The Triceps Gateways
THE THREE SPACES
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.
Clinical 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 (a rotator cuff muscle, external rotator).
- Teres Major is below (an adductor/internal rotator).
- The long head of triceps descends between them: it passes posterior to teres minor and anterior to teres major, dividing the medial triangular space from the lateral quadrangular space.
| 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 |
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 |
| Minor | Superior Teres Minor forms the upper border |
| Major | Inferior Teres Major forms the lower border |
| Triceps | Vertical Long Head splits them |
Hook:Major is heavy, so it sinks to the bottom.
AXEContents of Quadrangular Space
| A | Axillary Axillary Nerve |
| X | compleX Posterior CircumfleX Humeral Artery |
| E |
| A | Axillary Axillary Nerve |
| X | compleX Posterior CircumfleX Humeral Artery |
| E |
Hook:The Q-Space needs an AXE.
R-IRadial Nerve Space
| R | Radial Radial Nerve |
| I | Interval Triangular Interval |
| R | Radial Radial Nerve |
| I | Interval Triangular Interval |
Hook:Radials use Intervals, not Spaces.
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 |
| A | Articular Shoulder Joint | D | Deltoid Deltoid Motor supply |
| T | Teres Teres Minor Branch | C | Cutaneous Regimental Badge Area |
Hook:AC/DC rocks the shoulder.
Overview
The posterior scapulo-humeral region is organised around the intersection of teres minor (superiorly, a rotator cuff external rotator), teres major (inferiorly, an adductor/internal rotator), and the long head of triceps, which descends vertically between them. This arrangement creates three named passages that examiners repeatedly test because each carries a different neurovascular structure:
- Quadrangular (quadrilateral) space — lateral; transmits the axillary nerve and posterior circumflex humeral artery (PCHA).
- Triangular space — medial; transmits only the circumflex scapular artery (no major nerve).
- Triangular interval — inferior; transmits the radial nerve and profunda brachii artery into the spiral groove.
Mastery of these boundaries underpins safe posterior shoulder approaches, the deltoid-splitting safe zone, recognition of axillary nerve injury after dislocation or proximal humerus fracture, and the diagnosis of two uncommon but high-yield entrapment syndromes (quadrilateral space syndrome and triangular interval syndrome). The same anatomy explains the circumflex scapular pedicle used in scapular and parascapular free flaps.
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.
Differential Diagnosis of Posterior Shoulder Pain & Teres Minor Atrophy
| Condition | Distinguishing Feature | Atrophy Pattern | Key Test |
|---|---|---|---|
| Quadrilateral space syndrome | Overhead athlete, dynamic posterior pain | Isolated teres minor (axillary branch) | Dynamic angiography / abduction-ER provocation |
| Parsonage-Turner (neuralgic amyotrophy) | Acute severe pain then patchy palsy | Teres minor +/- deltoid +/- other muscles | EMG: multifocal denervation |
| Rotator cuff tear / traction | Older patient, cuff pathology on MRI | Teres minor with humeral decentering | MRI cuff integrity |
| Suprascapular neuropathy | Spinoglenoid/paralabral cyst | Infraspinatus +/- supraspinatus (NOT teres minor) | MRI cyst; EMG SSN |
| C5-C6 radiculopathy | Neck pain, dermatomal sensory loss | Multi-muscle myotomal | Spurling, cervical MRI |
| Triangular interval syndrome | Posterior arm/forearm pain, radial bias | Triceps weakness, no teres minor atrophy | Radial nerve tension test, no cervical provocation |
The Discriminator
Isolated teres minor atrophy points to the axillary nerve in the quadrilateral space. Isolated infraspinatus atrophy points to the suprascapular nerve at the spinoglenoid notch. Combined deltoid + teres minor denervation localises to the main axillary nerve trunk (e.g. dislocation, Parsonage-Turner).
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-related, and provoked by sustained overhead activity.
- Vascular form: digital ischaemia, cold hand, or splinter haemorrhages from PCHA thrombus embolisation.
