The most commonly reported failure modes of cementless hip stems are loosening and thigh pain both are attributed to the relative motion at the bone-implant interface due to failure to achieve sufficient primary fixation. The main aim of the current study is to investigate, using Finite Element Analysis, various factors that could affect micromotion and could compromise the stability of cementless femoral stems. We propose a novel technique for predicting hip stem instability to analyse these problems. The designs of cementless hip stems are crucial to its success. We first categorize them into three major types based on the overall geometry and they are all found to be stable under physiological loadings. Tsoelastic' stems are found to increase interface micromotion, but if tight fit is achieved distally, the stem would still be stable. Having shorter stems for primary arthroplasty is beneficial if revision surgery is required, but these produce larger relative motion. The results from this study show that if sufficient cortical contact is achieved distally, stability is not impaired. Two types of hip stems' fixation are also compared the proximal fixation design is found to be less stable than the distal fixation design, but stability can be improved with the use of proximal macrofeatures. The strength of primary fixation also depends on surgical parameters imprecise surgical procedures can cause interfacial gaps, implant undersizing and implant malalignment. The FE results show that undersizing should be avoided because it increases micromotion and instability, especially in stems with cylindrical design. Hip stems with varus malalignment are found to be relatively stable compared to the normally aligned undersized stem. Interfacial gaps due to surgical error are not found to impair the stem's fixation as long as maximum press-fit is achieved. Successful implant fixation also depends on the quality of the bone. Bone with skeletal diseases of osteoporosis and osteoarthritis are analysed and compared with the results from a normal healthy bone. The hip stem in the osteoporotic bone is found to have the largest micromotion and is the most unstable, especially during stair-climbing activity.

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