-anterior shear forces from 0 to 200 N. The slope of the loading curve was assessed to determine shear stiffness.

RESULTS

Homogenous load-displacement curves were obtained for all specimens. The average intact anterior displacement was 1.2 mm. After lesion, the displacement increased by 0.6mm compared to intact (P=0.032). The instrumentation decreased the displacement by 0.5 mm compared to lesion (P=0.046). The stiffness's were: 162 N/mm for intact, 106 N/mm for lesion, 148 N/mm for instrumentation. The difference was not significant between instrumented and intact segments (P=0.591).

CONCLUSIONS

Facetectomy plus undercutting laminectomy decreases segmental shear stiffness and increases anterior translational L4-L5 displacement. Shear stiffness of the instrumented segment is higher with the device and anterior displacements under shear loading are similar to the intact spine. This condition could theoretically be interesting for the simulation of non-fusion instrumentation in degenerative spondylolisthesis.

Single level lumbar laminectomy alters segmental biomechanical behavior without affecting adjacent segments

BACKGROUND

Degenerative lumbar spinal stenosis causes neurological symptoms due to neural compression. Lumbar laminectomy is a commonly used treatment for symptomatic degenerative spinal stenosis. However, it is unknown if and to what extent single level laminectomy affects the range of motion and stiffness of treated and adjacent segments. An increase in range of motion and a decrease in stiffness are possible predictors of post-operative spondylolisthesis or spinal failure.

METHODS

Twelve cadaveric human lumbar spines were obtained. After preloading, spines were tested in flexion-extension, lateral bending, and axial rotation. Subsequently, single level lumbar laminectomy analogous to clinical practice was performed at level lumbar 2 or 4. Thereafter, load-deformation tests were repeated. The range of motion and stiffness of treated and adjacent segments were calculated before and after laminectomy. Untreated segments were used as control group. Effects of laminectomy on stiffness and range of motion were tested, separately for treated, adjacent and control segments, using repeated measures analysis of variance.

FINDINGS

Range of motion at the level of laminectomy increased significantly for flexion and extension (7.3%), lateral bending (7.5%), and axial rotation (12.2%). Range of motion of adjacent segments was only significantly affected in lateral bending (-7.7%). Stiffness was not affected by laminectomy.

INTERPRETATION

The increase in range of motion of 7-12% does not seem to indicate the use of additional instrumentation to stabilize the lumbar spine. If instrumentation is still considered in a patient, its primary focus should be on re-stabilizing only the treated segment level.

A new device used in the restoration of kinematics after total facet arthroplasty

Facet degeneration can lead to spinal stenosis and instability, and often requires stabilization. Interbody fusion is commonly performed, but it can lead to adjacent-segment disease. Dynamic posterior stabilization was performed using a total facet arthroplasty system. The total facet arthroplasty system was originally intended to restore the natural motion of the posterior stabilizers, but follow-up studies are lacking due to limited clinical use. We studied the first 14 cases (long-term follow-up) treated with this new device in our clinic. All patients were diagnosed with lumbar stenosis due to hypertrophy of the articular facets on one to three levels (maximum). Disk space was of normal height. The design of this implant allows its use only at levels L3-L4 and L4-L5. We implanted nine patients at the L4-L5 level and four patients at level L3-L4. Postoperative follow-up of the patients was obtained for an average of 3.7 years. All patients reported persistent improvement of symptoms, visual analog scale score, and Oswestry Disability Index score. Functional scores and dynamic radiographic imaging demonstrated the functional efficacy of this new implant, which represents an alternative technique and a new approach to dynamic stabilization of the vertebral column after interventions for spine decompression. The total facet arthroplasty system represents a viable option for dynamic posterior stabilization after spinal decompression. For the observed follow-up, it preserved motion without significant complications or apparent intradisk or adjacent-disk degeneration.

Quantitative Analysis of the Nonlinear Displacement–Load Behavior of the Lumbar Spine

There is currently no universal model or fitting method to characterize the visco-elastic behavior of the lumbar spine observed in displacement versus load hysteresis loops. In this study, proposed methods for fitting these loops, along with the metrics obtained, were thoroughly analyzed. A spline fitting technique was shown to provide a consistent approximation of spinal kinetic behavior that can be differentiated and integrated. Using this tool, previously established metrics were analyzed using data from two separate studies evaluating different motion preservation technologies. Many of the metrics, however, provided no significant differences beyond range of motion analysis. Particular attention was paid to how different definitions of the neutral zone capture the high-flexibility region often seen in lumbar hysteresis loops. As a result, the maximum slope was introduced and shown to be well defined. This new parameter offers promise as a descriptive measurement of spinal instability in vitro and may have future implications in clinical diagnosis and treatment of spinal instability. In particular, it could help in assigning treatments to specific stabilizing effects in the lumbar spine.

