Biomechanics of Endoprostheses of the Hip and Adaptive Reactions of the Bone

Summary

Following the hypothesis of Pauwels [1–3], the bone adapts to its actual mechanical stress by densification and apposition of new material or by decalcification and resorption. Increasing stress stimulates bone formation; decreasing stress stimulates resorption. However, extreme magnitudes of stress may lead to paradoxical loss of bone substance. Implantation of an endoprosthesis changes the mechanical stress of the bone considerably. It must be expected, therefore, that the structure and distribution of the bone material will change remarkably. These changes can be visualized and quantified by the densitometry of X-ray pictures. For this purpose, 5-mm-thick cross-sectional slices were taken from anatomical specimens of femora with implanted endoprostheses. X-rays of these slices were analyzed by computer-aided densitometry. Bone condensation was found at the sites of stress concentration. More transparent areas, however, could be attributed to either extremely low or extremely high stresses. This loss of bone material is responsible for the loosening of endoprostheses. These phenomena are demonstrated on examples of cementless and cemented femoral endoprostheses.