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Flexor Digitorum Profundus (FDP)

I – First FDP to Index Finger (2nd Digit)
II – Second FDP to Long Finger (3rd Digit)
III – Third FDP to Ring Finger (4th Digit)
IV – Fourth FDP to Small Finger (5th Digit)
Key Points:
• Flexes the distal interphalangeal joint.
• Index finger may have individual muscle belly while the long, ring, and small share a common muscle belly.
• Extrinsic and deep flexor muscle.
Muscle Function:
Primary: Flexes the distal interphalangeal joint of the index, long, ring and small fingers.
• Assists in flexion of the proximal interphalangeal joint (with flexor digitorum superficialis).
• Assists in flexion of the metacarpal-phalangeal joint (with interosseous and lumbrical).
• May weakly assist in flexion of the wrist.  (We have found that some patients with intact extrinsic finger flexors but loss of the flexor carpi ulnaris and flexor carpi radialis have no functional wrist flexion.) 
• Note that flexor digitorum profundus to the index can provide independent motion if there is an independent muscle belly. However, flexor digitorum profundus to the long, ring, and small fingers always have a common muscle belly.
Palpation: Palpate the tendon of the flexor digitorum profundus over the middle phalanx of the finger during resisted distal interphalangeal joint flexion.
Strength Testing: Position – forearm and hand on table with palm up. Stabilize – metacarpal-phalangeal and proximal interphalangeal joints in extension. Resist – distal interphalangeal joint flexion (in the direction of extension). Test each finger individually.  Or stabilize as above and put patient’s distal interphalangeal joint into flexion, then tell the patient to keep that joint in that position and the examiner tries to extend the distal interphalangeal joint.
Possible Substitutions:
• Tenodesis effect occurs when active wrist extension results in finger flexion leading to illusion of active finger flexion. 
• Finger flexion that results after relaxation from finger hyperextension. Relaxation of the extensors leads to relative flexor muscle shortening, but is not necessarily active finger flexion. 
Relevant Anatomy:
• Roots: C8, T1.
• Nerve:
• Lateral Half (FDP I, II) – Median nerve, anterior interosseous nerve.
• Medial Half (FDP III, IV) – Ulnar nerve.
• Innervation Route: C8, T1 →
• Median nerve → anterior interosseous nerve → flexor digitorum profundus I, II branches.
• Note, in some patients there is a small separate branch directly off the median nerve that innervates the flexor digitorum superficialis III.  The branch comes off of the median nerve on the ulnar side of the nerve just distal to the takeoff of the flexor carpi radialis/palmaris longus branch.
• Ulnar nerve → flexor digitorum profundus III, IV branches.
Origins: Anterior ulnar surface of the proximal ulna, interosseous membrane, and deep antebrachial fascia.
Insertions: By four separate tendons to each index, long, ring, and small finger into the palmar base of the distal phalanx.