Shoulder dysfunction is usually caused by the shoulder capsule being too tight or too loose, by the rotator cuff musculotendinous unit being damaged or fatigued and less frequently from damage to the articular cartilage of the glenohumeral joint or acromioclavicular joint.

AC joint pain is the one shoulder disorder where pain is localized. Stiffness can be confirmed by testing external rotation motion and looseness by an apprehension test. Rotator cuff function can be checked, by assessing power of external rotation and supraspinatus, and impingement signs.

X-rays are useful for diagnosing calcific tendonitis, glenohumeral joint arthritis and cuff-tear arthropathy. Ultrasound is used to confirm a rotator cuff tear. A single corticosteroid injection in the relevant location is a useful first-line treatment for AC joint pain and impingement.

Persistent impingement, instability, stiffness and many rotator cuff tears can be restored effectively with minimally invasive arthroscopic surgery. Shoulder replacement is effective for treating severe gleno-humeral joint arthritis.