The bone-implant interface plays a key role in the primary stability achieved at the time of surgery and shortly after. While the mechanical properties of the bone dominate the structural behaviour of the bone-implant complex, friction between bone and implant contributes to primary stability. Although friction between bone and implant has been explored in isolated studies, its systematic investigation is missing. This study investigated the influence of applied load, sliding speed, material and sliding orientation on the friction properties of bovine bone and metallic implant materials.
Tribological experiments were performed using a pin-on-disk system and the coefficient of friction (CoF) was recorded. The applied load spanned from 1 N (elastic regime) to 50 N (irreversible deformation), sliding velocity ranged from 100 to 10'000 µm/s. Three implant materials were tested: 316L steel, Ti6Al7Nb and pure Ti. Pure Ti was also provided with different surface treatments. The effect of sliding direction (along and across the bone fibres) were compared. All experiments were done in saline solution at room temperature.
The results showed that the coefficient of friction between bone and implant depends strongly on both load and speed. However, load affects the CoF more than than the sliding speed: an increase of load from 1 N to 50 N lead to twofold decrease of CoF whereas to achieve a three-fold decrease of CoF, the sliding velocity had to be increased thousand times. At conditions relevant to post-operative activities (gait), the CoF was found to be ~0.4 for the 316L steel and ~0.6 for other Ti-based materials. The various surface treatments of the Ti implants did not strongly affect the CoF. Also very similar values of CoF (~0.45) was observed for both sliding directions.
Our investigations suggest that the CoF of bone versus implant is strongly affected by the load and the sliding speed. Its decrease with both variables suggests that, under the selected conditions, the contact is occurring in a mixed lubrication regime with an increasing load-bearing of the fluid at larger speeds. The CoF for realistic physiological conditions was determined for the three tested material and it was found that surface treatment as well as sliding direction influences the CoF less than the material stiffness.