A simple model for the function of proteoglycans and collagen in the response to compression of the intervertebral disc

Regeneration in Spinal Disease: Therapeutic Role of Hypoxia-Inducible Factor-1 Alpha in Regeneration of Degenerative Intervertebral Disc

The intervertebral disc (IVD) is a complex joint structure comprising three primary components—namely, nucleus pulposus (NP), annulus fibrosus (AF), and cartilaginous endplate (CEP). The IVD retrieves oxygen from the surrounding vertebral body through CEP by diffusion and likely generates ATP via anaerobic glycolysis. IVD degeneration is characterized by a cascade of cellular, compositional, structural changes. With advanced age, pronounced changes occur in the composition of the disc extracellular matrix (ECM). NP and AF cells in the IVD possess poor regenerative capacity compared with that of other tissues. Hypoxia-inducible factor (HIF) is a master transcription factor that initiates a coordinated cellular cascade in response to a low oxygen tension environment, including the regulation of numerous enzymes in response to hypoxia. HIF-1α is essential for NP development and homeostasis and is involved in various processes of IVD degeneration process, promotes ECM in NP, maintains the metabolic activities of NP, and regulates dystrophic mineralization of NP, as well as angiogenesis, autophagy, and apoptosis during IVD degeneration. HIF-1α may, therefore, represent a diagnostic tool for early IVD degeneration and a therapeutic target for inhibiting IVD degeneration

Degeneration alters structure-function relationships at multiple length-scales and across interfaces in human intervertebral discs

Intervertebral disc (IVD) degeneration and associated back pain place a significant burden on the population. IVD degeneration is a progressive cascade of cellular, compositional, and structural changes, which results in a loss of disc height, disorganization of extracellular matrix architecture, tears in the annulus fibrosus which may involve herniation of the nucleus pulposus, and remodeling of the bony and cartilaginous endplates (CEP). These changes to the IVD often occur concomitantly, across the entire motion segment from the disc subcomponents to the CEP and vertebral bone, making it difficult to determine the causal initiating factor of degeneration. Furthermore, assessments of the subcomponents of the IVD have been largely qualitative, with most studies focusing on a single attribute, rather than multiple adjacent IVD substructures. The objective of this study was to perform a multiscale and multimodal analysis of human lumbar motion segments across various length scales and degrees of degeneration. We performed multiple assays on every sample and identified several correlations between structural and functional measurements of disc subcomponents. Our results demonstrate that with increasing Pfirrmann grade there is a reduction in disc height and nucleus pulposus T2 relaxation time, in addition to alterations in motion segment macromechanical function, disc matrix composition and cellular morphology. At the cartilage endplate-vertebral bone interface, substantial remodeling was observed coinciding with alterations in micromechanical properties. Finally, we report significant relationships between vertebral bone and nucleus pulposus metrics, as well as between micromechanical properties of the endplate and whole motion segment biomechanical parameters, indicating the importance of studying IVD degeneration as a whole organ.

Multiscale and multimodal structure-function analysis of intervertebral disc degeneration in a rabbit model

OBJECTIVES

The objective of this study was to perform a quantitative analysis of the structural and functional alterations in the intervertebral disc during in vivo degeneration, using emerging tools that enable rigorous assessment from the microscale to the macroscale, as well as to correlate these outcomes with noninvasive, clinically relevant imaging parameters.

DESIGN

Degeneration was induced in a rabbit model by puncturing the annulus fibrosus (AF) with a 16-gauge needle. 2, 4, 8, and 12 weeks following puncture, degenerative changes in the discs were evaluated via magnetic resonance imaging (MRI), whole motion segment biomechanics, atomic force microscopy, histology and polarized light microscopy, immunohistochemistry, biochemical content, and second harmonic generation imaging.

RESULTS

Following puncture, degeneration was evident through marked changes in whole disc structure and mechanics. Puncture acutely compromised disc macro and microscale mechanics, followed by progressive stiffening and remodeling. Histological analysis showed substantial anterior fibrotic remodeling and osteophyte formation, as well as an overall reduction in disc height, and disorganization and infolding of the AF lamellae into the NP space. Increases in NP collagen content and aggrecan breakdown products were also noted within 4 weeks. On MRI, NP T2 was reduced at all post-puncture time points and correlated significantly with microscale indentation modulus.

CONCLUSION

This study defined the time dependent changes in disc structure-function relationships during IVD degeneration in a rabbit annular injury model and correlated degeneration severity with clinical imaging parameters. Our findings identified AF infolding and occupancy of the space as a principle mechanism of disc degeneration in response to needle puncture, and provide new insights to direct the development of novel therapeutics.

