Joint stiffness and 'articular gelling': inhibition of the fusion of articular surfaces by surfactant.

It was proposed some years ago that, in osteoarthritis, one source of joint stiffness arises from 'articular gelling', but, if so, why does this not occur in the normal joint? In a preliminary experiment using agar gels, it is shown how such fusion of gel surfaces can be inhibited by surface-active phospholipid (SAPL)--both synthetic and human--as quantified by the shear stress needed to cause cleavage between samples after prolonged contact. On the other hand, normal bovine articular cartilage (BAC) does not fuse to itself, but can be made to do so if rinsed with a powerful lipid solvent known to remove the outermost layer of adsorbed SAPL along with the hydrophobicity so characteristic of the normal 'waxy' surface it imparts. It is then shown how the inhibition of gel fusion can be restored by treating both bovine and human articular surfaces with exogenous SAPL derived from human AC and with synthetic SAPL. Samples of human articular cartilage excised from osteoarthritic hips and knees during total joint replacement showed a 55% greater tendency to fuse together than normal BAC. This was exacerbated by solvent rinsing and can be attributed to a deficiency in the outermost lining of SAPL previously studied as a load-bearing boundary lubricant capable of reducing friction and wear to the remarkably low levels observed physiologically. Hence, joint stiffness can be attributed, in part, to a deficiency in the lubricating layer of SAPL lining the normal articular surface where it can inhibit articular gelling/gel fusion, possibly imparting other desirable physiological functions. The possibility of clinical replenishment of SAPL in the osteoarthritic joint is discussed.