A fresh look at the nucleus-endplate region: new evidence for significant structural integration.

The disc nucleus is commonly thought of as a largely unstructured gel. However, exactly how the nucleus integrates structurally with the endplates remains somewhat ambiguous. The purpose of this study was to investigate whether a substantial level of structural/mechanical cohesion does, in fact, exist across the nucleus-endplate junction. Vertebra-nucleus-vertebra samples were obtained from mature ovine lumbar motion segments and subjected to a novel technique involving circumferential transverse severing (i.e. ring-severing) of the annulus fibrosus designed to eliminate its strain-limiting influence. These samples were loaded in tension and then chemically fixed in order to preserve the stretched nucleus material. Structural continuity across the nucleus-endplate junctions was sufficient for the samples to support, on average, 20 N before tensile failure occurred. Microscopic examination revealed nucleus fibres inserting into the endplates and the significant level of load carried by the nucleus material indicates that there is some form of structural continuity from vertebra to vertebra in the central nucleus region.