Long-term mechanical function and integration of an implanted tissue-engineered intervertebral disc

Tissue engineering holds great promise for the treatment of advanced intervertebral disc degeneration. However, assessment of in vivo integration and mechanical function of tissue-engineered disc replacements over the long term, in large animal models, will be necessary to advance clinical translation. To that end, we developed tissue-engineered, endplate-modified disc-like angle ply structures (eDAPS) sized for the rat caudal and goat cervical spines that recapitulate the hierarchical structure of the native disc. Here, we demonstrate functional maturation and integration of these eDAPS in a rat caudal disc replacement model, with compressive mechanical properties reaching native values after 20 weeks in vivo and evidence of functional integration under physiological loads. To further this therapy toward clinical translation, we implanted eDAPS sized for the human cervical disc space in a goat cervical disc replacement model. Our results demonstrate maintenance of eDAPS composition and structure up to 8 weeks in vivo in the goat cervical disc space and maturation of compressive mechanical properties to match native levels. These results demonstrate the translational feasibility of disc replacement with a tissue-engineered construct for the treatment of advanced disc degeneration.

Nondestructive, indirect assessment of the biomechanical properties of the rat intervertebral disc using contrast-enhanced μCT

Mechanical characterization of the intervertebral disc involves labor-intensive and destructive experimental methodology. Contrast-enhanced micro-computed tomography is a nondestructive imaging modality for high-resolution visualization and glycosaminoglycan quantification of cartilaginous tissues. The purpose of this study was to determine whether anionic and cationic contrast-enhanced micro-computed tomography of the intervertebral disc can be used to indirectly assess disc mechanical properties in an ex vivo model of disc degeneration. L3/L4 motion segments were dissected from female Lewis rats. To deplete glycosaminoglycan, samples were treated with 0 U/ml (Control) or 5 U/ml papain. Contrast-enhanced micro-computed tomography was performed following incubation in 40% Hexabrix (anionic) or 30 mg I/ml CA⁴⁺ (cationic) for 24 h (n = 10/contrast agent/digestion group). Motion segments underwent cyclic mechanical testing to determine compressive and tensile modulus, stiffness, and hysteresis. Glycosaminoglycan content was determined using the dimethylmethylene blue assay. Correlations between glycosaminoglycan content, contrast-enhanced micro-computed tomography attenuation, and mechanical properties were assessed via the Pearson correlation. The predictive accuracy of attenuation on compressive properties was assessed via repeated random sub-sampling cross validation. Papain digestion produced significant decreases in glycosaminoglycan content and corresponding differences in attenuation and mechanical properties. Attenuation correlated significantly to glycosaminoglycan content and to all compressive mechanical properties using both Hexabrix and CA⁴⁺ . Predictive linear regression models demonstrated a predictive accuracy of attenuation on compressive modulus and stiffness of 79.8-86.0%. Contrast-enhanced micro-computed tomography was highly predictive of compressive mechanical properties in an ex vivo simulation of disc degeneration and may represent an effective modality for indirectly assessing disc compressive properties. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2030-2038, 2018.

Promise, progress, and problems in whole disc tissue engineering

Intervertebral disc degeneration is frequently implicated as a cause of back and neck pain, which are pervasive musculoskeletal complaints in modern society. For the treatment of end stage disc degeneration, replacement of the disc with a viable, tissue-engineered construct that mimics native disc structure and function is a promising alternative to fusion or mechanical arthroplasty techniques. Substantial progress has been made in the field of whole disc tissue engineering over the past decade, with a variety of innovative designs characterized both in vitro and in vivo in animal models. However, significant barriers to clinical translation remain, including construct size, cell source, culture technique, and the identification of appropriate animal models for preclinical evaluation. Here we review the clinical need for disc tissue engineering, the current state of the field, and the outstanding challenges that will need to be addressed by future work in this area.

Translation of an injectable triple-interpenetrating-network hydrogel for intervertebral disc regeneration in a goat model

Degeneration of the intervertebral discs is a progressive cascade of cellular, compositional and structural changes that is frequently associated with low back pain. As the first signs of disc degeneration typically arise in the disc's central nucleus pulposus (NP), augmentation of the NP via hydrogel injection represents a promising strategy to treat early to mid-stage degeneration. The purpose of this study was to establish the translational feasibility of a triple interpenetrating network hydrogel composed of dextran, chitosan, and teleostean (DCT) for augmentation of the degenerative NP in a preclinical goat model. Ex vivo injection of the DCT hydrogel into degenerated goat lumbar motion segments restored range of motion and neutral zone modulus towards physiologic values. To facilitate non-invasive assessment of hydrogel delivery and distribution, zirconia nanoparticles were added to make the hydrogel radiopaque. Importantly, the addition of zirconia did not negatively impact viability or matrix producing capacity of goat mesenchymal stem cells or NP cells seeded within the hydrogel in vitro. In vivo studies demonstrated that the radiopaque DCT hydrogel was successfully delivered to degenerated goat lumbar intervertebral discs, where it was distributed throughout both the NP and annulus fibrosus, and that the hydrogel remained contained within the disc space for two weeks without evidence of extrusion. These results demonstrate the translational potential of this hydrogel for functional regeneration of degenerate intervertebral discs.

