From the Research
The normal spacing in the acromioclavicular (AC) joint is typically 1.6 to 2.9 mm in adults, as measured by the width of the AC joint capsular insertion on the acromion and clavicle 1. This measurement represents the vertical distance between the inferior aspect of the distal clavicle and the superior aspect of the acromion process of the scapula. Variations can exist based on individual anatomy, gender, and body size, with men often having slightly wider spacing than women. When evaluating AC joint spacing on radiographs, it's essential to compare with the contralateral (opposite) side when possible, as this provides the best reference for what is normal for that particular individual. Increased spacing beyond 5-6 mm often indicates AC joint separation or instability, which may be classified according to severity (Rockwood classification). The AC joint spacing is maintained by the acromioclavicular and coracoclavicular ligaments, which provide stability to this junction. When these ligaments are injured, the clavicle may displace superiorly relative to the acromion, resulting in abnormal widening of the AC joint space. Some studies suggest that a joint space wider than 7 mm in men and 6 mm in women is abnormal 2. However, the most recent and highest quality study available, which is a cadaveric study published in 2021, does not provide a specific measurement for the normal AC joint spacing 3. Therefore, the recommendation is based on the study by 1, which provides a detailed analysis of the AC joint capsular insertion and ligamentous anatomy. Key points to consider when evaluating AC joint spacing include:
- The width of the AC joint capsular insertion on the acromion and clavicle
- The distance from the medial acromion to the coracoacromial ligament insertion
- The axial and coronal angle of the AC joint
- The presence of any ligamentous injuries or instability.