The most important findings of this study are that sectioning the AC and trapezoid ligaments causes AC joint instability and that the clavicle can override the acromion on cross-body adduction view regardless of conoid ligament sectioning. The findings of this study confirm our hypotheses. Alexander [13] described the shoulder-forward view (the Alexander view), which can be used to assess AC joint injuries by thrusting the shoulder forward and evaluating acromion displacement anteriorly and inferiorly under the distal end of the clavicle. A similar cross-body adduction view was reported and used by Barnes et al. [14] to help identify whether the AC joint is stable. ISAKOS [8] suggested distinguishing two variations of type III Rockwood injury using a cross-body adduction view. These reports suggest that evaluating ligament injuries is difficult using only plain radiographic examination. Zumstein et al. [15] reported the use of the acromial center line spanning to the dorsal clavicle (AC-DC) to assess vertical displacement and the use of the glenoid center line spanning to the posterior clavicle (GC-PC) to assess horizontal displacement; performing these evaluations in a single Alexander view is recommended to guide the appropriate management of AC joint dislocations. Karargyris et al. [16] reported that AC-DC measurement seemingly represents a more realistic AC joint injury magnitude than the CC distance. Murphy et al. [10] validated a novel radiological parameter, the circle measurements, that objectively evaluates AC joint displacement on the Alexander view, providing novel therapeutic recommendations. Increasing attention has been given to the relationship between Alexander view evaluation and AC joint stability. In this study, we referred to radiological AC joint dislocation evaluation and investigated the relationship between ligament injury, cross-body adduction view, and AC joint instability. AC joint instability in both superior and posterior directions will make the distal clavicle override the acromion.
Regarding AC joint superior stability, Fukuda et al. [17] described the contributions of individual ligaments to joint stability by performing biomechanical loading experiments using fixed displacements and sequential ligament sectioning. At small displacements, the AC ligament primarily restraints the superior direction of the AC joint, and at large displacements, the conoid ligament primarily restraints the superior direction of the AC joint. Kurata et al. [9] reported that the AC ligament significantly contributes to AC joint stability in the superior direction, and AC joint displacement >50% may occur combined with isolated AC ligament injury in biomechanical loading experiments.
Regarding posterior stability, Fukuda et al. [17] reported that the AC ligament primarily restraints the posterior direction of the AC joint, and Dawson et al. [18] and Dyrna et al. [19] biomechanically investigated the kinematics of the AC joint and reported that the AC ligament strongly contributes to the horizontal stability of the AC joint. Oki et al. [11] conducted a biomechanical investigation using whole cadaver models and evaluated the function of the AC and CC ligaments in scapular and clavicular motions during humerothoracic motions and found that the AC ligament restraints clavicular retraction and that the trapezoid ligament restraints scapular internal rotation during horizontal plane adduction. Debski et al. [20] reported that the trapezoid ligament plays an important role in the posterior direction of the AC joint. Kurata et al. [9] reported that sectioning the trapezoid ligament after sectioning the AC ligament causes AC joint instability.
According to previous studies that examined superior and horizontal AC joint stability [9, 11, 17, 18, 19, 20], superior AC joint stability relies on the AC and conoid ligaments, whereas posterior AC joint stability relies on the AC and trapezoid ligaments. The results of this study revealed that five of the 6 clavicles overrode the acromion at stage 2, regardless of conoid ligament sectioning. Although the average superior displacement of the five clavicles was not large (average, 6.7 mm), sectioning the AC and trapezoid ligaments alone may cause AC joint instability enough to override the clavicle on the acromion in the horizontal adduction position. Our results suggest that there are variations of CC ligament injuries in type IIIB (unstable) injury of AC joint dislocation. There are many reports about arthroscopic CC ligament reconstruction for AC joint dislocation, and double bundle reconstruction is becoming a more common treatment [21, 22, 23]. Taking into account our study, it is possible that double bundle reconstruction includes reconstruction of the remaining ligament in type IIIB injury. Reconstructing only injured ligaments without altering remaining ligament could be used to reduce complications related to drilling holes in the distal clavicle and coracoid process [24, 25]. Diagnosis of AC joint dislocation should not only be based on the radiographic appearance of the AC joint. Recently, there are many reports about evaluation of the AC joint dislocation using magnetic resonance imaging (MRI) and the accuracy of diagnosing ligament injuries [26, 27]. For cases of type IIIB injuries, we recommend additional MRI examination.
This study has some limitations. First, ligament sectioning was performed according to previous studies, and the sequences of ligament sectioning may differ from clinical settings. Second, this study used the cadavers of old people; yet, AC joint dislocation is common among young people. Third, when resecting the trapezoid and conoid ligaments, the deltoid muscle was incised in the direction of the muscle fibers, and the ligaments were resected after clearly viewing them. Fourth, the direction of the AC joint changed, and we did not evaluate the horizontal translation of the AC joint in the horizontal adduction position. Fifth, we used a limited number of specimens.