STATEMENT OF SIGNIFICANCE

The results of this work demonstrate that a radiopaque hydrogel is capable of normalizing the mechanical function of the degenerative disc, is supportive of disc cell and mesenchymal stem cell viability and matrix production, and can be maintained in the disc space without extrusion following intradiscal delivery in a preclinical large animal model. These results support evaluation of this hydrogel as a minimally invasive disc therapeutic in long-term preclinical studies as a precursor to future clinical application in patients with disc degeneration and low back pain.

Disc Rehydration after Dynamic Stabilization: A Report of 59 Cases

Study Design

A retrospective study investigating decrease in the nucleus pulposus signal intensity or disc height on magnetic resonance imaging (MRI) and disc degeneration.

Purpose

Although a degenerated disc cannot self-regenerate, distraction or stabilization may provide suitable conditions for rehydration and possible regeneration. This study aimed to evaluate clinical outcomes and disc regeneration via MRI in a series of patients with degenerative disc disease (DDD) who underwent lumbar stabilization with a dynamic stabilization system (DSS).

Overview of Literature

A dynamic system provides rehydration during early DDD.

Methods

Fifty-nine patients (mean age, 46.5 years) who undedwent stabilization with DSS for segmental instability (painful black disc) between 2004 and 2014 were retrospectively evaluated. All patients underwent MRI preoperatively and 12 months postoperatively. Intervertebral disc (IVD) degeneration grades at the implanted segment were categorized using the Pfirrmann classification system. Patients were followed for a mean of 6.4 years, and clinical outcomes were based on visual analog scale (VAS) and Oswestry disability index (ODI) scores.

Results

Significant improvements in back pain VAS and ODI scores from before surgery (7 and 68%, respectively) were reported at 6 (2.85 and 27.4%, respectively) and 12 months postoperatively (1.8 and 16.3%, respectively). Postoperative IVD changes were observed in 28 patients. Improvement was observed in 20 patients (34%), whereas progressive degeneration was observed in eight patients (13.5%). Thirty-one patients (52.5%) exhibited neither improvement nor progression. Single Pfirrmann grade improvements were observed in 29% of the patients and two-grade improvements were observed in 5%.

Conclusions

Our observations support the theory that physiological movement and a balanced load distribution are necessary for disc regeneration. We conclude that DSS may decelerate the degeneration process and appears to facilitate regeneration.

Percutaneous Treatment of Herniated Lumbar Discs with Ozone: Investigation of the Mechanisms of Action

PURPOSE

To elucidate the mechanism of action of intradiscal oxygen-ozone therapy for herniated intervertebral disc therapy.

METHODS

Ozone's mechanism of action was investigated using 3 approaches: mathematical models of intervertebral disc space to explore the relationship between disc pressure and volume; ozonolysis experiments using glycosaminoglycans (GAGs) from a Chinese hamster ovary cell line that were similar in composition to GAGs found in human nucleus pulposus; and experiments in which live Yucatan miniature pigs received various concentrations of percutaneous, image-guided intradiscal oxygen-ozone treatment and were examined (after sacrifice) with histology and semiquantitative analysis of disc cytokine concentrations.

RESULTS

Engineering calculations support observations that a small (6%) disc volume reduction can result in considerable (9.84%) intradiscal pressure reduction. Porcine disc histology and Chinese hamster ovary GAG ozonolysis results showed that administered ozone reacted with and fragmented disc proteoglycans, reducing disc volume through disc dehydration. Cytokine analysis of porcine discs found that each of 4 cytokines measured (interleukin [IL]-1β, IL-6, IL-8, and tumor necrosis factor α) increased in concentration after 2 wt% ozone treatment.

CONCLUSIONS

Oxygen-ozone therapy breaks down proteoglycan GAGs that maintain disc osmotic pressure, dehydrating the nucleus pulposus and reducing intervertebral disc volume. This is likely a primary mechanism by which ozone relieves nerve root compression and alleviates herniated disc-related pain. Additionally, 2 wt% ozone appears to interact with intradiscal cytokines, generating an antiinflammatory response that may contribute to symptom improvement.

Anisotropic Multishell Analytical Modeling of an Intervertebral Disk Subjected to Axial Compression

Studies on intervertebral disk (IVD) response to various loads and postures are essential to understand disk's mechanical functions and to suggest preventive and corrective actions in the workplace. The experimental and finite-element (FE) approaches are well-suited for these studies, but validating their findings is difficult, partly due to the lack of alternative methods. Analytical modeling could allow methodological triangulation and help validation of FE models. This paper presents an analytical method based on thin-shell, beam-on-elastic-foundation and composite materials theories to evaluate the stresses in the anulus fibrosus (AF) of an axisymmetric disk composed of multiple thin lamellae. Large deformations of the soft tissues are accounted for using an iterative method and the anisotropic material properties are derived from a published biaxial experiment. The results are compared to those obtained by FE modeling. The results demonstrate the capability of the analytical model to evaluate the stresses at any location of the simplified AF. It also demonstrates that anisotropy reduces stresses in the lamellae. This novel model is a preliminary step in developing valuable analytical models of IVDs, and represents a distinctive groundwork that is able to sustain future refinements. This paper suggests important features that may be included to improve model realism.

