Posterior compartment of forearm

Question 1

Outline the posterior compartment of the forearm.

Answer 1

• A dozen muscles occupy the extensor compartment.
• At the upper part are anconeus (superficial) and supinator (deep).
• From the lateral part of the humerus arise 3 muscles that pass along the radial side (brachioradialis and extensor carpi radialis longus and brevis) and 3 that pass along the posterior surface of the forearm (extensor digitorum, extensor digiti minimi and carpi ulnaris).
• At the lower end of the forearm, these 2 groups are separated by 3 muscles that emerge from deeply in between them and go to the thumb (abductor pollicis longus and extensors pollicis longus and brevis).
• Finally, 1 muscle for the forefinger runs deeply to reach the back of the hand (extensor indicis).
• The nerve of the extensor compartment is the posterior interosseous nerve, which reaches it by passing around the radius (compare the peroneal nerve in the leg).
• The artery is the posterior interosseous, which gains the extensor compartment by passing between the 2 bones (compare the anterior tibial artery in the leg). The artery is small and the blood supply of the posterior compartment is reinforced by the anterior interosseus artery.
• The 6 long muscles that come from the lateral side of the humerus have not enough area available at the lateral epicondyle. 2 of them arise above this, from the lateral supracondylar ridge and the lateral intermuscular septum.

Question 2

Brachioradialis:
* Origin
* Insertion
* Nerve supply
* Action

Answer 2

• Origin - upper 2/3rds of the lateral supracondylar ridge
• Insertion - base of radial styloid.
• Nerve supply - radial nerve (C5, 6) by a branch arising above the elbow joint.
• Action - its action is to flex the elbow joint.
  It acts most powerfully when the forearm is semipronated.

Question 3

Extensor carpi radialis longus:
* Origin
* Insertion
* Nerve supply
* Action

Answer 3

• Origin - lower 1/3 of the lateral supracondylar ridge of the humerus.
• Insertion - base of second metacarpal
• Nerve supply - radial nerve (C6, 7) by a branch arising above the elbow.
• Action - extensor and abductor of the wrist.
  It is indispensable to the action of making a fist, acting as a synergist during finger flexion. It assists in flexion of the elbow.

Question 4

Extensor carpi radialis brevis:
* Origin
* Insertion
* Nerve supply
* Action

Answer 4

• Origin - common extensor origin on the front of the lateral epicondyle of the humerus.
• Insertion - base of 3rd metacarpal. It and the longus are inserted into the same metacarpals as flexor carpi radialis.
• Nerve supply - posterior interosseous nerve (C7, 8).
• Action - wrist extensor like its longus companion it contracts in making a fist.

Question 5

Extensor digitorum:
* Origin
* Insertion
* Nerve supply
* Action

Answer 5

• Origin - common extensor origin.
• Insertion - extensor expansion of the medial four digits.
• Nerve supply - posterior interosseous nerve on the back of the forearm (C7, 8).
• Action - extensor of the wrist, metacarpophalangeal and interphalangeal joints.

Question 6

Extensor digiti minimi:
* Origin
* Insertion
* Nerve supply
* Action

Answer 6

• Origin - common extensor origin.
• Insertion - It may or may not divide into two as it crosses the hand. It inserts in the base of the fifth proximal phalange on the dorsal side. The tendon of extensor digitorum to the little finger commonly joins them as a band near the MCPJ and they all form an expansion on the dorsum of the little finger, which behaves as the other extensor expansions.
• Nerve supply - posterior interosseous nerve (C7, 8).
• Action - assists extensor digitorum in extension of the 5th finger and wrist joint.

Question 7

Extensor carpi ulnaris:
* Origin
* Insertion
* Nerve supply
* Action

Answer 7

• Origin - lateral epicondyle of the distal humerus and the posterior aspect of the ulna. This aponeurosis arises in common with that of flexor carpi ulnaris, the 2 passing in opposite directions into the extensor and flexor compartments.
• Insertion - base of 5th metacarpal.
• Nerve supply - posterior interosseous nerve (C7, 8) at the back of the forearm.
• Action - extensor and adductor of the wrist.
  It acts as a synergist during finger flexion and is indispensable in making a fist.

Question 8

Anconeus:
* Origin
* Insertion
* Nerve supply
* Action

Answer 8

• Origin - posterior surface of the lateral epicondyle.
• Insertion - lateral side of the olecranon and adjacent shaft of the ulna.
• Nerve supply - radial nerve (C7, 8) by a branch that leaves the trunk in the radial groove and passes through the triceps, supplying it as well.
• Action - produces small amount of posterolateral movement of the ulna that occurs during pronation. Stabilises ulna during pronation.

