Summary Card
Overview
Compartments are fascial “rooms” in the upper limb that house specific muscles, nerves, and vessels. Knowing their arrangement helps predict function, guide surgical exposure, and diagnose compartment syndrome.
Upper Arm Compartments
The brachium contains two large compartments encased by the brachial fascia. Fascial septa extend from the humerus to partition the arm into anterior and posterior spaces and provide surfaces for muscle attachment.
Forearm Compartments
The forearm spans from the elbow to the wrist and contains three main compartments created by the interosseous membrane between the radius and ulna and the lateral intermuscular septum.
Compartment Syndrome
Rigid fascial boundaries mean that swelling or bleeding within a compartment raises pressure, impairing venous return and arterial inflow. Recognizing compartment syndrome early prevents irreversible ischemia.
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Overview of Upper Limb Compartments
Compartments are fascial “rooms” in the upper limb that house specific muscles, nerves, and vessels. Knowing their arrangement helps predict function, guide surgical exposure, and diagnose limb‑threatening compartment syndrome.
The upper limb extends from the shoulder to fingertips and is organized into mechanical compartments by strong layers of deep fascia and interosseous membranes. These compartments compartmentalize muscles performing similar actions and restrict swelling or infection to a defined space.
Surgeons must know which muscles, nerves, and vessels lie within each compartment to choose appropriate incisions and to recognize when elevated pressure threatens perfusion.
The compartments of the arm and forearm are key to understanding surgical approaches and safe operative technique.
- The arm divides into,
- Anterior compartment
- Posterior compartment
- The forearm contains three compartments separated by the interosseous membrane and the lateral intermuscular septum.
- Volar (flexor-pronator) compartment
- Dorsal (extensor-supinator) compartment
- Mobile wad of Henry (lateral forearm group)
This knowledge is essential for fracture fixation, tendon transfer planning, and compartment release.
Deep fascia doesn’t just wrap muscles. It mechanically separates them into compartments, limiting swelling and infection spread.
Upper Arm Compartments
The brachium contains two large compartments encased by the brachial fascia. Fascial septa extend from the humerus to partition the arm into anterior and posterior spaces and provide surfaces for muscle attachment.
Upper arm compartments also guide major neurovascular bundles.
Anterior Compartment (Arm)
The anterior compartment contains the elbow flexors and key neurovascular structures essential for upper limb function.
Muscles
- Biceps brachii
- Origin: Supraglenoid tubercle (long head) and coracoid process (short head) of the scapula.
- Insertion: Radial tuberosity and bicipital aponeurosis.
- Action: Flexes the elbow and powerfully supinates the forearm, also assists in shoulder flexion.
- Nerve Supply: Musculocutaneous nerve.
- Brachialis
- Lies deep to the biceps.
- Action: Primary elbow flexor, acting regardless of forearm position.
- Nerve supply: Musculocutaneous nerve.
- Coracobrachialis
- Origin: Coracoid process.
- Insertion: Medial surface of the humeral shaft.
- Action: Weak flexor and adductor of the arm.
- Nerve Supply: Musculocutaneous nerve.
Neurovascular Structures
- Brachial artery travels through this compartment, giving muscular branches before dividing into the radial and ulnar arteries near the cubital fossa.
- The musculocutaneous nerve innervates all muscles in this compartment before continuing as the lateral cutaneous nerve of the forearm.
Posterior Compartment (Arm)
The posterior compartment contains the extensors of the elbow and key neurovascular structures that must be protected during surgery.
Muscle
- Triceps Brachii
- Heads: Long (from the scapula), lateral, and medial (from the humerus).
- Insertion: Common tendon on the olecranon.
- Action: Sole extensor of the elbow.
- Nerve Supply: Radial nerve.
Neurovascular Structures
- Radial Nerve: Travels through the radial groove of the humerus with the profunda brachii artery. Supplies the triceps and skin of the posterior arm.
- Profunda Brachii Artery: Branch of the brachial artery accompanying the radial nerve.
Two compartments, two nerves: musculocutaneous for flexors, radial for extensors.
Forearm Compartments
The forearm spans from the elbow to the wrist and contains three main compartments created by the interosseous membrane between radius and ulna and the lateral intermuscular septum.
Each forearm compartment groups muscles of similar function and determines the path of nerves and vessels.
Volar (Anterior/Flexor-Pronator) Compartment
This compartment houses the muscles that flex the wrist and fingers and pronate the forearm. Most muscles originate from the medial epicondyle and are supplied primarily by the median nerve with the ulnar nerve innervating the flexor carpi ulnaris and the medial half of the flexor digitorum profundus.
The volar compartment is organized into superficial, intermediate, and deep layers.
Superficial Layer
- Pronator Teres: Pronates the forearm and assists in elbow flexion.
- Flexor Carpi Radialis (FCR): Flexes and abducts the wrist.
- Palmaris Longus (PL): Weak wrist flexor; absent in ~15 % of people.
- Flexor Carpi Ulnaris (FCU): Flexes and adducts the wrist; innervated by the ulnar nerve.
These muscles share a common flexor origin at the medial epicondyle. They are supplied by branches of the ulnar and radial arteries, with the FCU receiving branches from the ulnar artery.
Intermediate Layer
- Flexor digitorum superficialis (FDS): Splits into four tendons to flex the proximal interphalangeal (PIP) joints of digits 2-5. It is innervated by the median nerve.
The FDS forms a bridge over the median nerve and ulnar artery as they enter the forearm.
