The Annular Fiber’ Anchorage Within Endplate

Objective:To describe the microarchitecture of the annulus-endplate region, providing new insights into the structural mechanisms that give rise to its impressive mechanical properties.Methods:20motion segments (L1-L2. L2-L3, L3-L4. L4-L5) from5human cadaveric lumbar spines were harvested (4male,l female, mean age44.3years, range22-57). Annulus-endplate junction section of each segment9(divided as anterior, lateral and posterior) were dissected, chemically fixed and decalcified. After being routinely embedded in paraffin, sections were cut into4μm slices, stained with HE and Masson trichrome. The annulus-endplate junction section structure were observed with light microscope.Results:Compared with the annulus in the anterior and lateral region, annulus from the posterior region, arranges more intensively and irregularly. The annulus presents curvature structure outside the cartilaginous endplate, the outer annulus arranges regularly, while the inner shows disordered arrangement. The curvature structure disappears after the insertion of cartilaginous endplate, and mechanical anchorage is achieved via optically resolvable branched subbundles that appear to insert to a depth approximately equal to the thickness of the cartilaginous endplate without progressing any further in this form into the vertebral end plate bone. The subbundles formed a three-dimensional multileaf morphology with each leaf separated by cartilage endplate matrix, the outer fiber bundles are wider than inner fiber bundles. Calcification line can be seen at the interface of annulus-cartilage and cartilage-bone.Conclusion:The annular fiber bundles on entering the end plate are shown to subdivide into subbundles to increase the interface area between bundle and matrix, the increased interface area created by this branched configuration reduces the magnitude of shear stress generated at the interface by any tensile force applied to the fiber bundle. Calcified anchorages of annulus at annulus-cartilage and cartilage-bone interface provide enhancement in these regions, and enables higher loads to be carried by the annular fibers before end plate tearing occurs...