The lubrication of animal joints in relation to surgical reconstruction by arthroplasty.

The accepted theory of joint lubrication is that of MacConaill (1932), who assembled arguments to show that it was an example of hydrodynamic or full-film lubrication. The principal argument in applying the hydrodynamic theory to animal joints was the anatomical fact that in joints the curvatures of articulating surfaces are not completely congruous throughout their whole arc of movement. It was observed by Walmsley (1917, 1928) that the surfaces of the knee and hip are exactly congruous only in the position of full extension, which is the position of the motionless erect stance, but that in the arc employed for walking or running the convex surface has a slightly smaller radius than the concave surface. This presented to MacConaill's mind the existence of wedge-shaped spaces, occupied by wedge-shaped films of synovial fluid; it is the essence of the hydrodynamic theory that fluid pressure is generated in a wedge of lubricant as a result of the motion of the surfaces and that this pressure supports the load and keeps the sliding surfaces separated on a cushion of lubricant (Fig. 1). Even before examining the problem experimentally in animal joints, there are theoretical criticisms which make the hydrodynamic theory unlikely. Firstly, the articular cartilage, which is present as layer about I in. thick over the sliding surfaces of the joint, is very resilient, being easily indented by pressure of the thumb-nail. An ankle-joint of an adult male has a projected surface area of less than 2 sq. in., so that a man weighing 12 st. carrying a l-cwt. load on his shoulders will expose an anklejoint to pressures of about 150 lb. per sq. in. There can be little doubt that these resilient surfaces are intimately applied to each other over the whole area especially when carrying loads.