Biomechanical analysis and design of a dynamic spinal fixator using topology optimization: a finite element analysis

Surgeons often use spinal fixators to manage spinal instability. Dynesys (DY) is a type of dynamic fixator that is designed to restore spinal stability and to provide flexibility. The aim of this study was to design a new spinal fixator using topology optimization [the topology design (TD) system]. Here, we constructed finite element (FE) models of degenerative disc disease, DY, and the TD system. A hybrid-controlled analysis was applied to each of the three FE models. The rod structure of the topology optimization was modelled at a 39 % reduced volume compared with the rigid rod. The TD system was similar to the DY system in terms of stiffness. In contrast, the TD system reduced the cranial adjacent disc stress and facet contact force at the adjacent level. The TD system also reduced pedicle screw stresses in flexion, extension, and lateral bending.

A novel pedicle screw with mobile connection: a pilot study

To prevent adjacent disc problems after spinal fusion, a pedicle screw with a mobile junction between the head and threaded shaft was newly developed. The threaded shaft of the screw has 10 degrees mobility in all directions, but its structure is to prevent abnormal translation and tilting. This screw was evaluated as follows: (1) endurance test: 10⁶ times rotational stress was applied; (2) biological reactions: novel screws with a mobile head and conventional screws with a fixed head were inserted into the bilateral pedicles of the L3, L4, and L5 in two mini pigs with combination. Eight months after surgery, vertebral units with the screw rod constructs were collected. After CT scan, the soft and bony tissues around the screws were examined grossly and histologically. As a result, none of the screws broke during the endurance test stressing. The mean amount of abrasion wear was 0.0338 g. In the resected mini pig section, though zygapophyseal joints between fixed-head screws showed bony union, the amount of callus in the zygapophyseal joints connected with mobile-head screws was small, and joint space was confirmed by CT. No metalloses were noted around any of the screws. Novel screws were suggested to be highly durable and histologically safe.

Pedicle-Based Non-fusion Stabilization Devices: A Critical Review and Appraisal of Current Evidence

Over the last decades, spinal fusion has become one of the most important principles in surgical treatment of spinal pathologies. Despite the undoubted benefits of fusion surgery, there are several drawbacks associated with this technique, including adjacent segment degeneration and pseudoarthrosis. Based on biomechanical data, dynamic stabilization of the spine is intended to ameliorate adjacent level degeneration by stabilizing vertebral motion in defined planes and mimicking natural spine movements.In this paper, we review the literature and discuss past and present pedicle-based non-fusion dynamic stabilization devices. Although there is a paucity of high-quality prospective trials, studies have indicated both promising and disappointing results. In comparison to 360° fusion surgery, the perioperative risk seems to be lower. Other complications like screw loosening, however, have been reported with various systems, while a reduction of adjacent segment disease has not yet been demonstrated. The necessary degree of restabilization to achieve pain-free motion seems to vary greatly between patients and current systems are far from perfection. If these problems can be solved, dynamic stabilization may nevertheless be an important option of spinal surgery in the future.

Influence of an auxiliary facet system on intervertebral discs and adjacent facet joints

BACKGROUND CONTEXT

Facet supplementation stabilizes after facetectomy and undercutting laminectomy. It is indicated in degenerative spondylolisthesis with moderate disc degeneration and dynamic stenosis.

PURPOSE

To determine the influence of an auxiliary facet system (AFS) on the instrumented disc, adjacent levels' discs, and facet joints and to compare it with fusion.

STUDY DESIGN

Finite element study.

METHODS

L3-L4, L4-L5, and L5-S1 were studied using a validated finite element model with prescribed displacements for an intact spine, lesion by facetectomy and undercutting laminectomy, AFS, and fusion at L4-L5. The distribution of segmental range of motion (ROM) and applied moments, von Mises stress at the annulus, and facet joint contact forces were calculated with rotations in all planes. Institutional support for implant evaluation and modeling was received by Clariance.

