Kelly R. Wade

How healthy discs herniate: a biomechanical and microstructural study investigating the combined effects of compression rate and flexion.

Study Design. Microstructural investigation of compression-induced disruption of the flexed lumbar disc. Objective. To provide a microstructural analysis of the mechanisms of annular wall failure in healthy discs subjected to flexion and an elevated rate of compression. Summary of Background Data. At the level of the motion segment failure of the disc in compression has been extensively studied. However, at the microstructural level the exact mechanisms of disc failure are still poorly understood, especially in relation to loading posture and rate. Methods. Seventy-two healthy mature ovine lumbar motion segments were compressed to failure in either a neutral posture or in high physiological flexion (10°) at a displacement rate of either 2 mm/min (low) or 40 mm/min (high). Testing at the high rate was terminated at stages ranging from initial wall tearing through to facet fracture so as to capture the evolution of failure up to full herniation. The damaged discs were then analyzed microstructurally. Results. Approximately, 50% of the motion segments compressed in flexion at the high rate experienced annulus or annulus-endplate junction failure, the remainder failed via endplate fracture with no detectable wall damage. The average load to induce disc failure in flexion was 18% lower (P < 0.05) than that required to induce endplate fracture. Microstructural analysis indicated that wall rupture occurred first in the posterior mid-then-outer annulus. Conclusion. Disc wall failure in healthy motion segments requires both flexion and an elevated rate of compression. Damage is initiated in the mid-then-outer annular fibers, this a likely consequence of the higher strain burden in these same fibers arising from endplate curvature. Given the similarity in geometry between ovine and human endplates, it is proposed that comparable mechanisms of damage initiation and herniation occur in human lumbar discs. Level of Evidence: N/A

Micromechanics of annulus–end plate integration in the intervertebral disc

BACKGROUND CONTEXT The intervertebral disc plays a major functional role in the spinal column, providing jointed flexibility and force transmission. The end plate acts as an important structural transition between the hard vertebral tissues and the compliant disc tissues and is therefore a region of potentially high stress concentration. The effectiveness of anchorage of the tough annulus fibers in the end plate will have a major influence on the overall strength of the motion segment. Failure of the end plate region is known to be associated with disc herniation. PURPOSE The aim of this study was to investigate the mechanism of anchorage of the annular fibers in the end plate. STUDY DESIGN A microstructural analysis of the annulus-end plate region was carried out using motion segments obtained from the lumbar spines of mature ovine animals. METHODS Motion segments were fixed and then decalcified. Samples incorporating the posterior annulus-end plate were then removed and cryosectioned along the plane of one of the lamellar fiber directions to obtain oblique interlamellar sections. These sections were imaged in their fully hydrated state using differential interference contrast optical microscopy. RESULTS The annular fiber bundles on entering the end plate are shown to subdivide into subbundles to form a three-dimensional multileaf morphology with each leaf separated by cartilaginous end plate matrix. This branched morphology increases the interface area between bundle and matrix in proportion to the number of subbundles formed. CONCLUSIONS Given both the limited thickness of the end plate and the intrinsic strength of the interface bond between bundle and end plate matrix, the branched morphology is consistent with a mechanism of optimal shear stress transfer wherein a greater strength of annular fiber anchorage can be achieved over a relatively short insertion distance.

ISSLS Prize Winner: A Detailed Examination of the Elastic Network Leads to a New Understanding of Annulus Fibrosus Organization.

Study Design. Investigation of the elastic network in disc annulus and its function. Objective. To investigate the involvement of the elastic network in the structural interconnectivity of the annulus and to examine its possible mechanical role. Summary of Background Data. The lamellae of the disc are now known to consist of bundles of collagen fibers organized into compartments. There is strong interconnectivity between adjacent compartments and between adjacent lamellae, possibly aided by a translamellar bridging network, containing blood vessels. An elastic network exists across the disc annulus and is particularly dense between the lamellae, and forms crossing bridges within the lamellae. Methods. Blocks of annulus taken from bovine caudal discs were studied in either their unloaded or radially stretched state then fixed and sectioned, and their structure analyzed optically using immunohistology. Results. An elastic network enclosed the collagen compartments, connecting the compartments with each other and with the elastic network of adjacent lamellae, formed an integrated network across the annulus, linking it together. Stretching experiments demonstrated the mechanical interconnectivities of the elastic fibers and the collagen compartments. Conclusion. The annulus can be viewed as a modular structure organized into compartments of collagen bundles enclosed by an elastic sheath. The elastic network of these sheaths is interconnected mechanically across the entire annulus. This organization is also seen in the modular structure of tendon and muscle. The results provide a new understanding annulus structure and its interconnectivity, and contribute to fundamental structural information relevant to disc tissue engineering and mechanical modeling. Level of Evidence: N/A

Surprise Loading in Flexion Increases the Risk of Disc Herniation Due to Annulus-Endplate Junction Failure: A Mechanical and Microstructural Investigation.

