Purpose of Review: The glenohumeral (GH) joint is the most commonly dislocated joint in the human body. In acute dislocations, radiographs remain the first line of study. Despite their frequency, important secondary signs of anterior dislocation (such as impaction defects of the humeral head and glenoid rim fractures) are often subtle and easily missed.
Recent Findings: By using a dedicated search strategy, one that applies a comprehensive search for impaction defects at vulnerable sites, glenoid rim fractures, and changes in alignment, one may maximize the diagnostic accuracy of radiographic interpretation.
Summary: This article will provide a review of important indicators of anterior and posterior glenohumeral joint dislocations.
Keywords: Shoulder, Trauma, Fracture, Dislocation, Glenoid, Humerus
The glenohumeral (GH) joint is the most commonly dislocated joint in the human body and accounts for over 50% of all dislocations. These injuries occur most frequently in young males in the setting of trauma . Despite their frequency, important secondary signs of anterior dislocation (such as impaction defects of the humeral head and glenoid rim fractures) are often subtle and easily missed without a dedicated search strategy. Frequently, a posterior dislocation is overlooked, as 50% of posterior dislocations are not recognized in their initial presentation. This article serves as a primer on the primary and secondary radiographic signs of anterior and posterior shoulder dislocations and their associated injuries. The following sections are intended to help radiologists and clinicians to better understand the vulnerable areas in the shoulder that are associated with dislocations and the importance of applying a standardized search pattern to optimize diagnostic accuracy.
Anatomy of the glenohumeral joint
The glenohumeral joint is one of two spheroidal (ball-andsocket) joints in the body, the other being the hip joint. It is not a perfect ball-and-socket since the glenoid “socket” only covers one-third of the articular surface of the humeral head “ball” . The glenoid also has a large radius of curvature that, in combination with its relatively small size, allows the shoulder joint to have tremendous mobility. However, this heightened mobility also greatly increases risk of glenohumeral joint dislocation. Furthermore, the shoulder has a relatively weak joint capsule, further contributing to the ease of dislocation of the glenohumeral joint and ultimately making it the most dislocated joint in the human body .
Radiography of the shoulder
In any suspected shoulder dislocation, radiographs are the first imaging study performed. It is recommended that conventional radiographs of the shoulder be obtained before reduction to assess the position of the humeral head and assess for fractures. The American College of Radiology (ACR) recommends a minimum of three radiographic views . A typical trauma shoulder series contains an anteroposterior (AP), Grashey, transcapular Y, and when possible an axillary or similar type view.
The AP view is typically obtained with the arm in internal rotation (IR). With the patient’s back against the image cassette, the x-ray beam is directed in a true AP direction . The glenoid fossa and humeral head overlap in this view because the scapula is tilted horizontally about 35 to 45 degrees from the coronal plane. When the humerus is internally rotated, the head appears spherical and, in combination with the proximal shaft, gives an appearance similar to the profile of a lightbulb.
The Grashey view is regarded as the true AP radiograph of the shoulder since it compensates for the scapular oblique inclination . With the patient’s back toward the image receptor, the thorax is rotated 35 to 45 degrees towards the shoulder of interest. The preferred position of the arm in this view is in external rotation. There are two major advantages of this projection; it outlines the GH joint as a space between the glenoid fossa and articular surface of the humeral head without osseous overlap, and it depicts a smooth visual arc consisting of the lateral border of the scapular body and neck with the humeral neck and shaft (referred to as Moloney’s scapulohumeral arch). If the humeral head overlaps the scapular glenoid in this projection, it is suggestive that the glenohumeral joint may not be in anatomic alignment.
The scapular Y-view provides a true lateral view with the anterior aspect of the affected side rotated 30 to 45 degrees toward the cassette . In this view, several structures make up what appears to be the letter “Y”. The upper arms of the Y are formed by the coronoid process anteriorly and the scapular spine posteriorly, while the vertical arm of the Y is formed by the body of the scapula. The glenoid fossa is seen en face at the intersection of these three arms. As accurate positioning is not needed, this view can be advantageous in cases of severe pain or limited mobility in which patient arm movement is restricted. It is possible to obtain this view even when the arm is immobilized in an arm support.
The axillary view is obtained with the arm abducted 70 to 90 degrees in a supine patient. The beam is centered over the GH joint and directed cephalad toward the cassette. There are several alternative variations to this view. It is useful for depicting dislocations and subluxation of the humeral head and in depicting glenoid rim fractures . In contrast to the scapular-Y view, the axillary view may not be possible in settings of acute trauma if the patient is unable to abduct their arm. In these cases, modified axillary radiographs may be obtained such as the Velpeau axillary lateral projection or trauma axillary lateral projection.
