Effect of clearance on friction and lubrication of hip implants

Friction and lubrication behaviour of 50mm diameter Birmingham Hip Resurfacing implants with diametral clearances 80 to 300µm, using blood (clotted and whole blood), a combination of bovine serum with hyaluronic acid and carboxymethyl cellulose (CMC) adjusted to a range of viscosities (0.001-0.2 Pas), and bovine serum with CMC adjusted to a similar range of viscosities, have been investigated. The result of this investigation has suggested strongly that the optimum clearance for the 50 mm diameter MOM BHR implants to be ≥150µm and <235µm, so as to avoid high frictions (i.e. avoid friction factors >0.2) and be able to accommodate a mixed lubrication mode and hence lower the risk of micro- or even macro-motion specially immediately after hip implantation. These suggested optimum clearances will also allow for low friction (i.e. friction factors of <0.2-0.07) and reasonable lubrication (dominantly mixed regime) for the likely cup deflection occurring as a result of press-fit fixation.