Degeneration model of the porcine lumbar motion segment: effects of various intradiscal procedures

Establishment of intervertebral disc degeneration models; A review of the currently used models

One of the frequent causes of low back pain is intervertebral disc degeneration (IDD), which is followed by discogenic pain. Some significant risk factors that have been linked to the onset and progression of IDD include age, mechanical imbalance, changes in nutrition and inflammation. According to recent studies, five types of animal models are established for producing IDD: the spontaneous models, the puncture models, the biomechanical models, the chemical models and the hybrid models. These models are crucial in studying and understanding IDD's natural history and identifying potential treatment targets for IDD. In our study, we'll talk about the technical aspects of these models, the time between model establishment and the apparition of observable degradation, and their potential in various research. Each animal model should be compared to the human natural IDD pathogenesis to guide future research efforts in this area. By improving knowledge and appropriate application of various animal models, we seek to raise awareness of this illness and further translational research.

Combined nucleus pulposus augmentation and annulus fibrosus repair prevents acute intervertebral disc degeneration after discectomy

Tissue-engineered approaches for the treatment of early-stage intervertebral disc degeneration have shown promise in preclinical studies. However, none of these therapies has been approved for clinical use, in part because each therapy targets only one aspect of the intervertebral disc's composite structure. At present, there is no reliable method to prevent intervertebral disc degeneration after herniation and subsequent discectomy. Here, we demonstrate the prevention of degeneration and maintenance of mechanical function in the ovine lumbar spine after discectomy by combining strategies for nucleus pulposus augmentation using hyaluronic acid injection and repair of the annulus fibrosus using a photocrosslinked collagen patch. This combined approach healed annulus fibrosus defects, restored nucleus pulposus hydration, and maintained native torsional and compressive stiffness up to 6 weeks after injury. These data demonstrate the necessity of a combined strategy for arresting intervertebral disc degeneration and support further translation of combinatorial interventions to treat herniations in the human spine.

Efficacy of HYADD®4-G single intra-discal injections in a rabbit model of intervertebral disc degeneration

BACKGROUND

Various biomaterials/technologies have been tested for treatment of intervertebral disc (IVD) degeneration (IDD). Only few non-surgical options exist.

OBJECTIVE

Assessment of efficacy and safety of the hyaluronic acid derivative hydrogel HYADD®4-G in IDD using a well-established rabbit annular puncture model.

METHODS

Rabbits were punctured at two IVDs to induce IDD. Thirty days after, IVDs were injected with HYADD®4-G or saline. IVD hydration, height, appearance and tissue organization were assessed by radiographs, MRI and histopathology. Safety of HYADD®4-G injection was evaluated in non-punctured IVDs.

RESULTS

HYADD®4-G injection restored disc height to over 75% of the pre-punctured disc, saline injections led to 50% of initial disc height. Compared to saline, HYADD®4-G treatment resulted in improved water retention as revealed by MRI quantification. 83.3% of HYADD®4-G injected discs had normal appearance and reached grade I of the Pfirrmann scale. Regarding tissue organization and cellularity, HYADD®4-G treatment resulted in significantly lower IDD scores than saline (p < 0.01). HYADD®4-G injected into healthy IVDs did not induce inflammation or foreign body reactions.

CONCLUSIONS

Intra-discal HYADD®4-G injection is safe and has therapeutic benefits: IDD could be limited through restoration of disc height and hydration and maintenance of normal IVD tissue organization.

