Posterior Shoulder Dislocation
CASE
A 46-year-old male with no past medical history presents to the ED via ambulance after crashing his bicycle. He was traveling at 25 mph when a dog ran out in front of him, causing him to swerve and fall onto his left side. He was helmeted and did not lose consciousness.
On arrival, he is reporting extreme pain in his left shoulder. It is a 10/10, constant throbbing pain that radiates throughout his arm and into his chest, limiting his ability to move the shoulder.
On exam, vital signs are: BP 170/100, HR 72, temperature 98.2, RR 13, SpO2 97%. He is in acute distress. There is left upper extremity tenderness throughout the elbow, bicep, and clavicle without obvious deformity. There is left lateral neck tenderness without midline spinal tenderness. The left extremity is neurovascularly intact. Examination is otherwise unremarkable except for some superficial abrasions on the extremities.
Imaging of tender areas is undertaken. A CT of his brain and C-spine shows no abnormalities, and X-rays of the pelvis, hip, chest, wrist, and elbow are all normal.
Initial X-ray of the shoulder shows no obvious abnormalities but views are limited by the patient’s extreme pain with movement. There is possible posterior dislocation and AC separation, but it is unclear whether this is an artifact.
Figure 1. X-ray imaging of left shoulder
The patient is still in distress despite aggressive pain control, so a CT of the shoulder is performed. This shows a posterior dislocation of the humeral head with an associated reverse Hill-Sachs deformity, as well as a grade 2 AC separation.
Figure 2. Posterior Shoulder Dislocation on CT
DISCUSSION
Posterior Dislocation
Posterior dislocations of the shoulder are uncommon, making up less than 5% of all shoulder dislocations. They can occur from an anterior blow or from violent muscle contractions during seizures. On exam, the patient will have the arm adducted and internally rotated and will be unable to externally rotate it. There will also be anterior flattening of the shoulder with a posterior prominence.
This type of dislocation is difficult to visualize on standard AP X-ray and is not detected half of the time. Subtle signs of this injury include:
- Light bulb sign: circular appearance of the humeral head due to internal rotation of the tuberosities
- Rim sign: disappearance or widening of the space between the humeral head and anterior glenoid rim
- Trough line sign: two parallel lines of cortical bone on the medial cortex of the humeral head, with one representing the cortex and the other is the trough of a reverse Hill-Sachs impaction fracture (discussed later)
CT of the shoulder may be useful if X-ray is not diagnostic. The CT scan will show a posterior dislocation more clearly, and it can also be used to evaluate the size of a reverse Hill-Sachs fracture.
Reverse Hill-Sachs Deformity
The reverse Hill-Sachs deformity represents an impaction fracture of the anteromedial aspect of the humeral head, caused by posterior dislocation of the humerus. It is seen on CT as a loss of convexity. In contrast, a traditional Hill-Sachs deformity would be caused by an anterior shoulder dislocation and would be seen as a posterolateral impaction of the humeral head.
AC Separation
The acromioclavicular (AC) joint is the articulation between the distal clavicle and the acromion of the scapula. It is vulnerable to separation from direct trauma to the superior or lateral aspect of the shoulder with the arm adducted. Minimal intervention is needed for incomplete injuries (type I and II), while operative reduction is necessary for more severe injuries.
CASE RESOLUTION
The patient is sedated and the dislocation is reduced using traction. The reduction is confirmed by X-ray, which shows that AC separation is still present. The left upper extremity is still neurovascularly intact.
Figure 3. Post-Reduction films
The patient is put in a shoulder immobilizer and is discharged home for outpatient orthopedic follow-up. One month later he undergoes an MRI for persistent shoulder pain. This redemonstrates the reverse Hill-Sachs deformity and complete AC joint separation, with possible labrum, supraspinatus and subscapularis tendon tears.
Author: Katherine Barry is a fourth year medical student at the Warren Alpert Medical School of Brown University.
Faculty Reviewer: Kristina McAteer, MD is an attending physician at Rhode Island Hospital and Newport Hospital.
REFERENCES
“Acromioclavicular Joint Injuries (‘Separated’ Shoulder).” UpToDate, www.uptodate.com/contents/acromioclavicular-joint-injuries-separated-shoulder.
Kelly, M. J., et al. "The aetiology of posterior glenohumeral dislocations and occurrence of associated injuries: A systematic review." Bone Joint J 101.1 (2019): 15-21.
“Shoulder Dislocation and Reduction.” UpToDate, www.uptodate.com/contents/shoulder-dislocation-and-reduction.