Three apertures in the scapular region, formed between muscles arising from the lateral border and posterior surface of the scapula. Divided by the long head of triceps brachii (common boundary of all three).

Overview

1. Quadrangular Space

Boundaries:

Contents: Axillary nerve (C5, C6) + Posterior circumflex humeral artery and vein

Most clinically important space — axillary nerve passes through it.

2. Upper Triangular Space

Boundaries: Teres minor (superior), Long head of triceps (lateral), Teres major (inferior)

Contents: Circumflex scapular artery (branch of subscapular artery) → winds around lateral border of scapula → infraspinous fossa → anastomoses with suprascapular artery

3. Lower Triangular Space

Boundaries: Teres major (superior), Long head of triceps (medial), Shaft of humerus (lateral)

Contents: Radial nerve + Profunda brachii artery and vein → enter spiral groove of humerus

Comparison Table

Long head of triceps = medial wall of quadrangular and lower triangular spaces; lateral wall of upper triangular space.

Applied Anatomy

Axillary Nerve Injury — Quadrangular Space

Causes: Fracture of surgical neck of humerus; inferior dislocation of shoulder; direct trauma; fibrous bands (quadrangular space syndrome)

Features:

• Paralysis of deltoid → loss of abduction (15°–90°)
• Paralysis of teres minor → loss of lateral rotation
• Flattening of shoulder — acromion prominent
• Sensory loss over lower half of deltoid (regimental badge area) — upper lateral cutaneous nerve of arm

Quadrangular Space Syndrome

Compression of axillary nerve + PCHA by fibrous bands/hypertrophied muscles → point tenderness over space; pain and paraesthesia on abduction and external rotation

Radial Nerve — Lower Triangular Space

Exits lower triangular space → enters spiral groove → midshaft fracture of humerus → radial nerve injury → wrist drop

The intermuscular spaces (subscapular spaces) are three apertures in the scapular region formed between the muscles that arise from the lateral border and posterior surface of the scapula. They are visible from behind after reflecting the posterior part of the deltoid. Knowledge of these spaces is essential during surgery in the shoulder region.

The key structure dividing these spaces is the long head of triceps brachii, which passes vertically through the region from its origin at the infraglenoid tubercle.

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Overview

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1. Quadrangular Space

Boundaries

Contents

• Axillary nerve (C5, C6) — passes posteriorly through this space before winding around the surgical neck of the humerus
• Posterior circumflex humeral artery and vein — accompany the axillary nerve

The quadrangular space is the most clinically important of the three spaces because of the axillary nerve passing through it.

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2. Upper Triangular Space

Boundaries

Contents

• Circumflex scapular artery — a branch of the subscapular artery; enters this space, interrupts the origin of teres minor, winds around the lateral border of the scapula, and enters the infraspinous fossa to anastomose with the suprascapular artery

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3. Lower Triangular Space

Boundaries

Contents

• Radial nerve — passes through this space with the profunda brachii vessels before entering the spiral groove of the humerus
• Profunda brachii artery and vein — accompany the radial nerve

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Comparison Table

Memory: Long head of triceps is a common boundary of all three spaces — it forms the medial wall of the quadrangular space and the lower triangular space, and the lateral wall of the upper triangular space.

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Applied Anatomy

Axillary Nerve Injury in the Quadrangular Space

The axillary nerve (C5, C6) is closely related to the medial aspect of the surgical neck of the humerus immediately inferior to the capsule of the shoulder joint as it passes through the quadrangular space.

Causes of injury:

• Fracture of the surgical neck of the humerus
• Inferior dislocation of the shoulder joint
• Direct trauma to the quadrangular space
• Fibrous bands compressing the nerve within the space (quadrangular space syndrome)

Features of axillary nerve injury:

• Paralysis of deltoid → loss of abduction of the arm (15°–90°)
• Paralysis of teres minor → loss of lateral rotation (contribution)
• Flattening of shoulder — loss of the rounded deltoid contour; acromion becomes prominent
• Sensory loss over the lower half of deltoid (regimental badge area) — due to injury to the upper lateral cutaneous nerve of the arm (terminal branch of axillary nerve)

Quadrangular Space Syndrome

Compression of the axillary nerve and posterior circumflex humeral artery within the quadrangular space by fibrous bands or hypertrophied muscles. Features include point tenderness over the quadrangular space, pain and paraesthesia in the shoulder on abduction and external rotation.

Radial Nerve in the Lower Triangular Space

The radial nerve exits through the lower triangular space before entering the spiral groove of the humerus. A midshaft fracture of the humerus may injure the radial nerve here → wrist drop (Saturday night palsy mechanism at the spiral groove).

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