Pronator Quadratus

Pronator Quadratus

The Pronator quadratus is a thin, short, flat, square-shaped muscle present in the anterior compartment of the forearm that acts to pronate the hand.

There are three muscles named flexor digitorum profundus, flexor pollicis longus, and pronator quadratus present in the deep group of forearm flexor; while the superficial group of forearm flexor overlays these three muscles. The Pronator quadratus’s primary function is forearm pronation, for which it extends across the distal parts of the radius and ulna.

Size:

The Pronator quadratus is 6cm in length and 3.5 cm in width.

Origin and insertion:

The Pronator quadratus is a quadrilateral short, flat muscle that originates from the anterior surface of an aponeurosis and the distal ulna shaft. Both structures partially cover the pronator quadratus muscle.

Superficial muscle fibers of the Pronator quadratus project distally and laterally towards the anterior surface of the distal shaft of the radius. The deeper fiber is superiorly inserted to the ulnar notch of the radius.

Innervation and Blood supply:

The Pronator quadratus is innervated by the anterior interosseous nerve of the forearm, with the main contribution of C7 and C8 spinal nerves. The anterior interosseous nerve is a branch of the median nerve, which comes from the brachial plexus.

The blood is supplied to the Pronator quadratus through the anterior interosseous artery, which stems from the common interosseous artery. The interosseous artery is the main branch of the ulnar artery.

Relations:

In the anterior compartment of the forearm, the pronator quadratus is the deepest muscle. That is why the location of the pronator quadratus is directly below the remaining deep forearm flexor, which is digitorum profundus and flexor pollicis longus.

The Pronator quadratus is located distally in the forearm and superficially covers the interosseous membrane. The anterior interosseous artery makes a hole in the interosseous membrane proximal to the pronator quadratus because it passes from the anterior to the posterior compartment of the forearm.

The anterior interosseous artery branch moves downward to the pronator quadratus towards the palmar arch. The radial artery travels deep into the pronator quadratus, and its palmar carpal branch emerges close to the distal border of the pronator quadratus muscle. The anterior interosseous nerve travels to the deep surface of the pronator quadratus courses posteriorly. 

Spinal tracts:

The lateral corticospinal tract is accountable for the motor pathway of the pronator quadratus. The track begins in the precentral gyrus, which is the anatomical location of the primary motor cortex. Through the peduncles of the midbrain and the progression tracts of the internal capsule, the signal is transmitted to the upper motor nerve after crossing these two tracts.

The signal then crosses the medulla, travels down to the lateral corticospinal tract, and moves forward in the spinal cord’s lateral column. After that, it further crosses the spinal cord and synapses the anterior horn of the lower motor neurons of the skeletal muscles. 

The cuneate fasciculus tract is responsible for the perception of the pronator quadratus movement, position, visceral pain, deep touch, and vibration. The cuneate fasciculus tract starts in the dorsal nerve root, from which the signal is transmitted to the posterior column of the spinal cord.

After passing through the spinal cord, the signal synapses to an interneuron in the gracile nucleus. It further crosses the medial lemniscus of the medulla and travels through the cuneate nucleus and medial lemniscus of the midbrain to synapse the thalamus.

After that, it synapses with a third-order neuron to transmit the signal to the postcentral gyrus of the somesthetic cortex. The signal from the cuneate fasciculus tract is not specific for pronator quadratus muscle and can apply to any muscle present in the upper limb.

Function:

As the name indicated, the main function of the Pronator quadratus is forearm pronation. The Pronator quadratus produces forearm pronation by working on the proximal radioulnar joint. During this pronation movement, the head of the radius rotates the ulna, which helps to turn the arm inferiorly or posteriorly if the forearm is flexed.

This action of the pronator quadratus is performed with the help of pronator teres and brachioradialis muscle. Due to the pronator quadratus, an individual can rotates his forearm and palm while writing or tying. This action is possible due to the median nerve, which is present in this muscle and helps in nerve transmission.

The Pronator quadratus performs a protective role due to its location in the forearm. For example, when upward pressure is applied on the forearm during weight-bearing activities, the pronator quadratus holds the radius and ulna’s distal ends.

It performs a significant role in protecting and maintaining the distal radioulnar joint. The pronator quadratus’s deep fiber also helps keep the two bones of the forearm bound together. Moreover, it also protects the interosseous membrane by removing the force pulling on the membrane during rapid or forced forearm rotation.

Palpation:

This muscle can neither be palpated nor be observed because this is the deepest muscle of the forearm.

References:

  • https://www.kenhub.com/en/library/anatomy/pronator-quadratus-muscle#:~:text=Pronator%20quadratus%20is%20a%20quadrangular,superficial%20group%20of%20forearm%20flexors retrieved on March 23, 2022. 
  • https://en.m.wikipedia.org/wiki/Pronator_quadratus_muscle retrieved on March 23, 2022. 
  • https://www.physio-pedia.com/Pronator_Quadratus retrieved on March 23, 2022.
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