The Pronator Quadratus (PQ) is a small, deep muscle of the forearm that plays a significant role in hand and arm function. Its specific neural supply allows for precise motor control over a fundamental forearm movement. Understanding its function and the consequences of its injury is important for forearm anatomy.
Location and Structure of the Pronator Quadratus
The Pronator Quadratus is situated deep within the distal forearm, making it the deepest of the anterior forearm muscles. It has a distinct flat, quadrilateral shape, spanning the distance between the ulna and the radius.
The muscle originates on the anterior surface of the ulna’s distal quarter and inserts onto the anterior surface of the radius’s distal quarter. Its fibers run almost perpendicular to the arm, acting like a tight band connecting the two bones. This deep location means it is covered by the tendons of other forearm muscles, stabilizing the distal radioulnar joint.
The Anterior Interosseous Nerve Connection
The muscle receives its neural input from the Anterior Interosseous Nerve (AIN), a motor branch of the Median Nerve. The Median Nerve gives rise to the AIN in the proximal forearm, typically branching off about five to eight centimeters from the lateral epicondyle.
The AIN is a pure motor nerve, carrying signals only for muscle movement and not sensation. It travels down the forearm along the interosseous membrane, the fibrous sheet connecting the ulna and radius. As it descends, the AIN supplies the deep muscles of the forearm before reaching the Pronator Quadratus near the wrist joint. The nerve enters the PQ on its deep surface, supplying the necessary motor signals.
Primary Role in Forearm Movement
The primary action of the Pronator Quadratus is pronation, the movement that turns the palm downward. It achieves this by pulling the radius over the ulna, a movement essential for many daily activities. While the Pronator Teres assists in initiating this motion, the PQ is considered the most powerful pronator.
Its transverse fiber direction is suited to efficiently pull the distal radius across the ulna. The PQ is active when the forearm is partially flexed or when the movement requires controlled pronation. Beyond movement, the deep fibers of the muscle also provide a stabilizing force, holding the radius and ulna together at the wrist joint.
What Happens When the Nerve is Injured
Damage to the Anterior Interosseous Nerve results in Anterior Interosseous Nerve Syndrome (AIN Syndrome). Since the AIN is purely a motor nerve, this syndrome presents without loss of sensation, distinguishing it from other nerve injuries. The injury causes weakness or paralysis in the three muscles the AIN supplies, including the Pronator Quadratus.
When the PQ is weakened, a person experiences difficulty performing smooth, controlled forearm pronation. Testing for PQ weakness is done by having the patient attempt pronation while the elbow is flexed, minimizing the contribution of the weaker Pronator Teres. Loss of function in the AIN-supplied muscles also impairs the ability to make a proper “OK” sign, as the thumb and index finger cannot flex the tip joints.