Quadrangular & Triangular Spaces

The posterior scapulo-humeral region is organised around three muscles. Teres minor lies above, a rotator cuff external rotator; teres major lies below, an adductor and internal rotator. The long head of triceps descends vertically between them, passing posterior to teres minor and anterior to teres major, and divides the medial triangular space from the lateral quadrangular space.

The result is three named passages, each carrying a different neurovascular structure:

• Quadrangular (quadrilateral) space, lateral: the axillary nerve and the posterior circumflex humeral artery (PCHA)
• Triangular space, medial: only the circumflex scapular artery, with no major nerve
• Triangular interval, inferior: the radial nerve and profunda brachii artery, on their way into the spiral groove

Why it matters. These boundaries underpin safe posterior shoulder approaches, the deltoid-splitting safe zone, and the recognition of axillary nerve injury after dislocation or proximal humerus fracture. They also explain two uncommon but high-yield entrapments, quadrilateral space syndrome and triangular interval syndrome, and the circumflex scapular pedicle used in scapular and parascapular free flaps.

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The boundaries. The triangular space has no medial border: it is the space between the muscles as they merge medially. The triangular interval, also called the lower triangular space, lies below teres major and lateral to the long head.

The quadrangular space transmits structures from the axilla to the posterior shoulder. The axillary nerve passes posterior to the neck of the humerus with the PCHA, which anastomoses with the anterior circumflex humeral artery. Both wind around the surgical neck, where the nerve is at risk in dislocation and proximal humerus fracture.

The triangular space carries the circumflex scapular artery, a branch of the subscapular artery, which curves around the lateral border of the scapula. Collateral circulation around the scapula is robust.

The triangular interval is the entry point into the spiral groove. The radial nerve is visible here, with the profunda brachii artery beside it, before it passes deep to the lateral head of triceps.

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The axillary nerve's branches decide both the deltoid-split safe zone and the pattern of quadrilateral space syndrome:

• Articular branch to the shoulder joint
• Anterior (deep) branch, which winds transversely around the surgical neck deep to deltoid with the PCHA and supplies the anterior and middle deltoid, plus a few sensory twigs. It is the branch endangered by the lateral deltoid-splitting approach and by surgical-neck fractures and implants
• Posterior branch, which supplies the posterior deltoid and teres minor and continues as the superior lateral cutaneous nerve of the arm, the regimental-badge sensory area

A selective posterior-branch lesion gives isolated teres minor, with or without posterior deltoid, denervation: exactly the quadrilateral space syndrome pattern. Deltoid may be spared if the anterior branch is unaffected or if the compression is distal to where the main trunk branches, which is variable.

Quadrilateral space syndrome (QSS) is compression of the axillary nerve and the PCHA in the quadrangular space. The causes are fibrous bands, hypertrophy of the muscles in pitchers and swimmers, and paralabral cysts. The PCHA is the vessel compressed, and in overhead athletes it is prone to aneurysm.

Classification. Compression is dynamic, static or traumatic, and most cases of QSS are dynamic, or functional:

• Dynamic: functional compression, in athletes
• Static: a structural mass, such as a cyst, tumour or osteophyte
• Traumatic: haematoma or scarring after trauma

Triangular interval syndrome is entrapment of the radial nerve at the lower border of teres major.

QSS symptoms. Pain is vague and posterior, with paraesthesia in the regimental-badge area, although this is often absent in chronic cases. Weakness is subtle, fatigue-related and provoked by sustained overhead activity.

The vascular form. Thrombus in the PCHA can embolise distally, giving digital ischaemia, a cold hand or splinter haemorrhages.

QSS signs. Point tenderness over the quadrilateral space, lateral to the scapula, and symptoms reproduced by 1 minute of abduction-external rotation. Atrophy is often the only objective sign.

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Triangular interval syndrome. Pain radiates down the posterior arm and forearm, with triceps weakness in which the medial head is often relatively spared, as its branch arises more proximally. The wrist and finger extensors weaken if the compression is severe or proximal.

The C7 mimic. Differentiation from C7 radiculopathy is essential. Triangular interval syndrome reproduces symptoms with radial nerve tension testing and local scapular palpation, without cervical provocation.

