Stress shielding and bone resorption in shoulder arthroplasty.

The radiographs of 64 patients with 70 humeral head replacements were reviewed for signs of stress shielding. Of these, 49 were implanted for rheumatoid arthritis and 21 for osteoarthritis. The radiographic follow-up averaged 5.3 years. Measurements of cortex thickness were performed in 4 regions along the stem of the implant, and the differences between the postoperative radiograph and the radiograph at follow-up were calculated. The size of the stem in relation to the diameter of the humerus was calculated with the use of validated measures, resulting in the relative stem size. In 6 patients (9%) a significant reduction in cortical thickness was observed in the proximal-lateral region of the humeral stem, 5 in rheumatoid patients and 1 in an osteoarthritic patient. In the stress shielding group, the relative stem size was found to be significantly higher than that in the non-stress shielding group (0.58 vs 0.48). Osteoporosis, especially present in rheumatoid arthritis, could well be a risk factor. It was concluded that stress shielding is a long-term complication of shoulder arthroplasty and that the relative stem size is an important factor in its genesis.