The subchondral bone plate supports the articular cartilage in diarthrodial joints. It has a significant
mechanical function in transmitting loads from the cartilage into the underlying cancellous bone and has
been implicated in the destruction of cartilage in osteoarthritis (OA) and its sparing in osteoporosis (OP),
but little is known of its composition, structure or material properties. This study investigated the
microscopic appearance and mineral composition of the subchondral bone plate in femoral heads from
patients with OA or OP to determine how these correspond to changes in composition and stiffness found in
other studies. Freeze-fractured full-depth samples of the subchondral bone plate from the femoral heads of
patients with osteoarthritis, osteoporosis or a matched control group were examined using back scattered
and secondary emission scanning electron microscopy. Other samples were embedded and polished and
examined using back-scattered electron microscopy and electron probe microanalysis. The appearances of
the samples from the normal and osteoporotic patients were very similar, with the subchondral bone plate
overlayed by a layer of calcified cartilage. Osteoporotic samples presented a more uniform fracture surface
and the relative thicknesses of the layers appeared to be different. In contrast, the OA bone plate appeared
to be porous and have a much more textured surface. There were occasional sites of microtrabecular bone
formation between the trabeculae of the underlying cancellous bone, which were not seen in the other
groups, and more numerous osteoclast resorption pits. The calcified cartilage layer was almost absent and
the bone plate was apparently thickened. The appearance of the osteoarthritic subchondral bone plate was,
therefore, considerably different from both the normal and the osteoporotic, strongly indicative of abnormal
cellular activity.