Primary Stability of Collared and Collarless Cementless Femoral Stems – A Finite Element Analysis Study

BACKGROUND: Primary stability of the femoral stem is important for the long-term results of cementless total hip arthroplasty. Cementless collared stems have been known to have higher stability than collarless stems when there is a contact between the collar and the calcar. The purpose of this study was to compare the stabilities of collared stem and collarless stem in 2 loading conditions: 1) flat walking and 2) stair climbing. METHODS: We constructed 3 finite element models. In the first model, the collar had contact with the calcar. The second model had a 1 mm gap between the calcar and the collar. The third model was constructed with a collarless stem. The proximal femur around the stem was divided into 3 zones: the upper zone (Gruen zones 1 and 7), the middle zone (Gruen zones 2 and 6), and the lower zone (Gruen zones 3 and 5). The micromotion at the stem/bone interface was measured at each zone of the 3 models under the 2 loading conditions. RESULTS: The results showed that collared stems were more stable when the collar was in contact with the calcar than when a gap was left between the collar and the calcar. In particular, collar contact was highly effective in suppressing the micromotion proximal to the stem. CONCLUSIONS: Compared to the collarless stem, the collared stem had comparable stability when there was a gap at the collar and calcar interface and higher stability when there was contact between the collar and the calcar.