Question 9

Supinator:
* Origin
* Insertion
* Nerve supply
* Action

Answer 9

• Origin - broad origin from humerus and ulna (superficial and deep layers) - supinator crest of the ulna, the lateral epicondyle of the humerus, the radial collateral ligament and the annular radial ligament.
• Insertion - proximal third of the radius on the anterolateral and posterior surface, distal to the radial tuberosity.
• Nerve supply - posterior interosseous nerve (C6, 7).
• Action - supination of forearm. This can be done with the elbow in any position of flexion or extension. Supinator works with biceps brachii if powerful supination is required. However, biceps brachii is unable to supinate when the elbow is extended.

Question 10

Abductor pollicis longus:
* Origin
* Insertion
* Nerve supply
* Action

Answer 10

• Origin - superficial and deep part which originates from the ulna, interosseous membrane and radius. The ulnar origin is more proximal than the radial.
• Insertion - The tendon of the muscle usually divides into 2 slips, one being attached to the base of the 1st metacarpal, and the other to the trapezium.
• Nerve supply - posterior interosseous nerve (C7, 8).
• Action - abduction of the thumb and extension of the first CMCJ. It also assists in radial deviation and flexion of the wrist.

Despite its name, this muscle extends the thumb at the CMCJ, displacing it laterally in the plane of the palm.

Question 11

Extensor pollicis brevis:
* Origin
* Insertion
* Nerve supply
* Action

Answer 11

• Origin - arises below abductor pollicis longus from the radius and the adjacent interosseous membrane.
• Insertion - base of proximal phalanx.
• Nerve supply - posterior interosseous nerve (C7, 8).
• Action - extends the thumb at the MCPJ and CMCJ. It is also a mild abductor of the thumb.

Question 12

Extensor pollicis longus:
* Origin
* Insertion
* Nerve supply
* Action

Answer 12

• Origin - mid-third of the ulna and interosseous membrane. Extends higher into the forearm than extensor pollicis brevis.
• Insertion - extends more distally also into the thumb, being inserted into the base of the distal phalanx.
• Nerve supply - posterior interosseous nerve (C7, 8)
• Action - extends the interphalangeal joint of the distal phalanx of the thumb. It also assists in extension and abduction of the wrist.

Question 13

Extensor indicis:
* Origin
* Insertion
* Nerve supply
* Action

Answer 13

• Origin - posterior surface of ulna (distal to Extensor Pollicis Longus muscle) and interosseous membrane.
• Insertion - base of the second proximal phalange and into the tendon of extensor digitorum.
• Nerve supply - posterior interosseus nerve (C7, 8)
• Action - extension of second digit at MCPJ and interphalangeal joints.

Question 14

Describe the extensor retinaculum. What are the 6 compartments of the extensor tunnel?

Answer 14

• The extensor retinaculum is a band-like thickening in the deep fascia of the forearm, about 2.5 cm wide, which lies obliquely across the extensor surface of the wrist.
• Its proximal attachment is to the anterolateral border of the radius above the styloid process. It is not attached to the ulna; its distal attachment is to the pisiform and triquetral bones.
• From the deep surface of the extensor retinaculum fibrous septa pass to the bones of the forearm, dividing the extensor tunnel into six compartments.
  1. The most lateral compartment lies over the lateral surface of the radius at its distal extremity, and through it pass the tendons of abductor pollicis longus and extensor pollicis brevis, each usually lying in a separate synovial sheath.
  2. The next compartment extends as far as the dorsal tubercle, and conveys the tendons of the radial extensors of the wrist (longus and brevis), each lying in a separate synovial sheath.
  3. The groove on the ulnar side of the radial tubercle lodges the tendon of extensor pollicis longus, which lies within its own compartment invested with a synovial sheath.
  4. Between this groove and the ulnar border of the radius is a shallow depression in which four tendons of extensor digitorum lie, crowded together over the tendon of extensor indicis. All five tendons in this compartment are invested with a common synovial sheath.
  5. The next compartment lies over the radioulnar joint and transmits the tendon of extensor digiti minimi in a synovial sheath. 6. Lastly, the groove near the base of the ulnar styloid transmits the tendon of extensor carpi ulnaris in its synovial sheath.