The surface contour of articular cartilage in an intact, loaded joint

The friction coefficients measured in diarthrodial joints are small. Theori es of joint lubrication attribute this efficiency to entrapment or movement of synovial fluid, yet anatomical models of the surface are based on studi es of isolated fragments of cartilage, not functional joints. To investigat e the functional interrelationship of joint surfaces and synovial fluid, th e ultrastructure of loaded joints was examined. Twenty-four New Zealand whi te rabbit knee joints were loaded either statically or moved ex vivo using simulated muscle forces and then plunge-frozen under load. After fixation i n the frozen/loaded state by freeze-substitution fixation, the medial joint compartments were embedded in epoxy resin while still articulated. Bone wa s trimmed away from the articular surfaces, permitting the cartilage to be sectioned for light and electron microscopy. These joint surfaces were then compared with controls which were not loaded, not moved or had been disart iculated prior to embedding. Articular surfaces of loaded joints were smoot h at magnifications from x 35 to x 7500, whereas the tibial surfaces of non loaded joints were irregular. Small pools of joint fluid were observed at t he meniscal edge and beneath the anterior horn of the meniscus. At magnific ations of x 40000, the joint surfaces were separated by a uniform 100 nm sp ace containing fluid. An amorphous, electron dense articular surface lamina was present but, when loaded, was thicker and flatter than previously repo rted. No surface pits or bumps were visible in embedded, loaded joints. Thi s is the first ultrastructural study of intact loaded joints. The findings suggest that fluid him lubrication is present in diarthrodial joints, but t he fluid sequestration postulated in several models is not apparent.