Collagenous fibril texture of the discoid lateral meniscus.

PURPOSE: To provide the theoretic basis for treatment and to increase the understanding of the tear patterns of the discoid meniscus, we observed the collagen orientation of the discoid meniscus.
METHODS: Ten meniscus specimens were used to observe the collagen fibril orientation of the complete type of the discoid lateral menisci. The samples were observed layer by layer under a polarizing filter microscope by using Sirius red staining, and they were also observed under a scanning electron microscope.
RESULTS: The lateral discoid meniscus is classified into 7 layers based on collagen fibril orientation. The femoral surface of the discoid meniscus is covered by dense and well-arranged thick fibrils, which very much resembles a bunched streak. The fibrils show a sagittal isotropic-arranged orientation. However, the tibial surface shows an irregular and anisotropically arranged orientation. In the outer layer, a meshwork of thin fibrils has been observed. The collagen fibrils in the inner layer are radially orientated from the lateral side to the medial side. In the central layer, the peripheral collagen fibrils are displayed as dense bundles running in a circumferential pattern, whereas its medial zone shows as thin, loosely, and irregularly arranged fibrils without a bundle formation. The anterior and posterior zones of the central layer show the collagen fibrils with a straight arrangement in the radial direction.
CONCLUSIONS: In the lateral middle zone of discoid meniscus, the collagen fibrils run parallel to the periphery of the meniscus. Therefore, it would be ideally suited for resisting hoop stresses. From this anatomic study, it is apparent that the peripheral portion of the meniscus is constructed to bear a load. CLINICAL RELEVANCE: It is strongly recommended that the peripheral portion of the discoid meniscus should be preserved when a resection of the meniscus is mandatory.