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. 2022 Sep 26;10(9):23259671221118943.
doi: 10.1177/23259671221118943. eCollection 2022 Sep.

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

Maxwell T Trudeau  1 , Jonathan J Peters  1 , Benjamin C Hawthorne  1 , Ian J Wellington  1 , Matthew R LeVasseur  1 , Michael R Mancini  1 , Elifho Obopilwe  1 , Giovanni Di Giacomo  2 , Simone Cerciello  3 , Augustus D Mazzocca  1
Affiliations

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

Maxwell T Trudeau et al. Orthop J Sports Med. .

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.

Keywords: acromioclavicular joint; biomechanics; dynamic stabilizer; shoulder; trapezius.

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Conflict of interest statement

One or more of the authors has declared the following potential conflict of interest or source of funding: A.D.M. has received consulting fees from Arthrex and Astellas Pharma, speaking fees from Arthrex and Kairos Surgical, and honoraria from Arthrosurface. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

Figures

Figure 1.
Figure 1.
(A) Study setup illustration featuring posterior view of a right shoulder specimen with clavicle fixated, scapula attached to the Material testing system, humerus loaded to neutral, and trapezial clavicular and scapular muscle fibers loaded at their native anatomic angle. (B) Additional illustration inverted for greater appreciation of testing setup.
Figure 2.
Figure 2.
Illustration demonstrating scapular internal rotation (protraction) and external rotation (retraction).
Figure 3.
Figure 3.
Superior view of left shoulder specimen depicting 8 sequential trapezial defects at clavicular and scapular spine attachment sites. Blue numbers indicate the order of alternating clavicular and scapular spine trapezial transections.
Figure 4.
Figure 4.
Mean percentage of rotary torque for 12° of scapular internal rotation in correlation with the intact (native) condition across sequential defect (SD) (P < .001). CD, clavicular defect.
Figure 5.
Figure 5.
Mean percentage of rotary torque for 12° of scapular external rotation in correlation with the intact (native) condition across sequential defect (SD) (P = .596). CD, clavicular defect.
See this image and copyright information in PMC

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