What is the most likely diagnosis for a patient with shoulder pain, limited movement, and significant loss of external rotation?

The most likely diagnosis for a patient presenting with shoulder pain, limited movement, and significant loss of external rotation is frozen shoulder, also known as adhesive capsulitis. This condition is characterized by the progressive stiffness and pain in the shoulder, leading to a significant reduction in the range of motion. One of the hallmark features of frozen shoulder is the pronounced loss of external rotation, especially when compared to the internal rotation and flexion capabilities.

In frozen shoulder, the inflammation and thickening of the shoulder capsule occur, which restricts movement over time. The pain can initially be severe and often worsens, while the range of motion continues to decline, making everyday activities challenging. Positioning the arm in various ways becomes especially difficult due to the restriction caused by the condition.

The other conditions might present with shoulder pain and limited movement but tend to have different characteristics. A rotator cuff tear often leads to weakness in arm elevation and may not exclusively limit external rotation. Shoulder bursitis typically causes pain and limited motion but does not usually result in such a severe decrease in external rotation. Osteoarthritis may also present with pain and stiffness but usually involves more generalized symptoms, including crepitus and often affects multiple joints rather than being isolated like frozen shoulder.

Thus, the symptoms