Abstract
The radiographs of sixty-four patients with seventy humeral head replacements were reviewed for signs of stress shielding. Forty-nine were implanted for rheumatoid arthritis, twenty-one for osteoarthritis. The radiographic follow-up averaged 5. 3 years. Measurements of cortex thickness were performed in four regions along the stem of the implant and the differences between the post-operative radiograph and radiograph at follow-up were calculated. The size of the stem in relation to the diameter of the humerus was calculated using validated measures, resulting in the relative stem size.
A reduction of 1.6 millimeters or more was considered to be a significant reduction, because this lay outside of the calculated 95% normal range for the group as a whole. In six patients (9%) a significant reduction, in cortical thickness was observed in the proximal lateral region of the humeral stem. Five of these had rheumatoid arthritis and one osteoarthritis. In the stress shielding group the relative stem size was found to be significantly higher (p=0. 013) than in the non-stress shielding group (0. 58 versus 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. These resorptive processes may lead to a higher risk of failure of the implant and gives an increased risk for mid-stem fractures, due to cantilever loading. It is also desirable to preserve the proximal bone stock, considering the difficulties that arise when, for whatever reason, revision of the implant is necessary.