RESULTS

In flexion-extension and lateral bending, fusion decreased L4-L5 ROM and increased adjacent levels' ROM. Range of motion was similarly distributed with intact lesion and AFS. In axial rotation, L4-L5 ROM represented 33% with intact, 55% after lesion, 25% with AFS, and 21% with fusion. Fusion increased annulus stress at adjacent levels in flexion-extension and lateral bending, but decreased stress at L4-L5 compared with AFS. In axial rotation, von Mises stress was similar with fusion and AFS. Facet loading increased in extension and lateral bending with fusion. It was comparable for fusion and AFS in axial rotation.

CONCLUSIONS

This study suggests that the AFS stabilizes L4-L5 in axial rotation after facetectomy and undercutting laminectomy as fusion does. This is because of the cross-link that generates an increased annulus stress in axial rotation at adjacent levels. With imposed displacements, without in vivo compensation of the hips, the solicitation at adjacent levels' discs and facet joints is higher with fusion compared with AFS. Fusion decreases intradiscal stress at the instrumented level.

Intervertebral disc degeneration alters lumbar spine segmental stiffness in all modes of loading under a compressive follower load

BACKGROUND CONTEXT

Previous studies have investigated the relationship between the degeneration grade of the intervertebral disc (IVD) and the flexibility of the functional spinal unit (FSU) but were completed at room temperature without the presence of a compressive follower load. This study builds on previous work by performing the testing under more physiological conditions of a compressive follower load at body temperature and at near 100% humidity.

PURPOSE

The present work evaluates the effects of IVD degeneration on segmental stiffness, range of motion (ROM), hysteresis area, and normalized hysteresis (hysteresis area/ROM). This study also briefly evaluates the effect of the segment level, temperature, and follower load on the same parameters.

STUDY DESIGN

In vitro human cadaveric biomechanical investigation.

METHODS

Twenty-one FSUs were tested in the three primary modes of loading at both body temperature and room temperature in a near 100% humidity environment. A compressive follower load of 440 N was applied to simulate the physiological conditions. Fifteen of the 21 segments were also tested without the follower load to determine the effects of the follower load on segmental biomechanics. The grade of degeneration for each segment was determined using the Thompson scale, and the torque-rotation curves were fit with the Dual-Inflection-Point Boltzmann sigmoid curve.

RESULTS

Intervertebral disc degeneration resulted in statistically significant changes in segmental stiffness, ROM, and hysteresis area in axial rotation (AR) and lateral bending (LB) and statistically significant changes in ROM and normalized hysteresis in flexion-extension (FE). The progression of these changes with increased degeneration is nonlinear, with changes in the FE and LB tending to respond in concert and opposite to the changes in AR. The lumbosacral joint was significantly stiffer and demonstrated a decreased ROM and hysteresis area as compared with other lumbar segments in AR and LB. Temperature had a significant effect on the stiffness and hysteresis area in AR and on the hysteresis area in LB. Application of a compressive follower load increased the stiffness in all three modes of loading but was significant only in AR and LB. It also reduced the ROM and increased normalized hysteresis in all three modes of loading.

CONCLUSIONS

The results from this testing quantify the effects of degeneration on spinal biomechanics. Because the testing was conducted under physiological conditions (including a compressive follower load and at body temperature), we expect the measured response to closely match the in vivo response. The testing results can be used to guide the selection of appropriate surgical treatments in the context of IVD degeneration and to validate the mathematical and engineering models of the lumbar spine, including finite element models.

Elastic resistance of the spine: Why does motion preservation surgery almost fail?

Single metamere motility should not be interpreted merely as a movement on the 3 planes but also, and above all, as elastic resistance to dynamic stress on these 3 planes. In the light of this consideration, the aim of motion preservation is to neutralize excessive movements while preserving the physiological biomechanical properties of the metamere involved to interrupt the progression of degenerative processes and to prevent adjacent segment disease. Despite the fact that a myriad of devices have been developed with the purpose of achieving dynamic neutralization of the spine, there are now some doubts regarding the true efficacy of these devices.

