Flexor Digitorum Profundus Muscle Anatomy

The Flexor Digitorum Profundus is one of three deep muscles in the forearm, alongside the flexor pollicis longus and the pronator quadratus.

Flexor Digitorum Profundus Muscle Anatomy

Flexor Digitorum Profundus Muscle Anatomy originates from the upper three-fourths of the anterior and medial surfaces of the ulna, the interosseous membrane, and the deep fascia of the forearm. The muscle extends into four tendons, each inserting into the palmar base of the distal phalanx of the second to fifth fingers.

The Flexor Digitorum Profundus lies deep to the flexor digitorum superficialis (FDS) but extends more distally. Its tendons pass through the tendons of the FDS, attaching to the distal phalanx, hence earning the FDP the nickname “perforating muscle.” These long tendons run through the carpal tunnel, facilitating the flexion of the distal interphalangeal joints of the fingers.

See Also: Forearm Muscles Anatomy & Function

The flexor digitorum profundus is unique as it is innervated by both the median and ulnar nerves:

• The medial portion of the muscle, responsible for flexing the fourth and fifth digits, is supplied by the ulnar nerve (C8, T1).
• The lateral portion, which flexes the second and third digits, is innervated by the anterior interosseous branch of the median nerve (C8, T1).

The Flexor Digitorum Profundus is the only muscle capable of flexing the distal interphalangeal joints of the fingers. It also contributes to the flexion of the proximal interphalangeal joints, the metacarpophalangeal joint, and the wrist joint. This allows for complex movements such as making a fist and curling the fingers. Although the flexor digitorum superficialis can flex the proximal interphalangeal joints, only the Flexor Digitorum Profundus can flex the distal joints independently, particularly the index finger.

Flexor Digitorum Profundus physiologic variants are prone to spontaneous flexor tendon ruptures. The 2 ulnar FDP tendons usually originate from the same muscle belly and separate in the forearm or carpal tunnel. However, the FDP tendons of digits 4 and 5 may fuse at the midpalmar level in some individuals. This anomaly weakens the FDP digit 5 tendon against shearing forces, predisposing it to rupture. A very rare variant is the FDP tendon without any muscle attachment to the distal phalangeal bases

To assess the integrity of the Flexor Digitorum Profundus, especially concerning nerve function, the proximal interphalangeal joint is held in an extended position while the patient attempts to flex the distal interphalangeal joint. Testing the index finger can evaluate the median nerve, while testing the little finger assesses the ulnar nerve.

Common Flexor Digitorum Profundus injuries include the following:

• Lumbrical plus finger: FDP tendon rupture or amputation distal to the lumbrical origins can cause it. This condition manifests as a paradoxical extension of the IP joints when attempting to flex the fingers.
• Quadrigia: This condition arises when the FDP tendons to the medial 3 digits fuse with the FDP tendon to the index finger. Common causes are FDP tendon adhesion or amputation, FDP tendon suturing to the extensor tendons, and excessive distal advancement of the FDP tendons during surgery. The patient cannot make a full fist and has a weak grasp since the tendons cannot move independently.
• Jersey finger: This is an FDP tendon rupture close to the base of a distal phalanx. This condition affects the 4th digit in 75% of cases. Jersey finger occurs when the FDP muscle belly is maximally contracted during forceful DIP joint extension. The Leddy and Packer classification is used to stratify jersey finger cases based on the level of tendon retraction and the presence of a fracture. Patients may complain of pain and tenderness over the volar distal finger at rest. The affected finger also lies in slight extension relative to the other fingers. The proximally retracted flexor tendon may be palpated along the flexor sheath. Jersey finger is a common cause of quadrigia.
• AIN injury: Damage to the AIN paralyzes the FPL and FDP. Consequently, patients cannot perform the “ok” sign.

References & More

1. Clinically Oriented Anatomy – 8th Edition
2. Clinical Anatomy by Regions, Richard S. Snell.
3. Lung BE, Burns B. Anatomy, Shoulder and Upper Limb, Hand Flexor Digitorum Profundus Muscle. [Updated 2023 Nov 13]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: Pubmed