We present radiological results regarding the development of stress shielding around the stem of the AML hip endoprosthesis and discuss the clinical significance of it and preventive measures against stress shielding.
391 patients (216 female and 122 male) with an AML-hip-endoprosthesis between 1991 and 1992 were followed clinically and radiologically during 5 years postoperatively. It was searched for a statistical correlation of stress shielding with a pre-existing osteoporosis of the proximal femur (Barnett-Nordin index), the diameter of the stem, a varus position of the stem and thigh pain.
Stress shielding occurred in only 13.5 % of all patients. Atrophy of the proximal femur was detectable within 1 year, in 5 cases already after 3 weeks. Distal sclerosis reached its maximum after 1-2 years. Remarkably, women showed a tendency to more frequent stress shielding. In women more often than in men, a pre-existing osteoporosis of the proximal femur occurred. Cases with osteoporosis of the proximal femur experienced more often stress shielding. The same was seen in prostheses with diameters >13.5 mm. A slight varus position of the stem also favoured stress shielding, but it indicated an incomplete canal fit of the stems, too. Patients with stress shielding more often were suffered from thigh pain.
Stress shielding is a result of the discrepancy between the elasticity of the bony femur and the stiffness of the stem. The AML-hip-prosthesis rarely shows this phenomenon. An osteoporosis of the proximal femur (coupled with an thinning of the cortex and generating higher elasticity of the bone), stems with higher diameters (what means higher stiffness) and incomplete canal fit more likely predisposed patients to stress shielding. To prevent stress shielding, the AML prosthesis should not be implanted in cases with known severe osteoporosis of the proximal femur. This prosthesis should be taken preferable in cases with diameters of the femoral canal smaller than 15 mm. Fully canal fit has to be achieved. The frequency of thigh pain, which occurs more often in patients with stress shielding, could be reduced by considering those factors.