Flexor Digitorum Profundus
It is one of the deep muscles of the anterior compartment (deep volar compartment) of the forearm. It is the mass action muscle so act as the main gripping power of the hand because the tendons of the flexor digitorum profundus arise at or below the wrist joint (whereas tendons of flexor digitorum superficially arise in distal 3rd of the forearm).
It is the most powerful, and most bulky muscle of the forearm forming the muscular elevation seen and felt on the posterior surface of the forearm medial to the subcutaneous posterior border of the ulna.
In palm, lumbricals originate from the sides of tendons of flexor digitorum profundus.
It is a hybrid or composite muscle as it is innervated by two different nerves.
- Upper three-fourths of the anterior and medial surface of the shaft of the ulna.
- Medial surface of the olecranon and coronoid processes of the ulna.
- Adjacent half part of the anterior surface of the interosseous membrane.
- The muscle forms 4 tendons for the medial 4 digits which enter the palm by passing deep to the flexor retinaculum( appear in carpal tunnel) in the ulnar bursa and digital synovial sheaths.
- Opposite the proximal phalanx of the corresponding digit, the tendon perforates the tendon of the flexor digitorum superficialis. Each tendon is inserted on the palmar surface of the base of the distal phalanx of digits 2nd to 5th.
It is the hybrid muscle supplied by two different nerves:
- Medial half (associated with ring and little fingers) is innervated by ulnar nerve
- Lateral half (associated with middle and index fingers) by anterior interosseous nerve ( C8, T1)- motor branch of median nerve.
Blood Supply and Lymphatic Drainage
The flexor digitorum profundus is supplied by the anterior interosseous artery, which is a branch of the common interosseous artery and is accompanied by the palmar interosseous branch of the median nerve.
The flexor digitorum profundus lymphatic drainage is part of the upper limb lymph system, consisting of superficial and deep lymphatic vessels. The superficial vessels around the basilic vein go to the cubital lymph nodes, which are proximal to the medial epicondyle of the humerus. The deep lymphatic vessels also drain lymph follow the major deep veins terminating in the humeral axillary lymph nodes.
- It is the chief gripping muscle. lt acts best when the wrist is extended.
- It is the sole flexors of Distal Interphalangeal (DIP) Joint of digit 2nd to 5th.
- It also acts as a flexor of the midcarpal (wrist), metacarpophalangeal, and proximal interphalangeal( PIP) joints of the index, middle, ring, and little fingers along with Flexor digitorum superficialis.
- Since the lumbricals arise from the flexor digitorum profundus tendons and insert into the dorsal extensor expansions of the proximal phalanges,flexor digitorum profundus muscle aids the lumbricals in fully extending the fingers at the interphalangeal joints and flexing the fingers at the metacarpophalangeal joints. 
- Lumbrical plus finger: When injury to the flexor digitorum profundus tendon distal to the lumbrical origin occurs by rupture or amputation, it manifests as a paradoxical extension of the interphalangeal joints when attempting to flex the finger.
- Quadrigia: Since the flexor digitorum profundus tendons to the 5th,4th and 3rd fingers have a common muscle belly, the tendons may not be able to move independently if they are interconnected with the separate tendon that runs to the index finger. this may lead to patient not being able to make a full fist and an associated weak grasp. This condition is known as Quadrigia. This may occur following adhesion or scarring of FDP after rupture and repair.
- Jersey finger: The condition in which rupture of the flexor digitorum profundus tendon from the distal attachment insertion at the base of the distal phalanx leading no active flexion of the distal interphalangeal joint is known as Jersey finger. Jersey finger is a zone 1 flexor tendon injury that involves the ring finger in 75% of cases which is because of the fact that FDP muscle belly is in maximal contraction during forceful distal interphalangeal joint extension.
- Anterior interosseous nerve (AIN) injury: AIN supplies three muscles in the forearm: flexor pollicis longus (FPL), pronator quadratus (PQ), and the lateral half of flexor digitorum profundus (FDP). The patient will be unable to perform an “OK” sign with the thumb or a positive Pinch Grip test (Froment’s sign) due to paralysis of the flexor pollicis longus and the flexor digitorum profundus muscles to flex the DIP joint.
- Compartment syndrome of forearm: Compartment syndrome following supracondylar, ulnar, or radius fractures is released via the procedure named Fasciotomy where both the superficial and deep volar compartments must be released.
- Ulnar claw hand deformity: Injury to the ulnar nerve at the distal region at wrist level with the signs of hyperextension of MCP joints and flexion of IP joints of the ring and little finger.
OK sign: Injury to the Anterior interosseous nerve- makes a person difficult to make OK as it needs flexion of index finger DIP joint ( flexor digitorum profundus) and flexion of interphalangeal joint of Thumb ( flexor pollicis longus).
Sign of Benediction is the inability of 4th and 3rd finger to flex at ( MCP and IP joints) due to the involvement of AIN.
Froment's sign: Test for the ulnar nerve palsy which may be entrapped at the cubital tunnel. Since FDP's medial half is supplied by ulnar nerve, pinch test ( Froment's sign ) test positive.
Mass hand gripping strength: Since FDP is the major muscle for hand gripping power, dynamometer (handgrip ergometer) can be used for measuring the hand gripping strength.For more detail, go for this link Wrist and Hand Examination
Flexor digitorum profundus is the main gripping muscle. So any injury ( ulnar nerve injury, AIN injury, tendon injury) causing weakness or decrease in prehensive and precisive function of the hand should focus on strengthening of FDP. Based upon the cause of injury, assessment findings, and patient-centered goals, an exercise regime should be prescribed. For more details about the type of exercise and graded exercise specific to a flexor tendon injury, go for the following link mentioned.
- Lung BE, Burns B. Anatomy, Shoulder and Upper Limb, Hand Flexor Digitorum Profundus Muscle. InStatPearls [Internet] 2019 Feb 10. StatPearls Publishing.
- Chaurasia BD. Human_Anatomy, Volume 1 - Upper Limb Thorax. 6th Edition. CBS Publishers and distributors Pvt Ltd
- Anatomy Of The Flexor Digitorum Profundus Muscle - Everything You Need To Know - Dr. Nabil Ebraheim. Available from:https://www.youtube.com/watch?v=FAlrNNCmaC0 [last accessed 28/05/2020]
- Claw Hand, Ulnar Claw Hand - Everything You Need To Know - Dr. Nabil Ebraheim. Available from: https://www.youtube.com/watch?v=GyqaKGg3HmM. [Last Accessed:30-2-2020]
- Kaufmann RA, Kozin SH, Mirarchi A, Holland B, Porter S. Biomechanical analysis of flexor digitorum profundus and superficialis in grip-strength generation. Am J Orthop. 2007 Sep;36(9).
- Sarah Tucker Oxford. Hand Examination; Finger Flexors FDS and FDP. Available from: https://www.youtube.com/watch?v=hT0nR5d4Rnk [last accessed 28/05/2020]
- Peters SE, Jha B, Ross M. Rehabilitation following surgery for flexor tendon injuries of the hand. Cochrane Database of Systematic Reviews. 2017(1).