Lateral Forearm Compartment

Lateral Fascial Compartment of the Forearm

• This compartment is sometimes viewed as part of the posterior compartment.
• Muscles found here include the brachioradialis and the extensor carpi radialis longus.
• The blood supply is the radial and brachial arteries.
• The muscles are innervated by the radial nerve.

Brachioradialis

• Assists in elbow flexion when the forearm is in midpronation.
• Arises from the lateral supracondylar ridge of the humerus.
• Inserts on the base of the styloid process of the radius.
• Innervated by the radial nerve before it bifurcates.
• Recieves arterial supply from the radial and brachial arteries.

Extensor Carpi Radialis Longus Action

• Extends and abducts the wrist.
• Arises from the lateral supracondylar ridge of the humerus above the brachioradialis.
• Inserts on the posterior surface of the base of the second metacarpal bone.
• Innervated by the radial nerve before it bifurcates.
• Receives arterial supply from branches of the radial and brachial arteries.

Radial Nerve

• The radial nerve, a branch of the posterior cord of the brachial plexus, passes through the lateral intermuscular septum in the lower arm.
• The radial nerve goes into the cubital fossa.
• It passes in front of the lateral epicondyle of the humerus, between the brachialis and the brachioradialis and extensor carpi radialis longus.
• At the level of the lateral epicondyle, it divides into superficial and deep branches.
• Muscular branches include the brachioradialis, and extensor carpi radialis longus.
• It gives off a small branch to the lateral part of the brachialis muscle
• Articular branches go to the elbow joint.
• The deep branch becomes the posterior interosseus nerve.
• The superficial branch is sensory.

Superficial Branch of the Radial Nerve

• Travels down the anterior compartment of the forearm under the brachioradialis muscle on the lateral side of the radial artery.
• In the distal forearm, the artery is exited by the nerve, which passes under the brachioradialis tendon.
• It divides into terminal branches at the posterior wrist and supplies the skin on the lateral two-thirds of the posterior hand.
• Also supplies the posterior surface over the proximal phalanges of the lateral three and a half fingers.

Posterior Fascial Compartment of the Forearm

• A superficial group of muscles are: extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus.
• A deep group of muscles are: supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis.
• The muscles in the posterior compartment share a common origin tendon.
• The common tendon connects to the lateral epicondyle of the humerus.
• Posterior and anterior interosseous arteries provide blood supply.
• The deep branch of the radial nerve (posterior interosseous nerve) provides nerve supply.

Extensor Carpi Radialis Brevis

• Originates from the common extensor tendon on the lateral epicondyle of the humerus.
• Inserts on the dorsal surfaces of the bases of the second and third metacarpals.
• Extends and abducts the wrist.
• It is innervated by the posterior interosseous nerve.
• It is deep to extensor carpi radialis longus.

Extensor Digitorum

• Common extensor tendon that attaches to the lateral epicondyle of humerus.
• Forms four tendons, with each passing into a finger.
• It extends all joints of the hand.
• It is innervated by the posterior interosseous nerve.
• Insertion is the base of dorsal surfaces of middle and distal phalanges.
• The dorsal digital expansion of connective tissue holds phalanges together.

Extensor Digiti Minimi

• It extends the MCP and IP joints of little finger.
• It is innervated by the posterior interosseous nerve.
• Accessory extensor to the little finger, is medial to extensor digitorum.
• It contains a common extensor tendon, is attached to the lateral epicondyle of humerus.
• The dorsal digital expansion of the little finger follows the tendon of the extensor digitorum.

Extensor Carpi Ulnaris

• Lies medial to the extensor digiti minimi.
• Inserts on the medial side of the base of the fifth metacarpal.
• Extends and adducts the wrist.
• Innervated by the posterior interosseous nerve.
• It contains a common extensor tendon, is attached to the lateral epicondyle of the humerus (ulnar side).
• The posterior border of ulna is connected to an aponeurosis.

Anconeus

• The anconeus is a small triangular muscle found behind the cubital joint.
• It is mainly a weak extensor of the elbow.
• It is innervated by the radial nerve before division.
• Originates from the posterior surface of lateral epicondyle of humerus.
• Insertion: Lateral aspect of Olecranon, 1/4th of post surface of ulna

Supinator

• Two heads make up the supinator.
• It supinates the forearm and hand.
• It is innervated by the posterior interosseous nerve.
• Origin (Superficial head): lateral epicondyle of humerus, radial collateral ligament of elbow, annular ligament
• Origin (Deep Head): supinator crest of ulna, post part of triangular area in front of it
• Insertion: Lateral surface of prox 1/3 of radius

Abductor Pollicis Longus

• Abducts and extends the thumb at the carpometacarpal joint.
• It is innervated by posterior interosseous nerve.
• Origin Proximal posterior surfaces of the radius, ulna, and interosseous membrane.
• Insertion on the radial side of base of 1st metacarpal and trapezium.

