Flexor digitorum superficialis muscleFlexor digitorum superficialis is the largest muscle of the anterior compartment of the forearm. It belongs to the superficial flexors of the forearm, together with pronator teres, flexor carpi
radialis, flexor carpi ulnaris and palmaris longus. Some sources alternatively classify this muscle as an independent middle/intermediate layer of the anterior forearm, found between the superficial and deep groups.
Based on its origin sites, flexor digitorum superficialis is divided into two heads; a humeroulnar head and radial head. Its large muscular belly courses distally towards the wrist, where it splits into four tendons and attaches to the middle phalanges of the second through fifth digits of the hand. Due to their superficial location, these tendons can be easily palpated on the distal part of the forearm.
|Origin||Humeroulnar head: Medial epicondyle of humerus, coronoid process of ulna
Radial head: Proximal half of anterior border of radius
|Insertion||Sides of middle phalanges of digits 2-5|
|Action||Metacarpophalangeal and proximal interphalangeal joints 2-5: Finger flexion|
|Innervation||Median nerve (C8, T1)|
|Blood supply||Ulnar artery, radial artery, median artery|
This article will discuss the anatomy and function of the flexor digitorum superficialis.
Origin and insertion
Flexor digitorum superficialis arises at two origin points/heads:
- Humeroulnar head: as the name suggests, this head originates from both the medial epicondyle of humerus via the common flexor tendon, as well as the medial margin of the coronoid process of ulna. Some fibers also originate from the anterior band of ulnar collateral ligament of the elbow joint.
- Radial head: arises from the upper two-thirds of the anterior border of the radius. It has a long linear origin extending from the radial tuberosity to the insertion of pronator teres in the distal forearm.
From these origin points, the muscular heads of the flexor digitorum superficialis courses distally across the anterior forearm. Usually, the muscle is arranged into superficial and deep layers, or strata. About halfway along the forearm, each layer gives off two tendons. The tendons of the superficial layer are directed to digits 3 and 4, whereas the tendons of the deep layer insert into digits 2 and 5. Additionally, the deep layer gives off a small slip that joins the tendon of the superficial layer directed to the digit 2. At the wrist joint, the tendons pass deep to the flexor retinaculum through the carpal tunnel, after which they diverge into two pairs.
As the tendons of flexor digitorum superficialis enter the hand, they course superficial to the corresponding flexor digitorum profundus tendon. Upon passing the metacarpophalangeal joint, the two tendons enter the digital flexor sheath. Before reaching the proximal interphalangeal joint, each tendon of flexor digitorum superficialis bifurcates to pass around the corresponding tendon of flexor digitorum profundus.
An easy way to remember this little fact is to keep in mind the following mnemonic.
- Superficialis Splits in two, to Permit Profundus Passing through
The split tendon of flexor digitorum superficialis reunites deep to the tendon of flexor digitorum profundus, around which it forms a loop. The flexor digitorum superficialis tendons then insert into the palmar surface of the base of the middle phalanx.
Flexor digitorum superficialis lies in the anterior compartment of the forearm lying superficial to flexor digitorum profundus and flexor pollicis longus, and deep to pronator teres, palmaris longus, flexor carpi radialis and flexor carpi ulnaris muscles. The two heads of flexor digitorum superficialis form a muscular arch, through which the median nerve and ulnar artery pass.
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Flexor digitorum superficialis is innervated by muscular branches of the median nerve, derived from roots C8 and T1 that arises from the medial and lateral cords of the brachial plexus. The skin that overlies the muscle is supplied by roots C6-8 and T1.
The primary arterial blood supply to the flexor digitorum superficialis is derived from the ulnar artery and its anterior recurrent branch. In addition to branches of the ulnar artery, the anterior and lateral surfaces of the muscle are supplied by branches of the radial artery; and its posterior surface also receives branches from the median artery.
The main function of flexor digitorum superficialis is flexion of the digits 2-5 at the proximal interphalangeal and metacarpophalangeal joints. Unlike the flexor digitorum profundus, flexor digitorum superficialis has independent muscle slips for all four digits. This allows it to flex the digits individually at their proximal interphalangeal joints. In addition, flexor digitorum superficialis aids the aids flexion of the wrist.
Carpal tunnel syndrome is a common condition that causes numbness, paresthesia and pain in the thumb, index finger, middle finger and the medial side of the ring finger. It is caused by a compression of the median nerve as it passes through the carpal tunnel, together with tendons of flexor digitorum superficialis, flexor digitorum profundus and flexor pollicis longus. Carpal tunnel syndrome is most commonly caused by a combination of factors that reduce the space around the median nerve, such as swelling due to injury, inflammation or a neoplasm. Rarely, carpal tunnel syndrome can be caused by an aberrant muscle belly arising from the tendon of flexor digitorum superficialis that compresses the median nerve.