Effect Of Edge Contact Of Metal-on-metal Hip Joint Replacements

The contact dynamics of conformal spherical bearing is the foundation of study on the wear and lubrication of total hip joint replacements. A general model of contact mechanics of hip joint replacements on the basis of the spherical-grid-data-model was developed, where the articulation between the femoral head and the acetabular cup was represented by a nominal ball-in-socket configuration. Finite element method is used for the numerical simulation of contact dynamics at articulating surfaces by considering different physiological cup inclination status oppositing with femoral head, exerting the increasing displacement and changing the geometry of femoral head successively. The actual analytic process was achieved by the following steps:(1)The model of the contact mechanics was subsequently employed to study the edge contact of a typical metal-on-metal hip joint replacement under the given cup inclination angles oppositing with femoral head. The maximum contact pressure decreased slightly and the contact area increased when the cup inclination angles during small range. When the cup inclination angles were more than 80 degree, the corresponding contact area moved to the cup edge and the maximum contact pressure increased, but the position of maximum contact pressure moved from the initial contact points to new ones in the direction of the increasing inclination angle since the corresponding contact pressure distribution needed to balance with the loaded human weight. The bigger cup inclination angles oppositing with femoral head caused different edge contact phenomena.(2)The dynamic contact mechanics of hip joint replacement was simulated by the finite element contact model under the increasing displacement between the acetabular cup and femoral head. It was found from the obtained results that the vertical component of the support force due to contact pressure for the edge contact of hip joint replacements with opposite cup inclination angle of 80 degree was smaller than that of 45 degree, while the corresponding horizontal component of the support force increased more significantly with the dynamic displacement. In addition, the corresponding contact pressure distributions and the contact areas of hip joint replacements under the two cup inclination status were different with the increasing contact displacement. The dynamic edge contact behaviour of hip joint replacements took place with the increasing displacement of the head to the cup, which could cause the sliding between contact surfaces and additional wear since the horizontal support force increased.(3)The different orientations of non-sphericity of bearing geometries were rebuilded by using the corresponding program code. They were applied to study the effect of contact mechanics and edge contact behaviour of an ellipsoidal head against a spherical cup in a typical metal-on-metal hip joint replacement. The results obtained showed that the maximum contact pressure of the non-sphericical bearing decreased and the contact area increased when the average radius of curvature of ellipsoidal head around the centre of contact zone was increased, while the effect of the cup opposite inclination status on the maximum contact pressure of non-sphericical bearing under the same loading was small, but the contact pressure distributions was different from the spherical bearing. A well-controlled non-sphericity can improve the magnitude and the distribution of contact pressure of metal-on-metal hip joint replacements.The contact dynamics plays a significant role on the lubrication of hip joint replacements. The study of the contact dynamics of artificial hip joints can provide a reference for the assessment of wear. The contact status can be improved by changing the geometry of articulating surfaces appropriately. The attention to the edge contact phenomenon should be paid in clinic application and manufacturing of hip joint replacements.