Current concepts on spinal arthrodesis in degenerative disorders of the lumbar spine

Back pain is a common chronic disorder that represents a large burden for the health care system. There is a broad spectrum of available treatment options for patients suffering from chronic lower back pain in the setting of degenerative disorders of the lumbar spine, including both conservative and operative approaches. Lumbar arthrodesis techniques can be divided into sub-categories based on the part of the vertebral column that is addressed (anterior vs posterior). Furthermore, one has to differentiate between approaches aiming at a solid fusion in contrast to motion-sparing techniques with the proposed advantage of a reduced risk of developing adjacent disc disease. However, the field of application and long-term outcomes of these novel motion-preserving surgical techniques, including facet arthroplasty, nucleus replacement, and lumbar disc arthroplasty, need to be more precisely evaluated in long-term prospective studies. Innovative surgical treatment strategies involving minimally invasive techniques, such as lateral lumbar interbody fusion or transforaminal lumbar interbody fusion, as well as percutaneous implantation of transpedicular or transfacet screws, have been established with the reported advantages of reduced tissue invasiveness, decreased collateral damage, reduced blood loss, and decreased risk of infection. The aim of this study was to review well-established procedures for lumbar spinal fusion with the main focus on current concepts on spinal arthrodesis and motion-sparing techniques in degenerative disorders of the lumbar spine.

Biomechanical evaluation of the Total Atlanto-odontoid Joint Arthroplasty System: an in vitro human cadaveric study

BACKGROUND

Atlanto-odontoid joint arthroplasty is a motion restoring procedure suggested as an alternative to rigid fixation after surgical decompression. The purpose of this study was to evaluate the kinematics and pullout strength of a novel Total Atlanto-odontoid Joint Arthroplasty System using human cadaveric specimens.

METHODS

Nondestructive biomechanical tests were performed on 24 fresh craniocervical specimens separated into two groups: 1) the prosthesis implantation group and 2) Harms transoral atlantoaxial plate fixation group. The following configurations were investigated: intact, after decompression, and instrumented. Range of motion and neutral zone were calculated for the C1-C2 segment. In a second experimental series, 8 sets of fresh atlantoaxial specimens were used to test the pullout strength of the atlas-axis components.

FINDINGS

Compared with Harms rigid fixation, the Total Atlanto-odontoid Joint Arthroplasty System significantly increased the range of motion and neutral zone in all directions (P<.001). In addition, compared with the intact state, the only significant change in the range of motion and neutral zone with the Total Atlanto-odontoid Joint Arthroplasty System implantation was an increase in lateral bending (P<.001). The pullout strength created by the anterior C2 transpedicular screw was greater than that of the C2 vertebral screw and C1 lateral mass screw (P<.001), and the C1 lateral mass screw was stiffer than the C2 vertebral screw (P=.02).

INTERPRETATION

Biomechanical analyses suggest that the Total Atlanto-odontoid Joint Arthroplasty System was able to provide reliable fixation strength and preserve the normal kinematics of the C1-C2 segment after decompressive procedures.

A short history of posterior dynamic stabilization

Interspinous spacers were developed to treat local deformities such as degenerative spondylolisthesis. To treat patients with chronic instability, posterior pedicle fixation and rod-based dynamic stabilization systems were developed as alternatives to fusion surgeries. Dynamic stabilization is the future of spinal surgery, and in the near future, we will be able to see the development of new devices and surgical techniques to stabilize the spine. It is important to follow the development of these technologies and to gain experience using them. In this paper, we review the literature and discuss the dynamic systems, both past and present, used in the market to treat lumbar degeneration.

Kinematic effects of a pedicle-lengthening osteotomy for the treatment of lumbar spinal stenosis

OBJECT

Lumbar spinal stenosis (LSS) may lead to disabling neurogenic symptoms and has traditionally been treated using open laminectomy. A new technique for correcting LSS involves lengthening the lumbar pedicles through bilateral percutaneous pedicle osteotomies. In this paper, the authors' goal was to evaluate the changes in spinal canal dimensions and kinematic behavior after pedicle-lengthening osteotomies.

METHODS

The kinematic behavior of 8 cadaveric lumbar segments was evaluated intact and after bilateral pedicle-lengthening osteotomies at the L-4, L-5, and L-4 and L-5 levels. Testing was conducted with and without a compressive preload using a custom kinematic apparatus that allowed for 3D tracking of each vertebra during flexion-extension, right-left bending, and right-left rotation. A validated finite element (FE) spine model was used to measure the changes in the cross-sectional area of the spinal canal and neural foramen after 2-, 3-, and 4.5-mm simulated pedicle-lengthening osteotomy procedures.

