Flexor digitorum profundus

The flexor digitorum profundus (FDP) is a deep, extrinsic muscle of the hand, forming part of the deep layer of ventral (flexor) forearm muscles along with the flexor pollicis longus and pronator quadratus.

Origin

The flexor digitorum profundus originates from the upper three-quarters of the anterior and medial surfaces of the ulna, as well as the adjoining part of the interosseous membrane and deep fascia of the forearm.

Course and Insertion

The muscle fans out into four distinct tendons, one for each of the second through fifth fingers. These long tendons extend down the forearm, pass through the carpal tunnel, and then attach to the palmar base of the distal phalanges of the fingers. The FDP lies deep to the flexor digitorum superficialis (FDS). However, because the FDP inserts more distally than the FDS, its tendons perforate the FDS tendons at the level of the proximal phalanx, making their way to the distal phalanges.

Nerve Supply (Composite Innervation)

The flexor digitorum profundus is a composite muscle, receiving innervation from two different nerves:

• The medial aspect of the muscle, which supplies the 4th (ring) and 5th (little) digits, is innervated by the ulnar nerve (C8, T1).

• The lateral aspect of the muscle, responsible for flexing the 2nd (index) and 3rd (middle) digits, is innervated by the median nerve, specifically its anterior interosseous branch (C8, T1).

Function

The flexor digitorum profundus is a powerful flexor, acting on the distal interphalangeal (DIP) joints, as well as assisting in flexion at the proximal interphalangeal (PIP), metacarpophalangeal (MCP), and wrist (midcarpal) joints.

• Relationship with Lumbricals

  The lumbrical muscles of the hand uniquely arise from the radial side of the FDP tendons. This connection is crucial for coordinated finger movements. As the lumbricals originate on the palmar side but insert onto the dorsal extensor expansion, power is transferred from the FDP via the lumbricals to simultaneously flex the metacarpophalangeal joints and extend the interphalangeal joints of the fingers.

It's important to note that the tension generated for distal interphalangeal joint flexion can diminish when the wrist is fully flexed, due to the shortening of the FDP muscle, which reduces the force transmitted from it.