A 35 year old male is biking when he crosses are area of sand on the road. The bike fishtails and he falls onto his left shoulder. He was helmeted and denies head trauma, neck pain, or LOC. The patient is holding his left arm close to his side and across his chest. On physical exam there is a prominent acromion and coracoid anteriorly.
What Injury does this mechanism and presentation correlate with?
Posterior Shoulder dislocations account for only 2-4% of all shoulder dislocations. Most commonly they are associated with or caused by seizure, electrical shock, FOOSH or a direct blow to the shoulder, as occurred in this patient. Given the atypical presentation, they are missed in up to 50% of cases on initial presentation.
On presentation the patient will usually presents with the arm adducted and internally rotated. More so than in an anterior shoulder dislocation, movement, especially abduction or external rotation is very painful. A prominent acromion and coracoid are palpable, with the humeral head displaced posteriorly.
The deformity can be very subtle on the Anterior/Posterior view and most reliably visualized on the Axillary or Transscapular Y views. On the anterior/posterior view classically shows a “light bulb sign”. This occurs when the humeral head is forced into internal rotation as it dislocates posteriorly, giving it the appearance of a light bulb.
Another characterstic sign on radiograph after a posterior shoulder dislocation is the “Trough Sign”. This may be present when the dislocation results in a Reverse Hill-Sachs deformity. Radiographs will show a loss of overlap of the humeral head and glenoid fossa.
In general, Traction - CounterTraction is the most efficacious means to reduce this dislocation. In addition to the traction, gentle pressure to the humeral head in the posterior and lateral directions can be helpful to disengage the humeral head from the posterior glenoid. In our case above, gentle pressure to the humeral head quickly disengaged the humerus from the glenoid and under procedural sedation the patients shoulder was reduced on first attempt.
Similar to anterior dislocations after a posterior shoulder dislocation is reduced, the patient is placed in a sling and swath and discharged with orthopedic follow-up and instructions for non-weight bearing and to keep the shoulder in the sling at all times.
The “Reverse Hill-Sachs” lesion mentioned above is one of the complications of a posterior shoulder dislocation. This “lesion” is an osteochondral impression fracture in the anteromedial portion of the humeral headand is known by many names: enoche fracture, McLaughin lesion or reverse Hill-Sachs lesion. In posterior dislocations the shoulder rotates internally causing a anteromedial fracture while in an anterior shoulder dislocation which can lead to a Hill-Sachs deformity which is in the posteriorlateral humeral head.
Reviewed by: Dr Kristina McAteer