RESULTS

The overall and segmental kinematics were not significantly altered after the pedicle-lengthening osteotomy procedure at the L-4 and/or L-5 pedicles. The kinematic signatures of the intact and lengthened states were similar for all motion pairs. The FE spine model yielded kinematics predictions within or close to the 95% confidence interval for the cadaveric data. The FE spine demonstrated substantial, pedicle length-dependent enlargement of the cross-sectional areas of the spinal canal and neural foramen after simulated pedicle lengthening.

CONCLUSIONS

Bilateral pedicle-lengthening osteotomies produced substantial increases in the cross-sectional areas of the spinal canal and neural foramen without significantly altering normal spinal kinematics. This technique deserves further study as a less invasive treatment option for LSS.

Asymmetric motion distribution between components of a mobile-core lumbar disc prosthesis: an explanation of unequal wear distribution in explanted CHARITÉ polyethylene cores

BACKGROUND

The biconvex mobile core of the CHARITÉ lumbar disc prosthesis forms two joints (spherical bearings) with the metal end plates. We quantified the intra-prosthesis motion to test the hypothesis that the total prosthesis motion would not be equally distributed between the two bearings of implanted CHARITÉ discs, which might explain the unequal wear distribution reported in explanted cores.

METHODS

The hypothesis was tested by studying the flexion-extension motion responses of (1) twenty-six monosegmental CHARITÉ III discs implanted in nineteen human cadaveric lumbar spines, and (2) twenty-one CHARITÉ III discs (fifteen monosegmental, six bisegmental) implanted in eighteen patients in other published clinical studies. Intra-prosthesis motions were quantified with use of a radiographic image analysis technique.

RESULTS

Eighty-eight percent of the CHARITÉ discs implanted in cadaveric specimens exhibited larger motion at the superior bearing, with 54% demonstrating more than twice as much motion at the superior bearing as at the inferior bearing. The ratio of motion at the superior bearing to motion at the inferior bearing averaged 2.68 ± 1.84, which was significantly larger than 1.0 (p < 0.001). Ninety percent of prostheses implanted in patients showed larger motion at the superior bearing. The motion ratio averaged 2.39 ± 2.47 for monosegmental cases and 2.55 ± 2.66 for all cases; both ratios were significantly larger than 1.0 (p < 0.05).

CONCLUSIONS

We found preferentially larger motion at the superior bearing of the CHARITÉ discs implanted in human cadaveric lumbar spines and in patients, regardless of the implanted level.

Stem fracture after total facet replacement in the lumbar spine: a report of two cases and review of the literature

BACKGROUND CONTEXT

A randomized controlled multicenter investigational device exemption clinical trial comparing the total facet arthroplasty system (TFAS) (Archus Orthopedics, Redmond, WA, USA) with posterior fusion was discontinued because of financial reasons. To our knowledge, no clinical outcomes or complications have yet been presented for the TFAS, and no device-related complications have been reported for any other lumbar facet replacement system.

PURPOSE

To report and discuss two cases of stem fracture after total facet replacement in the lumbar spine.

STUDY DESIGN

Case report and literature review.

PATIENT SAMPLE

A 55-year-old man with a body mass index (BMI) of 40 underwent total facet replacement at L4-L5 for Grade 1 spondylolisthesis with stenosis. After 9 months of pain relief, he experienced gradually increasing pain and radiographs showed a broken stem. A 60-year-old woman with a BMI of 31 underwent total facet replacement at L4-L5 for Grade 1 spondylolisthesis with stenosis. She experienced stem fracture 27 months postoperatively.

OUTCOME MEASURES

Visual analog scale for pain, Oswestry Disability Index for function, and computed tomography and X-ray for imaging.

RESULTS

After TFAS stem breakage, both patients underwent interbody fusion through a transpsoas approach and have done well over 24- and 12-month follow-up periods, respectively.

CONCLUSIONS

These are the first cases of stem fracture reported after total facet replacement in the lumbar spine. Biomechanics of TFAS stem breakage may be similar to those of pedicle screw breakage, including fatigue and three-point bending stress. Further biomechanical studies and failure analyses however are needed for adequate understanding to improve the biomechanics of dynamic pedicle-based devices.

