Walking on water: subchondral vascular physiology explains how joints work and why they become osteoarthritic.

This review of bone perfusion introduces a new field of joint physiology, important in understanding osteoarthritis. Intraosseous pressure (IOP) reflects conditions at the needle tip rather than being a constant for the whole bone. Measurements of IOP in vitro and in vivo, with and without proximal vascular occlusion confirm that cancellous bone is perfused at normal physiological pressures. Alternate proximal vascular occlusion may be used to give a perfusion range or bandwidth at the needle tip more useful than a single IOP measure. Bone fat is essentially liquid at body temperature. Subchondral tissues are relatively delicate but are micro-flexible. They tolerate huge pressures with loading. Collectively, the subchondral tissues transmit load mainly by hydraulic pressure to the trabeculae and cortical shaft. Normal MRI scans demonstrate subchondral vascular marks which are lost in early osteoarthritis. Histological studies confirm the presence of those marks and possible subcortical choke valves which support hydraulic pressure load transmission. Osteoarthritis appears to be at least partly a vasculo-mechanical disease. Understanding subchondral vascular physiology will be key to better MRI classification and prevention, control, prognosis and treatment of osteoarthritis and other bone diseases.