DIFFERENCES IN PATELLOFEMORAL JOINT CARTILAGE MATERIAL PROPERTIES ANDTHEIR SIGNIFICANCE TO THE ETIOLOGY OF CARTILAGE SURFACE FIBRILLATION

Objective: To determine if differences in biomechanical properties and biochemical composition exist between human patellar articular cartil age and the opposing femoral articular cartilage. Design: The biomecha nical properties and biochemical composition of the articular cartilag e of 17 knees from 13 donors were determined for four sites on the pat ella and three sites on the femur representing regions of contact at 3 0 degrees and 90 degrees of flexion. The material properties were dete rmined by biphasic indentation testing, yielding the compressive aggre gate modulus, H-A, permeability, k, and Poisson's ratio, v(s). The thi ckness of the cartilage at the indentation site, h, was also measured using a needle probe. Full-thickness samples of cartilage adjacent to each indentation site were used for wet weight, sulfated glycosaminogl ycan content and hydroxyproline content determinations. Results: The p atellar cartilage was found to have a lower compressive aggregate modu lus by 30% (P < 0.001), higher premeability to fluid flow by 66% (P < 0.001) and greater thickness by 23% (P = 0.017) than that of the oppos ing femoral cartilage. The Poisson's ratios for both surfaces were fou nd to be nearly zero. The water content of the patella was higher by 5 % (P = 0.031) and the proteoglycan content lower by 19% (P = 0.030) th an that of the femur. However, no differences were found between the c ollagen contents of the cartilages. Conclusions: Significant differenc es were found between the intrinsic material properties of the patella r cartilage and those of the femoral-trochlear cartilage. This variabi lity of cartilage material properties with the patellofemoral joint ma y help explain why patellar cartilage has been frequently observed cli nically to exhibit earlier and more severe fibrillation changes than t he opposing femoral cartilage.