Nonidentical and outlier duty cycles as factors accelerating UHMWPE wear in THA: A finite element exploration

Abstract

Wear rate and wear direction vary considerably within total hip arthroplasty (THA) patient cohorts. Third body effects and wide-ranging differences in patient activity levels are two factors suspected of contributing to wear variability. A sliding-distance-coupled contact finite element formulation was used to test the hypothesis that nonidentical duty cycles (differing activities, or change of third body challenge) produce accelerations in polyethylene wear. Effects of nonidentical duty cycles, time-variant femoral head roughening, and outlier gait inputs were investigated. Without femoral head roughening, combination walk/stair-climb wear simulations did not result in appreciably higher volumetric wear than a walk-only simulation, but when a roughened zone was included, walk/stair-climb volumetric wear increased by approximately 57% above that of a similarly roughened walk-only simulation. To investigate time-variant femoral head roughening, wear simulations were begun with femoral head roughening at one location on the femoral head, switching to another location halfway through the simulation. Results varied depending on roughening sites, but cases of substantial increase in wear involved a transient jump in wear rate shortly after the change of head roughening location. Outlier duty cycles were simulated by increasing or decreasing the joint contact force and range of motion inputs, to levels at the 97.5th and 2.5th percentiles of a population of normal subjects. The resulting wear showed an increase or decrease closely proportional to the percentage by which each input (force or range of motion) was changed. ß

The influence of wear paths produced by hip replacement patients during normal walking on wear rates

“…In a recent finite element study, no difference in predicted wear rates was found between the conditions of walking and those of walking and stair climbing combined for smooth femoral heads, but when roughened femoral heads were tested wear rate increased by 57% for the combined walking and stair climbing compared to walking alone. 9 The combined effect of altering loading patterns and femoral head roughness dominated over the prevalent gait pattern in this theoretical study. Furthermore, intermittent shock loading also accelerates wear in laboratory testing.…”

“…22 The potential importance of variations in individual THA patient's wear paths on wear rates has recently been recognized, with altered gait cycle inputs used to predict wear rate in a finite element model. 9 This study is the first to test the relationship between individual patient kinematics and clinical wear rates for a large group of patients at long-term follow-up.…”

“…5,7 Variation of wear paths between total hip arthroplasty patients has recently been proposed as a factor that may influence the wear properties of the UHMWPE cup. 9 In a previous study involving a small number of patients (n ¼ 19), the authors found a positive correlation between both measurements of multidirectional motion and sliding distance experienced at the patients' hip joints and the wear. 13 This study sought to confirm the relationships between multidirectional motion, sliding distance, and patient activity levels of individual patients and the wear rates experienced by patients within a large homogenous group at long-term follow-up.…”

“…3,4 Variation in the articulation patterns between the femoral head and the acetabular cup greatly influence wear rates in simulator tests [5][6][7][8] and computer models. 9 Unidirectional motion promotes orientation of the long polyethylene chains and subsequent hardening, 8 while multidirectional motion promotes an environment of continually changing shear. Consequently, unidirectional motion produces minimal wear rates while multidirectional motion produces greatly increased wear rates.…”

Preferential superior surface motion in wear simulations of the Charité total disc replacement

“…In this procedure, as in previous studies [4,13,14], the relative sliding kinematics and contact pressure of each node against the endplate were tracked for one motion cycle. At the end of the motion cycle, the linear wear depth at each node was calculated based on the Archard/Lancaster [1] wear law, augmented to include cross-shear…”