The Role of the Trapezius in Stabilization of the Acromioclavicular Joint: A Biomechanical Evaluation

Abstract

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 = .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.

Current Concepts in Management of Acromioclavicular Joint Injury

“…Similarly, the force produced by middle trapezius muscle contraction leads to the retraction of the scapula. These actions may alleviate the stress placed on the static AC capsuloligamentous structures and CC ligaments during thoracohumeral motion [ 11 ]. While this may have significant implications for protecting reconstructed ligaments, the synergistic effect of the static and dynamic AC joint stabilizers is still a relatively new concept and is currently poorly understood within the orthopedic community.…”

“…Detailed anatomical knowledge of the AC joint Is also important in planning rehabilitation strategies following injury. The dynamic stabilizers, particularly the deltoid and trapezius muscles, play a key role in compensating for the loss of ligamentous support after such injuries [ 11 ]. While specific strategies may vary based on individual patient needs and the nature of the injury, a focus on enhancing the strength and function of these muscles could be beneficial in both conservative and post-operative management of AC joint injuries.…”

Clinical and Instrumental Diagnosis of Dislocations of the Clavicle Acromial End

“…Правильне анатомічне співвідношення пласких за формою і малих за розміром суглобових кінців ключиці й акроміального відростка лопатки, а головне -стабілізація суглоба при рухах забезпечується за рахунок динамічних і статичних стабілізаторів. Важливими анатомічними структурами, які забезпечують статичну стабілізацію суглоба є суглобова капсула, акроміально-ключична та дзьобоподібно-ключична зв'язки [10][11][12].…”