Longitudinal Comparison of Enzyme- and Laser-Treated Intervertebral Disc by MRI, X-Ray, and Histological Analyses Reveals Discrepancies in the Progression of Disc Degeneration: A Rabbit Study

Regenerative medicine is considered an attractive prospect for the treatment of intervertebral disc (IVD) degeneration. To assess the efficacy of the regenerative approach, animal models of IVD degeneration are needed. Among these animal models, chemonucleolysis based on the enzymatic degradation of the Nucleus Pulposus (NP) is often used, but this technique remains far from the natural physiopathological process of IVD degeneration. Recently, we developed an innovative animal model of IVD degeneration based on the use of a laser beam. In the present study, this laser model was compared with the chemonucleolysis model in a longitudinal study in rabbits. The effects of the treatments were studied by MRI (T2-weighted signal intensity (T2wsi)), radiography (IVD height index), and histology (NP area and Boos' scoring). The results showed that both treatments induced a degeneration of the IVD with a decrease in IVD height and T2wsi as well as NP area and an increase in Boos' scoring. The enzyme treatment leads to a rapid and acute process of IVD degeneration. Conversely, laser radiation induced more progressive and less pronounced degeneration. It can be concluded that laser treatment provides an instrumental in vivo model of slowly evolving IVD degenerative disease that can be of preclinical relevance for assessing new prophylactic biological treatments of disc degeneration.

Minimal invasive annulotomy for induction of disc degeneration and implantation of poly (lactic-co-glycolic acid) (PLGA) plugs for annular repair in a rabbit model

Background

The rabbit disc model is useful for the study of intervertebral disc (IVD) degeneration and experimental therapeutic interventions. The annulotomy-induced disc models present several drawbacks, particularly an excessive disruption of disc integrity and a rapidly disc degeneration; therefore, this study sought to establish a minimal invasive annulotomy for induction of disc degeneration model, combined to annulus repair using implantation of a PLGA (poly lactic-co-glycolic acid) plug.

Methods

New Zealand white rabbits (n = 24) received annular injuries in three discs (L3/4, L4/5 and L5/6). The experimental discs were randomly assigned to four groups: (a) annular defect with a 1.8 mm diameter mini-trephine; (b) annular puncture by 16G needle; (c) annular defect with a PLGA plug implanted by press-fit fashion; (d) uninjured L2/3 disc served as control. Postsurgical x-ray, MRI examination, and real-time PCR analysis were performed at 1, 3 and 6 months. Gross morphology and histology were evaluated at postoperative 6 months.

Results

Radiographic examinations showed a slow, progressive disc space narrowing and a significant degree of disc degeneration on MRI grade in the injured discs at 6 months in all rabbits. Histological examinations and aggrecan, Col1A1, Col2A1 and matrix metalloprotease (MMP)-3 mRNA expression confirmed the disc degeneration, supporting the imaging results. The PLGA implantation reduced the marked loss of T2-weighted signal intensity seen at MRI in the injured discs and slowly decreased the disc height index (DHI) over the follow-up period. HE/Safranin O staining showed that annular defect was replaced by regenerated connective tissue with significant loss of proteoglycan content.

Conclusions

The minimally invasive approach for the creation of annular defects by an appropriately sized mini-Trephine is a suitable option for the study of disc degeneration in a rabbit model. Implantation of a suitable PLGA plug induced a successful repair of the annulus fibrosus within the degenerated disc, and retarded the degenerative process in the annular injury model.

ISSLS Prize winner: Mechanical influences in progressive intervertebral disc degeneration

STUDY DESIGN

Mechanical study on cadaver motion segments.

OBJECTIVE

To determine whether high gradients of compressive stress within the intervertebral disc are associated with progressive disc degeneration.

SUMMARY OF BACKGROUND DATA

Mechanical loading can initiate disc degeneration but may be unimportant in disease progression, because degenerative changes cause the disc to be increasingly "stress-shielded" by the neural arch. However, the most typical feature of advanced disc degeneration (delamination and collapse of the annulus) may not depend on absolute values of compressive stress but on gradients of compressive stress that act to shear annulus lamellae.

METHODS

A total of 191 motion segments (T7-T8 to L5-S1) were dissected from 42 cadavers aged 19 to 92 years. Each was subjected to approximately 1 kN compression, while intradiscal stresses were measured by pulling a pressure transducer along the disc's midsagittal diameter. "Stress gradients" in the annulus were quantified as the average rate of increase in compressive stress (MPa/mm) between the nucleus and the region of maximum stress in the anterior or posterior annulus. Measurements were repeated before and after creep loading and in simulated flexed and erect postures. Disc degeneration was assessed macroscopically on a scale of 1 to 4.