Anterior shoulder Dislocations
Anterior dislocations account for 95% of all shoulder dislocations . The prevalence of this injury is about 1% to 2% in the general population . Anterior dislocations frequently occur from fall-on-outstretched-hand (FOOSH) injuries involving extension, abduction, and external rotation of the arm. Dislocations are classified by the terminal location of the humeral head, and there are four types: subcoracoid (the most common type), subclavicular, subacromial, and intrathoracic. Typically, a single AP radiograph with a neutrally positioned arm along with another standard projection is sufficient to diagnose most anterior dislocations .
Once a shoulder dislocates, there is increased potential for dislocation to happen again. The most significant risk factors for recurrence are the age that the initial dislocation took place and the gender of the patient . Patients who have a dislocation before the age of 20 years have a 5-fold greater risk of redislocating the same shoulder compared to those over 40 years old. Young men have a 3-fold greater risk of recurrence than females.
Although chronic glenohumeral joint instability may occur with a single dislocation, it is much more likely to occur with repeated episodes. With each recurrence, the joint becomes increasingly unstable. This cumulative effect occurs because repeat dislocations have higher rates of injury to the inferior labroligamentous complex, the capsule, and the osseous structures that serve as stabilizers of the shoulder .
A Hill-Sachs lesion is an impaction fracture of the posterolateral surface of the humeral head that is pathognomonic for an anterior GH joint dislocation. Typically, it occurs when the humeral head forcibly strikes the harder compact bone of the anterior glenoid rim when it dislocates, producing a variably-sized impaction defect. This abnormality is optimally visualized on an AP radiograph with an internally rotated humerus since the average location of the lesion occurs about 209 degrees from the anterior margin of the articular surface . When the HS lesion is large, a prominent vertical line of condensation (initially described by Hill and Sachs) is apparent on internally rotated radiographs (Figure 1). However, because the impaction defect can also appear flattened, concave, or wedge-shaped, a high index of suspicion is required for detection. Once the shoulder has been reduced into anatomical position, a Hill-Sachs lesion may be the only radiographic indicator of a prior dislocation. In the literature, the incidence of a Hill-Sachs lesion has ranged from 25% to nearly 100% in patients with recurrent instability . Hill-Sachs lesions involving greater than 40% of the humeral head are nearly always clinically significant, associated with greater joint instability, heightened risk of recurrence, and increased likelihood for surgical treatment .
A new detection method, referred to as the Broken Circle sign, has recently reported sensitivity of 91% and specificity of 82% for Hill-Sachs lesions of any size . This technique makes use of the markup region of interest (ROI) function within the picture archive and communication system (PACS). This tool allows a circular marker to be superimposed over the cortex of the humeral head on internal rotation shoulder radiographs (Figure 2). Hill-Sachs lesions are revealed in areas where the cortex deviates or “breaks away” from the circle’s perimeter in the 10 o’clock to 12 o’clock zones (Figure 3). This technique enables detection of even subtle deviations from the reference circle, often seen with small or shallow defects when the vertical condensation line is absent. The Broken Circle detection method has been shown to improve reader accuracy and sensitivity without dramatically altering specificity .
An osseous (“bony”) Bankart lesion is a true fracture of the anteroinferior glenoid rim that occurs when the humeral head dislocates. It is important to differentiate an osseous Bankart lesion from a “classic” Bankart lesion that denotes an avulsion of the anterior labrum caused by the inferior glenohumeral ligament with an associated disruption of the anterior periosteum. A “classic” Bankart lesion is radiographically occult. Bony Bankart lesions are evident in about 5% to 8% of patients with acute anterior dislocations, but the majority are radiographically occult either because the fracture fragment is too small, or because the imaging projection is not optimal to depict the rim fracture. A high index of suspicion is required to detect small fractures, so that areas of increased density or dense slivers of bone adjacent to the anterior glenoid rim should be regarded as suspicious (Figure 4).
CT is the preferred imaging modality both to confirm the presence of an osseous Bankart lesion and to quantify its size . Bone deficits that are larger than 25% of the bone width of the inferior glenoid are associated with a high rate of instability and often require more invasive surgical management . This includes the Latarjet-Bristow procedure in which the coracoid process is transferred to the anterior glenoid, and then the ligament is re-anchored with a soft-tissue Bankart repair . When osseous Bankart lesions are identified, quantification of the size of the Hill-Sachs lesion and the glenoid defect together will influence management, so assessment of the glenoid surface involved by CT is usually performed pre-operatively .
