Fibrocartilage in the transverse ligament of the human acetabulum

Biomechanical experiments on isolated hip joints have suggested that the tr ansverse ligament acts as a bridle for the lunate articular surface of the acetabulum during load bearing, but there are inherent limitations in such studies because the specimens are fixed artificially to testing devices and there are no modifying influences of muscle pull. Further evidence is thus needed td substantiate the theory. Here we argue that if the horns of the lunate surface are forced apart under load, the ligament would straighten a nd become compressed against the femoral head. It would thus be expected to share some of the features of tendons and ligaments that wrap around bony pulleys and yet previous work has suggested that the transverse ligament is purely fibrous. Transverse ligaments were removed from 8 cadavers (aged 17 -39 y) and fixed in 90 % methanol. Cryosections were immunolabelled with an tibodies against collagens (types I,II,III,VI), glycosaminoglycans (chondro itins 4 and 6 sulphate, dermatan sulphate, keratan sulphate) and proteoglyc ans (aggrecan, link protein, versican, tenascin). A small sesamoid fibrocar tilage was consistently present in the centre of each transverse ligament, near its inner surface at the site where it faced the femoral head. Additio nally, a more prominent enthesis fibrocartilage was found at both bony atta chments. All fibrocartilage regions, in at least some specimens, labelled f or type II collagen, chondroitin 6 sulphate, aggrecan and link protein, mol ecules more typically associated with articular cartilage. The results sugg est that the ligament should be classed as containing a 'moderately cartila ginous' sesamoid fibrocartilage, adapted to withstanding compression. This supports the inferences that can be drawn from previous biomechanical studi es. We cannot give any quantitative estimate of the levels of compression e xperienced. All that can be said is that the ligament occupies an intermedi ate position in the spectrum of fibrocartilaginous tissues. It is more cart ilaginous than some wrap-around tendons at the wrist, but less cartilaginou s than certain other wrap-around ligaments, e.g. the transverse ligament of the atlas.