Wear testing of polyethylene in total joint replacements is common and required for any new device. Computational wear modelling has obvious utility in this context as it can be conducted with much greater economy than physical testing. Archard's law has become the accepted standard for wear simulation in total joints but it does not account for cross-shear, which is known to increase wear significantly relative to unidirectional sliding. The purpose of this study was to develop a robust cross-shear model applicable to any interface geometry under any kinematic conditions. The proposed metric, x (*), is distinguished from existing cross-shear models by the fact that it measures cross-path motion incrementally throughout a motion cycle and quantifies cross-shear based on incremental changes in sliding direction. Validation showed strong support for the predictive capability of x (*) when applied to pin-on-disc test data.
The purpose of this study was to determine if Control Entropy (CE) of MMG and EMG would decline during fatiguing isometric muscle actions. Procedures were approved by the Hope HSRB. Subjects performed one isometric leg extension (30° flexion) at the 1‐RM of the non‐dominant leg to volitional fatigue. MMG and EMG signals were recorded from the vastus medialis (VM) and rectus femoris muscles (RF) using commercial software (Biopac Systems, CA). The effort was divided into epochs (10% of time‐to‐fatigue) and mean and peak‐to‐peak (P‐P) amplitude MMG and EMG data were analyzed using repeated measures ANOVA (α=0.05). CE was calculated in Matlab (Mathworks, MA) using 2 and 8 bit partitions, and varying window lengths according to the following equation: CEKS[x(t)] =hKS[sign(dx/dt(t))]. P‐P amplitude of VM‐MMG and RF‐EMG declined (p<0.05), while RF‐MMG (p=0.58) trended lower over time. Mean amplitude of VM‐EMG declined (p<0.05). CE of the MMG declined over the course of the effort for males, but not for females. When accounting for gender, a significant effect was observed for P‐P of VM .MMG, RF‐EMG and mean VM‐EMG. In addition, CE of EMG exhibited an unexpected response whereby it increased at the onset of the muscle action. These results support previous reports of a differential MMG response in males vs females. The attenuated CE response of MMG in females vs. males may shed light on the nature of this difference between genders.
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