The mechanism. The quadrangular-space anatomy is most often tested through trauma. The axillary nerve is the most commonly injured nerve in anterior shoulder dislocation, is frequently injured in proximal humerus fractures, is vulnerable in inferior dislocation, and is the nerve most at risk in shoulder surgery. The injury is a traction or stretch as the humeral head displaces, usually a neurapraxia or axonotmesis in continuity rather than a transection.

Examine before and after reduction. Document deltoid motor function by palpating the contracting muscle belly, because formal abduction testing is painful and unreliable acutely. Test sensation over the regimental-badge area, the territory of the superior lateral cutaneous nerve. An axillary-nerve palsy is not a contraindication to reduction.

Recovery. The majority recover spontaneously over about 3 to 6 months. Where axonal continuity is preserved the nerve regenerates at roughly 1 mm per day.

When to act. A persistent complete deficit at around 3 to 6 months warrants EMG/NCS and consideration of surgical exploration with nerve grafting, or a nerve transfer, for example a triceps branch of the radial nerve to the axillary nerve. A complete brachial-plexus pattern, an axillary-artery injury, or an irreducible or locked dislocation raises the urgency.

MRI. T1 shows fatty infiltration and T2 or STIR shows oedema, the denervation change of the acute phase. Look for paralabral cysts at the inferior labrum and for fibrous bands.

Teres minor atrophy. Isolated fatty atrophy of teres minor is the imaging hallmark of QSS, and it points to QSS or to isolated axillary nerve pathology such as Parsonage-Turner. It is sensitive but poorly specific, and the controversy below sets out why a single finding does not settle the diagnosis.

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Vascular imaging. MR or CT angiography can show occlusion of the PCHA with abduction, the dynamic compression. Dynamic imaging is key for vascular compression.

Electrodiagnostics. EMG shows positive sharp waves and fibrillations in deltoid and teres minor. Its sensitivity is variable, and nerve conduction studies are often inconclusive.

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Algorithm

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Conservative first. Conservative care is first line for 3-6 months: stretching of the posterior capsule, massage and activity modification. Success is high for functional QSS, and avoidance of the provoking activity, such as overhead throwing, is curative.

Surgical decompression. Indicated for failure of conservative treatment or a definite structural mass such as a cyst, through a posterior approach:

1. Incise along the posterior axillary fold
2. Identify deltoid, retracted superiorly, and teres major, inferiorly
3. Identify the long head of triceps medially
4. Release the fibrous bands over the nerve

The PCHA can bleed heavily.

Posterior approach. The interval is between infraspinatus and teres minor. The axillary nerve exits the quadrangular space below teres minor, so stay superior to protect it as it exits.

Lateral deltoid split. The quadrangular space lies deep to deltoid, and the nerve winds around the humerus here. Its anterior branch runs roughly transversely 5 to 7 cm distal to the lateral or anterolateral acromion, but as close as about 3.5 cm in a small patient. Do not carry a deltoid split more than about 5 cm below the acromion, and place a transverse stay suture at the distal apex to stop the split propagating.

Deltopectoral approach. The nerve is also at risk inferiorly as it crosses subscapularis toward the inferior glenoid. Palpate it (the "tug test") before retracting.

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• Nerve injury: iatrogenic injury to the axillary nerve during release, or to the radial nerve during plating
• Recurrence: inadequate release of the fibrous bands
• Bleeding: from the PCHA or circumflex scapular artery, during retraction
• Cosmetic: hypertrophy of the posterior scar

Rehabilitation. Pendular exercises start immediately and rotator cuff strengthening from 6 weeks. Overhead athletes return to sport at 3-4 months.

Prognosis. Decompression gives good relief of pain, but atrophy may not reverse.

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.

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 or vascular QSS? Whether QSS is primarily an axillary nerve compression or a PCHA disease, with aneurysm or thrombosis and 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 that 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 or arthroscopic decompression? Evidence is limited to small case series, and no comparative trials define the optimal surgical technique, indications or timing. Most authorities reserve surgery for refractory cases or a demonstrable structural lesion: a paralabral cyst, mass or aneurysm.