Posterior shoulder dislocation: An unusual complication

A 65-year-old gentleman was involved in a road traffic accident (RTA). He sustained a posterior shoulder dislocation which was subsequently reduced. He continued to have symptoms with his left shoulder. On examination, active forward flexion and abduction was to approximately 60 degrees, with external rotation to approximately 70 degrees. An MRI (1.5T) was requested, following which he was reviewed in the clinic.

Pertinent findings include denervation oedema involving the teres minor muscle (Figure 1) with a normal appearance of the quadrilateral space (QLS) (Figures 2 and 3) and reverse Hill–Sachs lesion with associated marrow oedema (Figure 4). No fracture. No rotator cuff tendon (RCT) tear or labral tear.

Background

The QLS is bounded by teres minor, triceps and humeral neck [1,2]. Postero-superior to the QLS, the axillary nerve divides into posterior and anterior branches; the posterior branch divides, innervating teres minor and posterior deltoid [3]. The anterior branch innervates the anterior 2/3 of the deltoid. Posterior shoulder dislocations force the humeral head posteriorly where it may injure the axillary nerve branch to teres minor [4,5]. While acutely, this may manifest as muscle denervation leading to muscle oedema, chronically, there may be fatty replacement [1].

Clinical Perspective

While teres minor denervation is associated with quadrilateral space syndrome (QSS), it can also be associated with RCT tears, glenohumeral capsule injury and joint dislocations [5]. QSS was initially described as compression of the posterior humeral circumflex artery and axillary nerve or one of its major branches [6]. Given the variable axillary nerve branching pattern, there may be isolated oedema of teres minor with normal deltoid muscle [7]. Hence, commenting on specific anatomically diseased areas will help clinicians understand the level of neural irritation (according to nerve branching pattern) and underlying causes, such as a mass in the QLS or posterior instability causing neural traction.

Imaging Perspective

In shoulder dislocations with reduced range of motion (RROM) and ongoing pain, it is important to consider labral and/or (RCT) tears [5]. In this patient, the principal symptom was RROM, and the primary differential diagnosis was an RCT tear. Hence, a non-arthrographic MRI was performed. However, this showed diffuse oedema within the teres minor muscle—RCTs were intact.

Given the homogeneity, diffuse oedema and preserved muscle architecture, a neural injury was suspected rather than direct trauma, which would have otherwise resulted in focal oedema and muscle architecture loss. Muscle oedema indicates at least a grade II nerve injury on the Sunderland and Mackinnon Classification, implying axonotmesis [8,9]. Nerve conduction studies or high-resolution MRI neurography can be used to trace neural branches to help ensure nerve continuity and rule out a higher grade of nerve injury [8,9].

Outcome

As the patient still had mechanical symptoms, he underwent a follow-up MR arthrogram which demonstrated resolution of oedema (Figure 5).

Take Home Message / Teaching Points

1. Posterior shoulder dislocation can injure the axillary nerve and its branches.
2. Acute muscle denervation manifests as homogeneously diffuse muscle oedema with no muscle architecture loss.
3. Denervation muscle oedema implies at least grade II Sunderland and Mackinnon nerve injury (low-grade axonotmesis). This may be confirmed on nerve conduction studies.