Abstract
Introduction
In total hip arthroplasty the femoral head is connected to the stem based on a taper connection (Fig. 1). Implant manufacturers proclaim that the modular tapers are not standardized and can vary from manufacturer to manufacturer. The combination of different implant components from different manufacturers (Mix & Match) is not permitted. However, in case of revision surgery, where the stem is still well fixed, the surgeon may decide to use a femoral head of a different manufacturer (Mix & Match). This decision may be related to a limited availability of a manufacturer-identical head or the manufacturer can't be identified. In this study, different taper combinations were experimentally investigated to assess the effect of Mix & Match on taper strength.
Methods
Hip stems and metal heads (n=3) of four different implant manufacturers (Biomet, DePuy, Smith&Nephew, Zimmer) were used. Firstly, torque-off tests similar to the ISO draft (ISO/DIS 7206-13) were performed without mixing the components. Subsequently, the stems were combined with metal heads from other manufacturers. All taper connections were impacted with 3 kN and then an increasing torque was applied until the head disconnected using a material testing machine (MTS Mini Bionix), (Fig. 2). The maximal torque off value was used as a measure for the taper strength.
Results
Basically three different characteristics were identifed:
-
For the stems of the manufacturers Smith&Nephew and Zimmer there were no significant differences in taper strength using heads from other manufacturers.
-
The Biomet stems showed a significantly reduced taper strength (up to 14%) if femoral heads of DePuy or Smith&Nephew were used, while there was no significant difference using heads of Zimmer.
-
Interestingly it seems that DePuy stems in combination with the originally intended femoral heads lead to lower taper strengths compared to the use of heads of all other manufacturers.
Discussion
This study clearly shows that a general recommendation regarding the risk of Mix & Match of stem tapers and femoral heads can't be given. The results of this study suggest that mixing of components of different manufactures may affect the taper strength positively and negatively. Obviously, some implant designs are more robust regarding taper strength in a case of a Mix & Match situation than others. However, these results were only evaluated experimentally and were limited to the taper strength. Regarding the clinical situation further aspects like taper corrosion should be considered.
