How maturity influences annulus-endplate integration in the ovine intervertebral disc: a micro- and ultra-structural study.

The annulus-endplate anchorage system plays a vital role in structurally linking the compliant disc to its adjacent much more rigid vertebrae. Past literature has identified the endplate as a region of weakness, not just in the mature spine but also in the immature spine. The aim of this structural study was to investigate in detail the morphological changes associated with annulus-endplate integration through different stages of maturity. Ovine lumbar motion segments were collected from two immature age groups: (i) newborn and (ii) spring lamb (roughly 3 months old); these were compared with a third group of previously analysed mature ewe samples (3-5 years). Sections from the posterior region of each motion segment were obtained for microstructural analysis and imaged in their fully hydrated state via differential interference contrast (DIC) optical microscopy. Selected slices were further prepared and imaged via scanning electron microscopy (SEM) to analyse fibril-level modes of integration. Despite significant changes in endplate morphology, the annular fibre bundles in all three age groups displayed a similar branching mechanism, with the main bundle splitting into several sub-bundles on entering the cartilaginous endplate. This morphology, previously described in the mature ovine disc, is thought to strengthen significantly annulus-endplate integration. Its prevalence from an age as young as birth emphasizes the critical role that it plays in the anchorage system. The structure of the branched sub-bundles and their integration with the surrounding matrix were found to vary with age due to changes in the cartilaginous and vertebral components of the endplate. Microscopically, the sub-bundles in both immature age groups appeared to fade into the surrounding tissue due to their fibril-level integration with the cartilaginous endplate tissue, this mechanism being particularly complex in the spring lamb disc. However, in the fully mature disc, the sub-bundles remained as separate entities throughout the full depth of their anchorage into the cartilaginous endplate. Cell morphology was also found to vary with maturity within the cartilaginous matrix and it is proposed that this relates to endplate development and ossification.

A multiscale structural investigation of the annulus-endplate anchorage system and its mechanisms of failure.

BACKGROUND CONTEXT: The annulus-endplate anchorage system performs a critical role in the disc, creating a strong structural link between the compliant annulus and the rigid vertebrae. Endplate failure is thought to be associated with disc herniation, a recent study indicating that this failure mode occurs more frequently than annular rupture. PURPOSE: The aim was to investigate the structural principles governing annulus-endplate anchorage and the basis of its strength and mechanisms of failure. STUDY DESIGN: Loading experiments were performed on ovine lumbar motion segments designed to induce annulus-endplate failure, followed by macro- to micro- to fibril-level structural analyses. METHODS: The study was funded by a doctoral scholarship from our institution. Samples were loaded to failure in three modes: torsion using intact motion segments, in-plane tension of the anterior annulus-endplate along one of the oblique fiber angles, and axial tension of the anterior annulus-endplate. The anterior region was chosen for its ease of access. Decalcification was used to investigate the mechanical influence of the mineralized component. Structural analysis was conducted on both the intact and failed samples using differential interference contrast optical microscopy and scanning electron microscopy. RESULTS: Two main modes of anchorage failure were observed--failure at the tidemark or at the cement line. Samples subjected to axial tension contained more tidemark failures compared with those subjected to torsion and in-plane tension. Samples decalcified before testing frequently contained damage at the cement line, this being more extensive than in fresh samples. Analysis of the intact samples at their anchorage sites revealed that annular subbundle fibrils penetrate beyond the cement line to a limited depth and appear to merge with those in the vertebral and cartilaginous endplates. CONCLUSIONS: Annulus-endplate anchorage is more vulnerable to failure in axial tension compared with both torsion and in-plane tension and is probably due to acute fiber bending at the soft-hard interface of the tidemark. This finding is consistent with evidence showing that flexion, which induces a similar pattern of axial tension, increases the risk of herniation involving endplate failure. The study also highlights the important strengthening role of calcification at this junction and provides new evidence of a fibril-based form of structural integration across the cement line.

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.