High Yield Overview

AXILLARY NERVE ANATOMY

The Sentinel of the Shoulder

C5-C6Roots

Post.Cord

5cmSafe Zone

40%Palsy in Dislocation over 40yo

INJURY MECHANISMS

Trauma

PatternAnterior Shoulder Dislocation / Proximal Humerus #

TreatmentReduce → Observe

Compression

PatternQuadrangular Space Syndrome

TreatmentDecompression

Iatrogenic

PatternDeltoid splitting approach / Arthroscopy

TreatmentRepair / Transfer

Neuritis

PatternParsonage-Turner Syndrome

TreatmentMedical / Physio

Critical Must-Knows

Originates from Posterior Cord (with Radial nerve)
Passes through Quadrangular Space with PCHA
Divides into Anterior (Motor) and Posterior (Mixed) branches
Anterior Branch wraps horizontally around humerus (5-7cm from acromion)
Sensory: 'Regimental Badge' area (Upper Lateral Cutaneous Nerve of Arm)

Examiner's Pearls

"
Most commonly injured nerve in shoulder dislocation
"
Teres Minor is the first muscle recovered (Posterior Branch)
"
Deltoid extension lag is a sign of Axillary palsy (often missed)
"
Nerve Transfer: Triceps branch to Axillary is the gold standard salvage

Clinical Imaging

Imaging Gallery

Posterior view of upper limb showing axillary and radial nervesCredit: Henry Vandyke Carter via Gray's Anatomy (1918) via Wikimedia Commons (Public Domain)

Posterior scapular region showing quadrangular space and axillary nerve branchesCredit: Colorized Gray's Anatomy via Wikimedia Commons via Wikimedia Commons (Public Domain)

The 'Hidden' Palsy

Dislocation Mask

Pain Inhibits Testing.

In acute shoulder dislocation, patient cannot abduct.
Test: Check Sensation (Regimental Badge).
Test: Isometric deltoid contraction (palpate).
Warning: 10-40% of dislocations have nerve injury. Document status PRE-reduction.

Subtle Wasting

Chronic Injury.

Deltoid atrophy leads to "Squaring" of the shoulder.
Can be masked by strong Rotator Cuff (Supraspinatus initiates abduction).
Look for fatigue in overhead activity.

Branch	Supplies	Course	Injury Risk
Main Trunk	Shoulder Joint Capsule	Quadrangular Space	Dislocation / QSS
Anterior Branch	Deltoid (Ant/Mid)	Winds around surgical neck	Deltoid Splitting / #PH
Posterior Branch	Teres Minor + Deltoid (Post)	Deep to Deltoid	Retraction injury
Upper Lat Cutaneous	Skin over deltoid	Pierces deep fascia	Sensory loss only

Mnemonic

TAM-hum-triQuadrangular Space Boundaries

Superior

Teres Minor

(Subscapularis anteriorly)

Inferior

Teres Major

Medial

Triceps Long Head

Lateral

Humerus

(Surgical Neck)

Memory Hook:The Teres sandwich with Humerus and Triceps bread.

Mnemonic

BADGERegimental Badge

Blow

Direct blow or Dislocation

Axillary

Nerve injured

Deltoid

Wasting

Great Tuberosity

Sensation area

Extension

Lag is the key sign

Memory Hook:Soldiers wear the badge where the sensation is lost.

Mnemonic

SITSRotator Cuff Innervation

Supraspinatus

Suprascapular Nerve

Infraspinatus

Suprascapular Nerve

Teres Minor

Axillary Nerve

Subscapularis

Subscapular Nerves (Upper/Lower)

Memory Hook:Axillary only supplies the MINOR Teres.

Overview

The Axillary Nerve is a terminal branch of the Posterior Cord. It provides motor function to the Deltoid and Teres Minor, critical for shoulder abduction and external rotation. Its course around the surgical neck of the humerus makes it highly vulnerable in trauma.

Neurovascular

Gray's Anatomy illustration showing the suprascapular, axillary, and radial nerves — The suprascapular, axillary, and radial nerves (Gray's Anatomy, Plate 818). Posterior view of the right upper limb showing the course of these major brachial plexus branches highlighted in yellow. The axillary nerve is seen passing around the surgical neck of the humerus after exiting the quadrangular space, then dividing to supply deltoid and teres minor. The radial nerve descends in the spiral groove of the humerus, giving off the deep branch in the forearm.Credit: Henry Vandyke Carter via Wikimedia - Public Domain

Origin

Posterior Cord of Brachial Plexus (C5, C6).
Lies posterior to the Axillary Artery.
Located on the surface of Subscapularis.

Course

Runs inferiorly and laterally.
Exits axilla through the Quadrangular Space.
Travels with the Posterior Circumflex Humeral Artery (PCHA).
Winds posteriorly around the Surgical Neck of the Humerus.