RESULTS

As grade of disc degeneration increased from 2 to 4, nucleus pressure decreased by an average 68%, and maximum compressive stress in the annulus decreased by 48% to 64%, depending on location and posture. In contrast, stress gradients in the annulus increased by an average 75% in the anterior annulus (in flexed posture) and by 108% in the posterior annulus (in erect posture). Spearman rank correlation showed that these increases were statistically significant.

CONCLUSION

Despite stress-shielding by the neural arch, gradients of compressive stress increase with increasing grade of disc degeneration. Stress gradients act to shear adjacent lamellae and can explain progressive annulus delamination and collapse.

LEVEL OF EVIDENCE

N/A.

Retroperitoneal approach to the intervertebral disc for the annular puncture model of intervertebral disc degeneration in the rabbit

BACKGROUND CONTEXT

The rabbit annular puncture model of degeneration is among the most widely used models of intervertebral disc (IVD) degeneration. There are no published reports of the specific surgical technique used to produce this model.

PURPOSE

To describe the model in detail in an effort to reduce center-to-center variability and hopefully improve the reproducibility of future experimental results.

STUDY DESIGN

Technical report of surgical approach and experience.

PATIENT SAMPLE

New Zealand White Rabbits.

METHODS

A detailed report of the annular puncture technique in rabbits is provided including preparation of the animals, instrumentation, a description of retroperitoneal approach to the lumbar area, and the technique for IVD injury. Common pitfalls and complications of the procedure are described.

CONCLUSIONS

The methods described can serve to standardize the implementation of this important preclinical model of disc degeneration.

Injection of a polymerized hyaluronic acid/collagen hydrogel matrix in an in vivo porcine disc degeneration model

INTRODUCTION

Disc degeneration and re-herniation after nucleotomy procedures are common problems. Simultaneous application of hyaluronic acid (HA)-based matrix has been proposed to limit disc degeneration. This, however, is hampered by loss of the substituted matrix out of the disc. Hence, in situ polymerization of the injected matrix with ultraviolet light (UVL) directly used after injection may be useful. Therefore, this study evaluates a new HA/collagen hydrogel matrix with in situ polymerization after implantation in an established porcine nucleotomy model.

MATERIALS AND METHODS

12 mature minipigs were used. A total of 60 lumbar discs were analyzed. 36 discs underwent partial nucleotomy with a 16G biopsy needle. Of those, 24 discs received matrix (porcine nucleus pulposus collagenous scaffold component and chemically modified HA) which was in situ polymerized using UVL immediately after transplantation. 12 nucleotomized discs and 24 non-nucleotomized discs served as controls. After 24 weeks, animals were killed. X-rays, MRIs, histology, and gene expression analysis were done.

RESULTS

Disc height was reduced equally after sole nucleotomy and nucleotomy with HA treatment and in MRIs signal intensity decreased. For both nucleotomy groups, the nucleus histo-degeneration score showed a significant increase compared to controls. In histology, HA treatment resulted in more scarring and inflammation in the annulus. Gene expression of catabolic MMPs was up-regulated, whereas IFN-gamma, IL-6, and IL-1b were unchanged.

CONCLUSION

Although nucleotomy and administration of the implant material did not cause generalized inflammation of the disc, localized annular damage with annulus inflammation and scarring resulted in detrimental degenerative disc changes. As a result, therapeutic strategies should strongly focus on the prevention of annular damage or techniques for annular repair to remain disc integrity.

