Deficiency of lubricating surfactant lining the articular surfaces of replaced hips and knees.

While preceding papers have demonstrated that the active load-bearing agent in the boundary mode of joint lubrication is surface-active phospholipid (SAPL)--probably adsorbed as the outermost layer of articular cartilage--this study is designed to determine whether that layer is deficient in osteoarthritis (OA). This layer has been studied on 12 hips and 31 knees obtained from surgically replaced joints afflicted with OA. Measurement of the contact angle (theta) subtended by a droplet of saline clearly demonstrated a highly significant decrease in hydrophobicity, theta falling from 100 degrees for 13 bovine controls (78 degrees for five human controls) to 56 degrees for arthritic hips and 63 degrees and 68 degrees for the 'worn' and 'unworn' areas of arthritic knees, respectively. These changes were reflected in the quantities of SAPL (and proteolipid) recovered from the same articular surfaces by solvent rinsing, yields of SAPL being 36% lower in hips and 25% lower in 'worn' areas of knees, but not significantly different in 'unworn' areas. These results indicate that the outermost lubricating layer of SAPL deposited onto articular cartilage from SF is deficient in OA.