Friction coefficients at the curved bone–implant interface using a custom tribometer for K-wire fixation

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Abstract
Kirschner (K) wires are widely used for fracture fixation, where curved metal surfaces contact cortical bone under press-fit conditions. However, the tribological parameters at the curved bone–implant interface remain insufficiently characterized, despite their importance for initial construct stability and accurate boundary conditions in finite element analysis. In this study, we quantified the static and kinetic coefficients of friction between bone and two clinically used K-wire materials. A custom-designed curved-contact tribometer was developed to reproduce the geometric constraints imposed by cylindrical wire insertion. Semicircular grooves were machined into rabbit humeri, and either stainless steel or titanium alloy K-wires were placed within each groove. The bone specimen was then pulled at a constant speed under a controlled normal load to measure the frictional response. The static friction coefficient differed significantly between stainless steel (0.370 ± 0.115) and titanium alloy (0.456 ± 0.099), suggesting that surface-related properties may contribute more strongly to the observed material-dependent friction than contact geometry alone. Kinetic friction followed a similar trend. Inter-bone variability exceeded intra-bone variability across measurement grooves, indicating that biological heterogeneity may dominate local surface effects. These quantitative friction coefficients provide experimentally grounded parameters for bone–implant contact modeling and may help improve the predictive accuracy of computational simulations of fracture fixation. Future work should investigate the effects of lubrication and surface topography during actual K-wire insertion.

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