Clavicular-Sided Tears Were the Most Frequent Mode of Failure During Biomechanical Analysis of Acromioclavicular Ligament Complex Failure During Adduction of the Scapula

PURPOSE: The purpose of this study was to describe the force and failure pattern of the acromioclavicular ligament complex (ACLC) in an adducted scapula, potentially simulating an indirect force injury of the AC joint. By using a biomechanical simulation in which the scapula is moved and the clavicle is fixed, we are able to better replicate the in vivo motion of the joint. METHODS: Ten cadaveric shoulders (mean age of 62.0 ± 8.6) with a bone mineral density of .51 ± .18 g/cm(2) were used. A standard reproducible anatomic mounting system was used to secure the clavicle and move the scapula. Displacement control was used to adduct the scapula (inferior angle of the scapula moving toward the clavicle) with the clavicle fixed until specimen failure, producing torque and angle of rotation. The failure mode of the ACLC during this simulated adduction was analyzed with slow motion video analysis. Tears of the ACLC were characterized as clavicular, midline, or acromion-sided tears. RESULTS: The mean torque required for load to failure was found to be 27.75 N-m (95% CI [20.85 N-m, 34.65 N-m]). The mean rotary angle at failure was 30° (95% CI [25°, 35°]). The mean stiffness (resistance provided by the ACLC) was 1.64 N-m/° (95% CI [1.28 N-m/°, 2.01 N-m/°]. Mode of failure analysis showed there were 6 clavicle-sided tears, 1 acromion-sided tear, 2 acromion fractures, and 1 clavicle fracture. CONCLUSIONS: Clavicular side tears were the most frequent mode of failure compared to midline and acromion side tears. The first segment of the ACLC to fail most frequently during adduction was the posterosuperior ligament. CLINICAL RELEVANCE: This biomechanical study simulates a potential mechanism of AC ligament injury. Additional knowledge about the mode of failure provides a better understanding of the ACLC, allowing for new information for the purpose of AC ligament reconstruction.