Forearm flexors (anterior compartment)

Musculi antebrachii anteriores

Also known as: flexor–pronator group, common flexor origin, anterior forearm muscles, wrist flexors

The eight muscles of the anterior compartment of the forearm, arranged in three layers. They flex the wrist and digits and pronate the forearm, and nearly all are supplied by the median nerve.

Overview

The anterior (flexor–pronator) compartment lies in front of the radius, ulna and interosseous membrane. Many of its superficial muscles arise from the common flexor origin on the medial epicondyle of the humerus, which is why that epicondyle is larger than the lateral one. The muscles are arranged in three layers.

- Superficial layer, lateral → medial: pronator teres, flexor carpi radialis, palmaris longus, flexor carpi ulnaris ("Pass Fail Pass Fail"). Place the heel of your opposite hand on the medial epicondyle: thumb, index, middle and ring fingers then lie along PT, FCR, PL and FCU.
- Intermediate layer: flexor digitorum superficialis.
- Deep layer: flexor digitorum profundus, flexor pollicis longus, pronator quadratus.

Innervation: everything is supplied by the median nerve (or its anterior interosseous branch for the deep layer), except FCU and the medial half of FDP, which are supplied by the ulnar nerve, "one and a half muscles". The long flexor tendons pass through the carpal tunnel with the median nerve, except FCU, palmaris longus and part of the FCR tendon.

Blood supply is from the radial and ulnar arteries and the anterior interosseous artery. Because the compartment is enclosed by deep fascia and the interosseous membrane, it is at risk of compartment syndrome after supracondylar or forearm fractures, which can lead to Volkmann ischaemic contracture.

Location

Anterior (flexor) compartment of the forearm.

Parts & subdivisions

Pronator teres

Superficial. Pronates and flexes the elbow. Median nerve passes between its heads.

Flexor carpi radialis

Superficial. Flexes and abducts the wrist. Median nerve.

Palmaris longus

Superficial. Tenses the palmar aponeurosis. Absent in ≈14%. Median nerve.

Flexor carpi ulnaris

Superficial. Flexes and adducts the wrist. Ulnar nerve.

Flexor digitorum superficialis

Intermediate. Flexes the PIP joints of digits 2–5. Median nerve.

Flexor digitorum profundus

Deep. Flexes the DIP joints of digits 2–5. Anterior interosseous nerve (lateral half) and ulnar nerve (medial half).

Flexor pollicis longus

Deep. Flexes the thumb IP joint. Anterior interosseous nerve.

Pronator quadratus

Deep. Primary pronator. Anterior interosseous nerve.

Anatomy

Origin

Mainly the common flexor origin on the medial epicondyle of the humerus; also the coronoid process, radius, ulna and interosseous membrane (deeper muscles).

Insertion

Metacarpal bases (wrist flexors), the middle and distal phalanges (digital flexors) and the radius (pronators).

Action

Flexion of the wrist and fingers, pronation of the forearm, weak elbow flexion.

Innervation

Median nerve and its anterior interosseous branch; ulnar nerve for FCU and the medial half of FDP.

Nerve roots

C6–T1

Arterial supply

Radial, ulnar and anterior interosseous arteries (and the anterior ulnar recurrent artery).

Clinical significance

Medial epicondylitis ("golfer’s elbow")

Tendinopathy of the common flexor origin, mainly pronator teres and FCR. It causes pain at the medial epicondyle on resisted wrist flexion and pronation. The ulnar nerve lies just behind it.

Median nerve lesions

A lesion at the elbow causes loss of forearm pronation, weak wrist flexion with ulnar deviation, loss of flexion of the index and middle fingers and the thumb (the "hand of benediction" on attempting a fist), and thenar wasting. A lesion at the wrist (carpal tunnel) spares the forearm muscles.

Volkmann ischaemic contracture

Ischaemic necrosis of the deep flexors (FDP and FPL) after compartment syndrome leaves a fixed flexion contracture of the wrist and fingers.