Posterior Shoulder Dislocation: Axillary Nerve Injury, Symptoms, and Recovery

What nerve is affected in a posterior shoulder dislocation?

The primary nerve most commonly affected in a posterior shoulder dislocation is the axillary nerve, due to its vulnerable anatomical position as it courses around the surgical neck of the humerus, directly adjacent to the glenohumeral joint.

Understanding Posterior Shoulder Dislocation

A shoulder dislocation occurs when the head of the humerus (upper arm bone) separates from the glenoid fossa (shoulder socket). While most shoulder dislocations are anterior (the humeral head moves forward), posterior dislocations are far less common, accounting for only 2-5% of all shoulder dislocations. They often result from specific mechanisms such as direct trauma to the anterior shoulder, seizures, or electrocution, where strong internal rotation and adduction forces are applied to the arm. Given the significant force involved, neurovascular compromise is a critical concern requiring immediate assessment.

The Axillary Nerve: The Primary Concern

The axillary nerve (C5-C6) is the nerve most frequently injured in the context of a posterior shoulder dislocation. Its susceptibility stems directly from its anatomical course and close proximity to the glenohumeral joint capsule and the surgical neck of the humerus.

When the humeral head displaces posteriorly, it can directly compress, stretch, or even avulse the axillary nerve, leading to neurapraxia, axonotmesis, or, in severe cases, neurotmesis. Prompt diagnosis and reduction of the dislocation are crucial to minimize the duration and severity of nerve compression.

Anatomy and Function of the Axillary Nerve

The axillary nerve originates from the posterior cord of the brachial plexus. It travels inferiorly and laterally, passing through the quadrangular space (bounded by the teres minor, teres major, long head of triceps, and surgical neck of the humerus). After exiting this space, it wraps around the surgical neck of the humerus, lying in direct contact with the bone.

Its primary functions include:

• Motor innervation:
  • Deltoid muscle: The main muscle responsible for shoulder abduction (lifting the arm away from the body) and contributing to flexion and extension.
  • Teres minor muscle: A rotator cuff muscle that assists in external rotation of the shoulder.
• Sensory innervation: Provides sensation to the skin over the lateral aspect of the shoulder, often referred to as the "regimental badge" area.

Due to this critical innervation, injury to the axillary nerve can severely impair shoulder function and sensation.

Other Potentially Affected Nerves

While the axillary nerve is the most common, other nerves of the brachial plexus can be affected in cases of severe or complex posterior shoulder dislocations, or if the trauma is extensive. These include:

• Musculocutaneous nerve: Supplies the biceps brachii and brachialis, responsible for elbow flexion.
• Radial nerve: Supplies the triceps and wrist/finger extensors.
• Median and Ulnar nerves: Less commonly affected in isolated posterior dislocations but can be involved in major brachial plexus injuries.

A thorough neurovascular assessment is always paramount to identify any broader nerve involvement and ensure no other significant injuries are present.

Clinical Manifestations of Axillary Nerve Injury

Patients with axillary nerve injury following a posterior shoulder dislocation may present with a combination of motor and sensory deficits:

• Motor Weakness/Paralysis:
  • Difficulty or inability to abduct the arm (raise it sideways) due to deltoid dysfunction.
  • Weakness in external rotation of the shoulder due to teres minor involvement.
• Sensory Disturbances:
  • Numbness, tingling (paresthesia), or a complete loss of sensation over the lateral aspect of the shoulder (the "regimental badge" area).
• Pain: Localized pain in the shoulder region, distinct from the nerve-related symptoms.

These symptoms, combined with the mechanism of injury and the physical findings of a dislocated shoulder, strongly suggest axillary nerve involvement.

Diagnosis and Management

Diagnosis of a posterior shoulder dislocation often involves a clinical examination, which may reveal a flattened anterior shoulder contour and a prominent coracoid process. Imaging, particularly axillary or scapular Y-view X-rays, are crucial as posterior dislocations can be missed on standard anterior-posterior (AP) views.

Management includes:

• Prompt Reduction: The dislocated humeral head must be reduced back into the glenoid fossa as quickly as possible to alleviate pressure on the nerve and other soft tissues. This can be done via closed reduction techniques under sedation or, in complex cases, open reduction surgery.
• Neurovascular Assessment: A comprehensive assessment of nerve function (motor and sensory) and vascular status must be performed before and after reduction to document any deficits and monitor for changes.
• Immobilization: Following reduction, the shoulder is typically immobilized in a sling for a period to allow soft tissue healing.

Recovery and Rehabilitation

Recovery from axillary nerve injury can vary widely depending on the severity of the damage.

• Neurapraxia (temporary block of nerve conduction) often resolves within weeks to a few months.
• Axonotmesis (damage to axons with intact nerve sheath) can take longer, with recovery occurring over several months as axons regenerate at a rate of approximately 1mm per day.
• Neurotmesis (complete nerve transection) requires surgical repair and has the poorest prognosis for full recovery.

Rehabilitation plays a vital role, focusing on:

• Pain management and swelling reduction.
• Restoration of passive and active range of motion.
• Strengthening exercises for the deltoid, teres minor, and surrounding shoulder musculature once nerve recovery begins.
• Proprioceptive training to improve joint awareness and stability.
• Patient education on activity modification and proper body mechanics.

Conclusion

A posterior shoulder dislocation is a serious injury with the potential for significant neurovascular complications. The axillary nerve is the most vulnerable and commonly affected nerve, leading to impaired shoulder abduction, external rotation, and lateral shoulder sensation. Prompt diagnosis, careful reduction, and meticulous neurovascular assessment are critical to optimize patient outcomes and facilitate effective rehabilitation, ensuring the best possible recovery of shoulder function.