Tissue Engineering a Biological Repair Strategy for Lumbar Disc Herniation

The intervertebral disc is a critical part of the intersegmental soft tissue of the spinal column, providing flexibility and mobility, while absorbing large complex loads. Spinal disease, including disc herniation and degeneration, may be a significant contributor to low back pain. Clinically, disc herniations are treated with both nonoperative and operative methods. Operative treatment for disc herniation includes removal of the herniated material when neural compression occurs. While this strategy may have short-term advantages over nonoperative methods, the remaining disc material is not addressed and surgery for mild degeneration may have limited long-term advantage over nonoperative methods. Furthermore, disc herniation and surgery significantly alter the mechanical function of the disc joint, which may contribute to progression of degeneration in surrounding tissues. We reviewed recent advances in tissue engineering and regenerative medicine strategies that may have a significant impact on disc herniation repair. Our review on tissue engineering strategies focuses on cell-based and inductive methods, each commonly combined with material-based approaches. An ideal clinically relevant biological repair strategy will significantly reduce pain and repair and restore flexibility and motion of the spine.

Damping properties of the nucleus pulposus

BACKGROUND

The nucleus pulposus is extremely deformable and it is not uncommon to observe strain amplitudes as large as 12.5% in physiological loading conditions. It has been shown that the nucleus pulposus contributes to the damping properties of the intervertebral disc. The quantification of the damping properties of the nucleus pulposus under physiological large deformations is then a key aspect for its mechanical characterization and for the design of nucleus replacement devices.

METHODS

A specific mechanical device has been developed to encapsulate nucleus pulposus tissues into a deformable and permeable device, while quantifying its water content. The specific damping capacity was defined by dividing the energy loss by the work input. With this device and definition, the specific damping capacity of the bovine coccygeal nucleus pulposus was quantified in large compressive deformations (12.5%) and for frequencies ranging between 10(-2) and 10(1)Hz.

FINDINGS

It is found that the specific damping capacity of the nucleus pulposus of the bovine coccygeal ranged between 18 and 36%. The lowest values of specific damping capacity are found for frequencies corresponding to the dynamics of loads in all day activities such as walking (0.1 to 1Hz).

INTERPRETATION

The nucleus pulposus contributes to dissipate energy under physiological large deformations. However, it seems that the nucleus pulposus is designed so that damping is minimal for frequencies corresponding to moderate daily activities.

The effects of a new shape-memory alloy interspinous process device on the distribution of intervertebral disc pressures in vitro

This study was designed to measure the pressure distribution of the intervertebral disc under different degrees of distraction of the interspinous process, because of a suspicion that the degree of distraction of the spinous process may have a close relationship with the disc load share. Six human cadaver lumbar spine L2-L5 segments were loaded in flexion, neutral position, and extension. The L3-L4 disc load was measured at each position using pressure measuring films. Shape-memory interspinous process implants (SMID) with different spacer heights, ranging in size from 10 to 20 mm at 2 mm increments, were used. It was found that a SMID with a spacer height equal to the distance of the interspinous process in the neutral position can share the biomechanical disc load without a significant change of load in the anterior annulus. An interspinous process stabilizing device (IPD) would not be appropriate to use in those cases with serious spinal stenosis because the over-distraction of the interspinous process by the SMID would lead to overloading the anterior annulus which is a recognized cause of disc degeneration.

Effects of a new shape-memory alloy interspinous process device on pressure distribution of the intervertebral disc and zygapophyseal joints in vitro

OBJECTIVE

To quantify the pressure distribution of lumbar intervertebral discs and zygapophyseal joints with different degrees of distraction of the interspinous processes by using a new shape-memory interspinous process stabilization device, and to research the relationship between changing disc and zygapophyseal joint loads and the degree of distraction of interspinous processes, and thus optimize usage of the implant.

METHODS

Six cadaver lumbar specimens (L(2)-L(5)) were loaded. The loads in disc and zygapophyseal joints were recorded at each L(3-4) disc level. Implants with different spacer heights were then placed by turn and the pressure measurements repeated.

RESULTS

An implant with 10 mm spacer height does not significantly share the load. A 12 mm implant reduces the posterior annulus load, and meanwhile decreases the zygapophyseal joints pressure, but only in extension. A 14 mm implant shares the loads of posterior annulus, nucleus, and zygapophyseal joints in extension and the neutral position, but slightly increases the anterior annulus' load. Though 16-20 mm implants do decrease the loads in the posterior annulus and zygapophyseal joints, the anterior annulus' load was apparently increased.

CONCLUSION

Different degrees of distraction of the interspinous processes lead to different load distribution on the intervertebral disc. The implant tested is not appropriate in cases of serious spinal stenosis because of the contradiction that, while over-distraction of the interspinous processes decreases the posterior annulus and the zygapophyseal joints load and distracts the intervertebral foramina, it leads to a marked increase in the load of the anterior annulus, which is recognized to accelerate disc degeneration.

