Micromechanics of annulus-end plate integration in the intervertebral disc

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.

The aim of this study was to investigate the mechanism of anchorage of the annular fibers in the end plate.

A microstructural analysis of the annulus-end plate region was carried out using motion segments obtained from the lumbar spines of mature ovine animals.

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.

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.

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.