Tissue engineering for intervertebral disk degeneration

Many challenges confront intervertebral disk engineering owing to complexity and the presence of extraordinary stresses. Rebuilding a disk of native function could be useful for removal of the symptoms and correction of altered spine kinematics. Improvement in understanding of disk properties and techniques for disk engineering brings promise to the fabrication of a functional motion segment for the treatment of disk degeneration. Increasing sophistication of techniques available in biomedical sciences will bring its application into clinics. This review provides an account of current progress and challenges of intervertebral disk bioengineering and discusses means to move forward and toward bedside translation.

Methods to monitor distribution and metabolic activity of mesenchymal stem cells following in vivo injection into nucleotomized porcine intervertebral discs

Intervertebral disc (IVD) degeneration involves a series of biochemical and morphological changes leading to loss of spinal stability and flexibility. Cell therapy is promising to reconstitute IVDs with new cells, however, sustained metabolic activity seems crucial for an active contribution to regeneration. The aim of the present study was to establish methods for separate follow up of persistence and activity of autologous porcine mesenchymal stem cells (pMSC) after implantation into IVDs of Goettingen minipigs in vivo in order to conclude about the potential of such an intervention strategy. For quantitative follow up, the transfer matrix was supplemented with Al(2)O(3) particles and pMSC which were retrovirally labeled with firefly luciferase (pMSC-Luc). Six mature Goettingen minipigs underwent matrix based cell transfer after partial nucleotomy of lumbar IVDs (n = 24). Day 0 and day 3 segments were analyzed for retained volume of Al(2)O(3) particles by micro-computed-tomography (muCT) and for cell activity by luciferase enzyme assessment. Three days after injection a reduction of Al(2)O(3) particles (P = 0.028) to about 9% and of pMSC-Luc activity to about 7% of initial values (P = 0.003) was detected, which suggests loss of 90% of the implant material under in vivo conditions without evidence for reduced pMSC-Luc metabolic activity (P = 0.887). In conclusion, separate follow up of implant material and cell activity was possible and unravels problems with in vivo implant persistence after annular puncture rather than quick loss of cell activity. Therefore, IVD-regeneration-strategies should increasingly focus on annulus reconstruction in order to reduce implant loss due to annular failure.

A new porcine in vivo animal model of disc degeneration: response of anulus fibrosus cells, chondrocyte-like nucleus pulposus cells, and notochordal nucleus pulposus cells to partial nucleotomy

STUDY DESIGN

In vivo animal study.

OBJECTIVES

To describe a new porcine disc degeneration model, and to analyze disc remodeling and degeneration after nucleotomy with special view to the different nucleus pulposus (NP) cell types.

SUMMARY OF BACKGROUND DATA

Thus far, predominantly smaller animals were used for disc degeneration models; however, such small discs were inappropriate to investigate cell implementation therapies. Though notochordal cells (NCs) are important for disc formation and maintenance, differences in the amount of NCs between human and animal discs have often been neglected.

METHODS

Twenty-four Goettingen minipigs underwent partial nucleotomy with a 16G biopsy cannula, to remove approximately 10% of total NP volume. Animals were followed up for 3, or 24 weeks and analyzed by radiographs, MRIs, (immuno)histology, gene expression analysis, and biomechanical testing.

RESULTS

Three weeks after nucleotomy disc height was reduced by 26%, and magnetic resonance imaging signal intensity by 40%. At 24 weeks disc height was decreased by 32%. Increased degenerative changes were found in a histodegeneration score 3 and 24 weeks after nucleotomy, as well as considerable NP scarification after 3 weeks. In controls, cytokeratin-8 immunohistochemistry identified NCs in proximity to chondrocyte-like NP cells at approximately equal ratio. After nucleotomy, NCs were considerably reduced to <10% of total NP cells. Matrix genes were upregulated, except for aggrecan that decreased to 35% of initial values 3 weeks after nucleotomy. Matrix degrading factors (matrix metalloproteinases 13 and 3) were continuously upregulated, whereas transcripts of their inhibitors (tissue inhibitors of matrix metalloproteinase 2 and 3) were downregulated. No significant changes in segmental spinal flexibility or bone density were found after nucleotomy.

