BACKGROUND: Approximately 9% of shoulder girdle injuries involve the acromioclavicular joint (ACJ). There is no clear gold standard or consensus on surgical management of these injuries, in part perpetuated by our incomplete understanding of native ACJ biomechanics. We have therefore conducted a biomechanical study to assess the stabilizing structures of the ACJ in superior-inferior (SI) and anterior-posterior (AP) translation. METHODS: Twenty fresh frozen cadaver specimens were prepared and mounted to a robotic arm. The intact native joint was tested in SI and AP translations under 50N displacing force. Each specimen was re-tested after sectioning of its stabilizing structures in the following order; investing fascia, ACJ capsular ligaments, trapezoid ligament, and conoid ligament. Their contributions to resisting ACJ displacements were calculated. RESULTS: In the intact native ACJ, mean anterior displacement of the clavicle was 7.9 +/- 4.3mm, mean posterior displacement was 7.2 +/- 2.6mm, mean superior displacement 5.8 +/- 3.0mm, and mean inferior displacement 3.6 +/- 2.6mm. The conoid ligament was the primary stabilizer of superior displacement (45.6%). The ACJ capsular ligament was the primary stabilizer of inferior displacement (33.8%). The capsular ligament and conoid ligament contributed equally to anterior stability, with 23% and 25.2% respectively. The capsular ligament was the primary contributor to posterior stability (38.4%). CONCLUSION: The conoid ligament is the primary stabilizer of superior displacement of the clavicle at the ACJ and contributes significantly to AP stability. Consideration should be given to reconstruction of the ACJ capsular ligament for complete AP stability in high grade and horizontally unstable ACJ injuries.