Varag Abed
Acromioclavicular (AC) joint injury is a common condition, particularly among athletes, with an incidence of approximately 3.1 per 100,000 person-years1. The Rockwood classification system remains the traditional standard for categorizing these injuries. Types I and II are typically treated nonoperatively, whereas types IV through VI are generally treated surgically. The optimal treatment strategy for type-III injuries—whether operative or nonoperative—remains controversial2. The deltoid and trapezius muscles serve as dynamic stabilizers of the AC joint, with the trapezius being of particular interest because of its insertion on the clavicle, scapula, and AC ligament complex3. In a 3D magnetic resonance imaging study, Sahara et al. examined AC-joint kinematics during abduction and found that the trapezius attachment at the distal clavicle facilitated posterior translation, whereas the upper and middle trapezial fibers contributed to elevation and lateral rotation of the clavicle and scapula3,4. The recent anatomical study by Sugiura et al. aimed to further characterize the structure of the trapezial aponeurosis at the AC joint. Shoulder girdles from Japanese cadaveric specimens were examined both macroscopically and histologically. The aponeurosis length was greatest at the acromial insertion (mean ± standard deviation, 28.9 ± 5.4 mm), followed by the AC-joint region (20.3 ± 7.7 mm) and the clavicular insertion (7.2 ± 3.2 mm). Histological evaluation revealed that a fibrocartilaginous insertion was present only at the acromial attachment, located posterior to the AC joint. These findings further highlight the important role of the trapezius muscle in maintaining dynamic stability of the AC joint. One of the most important methodological considerations warranting further discussion is the nature of the cadaveric specimens analyzed in the study by Sugiura et al. Previous studies have demonstrated that AC-joint injuries occur predominantly in males between 10 and 30 years of age1. In contrast, the specimens utilized in this anatomical study had a mean age of 86 years and were predominantly from female donors (9 of the 13). Consequently, differences in tissue quality, anatomical characteristics, and demographic representation may limit the generalizability of these findings to the younger, predominantly male population most commonly affected by AC-joint injuries. Similarly, there have been concerns that anatomic assessment with use of cadavers may not directly correlate with living tissues5. Yammine found that most cadaveric studies in the literature assessed the musculoskeletal system in the elderly, with a mean age of 71.1 years6. As a result, findings may not be generalizable to younger populations. Although the limitations must be recognized, this study contributes to the growing body of literature exploring the role of the trapezius in AC-joint stability. A biomechanical analysis by Trudeau et al. involving 10 cadaveric shoulders demonstrated that trapezial injury led to AC-joint instability associated with scapular internal rotation (protraction)3. Similarly, Hawthorne et al. investigated the biomechanical contributions of the deltoid and trapezius muscles to AC-joint stability, with a focus on rotational control. In their study of 12 cadaveric shoulders, anatomic deltotrapezial repair was shown to enhance posterior rotational stability7. In conclusion, this study provides valuable anatomical insight into the role of the trapezius in maintaining AC-joint stability. Given that more than 100 surgical stabilization techniques have been proposed for AC-joint injuries7, a deeper understanding of the underlying anatomy, integrated with biomechanical evidence, may help to guide the development of a standardized, evidence-based approach to the treatment of AC-joint instability.