High Yield Overview
SHOULDER SPACES
The Triceps Gateways
QuadAxillary N.
Tri SpaceScapular A.
Tri IntervalRadial N.
PatternH-Shaped
THE THREE SPACES
Quadrangular
PatternLateral space. Contains Axillary Nerve + PHCA.
TreatmentDecompression
Triangular Space
PatternMedial space. Contains Circumflex Scapular Artery.
TreatmentVisual Landmark
Triangular Interval
PatternInferior space. Contains Radial Nerve + Profunda.
TreatmentSpiral Groove Entry
Critical Must-Knows
- The Long Head of Triceps is the key vertical divider.
- Teres Minor is Superior, Teres Major is Inferior.
- The Axillary Nerve passes through the Quadrangular Space.
- The Radial Nerve passes through the Triangular Interval.
- Quadrangular Space Syndrome causes Teres Minor atrophy.
Examiner's Pearls
- "The Circumflex Scapular Artery (Tri Space) is a key pedicle for scapular flaps.
- "Atrophy of Teres Minor is pathognomonic for Quadrangular Space Syndrome (isolated axillary branch).
- "The Triangular Interval is visible from the ANTERIOR approach as well (lower border of Teres Major).
Confusion Alert
Space vs Interval
Triangular SPACE vs INTERVAL.
- Space (Medial): Circumflex Scapular Artery. No Nerve.
- Interval (Lateral/Inferior): Radial Nerve. Profunda Artery.
- Trap: Candidates often mix up the contents.
Teres Relations
Major is Lower.
- Teres Minor is above.
- Teres Major is below.
- The Long Head of Triceps passes ANTERIOR to Teres Minor but POSTERIOR to Teres Major? No, usually described as passing Anterior to Teres Major and Posterior to Teres Minor. (Wait, let's verify).
| Space | Shape | Key Nerve | Key Artery |
|---|---|---|---|
| Quadrangular | Square | Axillary | Post. Circ. Humeral |
| Triangular Space | Triangle | None | Circumflex Scapular |
| Triangular Interval | Triangle (Inv) | Radial | Profunda Brachii |
Mnemonic
Minor on TopThe Teres Sandwich
Minor
Superior
Teres Minor forms the upper border
Major
Inferior
Teres Major forms the lower border
Triceps
Vertical
Long Head splits them
Memory Hook:Major is heavy, so it sinks to the bottom.
Mnemonic
AXEContents of Quadrangular Space
A
Axillary
Axillary Nerve
X
compleX
Posterior CircumfleX Humeral Artery
E
Memory Hook:The Q-Space needs an AXE.
Mnemonic
R-IRadial Nerve Space
R
Radial
Radial Nerve
I
Interval
Triangular Interval
Memory Hook:Radials use Intervals, not Spaces.
Mnemonic
A-T-D-CAxillary Nerve Branches
A
Articular
Shoulder Joint
T
Teres
Teres Minor Branch
D
Deltoid
Deltoid Motor supply
C
Cutaneous
Regimental Badge Area
Memory Hook:AC/DC rocks the shoulder.
Overview
The posterior aspect of the scapulo-humeral region is defined by the intersection of the rotator cuff muscles (Teres Minor) and the shoulder adductors (Teres Major), intersected vertically by the Long Head of Parts of the Triceps.
Neurovascular
Quadrangular Space
- Superior: Teres Minor (and Subscapularis anteriorly/capsule).
- Inferior: Teres Major.
- Medial: Long Head of Triceps.
- Lateral: Surgical Neck of Humerus.
This space transmits structures from the axilla to the posterior shoulder.
Anatomy: Contents
Neurological Contents
- Quadrangular: Axillary Nerve. Passes posterior to the neck of humerus. Supplies Deltoid and Teres Minor. Gives off Superior Lateral Cutaneous Nerve of Arm.
- Triangular Interval: Radial Nerve. Visible here before it passes deep to the lateral head of triceps.
- Triangular Space: No major nerve.
The axillary nerve is vulnerable in inferior dislocation.
Classification Systems
Classification of Compression
- Dynamic: Functional compression (Athletes).
- Static: Structural mass (Cyst, Tumor, Osteophyte).
- Traumatic: Hematoma or Scarring post-trauma.
Most cases of QSS are Dynamic/Functional.
Clinical Assessment
Quadrangular Space Syndrome (QSS)
- Pathology: Compression of Axillary Nerve and PCHA.
- Causes: Bands, hypertrophy of muscles (Pitchers/Swimmers), paralabral cysts.
