Attachments
The flexor compartment has four superficial muscles from the medial epicondyle, one intermediate muscle, and three deep muscles from the shafts of the radius and ulna.
| Muscle | Origin | Insertion | Nerve |
|---|---|---|---|
| Pronator teres | Medial epicondyle (humeral head); coronoid process (ulnar head) | Middle of the lateral surface of the radius | Median |
| Flexor carpi radialis (FCR) | Medial epicondyle | Bases of the second and third metacarpals | Median |
| Palmaris longus | Medial epicondyle | Flexor retinaculum and palmar aponeurosis | Median |
| Flexor carpi ulnaris (FCU) | Medial epicondyle; olecranon and posterior border of ulna | Pisiform, then hook of hamate and fifth metacarpal base by ligaments | Ulnar |
| Flexor digitorum superficialis (FDS) | Medial epicondyle and coronoid process; anterior oblique line of radius | Sides of the middle phalanges of the four fingers | Median |
| Flexor digitorum profundus (FDP) | Anterior and medial surfaces of the ulna; interosseous membrane | Bases of the distal phalanges of the four fingers | AIN (index, middle); ulnar (ring, little) |
| Flexor pollicis longus (FPL) | Anterior surface of the radius; interosseous membrane | Base of the distal phalanx of the thumb | AIN |
| Pronator quadratus | Distal quarter of the anterior ulna | Distal quarter of the anterior radius | AIN |
FDP is the only muscle that flexes the distal interphalangeal joints of the fingers, and FDS the only one that flexes the proximal interphalangeal joints independently. Pronator quadratus is the main pronator; pronator teres adds power when speed or force is needed. Brachioradialis and extensor carpi radialis longus lie on the lateral border of the forearm but belong to the posterior compartment and are supplied by the radial nerve.
Arteries and nerves
The brachial artery divides in the cubital fossa, at about the level of the radial neck, into the radial and ulnar arteries, which run down either side of the compartment.
Radial artery
It runs down the lateral forearm under cover of brachioradialis, with the superficial branch of the radial nerve lateral to it in the middle third. It gives the radial recurrent artery to the elbow anastomosis and, just above the wrist, the superficial palmar branch, which crosses the thenar muscles to complete the superficial palmar arch. At the wrist it lies lateral to the FCR tendon, where the pulse is felt, then winds round to the anatomical snuffbox.
Ulnar artery
The larger branch passes deep to the ulnar head of pronator teres and to FDS, lies on FDP, and is joined on its medial side by the ulnar nerve. Its common interosseous branch divides into the anterior interosseous artery, which runs on the interosseous membrane with the anterior interosseous nerve, and the posterior interosseous artery. It crosses the wrist superficial to the flexor retinaculum and forms most of the superficial palmar arch.
Nerves
The median nerve runs between FDS and FDP; its anterior interosseous branch (AIN) runs deep with the anterior interosseous artery. The ulnar nerve runs under FCU on FDP. The skin is supplied by the lateral cutaneous nerve of the forearm (musculocutaneous), the medial cutaneous nerve of the forearm (medial cord) and the posterior cutaneous nerve of the forearm (radial).
Relations at the wrist
At the front of the wrist the tendons and neurovascular bundles lie in a predictable order, which is how lacerations are assessed.
From lateral to medial: radial artery, FCR tendon, median nerve with palmaris longus superficial to it, the four FDS tendons, ulnar artery, ulnar nerve, FCU tendon. FPL lies deep on the lateral side, and the FDP tendons lie deep to FDS, with pronator quadratus behind them all on the lower radius and ulna.
Clinical relevance
The anterior forearm is examined for tendon, nerve and vascular injury and is the classic site of compartment syndrome.
- Compartment syndrome: after supracondylar or forearm fractures or crush, rising pressure in the flexor compartment causes pain on passive finger extension. The deep flexors, FDP and FPL, suffer first, and untreated ischaemia leaves Volkmann's ischaemic contracture. Fasciotomy decompresses it.
- Testing FDS and FDP: FDS is tested by holding the other fingers straight and asking the patient to bend the free finger at the proximal joint; FDP by holding the middle phalanx and asking for flexion of the fingertip.
- Anterior interosseous nerve palsy: loss of FPL and the index FDP makes the OK sign collapse, without sensory loss.
- Radial artery: used for blood gases, cannulation and coronary grafts. Allen's test checks that the ulnar artery can perfuse the hand through the palmar arches first.
- Palmaris longus: absent on one or both sides in some people; when present it is a common tendon graft donor.
On the specimen
The forearm stations show the flexor compartment opened from the front, with the superficial muscles either intact or reflected to reveal the deep layer.
- Superficial muscles: from lateral to medial, pronator teres, FCR, palmaris longus and FCU. Placing the heel of the opposite hand on the medial epicondyle with the fingers spread down the forearm matches them to thumb, index, middle and little finger, with FDS under the ring finger.
- Deep layer: FDP medially, FPL laterally, and pronator quadratus as a square sheet across the lower quarter of both bones.
- Lateral border: brachioradialis, with extensor carpi radialis longus behind it; do not call them flexors.
- Cubital fossa: the biceps tendon, the bicipital aponeurosis sweeping medially from it, the brachial artery dividing, and the median nerve medial to it. The median cubital vein and cephalic vein lie superficially.
- Arteries: the radial recurrent artery runs back up from the radial artery; the common interosseous artery is a short trunk from the ulnar artery that splits at the upper border of the interosseous membrane; the anterior interosseous artery and nerve run together on the membrane. The superficial palmar branch leaves the radial artery just above the wrist.