Original Research The Role of the Trapezius in Stabilization of the Acromioclavicular Joint A Biomechanical Evaluation Maxwell T. Trudeau,* † BS, Jonathan J. Peters, † BS, Benjamin C. Hawthorne, † BS, Ian J. Wellington, † MD, Matthew R. LeVasseur, † MD, Michael R. Mancini, † MD, Elifho Obopilwe, † MS, Giovanni Di Giacomo, ‡ MD, Simone Cerciello, § MD, and Augustus D. Mazzocca, † MS, MD Investigation performed at the Department of Orthopaedic Surgery, University of Connecticut School of Medicine, Farmington, Connecticut, USA Background: Acromioclavicular joint (ACJ) injuries are common, and many are adequately treated nonoperatively. Biomechanical studies have mainly focused on static ligamentous stabilizers. Few studies have quantified ACJ stabilization provided by the trapezius. Purpose/Hypothesis: To elucidate the stabilization provided by the trapezius to the ACJ during scapular internal and external rotation (protraction and retraction). It was hypothesized that sequential trapezial resection would result in increasing ACJ instability. Study Design: Controlled laboratory study. Methods: A biomechanical approach was pursued, with 10 cadaveric shoulders with the trapezius anatomically force loaded to normal. The trapezius was then serially transected over 8 trials, which alternated between clavicular defects (CD) and scapular defects (SD); each sequential defect consisted of 25% of the clavicular or scapular trapezial attachment. After each defect, specimens were tested with angle-controlled scapular internal and external rotation (12  ) with rotary torque measurements to evaluate ACJ stability. Results: The mean resistance in rotary torque for 12  of scapular internal rotation (protraction) with native specimens was 7.0 ± 2.0 Nm. Overall, internal rotation demonstrated a significant decrease in ACJ stability with trapezial injury (P < .001). Eight sequential defects resulted in the following significant percentage decreases in rotary torque from native internal rotation: 1.5% (25% CD; 0% SD), 5.6% (25% CD; 25% SD), 5.1% (50% CD; 25% SD), 6.5% (50% CD; 50% SD), 3.8% (75% CD; 50% SD), 7.1% (75% CD; 75% SD), 6.7% (100% CD; 75% SD), and 12.3% (100% CD 100% SD) (P < .001). The mean resistance in rotary torque for 12  of scapular external rotation (retraction) with native specimens was 7.1 ± 1.7 Nm. External rotation did not demonstrate a significant decrease in ACJ stability with trapezial injury (P ¼ .596). The 8 sequential defects resulted in decreases in rotary torque from native external rotation of 0%, 3.8%, 4.0%, 3.2%, 3.5%, 3.4%, 4.2%, and 0.7%. Conclusion: Trapezial injury resulted in increased instability in the setting of scapular internal rotation (protraction) of the ACJ. Clinical Relevance: These findings validate the inclusion of deltotrapezial fascial injury consideration in the modified Rockwood classification system. Repair of the trapezial insertion on the ACJ may provide improved outcomes in the setting of ACJ reconstruction. Keywords: shoulder; trapezius; dynamic stabilizer; biomechanics; acromioclavicular joint The surgical treatment of acromioclavicular joint (ACJ) injuries is still evolving, and the gold standard procedure has not been achieved in a reproducible manner for open or arthroscopic approaches. 2-5,10,17,18,20,23,27 Biomechanical analyses have investigated the important roles of the coracoclavicular (CC) ligaments, 4,5,18,23 acromioclavicular ligament complex (ACLC), 17,24 and coracoacromial ligament 18 in ACJ stabilization. Historically, most of these studies have focused on clavicular movement, supe- rior and inferior translation, 18 anterior and posterior translation, 17,18 and anterior and posterior rotation. 6,24 While these investigations have increased the knowledge base of static ACJ ligamentous stabilizers, incorporating these findings into surgical reconstructions has revealed that The Orthopaedic Journal of Sports Medicine, 10(9), 23259671221118943 DOI: 10.1177/23259671221118943 ª The Author(s) 2022 1 This open-access article is published and distributed under the Creative Commons Attribution - NonCommercial - No Derivatives License (https://creativecommons.org/ licenses/by-nc-nd/4.0/), which permits the noncommercial use, distribution, and reproduction of the article in any medium, provided the original author and source are credited. You may not alter, transform, or build upon this article without the permission of the Author(s). For article reuse guidelines, please visit SAGE’s website at http://www.sagepub.com/journals-permissions.