Which examination finding is most characteristic of AC joint instability?

Which examination finding is most characteristic of AC joint instability?

• A. Pain with palpation of the AC joint
• B. Positive O'Brien active compression test with Decreased symptoms when the thumb is oriented upward
• C. Pain at the top of the shoulder during resisted shoulder flexion only
• D. Paxinos cross-body adduction test result

Answer: (B)

AC joint instability is best identified when a test stresses the AC joint and then shows a change in pain with a simple adjustment of forearm position. The active compression test is performed with the arm flexed to about 90 degrees, the elbow straight, and the arm adducted across the chest while the patient resists a downward force. If this position reproduces pain at the top of the shoulder, it points toward AC joint involvement. Crucially, when the same maneuver is repeated with the forearm supinated (thumb oriented upward), AC joint pain often decreases. This pattern—pain with the compressed position that improves when the thumb is oriented upward—has high specificity for AC joint pathology and helps distinguish AC joint instability from intra-articular or labral problems. Other findings like pain with AC joint palpation or pain with resisted flexion alone are less specific for instability, and a cross-body adduction test can indicate AC joint issues but doesn’t as clearly demonstrate the instability pattern the O’Brien test reveals.

AC joint instability is best identified when a test stresses the AC joint and then shows a change in pain with a simple adjustment of forearm position. The active compression test is performed with the arm flexed to about 90 degrees, the elbow straight, and the arm adducted across the chest while the patient resists a downward force. If this position reproduces pain at the top of the shoulder, it points toward AC joint involvement. Crucially, when the same maneuver is repeated with the forearm supinated (thumb oriented upward), AC joint pain often decreases. This pattern—pain with the compressed position that improves when the thumb is oriented upward—has high specificity for AC joint pathology and helps distinguish AC joint instability from intra-articular or labral problems. Other findings like pain with AC joint palpation or pain with resisted flexion alone are less specific for instability, and a cross-body adduction test can indicate AC joint issues but doesn’t as clearly demonstrate the instability pattern the O’Brien test reveals.