Bone changes in total hip replacement

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Bone remodeling around an endoprostheses is dependent on the site of healing of the bone to the bone ingrowth stem and to the stiffness of the stem. The healing associated with the bone ingrowth pattern is characterized by the spot weld formation as described by Engh for fixation in the diaphysis. With healing in the metaphysis the pattern is one which we have termed the “buttress pattern of healing.” On x-ray film this appears as a medial and lateral condensation of bone that directs the load into the diaphyseal cortices. When the implant is stable, the endosteal healing progresses along the entire length of the stem moving from proximal to distal. The bone demonstrates less stress shielding if the bone is loaded parallel to its grain rather than across the grain. Implant retrieval demonstrates a neocortex of bone that forms along the length of the stem. If the implant is stable, the neocortex forms adjacent to the metal. With motion of the stem, the neocortex forms at the periphery of the motion and a radiolucent line is evident on the film. On implant retrieval all bone around a hip stem is seen to be osteoporotic. In revision bone the same patterns of healing are seen. The initial stability and healing of the implant is more dependent on the structure of the bone than the cellular function of the bone because this cellular function is much the same regardless of the amount of osteolysis.

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