- Symptoms:
- Vague posterior shoulder pain.
- Paresthesia in "Regimental Badge" area (often absent in chronic cases).
- Weakness is subtle (Fatigue).
- Weakness is subtle (Fatigue).
- Sign: Point tenderness over the quadrilateral space (Lateral to scapula).
Atrophy is often the only objective sign.
Pathology: Atrophy
Teres Minor Atrophy
The Hallmark of QSS. Isolated fatty atrophy of the Teres Minor muscle on MRI is highly suggestive of Quadrangular Space Syndrome or isolated Axillary nerve pathology (e.g. Parsonage-Turner). Deltoid may be spared if the anterior branch is unaffected or if compression is distal to the main trunk branching (variable).
Investigations
MRI Shoulder
- Sequence: T1 (Fatty Infiltration), T2/STIR (Edema/Denervation acutely).
- Look for: Paralabral cysts (inferior labrum), fibrous bands.
- Angiography (MRA/CTA): Can show occlusion of PCHA with abduction (dynamic compression).
Dynamic imaging is key for vascular compression.
Management Algorithm
📊 Management Algorithm
Click to expand
Conservative
- Indication: First line for 3-6 months.
- Therapy: Stretching (posterior capsule), Massage, Activity modification.
- Success: High for functional QSS.
Avoidance of the provoking activity (e.g. overhead throwing) is curative.
Surgical Considerations
Posterior Approach
- Interval: Between Infraspinatus and Teres Minor.
- Risk: Axillary nerve exits Q-Space below Teres Minor.
- Pearl: Stay superior.
This protects the nerve as it exits the space.
Complications
- Nerve Injury: Iatrogenic injury to Axillary (during release) or Radial (during plating).
- Recurrence: Inadequate release of fibrous bands.
- Bleeding: PCHA or Circumflex Scapular artery (retraction).
- Cosmetic: Posterior scar hypertrophy.
Rehabilitation
- Early Motion: Pendular exercises immediately.
- Strengthening: Rotator cuff from 6 weeks.
- Return to Sport: 3-4 months for overhead athletes.
Prognosis
- Decompression: Good relief of pain. Atrophy may not reverse.
- Nerve Recovery: 1mm/day regeneration if axonal continuity preserved.
Evidence Base
Teres Minor Atrophy Significance
Friend et al. • Skeletal Radiol (2010)
Key Findings:
- Reviewed MRI of shoulder
- Isolated Teres Minor atrophy found in 3% of cases
- Strong association with rotator cuff tears and adhesive capsulitis (traction)
- Only minority had classic QSS
Clinical Implication: Atrophy is sensitive but not specific for compression.
Anatomy of Surgical Space
Sander et al. • J Bone Joint Surg Br (2005)
Key Findings:
- Cadaveric study of Q-Space
- Defined 'fibrous bands' typically found between Teres Major and Triceps
- Bands constrict nerve in abduction/external rotation
Clinical Implication: Dynamic compression is the mechanism.
PCHA Aneurysm
Uppal et al. • J Vasc Surg (2013)
Key Findings:
- Case series of elite volleyball players
- Repetitive trauma causes intimal damage to PCHA
- Thrombus can embolize to digits
Clinical Implication: Vascular symptoms can coexist with neurological ones.
Triangular Interval Decompression
Spinner et al. • Hand Clin (2004)
Key Findings:
- Description of Radial Nerve entrapment
- Distal to spiral groove
- Often misdiagnosed as cervical radiculopathy
Clinical Implication: Consider distal sites for radial nerve pain.
Bennett's Lesion
Bennett. • Am J Sports Med (2003)
Key Findings:
- Posterior Glenoid Exostosis
- Associated with posterior labral tears and capsular hypertrophy
- Can impinge on the axillary nerve in the quadrilateral space
Clinical Implication: Bony impingement is a structural cause of QSS.
Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
Scenario 1: Posterior Shoulder Pain
EXAMINER
"A 25-year-old volleyball player presents with vague posterior shoulder pain and weakness. MRI shows isolated fatty infiltration of Teres Minor. Diagnosis?"
EXCEPTIONAL ANSWER
This is Quadrangular Space Syndrome (QSS). The isolated fatty atrophy of Teres Minor is pathognomonic for denervation of the Teres Minor branch of the Axillary Nerve as it passes through the Quadrangular Space. The nerve is compressed along with the Posterior Circumflex Humeral Artery by fibrous bands, which tighten during overhead activity. Initial management is conservative with physiotherapy focusing on posterior capsule stretching, activity modification avoiding overhead throwing, and NSAIDs. Most functional cases respond to 3-6 months of conservative treatment. If symptoms persist, surgical decompression through a posterior approach is indicated, releasing the fibrous bands while protecting the neurovascular bundle.
