Pronator Quadratus Muscle Anatomy

Location & Overview

In Latin, the word ‘pronator’ means ‘lying face downward’. This Latin word suggests how the pronator quadratus muscle gets its name. The pronator quadratus muscle can rotate the forearm into a pronated position, with the palm facing down. It also has a quadrilateral shape. The pronator quadratus is short and flat, with parallel-running fibers. It is the deepest muscle of the anterior forearm and cannot be palpated or observed superficially.^[1,2]

The pronator quadratus muscle is useful for activities that involve pronation of the forearm, such as turning a doorknob, using a screwdriver or a wrench, or using a computer mouse. It is also involved in activities that require fine motor skills, such as painting, drawing, or writing. The pronator quadratus also assists with sports that involve forearm rotation, such as baseball, where it is necessary to twist the wrist in order to throw a ball or hit the ball with a bat.

Origin & Insertion

The pronator quadratus originates from the anterior surface of the distal fourth of the ulna. More specifically, its ulnar attachment includes the oblique ridge on the anterior surface of the ulna and the adjacent medial part of this surface. Some of the medial part of the muscle is also covered by a strong aponeurosis.^[3,1]

From its origin on the ulna, the muscle fibers pass laterally toward the radius. The pronator quadratus inserts onto the lateral border and anterior surface of the distal fourth of the radius. Deeper fibers also attach to the triangular area above the ulnar notch of the radius. This arrangement allows the muscle to pull the distal radius across the ulna during pronation.^[3,1]

Action & Function

The main action of the pronator quadratus is pronation of the forearm. During pronation, the radius rotates around the ulna so that the palm turns downward or backward, depending on the starting position of the forearm. The pronator quadratus is considered the primary muscle for pronation, with the pronator teres contributing more when greater speed or force is required.^[4,3]

The pronator quadratus also contributes to stability at the distal radioulnar joint. Its deeper fibers help oppose separation of the distal radius and ulna, particularly when force is transmitted upward through the wrist, such as during weight-bearing through the hand. This stabilizing role is important because the distal radius moves around the relatively fixed distal ulna during pronation and supination.^[3,4]

The muscle is difficult to test in isolation because pronator teres commonly contracts at the same time. Testing usually involves resisted forearm pronation while attempting to keep the wrist and finger flexors relaxed.^[3]

Innervation

The pronator quadratus muscle is innervated by the anterior interosseous nerve (C7, C8). The anterior interosseous nerve is a branch of the median nerve and travels along the anterior surface of the interosseous membrane with the anterior interosseous artery. Before reaching the pronator quadratus, it supplies other deep anterior forearm muscles, including flexor pollicis longus and the lateral part of flexor digitorum profundus.^[4,5]

Near the distal forearm, the anterior interosseous nerve passes posterior to the pronator quadratus and enters the muscle from its dorsal surface. In a cadaveric study of 40 upper limbs, Svízenská et al. found that most distal branches of the anterior interosseous nerve entered the pronator quadratus. The median number of muscular branches was five, with a reported range of two to eight branches.^[5]

The anterior interosseous nerve may also give small terminal branches toward the wrist or distal radioulnar joint after passing through the pronator quadratus. In the same cadaveric study, an articular branch to the wrist joint or distal radioulnar joint was found in seven of 40 specimens. The authors emphasized that cutting the anterior interosseous nerve before it supplies the pronator quadratus may damage the innervation of a muscle involved in initiating pronation.^[5]

Blood Supply

Blood is supplied to the pronator quadratus mainly by the anterior interosseous artery. The anterior interosseous artery arises from the common interosseous artery, which is a branch of the ulnar artery. It descends along the anterior surface of the interosseous membrane with the anterior interosseous nerve before passing near the pronator quadratus in the distal forearm.^[3,6]

Thomas et al. studied the vascular basis of the pronator quadratus muscle flap using cadaveric dissections and arterial injection techniques. They found that the anterior interosseous vessels formed the major vascular pedicle of the muscle. In their specimens, the anterior interosseous pedicle entered the dorsal aspect of the pronator quadratus, supplied the deep head, and then continued to supply the superficial head.^[6]

The same study also described smaller vascular contributions from the radial and ulnar arteries. Radial artery branches were identified in all specimens, while ulnar artery branches were found in 20 of 26 muscles. These smaller branches entered from the radial and ulnar sides of the muscle and anastomosed with branches from the anterior interosseous artery within the muscle.^[6]