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Flexor Digitorum Superficialis (FDS)

Other Name: Flexor digitorum sublimis (FDS)
 
Key Points:
• Flexes the proximal interphalangeal joint.
• Extrinsic flexor muscle.
• Superficial flexor muscle.

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EXAMINATION
Muscle Function:
Primary: Flexes the proximal interphalangeal joints of the index, long, ring, and small fingers.
Secondary: Assists in metacarpal-phalangeal joint and 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.) 
Palpation: Palpate the flexor digitorum superficialis tendon at the volar wrist ulnar to the palmaris longus or flexor carpi radialis tendons. The long and ring finger tendons lie superficial to the index and small finger tendons. 
 
Strength Testing: Position – wrist and hand in neutral with palm up; keep the distal interphalangeal joints of the three fingers you are not testing in full extension. Stabilize – metacarpal-phalangeal joint in extension.  Resist – flexion at proximal interphalangeal joint by applying force at palmar middle phalanx (in the direction of extension). Test each finger individually (holding the other fingers not being testing in full extension as noted).
 
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 shortening, but is not necessarily active finger flexion. 
• Flexor digitorum profundus – avoid by maintaining distal interphalangeal joints in extended position.
Note: Many patients do not have a flexor digitorum superficialis of the small finger.
 
Relevant Anatomy:
Innervation:
• Roots: C7, C8, T1.
• Nerve: Median nerve.
• Innervation Route: C7, C8, T1 → median nerve → flexor digitorum superficialis branches.
 
• Often, two branches from the median nerve innervate the FDS (proximal and distal). 
Origin of Humeral Head: Common flexor tendon from the medial epicondyle of the humerus, ulnar collateral ligament of the elbow joint, and deep antebrachial fascia.
 
Origin of Ulnar Head: Medial side of the coronoid process of the ulna.
 
Origin of Radial Head: Anterior surface of radius by providing a smaller more tendinous contribution.
 
Insertion: By four separate tendons to each corresponding finger into the ulnar and radial aspect of the palmar middle phalanx.