Design and fabrication of cementless hip stems.

Theoretical and experimental studies before and after stem insertion demonstrated that stresses and strains were closer to normal for uncemented stems than for cemented stems. The values were affected by relative tightness of fit in the proximal and distal regions and by the presence or absence of a collar. For designing an optimal fit-stem, the average femoral geometry was first determined. The stem was used in a photoelastic coating study that showed a continuous strain field over the bone surface. The average proximomedial bone strains for intact, press-fit, loose press-fit with collar, and proximally cemented (to simulate ingrowth) designs were 100%, 65%, 101%, and 54%, respectively. Localized patches of high strain were seen on the proximal bone surface and beneath a collar, indicative of localized stem-bone contact points.