Role of the collar on the femoral stem of cemented total hip replacements

Three dimensional finite element models with realistic loading conditions were used to analyse the role of the collar of cemented total hip stems. Three types of implant were considered: large collared stems, small collared stems and totally collarless stems. A computer model of the natural femur provided an indication of the bone stresses before hip replacement, and thus aided the identification of possible mechanisms of bone resorption. In particular, resorption of the medial femoral neck was examined, and its effect on the stress distribution within the stem-cement-bone structure is discussed. The results indicate that loss of bone at the femoral neck is a serious symptom for any design of implant. The factors leading to the resorption are complex but it is clear that it is not a consequence of simple stress shielding of the bone. The most likely cause is the manner in which the stem moves within the femoral shaft, which is a function of whether the stem is collared or not.