Biomechanical experiments on isolated hip joints have suggested that the transverse 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 to substantiate the theory. Here we argue that if
the horns of the lunate surface are forced apart under load, the ligament would straighten and 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 antibodies against collagens (types I, II, III, VI),
glycosaminoglycans (chondroitins 4 and 6 sulphate, dermatan sulphate, keratan sulphate) and proteoglycans
(aggrecan, link protein, versican, tenascin). A small sesamoid fibrocartilage was consistently present in the
centre of each transverse ligament, near its inner surface at the site where it faced the femoral head.
Additionally, a more prominent enthesis fibrocartilage was found at both bony attachments. All
fibrocartilage regions, in at least some specimens, labelled for type II collagen, chondroitin 6 sulphate,
aggrecan and link protein, molecules more typically associated with articular cartilage. The results suggest
that the ligament should be classed as containing a ‘moderately cartilaginous’ sesamoid fibrocartilage,
adapted to withstanding compression. This supports the inferences that can be drawn from previous
biomechanical studies. We cannot give any quantitative estimate of the levels of compression experienced.
All that can be said is that the ligament occupies an intermediate position in the spectrum of
fibrocartilaginous tissues. It is more cartilaginous than some wrap-around tendons at the wrist, but less
cartilaginous than certain other wrap-around ligaments, e.g. the transverse ligament of the atlas.