Study Design. Microstructural investigation of compression-induced herniation of the flexed lumbar disc. Objective. To provide a microstructural analysis of the mechanisms of annular wall failure in healthy discs subjected to flexion and a rate of compression comparable with the maximum rate at which the muscles of the spinal column can generate a force. Summary of Background Data. Clinical evidence indicates the involvement of the endplate in herniation. It is known that both an elevated rate of compression and a flexed posture are necessary to cause disc failure either within the midspan of the annulus or at the annular-endplate interface. However, the question of what effect a sudden or “surprise” loading might have on the mode of failure is, as yet, unanswered. Methods. Twenty-four healthy mature ovine lumbar motion segments were compressed to failure in high physiological flexion (10º). This occurred over approximately 5 mm of crosshead displacement in 0.75 seconds that resulted in a displacement rate of 400 mm/min (defined as a “surprise” rate) and was intended to simulate the maximum rate at which the muscles of the spinal column can generate a force. The damaged discs were then analyzed microstructurally. Results. Fifty-eight percent of discs suffered annular-endplate junction rupture, 25% suffered midspan annular rupture, and the balance of 17% endplate fracture. Microstructural analysis indicated that annular rupture initiated at the endplate apical ridge in the mid-to-outer region of the annulus in both annular-endplate and midspan annulus rupture. Conclusion. Motion segments subjected to a “surprise” loading rate are likely to fail via some form of annular rupture. Failure under such sudden loading occurs mostly via rupture of the annular-endplate junction and is thought to arise from a rate-induced mechanostructural imbalance between the annulus and the endplate. Level of Evidence: N/A

On the Extent and Nature of Nucleus-Annulus Integration

Study Design. Mechanical and microstructural assessment of nucleus-annulus integration. Objective. To investigate the existence of structural integration between the nucleus and the inner annulus. Summary of Background Data. The nucleus is often viewed as a hydrostatically functioning entity that is largely separate from its surroundings. The boundary between nucleus and annulus is acknowledged as difficult to define. Methods. Ten-millimeter-thick sagittal slabs were cut from the central region of ovine lumbar discs. The annulus-nucleus transition region was isolated and the resulting samples subjected to transverse tensile loading up to failure. Similar samples were stretched to about 4 to 5 times their original separation and then subjected to microstructural examination to investigate structural integration across the inner annulus-nucleus region. Results. The annulus-nucleus boundary could support an average load of 5.7 N (range, 2–11.5 N). Tensile loading causes the fibrous structure of the nucleus to be drawn into an approximate alignment in the transverse stretch direction with an associated reverse inpulling of the inner annular layers. At high magnification, the horizontally aligned nucleus fibers can be seen to branch and blend with the inner annular structure. Conclusion. The nucleus contains a convoluted but highly structured network of fibers of varying length, which appear to integrate with the inner annulus and confer a significant degree of transverse interconnectivity that can be demonstrated mechanically. This new experimental evidence, together with that from a previous study demonstrating nucleus-endplate connectivity, makes it clear that the nucleus cannot be considered as a separate entity in the disc. We propose that this structural integration provides the nucleus with a form of tethered mobility that supports physiological functions distinct from the primary strength requirements of the motion segment.

