Influence of Acetabular Cup Rim Design on the Contact Stress During Edge Loading in Ceramic-on-Ceramic Hip Prostheses
Section snippets
Materials and Methods
A 28-mm diameter alumina ceramic-on-ceramic hip prostheses was modeled with a radial clearance of 40 μm and an acetabular cup thickness of 5 mm at an inclination angle of 45°. The elastic modulus and Poisson's ratio of alumina was 380 and 0.26 GPa, respectively. The effects of varying the cup thickness, radial clearance, cup inclination, and boundary conditions were described in a previous study [8]. Bearing surfaces were modeled as contact elements assuming friction was negligible. A vertical
Results
The maximum contact pressure at the edge as a function of translation distance is shown in Table 1 for the 3 acetabular cup edge designs considered in the study, namely, the “new,” “worn,” and “chamfered.” Under normal operating conditions, a maximum contact pressure of 81.6 MPa occurs under a load of 2500 N. No edge contact occurred for the any of the edge designs after 0 to 80 μm of translation; however, as the separation distance increased to 100 μm or greater, the contact area became
Discussion
Based on the geometry of the current design, it can be shown that when the bearing centers of the femoral head and acetabular cup translate approximately 100 μm, edge contact begins to occur. This is consistent with the present finite element analysis prediction. Eq. (1) shows that the distance for onset of rim contact (s) is inversely proportional to the cup inclination angle (θ) and proportional to the radial clearance (c) [8].[8]
The design of the edge of the cup also affects the
Conclusion
Interpreting finite element models of edge contacts is always difficult due to the sensitive nature of the model. The results of the study suggest that microseparation edge loading of alumina ceramic-on-ceramic hips increases the maximum tensile stress in the acetabular cup to a level where it may exceed the flexural strength of the ceramic material. Once the acetabular cup edge has worn, after the bedding-in period (0-500 000 cycles), stress levels are predicted to reduce by approximately 30%.
Acknowledgments
This work was supported by the NIHR (National Institute for Health Research, United Kingdom) as part of a collaboration with the LMBRU (Leeds Musculoskeletal Biomedical Research Unit).
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2015, Journal of the Mechanical Behavior of Biomedical MaterialsCitation Excerpt :Despite the wear tests performed on NCD, it is important to take into account that during the normal life of a patient, microseparation of the bearing centers of the femoral head and acetabular liner can occur. Microseparation causes edge loading, which changes the contact stress between the conforming spherical bearing surfaces and causes the presence of stripe wear in the ceramics (Walter et al., 2004; Mak et al., 2011; Tipper et al., 2002). Stripe wear is originated from the intragranular (Mak et al., 2011) and intergranular (Tipper et al., 2002; Zeng et al., 2012) fracture, with grain pullout (Walter et al., 2004).
No benefits or funds were received in support of the study.