CONCLUSION

We introduced a new disc degeneration model with relatively large discs that could be used for cell therapeutic approaches. The study gives further information about disc remodeling after nucleotomy and indicates the relevance of an altered cellular composition for the development of disc degeneration.

Stress distribution in the intervertebral disc correlates with strength distribution in subdiscal trabecular bone in the porcine lumbar spine

BACKGROUND

It is understood that an interdependence of properties exists between the intervertebral disc and the subdiscal trabecular bone. Determining the biomechanics of this relationship is important in the development of novel spinal implants and instruments. The aim of this study was to analyze this relationship for the porcine lumbar spine and to compare it with that of the human spine.

METHODS

The stress distribution within the intervertebral disc of 10 porcine lumbar (L4/L5) motion segments was recorded using a 1.5mm needle pressure transducer. For dynamic loading a specialized testing rig was developed to apply flexion/extension and medial/lateral bending while intervertebral disc stress was simultaneously recorded. The regional variation in mechanical properties of trabecular bone was also examined for an additional 10 porcine (L5) vertebral bodies. For compressive testing of the subdiscal bone, columns were prepared using a low speed cutting saw and subjected to axial compression.

FINDINGS

Under pure compressive loading, stress levels within the intervertebral disc were relatively uniform. However, during asymmetric loading large peak stresses were evident in the periphery of the intervertebral disc in areas underlying the annulus fibrosus. The mechanical properties of trabecular bone demonstrated regional variations within the vertebral body. The ratio of compressive yield strength of bone underlying the outer annulus fibrosus to that of bone underlying the nucleus pulposus averaged 1.36.

INTERPRETATION

Although the effects of stress distribution and bone mass adaptation cannot be directly related, it is probable that peak stresses arising in the annulus fibrosus during asymmetric loading provide greater stimuli for the underlying bone to undergo adaptive remodeling to withstand the greater forces experienced. Findings of intervertebral stress distribution and strength distribution of subdiscal trabecular bone for the porcine spine may aid in the development of strategies for preclinical animal testing of spinal implants.

Intermediate Screws in Short Segment Pedicular Fixation for Thoracic and Lumbar Fractures

To determine the effect of adding pedicle screws at the level of a burst fracture (intermediate screws) on the stiffness of a short segment pedicle fixation, an in vitro biomechanical study was carried out. Six fresh-frozen pig lumbar spine specimens were used. The flexibility of the intact specimens was examined in flexion, extension, lateral bending, and torsion. An unstable burst fracture model was created by the dropped-mass technique. The unstable spine specimens were instrumented with pedicle screws. The flexibility was tested again with and without intermediate screws. The addition of intermediate screws provided a smaller range of motion in flexion-extension (P<0.001), torsion (P<0.001), and lateral bending (P=0.014). The slopes of the load displacement curves increased in flexion (P<0.001), extension (P=0.003), lateral bending (P=0.003), and torsion (P=0.006), signifying a decrease in flexibility. The addition of intermediate screws at the level of a burst fracture significantly increases the stiffness of a short segment pedicular fixation.

Outer annulus tears have less effect than endplate fracture on stress distributions inside intervertebral discs: relevance to disc degeneration

BACKGROUND

Annulus tears and endplate fracture are common lesions in human intervertebral discs. Both cause degenerative changes in animal models, but the time course appears to be different. The purpose of the present experiment is to compare the effects of outer annulus tears and endplate fracture on intradiscal stresses. We hypothesise that endplate fracture provides a greater stimulus for disc degeneration.

METHODS

Seven cadaveric lumbar "motion segments" aged 49-70 years were compressed at 2 kN while the distribution of compressive stress was measured in each disc by pulling a 1.3 mm-diameter pressure transducer along its mid-sagittal diameter. Measurements were repeated after rim tears were simulated by 10 mm-deep scalpel cuts into the outer anterior annulus. The first cut was horizontal, 15 mm to the right of the disc midline, near the junction with the upper endplate. The second cut was vertical, 15 mm to the left of the disc midline, at mid-disc height. The third cut was horizontal, in the disc midline and at mid-disc height, so that the cut passed through the needle hole of the pressure transducer. Stress profiles were recorded in three postures and at two load levels, after each cut. Stress measurements were repeated a final time following compressive overload sufficient to fracture the endplate.

