THE FRICTION OF EXPLANTED HIP PROSTHESES

Charnley prostheses, retrieved at revision surgery, were studied to as sess the effects of friction on the total hip replacement procedure. F rictional resistance was measured using the Durham hip function simula tor under both dry and lubricated conditions. The friction factor valu es (f) for the explanted prostheses were found to have a non-Gaussian distribution with medians of 0.13 [inter-quartile range (IQR) 0.10-0.1 6] and 0.06 (IQR 0.605-0.08) for dry and lubricated (eta = 0.01 Pa s) regimes, respectively. New Charnley prostheses had values of f equal t o 0.11 +/- 0.025 and 0.04 +/- 0.01 under the same conditions, and show ed no large deviation from a Gaussian distribution. There was found to be a statistically significant difference in the medians of the frict ion factors for new and retrieved prostheses in the lubricated regime. Ingression of cement into the worn region of the cup was found to inc rease the friction factor significantly under dry conditions. There wa s no evidence of an increase in the friction factor or torque for thos e joints that had a loose socket with respect to those that were fixed at revision. A decrease in the frictional torque against number of cy cles undergone by the joint in vivo may indicate that a fatigue-type p rocess may have a role in the loosening of the socket. However, this r elationship was found not to be significant for friction measured unde r lubricated conditions and it seems unlikely that the frictional torq ue generated in this type of prosthesis will contribute significantly to the long-term loosening of the socket.