Novel indication for posterior dynamic stabilization: Correction of disc tilt after lumbar total disc replacement

Background

The increase in total disc replacement procedures performed over the last 5 years has increased the occurrence of patients presenting with postoperative iatrogenic deformity requiring revision surgery. Proposed salvage treatments include device retrieval followed by anterior lumbar interbody fusion or posterior fusion. We propose a novel approach for the correction of disc tilt after total disc replacement using a posterior dynamic stabilization system.

Methods

Pedicle screws can be inserted either in an open manner or percutaneously by standard techniques under fluoroscopy. The collapsed side is expanded, and the convex side is compressed. Universal spacers are placed bilaterally, with the spacer on the collapsed side being taller by 6 mm. Cords are threaded through the spacers and pulled into place with the tensioning instrument. Extra tension is applied to the convex side, and the wound is closed by standard techniques.

Results

Three patients presenting with tilted total disc replacement devices underwent corrective surgery with posterior dynamic stabilization. Radiographs confirmed correction of deformity in all cases.

Conclusions/Level of Evidence

This technical note presents a novel indication for posterior dynamic stabilization and describes its surgical application to the correction of disc tilt after total disc replacement. This is level V evidence.

Characterization of articulation of the lumbar facets in the human cadaveric spine using a facet-based coordinate system

BACKGROUND CONTEXT

The three-joint-complex, which includes the two facet joints and the intervertebral disc, significantly directs the movement of the lumbar spine. When studying the biomechanics of this complex, kinematics have traditionally been measured relative to a vertebral body-based reference frame. Recently, a facet-based reference system has been used to describe the same motions, but relative to the facets.

PURPOSE

To describe in-plane and out-of-plane motion during flexion-extension (FE), lateral bending (LB), and axial rotation (AR) in the lumbar facets of cadaveric spines.

STUDY DESIGN

Biomechanical in vitro study.

METHODS

Seven fresh-frozen and noninstrumented human cadaveric lumbar spines from L1 to sacrum were tested in FE, AR, and LB. Three coordinate systems were assigned to each vertebral body from L1 to L5: one corresponding to the vertebral body reference frame (X, Y, Z) and two corresponding to the left and right facet reference frames (A, B, C). The facet-based reference frame was aligned to each facet surface to provide descriptions of in-plane (articulation) and out-of-plane (separation) motion. For each of the three modes of loading, the magnitude of the translational range of motion (ROM) vector was calculated for the three reference frames.

RESULTS

During FE, there was a significant difference in the magnitude of the translational ROM vector of the right facet as compared with the vertebral body at L1-L2. During AR, there was a significant difference between the magnitude of translational ROM vector at L2-L3, L3-L4, L4-L5, and L1-L2 trended toward significance. During LB, there was a significant difference in the magnitude of the translational ROM vector at L1-L2, L2-L3, L3-L4, and L4-L5.

CONCLUSIONS

A facet-based reference frame accounts for variations in facet orientation and provides a valuable characterization of facet articulation and separation that is not possible to derive from translations in a vertebral body-based reference frame.

[Status quo of facet joint replacement]

This review, regarding facet joint replacement, includes the presentation of six different implant systems from five US firms and one Swiss company. The implant systems are introduced for motion retaining replacement of the lumbar facet joints. Biomechanical and clinical results are included as far as they have been available from an Internet search, publications,oral and poster presentations, and from companies directly. At the beginning anatomical, biomechanical, and clinical data of the natural facet joints are presented. Basic principles of the high morbidity in that topographical region and at the same time for an artificial replacement of the facet joints are derived from the data. An implant classification and the available results of these implants are the basis for this article, which presents the actual situation of the treatment method for motion retaining replacements of facet joints, which have been in general clinical use since 2005. Clinical studies are not yet finalized and there are not enough clinical data; therefore, no binding recommendations for treatment with artificial facet joints are possible.

Kinematics of total facet replacement (TFAS-TL) with total disc replacement

Background

Total disc replacement (TDR) and total facet replacement (TFR) have been the focus of recent kinematics evaluations. Yet their concurrent function as a total joint replacement of the lumbar spine's 3-joint complex has not been comprehensively reported. This study evaluated the effect of a TFR specifically designed to replace the natural facets and supplement the function with the natural disc and with TDR. The ability to replace degenerated facets to complement a pre-existing or simultaneously implanted TDR may allow surgeons to completely address degenerative pathologies of the 3-joint complex of the lumbar spine. We hypothesized that TFR would reproduce the biomechanical function of the natural facets when implanted in conjunction with TDR.

