Root Origin
- Spinal nerve roots: C5, C6
- Plexus: Brachial plexus
- Cord: Posterior cord
- Type: Mixed (motor and sensory)
Course
- Posterior cord. The axillary nerve branches from the posterior cord of the brachial plexus in the axilla, just posterior to the axillary artery. It is the more superior of the two terminal branches of the posterior cord (the other being the radial nerve).
- Quadrangular space. The nerve exits the axilla posteriorly through the quadrangular space — a gap bounded by the teres minor (superiorly), teres major (inferiorly), long head of the triceps (medially), and the surgical neck of the humerus (laterally). The posterior circumflex humeral artery travels with the nerve through this space. This is the primary entrapment site.
- Posterior shoulder. After exiting the quadrangular space, the nerve wraps around the surgical neck of the humerus from posterior to lateral, traveling on the deep surface of the deltoid. It divides into anterior and posterior branches.
- Anterior branch. Wraps around the surgical neck of the humerus with the posterior circumflex humeral artery, supplying the anterior and middle deltoid. This branch curves around the humerus approximately 5-7 cm below the acromion — a critical measurement for avoiding nerve injury during deltoid injections.
- Posterior branch. Supplies the teres minor and the posterior deltoid. It gives off the superior lateral cutaneous nerve of the arm — the sensory branch that supplies the "regimental badge" area.
Motor Distribution
| Muscle | Action | Notes |
|---|---|---|
| anatomy/muscles/deltoid (all three parts) | Abduction (middle fibers), flexion (anterior fibers), extension (posterior fibers) | The primary shoulder abductor from 15-90 degrees; supraspinatus initiates the first 15 degrees |
| anatomy/muscles/teres-minor | External rotation, weak adduction | Part of the rotator cuff; the only rotator cuff muscle not supplied by the suprascapular nerve |
Sensory Distribution
- Superior lateral cutaneous nerve of the arm (regimental badge area). The sensory branch supplies a patch of skin over the lateral deltoid — roughly where a military badge would sit on the upper arm. This area extends from just below the acromion to approximately halfway down the lateral arm.
- Clinical significance: Loss of sensation over the "regimental badge" area is the hallmark sensory finding of axillary nerve injury. Test this patch after any shoulder dislocation or humeral neck fracture. Numbness here with weak abduction confirms axillary nerve involvement — do not confuse this with C5 dermatomal loss, which covers a broader lateral arm area.
Entrapment Sites
1. Quadrangular Space
- Location: The quadrangular space — bounded by teres minor, teres major, long head of triceps, and the humeral neck
- Structure: The nerve and posterior circumflex humeral artery pass through this space together. Compression can occur from fibrous bands within the space, hypertrophy of the bordering muscles, or space-occupying lesions
- Condition: Quadrangular space syndrome
- Presentation: Poorly localized posterior shoulder pain, worsened by abduction and external rotation (which tightens the space). Paresthesia over the regimental badge area. In chronic cases, deltoid atrophy with a flattened shoulder contour. Often misdiagnosed as rotator cuff pathology.
- MT relevance: When working the posterior shoulder — particularly teres minor and the long head of triceps — be aware of the quadrangular space. Sustained deep compression directly over the space can reproduce symptoms. If posterior shoulder pain does not respond to rotator cuff treatment and is accompanied by lateral arm numbness, consider the nerve.
2. Surgical Neck of the Humerus
- Location: Where the nerve wraps around the surgical neck, approximately 5-7 cm below the acromion
- Structure: The nerve is tethered to the bone as it curves around the humerus. Fractures at the surgical neck can stretch, contuse, or sever the nerve. Anterior shoulder dislocations displace the humeral head inferiorly, stretching the nerve across the displaced bone.
- Presentation: Post-traumatic deltoid weakness and regimental badge numbness. Onset follows a shoulder dislocation or proximal humeral fracture.
- MT relevance: After a shoulder dislocation or humeral neck fracture, always screen for axillary nerve integrity before beginning rehabilitation. Test resisted abduction and regimental badge sensation. Nerve injury occurs in approximately 5-10% of anterior dislocations and up to 30-60% of proximal humeral fractures in older adults.
