Background: The acromioclavicular (AC) capsule and ligament have been found to play a major role in maintaining horizontal stability. To reconstruct the AC capsule and ligament, precise knowledge of their anatomy is essential. Purpose/Hypothesis: The purposes of this study were (1) to determine the angle of the posterosuperior ligament in regard to the axis of the clavicle, (2) to determine the width of the attachment (footprint) of the AC capsule and ligament on the acromion and clavicle, (3) to determine the distance to the AC capsule from the cartilage border of the acromion and clavicle, and (4) to develop a clockface model of the insertion of the posterosuperior ligament on the acromion and clavicle. It was hypothesized that consistent angles, attachment areas, distances, and insertion sites would be identified. Study Design: Descriptive laboratory study. Methods: A total of 12 fresh-frozen shoulders were used (mean age, 55 years [range, 41-64 years]). All soft tissue was removed, leaving only the AC capsule and ligament intact. After a qualitative inspection, a quantitative assessment was performed. The AC joint was fixed in an anatomic position, and the attachment angle of the posterosuperior ligament was measured using a digital protractor. The capsule and ligament were removed, and a coordinate measuring device was utilized to assess the width of the AC capsule footprint and the distance from the footprint to the cartilage border of the acromion and clavicle. The AC joint was then disarticulated, and the previously marked posterosuperior ligament insertion was transferred into a clockface model. The mean values across the 12 specimens were demonstrated with 95% CIs. Results: The mean attachment angle of the posterosuperior ligament was 51.4° (95% CI, 45.2°-57.6°) in relation to the long axis of the entire clavicle and 41.5° (95% CI, 33.8°-49.1°) in relation to the long axis of the distal third of the clavicle. The mean clavicular footprint width of the AC capsule was 6.4 mm (95% CI, 5.8-6.9 mm) at the superior clavicle and 4.4 mm (95% CI, 3.9-4.8 mm) at the inferior clavicle. The mean acromial footprint width of the AC capsule was 4.6 mm (95% CI, 4.2-4.9 mm) at the superior side and 4.0 mm (95% CI, 3.6-4.4 mm) at the inferior side. The mean distance from the lateral clavicular attachment of the AC capsule to the clavicular cartilage border was 4.3 mm (95% CI, 4.0-4.6 mm), and the mean distance from the medial acromial attachment of the AC capsule to the acromial cartilage border was 3.1 mm (95% CI, 2.9-3.4 mm). On the clockface model of the right shoulder, the clavicular attachment of the posterosuperior ligament ranged from the 9:05 (range, 8:00-9:30) to 11:20 (range, 10:00-12:30) position, and the acromial attachment ranged from the 12:20 (range, 11:00-1:30) to 2:10 (range, 13:30-14:40) position. Conclusion: The finding that the posterosuperior ligament did not course perpendicular to the AC joint but rather was oriented obliquely to the long axis of the clavicle, in combination with the newly developed clockface model, may help surgeons to optimally reconstruct this ligament. Clinical Relevance: Our results of a narrow inferior footprint and a short distance from the inferior AC capsule to cartilage suggest that proposed reconstruction of the AC joint capsule should focus primarily on its superior portion.
