FRICTIONAL CHARACTERIZATION OF EXPLANTED CHARNLEY HIP PROSTHESES

The frictional resistances of 54 explanted and 5 new Charnley hip pros theses were measured using one of two Durham hip function simulators. The friction factor f was measured in both dry and lubricated environm ents up to a viscosity eta of approximately 0.1 Pa s. The friction fac tor values for the new prostheses were found to be 0.11 (+/-0.025) and 0.04 (+/-0.01) for dry and lubricated (eta=0.01 Pa s) regimes respect ively. The distributions of f for the new prostheses were found to be approximately Gaussian. However, the distributions for the explanted p rostheses were skewed, with most retaining friction values similar to those of new prostheses, but a significant proportion exhibiting incre ased frictional resistance. For dry regimes 30% of the prostheses show ed values of f in excess of 0.16, while for lubricated regimes (eta=0. 01 Pa s) 39% showed values of f greater than 0.07. All the joints indi cated a mixed lubrication regime when analysed in terms of a Stribeck plot. A weak negative correlation (P<0.1) between penetration depth an d friction factor, under lubricated conditions, was found for the expl anted prostheses and indicates that f is not the cause of the increase d occurrence of loosening found with increased penetration in this typ e of prosthesis.