Dynamic bulging of intervertebral discs in the degenerative lumbar spine

STUDY DESIGN

The effect of postural change on degenerative lumbar discs was quantified using novel kinematic magnetic resonance imaging (kMRI).

OBJECTIVE

The purpose is to describe the bulging of degenerative intervertebral lumbar discs in vivo subjected to different postural loads using a novel kMRI.

SUMMARY OF BACKGROUND DATA

Symptomatic lumbar disc degeneration is a leading cause of pain and disability throughout the world. Over 70% of US citizens will experience a debilitating episode of low back pain. Earlier reports of degenerative disc changes are cadaver studies or are performed with recumbent MRI that eliminates the functional effects of gravity and muscle power. Little data are available on the behavior of degenerative intervertebral discs in vivo under physiologic loads.

METHODS

A total of 513 patients obtained kMRI. Disc bulging beyond the intervertebral space was quantified during upright neutral, flexion, and extension imaging. The degree of intervertebral disc degeneration was correlated using the Pfirrmann Classification.

RESULTS

Moderately degenerated intervertebral discs (grade III and IV) demonstrated greater bulging than mildly degenerated discs (grade II). Severely degenerated discs (grade V) also showed a trend toward greater bulging, but this was not significant. Grade I discs at all levels moved posteriorly in flexion and anteriorly in extension when compared to neutral posture. However, mild to severe (grade II-V) degenerative discs behaved differently in response to postural loads. Extension resulted in significant posterior bulging, while flexion did not demonstrate obvious anterior derangement.

CONCLUSION

Disc bulging increases with the severity of disc degeneration. Grade I discs demonstrate the expected sagittal migration in response to postural load. However, more degenerative discs behave less predictably, and spine extension may result in significant posterior disc bulging. Degenerative changes in the intervertebral disc significantly affect the kinematic patterns under postural load in vivo. kMRI is a useful tool to quantify the kinematic behavior of degenerative intervertertebral discs.

Nucleus pulposus glycosaminoglycan content is correlated with axial mechanics in rat lumbar motion segments

The unique biochemical composition and structure of the intervertebral disc allow it to support load, permit motion, and dissipate energy. With degeneration, both the biochemical composition and mechanical behavior of the disc are drastically altered, yet quantitative relationships between the biochemical changes and overall motion segment mechanics are lacking. The objective of this study was to determine the contribution of nucleus pulposus glycosaminoglycan content, which decreases with degeneration, to mechanical function of a rat lumbar spine motion segment in axial loading. Motion segments were treated with varying doses of Chondroitinase-ABC (to degrade glycosaminoglycans) and loaded in axial cyclic compression-tension, followed by compressive creep. Nucleus glycosaminoglycan content was significantly correlated (p < 0.05) with neutral zone mechanical behavior, which occurs in low load transition between tension and compression (stiffness: r = 0.59; displacement: r = -0.59), and with creep behavior (viscous parameter eta(1): r = 0.34; short time constant tau(1): r = 0.46). These results indicate that moderate decreases in nucleus glycosaminoglycan content consistent with early human degeneration affect overall mechanical function of the disc. These decreases may expose the disc to altered internal stress and strain patterns, thus contributing through mechanical or biological mechanisms to the degenerative cascade.

Trans-Endplate Nucleotomy Increases Deformation and Creep Response in Axial Loading

Knowledge of the functional role of the nucleus pulposus is critical for the development and evaluation of disc treatment strategies to restore mechanical function. While previous motion segment studies have shown that nucleotomy alters disc mechanics, disruption of the annulus fibrosus may have influenced these experiments. The objective of this study was to determine the mechanical role of the nucleus pulposus in support of axial loads via a trans-endplate nucleotomy procedure. Sheep motion segments were randomly assigned to three groups: control, limited nucleotomy, and radical nucleotomy. Mechanical testing consisted of 20 cycles of compression-tension, a 1-h creep, and a slow constant-rate compressive ramp test. Nucleotomy led to increased axial deformations, in particular an elongated neutral zone, a greater range of motion, and altered creep behavior. In general, the elastic properties exhibited a graded response with respect to the amount of nucleus material removed. This graded effect can be attributed to swelling of the nucleus pulposus in the limited nucleotomy group, whereas little swelling was observed in the radical group. The findings of the present study indicate that functional evaluation of nucleus pulposus replacements and disc implants should include range of motion measures (including neutral zone) and viscoelastic creep experiments in addition to considering compressive stiffness.

Dynamic stabilization in the surgical management of painful lumbar spinal disorders

STUDY DESIGN

A literature review.

OBJECTIVE

To evaluate the mechanisms of action and effectiveness of posterior dynamic stabilization devices in the management of painful spinal disorders.

SUMMARY OF BACKGROUND DATA

Dynamic stabilization may provide pain relief by altering the transmission of abnormal loads across the degenerated disc space.

METHODS

A Medline search was conducted.

