Wear of bearing surfaces is possibly the greatest limitation to the longevity of hip joint replacement components. The bearing stresses from normal joint loading may be well studied;however, load cases involving edge loading are less well understood, though they appear to be closely linked to important failure modes such as rim cracking in polyethylene liners and stripe wear and squeaking in ceramic bearings. This proposed research will elucidate the bearing stresses in cases of edge loading involving both polyethylene and ceramic bearings. Further, it will induce these stresses in wear tests developed for finished components and for research material samples. Replacement of worn implants is not only traumatic for patients, but is also a significant cost in the nation's healthcare budget. Mitigating these problems hinges upon improving the durability of implant bearing materials, because reducing wear and its resulting debris is essential to maximizing implant longevity. This proposed research will address these important health needs by its attention to bearing load and stress cases that are poorly understood but which are responsible for serious and current wear- based failure modes. The project will examine two examples of edge loading. The first is a relatively static case called subtle subluxation, and the second is a more dynamic case involving microseparation and impact. For both cases, the first aim is simulate the load case in a laboratory contact study, and then to analytically model the load case to calculate immeasurable quantities such as the contact stress. Both Hertzian and finite element analytic models will be evaluated, and the models will be validated by comparing the calculated contact area with the experimental results. With the contact stresses identified, two different wear tests will be developed for each load case. The first test will apply to real components, and it will impart repetitive loads similar to the single-instance load from the first aim. The second test, also a repetitive-load wear test, will apply to surrogate test samples that are more simply shaped than finished components. The validity of the second test as a simpler proxy for the first test will be predicated upon establishing closely equivalent contact stress fields between the two test methods, and will be evaluated by comparison of empirical results.
Revision surgery to replace failed or worn-out artificial joints is traumatic to patients and is a significant drain on heath-care resources. Rim cracking of polyethylene acetabular liners and squeaking in ceramic components are two important potential failure modes of hip implants, but the loads and stresses that cause such failures are not well understood. This research project will analyze contact stresses in hip implants under worst case load conditions and will develop new wear test methods to improve the pre-clinical evaluation of next-generation implants and their materials.
