This case-control study introduced the acromion index, a radiographic ratio measuring how far the acromion extends laterally over the humeral head. It asks whether a larger lateral acromial extension is associated with full-thickness rotator cuff tears compared with intact cuffs. The index is measured on a true AP radiograph and compared across tear, osteoarthritis, and normal control groups.
When you read a shoulder AP for cuff pathology, the acromion index gives you a quick lateral-coverage number to add to Bigliani type and the lateral acromion angle. A high index (tear group averaged 0.73) means the acromion overhangs the humeral head, which the authors argue tilts the deltoid force vector upward and drives the head against the coracoacromial arch.
The mental model is useful: a wide acromion favors impingement and cuff tearing, while a narrow acromion pushes force into the joint and may favor glenohumeral arthritis. This explains why the OA group had the lowest index. Measure it correctly. It is only valid on a true AP with the arm in neutral or internal rotation, since external rotation and scapular malrotation inflate the number.
Remember the ceiling on this evidence: Level IV, association only. The authors explicitly could not say whether the large acromion causes tears or results from them.
This case-control study introduced the acromion index, a radiographic ratio measuring how far the acromion extends laterally over the humeral head. It asks whether a larger lateral acromial extension is associated with full-thickness rotator cuff tears compared with intact cuffs. The index is measured on a true AP radiograph and compared across tear, osteoarthritis, and normal control groups.
When you read a shoulder AP for cuff pathology, the acromion index gives you a quick lateral-coverage number to add to Bigliani type and the lateral acromion angle. A high index (tear group averaged 0.73) means the acromion overhangs the humeral head, which the authors argue tilts the deltoid force vector upward and drives the head against the coracoacromial arch.
The mental model is useful: a wide acromion favors impingement and cuff tearing, while a narrow acromion pushes force into the joint and may favor glenohumeral arthritis. This explains why the OA group had the lowest index. Measure it correctly. It is only valid on a true AP with the arm in neutral or internal rotation, since external rotation and scapular malrotation inflate the number.
Remember the ceiling on this evidence: Level IV, association only. The authors explicitly could not say whether the large acromion causes tears or results from them.