Subacromial space measurement: a reliable method indicating fatty infiltration in patients with rheumatoid arthritis.

Proximal migration of the humeral head is thought to indicate fatty infiltration of the rotator cuff muscles or rotator cuff tears. We sought to evaluate the influence of these rotator cuff abnormalities on the subacromial space. Using anteroposterior radiographs, ultrasound, and computed tomography, we analyzed 54 shoulders in 29 patients with rheumatoid arthritis. The upward migration index was defined as proximal migration of the humeral head relative to its size. The mean muscle density from computed tomography images was used to indicate fatty infiltration. Fatty infiltration of the infraspinatus muscle showed the strongest correlation with proximal migration. After correcting for age, cuff tears, and rheumatoid disease, the partial correlation coefficient between both remained strong. A subdivision in proximal migration is proposed to screen for rotator cuff abnormalities. A large amount of fatty infiltration was indicated by an upward migration index less than 1.25, a medium amount by an upward migration index between 1.25 to 1.35, and a small amount by an upward migration index greater than 1.35. Measurement of proximal migration using the upward migration index provides a reliable screening method indicating fatty infiltration of the rotator cuff.