Effect of deltoid–trapezius overlap repair on acromioclavicular joint stability: A biomechanical study using fresh-frozen cadavers

Background: Severe acromioclavicular (AC) joint dislocation is associated with damage to the deltoid and trapezius muscles. However, studies on the role of these muscles in AC joint stability are limited. To evaluate AC joint stability achieved by deltoid–trapezius overlap repair (DTOR) and compare it with that provided by the coracoclavicular (CC) ligament. We hypothesized that DTOR would improve the vertical and horizontal stability of the AC joint. Methods: This study was conducted using six fresh-frozen cadaveric shoulders from three male and three female specimens (mean age, 76.5 years; range, 67–92 years). The study involved fixing the left side of the trunk, upper limb, and spine and measuring clavicular displacement via an electromagnetic tracker. Distal clavicular dislocation was simulated with sequential sectioning of the deltoid, trapezius, AC, and CC ligaments, followed by DTOR as follows: Stage 0, intact; Stage 1, the deltoid, trapezius, AC ligament, and AC joint capsule were sectioned; Stage 2, the trapezoid ligament was sectioned; Stage 3, the conoid ligament was sectioned; and Stage 4, DTOR was performed. The superior displacement was measured during inferior traction of the upper limb, and the posterior and lateral displacements were measured during horizontal adduction of the upper limb. Results: After sectioning the conoid ligament, the superior displacement significantly increased. Although performing DTOR afterward significantly reduced the superior displacement, it remained significantly larger than the displacement observed before sectioning the conoid ligament. Posterior and lateral displacements significantly increased after sectioning the conoid ligament but did not significantly decrease after DTOR. Conclusion: The DTOR-induced restraint of the AC joint in the superior, posterior, and lateral directions to improve its dynamic stability of the AC joint was inferior to the static stability provided by the CC ligament. However, because DTOR can achieve superior stability of the AC joint, it can be performed as an additional reinforcement procedure for treating AC joint dislocation.