Abstract / Summary
Procedures that predict or test wear in total hip replacements use the contact force waveform as their principal input. This approach rests on the assumption that activities producing large contact forces also impose a large frictional burden. Because friction is governed by sliding at the articulating surfaces and by the state of the lubricating film, the correspondence between the two quantities requires verification. The present study reanalysed publicly available measurements from an instrumented hip implant to test whether the ranking of peak contact force across activities agrees with the ranking of peak friction moment, and to establish how that result depends on how the metric is defined. Hip Joint III data from the OrthoLoad database were used. Of the activities performed by ten subjects, the eight with a cyclic structure were analysed, comprising 72 individual trials; one-legged stance was treated separately because the timing of its peaks is not defined. The accuracy of the data import was confirmed by comparing the imported HIGH100 standardised loads with the originally published values. At the activity level, an exhaustive permutation test over all 8! = 40,320 orderings was applied; at the trial level, a linear mixed model including the activity-by-metric interaction was fitted. The rankings of contact force and friction moment diverged at the peak (Spearman rho = 0.67; mean rank shift 1.25). In the trial-level model, the activity-by-metric interaction was significant (Wald chi-square = 40.3, df = 7, p = 1 × 10−6) and was largest for jogging (−1.74 ± 0.43 standard deviations, p = 0.0001). When the same data were expressed as cycle impulse, the rankings agreed almost completely (rho = 0.98; mean rank shift 0.25) and the interaction was not significant (chi-square = 1.7, p = 0.975). The instant at which the friction moment peaked differed by activity: in walking, stair ascent and descent, and jogging it occurred 29 to 37% of the cycle after the contact force peak, whereas in postural transitions and cycling the two instants coincided. The discordance between contact force and friction moment is confined to peak values and does not appear in cumulative quantities. The delayed friction moment peak is consistent with a within-cycle change in lubrication regime, in which the film thins under sustained load and the greatest interfacial shear arrives after the greatest load. Given that polyethylene wear depends on the shear at the articulating surfaces, a load waveform alone may not reproduce the phase of the cycle in which wear actually occurs.