RESULTS

Articles supporting abnormal load transmission across the disc space and clinical reviews of currently available posterior dynamic systems were included.

CONCLUSIONS

Posterior dynamic stabilization systems may provide benefit comparable to fusion techniques, but without the elimination of movement. Further study is required to determine optimal design and clinical indications.

Biomechanics of nonfusion implants

Although spine fusion is a versatile and effective technique in the treatment of spinal disorders, increased stresses on adjacent unfused levels lead to symptomatic adjacent level degeneration in many patients. The goal of nonfusion devices in spine surgery is to ablate or unload painful structures while preserving segmental motion. The intended performance of nonfusion devices such as disc replacement, nucleus pulposus replacement, and posterior stabilization devices can be understood from the biomechanics of the functional spinal unit in health and disease and the interplay between the motion segment and the device. Implant design issues can also markedly affect performance.

Lumbar disc arthroplasty

Degenerative disc disease is the leading cause of pain and disability in adults in North America, and spinal fusion is the standard treatment. Despite this, it has been discussed among surgeons that (1) spinal fusion deserves reexamination in terms of its long-term consequences and benefits and (2) modern clinical research and development in disc arthroplasty strongly supports its emergence as an alternative. The ability to relieve pain by maintaining motion may be a critical factor in obtaining not only greater pain relief but in preventing adjacent segment degeneration. Early research in arthroplasty devices was promising but cut short. New knowledge in the functional anatomy and biomechanics of the spine has made possible the development of modern arthroplasty devices (eg, Charite Artificial Disc, ProDisc, Maverick device, FlexiCore device) of different constructions and materials (metal-on-plastic, metal-on-metal) and various ranges of motion/mobility that provide a basis for a classification of spinal mode and an assistance in implant selection. Current research also is confronting the critical obstacles of wear and tear and axial compression. Several devices currently are in clinical trials. A detailed review of their characteristics shows the exciting progress of a new treatment era of total disc replacement in spine-lumbar disc arthroplasty.

Why a mechanical disc?

Low back pain secondary to degenerative disc disease is an overwhelming and growing problem in the United States and Western countries. Most degenerative disc disease can be treated nonoperatively. There are, however, substantial numbers of patients who have not benefited from exhaustive nonoperative treatments and subsequently seek surgical solutions to their incapacitating back pain. Lumbar fusion for back pain and/or leg pain associated with degenerative disc disease is considered the gold standard by which other treatments are judged. A challenge to spinal fusion for degenerative disc disease is now being offered in the form of the artificial disc. The implantation of an artificial lumbar disc allows for maintenance or restoration of physiologic movement at affected segments. A major long-term complication of spinal fusion is degeneration of a disc adjacent to the fused segments. Theoretically, the maintenance of motion could minimize development of adjacent disc degeneration as seen with spinal fusion. It is interesting to note that fusion of the hip or knee is not considered a primary procedure, but fusion is a primary procedure for the lumbar spine. Four artificial lumbar discs are discussed in this article. Early results are promising in terms of clinical results and movement, but long-term follow-up clinical trials must be done in order to gain an accurate comparison with spinal fusion. Trials are currently ongoing. The clinical results up to now and the potential for maintaining lumbar mobility throughout life warrant continuation of this surgical procedure.

Altered disc mechanics in mice genetically engineered for reduced type I collagen

STUDY DESIGN

Mechanically test lumbar discs of transgenic mice in compression-tension and torsion.

OBJECTIVES

Determine if a reduction in type I collagen results in decreased disc mechanics.

SUMMARY OF BACKGROUND DATA

Quantitative relationships between disc structure and function would improve the understanding of disc generation and are essential relationships for functional tissue engineering. The reduced type I collagen transgenic mouse has been used in structure-function studies of bone and tendon, but not intervertebral discs. Methods for testing mouse discs have recently been developed, making disc structure-function studies possible.

METHODS

Microradiographed and mechanically tested lumbar discs from control and collagen-reduced mice in both compression-tension and torsion were used. Disc area and polar moment of inertia were determined from radiographic data, stiffness from mechanical data, and apparent modulus from geometric and mechanical data.

RESULTS

Collagen-reduced discs had a larger area and polar moment of inertia compared to controls. The linear and torsional stiffness of collagen-reduced and control discs were not significantly different. Finally, the apparent modulus of collagen-reduced discs was significantly less than controls in compression (73% of control) and torsion (50%).

CONCLUSIONS

Compared to controls, collagen-reduced discs had reduced apparent modulus in both loading directions, suggesting that the transgenic disc tissue was mechanically inferior to controls. These results are consistent with the widely accepted functional role of type I collagen in disc mechanics, and therefore supports the use of transgenic mice to study structure-function relationships of the disc. Future work will focus on quantifying structure-function relationships related to degeneration, as well as those relevant to the design of tissue-engineered disc replacements.