Clinical Tests
| Test | Procedure | Positive Finding | What It Tells You |
|---|---|---|---|
| Resisted shoulder abduction | Patient abducts the arm to 90 degrees against resistance (or attempts to). | Weakness or inability to hold the arm in abduction. | Deltoid motor function via the axillary nerve. Compare sides. Grade 0-5. Cannot abduct past 15 degrees without deltoid (supraspinatus alone only initiates). |
| Regimental badge sensation | Light touch and pinprick over the lateral deltoid area — the patch of skin directly over the deltoid insertion. | Decreased or absent sensation compared to the opposite side. | Sensory function of the superior lateral cutaneous nerve of the arm (axillary branch). This is the quickest screen for axillary nerve integrity. |
| Resisted external rotation (teres minor) | Patient externally rotates the arm against resistance with elbow at 90 degrees and arm at the side. | Weakness of external rotation. | Teres minor motor function. However, infraspinatus (suprascapular nerve) is the primary external rotator — isolated teres minor weakness is difficult to detect clinically. Combined weakness with deltoid involvement confirms axillary nerve. |
Clinical Notes
- Post-dislocation screening is mandatory. After every anterior shoulder dislocation, test the axillary nerve: resisted abduction and regimental badge sensation. The nerve is stretched during the dislocation, and injury rates of 5-10% make this a common post-traumatic neuropathy. Document the findings — nerve recovery can take months, and baseline documentation matters for tracking progress.
- Deltoid atrophy changes the shoulder contour. Loss of the deltoid's bulk produces a visibly squared-off shoulder — the normal rounded contour is replaced by a flattened lateral profile where the acromion becomes prominent. This is visible from across the room in established cases. Compare sides. Deltoid atrophy can also result from disuse, but asymmetric atrophy with regimental badge numbness is axillary nerve until proven otherwise.
- Injection safety: the 5-7 cm rule. Intramuscular injections into the deltoid must be given well below the acromion but above the deltoid insertion — the "safe zone" is the middle third of the deltoid, at least 5 cm below the acromion. The axillary nerve wraps around the humerus at approximately this level. Injections too close to the acromion risk the subacromial bursa; too low risk the nerve.
- Quadrangular space syndrome mimics rotator cuff pathology. Both present with posterior shoulder pain worsened by overhead activity. The differentiator is the regimental badge numbness — rotator cuff tears do not produce sensory changes on the lateral arm. If posterior shoulder treatment plateaus and there is lateral arm paresthesia, suspect the quadrangular space.
- Sleeping posture matters. Side-lying on the affected shoulder compresses the quadrangular space. Patients with axillary nerve irritability should avoid sleeping on the affected side. This is a simple intervention that can significantly reduce symptoms.
Related Nerves
- anatomy/nerves/suprascapular-nerve — C5-C6. Supplies the other rotator cuff muscles (supraspinatus, infraspinatus). Together, the axillary and suprascapular nerves innervate the entire rotator cuff. Both carry C5-C6 fibers.
- anatomy/nerves/radial-nerve — C5-T1, posterior cord. The axillary nerve's "sibling" from the posterior cord. A posterior cord lesion affects both nerves — causing deltoid weakness (axillary) plus triceps weakness and wrist drop (radial). If you find axillary deficits, test the radial nerve to rule out a cord-level lesion.
- anatomy/nerves/musculocutaneous-nerve — C5-C7, lateral cord. Shares C5-C6 roots but from a different cord. Upper trunk lesions (Erb's palsy) affect both nerves — producing the classic "waiter's tip" position with shoulder and elbow dysfunction.
Key Takeaways
- Supplies deltoid (shoulder abduction) and teres minor (external rotation) — test both after any shoulder dislocation or humeral neck fracture.
- The "regimental badge" area on the lateral deltoid is the signature sensory territory — numbness here is the quick screen for axillary nerve injury.
- Wraps around the surgical neck of the humerus at 5-7 cm below the acromion — vulnerable in fractures (30-60% in elderly) and dislocations (5-10%).
- Quadrangular space syndrome mimics rotator cuff pathology but includes lateral arm numbness — the key differentiator.