Rheumatoid arthritis results in pain and loss of function due to gradual destruction of articular cartilage. The shoulder joint is frequently involved and a prosthetic replacement of the humeral head can restore function and relieve pain. Deficiency of the rotator cuff is common in patients with rheumatoid arthritis. Longevity of movement at the intraprosthetic interface of the bipolar shoulder prosthesis is debatable and has not previously been studied in rheumatoid arthritis.

We report a radiological study of the intraprosthetic movements of a bipolar shoulder replacement in 25 shoulders in 20 patients with rheumatoid arthritis of mean age 66 years (SD 10 years). Shoulders were X-rayed at a minimum of 3 and a maximum of 10 years from surgery. Measurements were repeated in 12 shoulders 3 years later. The patient was positioned in the scapular plane. An initial X-ray was taken with the arm in neutral and a further X-ray taken with the arm in full active abduction. Measurements were taken to determine the movement at the intraprosthetic interface and at the prosthesis/glenoid interface. Interobserver error and intraobserver error were determined using an intraclass correlation coefficient (ICC). A paired T-test and Pearson Correlation Coefficient were used to compare intraprosthetic movement with prosthesis/glenoid movement.

We found that intraprosthetic movement was preserved up to 10 years from surgery. However, there was no significant difference between intraprosthetic movement and shell/glenoid movement, with some shoulders exhibiting paradoxical movement at the intraprosthetic interface. Repeating the measurements after a 3 year interval in a subgroup of 12 shoulders showed a significant difference in intraprosthetic movement. Interobserver and intraobserver reliability for measurements of the movement at the intraprosthetic interface were excellent with a Kappa value of 0.92 for intraobserver error and a Kappa value of 0.94 for interobserver error.

We conclude that movement of the bipolar shoulder prosthesis in rheumatoid shoulders at the intraprosthetic interface is preserved up to 10 years from operation but is not related to or significantly different from prosthesis/glenoid movement and requires further investigation.