FINDINGS

Outer annulus tears had negligible effect on compressive stress distributions in the annulus fibrosus more than 15 mm from the scalpel cut, and they caused nucleus pressure to fall by only 1% (SD 1.3%, NS). In contrast, endplate fracture reduced nucleus pressure by 37% (P=0.004) and increased maximum stress within the posterior annulus by 93% (P=0.033).

INTERPRETATION

Outer annulus tears have less (immediate) effects on intradiscal compressive stresses than endplate fracture, supporting our hypothesis.

Animal models for human disc degeneration

Despite the significant impairment associated with degenerative disc disease, a clear understanding of its pathogenesis is still lacking. Currently, no particular model parallels the complex nature of human disc degeneration. Naturally occurring animal models have the drawback that the basis for the high rate of disc degeneration is not known. Although the interventions in artificial animal models that create disc degeneration are known, the relationship of those to the events leading to disc degeneration in humans is not. With the recent progress in biomechanics, cell biology and molecular biology, an easily reproducible and valid animal model may help unlock the complex cascade of events surrounding human disc degeneration.

Spine biomechanics

Current trends in spine research are reviewed in order to suggest future opportunities for biomechanics. Recent studies show that psychosocial factors influence back pain behaviour but are not important causes of pain itself. Severe back pain most often arises from intervertebral discs, apophyseal joints and sacroiliac joints, and physical disruption of these structures is strongly but variably linked to pain. Typical forms of structural disruption can be reproduced by severe mechanical loading in-vitro, with genetic and age-related weakening sometimes leading to injury under moderate loading. Biomechanics can be used to quantify spinal loading and movements, to analyse load distributions and injury mechanisms, and to develop therapeutic interventions. The authors suggest that techniques for quantifying spinal loading should be capable of measurement "in the field" so that they can be used in epidemiological surveys and ergonomic interventions. Great accuracy is not required for this task, because injury risk depends on tissue weakness as much as peak loading. Biomechanical tissue testing and finite-element modelling should complement each other, with experiments establishing proof of concept, and models supplying detail and optimising designs. Suggested priority areas for future research include: understanding interactions between intervertebral discs and adjacent vertebrae; developing prosthetic and tissue-engineered discs; and quantifying spinal function during rehabilitation. "Mechanobiology" has perhaps the greatest future potential, because spinal degeneration and healing are both mediated by the activity of cells which are acutely sensitive to their local mechanical environment. Precise characterisation and manipulation of this environment will be a major challenge for spine biomechanics.

Limitations of the cervical porcine spine in evaluating spinal implants in comparison with human cervical spinal segments: a biomechanical in vitro comparison of porcine and human cervical spine specimens with different instrumentation techniques

STUDY DESIGN

Porcine and human cervical spine specimens were in vitro biomechanically compared with different instrumentation techniques.

OBJECTIVES

To evaluate whether subaxial porcine cervical spines are a valid model for implant testing in a single level corpectomy.

SUMMARY OF BACKGROUND DATA

Biomechanical in vitro tests are widely used for implant tests, mainly with human spine specimens. The availability of human cadavers is limited and the properties of the specimen regarding age, bone mineral density, and grade of degenerative changes is inhomogeneous.

METHODS

Six porcine and six human cervical specimens were loaded nondestructively with pure moments: 1) in an intact state; 2) after a corpectomy of C5 and substitution by a cage with integrated force sensor; 3) after additional instrumentation with a posterior screw and rod system with: a) lateral mass and b) pedicle screws; 4) after instrumentation with an anterior plate; and 5) with a circumferential instrumentation. The unconstrained motion and the axial loads occurring in the corpectomy gap were measured, as well as the bone mineral density of the specimen before testing.

