Posterior Shoulder Dislocation

Posterior Shoulder Dislocations happen when the head of the humerus is pushed backwards, displacing it from the shoulder joint. Posterior shoulder dislocations account for <4% of all shoulder dislocations. 

What is a Dislocation?

Dislocation is defined as the complete separation of the two articulating surfaces of a joint. 

What is a Posterior Dislocation of the Shoulder?

Posterior dislocation is defined when the head of the humerus is separated from the glenoid and pushed posteriorly. One of the most missed or the late diagnosed dislocation of the shoulder is the posterior one(1).

This is because the shoulder complex is guarded posteriorly by the scapula and a thick musculature covering it. This makes the posterior dislocation less frequent with about 2 -5% of the occurrence, compared to the anterior one.

About 60 – 79% of the posterior dislocations are missed during the initial examination, hence it warrants a keen examination for a good prognosis(1,2)

Aa per the basic classification, posterior dislocations can be classified as Traumatic and Atraumatic, of which the traumatic being more common.

Posterior dislocations are classified more precisely according to the etiopathogenesis and presentation as- 

  1. Acute dislocation
  2. Chronic (Fixed/ Locked) dislocation
  3. Recurrent Posterior subluxation (more common)

Mechanism of Injury

Trauma

Trauma is the most common mechanism of posterior dislocation. 67% of all posterior dislocations occur with a high energy force directed axially to the shoulder, with the upper extremity in an internally rotated and adducted position(3).

This may be observed in the case of grabbing a dashboard during a motor vehicle collision or falling on the hand(2).

A sudden force while guarding against an opponent in combat or high-intensity sports.

Seizures

Seizures can cause posterior shoulder dislocations. In fact, 31% of the dislocations reported were being accountable due to the seizures of all the posterior dislocations(3).

A seizure is characterised by a sudden onset of high-intensity muscle contraction.

This can cause the larger muscle mass of internal rotators, consisting of the subscapularis, latissimus dorsi, and pectorals to overpower the external rotators mainly consisting of the rotator cuff muscles to posteriorly dislocate the shoulder.

Electrocution

A sudden passage of electric current has an incidence of 2% of total posterior dislocations (3).

Posterior Instability / Microtrauma

The posterior structures of the shoulder are under tension due to repeated episodes of loading. This can be seen in line backs in American Football, goalkeepers,  weightlifters, and overhead athletes who repeatedly are subjected to posteriorly directed force (3,5). A recent study on shoulder instability in the NFL has shown a greater incidence of posterior instability in quarterbacks and linebackers compared to other instabilities(8).  The mechanism of this can be explained as the structures getting stretched. This is seen as a continuum in fact a posterior instability gives rise to frank dislocation during the season, and hence preseason screening – prevention strategies can help in managing the injuries better. 

Chart showing Shoulder Instability events for NFL players
The chart shows Shoulder Instability Events for  NFL players, grouped by what position they play. Defensive Secondary players have the highest occurrence of shoulder instability events. Offensive Linemen suffer the highest proportion of Posterior Shoulder Instability Events.  Image credits: Orthopaedic Journal of Sports Medicine (8)

Anterior vs Posterior Shoulder Dislocation:

The differences between anterior and posterior shoulder dislocations are shown in this table:

ANTERIOR POSTERIOR
Position of the Humeral head In front and inferior to glenoid Backwards and inferior to glenoid
Mechanism of Trauma Anteriorly directed force with the arm is externally rotated and abducted position Posteriorly directed force with the arm is internally rotated and adducted position
Clinical Presentation
  1. Anterior Fullness, humeral head palpable anteriorly. Acromion prominent.
  1. Loss of contour of deltoid
  2. Affected arm supported by another hand in an Abducted and externally rotated position.
  1. Discomfort and pain in Internal Rotation and Adduction
  2. Inability to touch the opposite shoulder (Dugas test)
  3. Reduced axillary concavity (11)
  1. Posterior fullness, humeral head palpable posteriorly coracoid and acromion prominent
  1. Loss of anterior shoulder contour
  2. Affected arm in an internally rotated and adducted position.
  1. Discomfort and pain in External rotation and Abduction
  2.  Reduced supination in the dislocated forearm
  3. Neurovascular injuries are less common than anterior ones (6,7)
ASSOCIATED BONY AND LABRAL LESIONS
  1. Anterior capsulolabral injury called Bankart lesion 
  1. The bony impaction and compression fracture of the posterior lateral humeral head – is called “Hill Sach’s Lesion”
  2. Associated with mostly fractured tuberosities, neuropraxia of axillary nerves, and rotator cuff tears in elderly(11)
1. Posterior Capsulolabral injury, when the posterior- inferior labrum is injured and concealed is called Kim’s lesion.

