Finger Flexion

There are two long flexors to each finger:

• Flexor digitorum profundus (FDP) inserts into the base of the distal phalanx and flexes the distal interphalangeal joint (DIPJ)
• Flexor digitorum superficialis (FDS) inserts into the sides of the middle phalanx and flexes the proximal interphalangeal joint (PIPJ)

Nerves

The ulnar nerve supplies the FDP tendons to the little and ring fingers; the other FDP tendons are supplied by the median nerve. The ulnar nerve supplies sensation over the ulnar side of the hand and little finger. It also supplies the dorsal ulnar region of the hand via the dorsal cutaneous branch of the ulnar nerve.

The median nerve supplies sensation over the palmar index, middle fingers, thumb, and proximal palm near the thenar eminence. Motor function can be tested by abductor pollicis brevis action test thumb abduction with palm up, raising the thumb to perpendicular.

Weakness or absence of flexion of the IPJ of the thumb (FPL) and the DIPJ of the index finger (FDP) against resistance, if present, are due to a more proximal lesion (anterior interosseous nerve).

Tendons (FDS and FDP)

The FDS and the FDP tendons travel distally from the forearm through the carpal tunnel, after which they traverse a fibro-osseous tunnel in each digit to insert in the respective phalanges.

The profundus tendon pierces that of the superficialis over the proximal phalanx.

The metacarpal heads, phalanges and intervening joints form the dorsal wall, with the anterolateral wall comprising of the annular pulley system and fibrous flexor sheath (condensed deep fascia attached to the margins of the phalanx).

The fibrous sheaths are lined by the synovial membrane, which reflects around each tendon.

Tendons (synovial sheaths)

The tendons of the index, middle and ring fingers have synovial sheaths that extend from the level of the FDP insertion, closing off proximally at the mid palm level at the metacarpal head.

However, the synovial sheaths of the thumb and little finger extend proximally into the palm as the radial and ulnar bursae respectively. These extend below the flexor retinaculum and communicate in about 50% of patients [5]. While more proximally, the radial and ulnar bursae are contiguous with a potential deep space (space of Perona) lying between the pronator quadratus and the FDP tendons, where an abscess may occur in hand infections.

Pulleys

The annular and cruciform pulleys form an intricate constraining sheath to keep the tendons close to the bone, preventing bowstringing when flexing the metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints.

Three cruciform pulleys (C1-C3) and five annular pulleys (A1-A5) exist. From a biomechanical advantage point the A2 and A4 pulleys are considered the most important to prevent bowstringing.