Flexor Digitorum Superficialis Muscle Anatomy

Location & Overview

The flexor digitorum superficialis is a forearm muscle that is commonly considered a superficial muscle. The flexor digitorum superficialis crosses the elbow joint via the common flexor tendon, with the flexor digitorum profundus passing deep to it. The common flexor tendon is a band of fibrous connective tissue that attaches to the medial epicondyle of the humerus. The muscles that attach to the common flexor tendon include the pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, and the flexor carpi ulnaris. Distally, at the wrist, the flexor digitorum superficialis divides into four tendons. These four tendons travel deep to the flexor retinaculum via the carpal tunnel. The carpal tunnel is a narrow passageway formed by the bones and ligaments of the wrist joint. The flexor retinaculum forms the anterior roof of the carpal tunnel. When the flexor digitorum superficialis reaches the distal part of the proximal phalanx of a finger, an opening, or split, appears in the muscle’s tendon. This opening allows the tendon of the flexor digitorum profundus muscle to pass through it.^[1,2] Images of this are included below.

Origin & Insertion

There are two points of origin for the flexor digitorum superficialis muscle. The first is the humeroulnar head of the muscle, which originates from the medial epicondyle of the humerus and the coronoid process of the ulna. The second is the radial head, originating from the proximal half of the anterior border of the radius. The flexor digitorum superficialis then continues distally, where it crosses the wrist joint and reaches the fingers. It then inserts onto the bases of the middle phalanges of digits two to five.^[1,2]

Action & Function

The flexor digitorum superficialis flexes the proximal interphalangeal joints and metacarpophalangeal joints of digits two to five. It can also assist during flexion of the wrist. The primary flexors of the wrist are the flexor carpi radialis and the flexor carpi ulnaris. The flexor digitorum superficialis can flex each of the fingers to which it connects independently.^[1,2]

Innervation

The flexor digitorum superficialis muscle is innervated by the median nerve (C7, C8, T1). This means that the nerve supply comes through the median nerve, with nerve fibers mainly associated with the lower cervical and first thoracic spinal nerve levels. The key point is that the flexor digitorum superficialis is a median nerve muscle, unlike flexor carpi ulnaris and the medial part of flexor digitorum profundus, which are supplied by the ulnar nerve.^[3,4]

The median nerve passes into the forearm near the elbow and travels between the humeroulnar and radial heads of the flexor digitorum superficialis. This relationship is useful because it helps explain why the muscle is closely associated with the main nerve pathway through the anterior forearm. Branches of the median nerve supply flexor digitorum superficialis near the elbow, with the branch to the part of the muscle serving the index finger arising nearer the mid-forearm and sometimes coming from the anterior interosseous nerve.^[4]

This innervation is useful to understand because weakness of the flexor digitorum superficialis can affect flexion of the proximal interphalangeal joints of digits two to five. In simple terms, this muscle helps bend the middle joints of the fingers. If its median nerve supply is impaired, a person may have difficulty flexing those finger joints independently, especially when trying to test one finger at a time.^[3]

Blood Supply

The blood supply to the flexor digitorum superficialis is supplied by branches from the ulnar and radial arteries, with additional supply from the anterior ulnar recurrent artery and the median artery. A simple way to understand this is that different parts of the muscle receive blood from nearby vessels as they pass through the anterior forearm.^[4]

The humeral head of the flexor digitorum superficialis is supplied by the anterior ulnar recurrent artery. The main body of the muscle receives branches from both the ulnar and radial arteries on its anterior surface. Its posterior surface receives blood from the ulnar artery and median artery, while its lateral surface receives additional branches from the radial artery.^[4]