The friction of explanted hip prostheses

Charnley prostheses, retrieved at revision surgery, were studied to assessthe effects of friction on the total hip replacement procedure. Frictionalresistance was measured using the Durham hip function simulator under bothdry and lubricated conditions. The friction factor values (f) for theexplanted prostheses were found to have a non- Gaussian distribution withmedians of 0.13 [inter-quartile range (IQR) 0.10-0.16] and 0.06 (IQR0.005-0.08) for dry and lubricated (n = 0.01 Pa s) regimes, respectively.New Charnley prostheses had values of f equal to 0.11 +/- 0.025 and 0.04+/- 0.01 under the same conditions, and showed no large deviation from aGaussian distribution. There was found to be a statistically significantdifference in the medians of the friction factors for new and retrievedprostheses in the lubricated regime. Ingression of cement into the wornregion of the cup was found to increase the friction factor significantlyunder dry conditions. There was no evidence of an increase in the frictionfactor or torque for those joints that had a loose socket with respect tothose that were fixed at revision. A decrease in the frictional torqueagainst number of cycles undergone by the joint in vivo may indicate that afatigue-type process may have a role in the loosening of the socket.However, this relationship was found not to be significant for frictionmeasured under lubricated conditions and it seems unlikely that thefrictional torque generated in this type of prosthesis will contributesignificantly to the long-term loosening of the socket.