Methods

Lumbar spines (L1-5, 51.3 ± 14.2 years, N = 6) were tested sequentially as follows: (1) intact, (2) after TDR implantation, and (3) after TFR implantation in conjunction with TDR, all at L3-4. Specimens were tested in flexion-extension (+ 8 Nm to − 6 Nm), lateral bending (± 6 Nm), and axial rotation (± 5 Nm). A 400 N compressive follower preload was applied during flexion-extension tests. Three-dimensional segmental motion was recorded and analyzed using analysis of variance in Systat (Systat Software Inc., Chicago, Illinois) and multiple comparisons with Bonferroni correction.

Results

The TDR implantation (TDR + natural facets) allowed similar lateral bending (P = .66), but it generally increased flexion-extension (P = .06) and axial rotation (P < .05) range of motion (ROM) at the implanted level compared to intact. The TFR + TDR (following replacement of the natural facets with TFR) decreased ROM to levels similar to intact in lateral bending (P = .70) and axial rotation (P = .23). The TFR + TDR flexion-extension ROM was reduced in comparison to intact and TDR + natural facets (P < .05).

Conclusions

The TFR with TDR was able to restore stability to the lumbar segment after bilateral facetectomy, while allowing near-normal motions in all planes.

L5 - s1 segmental kinematics after facet arthroplasty

Background

Facet arthroplasty is a motion restoring procedure. It is normally suggested as an alternative to rigid fixation after destabilizing decompression procedures in the posterior lumbar spine. While previous studies have reported successful results in reproducing normal spine kinematics after facet replacement at L4-5 and L3-4, there are no data on the viability of facet replacement at the lumbosacral joint. The anatomy of posterior elements and the resulting kinematics at L5-S1 are distinctly different from those at superior levels, making the task of facet replacement at the lumbosacral level challenging. This study evaluated the kinematics of facet replacement at L5-S1.

Methods

Six human cadaveric lumbar spines (L1-S1, 46.7 ± 13.0 years) were tested in the following sequence: (1) intact (L1-S1), (2) complete laminectomy and bilateral facetectomy at L5-S1, and (3) implantation of TFAS-LS (Lumbosacral Total Facet Arthroplasty System, Archus Orthopedics, Redmond, Washington) at L5-S1 using pedicle screws. Specimens were tested in flexion (8Nm), extension (6Nm), lateral bending (LB, ± 6Nm), and axial rotation (AR, ± 5Nm). The level of significance was α = .017 after Bonferroni correction for three comparisons: (1) intact vs. destabilized, (2) destabilized vs. reconstructed, and (3) intact vs. reconstructed.

Results

Laminectomy-facetectomy at L5-S1 increased the L5-S1 angular range of motion (ROM) in all directions. Flexion-extension (F-E) ROM increased from 15.3 ± 2.9 to 18.7 ± 3.5 degrees (P < .017), LB from 8.2 ± 1.8 to 9.3 ± 1.6 degrees (P < .017), and AR from 3.7 ± 2.0 to 5.9 ± 1.8 degrees (P < .017). The facet arthroplasty system decreased ROM compared to the laminectomy-facetectomy condition in all tested directions (P < .017). The facet arthroplasty system restored the L5-S1 ROM to its intact levels in LB and AR (P > .017). F-E ROM after the facet arthroplasty system implantation was smaller than the intact value (10.1 ± 2.2 vs. 15.3 ± 2.9 degrees, P < .017). The load-displacement curves after the facet arthroplasty system implantation at L5-S1 were sigmoidal, and quality of motion measures were similar to intact, demonstrating graded resistance to angular motion in F-E, LB and AR.

Conclusions

The facet arthroplasty system was able to restore stability to the lumbosacral segment after complete laminectomy and bilateral facetectomy, while also allowing near-normal kinematics in all planes. While F-E ROM after the facet arthroplasty system implantation was smaller than the intact value, it was within the physiologic norms for L5-S1. These results are consistent with previous studies of facet arthroplasty at L3-L4 and L4-L5 and demonstrate that TFAS technology can be adapted to the lumbosacral joint with functionality comparable to its application in superior lumbar levels.