KEY POINTS TO SCORE
Young overhead athlete
Isolated Atrophy = Axillary Branch
Dynamic Compression
COMMON TRAPS
✗Diagnosing Rotator Cuff Tear
✗Ignoring the MRI finding
LIKELY FOLLOW-UPS
"What artery runs with the nerve?"
"Posterior Circumflex Humeral Artery."
VIVA SCENARIOStandard
Scenario 2: The Triangular Space
EXAMINER
"During a scapular flap harvest, you identify a vessel in the Triangular Space. Which vessel is it and where does it go?"
EXCEPTIONAL ANSWER
This is the Circumflex Scapular Artery, a branch of the Subscapular Artery. It passes through the Triangular Space (bordered by Teres Minor superiorly, Teres Major inferiorly, and Long Head of Triceps laterally) to reach the posterior aspect of the scapula. Once through, it curves around the lateral border of the scapula to enter the Infraspinous fossa, where it anastomoses with the Suprascapular Artery (from thyrocervical trunk) and Dorsal Scapular Artery. This scapular anastomosis is clinically significant for scapular flap harvest in reconstructive surgery. Importantly, there is NO major nerve in this space - only the Circumflex Scapular Artery - which differentiates it from the Quadrangular Space (Axillary Nerve) and Triangular Interval (Radial Nerve).
KEY POINTS TO SCORE
Circumflex Scapular Artery
Triangular Space (Medial)
Scapular Anastomosis
COMMON TRAPS
✗Confusing with Transverse Cervical
✗Thinking there is a nerve in this space
LIKELY FOLLOW-UPS
"What are the borders of this space?"
"Teres Minor, Teres Major, Long Head Triceps."
VIVA SCENARIOStandard
Scenario 3: Radial Nerve Course
EXAMINER
"Trace the course of the Radial Nerve relative to the Triceps heads."
EXCEPTIONAL ANSWER
Exits axilla posterior to the artery. Enters the Triangular Interval (between Long and Lateral heads/Humerus). Runs in the Spiral Groove. Pierces the Lateral Intermuscular Septum to enter the anterior compartment.
KEY POINTS TO SCORE
Triangular Interval vs Space
Spiral Groove
Lateral Septum
COMMON TRAPS
✗Suggesting it passes through Quadrangular space
LIKELY FOLLOW-UPS
"Where is the Spiral Groove?"
"Posterior face of the humerus, between medial and lateral heads of triceps."
MCQ Practice Points
Quadrangular Contents
Q: Which structure passes through the Quadrangular Space? A: Axillary Nerve. (And PCHA).
Triangular Interval
Q: The Triangular Interval transmits which nerve? A: Radial Nerve.
Teres Major Relation
Q: The Triangular Space is bounded inferiorly by which muscle? A: Teres Major.
Arterial Supply
Q: The Circumflex Scapular Artery is found in which space? A: Triangular Space.
Regimental Badge
Q: What sensory area is supplied by the axillary nerve after exiting the Quadrangular Space? A: The 'Regimental Badge' area - lateral upper arm over the deltoid insertion. This is via the Superior Lateral Cutaneous Nerve of Arm branch.
Australian Context
- Sports Medicine: High relevance in cricket (bowlers) and swimming populations in Australia.
- Imaging: Musculoskeletal ultrasound is widely used in Australia for dynamic assessment of these spaces.
High-Yield Exam Summary
Quadrangular Space
- •Superior: Teres Minor (Subscapularis anteriorly)
- •Inferior: Teres Major
- •Medial: Long Head Triceps
- •Lateral: Surgical Neck Humerus
- •Content: Axillary Nerve
- •Content: Posterior Circumflex Humeral Artery
Triangular Space
- •Superior: Teres Minor
- •Inferior: Teres Major
- •Lateral: Long Head Triceps
- •Content: Circumflex Scapular Artery
- •Note: No major nerve
- •Clinical: Scapular Anastomosis
Triangular Interval
- •Superior: Teres Major
- •Medial: Long Head Triceps
- •Lateral: Humerus Shaft
- •Content: Radial Nerve
- •Content: Profunda Brachii Artery
- •Clinical: Interval Syndrome (Radial palsy)