Fig. 7.

Endplate morphology and disruption susceptibility. The disc wall can be divided into three morphologically distinct zones: inner, mid and outer. Between the nucleus and inner zone exists the transition zone. The transition zone contains fibres that have a well-defined superior–inferior orientation, but are not organized into distinct lamellae. Inner zone (c): the inner zone begins at the first well-defined lamella. Whilst no calcification of the cartilaginous endplate is present adjacent to the nucleus (B), adjacent to the transition and inner zones both calcified and uncalcified cartilaginous endplate exist (C). The boundary between the calcified and uncalcified cartilage, the tidemark, is indicated by arrow tm in C. The boundary between the calcified cartilage and the vertebral endplate, the cement line, is indicated by arrow cl in C. The ratio of calcified to uncalcified cartilage increases with increasing radial distance away from the nucleus. Mid-zone (d): the cartilaginous endplate is fully calcified. The cement line is well defined. There appears to be little to no connection between the annular fibres penetrating the cartilaginous endplate and the vertebral endplate. It is the endplate in this zone that is always implicated in radial annular-endplate tears and appears to be the most prone to mechanical disruption (Figs. 3C, 4D). Outer zone (e): the cartilaginous endplate is fully calcified. The cement line is less prominent. Annular fibres penetrating the cartilaginous endplate appear to integrate with the vertebral endplate forming a robust disc/vertebra connection. The tissue section shown is from the mediolateral posterior aspect of a ewe’s L56 disc (vertebral growth plates unfused). Note that the posterior longitudinal ligament has been removed; if present it would exist to the right of the outer zone