Weight transmission through the sacrum in man.

An attempt has been made to find the route and relative magnitude of weight passing through different components of the sacrum. In 44 adult male dry sacra, the combined surface area of the body and the two facets was compared with the combined surface area of the two auricular surfaces. It was found that the forces acting on the body and articular facets, at the upper end of the sacrum, are ultimately transmitted through the two auricular surfaces with an appreciable part of the load passing directly from the transverse process of the fifth lumbar vertebra to the ala of the sacrum through the lumbosacral ligament. The direction of the trabecular bone indicates the route of load transmission in the sacrum. From the various parts of the sacrum (body, facets, alae and laminae) distinct sets of trabeculae extend towards the auricular surface. Observations on the sacra with the anomalous articular processes provided strong evidence for the role of the neural arch elements in the load transmission. In specimens where the articular facet was absent on one or both sides, there was always an accessory facet on the ala of the sacrum so that the load was transmitted to this facet from the transverse process of the fifth lumbar vertebra.