Pronator Quadratus

What is the Pronator Quadratus

The pronator quadratus is one of the three deep muscles in the anterior compartment of th forearm, the other two being the flexor digitorum profundus and flexor pollicis longus. The small, flat, squarish intrinsic muscle acts on the forearm and hand, helping it turn downwards.

Anatomy

Location and Attachments

Origin	The anterior surface of the distal side of the ulna
Insertion	The anterior surface of the distal side of the radius

The muscle originates from the distal end of the ulna at the pronator ridge on the anterior surface. A few muscle fibers also arise from the aponeurosis that covers the medial side of the muscle.

After originating, the muscle fibers course laterally and slightly distally to insert into the lateral border on the anterior surface of the most distal one-fourth of the radial shaft. It results in the muscle stretching across the distal ends of the ulna and radius near the wrist joint.

Some deeper fibers of the pronator quadratus also insert into a higher point into the ulnar notch of the radius. This second insertion point is comparable with the point of origin of the posterior compartment muscle supinator, which rises from below the radial notch of the ulna (the supinator crest).

Relations With Surrounding Muscles and Structures

The pronator quadratus constitutes the deepest layer of the deep anterior compartment. The other two muscles in this group, the flexor digitorum profundus and flexor pollicis longus lie superficial to it. Since the small muscle lies on the distal side of the forearm, it is superficial to the interosseous membrane, a thick fibrous connective tissue between the radius and ulna.

Several vital nerves and blood vessels of the hand and arm also pass from around the muscle. The anterior interosseous passes from the anterior compartment of the forearm to the posterior through the part of the interosseous membrane that proximally covers the pronator quadratus. An anterior interosseous artery branch runs deep into this muscle as it continues to the palmar arch, the arterial network in the palm. Lastly, the radial artery passes from the anterior side of the pronator quadratus. The palmar carpal branch of this artery also rises near the muscle’s distal border. 

The anterior interosseous nerve travels from the back of the muscle’s deep surface.

Function

Action

Pronation of the forearm

Pronating the Forearm

As suggested by its name, this muscle’s primary function is to pronate the forearm and the hand, which means rotating it downwards, so the palm faces the ground.

During pronation, the radial head pivots around the ulna at the proximal radioulnar joint, which turns the palm inwards or outwards as the forearm is flexed. The muscle, with its attachments at the radius and ulna, aids with this movement.

The forearm’s superficial anterior compartment muscle pronator teres and the superficial posterior compartment muscle brachioradialis help the pronator quadratus in this action. 

Keeping the Radius and Ulna Together 

With its attachment across the distal radial and ulnar ends, this muscle also plays a vital role in keeping the humerus and ulna together.

When an upward force is applied on the forearm, like you landing onto your outstretched arms after a fall, the muscle ensures that the distal ends of the radius and ulna stay together instead of being pushed apart. So, the muscle is instrumental in stabilizing and protecting the distal radioulnar joint.

Protecting the Interosseous Membrane

The pronator quadratus also keeps the interosseous membrane in place by distributing any extra force that might pull the membrane out of place when a sudden external force is applied to the arms.

The primary antagonist of the pronator quadratus is the supinator muscle.

Innervation

Nerve

Anterior interosseous nerve (C8 and T1)

The muscle is innervated by the anterior interosseous nerve, which branches off the median nerve, a branch of the brachial plexus.

Blood Supply

Artery

Anterior interosseous artery

The anterior interosseous artery that supplies this muscle is a branch of the common interosseous artery, which itself branches from the ulnar artery.