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Diagnostic Imaging Review

Distinguishing three types of shoulder dislocation


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Discussed here are three patients, each of whom presented with a different type of shoulder dislocation.

The first patient was a 22-year-old male who appeared in the emergency department (ED) with shoulder pain after being tackled while playing football. Range of motion (ROM) of the affected shoulder was decreased. The patient was guarding his arm and did not want it manipulated. Deformity of the shoulder was noticeably visible. He was neurovascularly intact. Figure 1 is an anteroposterior (AP) radiograph of his injury.

Figure 1:
. Anterior shoulder dislocation shows that the humerus is inferior to the coracoid process and medial to the glenoid process (AP view).

The second patient was a 30-yearold male who presented to the ED with shoulder pain after having a seizure. He was holding the affected arm close to his body and was unable to externally rotate it. No obvious deformity was seen on physical examination, and findings on his neurologic and vascular examinations were normal. He, too, guarded his shoulder and had decreased ROM. Figure 2 is an anterior radiograph of his injury.

Figure 2:
. AP view of a posterior shoulder dislocation demonstrates the humerus fixed in internal rotation and a distance of 12 mm between the humeral head and the glenoid process (positive rim sign).

The third patient was a 29-year-old male who was working on the gutters of his house when the ladder fell out from under him. He caught himself with one arm on the roof but immediately felt pain and let go, landing on the ground feet first. At presentation, one of his arms was extended overhead with the forearm resting on his head. He could not bring the arm down. After findings from a neurovascular examination were found to be normal, radiographs were obtained (Figure 3). What do the three radiographs show?

Figure 3:
. AP view of an inferior shoulder dislocation is similar to that of an anterior dislocation (Figure 1), but the arm is extended upward in a fixed position.


Figures 1, 2, and 3 demonstrate various presentations of shoulder dislocations. The shoulder is the most commonly dislocated joint in the body and a frequent cause of ED visits.1–4 Approximately 50% of all joint dislocations occur in the shoulder.2

Figure 1 shows an anterior dislocation. The anterior dislocation is the most frequently encountered shoulder dislocation, comprising approximately 95% to 97% of cases.1,5 Posterior dislocations (Figure 2) occur in 2% to 4% of cases, and inferior dislocations (Figure 3) are rare.1,2,5 Patients with shoulder dislocation of any type typically present with a history of trauma or feeling the shoulder pop, followed by pain. Those with a posterior dislocation may report having a seizure, suffering a blow to the front of the shoulder, or being struck by electricity.

Physical examination Any shoulder dislocation will manifest with decreased ROM of the affected extremity. The patient may also experience tingling or numbness. A neurovascular examination must be performed, as the axillary nerve and artery can be injured. Brachial plexus injury is rare. The shoulder may be visibly deformed.

Patients with an anterior dislocation may hold the affected extremity in a position of abduction and external rotation, whereas patients with a posterior dislocation may hold the arm close to the body, adducted, and in internal rotation. Posterior dislocations can be the most difficult to diagnose and are often missed initially. In the inferior dislocation, the arm is held overhead, typically with the forearm resting on the head. Inferior shoulder dislocations are also known as luxatio erecta. Most dislocations occur as a result of trauma, but a small percentage of patients do not report a history of trauma. Such patients may have ligament laxity, a congenital abnormality of the humerus or glenoid process of the scapula, axillary nerve injury, or a neuromuscular disorder that predisposes them to shoulder dislocations.

Radiographic views Even if the diagnosis of a shoulder dislocation is clinically obvious, radiographs are useful to evaluate for possible associated fracture before reduction is attempted. At least two views of the shoulder should be obtained. Anteroposterior and axillary views are preferred, but if the axillary view cannot be obtained, a scapular-Y view or posterior oblique projection may be helpful. Rarely is CT needed. On the AP view of the patient with an anterior shoulder dislocation, the humerus is projected inferior to the coracoid process and medial to the glenoid process (Figure 1).

