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.

Hemi-arthroplasty of the hip (an artificial femoral head articulating against the natural acetabulum) is used to treat fractured necks of femur; however, there is evidence that articulation causes erosion of the cartilage, resulting in pain for the patient. Parameters that may influence this cartilage erosion include head material and roughness, clearance between the head and acetabulum and activity levels of the patient. This study has assessed the effect of clearance of hemi-arthroplasty articulations on the contact stress, friction and cartilage deformation in an in vitro tribological simulation of the hemi-arthroplasty joint that applied dynamic loads and motion. It has been demonstrated that peak contact stress increased from 5.6 to 10.6 MPa as radial clearance increased from small (<0.6 mm) to extra-large (>1.8 mm). In all samples, friction factor increased with time and was significantly less with extra-large clearances compared to small (<0.6 mm), medium (0.6-1.2 mm) and large (1.2-1.8 mm) clearances. The cartilage deformation observed was significantly greater in acetabulum samples paired to give small or extra-large clearances compared to those with medium or large clearances.

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