Dynamic stabilization devices in the treatment of low back pain

Soft stabilization has an important role in the treatment of the degenerative lumbar spine. Fusion of one or two motion segments may not make a big difference in the total range of motion of the lumbar spine, but preserving flexibility of a motion segment may prevent adjacent segment disease and may permit disc replacement, even when facet joints need to be excised. If a favorable environment is created in the motion segment by unloading the disc and permitting near normal motion, the disc may be able to repair itself or may supplement the reparative potential of gene therapy. Although soft stabilization seems promising, one should take a cautious approach to any new implant system. An implant for fusion only has to serve a temporary stabilization until fusion has taken place; on the other hand, a soft stabilization system has to provide stability throughout its life. Implant loosening following fusion surgery is common in the presence of pseudarthrosis. After soft stabilization, the implant has to stay anchored to the bone despite allowing movement. This sounds like a daunting task. The flexibility of the implant system, however, should be able to protect it from loosening at the anchor point into the bone. Finally, the soft stabilization system is intended to load-share with the disc and the facet joint only partially and unloads the motion segment. Any mismatch between the kinematics of the implant system and the motion segment, in particular any discrepancy between their IAR, would result in the implant bearing unexpected load at certain ranges of motion. If that happens, it would guarantee an early implant failure or loosening. The need for strict bench testing in the laboratory, therefore, cannot be over-emphasized. The few soft stabilization systems that have had clinical applications so far have produced a clinical outcome comparable to that of fusion. No prospective randomized controlled trial has been reported yet, which is an essential requirement for practice of evidence-based medicine.

Sheep lumbar intervertebral discs as models for human discs

OBJECTIVE

To determine the water content, collagen content and collagen orientation angle in different regions of sheep lumbar discs.

DESIGN

A laboratory study of sheep discs obtained from an abattoir.

METHODS

A total of 21 sheep lumbar discs were obtained from three lumbar spines. Water content was determined by oven drying (60 degrees C) to constant mass. Collagen content was determined by hydroxyproline analysis. Fibre orientation angles were determined by X-ray diffraction.

RESULTS

Water content increased from 74% of total tissue mass in the outer annulus, to 82% in the inner annulus, to 86% in the nucleus. Collagen content decreased from 30% of total tissue mass in the outer region to 20% in the inner region of the anterior and lateral annulus; it was 16% in the posterior annulus. The orientation angle of the collagen fibres decreased from 59 degrees in the outer region to 56 degrees in the inner region of the anterior and lateral annulus; it was 51 degrees in the posterior annulus.

CONCLUSIONS

Sheep lumbar intervertebral discs provide a reasonable model for human lumbar intervertebral discs.

RELEVANCE

Sheep lumbar discs have been used to investigate the effects of removing and replacing the nucleus. These studies indicate that removal of nucleus may lead to further disc degeneration and indicate the material properties required for an implant material. The relevance of these previous studies is increased if human and sheep lumbar discs have a similar composition and structure.

Changes in posterior disc bulging and intervertebral foraminal size associated with flexion-extension movement: a comparison between L4-5 and L5-S1 levels in normal subjects

BACKGROUND CONTEXT

No previous study has used magnetic resonance imaging (MRI) to evaluate changes of posterior disc bulging and intervertebral foraminal size in the normal spine with flexion-extension movement, comparing L4-5 versus L5-S1 intervertebral levels.

PURPOSE

To determine changes in posterior disc bulging and intervertebral foraminal size with flexion-extension movement, comparing L4-5 versus L5-S1 intervertebral levels.

STUDY DESIGN

An in vivo study of magnetic resonance kinematics with spine flexion extension.

METHODS

Spines of three volunteers with no history of low back pain were scanned in neutral, flexion, and extension positions in a vertically open MRI system. MRI was repeated after 6 hours of normal activity and an additional 4 hours of heavy activity with a weighted vest. Posterior bulging of the intervertebral disc and the size of intervertebral foramen were measured at the L4-5 and L5-S1 levels.

RESULTS

With spine flexion, posterior bulging of the discs increased at L4-5 in eight of nine measurements (three different spine-loading states for each of three subjects) and L5-S1 discs in six of nine measurements. In most cases, posterior bulging decreased with extension. No significant difference was noted in the degree of disc bulge between levels. Foraminal size at L4-5 increased with flexion and decreased with extension, and the extent of these changes was greater at the L4-5 level than at L5-S1.

CONCLUSIONS

This pilot study demonstrates two distinct behavior characteristics of the normal spine with flexion-extension movement.

Effect of removing the nucleus pulposus on the deformation of the annulus fibrosus during compression of the intervertebral disc

Eighteen frozen ovine discs were bisected, in the mid-sagittal plane, to produce 36 specimens. The cut surfaces were marked at the inner and outer annulus boundaries of the annulus fibrosus, both anteriorly and posteriorly, with Alcian blue stain. The sections were sealed by a transparent plate, and thawed. A compression of 1mm at a rate of 0.2mms(-1) was applied. The displacements of the Alcian blue marks were measured from the video images, recorded during the tests, using interactive image analysis software. Before removal of the nucleus, the inner boundaries of the annulus moved outwards during compression (P<0.001, anterior; P=0.01, posterior). However, after removal of the nucleus, both inner boundaries moved inwards (P<0.001, anterior and posterior). The outer boundaries moved outwards both before and after removal of the nucleus (P<0.001). The results showed that total removal of the nucleus changes the response of the annulus to compression.

