Pronator Quadratus Muscle: Anatomy, Function, MRI & CT Imaging

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Pronator quadratus muscle

The pronator quadratus (PQ) is a flat, quadrilateral muscle located in the distal anterior forearm, deep to the flexor tendons. It is the primary pronator of the forearm, acting near the wrist joint. The PQ stabilizes the distal radioulnar joint (DRUJ) and maintains forearm alignment during pronation and supination.

It is considered a key deep muscle of the anterior compartment and has significant clinical relevance in distal forearm fractures, surgical exposures, and volar plate fixations.

Synonyms

Quadrate pronator muscle
Deep pronator muscle

Origin, Course, and Insertion

Origin: Distal one-fourth of the anterior surface of the ulna.
Course: Fibers run transversely across the anterior distal forearm, deep to the flexor digitorum profundus and flexor pollicis longus tendons.
Insertion: Distal one-fourth of the anterior surface of the radius.

Relations

Anteriorly: Covered by the flexor digitorum profundus and flexor pollicis longus tendons.
Posteriorly: Lies directly on the distal radius, ulna, and interosseous membrane.
Laterally: Radial artery and distal radius.
Medially: Distal ulna and ulnar periosteum.
Superiorly: Distal forearm fascia and pronator teres tendon (proximal to PQ).
Inferiorly: Wrist joint capsule and distal radioulnar joint.

Nerve Supply

Anterior interosseous nerve, a branch of the median nerve (C7–C8 fibers, primarily C8).

Arterial Supply

Anterior interosseous artery (branch of the common interosseous artery from the ulnar artery).
Small collateral contributions from radial and ulnar arteries at the wrist.

Function

Forearm pronation: Primary pronator of the forearm, turning the palm posteriorly or downward.
Distal radioulnar joint stability: Maintains congruency of distal radius and ulna during pronation and supination.
Assists in grip: Stabilizes the forearm during grasping movements.
Protective role: Shields the anterior distal radius and ulna, important during surgical approaches.

Clinical Significance

Pronator quadratus syndrome: Compression of the anterior interosseous nerve causing weakness of thumb and index finger flexion.
Fracture stability: PQ often elevated during volar plate fixation for distal radius fractures and may be repaired for soft-tissue coverage.
Tendon or muscle injury: Rare but may occur after distal forearm trauma.
Imaging relevance: Evaluation of PQ thickness, signal alteration, or atrophy can indicate nerve injury, compartment syndrome, or inflammation.
Postoperative relevance: Used as a soft-tissue flap to cover hardware in distal radius surgery.

MRI Appearance

T1-weighted images:
- Normal muscle: intermediate signal intensity, clearly delineated against bright subcutaneous fat.
- Tendinous attachments at radius and ulna: low signal (dark).
- Fatty infiltration or chronic atrophy: increased brightness within muscle belly.
- Bone cortex beneath PQ: dark linear margin.
T2-weighted images:
- Normal muscle: intermediate-to-low signal, slightly darker than on T1.
- Tendons: low signal, continuous with periosteum.
- Pathology (strain, inflammation, edema): bright hyperintense regions within or around the muscle.
- Post-traumatic changes: subtle high-signal streaks near the distal forearm.
STIR:
- Normal muscle: intermediate-to-dark signal.
- Pathology: bright hyperintense edema in acute strain, inflammation, or post-fracture irritation.
- Useful for detecting anterior interosseous nerve denervation (shows early T2/STIR hyperintensity before atrophy).
Proton Density Fat-Saturated (PD FS):
- Normal PQ: intermediate-to-dark, homogeneous signal.
- Pathologic muscle: bright hyperintense areas indicate myositis, strain, or early denervation edema.
- Fat suppression highlights perimuscular inflammation or fascial plane effusions.
T1 Fat-Sat Post-Contrast:
- Normal: mild uniform enhancement.
- Myositis or inflammation: patchy or diffuse enhancement pattern.
- Postoperative or scar tissue: focal linear enhancement along volar plate site.
- Chronic denervation: reduced enhancement with fatty infiltration.

CT Appearance

Non-Contrast CT:

Muscle: homogeneous soft-tissue density, seen between distal radius and ulna.
Attachments: well-defined cortical margins at radius and ulna.
Fat planes anterior to PQ: preserved and bright, separating it from flexor tendons.
Pathology: muscle atrophy, postoperative scarring, or hematoma may appear as soft-tissue asymmetry.

Post-Contrast CT (standard):

Normal PQ enhances mildly and homogeneously.
Inflamed or vascularized muscle: shows focal increased enhancement.
Post-surgical changes: linear enhancement or thickening along repaired PQ flap.

MRI image

CT image

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