The effect of contact stress on cartilage friction, deformation and wear.

Following hip hemiarthroplasty, a metal femoral head articulates against natural acetabular cartilage. Cartilage friction and wear may be influenced by variables including loading time, contact stress, contact area, sliding distance, and sliding speed. The aim of this study was to investigate the effect of these variables on cartilage friction, deformation and wear in a simulation using idealized geometry model. Bovine cartilage pins were reciprocated against metal plates to mimic a hemiarthroplasty articulation under static loading. The effective coefficient of friction (micro elf) under contact stresses (0.5 to 16 MPa), contact areas (12 and 64 mm2), stroke lengths (4 and 8 mm), sliding velocities (4 and 8 mm/s), and loading time (1 and 24 hours) were studied. The permanent deformation of cartilage (after 24 hours of recovery) with and without motion was recorded to assess cartilage linear wear. The micro eff was found to remain < 0.35 with contact stresses < or =4 MPa. Severe damage to the cartilage occurred at contact stresses > 8 MPa and significantly increased micro eff after 12 hours of reciprocation. In long-term, contact area had no significant effect on micro eff, and sliding distance and velocity only affected micro eff under low contact stresses. The cartilage linear wear increased with contact stress, sliding distance and velocity.