The FDS tendons split into two slips at each finger A1 pulley to pass around and insert on the middle phalanx base, allowing the FDP tendon to pass through the FDS split (Camper chiasm) to reach the distal phalanx. FDS specifically flexes the PIP joint and can be tested in isolation by holding the other fingers in extension (preventing FDP contribution). Zone II flexor tendon injuries involve both FDS and FDP in the confined digital sheath.
| Muscle | |
|---|---|
| Attachment | Four tendons through the carpal tunnel, splitting over the proximal phalanx to insert on the sides of the middle phalanges of fingers 2-5 |
PIP joint flexion of fingers 2-5
In the carpal tunnel the middle and ring finger FDS tendons lie superficial to those of the index and little fingers, an arrangement to respect in wrist-level repairs. Synovial thickening around these tendons in inflammatory arthritis can cause wrist swelling, finger stiffness and carpal tunnel symptoms. Zone II lacerations within the digital sheath often divide both FDS and FDP; primary repair with core and epitendinous sutures and early controlled motion is now standard. The FDS tendon can be transferred as the motor for opposition reconstruction (ring finger FDS to APB, Bunnell opposition transfer) providing a reliable tendon with excellent excursion for restoring thumb opposition in median nerve palsy.
Ring finger FDS transfer to restore thumb opposition in median nerve palsy, the Bunnell opposition transfer.
Division of FDS, often with FDP, inside the digital sheath, producing loss of PIP flexion, treated by primary repair and early controlled mobilisation.
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