Structure, strain and function of the transverse acetabular ligament.

To assess the morphology and function of the transverse acetabular ligament (TAL), its internal structure and insertion sites were analyzed, and its strain was determined during hip joint loading. Fifteen specimens were embedded in methyl methacrylate and cut into 500-micron sections parallel to the inlet of the acetabulum. Polarized light microscopy and contact radiography revealed important differences between the ventral and dorsal attachment of the TAL. Ventrally, the fibers insert close to the labrum, at a rather extensive site consisting of fibrous cartilage. Dorsally, the ligament arises from a wide attachment at the bony region of the posterior horn, where it is continuous with the periosteum and the joint capsule. In a second part of the study, six specimens (four fixed, two fresh) were investigated, using a testing machine to simulate hip joint loading. The strains of the TAL and of two sites of the acetabular labrum were measured with strain gauges. Loads of 10-2,800 N were applied and the degree of rotation of the femoral head was varied. At 400% body weight, the TAL encountered tensile strains of up to 3.7%, but the labrum only of up to 0.5%. The contact areas of the hip joints were determined with a polyether casting material. At 50% body weight they were found to lie in the anterior and posterior aspects of the lunate surface. These results indicate that, due to a natural incongruity, the acetabular notch is widened during loading and the TAL submitted to tension.