Abstract
The effects of metal ion release and wear particle debris in metal-on-metal articulation warrants an investigation of alternative material, like ceramics, as a low-wear bearing couple [1]. Short-stem resurfacing femoral implant, with a stem-tip located at the centre of the femoral head, appears to provide a better physiological load transfer within the femoral head and therefore seems to be a promising alternative to the long-stem design [2]. The objective of this study was to investigate the effect of evolutionary bone adaptation on load transfer and interfacial failure in cemented metallic and ceramic resurfacing implant.
Bone geometry and material properties of 3D finite element (FE) models (intact, short-stem metallic and ceramic resurfaced femurs of 44 mm head diameter) were derived from the CT scan data. The FE models consisted of 170352 quadratic tetrahedral elements and 238111 nodes with frictional contact at the implant-cement (μ = 0.3) and stem-bone interfaces (μ = 0.4) and fully bonded cement-bone interface. Normal walking and stair climbing were considered as two different loading conditions. A time-dependant “site specific” bone remodelling simulation was based on the strain energy density and internal free surface area of bone [3]. The variable time-step was determined after each remodelling iteration. The Hoffman failure criterion was used to assess cement-bone interfacial failure.
Predicted change in bone density due to bone remodelling was very much similar in both the metallic and ceramic resurfaced femurs (Fig. 1). Both the metallic and ceramic implant resulted in strain reduction in the proximal regions (Region of interest, ROI 2 and 4) and subsequent bone resorption, average bone density reduction by 72% (Fig. 1). Higher strains were generated in ROI 5 and 7, which caused bone apposition, an average increase in bone density of 145% (Fig. 1). The tensile stresses in the resurfacing implants increased with change in bone density; a maximum stress of 83 MPa and 63 MPa were observed in the ceramic and the metallic implants, respectively. The tensile stress in the cement mantle also increased with bone remodelling. Although the cement-bone interface was secure against interface debonding in the post-operative situation, calculations of Hoffman number indicated that risk of cement-bone interfacial failure was increased with peri-prosthetic bone adaptation.
During the remodelling simulation, maximum tensile stress in the implant and the cement was far below its strength. However, with bone adaptation greater volume of cement mantle was exposed to higher stresses which, in-turn, resulted in greater risk of interfacial failure around the periphery of the cement mantle. Both the short-stem ceramic and metallic resurfacing component, under debonded stem-bone interface, resulted in more physiological stress distribution across the femoral head. Based on these results, short-stem ceramic resurfacing component appears to be a viable alternative to the metallic design.
