Investigation of relative micromotion at the stem-cement interface in total hip replacement

Cemented total hip replacement has become a standard surgical technique to treat patients with osteoarthritis and osteonecrosis. The stem-cement interface experiences fretting wear in vivo due to low-amplitude oscillatory micromotion under physiological loading, and this wear is currently becoming important as a potential mechanism for the overall wear of cemented total hip replacements. However, the relative micromotion at the stem-cement interface has not been widely reported. In the present study, a new micromotion sensor is developed that is based on the deformation of a strain gauge, and this sensor is used to probe the migration of a polished Exeter stem within a Simplex P cement mantle through an in vitro wear simulation. It is demonstrated that the stem migration value generally increases with an increase in the number of loading cycles, with a gradual decrease of migration rate. Additionally, fretting wear is successfully replicated on the stem surface, and the micropores in the cement surface are considered to contribute to initiation and propagation of the fretting damage on the stem. This is confirmed by the observation that no evidence of fretting wear is detected on the stem where the surface is in contact with the pore-free areas on the cement. This study allows a deep insight into the micromotion at the stem-cement interface, and provides evidence highlighting the significance of the micropores in the cement surface in the generation of fretting wear on a polished femoral stem.