RESULTS

The range of motion in the intact state, as well as for the different instrumentations, was comparable for flexion-extension. In lateral bending and axial rotation, marked differences in the intact state as well as for pedicle screw instrumentations occurred.

CONCLUSIONS

The subaxial porcine cervical spine is a potential model in flexion-extension because of its biomechanical similarity. For lateral bending and axial rotation, the marked differences severly restrict the comparability.

A fibronectin fragment stimulates intervertebral disc degeneration in vivo

STUDY DESIGN

A radiographic, histologic, biochemical, and gene expression study was conducted in vivo in a rabbit model to determine the effect of injection of the N-terminal 30 kDa fibronectin fragment (Fn-f) into the intervertebral disc along with various control substances.

OBJECTIVE

To determine if the Fn-f is able to induce disc degeneration in vivo.

SUMMARY OF BACKGROUND DATA

Animal models of disc degeneration are crucial to defining the molecular events involved in disc degeneration. Although spontaneous and induced models of disc degeneration have been described, none is ideal for molecular studies. A better understanding of disc degeneration at the molecular level is necessary to promote rational design of therapies for degenerative disc disease.

MATERIALS AND METHODS

Thirty-one New Zealand white rabbits underwent injection of Fn-f and control substances into the central region of separate lumbar discs using a fine needle. Euthanasia was performed at the 2-, 4-, 8-, 12-, and 16-week time points and the discs were examined radiographically, histologically, biochemically, and with gene expression.

RESULTS

Radiographs demonstrated anterior osteophyte formation at Fn-f-injected disc spaces by the 12-week time point. Histology demonstrated a progressive loss of the normal architecture of the nucleus pulposus and anulus fibrosus over the 16-week study period. A progressive loss of proteoglycans was documented using GAG assay but total collagen did not appear to change appreciably. Gene expression studies demonstrated a significant down-regulation of both aggrecan and type II collagen mRNA between the 8- and 16-week time points.

CONCLUSION

Fn-f appears to induce a progressive degenerative process within the intervertebral disc after injection that resembles degenerative disc disease. This model has several significant advantages for the study of disc degeneration at the molecular level. Further studies are warranted to elucidate the mechanism and molecular events associated with Fn-f-mediated disc degeneration.

Mechanical initiation of intervertebral disc degeneration

STUDY DESIGN

Mechanical testing of cadaveric lumbar motion segments.

OBJECTIVES

To test the hypothesis that minor damage to a vertebral body can lead to progressive disruption of the adjacent intervertebral disc.

SUMMARY OF BACKGROUND DATA

Disc degeneration involves gross structural disruption as well as cell-mediated changes in matrix composition, but there is little evidence concerning which comes first. Comparatively minor damage to a vertebral body is known to decompress the adjacent discs, and this may adversely affect both structure and cell function in the disc.

METHODS

In this study, 38 cadaveric lumbar motion segments (mean age, 51 years) were subjected to complex mechanical loading to simulate typical activities in vivo while the distribution of compressive stress in the disc matrix was measured using a pressure transducer mounted in a needle 1.3 mm in diameter. "Stress profiles" were repeated after a controlled compressive overload injury had reduced motion segment height by approximately 1%. Moderate repetitive loading, appropriate for the simulation of light manual labor, then was applied to the damaged specimens for approximately 4 hours, and stress profilometry was repeated a third time. Discs then were sectioned and photographed.

RESULTS

Endplate damage reduced pressure in the adjacent nucleus pulposus by 25% +/- 27% and generated peaks of compressive stress in the anulus, usually posteriorly to the nucleus. Discs 50 to 70 years of age were affected the most. Repetitive loading further decompressed the nucleus and intensified stress concentrations in the anulus, especially in simulated lordotic postures. Sagittal plane sections of 15 of the discs showed an inwardly collapsing anulus in 9 discs, extreme outward bulging of the anulus in 11 discs, and complete radial fissures in 2 discs, 1 of which allowed posterior migration of nucleus pulposus. Comparisons with the results from tissue culture experiments indicated that the observed changes in matrix compressive stress would inhibit disc cell metabolism throughout the disc, and could lead to progressive deterioration of the matrix.

