In vivo strain measurements from bone and prosthesis following total hip replacement. An experimental study in sheep

The mechanical response of the proximal part of the femur after total hip replacement was investigated in vivo in sheep by attaching rosette strain gauges to both femora. On one side the femur was intact and on the other side a total hip replacement was performed using a femoral component to which single-element strain-sensitive gauges had been attached. Strains were recorded in the intact femora and in those containing the prosthesis as well as in the stem of the femoral component while the animals walked at a speed of one meter per second. In the region of the calcar femorale of the intact femur the direction of the peak principal compressive strain passed through the femoral head, and the peak principal tensile strain therefore was almost transverse. Strains on the lateral surface of the femoral shaft (away from the calcar femorale) suggested that here there was torsional loading around the femur's long axis. A few days after total hip replacement the direction of the peak principal compressive strain in the calcar femorale during walking was the same as that in the calcar femorale of the intact femora but the amount of strain was reduced by 58 per cent. The peak principal tensile strain, however, was increased by 31 per cent. Semi-analytical treatment of the strain data obtained from the intact femora and from the prostheses and femora containing them suggested micromovement and incomplete transference of load between the prosthesis and the proximal part of the femur. This so-called looseness was present a few days after insertion of the prosthesis and almost certainly occurred at the bone-cement interface. In four animals in which recordings were made between thirty-eight and 141 days after implantation of the prosthesis, the transference of load to the bone at the level of the calcar appeared to decrease further with time. Clinical relevance: The extensive resorption of the calcar femorale that may occur after total hip replacement cannot be explained solely by a decrease in compressive strain in the calcar, but probably results from the total reorganization of the strain pattern (including compression and tension) in that region. Some looseness between the prosthesis and bone is present immediately after implantation, but the resorption induced by the inappropriate strain pattern leads to further loosening and decreased transference of load from the prosthesis to the bone at the level of the calcar femorale. These two features of inappropriate strain and progressive looseness may be part of a self-perpetuating sequence from which there is no possibility of reversal.