On how nucleus-endplate integration is achieved at the fibrillar level in the ovine lumbar disc

The intervertebral disc nucleus has traditionally been viewed as a largely unstructured amorphous gel having little obvious integration with the cartilaginous endplates (CEPs). However, recent work by the present authors has provided clear evidence of structural cohesion across the nucleus‐endplate junction via a distinctive microanatomical feature termed insertion nodes. The aim of this study was to explore the nature of these insertion nodes at the fibrillar level. Specially prepared vertebra‐nucleus‐vertebra composite samples from ovine lumbar motion segments were extended axially and chemically fixed in this stretched state, and then decalcified. Sections taken from the samples were prepared for examination by scanning electron microscopy. A close morphological correlation was obtained between previously published optical microscopic images of the nodes and those seen using low magnification SEM. Progressively high magnifications provided insight into the fibrillar‐level modes of structural integration across the nucleus‐endplate junction. The closely packed fibrils of the CEP were largely parallel to the vertebral endplate and formed a dense, multi‐layer substrate within which the nodal fibrils appeared to be anchored. Our idealised structural model proposes a mechanism by which this integration is achieved. The nodal fibrils, in curving into the CEP, are locked in place within its close‐packed layers of transversely aligned fibrils, and probably at multiple levels. Secondly, there appears to be a subtle interweaving of the strongly aligned nodal fibrils with the multi‐directional endplate fibrils. It is suggested that this structural integration provides the nucleus with a form of tethered mobility that supports physiological functions quite distinct from the primary strength requirements of the disc.

ISSLS Prize Winner: Vibration Really Does Disrupt the Disc: A Microanatomical Investigation.

Study Design. Microstructural investigation of vibration-induced disruption of the flexed lumbar disc. Objective. The aim of the study was to explore micro-level structural damage in motion segments subjected to vibration at subcritical peak loads. Summary of Background Data. Epidemiological evidence suggests that cumulative whole body vibration may damage the disc and thus play an important role in low back pain. In vitro investigations have produced herniations via cyclic loading (and cyclic with added vibrations as an exacerbating exposure), but offered only limited microstructural analysis. Methods. Twenty-nine healthy mature ovine lumbar motion segments flexed 7° and subjected to vibration loading (1300 ± 500 N) in a sinusoidal waveform at 5 Hz to simulate moderately severe physiologic exposure. Discs were tested either in the range of 20,000 to 48,000 cycles (medium dose) or 70,000 to 120,000 cycles (high dose). Damaged discs were analyzed microstructurally. Results. There was no large drop in displacement over the duration of both vibration doses indicating an absence of catastrophic failure in all tests. The tested discs experienced internal damage that included delamination and disruption to the inner and mid-annular layers as well as diffuse tracking of nucleus material, and involved both the posterior and anterior regions. Less frequent tearing between the inner disc and endplate was also observed. Annular distortions also progressed into a more severe form of damage, which included intralamellar tearing and buckling and obvious strain distortion around the bridging elements within the annular wall. Conclusion. Vibration loading causes delamination and disruption of the inner and mid-annular layers and limited diffuse tracking of nucleus material. These subtle levels of disruption could play a significant role in initiating the degenerative cascade via micro-level disruption leading to cell death and altered nutrient pathways. Level of Evidence: 5

Differential Response of Bovine Mature Nucleus Pulposus and Notochordal Cells to Hydrostatic Pressure and Glucose Restriction

Objective The nucleus pulposus of the human intervertebral disc contains 2 cell types: notochordal (NC) and mature nucleus pulposus (MNP) cells. NC cell loss is associated with disc degeneration and this process may be initiated by mechanical stress and/or nutrient deprivation. This study aimed to investigate the functional responses of NC and MNP cells to hydrostatic pressures and glucose restriction. Design Bovine MNP and NC cells were cultured in 3-dimensional alginate beads under low (0.4-0.8 MPa) and high (1.6-2.4 MPa) dynamic pressure for 24 hours. Cells were cultured in either physiological (5.5 mM) glucose media or glucose-restriction (0.55 mM) media. Finally, the combined effect of glucose restriction and high pressure was examined. Results Cell viability and notochordal phenotypic markers were not significantly altered in response to pressure or glucose restriction. MNP cells responded to low pressure with an increase in glycosaminoglycan (GAG) production while high pressure significantly decreased ACAN gene expression compared with atmospheric controls. NC cells showed no response in matrix gene expression or GAG production with either loading regime. Glucose restriction decreased NC cell TIMP-1 expression but had no effect on MNP cells. The combination of glucose restriction and high pressure only affected MNP cell gene expression, with decreased ACAN, Col2α1, and ADAMTS-5 expression. Conclusion This study shows that NC cells are more resistant to acute mechanical stresses than MNP cells and provides a strong rationale for future studies to further our understanding the role of NC cells within the disc, and the effects of long-term exposure to physical stresses.