Extensor Pollicis Brevis

• Extends the proximal phalanx and metacarpal of the thumb.
• Innervated by the posterior interosseous nerve.
• Originates from the posterior surface of radius, interosseous membrane.
• Inserts on the dorsal surface of the base of the proximal phalanx of thumb.

Extensor Pollicis Longus

• Extends all joints of the thumb.
• It is innervated by the posterior interosseous nerve.
• Originates from the Posterior surface of ulna interosseous membrane.
• Inserts on the dorsal surface of the distal phalanx of the thumb

Extensor Indices

• Extension of the index finger.
• It is innervated by the posterior interosseous nerve.
• Originates from the posterior surface of ulna distal to extensor pollicis longus, interosseous memb.
• Insertion on the ulnar side of tendon of extensor digitorum for the index finger plus dorsal digital expansion.

Arteries of the Posterior Fascial Compartment of the Forearm

• The anterior and posterior interosseous arteries come from the common interosseous artery, a branch of the ulnar artery.
• They supply the adjoining muscles and bones.
• They pass downward on the anterior and posterior surfaces of the interosseous membrane, .
• They end near the wrist joint by anastomosing
• Anterior part: goes into the anterior compartment of forearm and pierces into the posterior compartment near the wrist.
• The common interosseous artery of the ulnar artery gives anterior and posterior interosseous arteries
• Posterior part: pierces into the posterior compartment of forearm and anastomes near the wrist.

Deep Branch of the Radial Nerve

• Arises from the radial nerve in front of the lateral epicondyle of the humerus in the cubital fossa
• It pierces the supinator and runs around the lateral aspect of the neck of the radius.
• Afterward it goes into the posterior compartment of the forearm.
• It runs in the space between the superficial and deep groups of muscles and reaches the posterior wrist joint.
• Muscular and Articular branches to the wrist and carpal joints are two branches the deep radial nerve has.

Extensor Retinaculum

• The extensor retinaculum thickens the deep fascia, stretching along the rear of the wrist.
• It keeps holding long extensor tendons.
• Grooves are on the distal ends of the radius + ulna and converted into six separate tunnels designed for extending long tendons.
• Every tunnel is lined + extends above/below the retinaculum .
• Tunnels separated via fibrous septa traveling along from deep surface to the related bones.
• Retinaculum attaches to the pisiform bone + hook (hamate at the distal radius end.)
• The upper + lower borders are attached with the deep fascia on their forearm + hand.
• It creates fixation for the tendons.

Structures Passing Beneath the Extensor Retinaculum (Medial to Lateral)

1. Extensor carpi ulnaris tendon grooves the posterior aspect of the head of the ulna into the 5th metacarpal bone.
2. Extensor digiti minimi tendon, situated posterior to the distal radioulnar joint.
3. Extensor digitorum and extensor indicis tendons are located on the posterior surface of radius + contain a common synovial sheath.
4. Extensor pollicis longus tendon goes around on the inside of the dorsal tubercle in the radius
5. Extensor carpi radialis longus and brevis tendons share its synovial sheath + are on the back of radius' lateral side.
6. Abductor pollicis longus/extensor pollicis brevis tendons possess distinct synovial sheaths yet have a single compartment.

Carpal Tunnel

• The carpal tunnel is created by the hand bones and flexor retinaculum.
• The median nerve sits in a restricted area between the tendons for digitorum superficialis' and carpi radialis' muscles.

Structures on the Anterior Aspect of the Wrist (Medial to Lateral)

• Pass superficial to the flexor retinaculum

1. Flexor carpi ulnaris tendon - to the pisiform bone/attachment.
2. Ulnar Nerve - placed away pisiform's (the bone) edges.
3. Ulnar Artery and Vein - lies lateral to the ulnar nerve.
4. Palmar cutaneous branch of the ulnar nerve.
5. Palmaris longus tendon - enters the flexor retinaculum + palmar aponeurosis (if present).
6. Palmar cutaneous branch of the median nerve.

Structures Passing Beneath the Flexor Retinaculum (Medial to Lateral)

• Flexor digitorum superficialis tendons and, posterior, digitorum profundus tendons; each group shares the same synovial.
• Median Nerves
• Flexor pollicis longus tendons are wrapped via synovial
• Flexor carpi radialis tendons are slit down + surrounds synovial.

Structures on the Posterior Aspect of the Wrist (Medial to Lateral)

• Dorsal (posterior) cutaneous branch of the ulnar nerve.
• Basilic vein extends from the little finger.
• Cephalic extends from the thumb.
• Superficial branch of the radial nerve.