Localized stresses in the intervertebral disc resulting from a loose fragment. A theory for fissure and fragment

STUDY DESIGN

A theoretical analysis of estimate the stresses generated in the anulus fibrosus by a loose fragment.

OBJECTIVES

To test the hypothesis that a fragment can generate stress concentrations in an intervertebral disc that could lead to the progression of a fissure, and to provide a theoretical foundation for the laboratory model of lumbar disc protrusion, which proposes that a fragment is formed first and that prolapse is the final event of a chronic process.

SUMMARY OF BACKGROUND DATA

A laboratory model was developed by Brinckmann and Porter in 1994, which indicated that introducing a fragment into an otherwise intact disc resulted in rapid failure of the disc under physiologic loads and flexion. Identical in vitro surgery with no reintroduction of fragments did not result in failure.

METHODS

A theoretical model was developed in which a spherical fragment was impressed against a plane surface, representing the inner surface of the anulus. The stiffness of the sphere and that of the surface were varied, and the stresses generated in the anulus were calculated as a function of applied load.

RESULTS

The contact stress and the shear stress increased as the stiffness of the fragment approached and exceeded that of the matrix. Increases in stiffness to more than four times that of the matrix resulted in little further increase in stress. Peak shear stresses are developed in the bulk matrix beyond the point of contact between the fragment and the anulus. For relatively small forces on the fragment, these stresses were comparable with those found in diarthrodial joints.

CONCLUSIONS

It is shown that a fragment in an intervertebral disc produces localized areas of increased stress. Fatigue over long periods at these stresses could result in fissuring and premature failure of the tissue.

Proteoglycan expression in the rat temporomandibular joint in response to unilateral bite raise

The vertebrate articular tissue consists of collagen fibers embedded in a ground substance. Collagen resists tensile forces, while proteoglycans in the ground substance provide resilience and resistance to compression. It was hypothesized that unilateral bite raise would induce increasing expression of proteoglycans in TMJ articular tissues. As a test of this hypothesis, six- and nine-week-old Sprague-Dawley rats received unilateral bite-raising appliances bonded to their right upper molars for 4 wks. A group of nine-week-old rats was housed for an additional 4 wks after removal of the appliances they had worn for 4 wks. Proteoglycans that carry abundant chondroitin sulfate and keratan sulfate side-chains, most likely aggrecans, were detected by safranin O in the fibrocartilaginous zone of the condyle in parasagittal sections. A monoclonal antibody against a large chondroitin sulfate proteoglycan related to versican reacted strongly in the surface fibrous layer of the mandibular condyle and moderately in the discs of the treated specimens. Computer quantification for safranin O and anti-versican antibody staining revealed that the average intensities of the treated specimens were significantly higher than those of their corresponding sham-operated controls, and the average intensities of the treatment-reversal specimens had no significant differences from their corresponding sham-operated controls. Thus, unilateral bite raise appeared to have induced an increase in the expression of aggrecan in the condylar cartilage and a proteoglycan related to versican in the TMJ disc and the articular surface of the condyle. The elevated proteoglycan expression is interpreted to suggest that unilateral bite raise leads to an increase in the magnitude of compressive forces in the rat temporomandibular joint.

Migration of the nucleus pulposus within the intervertebral disc during flexion and extension of the spine

STUDY DESIGN

Magnetic resonance images were obtained of the lumbar spines of three volunteers in neutral, flexed, and extended postures.

OBJECTIVES

To measure migration of the nucleus pulposus within the intervertebral disc during flexion and extension of the spine in living people.

SUMMARY OF BACKGROUND DATA

Results of experiments on bisected cadaveric spines have indicated that the nucleus migrates posteriorly during flexion and anteriorly during extension in nondegenerate discs. Degenerate discs may have faults or fissures that result in abnormal motion of the nucleus.

METHODS

Proton density weighted, sagittal, magnetic resonance images were obtained from the lumbar spines of three volunteers. Measurements of the positions of the anterior and posterior margins of the nucleus and of flexion and extension angles were made on tracings of the images corresponding to neutral, flexed, and extended postures.

RESULTS

The observed frequency (22 of 24 measurements) at which the margins of the nucleus migrated in the directions predicted by results of cadaveric studies was significantly greater than the frequency that would be expected by chance (P < 0.001). The two exceptions may be a result of disc degeneration. There was a significant (P < 0.05) linear correlation between the migration of the anterior margin and the flexion-extension angle and a highly significant (P < 0.001) correlation for the posterior margin and the flexion-extension angle.

CONCLUSIONS

Flexion of an intervertebral disc in a living person tends to be accompanied by posteriorly directed migration of the nucleus pulposus within the disc. Extension tends to be accompanied by an anteriorly directed migration.