The importance of stemgeometryfor load transfer andprimary stability in uncementedhip arthroplasty: Experimental laboratory studies on human cadaverfemurs
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Anatomically shaped- and customized uncemented femoral stems are designed to fit and lock to the endocotical bone in the metaphyseal part of the femur and to avoid filling and locking in the diaphyseal part of the femur to achieve proximal load transfer from implant to bone. The geometry of the stems should also ensure primary stability of the stems, with a minimal cyclical movement at the bone implant interface to allow bone ingrowth into the surface of the implant. In this thesis we inserted the anatomical ABG I stem and the customized Unique stem with different proximal fit and fill and also different stemlength in cadaver femurs. The femurs were loaded in a hip simulator corresponding to single leg stance and stair climbing. Alterations in cortical strain pattern were measured after insertion of the different stems. Our results show that insertion of the anatomical ABG-I stem causes a stress shielding in the proximal femur that can explain the clinically reported negative bone remodelling. Insertion of the customized Unique stem resulted in a more physiological pattern of strain in the proximal femur compared to the anatomical ABG I stem. The more favourable pattern of strain observed for the Unique stem can be explained by the more optimal proximal fit of the stem, but the shorter stemlength of the Unique stem could also contribute to the more physiological pattern of strain. To study the relationship between the length of the stem and changes in the pattern of strain we gradually shortened the stem length of the ABG-I stem. Shortening of the distal part of the ABG-I stem resulted in a more physiological pattern of cortical strain in the proximal femur. A shortening of 40-50 mm of the stem nearly normalized the cortical strains measured in the lower metaphysis and in the upper diaphysis. The strain shielding observed in the calcar region was not related to stem length. In this thesis we also present a reproducible indirect method for in-vitro measurements of implant-bone interface movements in all six degrees of freedom at any wanted levels of the stem, leaving the femur intact for further analysis. In our studies both the anatomical and customized stems achieved primary stability allowing bone ingrowth and permanent fixation of the stems. The customized stem with optimal proximal fit and fill provided the best initial stability for rotation in retroversion, whereas the longer anatomical stem was more resistant to permanent rotation into varus. Shortening of the anatomical stem ABG-I stem did not affect the cyclical micromotion of the stem, indicating that the distal 50 mm of this stem is not needed to ensure biological osseointegration of the HA-coated part of the stem in clinical use.