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Objectives: Osteoarthritis (OA) affects over 21 million U.S. adults, many of them veterans, elderly, or both. Severe OA is treated with joint replacements. The bearing surface of these implants is often composed of polyethylene, which wears out over time. To date, efforts aimed at understanding this wear process have assumed a priori a particular joint loading to obtain stress and deformation, used to quantify wear. The objective of this research is to couple finite element stress analysis with a dynamic model of human gait, offering a simulation that better represents in vivo loading conditions.
Methods: In this study, we created a finite element model of the human femoral head, attached to rigid elements representing legs. A load simulating the head-arms-trunk (HAT) was applied to the stance leg through the hip joint. The stress distributions and deformations in the hip for one complete gait cycle were obtained.
Results: As the stance leg moves from the initial position, associated with heel strike, to the final position, representing toe off, the hydrostatic stress is observed to peak, just below the point of load application, during the mid-stance. This result is consistent with the ballistic gait framework because the rigid leg produces the greatest reaction force against the HAT at mid-stance. This result is expected to change from a single stress peak to a bi-modal stress peak, seen just after heel strike and just prior to toe-off, as the gait model is refined to more-closely match physiological gait.
Conclusions: Coupling stress analysis with human gait allows for the evaluation and improvement of existing methods for computing joint wear that presuppose joint loading. The approach advocated here offers clinical relevance because it can help lead to better implant designs by producing polyethylene wear results that better match in vivo wear patterns. Additionally, this research may offer insight not previously available. For example, the coupled approach allows for alterations of gait patterns to be used, which may help elucidate how humans adapt locomotion strategies (i.e., limping after trauma or surgery) to minimize joint stress.
Acknowledgments: We gratefully acknowledge the Department of Veterans Affairs, Rehabilitation R&D Service, Pre-Doctoral Research Fellowship (CBH).