The acromioclavicular (AC) joint is a diarthrodial joint in the shoulder between the distal clavicle and the acromion. The AC joint is stabilized by muscular and ligamentous structures including the deltoid and trapezoid muscles and the capsular, AC, and coracoclavicular ligaments [1]. AC joint separation is common among athletes and occurs following a traumatic injury to the shoulder [2]. This injury is roughly 10 times more frequent in men than in women [3]. AC joint separation accounts for 9% of all shoulder injuries but 40% of all sports-related injuries [4,5]. The mechanism of injury is typically a fall or a direct blow to the shoulder with the arm adducted [6]. Many classifications have been proposed to describe AC joint dislocations. The Cadenat classification was published in 1917, followed by the Tossy classification in 1963 and the Rockwood classification in 1984 [7,8]. The Rockwood classification uses plain radiographs to categorize AC joint dislocations into six grades of severity and nature of ligament injuries and joint dislocation. The grades range from mild sprain in grade I to complex ligament rupture and dislocation in grades IV to VI [3]. The higher is the classification of the injury, the worse is the prognosis.

Recent reports show that intra-articular glenohumeral pathologies (IAPs) often accompany low-grade AC joint dislocations, which challenges the notion that they are only seen in high-grade injuries [6]. Despite this, there is a lack of comprehensive studies on the occurrence of labral tears alongside AC joint dislocations. To fill this gap, our systematic review sought to accomplish two main objectives. First, we evaluated the prevalence of labral tears in AC joint dislocations. Second, we attempted to identify risk factors that may contribute to this association, such as age and grade of AC joint dislocation. We hypothesize that labral tears are a commonly overlooked pathology that may frequently co-occur in both low- and high-grade AC joint dislocations. We also suspect that certain risk factors make this co-occurrence more likely.

The prevalence of concomitant labral injuries and AC joint dislocation was approximately 27%. This prevalent association may be explained by the shared mechanism of injury in these two entities. Direct or indirect trauma is the most common cause of AC-joint separations, which often result from a fall, accident, or direct blow to the upper extremity with scapular depression [6]. Alternatively, an indirect injury can cause AC-joint separation because impact on the adducted hand or elbow drives the humeral head into the acromion [14]. These same mechanisms of injury can also damage active or passive stabilizers, such as muscle attachments, ligaments, labrum, and bony or cartilaginous structures. Similar mechanisms may lead to compression or traction injuries (in the case of traumatic SLAP lesions), which are frequently caused by a fall to the abducted or flexed arm [15-17]. These phenomena can explain the association between ACJIs and labral lesions. Furthermore, Clavert et al. [18] demonstrated in a biomechanical study that SLAP type II lesions occurred in every simulated fall onto outstretched arms. In that study, a customized shoulder testing device that can imitate muscular stresses was attached to a servohydraulic testing system and used on 10 cadaveric shoulders to simulate a forward or backward fall [18]. Five SLAP lesions were discovered in the five shoulders used to simulate a forward fall. In contrast, only two SLAP lesions were identified in the five shoulders that were used to simulate backward falls. Shearing forces appeared to play a significant role in the development of this injury [18].

AC-joint reconstruction is performed solely through an all-arthroscopic trans-articular approach, and diagnostic shoulder arthroscopy is also conducted in these cases. However, for surgeons who use an open approach for AC-joint repair, preoperative MRI scans may be useful to determine which patients should undergo arthroscopy before the open procedure. For instance, Shah et al. [6] employed MRA, which has a reported sensitivity ranging from 89% to 100%, specificity ranging from 69% to 91%, and accuracy ranging from 74% to 92% for the detection of labral/SLAP lesions. Therefore, MRA may be an important diagnostic modality that clinicians should consider when it is clinically available [19-21]. Although an acute physical examination may be unreliable, routine use of MRA in the setting of acute AC joint separation may not be fiscally feasible and should be reserved for cases in which non-operative management has failed.

As for possible risk factors, the findings are still contradictory and inconclusive. Shah et al. [6] found no significant age difference between patients with and without concomitant labral pathologies. This was also demonstrated by Ruiz Ibán et al. [12] and Tischer et al. [2], who showed that the prevalence of associated lesions was not correlated with the subjects' sporting activity level, work site functional requirements, or age. Furthermore, Markel et al. [1] and Pauly et al. [11] did not find any difference in the frequency of associated labral lesions between the different Rockwood types. However, according to Tischer et al. [2], there is a greater incidence of SLAP lesions in cases of Rockwood type V than there is in milder forms of AC joint instability.

Clinicians face several challenges when analyzing the association between AC joint dislocation and intraarticular pathologies. First, it is difficult to differentiate between pre-existing, asymptomatic labral lesions and those that were caused by the same trauma that caused the ACJI [11,22]. In fact, several studies have revealed that previous injuries or overuse may result in asymptomatic findings that can lead to unnecessary treatments [23-25]. Nevertheless, Pauly et al. [11] found that only a small proportion of IAPs (14.4%) in high-grade AC separation cases were unrelated to recent trauma based on the tissue appearance during arthroscopic surgery. This problem is even more prevalent in low grade AC joint dislocations. According to Mouhsine et al. [26], approximately 27% of grade 1 and 2 AC separation cases treated nonoperatively required additional surgery. Another study by Petri et al. [27] showed that more than 65% of patients who failed nonoperative management of grade 3 AC separation had intra-articular pathologies (IAPs) identified during eventual operative management. Since nonsurgical treatment is the primary method of managing low-grade AC joint separation, there is limited literature on its associated pathologies [1]. Second conducting a thorough examination of an acute AC separation can be difficult because it requires painful restriction of shoulder motion and strength. In addition, the clinical tests for SLAP tears are similar to those used for AC separation [6]. Furthermore, it is currently uncertain whether repair is necessary for these injuries, such as partial tears of labral structures, which may heal without surgery. Additionally, there is a lack of data on revisions after failed AC-joint repair due to overlooked intra-articular pathologies that may be due to the injury but are not problematic enough to require revision surgery [10]. However, ongoing shoulder pain after successful AC-joint repair may be caused by other injuries. It is the surgeon’s responsibility to identify and treat these accompanying injuries [10].

This study has several limitations. For instance, there was a small number of included studies despite the extensive search method used across three databases. Furthermore, the inclusion and exclusion criteria for patients were different. The data used for the analysis were pooled, and the individual patient data were unavailable, which could have limited comprehensive analyses. Regardless, this was the first systematic review studying the prevalence of the association between acute AC joint dislocation and labral tears.

Our findings shed light on the intricate relationship between labral lesions and AC joint dislocations, which have a prevalence of approximately 27%. Notably, SLAP lesions emerged as the predominant type of labral tear and exhibited a grade-dependent pattern. Higher-grade AC joint dislocations are often associated with a higher prevalence of these tears. However, it is unknown whether these labral injuries were caused by the AC injury itself, or if they were present prior to the dislocation. Therefore, further high-quality research is warranted to clarify this prevalent association.