CONCLUSIONS

Minor damage to a vertebral body endplate leads to progressive structural changes in the adjacent intervertebral discs.

Stress distributions inside intervertebral discs: the validity of experimental "stress profilometry'

This paper evaluates a technique for measuring the distribution of compressive stress within cadaveric intervertebral discs. A strain-gauged pressure transducer, side-mounted near the tip of a 1.3 mm diameter needle, was inserted into cubes of disc tissue and into intact discs. Regardless of the position and orientation of the transducer within the tissue or disc, its output was found to be proportional to the compressive force applied to the specimen. The distribution of compressive stress was measured by pulling the instrumented needle through the specimen and the resulting stress profiles were reproducible to within 20 per cent. Profiles obtained at different applied loads showed a similar distribution of stress within the disc, suggesting that the compressive stress at any location and direction increased in proportion to the applied load. Since transducer output was also proportional to applied load, it was reasoned that it must be proportional to compressive stress within the disc. The average vertical compressive stresses acting on various regions within a disc were calculated from the stress profiles and multiplied by the cross-sectional area of each region: the resulting force was then compared with the known applied force in order to assess the calibration coefficient of the transducer. Agreement between the two forces was good, indicating that the calibration coefficient established in a saline bath was applicable to disc tissues also. However, artifactual stress peaks could be generated if the transducer was pulled across a bony asperity. It is concluded that the transducer measures the mean compressive stress acting upon it within disc tissues. Errors associated with the technique are small compared to differences in stress distributions which occur naturally, for example when intervertebral discs are loaded to simulate different postures in a living person.

Sustained loading generates stress concentrations in lumbar intervertebral discs

STUDY DESIGN

Cadaveric motion segment experiment. Measurements on each specimen were compared before and after creep loading.

OBJECTIVES

To show how sustained "creep" loading affects stress distributions inside intervertebral discs.

SUMMARY OF BACKGROUND DATA

The central region of an intervertebral disc acts like a hydrostatic "cushion" between adjacent vertebrae. However, this property depends on the water content of the tissues and may be lost or diminished after creep.

METHODS

Twenty-seven lumbar motion segments consisting of two vertebrae and the intervening disc and ligaments were loaded to simulate erect standing postures in life. The distribution of compressive stress in the disc matrix was measured by pulling a miniature pressure transducer through the disc in the midsagittal plane. Profiles of vertical and horizontal compressive stress were repeated after each specimen had been creep loaded in compression for 2-6 hours.

RESULTS

Creep reduced the hydrostatic pressure in the nucleus by 13-36%. Compressive stresses in the anulus were little affected when the profiles were measured at 1 kN, but at 2 kN, localized peaks of compressive stress appeared (or grew in size) in the posterior anulus after creep.

CONCLUSIONS

Increased loading of the apophysial joints causes an overall reduction in intradiscal stresses after creep. In addition, water loss from the nucleus causes a transfer of load from nucleus to anulus. Stress concentrations may lead to pain, structural disruption, and alterations in chondrocyte metabolism. Disc mechanics depend on loading history as well as applied load.

Recent advances in lumbar spinal mechanics and their clinical significance

Of the many problems associated with low back pain, those which are most amenable to biomechanical investigation are identified. Recent advances in lumbar spinal mechanics are then reviewed in five sections dealing with mechanical function, mechanisms of failure, movements in vivo, loading in vivo, and the biological consequences of mechanical loading. The discussion suggests that mechanical fatigue damage may frequently be the underlying cause of low back pain, even when degenerative changes are evident in the tissues, and the review ends by suggesting some priority areas for future research.