Micromotion at the Bone-Stem Interface during the Gait Cycle after Cementless Total Hip Replacement: Influence of Stem Design and Loading Level

Abstract

The micromotion distribution of a cementless stem relative to femoral bone and the stress transfer were calculated with a three dimensional model including bone non-homogeneity, anisotropy and discontinuity of the bone-implant interface. Two implant designs (anatomical, straight) were investigated to point out the geometry effects. Micromotion distributions were calculated over the entire interface during a complete gait cycle taking into account the hip musculature forces. Different loading levels (from 20% to 100% of total load) were stimulated. For the two implants, peak values of distractive and shear micromotions were maximal at the Single Limb Stance phase. The implant geometry had no sensible effects on distractive micromotion magnitude. Conversely, the implant design had noticeable influences on shear micromotions. Regarding the effects of loading levels, at 20% of the full load, interface shear micromotions were lower than 30μm over the entire interface allowing bony ingrowth and expected to ensure initial stability for both implants. At 40%, shear micromotions exceeding 30μm appeared and were located differently according to the stem design.