Acromioclavicular stability: a biomechanical comparison of acromioplasty to acromioplasty with coplaning of the distal clavicle

PURPOSE: The purpose of the study was to compare the laxity of the acromioclavicular (AC) joint in the superior, posterior, and anterior planes after isolated acromioplasty and after acromioplasty with inferior clavicular coplaning. TYPE OF STUDY: In vitro (cadaveric) analysis. METHODS: Eight fresh-frozen cadaveric shoulders were evaluated using a hydraulic actuator. While the scapula was stabilized, a 30-N force was applied to the distal clavicle perpendicular to the AC joint in the superoinferior plane and parallel to the joint in the anteroposterior plane. Laxity of the AC joint in the superior, anterior, and posterior directions was evaluated via load-displacement analysis after acromioplasty and after acromioplasty with coplaning. RESULTS: Coplaning the distal clavicle increased superior AC laxity by 53% compared with acromioplasty alone (P =.012). With regard to anteroposterior laxity, coplaning increased anterior translation by 19% (P =.047) and increased posterior translation by 16% (P =.237). Bony impingement was seen to limit posterior translation in 3 specimens. CONCLUSIONS: Acromioplasty with coplaning increases AC laxity significantly in the superior and anterior directions as compared with acromioplasty alone. A trend toward increased posterior translation was found; posterior bony impingement may limit posterior laxity.