The flexor digitorum superficialis is the superficial digital flexor of the forearm, inserting on the middle phalanges and acting primarily at the proximal interphalangeal joints of the four fingers. At the level of the digit it splits into two slips around the flexor digitorum profundus tendon at Camper chiasm before reinserting on the middle phalanx, creating a pulley mechanism that helps guide the profundus tendon distally.
| Origin | Medial epicondyle via the common flexor origin and coronoid process of the ulna; Anterior oblique line of the radius |
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| Insertion | Sides of the middle phalanges of fingers 2 through 5, after splitting around the flexor digitorum profundus tendon at the chiasm of Camper |
| Nerve Supply | Median nerve (C7, C8, T1) |
| Blood Supply | Ulnar artery |
| Actions | Flexion of the proximal interphalangeal joints of fingers 2 to 5; Assists flexion of the metacarpophalangeal joints; Assists wrist flexion |
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Testing the FDS in isolation requires blocking the FDP contribution by holding all other fingers in extension while the test finger flexes at the PIP joint only, confirming that FDS is functioning independently.
The FDS is the tendon most commonly involved in flexor tendon injuries in zone II of the finger, the critical no-man's land between the A1 pulley at the palm and the insertion of the FDS at the middle phalanx. Injuries in this zone are technically demanding to repair and rehabilitate because of the complex anatomy of the digital sheath. FDS tendon slips are also used in surgical procedures for ulnar claw correction and dynamic tenodesis.
The FDS is palpable as the superficial of the two flexor tendons in the palm and finger, and individual FDS tendons can be isolated by the FDS test of holding adjacent fingers in extension while asking the patient to flex the test finger at the PIP joint.
Tendon division in the digital flexor sheath from a penetrating injury, producing loss of PIP joint flexion that requires surgical repair and structured hand therapy rehabilitation.
Avulsion of the FDP with preserved FDS producing inability to flex the DIP joint, most common in ring finger during a tackle when the finger catches on a jersey.
