Role of uppermost superficial surface layer of articular cartilage in the lubrication mechanism of joints

The uppermost superficial surface layer of articular cartilage, the 'lamina splendens' which provides a very low friction lubrication surface in artic ular joints, was investigated using atomic force microscopy (AFM). Compleme ntary specimens were also observed under SEM at - 10 degreesC without dehyd ration or sputter ion coating. Fresh adult pig osteochondral specimens were prepared from the patellas of pig knee joints and digested with the enzyme s, hyaluronidase, chondroitinase ABC and alkaline protease. Friction coeffi cients between a pyrex glass plate and the osteochondral specimens digested by enzymes as well as natural (undigested) specimens were measured, using a thrust collar apparatus. Normal saline, hyaluronic acid (HA) and a mixtur e of albumin, globulin, HA (AGH) were used as lubrication media. The surfac e irregularities usually observed in SEM studies were not apparent under AF M. The articular cartilage surface was resistant to hyaluronidase and also to chondroitinase ABC, but a fibrous structure was exhibited in alkaline pr otease enzymes-digested specimens. AFM analysis revealed that the thickness of the upper-most superficial surface layer of articular cartilage was bet ween 800 nm and 2 mum in adult pig articular cartilage. The coefficient of friction (c.f.) was significantly higher in chondroitinase ABC and alkaline protease enzymes digested specimens. Generally, in normal saline lubricati on medium, c.f. was higher in comparison to HA and AGH lubrication media. T he role of the uppermost, superficial surface layer of articular cartilage in the lubrication mechanism of joints is discussed.