Approximately 15% to 25% of anterior dislocations are associated with greater tuberosity fractures, usually occurring adjacent to a Hill-Sachs lesion [1,13]. These fractures may be comminuted or displaced (Figure 5).
Other fractures that may occur are much less frequent and may involve the proximal humerus at the surgical neck, the body or acromion process of the scapula, or the clavicle.
A soft tissue “classic” Bankart lesion is defined as an avulsion of the anterior labrum by the anterior band of the inferior glenohumeral ligament with an associated disruption of the anterior periosteum. There are a variety of Bankart variants that can be categorized by the involvement of surrounding structures, such as the labrum, articular cartilage, bony periosteum, or bone. A humeral avulsion by the glenohumeral ligament (HAGL), anterior labroligamentous periosteal sleeve avulsion (ALPSA), and glenoid labral articular disruption (GLAD) lesions are important in dislocations and are most optimally depicted and characterized with MR arthrography .
Posterior shoulder dislocations
Posterior dislocations account for < 2-4% of all shoulder dislocations and are most often caused by seizures, electrocution, or from falls with the arm adducted, flexed, and internally rotated . Posterior dislocations may also be caused by direct trauma in which the humeral head is internally rotated and forcefully displaced posteriorly . Seizures may result in bilateral dislocations. There are three types: subacromial (the most common at about 98%), as well as the infrequently occurring subglenoid and subspinous types . About 50% of posterior shoulder dislocations are not recognized on initial inspection usually owing to subtle radiographic findings. To increase the diagnostic accuracy, each radiographic projection of the shoulder must be closely scrutinized for clues. In addition to a true AP view and Y view, an axillary radiograph, when obtained, can be essential for diagnosis of posteriorly subluxed or dislocated humeral head .
Trough (Reverse Hill-Sachs) Lesion
A trough (or reverse Hill-Sachs) lesion is an impaction fracture of the anteromedial aspect of the humeral head that occurs with a posterior dislocation (Figure 6). It is an important radiographic finding, appearing as a vertically oriented dense linear or curvilinear band that parallels the medial cortex of the humeral head on internally rotated radiographs of the shoulder . If the impaction defect affects greater than 25% of the humeral head, operative management may be considered; if the lesion affects greater than 50% of the head, shoulder arthroplasty becomes the preferred treatment consideration [11,15].
Reverse osseous Bankart lesions and other labrocapsular complex lesions occur with high frequency in traumatic posterior shoulder dislocation . These lesions are extremely difficult to detect radiographically (Figure 7). This is attributable to both the size and position of the posterior rim fracture, as the lesion may displace posteriorly and overlay the cortex of the glenoid fossa. CT is usually required for confirmation particularly when the fragment is small.
Additionally, approximately 25% of patients with a posterior dislocation will present with a lesser tuberosity fracture and rotator cuff tears (Figure 8) [1,18]. Rarely, the shaft of the humerus may also fracture .
Associated Radiographic Signs
Because the diagnosis may be elusive in some cases, there are important radiographic observations that may be indicative of a posterior shoulder dislocation [1,6]. The rim sign indicates widening of the glenohumeral joint space in excess of 6 mm (Figure 9).
The lightbulb sign refers to an arm that is “locked” in internal rotation such that all of the views of the shoulder series appear similar because the impaction defect of the head is perched on the posterior glenoid rim.
A disrupted arch sign indicates that there is discontinuity of the scapulohumeral arch owing to subluxation of the humeral head (Figure 10). The absent half-moon, or crescent, sign refers to the absence of the normal overlap of bone between the glenoid and humeral head on an AP shoulder radiograph (Figure 11).
The crossover sign refers to overlapping profiles of the glenoid and the humeral head on a Grashey view. The number of radiographic secondary signs underscores the difficulty of prospectively identifying a posterior dislocation in the urgent or emergent setting.
The glenohumeral joint is the most commonly dislocated joint in the body. Utilizing a standardized search pattern with meticulous evaluation of the glenohumeral joint in search of primary and secondary signs of dislocations is essential for diagnosis. Accurate diagnosis establishes proper treatment early and prevents complications that may arise from delayed detection. Each radiographic projection yields important information, but knowledge of the strengths of each view is essential in the search strategy. A Hill-Sachs lesion is pathognomonic for anterior shoulder dislocations, and the new Broken Circle sign provides a novel and effective method for recognizing these lesions. There are a number of secondary signs associated with posterior shoulder dislocation and it is important to recognize them in post-traumatic patients so that diagnosis and timely treatment may be initiated.
Declaration of conflict of interest
The authors have no financial or non-financial conflict of interest that may be directly or indirectly related to the contents of this manuscript.