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The ‘Lightbulb Sign’ doesn’t always mean a posterior dislocation

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The Primary Exam
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A 35 yo woman presents to the emergency department with a complaint of shoulder dislocation. There is a previous history of multiple dislocations, including two posterior dislocations. There has been no trauma, or seizure or other major force, as might be expected to cause a posterior dislocation, although posterior dislocation can occur in the absence of these.

On examination, the patient is sitting comfortably with the left shoulder lower than the right, plus some inward rotation of the humerus. There is no flattening of the shoulder and the humeral head appears to be in the right position on palpation. The arm is neurovascularly intact.

At this point the patient is refusing an X-ray, as she has had several X-rays before and states that the shoulder has been put in previously and a single x-ray is done post reduction, to minimise the number of X-rays done. There is no record of this patient in our hospital, however discharge notes from another department show that the shoulder was recently reduced.

Following discussion with the patient and because the clinical picture doesn’t fit, an x-ray of the joint is taken as shown below. Is this shoulder dislocated? What is the sign?

The trans-scapular Y view is shown here. The humeral head should normally be centered over the center of the Y formed by the coracoid, blade of the scapula and spine of the scapula (acromion). In this view it isn’t as centred as we would expect it to be.

The trans-scapular Y view

 

 


In this AP view we see the ‘lightbulb sign’. This is a radiographic appearance. It occurs because a posteriorly dislocated shoulder is internally rotated, causing a lightbulb appearance in the AP view.


 

Is this a posterior dislocation?
Posterior Glenohumeral joint dislocations account for about 4% of shoulder dislocations and result from the humeral head being forced posteriorly, whilst in internal rotation, with the arm abducted. These types of dislocations usually result from significant forces, such as trauma, electrocution and seizures, but can also occur in rotator cuff muscle abnormalities.

The dislocation can be missed on AP views. Important signs to look for include:

  1. The Lightbulb sign: As seen above, is due to internal rotation of the humeral head.
  2. The Rim sign, which is a widening of the glenohumeral joint, usually > 6mm
  3. The Trough Line sign, which is a vertical line on the medial aspect of the humeral head, that results from humeral head impaction.

There are several other signs, however these are the most commonly quoted.


In this particular patient there did not appear to be a dislocation clinically, regardless of the ‘lightbulb sign’. In situations such as this, an axial film can be done. It is shown below. This is not a dislocation. The humeral head articulates with the glenoid.


 

 

Axial view of patient

 

Axial View Anatomy

 

 

 

 

 

 

 

 

 

 

 

 


Below is an example that quite clearly demonstrated disruption of the joint.


 

Actual Left shoulder dislocation: Case courtesy of A.Prof Frank Gaillard, Radiopaedia.org. From the case rID: 10778

 


What happened next? We discussed the findings with the patient. The patient was told that the joint was not dislocated and no further analgesia would be given. The patient then moved the arm normally and discharged.

The lesson here is that the lightbulb sign, is purely an appearance on the x-ray and doesn’t mean the joint is dislocated. Posterior dislocations can also be difficult to diagnose and can be missed, especially in the elderly. When in doubt, look for the signs mentioned above and remember that an axial view is helpful.


Peter Kas

 

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