This course explains the high risk during inferior capsule release.

Classification Systems

Seddon Classification (1943)

Neuropraxia: Conduction block. No structural damage. Expect full recovery.
Axonotmesis: Axon interruption. Sheath intact. Wallerian degeneration occurs. Recovery 1mm/day.
Neurotmesis: Complete transection. No recovery without surgery.

This system is universally used for nerve injuries.

Clinical Assessment

Motor Testing

Deltoid: Abduction (check greater than 90 deg, as Supraspinatus does 0-30). Also Extension (Post Deltoid).
Teres Minor: External Rotation (Hornblower's Sign).
Look for "Swallow Tail" sign on extension (lag of posterior deltoid).

Sensory Testing

Regimental Badge: Area over the middle deltoid.
Supplied by Superior Lateral Cutaneous Nerve (branch of Posterior division).
Sensation may be preserved even with motor injury (incomplete palsy).

Differential Diagnosis

Condition	Weakness	Sensory Loss	Key Feature
Axillary Palsy	Deltoid/Teres Minor	Regimental Badge	Hx of Trauma/Dislocation
C5 Radiculopathy	Deltoid + Biceps	C5 Dermatome (Thumb)	Neck Pain, Biceps Reflex loss
Rotator Cuff Tear	Supraspinatus (Abd)	None (usually)	Painful Arc, Weak ER
Parsonage-Turner	Patchy / Severe Pain	Variable	Viral prodrome, profound waste

Investigations

MRI Shoulder

Acute: Identify edema in Deltoid/Teres Minor (denervation edema).
Chronic: Fatty infiltration and atrophy.
Quadrangular Space: Visualize space-occupying lesions (Paralabral cyst, Lipoma) compressing the nerve.
Hypertrophy: Teres Minor hypertrophy is a sign of chronic rotator cuff tear (compensation), NOT axillary nerve path.

MRI is also crucial for excluding rotator cuff tears in elderly patients.

Management Algorithm

Trauma (Dislocation/#)

Closed Injuries: 80-90% are neuropraxia and recover spontaneously.
Protocol:
- Reduce shoulder gently.
- Re-examine nerve.
- Physio (Range of motion).
- NCS at 3-4 weeks if complete palsy.
- Repeat NCS at 3 months.
- Surgery: If no recovery by 3-6 months → Exploration / Graft / Transfer.

Timing is critical for successful re-innervation.

Treatment Summary

Scenario	Timeframe	Action
Acute Dislocation	Day 0	Reduce, Confirm Neuro status
Palsy Post-Reduc	Week 3-4	EMG to baseline. Physio.
No Recovery	Month 3-4	Repeat EMG. If denervation + no MUAPs → Plan Surg
Exploration	Month 4-6	Neurolysis vs Graft vs Transfer

Surgical Technique

Posterior Approach

Incision: Vertical incision over posterior deltoid border.
Interval: Deltoid (Axillary) and Triceps (Radial).
Landmark: Teres Major (Inferior border of Q-Space).
Identify: Axillary nerve emerging from Quadrangular Space.
Action: Neurolysis (release bands) or Sural Graft.

The key landmark is the Teres Major muscle belly inferiorly.

The Safe Zone

In the Deltoid Splitting Lateral Approach, the axillary nerve runs horizontally approx 5-7 cm distal to the acromion.

Stay within 5cm of the acromion to stay safe.
If extending distally, palpate the nerve or peel deltoid from insertion.
The "Safe Zone" is shorter in smaller patients!

Complications

Shoulder stiffness: Adhesive capsulitis from prolonged immobilization.
Fail to recover: Permanent deltoid wasting.
Donor site morbidity: Sural nerve (numb foot), Triceps weakness (rare grade 4).
Neuroma: Painful neuroma in continuity.

Early identification of stiffness is key.

Rehabilitation Protocol

Phase	Timeframe	Goals	Precautions
Protection	0-3 Weeks	Sling, Pendulums	No Active Abduction
Activation	3-8 Weeks	Gravity eliminated abduction	Observe Scapular rhythm
Strengthening	8+ Weeks	Deltoid strengthening	Full ROM

Specific Exercises

Gravity Eliminated: Lying supine or on side, sliding arm.
Scapular Setting: Crucial to prevent "hitching" compensation.

Special Scenarios

Quadrangular Space Syndrome

Pathology: Compression of Axillary Nerve and PCHA in the space.
Causes: Hypertrophy of muscles (pitchers, swimmers), Fibrous bands, Paralabral cysts.
Presentation: Vague posterior shoulder pain, paresthesia in badge area, weakness in ER/Abd (fatigue).
Diagnosis: MRI (atrophy), Angiogram (PCHA occlusion on abduction).
Treatment: Surgical decompression.

Decompression involves releasing fibrous bands in the space.

