Flexor digitorum superficialis
Musculus flexor digitorum superficialis
Also known as: FDS, flexor digitorum sublimis
The large intermediate-layer flexor. Its four tendons split to insert on the middle phalanges of digits 2–5, and it is the main flexor of the PIP joints.
Overview
FDS has a humero-ulnar head (medial epicondyle and coronoid process) and a radial head (anterior oblique line of the radius). The two are joined by a fibrous arch under which the median nerve and ulnar artery pass. In the distal forearm its four tendons are arranged in pairs: the tendons to the middle and ring fingers lie superficial to those to the index and little fingers. They pass through the carpal tunnel with FDP and FPL and enter the fibrous flexor sheaths. Over the proximal phalanx each tendon splits (Camper chiasm) to let the FDP tendon pass through, then reunites to insert on both sides of the middle phalanx.
FDS flexes the proximal interphalangeal (PIP) joints, then the MCP joints and the wrist. Each finger's slip can act independently, unlike FDP, whose tendons share a common belly except for the index finger.
Anatomy
- Origin
Humero-ulnar head: medial epicondyle and coronoid process. Radial head: anterior oblique line of the radius.
- Insertion
Sides of the middle phalanges of digits 2–5.
- Action
Flexion of the PIP joints, then the MCP joints and the wrist.
- Innervation
Median nerve.
- Nerve roots
C7–T1 (C8)
- Arterial supply
Ulnar and radial arteries.
- Relations
The median nerve runs on its deep surface (adherent to it) in the forearm. Its tendons pass through the carpal tunnel.
Function
- Flexes the PIP joints of digits 2–5
- Flexes the MCP joints and the wrist
Clinical significance
Testing FDS
Hold the other fingers in full extension (which blocks FDP, since it shares one belly) and ask the patient to flex the free finger. Flexion at the PIP joint with a floppy DIP joint confirms that FDS is intact.
Flexor tendon injuries
Lacerations in zone II ("no man's land", from the distal palmar crease to the middle of the middle phalanx), where FDS and FDP share a tight sheath, have the poorest outcomes because of adhesions.
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.