What nerve is most likely injured in a humeral dislocation?

Axillary Nerve Injury in Humeral Dislocation

The axillary nerve is the most commonly injured nerve structure in humeral dislocations, occurring in up to 58% of cases. 1

Anatomical Basis for Axillary Nerve Vulnerability

The axillary nerve is particularly vulnerable during shoulder dislocations due to:

• Its anatomical course through the quadrilateral space 2
• Its close proximity to the glenohumeral joint capsule 3
• The traction force applied during dislocation 4

The nerve typically gets injured just proximal to the quadrilateral space, where it has limited mobility and is subject to stretching forces during the dislocation event 4.

Clinical Presentation of Axillary Nerve Injury

When evaluating a patient with humeral dislocation, look for:

• Weakness in shoulder abduction (deltoid muscle function)
• Sensory loss over the lateral aspect of the shoulder (the "regimental badge" area)
• Atrophy of the deltoid muscle (in chronic cases)
• Inability to actively abduct the arm beyond 15-20 degrees

Diagnostic Approach

1. Clinical examination: Test deltoid strength and sensation over the lateral shoulder

2. Electromyography (EMG): Should be obtained within 4 weeks of injury 4

   • Helps establish baseline nerve function
   • Follow-up EMG at 12 weeks to assess recovery

3. Imaging:

   • Standard radiographs to evaluate for associated fractures 5
   • MRI without contrast is preferred for soft tissue evaluation if nerve injury is suspected 5

Management

Most axillary nerve injuries following humeral dislocation recover spontaneously:

• Initial management: Rest the shoulder and provide appropriate analgesia
• Rehabilitation: Begin gentle range of motion exercises once acute pain subsides
• Follow-up: Clinical reassessment at 6 weeks to evaluate progress 6
• EMG monitoring: Repeat at 12 weeks to assess nerve recovery 4

If no clinical or electromyographic improvement is noted by 3-6 months, surgical exploration may be indicated 4.

Prognosis

The prognosis for axillary nerve injury following humeral dislocation is generally favorable:

• Most patients recover with non-operative treatment 4
• Complete recovery can take 6-12 months
• Results of surgical repair (if needed) are better than for other peripheral nerve injuries due to the monofascicular composition of the nerve 4

Common Pitfalls

1. Missed diagnosis: Axillary nerve injury is often overlooked during initial evaluation of shoulder dislocations
2. Delayed treatment: Surgical intervention, if needed, should be performed within 3-6 months for optimal results 4
3. Inadequate follow-up: Failure to monitor nerve recovery with clinical examination and EMG
4. Attributing all shoulder weakness to pain: Distinguish between pain-related weakness and true neurological deficit

Remember that axillary nerve injury may occur in combination with rotator cuff tears and shoulder dislocation, comprising the "unhappy triad" of the shoulder joint 3.