Prognosis

Spontaneous Recovery: 80-90% of dislocation-associated palsies recover by 3-6 months.
Surgical Repair: Direct repair outcomes are better than graft.
Nerve Transfer: Triceps to Axillary yields M4 strength in over 85% of patients.
Quality of Life: Persistent deltoid weakness is well tolerated if Rotator Cuff is intact (Supraspinatus compensation).

Evidence Base

Nerve Transfer vs Grafting

Bertelli et al. • J Hand Surg (2014)

Key Findings:

Triceps to Axillary transfer yielded equivalent or better strength compared to splicing
Recovery time was shorter for transfer (distal target)
No functional deficit in triceps

Clinical Implication: Nerve transfer is becoming the gold standard for proximal injuries.

Limitation: Retrospective.

Safe Zone Anatomy

Cetik et al. • JBJS Am (2006)

Key Findings:

Distance from acromion to nerve varies by height
Average distance 5.6 cm
Can be as proximal as 4.2 cm
Safe zone recommendation: stay proximal to 4 cm

Clinical Implication: The '5cm rule' is a guide but is not absolute. Be careful.

Limitation: Cadaveric study.

Outcomes of Nerve Transfer

Lee et al. • JBJS Am (2012)

Key Findings:

Triceps to Axillary transfer achieves M3 or M4 power in 90% of cases
Average time to recovery: 6 months
No significant donor morbidity

Clinical Implication: Reliable salvage for failed primary repair or late presentation.

Axillary Nerve Palsy in Shoulder Dislocation

De Laat EAT et al. • J Shoulder Elbow Surg (1994)

Key Findings:

Axillary nerve injury in 9-18% of anterior dislocations
Higher incidence with age over 50
Most are neuropraxia with full recovery
Document pre-reduction status

Clinical Implication: Always document neurological status before and after reduction.

Quadrangular Space Syndrome

Cahill BR, Palmer RE • J Hand Surg Am (1983)

Key Findings:

First description of QSS in athletes
Posterior shoulder pain with paresthesia
PCHA occlusion on angiography
Surgical decompression is curative

Clinical Implication: Consider QSS in overhead athletes with posterior shoulder pain and atrophy.

Viva Scenarios

Practice these scenarios to excel in your viva examination

VIVA SCENARIOStandard

Scenario 1: Anterior Dislocation

EXAMINER

"A 25-year-old male has an anterior shoulder dislocation. Reduced in ED. You see him next day. He has numbness over the deltoid. What do you do?"

EXCEPTIONAL ANSWER

This represents an Axillary Nerve injury, common in dislocation (neurapraxia). 1. Document full neurological exam (check distal pulses/nerves too). 2. Confirm reduction X-rays (Greater Tuberosity #?). 3. Manage expectantly: Physio for ROM. 4. Review at 3 weeks. If still palsy → EMG. Most recover. Operative exploration is reserved for no recovery at 3-4 months.

KEY POINTS TO SCORE

Neuropraxia is most common

Documentation is Medicolegal critical

Wait 3 months before surgery

COMMON TRAPS

✗Rushing to MRI (unless cuff tear suspected)

✗Ignoring the palsy

LIKELY FOLLOW-UPS

"What if he is 60 years old?"

"Massive rotator cuff tear is the main differential for weakness."

VIVA SCENARIOStandard

Scenario 2: Failed Cuff Repair

EXAMINER

"A patient had a rotator cuff repair via a mini-open lateral approach. Now has no active abduction. Cuff appears intact on Ultrasound. Diagnosis?"

EXCEPTIONAL ANSWER

Consider iatrogenic Axillary Nerve injury (Anterior Branch). The lateral splitting approach puts the nerve at risk if extended beyond 5cm. I would order an EMG to look for denervation in the anterior/middle deltoid. If confirmed, this is a clean transection or suture entrapment. It requires urgent exploration and repair/grafting (unlike blunt trauma neuropraxia).

KEY POINTS TO SCORE

Iatrogenic injury = Explore early

Anterior branch is motor only

Different to dislocation management

COMMON TRAPS

✗Waiting 3-6 months 'to see'

✗Assuming it's a recurrent tear

LIKELY FOLLOW-UPS

"What transfer options exist?"

"Triceps to Axillary."

VIVA SCENARIOCritical

Scenario 3: Quadrangular Space Syndrome

EXAMINER

"A 19-year-old elite swimmer complains of vague posterior shoulder pain and fatigue during late laps. MRI shows isolated atrophy of Teres Minor. Proximal humerus is normal."

EXCEPTIONAL ANSWER

This is classical Quadrangular Space Syndrome. The hypertrophy of the Teres muscles compresses the Axillary nerve. The isolated Teres Minor atrophy suggests compression of the posterior branch or main trunk. Management: 1. Confirm with Angiogram (PCHA occlusion in abduction) or dynamic EMG. 2. Stop aggravating activity. 3. Surgical decompression if non-operative management fails (release fibrous bands).

