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Palmar Aponeurosis and Tendon Sheaths

Q: What structures form the boundaries of the quadrangular space?

The boundaries of the quadrangular space (foramen quadrilaterum) are formed by two muscles, the long head of the triceps, and a bone. Superior border — subscapularis muscle (m. subscapularis) and teres minor muscle (m. teres minor).Inferior border — teres major muscle (m. teres major).Medial border — long head of the triceps brachii muscle (caput longum m. tricipitis brachii).Lateral border — surgical neck of the humerus (collum chirurgicum humeri).

Aponeurosis palmaris

For medical students2 min readUpdated 2026-10-10

The palmar aponeurosis and tendon sheaths form a complex protective and gliding apparatus of the hand. Knowledge of these structures is essential for understanding finger biomechanics and diagnosing pathologies such as tunnel syndromes and contractures.

Aponeurosis shapeTriangular fibrous plate.

Tunnel syndromeCompression of the median nerve beneath the transverse carpal ligament.

Radial bursaIsolated synovial sheath for the FPL tendon.

Lymph nodesLymph nodes are divided into three levels relative to the pectoralis minor muscle.

Palmar Aponeurosis

The palmar aponeurosis (aponeurosis palmaris) is a dense fibrous plate with a characteristic triangular shape. This structure is an essential element of the superficial fascia of the hand.

The tendon of the palmaris longus muscle (m. palmaris longus) blends into the proximal part of the aponeurosis.
Distally, the aponeurosis divides into prominent longitudinal bands that extend toward the bases of the fingers.

Clinically, this area is critically important: scarring and shortening of the longitudinal bands leads to Dupuytren's contracture, a condition resulting in permanent finger flexion and loss of hand function.

Carpal Tunnel and Flexor Retinaculum

On the palmar surface of the wrist lies the transverse carpal ligament, or flexor retinaculum (retinaculum flexorum). Spanning between bony prominences, it forms a narrow space known as the carpal tunnel.

Contents of the carpal tunnel:

Median nerve (n. medianus).
Tendons of the flexor digitorum superficialis and profundus (FDS and FDP).
Tendon of the flexor pollicis longus (FPL), which is enclosed in a separate synovial sheath called the "radial bursa".

Inflammation of the synovial sheaths or tissue edema in this zone compresses the n. medianus, clinically recognized as carpal tunnel syndrome.

Synovial Sheaths of the Hand

To ensure smooth tendon gliding, synovial sheaths are present. Two major bursae are identified on the palmar surface:

Common flexor sheath (ulnar bursa): encloses the FDS and FDP tendons, extending distally all the way to the distal phalanx of digit V.
Radial bursa: isolated sheath surrounding the FPL tendon.

On the dorsal side of the hand, tendons also feature synovial sheaths located within specialized canals beneath the extensor retinaculum.

Topography of the Axillary Cavity

Other important topographical and anatomical structures of the upper limb include the axillary cavity. Its contents include vital neurovascular and lymphatic structures:

Cords of the brachial plexus (plexus brachialis).
Axillary artery (a. axillaris) with its branches and axillary vein (v. axillaris) with its tributaries.
Axillary lymph nodes (axillary lymph nodes), surgically classified into levels I–III based on their relationship to the pectoralis minor muscle (m. pectoralis minor).

Potential vascular and nerve "gaps" form on the posterior wall of the axillary cavity:

Quadrangular space: transmits the axillary nerve (n. axillaris) and posterior circumflex humeral artery (a. circumflexa humeri posterior).
Triangular space: transmits the circumflex scapular artery (a. circumflexa scapulae).

Mnemonic

To remember the synovial bursae: "The pinky and thumb have long bursae (ulnar and radial bursa), while digits II, III, and IV have short isolated sheaths."

Frequently asked questions

Which bones form the floor and walls of the carpal tunnel?

The floor and walls of the carpal tunnel are formed by the carpal bones, which create the carpal sulcus, and the flexor retinaculum.

Floor (deep layer) — carpal bones and ligaments.
Medial wall — pisiform bone (os pisiforme) and hook of the hamate (hamulus ossis hamati).
Lateral wall — tubercle of the scaphoid (tuberculum ossis scaphoideum) and tubercle of the trapezium (tuberculum ossis trapezii).
Anterior wall — flexor retinaculum (retinaculum musculorum flexorum).

How are the synovial sheaths of digits II, III, and IV structured?

The synovial tendon sheaths of digits II, III, and IV are isolated and extend from the level of the metacarpal heads to the bases of the distal phalanges. The segments of the tendons within the connective tissue of the middle palmar space are not covered by a synovial membrane.

Structure of the synovial sheath (vagina synovialis):

Parietal layer — lines the inner surface of the fibrous sheath.
Visceral layer — covers the tendon itself (epitenon).
Mesotendineum — formed at the transition between the layers, containing nourishing vessels and nerves.

What structures form the boundaries of the quadrangular space?

The boundaries of the quadrangular space (foramen quadrilaterum) are formed by two muscles, the long head of the triceps, and a bone.

Superior border — subscapularis muscle (m. subscapularis) and teres minor muscle (m. teres minor).
Inferior border — teres major muscle (m. teres major).
Medial border — long head of the triceps brachii muscle (caput longum m. tricipitis brachii).
Lateral border — surgical neck of the humerus (collum chirurgicum humeri).

What causes carpal tunnel syndrome?

The syndrome is caused by compression of the median nerve within the narrow carpal tunnel beneath the flexor retinaculum, often secondary to synovial sheath inflammation.

Which tendon is associated with the common flexor synovial sheath?

It encloses the tendons of the flexor digitorum superficialis and profundus and extends to the little finger (digit V).

What passes through the quadrangular space?

The quadrangular space transmits the axillary nerve (n. axillaris) and the posterior circumflex humeral artery (a. circumflexa humeri posterior).

Go deeper

Anatomy of the extensor compartments on the dorsal wrist
Detailed branching of the axillary artery by pectoral triangles
Formation and branches of the brachial plexus cords
Classification of axillary lymph nodes (levels I, II, III) and lymphatic drainage pathways