Lesions that are commonly seen with an anterior dislocation include the Hill-Sachs fracture and the Bankart fracture. A Hill-Sachs fracture is a fracture of the humeral head. It occurs along the posterior and superior aspect and is caused by the impaction of the humeral head on the inferior aspect of the glenoid process. A Bankart fracture is caused by the same mechanism, but it is a fracture of the inferior aspect of the glenoid process. Figure 4 is the postreduction image of the anterior dislocation shown in Figure 1. The patient had both a Hill-Sachs fracture and a Bankart fracture. Other complications of an anterior dislocation include fracture of the greater tuberosity of the humerus, ligamentous injury, labral injury, rotator cuff tears, capsular abnormalities, and intra-articular loose bodies.6 MRI can be helpful in evaluating many of these injuries if necessary.

Figure 4:
. Postreduction radiograph of the anterior dislocation shown in Figure 1 demonstrates both a Hill-Sachs fracture and a Bankart fracture (AP view).

Posterior dislocations can be more difficult to diagnose and may be missed clinically and radiographically. Figure 2 is an AP radiograph of a patient with a posterior dislocation. Initially, internal and external rotation AP views of the shoulder were obtained, but because the patient was not able to externally rotate the arm, both views appeared the same. The similarity of the radiographs combined with the inability to externally rotate the arm may be the only clue on AP radiographs to suggest a posterior dislocation. Fortunately, this was observed before the patient was discharged, and an axillary view was obtained. Figure 5 demonstrates the humeral head impacted on the posterior aspect of the glenoid process. Other clues that may be visualized on the AP view of a posterior dislocation include the trough sign and the rim sign and the loss of the crescent sign. The trough sign is a vertical or archlike fracture line seen running parallel and lateral to the articular surface of the humeral head.2,5 It is caused by impaction of the humeral head on the posterior glenoid process rim. The rim sign involves measuring the distance between the medial border of the humeral head and the anterior border of the glenoid rim2 (Figure 2). If this distance measures greater than 6 mm, then the patient is considered to have a positive rim sign and possible posterior shoulder dislocation. The rim sign can also be positive in patients with a joint effusion or lymphedema. The crescent sign describes the crescent appearance formed by the overlapping of the humeral head and the glenoid process.7 Loss of the crescent sign may indicate a posterior dislocation. Complications of posterior dislocations include labral and ligamentous injuries, reverse Hill-Sachs fracture, and teres minor tears.

Figure 5:
. Posterior shoulder dislocation shows impaction of the humeral head on the posterior aspect of the glenoid process (axillary view).

With an inferior dislocation, the shoulder is in a fixed overhead position, which is obvious clinically. In Figure 3, the humeral head is displaced medial and inferior to the glenoid process, similar to an anterior dislocation, but notice that the upper arm is extended upward.

Treatment Shoulder dislocations require reduction and physical therapy. Surgery is typically reserved for patients with recurrent shoulder instability. Once patients have suffered a shoulder dislocation, they are at increased risk of recurrence. Younger patients are more likely to have recurrent dislocations than older patients. This is thought to be the result of younger patients being more physically active. The presence of a glenoid fracture, Hills-Sachs fracture, or rotator cuff tear also increases the risk of recurrent dislocation. Joint stability is the ultimate goal of treatment. MRI can be useful in further evaluating shoulder instability and is usually performed in conjunction with arthrography. CT is typically reserved for patients who have a contraindication to MRI.


1. Cutts S, Prempeh M, Drew S. Anterior shoulder dislocation. Ann R Coll Surg Engl. 2009;91(1):2-7.
2. Gor DM. The trough line sign. Radiology. 2002;224(2): 485-486.
3. Seade LE. Shoulder dislocation. Medscape Reference Web site. Updated October 3, 2012. Accessed April 8, 2013.
4. Shoulder dislocation and instability. Virtualmedicalcentre. com Web site. Reviewed March 21, 2011. Accessed April 8, 2013.
5. Greenspan A. Orthopedic Imaging: A Practical Approach. 4th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2004:114-124.
6. Tseng GY. Shoulder dislocation imaging. Medscape Reference Web site. Updated September 2, 2011. Accessed April 8, 2013.
7. Brant W, Helms C. Fundamentals of Diagnostic Radiology. 2nd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 1999: 1011-1015.
© 2013 American Academy of Physician Assistants.