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Flexor Digitorum Superficialis Tendon

tendines musculi flexoris digitorum superficialis

The FDS tendons lie superficial to the FDP tendons in the carpal tunnel and in the finger flexor sheaths. At the proximal phalanx level each FDS splits into two slips (Camper chiasm) that spiral around the FDP tendon and reunite to insert on the sides of the middle phalanx. This split allows the deeper FDP to pass through the FDS to reach the distal phalanx. The FDS is tested by holding adjacent fingers in extension and asking the patient to flex the tested finger — isolating the FDS from the FDP.

Region: Hand & Wrist

Biomechanics

Function & Actions

PIP joint flexion of fingers 2 through 5; independent digital flexion at the PIP level

Clinical Relevance

Clinical Notes

The FDS test isolates the FDS by blocking FDP function through tethering adjacent fingers — if the patient can flex the PIP joint, the FDS is intact. In Zone II flexor tendon injuries both FDS and FDP must be repaired with meticulous technique to prevent adhesion formation in the constrained tendon sheath. Selective FDS tenolysis from a single finger for ring sublimis transfer restores thumb opposition in low median nerve palsy.

Pathology

Common Injuries & Conditions

Zone II Flexor Tendon Laceration

FDS and FDP division in the digital sheath requiring primary repair with controlled active mobilisation to prevent adhesions.