Anchorage of annulus fibrosus within the vertebral endplate with reference to disc herniation

ABSTRACTPurpose: To describe the microarchitecture of the annulus‐endplate region with reference to disc herniation. Methods: Twenty‐five motion segments (L1‐2 to L5‐S1) were harvested from five cadaveric spines (four males, one female). Superior and inferior annulus‐endplate junctions were divided into anterior, lateral and posterior regions. Tissue blocks were fixed, decalcified, and embedded in paraffin. Thin sections (4 µm) were stained with H&E and Masson's trichrome for light microscopy. Results: Annulus fibrosus presents a laminated structure, with lamellae becoming wider and more disordered close to the nucleus. Each lamella comprises parallel bundles of collagen fibers. On entering the hyaline cartilage endplates, fiber bundles from the outer annulus often change direction, branch into sub‐bundles separated by hyaline cartilage matrix, and no longer exhibit the undulating “crimp” structure of the annulus. Their three‐dimensional multileaf morphology appears to provide direct mechanical anchorage in vertebral bone. Calcification is present at the annulus‐cartilage and cartilage‐bone interfaces, especially posteriorly. In the inner annulus and nucleus, annulus fibers branch and merge obliquely with the hyaline cartilage matrix. The diameter of collagen fiber bundles and sub‐bundles increased significantly in the outer annulus compared with middle annulus, and increased at lower spinal levels, but no significant differences were observed between superior and inferior endplates. Conclusions: The branching of annulus collagen fiber bundles within the endplates increases the interface area with surrounding matrix, reducing shear stresses and (probably) increasing strength. Calcification may also increase strength. These observations can explain why disc herniations often contain cartilage and bone fragments from the endplate. Microsc. Res. Tech. 78:754–760, 2015. © 2015 Wiley Periodicals, Inc.