KEY POINTS TO SCORE

Young overhead athletes

Isolated atrophy on MRI

Dynamic vascular occlusion

COMMON TRAPS

✗Diagnosing 'Rotator Cuff Tendinitis'

✗Missing the atrophy on MRI

LIKELY FOLLOW-UPS

"What artery runs with the nerve?"

"Posterior Circumflex Humeral Artery."

MCQ Practice Points

Quadrangular Space Contents

Q: What accompanies the Axillary nerve in the Quadrangular Space? A: Posterior Circumflex Humeral Artery (PCHA). The Circumflex Scapular Artery is in the Triangular Space. The Profunda Brachii is in the Triangular Interval.

Safe Zone

Q: What is the average distance of the axillary nerve from the acromion? A: 5-7 cm. However, in shorter individuals, it can be 4cm. A safe rule of thumb is 3-4 cm.

First Sign of Recovery

Q: Which muscle recovers first after a proximal axillary nerve injury? A: Teres Minor. It is supplied by the posterior branch and is closer to the origin than the deltoid branches. Recovery of external rotation often precedes abduction.

Hornblower's Sign

Q: What does Hornblower's sign test for? A: Teres Minor dysfunction. The patient cannot externally rotate the arm in 90 degrees of abduction. The hand falls towards the mouth (like blowing a horn).

Nerve Transfer Donor

Q: Which nerve is most commonly used as a donor for Axillary Nerve reconstruction? A: Radial Nerve (Branch to Long Head Triceps). It is anatomically close, has redundant innervation to the triceps, and provides excellent motor axon match.

Australian Context

Rugby/AFL: Shoulder dislocations are extremely common. High index of suspicion for nerve injury.
Guidelines: Early MRI in patients over 40 years with dislocation to rule out cuff tear (often co-exists with nerve injury).
Referral: Peripheral nerve injuries often managed in major trauma units (Royal North Shore, Alfred, Royal Brisbane).

High-Yield Exam Summary

Anatomy High Yield

•Roots: C5-C6
•Cord: Posterior
•Space: Quadrangular
•Vessel: PCHA

Clinical Signs

•Badge: Sensory loss
•Square Shoulder: Wasting
•Swallow Tail: Post Delt Lag
•Hornblower: Teres Minor (ER)

Key Numbers

•5cm: Safe zone from acromion
•30%: Incidence of palsy in dislocations
•3 months: Time to EMG
•1mm/day: Nerve regrowth rate

Differentials

•Rotator Cuff Tear (Pain/Weakness)
•C5 Radiculopathy (Biceps involved)
•Parsonage Turner (Pain +++)
•Frozen Shoulder (Stiffness)

Branch

Supplies

Course

Injury Risk

Main Trunk

Shoulder Joint Capsule

Quadrangular Space

Dislocation / QSS

Anterior Branch

Deltoid (Ant/Mid)

Winds around surgical neck

Deltoid Splitting / #PH

Posterior Branch

Teres Minor + Deltoid (Post)

Deep to Deltoid

Retraction injury

Upper Lat Cutaneous

Skin over deltoid

Pierces deep fascia

Sensory loss only

Origin

Posterior Cord of Brachial Plexus (C5, C6).
Lies posterior to the Axillary Artery.
Located on the surface of Subscapularis.

Course

Runs inferiorly and laterally.
Exits axilla through the Quadrangular Space.
Travels with the Posterior Circumflex Humeral Artery (PCHA).
Winds posteriorly around the Surgical Neck of the Humerus.

This course explains the high risk during inferior capsule release.

Condition

Weakness

Sensory Loss

Key Feature

Axillary Palsy

Deltoid/Teres Minor

Regimental Badge

Hx of Trauma/Dislocation

C5 Radiculopathy

Deltoid + Biceps

C5 Dermatome (Thumb)

Neck Pain, Biceps Reflex loss

Rotator Cuff Tear

Supraspinatus (Abd)

None (usually)

Painful Arc, Weak ER

Parsonage-Turner

Patchy / Severe Pain

Variable

Viral prodrome, profound waste

Scenario

Timeframe

Action

Acute Dislocation

Day 0

Reduce, Confirm Neuro status

Palsy Post-Reduc

Week 3-4

EMG to baseline. Physio.

No Recovery

Month 3-4

Repeat EMG. If denervation + no MUAPs → Plan Surg

Exploration

Month 4-6

Neurolysis vs Graft vs Transfer

Phase

Timeframe

Goals

Precautions

Protection

0-3 Weeks

Sling, Pendulums

No Active Abduction

Activation

3-8 Weeks

Gravity eliminated abduction

Observe Scapular rhythm

Strengthening

8+ Weeks

Deltoid strengthening

Full ROM