- Sign: Point tenderness over the quadrilateral space (lateral to scapula); symptoms reproduced by 1 minute of abduction-external rotation.
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
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.
Controversies & Areas of Uncertainty
Do fibrous bands really cause QSS?
Cahill and Palmer's original cadaveric work did not consistently identify discrete bands, yet later dissection (McClelland and Paxinos) found bands in 14 of 16 shoulders, most often between teres major and the long head of triceps. Their pathological significance versus normal anatomical variation remains debated.
Neurogenic vs vascular QSS
Whether QSS is primarily an axillary nerve compression or a PCHA disease (aneurysm/thrombosis with distal embolisation) is unsettled. The two may be distinct entities sharing a name, which affects whether decompression or vascular intervention is appropriate.
Is isolated teres minor atrophy diagnostic?
MRI series show isolated teres minor atrophy is far more often associated with cuff tears, humeral decentering, or idiopathic traction than with true compressive QSS, which is rare. The finding is sensitive but poorly specific.
Open vs arthroscopic decompression
Evidence is limited to small case series; no comparative trials define the optimal surgical technique, indications, or timing. Most authorities reserve surgery for refractory cases or a demonstrable structural lesion (paralabral cyst, mass, aneurysm).
Evidence Base
Quadrilateral Space Syndrome (original description)
- Landmark paper that defined the syndrome: compression of the axillary nerve and PCHA in the quadrilateral space
- Symptoms aggravated by forward flexion, abduction and external rotation; constant point tenderness posteriorly
- Subclavian arteriography showed PCHA occlusion with the arm in abduction-external rotation
- Of 18 patients operated via a posterior approach: 8 complete relief, 8 improved, 2 no improvement
Cadaveric Anatomy of Fibrous Bands
- 16 cadaveric shoulders dissected to clarify the anatomy predisposing to QSS
- Fibrous bands present in 14 of 16 shoulders, most commonly between teres major and the long head of triceps
- Where bands were present, both internal and external rotation reduced the cross-sectional area of the quadrilateral space
Viva Scenarios
Use these scenarios to practise clinical reasoning and management decisions
Scenario 1: Posterior Shoulder Pain
"A 25-year-old volleyball player presents with vague posterior shoulder pain and weakness. MRI shows isolated fatty infiltration of Teres Minor. Diagnosis?"
Scenario 2: The Triangular Space
"During a scapular flap harvest, you identify a vessel in the Triangular Space. Which vessel is it and where does it go?"
Scenario 3: Radial Nerve Course
"Trace the course of the Radial Nerve relative to the Triceps heads."
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.
Guidelines, Registries & Global Practice
Global epidemiology
- Quadrilateral space syndrome is rare and there is no dedicated registry; most evidence is cadaveric or small case series. It predominates in young (20-40y) overhead athletes (volleyball, baseball/cricket bowling, swimming, tennis) and in patients with paralabral cysts.
- Symptoms of digital ischaemia from PCHA disease have been reported in up to roughly a third of elite volleyball players, underlining the vascular variant of the condition.
Society/guideline positions (no condition-specific guideline exists)
- AAOS / ASES (US) and BOA-BESS (UK): no formal guideline for QSS; consensus from upper-limb literature favours prolonged conservative management first, with surgery reserved for refractory cases or a demonstrable structural lesion.
- AO Foundation / EFORT: relevant guidance is on the axillary nerve safe zone in deltoid-splitting and posterior approaches (protect the nerve 5-7 cm distal to the acromion), rather than on QSS itself.
- IOC / sports-medicine consensus: emphasise vascular work-up (Doppler/duplex, angiography) in overhead athletes with hand ischaemia before attributing symptoms to nerve compression alone.
Imaging & practice variation
- High-resource settings: MRI (denervation signal, paralabral cysts) plus MRA/CTA or dynamic duplex ultrasound for the vascular component; arthroscopic or open decompression available.
- Limited-resource settings: diagnosis rests on clinical provocation testing and plain radiographs to exclude bony causes; ultrasound (where skilled operators exist) is a low-cost dynamic tool; management is predominantly conservative.
Clinical 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)