Deep Layer
- Flexor digitorum profundus (FDP): Flexes the distal interphalangeal (DIP) joints. Lateral half is innervated by the anterior interosseous branch of the median nerve, while the medial half receives ulnar nerve branches.
- Flexor pollicis longus (FPL): Flexes the thumb’s interphalangeal joint and is supplied by the anterior interosseous nerve.
- Pronator quadratus (PQ): Pronates the forearm; forms a square muscle between the distal radius and ulna. Supplied by the anterior interosseous nerve.
The anterior interosseous artery, a branch of the ulnar artery, runs along the interosseous membrane and supplies the deep flexors. The median nerve travels between the FDS and FDP, entering the carpal tunnel to supply the hand.
Dorsal (Posterior/Extensor-Supinator) Compartment
Contains muscles that extend the wrist and fingers and supinate the forearm. Innervation is via the radial nerve and its posterior interosseous branch.
This compartment is also arranged into superficial and deep layers.
Superficial Layer
- Extensor digitorum (ED): Extends the fingers (metacarpophalangeal and interphalangeal joints). Its tendon divides into four slips forming the extensor hoods.
- Extensor digiti minimi (EDM): Extends the little finger; its tendon runs ulnar to the extensor digitorum tendon.
- Extensor carpi ulnaris (ECU): Extends and adducts the wrist.
- Anconeus: Assists triceps in elbow extension and stabilizes the elbow joint.
These muscles are innervated by the posterior interosseous nerve (PIN), a branch of the radial nerve. Blood supply derives from the posterior interosseous artery, a branch of the ulnar artery, which travels with the PIN along the extensor compartment.
Deep Layer
- Supinator: Unwinds the radius around the ulna to supinate the forearm.
- Abductor pollicis longus (APL): Abducts and extends the thumb.
- Extensor pollicis brevis (EPB): Extends the thumb’s metacarpophalangeal joint.
- Extensor pollicis longus (EPL): Extends the thumb’s interphalangeal joint.
- Extensor indicis (EI): Extends the index finger and assists the extensor digitorum.
All of these deep extensors share innervation from the posterior interosseous nerve. Their arterial supply is mainly via the posterior interosseous artery.
Mobile Wad of Henry (Lateral Compartment)
This compartment houses three muscles (brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis) that flex the elbow during rapid or resisted movement and assist in radial deviation of the wrist. These muscles bridge functions between flexors and extensors and are supplied by the radial nerve before its bifurcation.
The mobile wad sits on the lateral aspect of the forearm.
- Brachioradialis: Originates from the proximal lateral supracondylar ridge of the humerus and inserts near the radial styloid. It is the strongest forearm flexor, especially when the forearm is in mid‑pronation.
- Extensor carpi radialis longus (ECRL): Extends and abducts the wrist. Originates from the distal lateral supracondylar ridge and inserts on the base of the second metacarpal.
- Extensor carpi radialis brevis (ECRB): Extends the wrist and assists in radial deviation, inserts on the base of the third metacarpal.
The radial nerve innervates all three muscles before dividing into its superficial sensory branch and deep motor branch (posterior interosseous nerve). The radial artery, running along the lateral forearm, supplies the mobile wad.
‘F-PRAM’: Flexor-Pronator, Anterior, Median nerve — a quick way to recall volar compartment anatomy.
Compartment Syndrome & Clinical Considerations
Rigid fascial boundaries mean that swelling or bleeding within a compartment raises pressure, impairing venous return and arterial inflow. Recognizing compartment syndrome early prevents irreversible ischemia.
Each muscle compartment is bound by inelastic fascia. When fluid volume rises due to fractures, vascular injury, reperfusion, burns, or tight casts, the pressure within the compartment increases. Fascia cannot stretch, so elevated pressure first collapses thin-walled veins, reducing venous outflow. As pressure climbs, arterial inflow also falls, leading to ischemia and eventual necrosis.
Symptoms include severe pain (often out of proportion to the injury), tense swelling, paresthesia, and pain with passive stretch, while pulses may remain palpable. In the upper limb, compartment syndrome most commonly affects the forearm, but it can occur anywhere there is a closed fascial space. Pressure measurements > 30 mmHg help confirm the diagnosis, but serial clinical examinations are paramount. Treatment is emergent fasciotomy to release the fascia and restore perfusion.
For more details, read our article on compartment syndrome of the hand.
Compartment syndrome pain is often out of proportion. Don’t wait for pulse loss as that’s a late sign.
Conclusion
1. The Role of Deep Fascia: Understand how fascial septa compartmentalize muscles and why swelling within a closed osteofascial space can lead to compartment syndrome.
2. Upper Limb Compartments: Recognize that the arm has two compartments (anterior and posterior) and the forearm three (volar, dorsal, mobile wad) and list the muscles contained in each.
3. Nerve and Vascular Supply: Match the major nerves (musculocutaneous, median, radial, ulnar, posterior interosseous) and arteries (brachial, ulnar, radial, interosseous) to their respective compartments.
4. Clinical Signs of Compartment Syndrome: Describe the pathophysiology of acute compartment syndrome and appreciate the urgency of fasciotomy when intracompartmental pressure rises.
Further Reading
- Chaudhry MA, Hafeez AM, Sinkler MA, Arain A. Forearm Compartments. In: StatPearls. Treasure Island (FL).
- Alshammari SM, Bordoni B. Arm Muscles. In: StatPearls. Treasure Island (FL).
- Torlincasi AM, Lopez RA, Waseem M. Acute Compartment Syndrome. In: StatPearls. Treasure Island (FL).