2. The bony impaction and compression fracture of the anteromedial humeral head (Hill Sach’s Lesion”

3. Associated with fracture of neck of humerus, sometimes tuberosities, and rotator cuff tears mostly in elderly(6,7).

Diagnostic X-ray views
  1. Antero-posterior view
  2. Lateral or Scapular Y
  3. Stryker view
  4. Axillary view
  1. True Anteroposterior view
  2. Lateral or Scapular Y
  3. The axillary view is generally diagnostic
  4. Modified Axial or Velpeau view

Shoulder Dislocation X-Ray

The images below show X-Rays of anterior and posterior shoulder dislocations.

Anterior Shoulder Dislocation X-ray
Fig 1. Anterior Dislocation. Humerus is externally rotated and can be seen in front of the glenoid. X-Ray image by The Radswiki, www.radiopaedia.org, published under Creative Commons License. 
Posterior Dislocation of the shoulder X-ray. Humerus is internally rotated and vacant glenoid can be seen
Fig 2. Posterior Dislocation of the Shoulder. Humerus is internally rotated and vacant glenoid can be seen anteriorly. X-Ray image by Andrew Murphy www.radiopaedia.org Published under Creative Commons license.  
X-ray of Anterior Dislocation of the shoulder. Lateral View
Fig 3. Anterior Dislocation of the shoulder, lateral view. X-Ray image by The Radswiki, www.radiopaedia.org, published under Creative Commons License. 
X-ray of posterior dislocation of the shoulder. Lateral view
Fig 4. X-ray of Posterior Dislocation of the Shoulder. Lateral view. X-Ray image by Andrew Murphy www.radiopaedia.org published under Creative Commons license.  

Posterior Shoulder Dislocation MRI

MRI helps in diagnosing the associated soft-tissue injuries like rotator cuff tears , bicep tendon inflammation, extent of reverse hill sach lesion and vascular supply of the humeral head(6,7).

MRI Scans of posterior shoulder dislocation
Fig.5.  MRI Findings of a 58 year old male, showing reverse hill each lesion with increasing size defect in the humeral head as indicated by the arrows, a) Less than 25% of humeral head defect, b) 25 – 50% of humeral head defect, c) More than 50% of humeral head defect, d) Medium-sized defect of locked posterior dislocation. Image from Saupe et al, published under Creative Commons license.  

Hence, posterior dislocation of the shoulder, one of the trickiest shoulder instability conditions to diagnose, needs a thorough assessment, a good reason for doubt, and preventive strategies in athletic shoulders, and a series of radiology investigations.

One of the main differentials in the elderly population is frozen shoulder which can give similar signs of restriction in external rotation and abduction. If misdiagnosed as the frozen shoulder the patient loses time and the viable humeral head blood supply(1). Even reduction in more than 3 weeks old chronic cases should be only taken with an in-depth investigation and consultation with a shoulder surgeon.

REFERENCES

  1. Perron AD, Jones RL. Posterior shoulder dislocation: avoiding a missed diagnosis. The American journal of emergency medicine. 2000 Mar 1;18(2):189-91.
  2. Paul J, Buchmann S, Beitzel K, Solovyova O, Imhoff AB. Posterior shoulder dislocation: systematic review and treatment algorithm. Arthroscopy: The Journal of Arthroscopic & Related Surgery. 2011 Nov 1;27(11):1562-72.
  3. Robinson CM, Seah M, Akhtar MA. The epidemiology, risk of recurrence, and functional outcome after an acute traumatic posterior dislocation of the shoulder. JBJS. 2011 Sep 7;93(17):1605-13.
  4. Rouleau DM, Hebert-Davies J. Incidence of associated injury in posterior shoulder dislocation: systematic review of the literature. Journal of orthopaedic trauma. 2012 Apr 1;26(4):246-51.
  5. Tannenbaum E, Sekiya JK. Evaluation and management of posterior shoulder instability. Sports health. 2011 May;3(3):253-63.
  6. Basal O, Dincer R, Turk B. Locked posterior dislocation of the shoulder: A systematic review. EFORT Open Reviews. 2018 Jan 15;3(1):15
  7. Paparoidamis G, Iliopoulos E, Narvani AA, Levy O, Tsiridis E, Polyzois I. Posterior shoulder fracture-dislocation: A systematic review of the literature and current aspects of management. Chinese Journal of Traumatology. 2021 Jan 1;24(01):18-24.
  8. Anderson MJ, Mack CD, Herzog MM, Levine WN. Epidemiology of shoulder instability in the national football league. Orthopaedic Journal of Sports Medicine. 2021 Apr 27;9(5):23259671211007743.
  9. Chan O, editor. ABC of emergency radiology. John Wiley & Sons; 2012 Dec 12.
  10. https://www.ortho-teaching.feinberg.northwestern.edu/XRreading/shoulder/yview.html
  11. Abrams R, Akbarnia H. Shoulder dislocations overview. InStatPearls [Internet] 2021 Aug 13. StatPearls Publishing.
  12. https://radiopaedia.org/articles/posterior-shoulder-dislocation

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