Interface stresses in the resurfaced hip: Finite element analysis of load transmission in the femoral head

Abstract

The load transmission and interface stresses in the Wagner resurfaced femoral head were evaluated for the purpose of studying possible failure mechanisms. We found that unnatural stress patterns occur in the head and at the implant-bone interfaces, in addition to regions of stress protection in the bone, possibly enhancing interface failure and bone remodelling. However, these stresses are not higher than those reported for other kinds of prostheses, e.g. acetabular cup, tibial plateau. From these findings, together with clinical observations, it is hypothesized that the femoral surface cup is more sensitive to local loosening than other prostheses. This hypothesis would indicate that prosthetic designs should be analysed relative to their potential to provoke failure propagation, rather than only initiation of mechanical failure and loosening.

Influence of femur size and morphology on load transfer in the resurfaced femoral head: A large scale, multi-subject finite element study

“…Despite this, resurfacing implants alter load transfer through the proximal femur; with strain being reduced in the femoral head and increased in the femoral neck (Huiskes et al, 1985;Watanabe et al, 2000;Ong et al, 2006;Cristofolini et al, 2009). The major observed complication with recent designs is neck fracture, accounting for over 40% of revisions in the Australian Orthopaedic Association National Joint Replacement Registry (2008) with an incidence of approximately 2% (Shimmin and Back, 2005).…”

A clinical, radiological and biomechanical study of the TARA hip prosthesis

“…Cook et al [11] compared the load transfer in the TARA prosthesis to that in four other resurfacing prostheses, using finite element method. He noted substantial differences in the stress patterns when a TARA prosthesis was placed, This is, of course, important, because the stress concentrations are though to be responsible for loosening [24,25,32,34].…”

“…Although the precise causes of the failures have not been demonstrated beyond doubt, a number of hypotheses have been suggested, such as excessive wear particles causing osteolysis of the interfaces, the difficulty of the surgery, inviting errors, the thin polyethylene cup, disturbances of the vascularity under the metal head, and biomechanical causes related to load transfer, stress shielding and interface failure. Biomechanical finite eleCorrespondence to: M. C. deWaalMalefijt, MD, PhD ment analyses of the Tharies prosthesis [32], the ICLH prosthesis [31], the Wagner prosthesis [24,25] and the Gerard prosthesis [39] have been published. In all cases, it was found that the relatively rigid metal resurfacing cup produces unnatural stress-shielding effects in the underlying bone and stress concentrations in the bone around the cup rim.…”

Curved-stem Hip Resurfacing

“…This was despite the effort made to downsize the femoral head that resulted in femoral neck notching in some cases. Placing the femoral component in valgus reduces the stresses in the superior aspect of the femoral head and neck [23,27,45]. Femoral components placed in 5°valgus have a factor of 6.1 reduction in the relative risk of an adverse outcome [7].…”