Lab 13 - Extensor compartment & hand

Pre-lab Exercise: Vessels of the Upper Limb

Primary Lab Objectives:

  1. Remove the skin on the extensor surface of forearm and hand.Identify brachioradialis and the superficial group of extensors.

  2. Lift the brachioradialis to expose the radial nerve splitting into deep and superficial branches. Verify that the radial nerve innervates all the muscles of the extensor group of the forearm. Note that brachioradialis is an exception because it operates as a flexor although its origin and innervation is that of an extensor.

  3. Identify the deeper group of extensors.  Three extensors go to the thumb and one to the index finger. Examine the radial artery running through the anatomical “snuffbox."

  4. Trace the course of the posterior interosseous artery through the extensor compartment.

  5. Identify the extensor retinaculum at the wrist and note how it keeps the tendons of the extensor muscles from “bowstringing”. Sever the extensor retinaculum to allow for exploration of the dorsum of the hand. Examine the dorsal interosseous muscles and trace the radial artery through the first dorsal interosseous to reach the deep palmar arch. Dissect the extensor expansion on the third digit. Understand how the lumbricals attach to it.


Dissection Instructions

There are 11 muscles in the extensor compartment of the forearm:

  1. Superficial group:
    1. Brachioradialis
    2. Extensor carpi radialis longus (ECR longus)
    3. Extensor carpi radialis brevis (ECR brevis)
    4. Extensor carpi ulnaris (ECU)
    5. Extensor digitorum
    6. Extensor digiti minimi
  2. Deep group:
    1. Abductor pollicis longus
    2. Extensor pollicis longus
    3. Extensor pollicis brevis
    4. Extensor indicis
    5. Supinator

The distinction of these extensor muscles as superficial or deep is much less precise than it was in the flexor compartment. Some consider there to be a single extensor compartment with digital and thumb muscles. The deep group comprises all the extensors of the thumb, plus a separate extensor for the index finger and a supinator that originates on the humerus.

The superficial group originates exclusively from the humerus and includes extensors to the four fingers, as well as the wrist extensors. The deep layer sends tendons of insertion out toward the thumb. The radial nerve supplies the extensor compartments of both the arm and forearm.



  1. Remove the skin from the dorsal surface of the forearm and hand.
  2. Break through the deep fascia and separate the muscles of the superficial layer.
  3. From the lateral edge of the distal humerus, follow the brachioradialis and the two radial extensors of the wrist (extensor carpi radialis longus and brevis) running along the radial edge of the flexor (anterior) compartment.

Atlas Images:

  1. Check to see if your cadaver has an identifiable anconeus muscle near the elbow joint.
  2. Identify the extensor digitorum with its four tendons heading to digits II-V.
  3. Locate the most medial muscle in this group: extensor carpi ulnaris.
  4. In between the extensor digitorum and the extensor carpi ulnaris a separate slip known as the extensor digiti minimi.
  5. Once you have identified all of the superficial extensor muscles, make staggered cuts through these muscles so that you can find the deeper structures.



Supinator has two heads - a superficial and deep. The superficial head originates from the lateral epicondyle of the humerus, and the deep head from the radius and ulna. Between the superficial and deep parts of the supinator, the radial nerve passes back into the extensor compartment of the forearm. Before it pierces supinator, the radial nerve sends motor fibers to the lateral group of superficial extensors, then to the other extensors after it reaches the posterior compartment (deep branch or posterior interosseous nerve). The superficial branch of the radial nerve running down the forearm under the brachioradialis is entirely a cutaneous sensory nerve to the hand.

  1. Lift brachioradialis up at the elbow and expose the radial nerve splitting into superficial and deep branches.
  2. Follow the deep branch (posterior interosseous nerve) until it disappears into the supinator muscle. Supinator will be found close to the elbow with the muscle belly lateral to abductor pollicis longus.
  3. Follow the superficial branch of the radial nerve to the wrist.

Atlas Images (note, the first image is as viewed from the flexor surface):



The deep layer of extensors includes muscles that originate from the radius, ulna, and the interosseous membrane – like the deep flexor layer on the other side of the forearm. Three of these are thumb muscles: an abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus, which insert in that order on the bases of the three bones of the thumb.

  1. Locate the extensor indicis arising from the ulna and heading to the index finger (digit II). It gives the index finger added independence from the medial three fingers.
  2. Locate the abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus. These muscles arise in the mid-forearm and insert on the thumb. 

Atlas Images:

Extensor pollicis longus passes through a different compartment of the extensor retinaculum producing a hollow called the “anatomical snuffbox” between its tendon and those of the other two thumb elevator muscles. The radial artery appears in this hollow between the tendons.

  1. Locate your “anatomical snuffbox” by extending and abducting your thumb.
  2. With your forefinger, feel for the radial pulse here.
  3. Now trace the radial artery through the cadaver's anatomical snuffbox.



  1. Find the origin of the posterior interosseous artery (branching from the common interosseous artery of the ulnar artery) and trace it back to where it enters the extensor compartment (i.e., around the superior free edge of the interosseous membrane).
  2. Look for the branches of the posterior interosseous vessels piercing the supinator to supply the extensor compartment.

Atlas Images:

The medial elements of the superficial layer of the extensors receive some arterial supply from branches of the radial artery near the epicondyle, which you saw previously accompanying the deep radial branch through the supinator. Additionally, arterial supply for the extensor compartment comes from the posterior interosseous branch of the ulnar artery.



The extensor retinaculum overlies the extensor tendons over the back of the wrist. The deeper fibers of the retinaculum are bound to grooves on the radius, thus producing multiple fibrous tunnels lined with tubular synovial sheaths (overlying each tendon).

  1. Consult a skeleton and note the grooves in the back of the radius, which correspond to various extensor tendons.
  2. Demonstrate on an articulated skeleton that a tight extensor retinaculum of the wrist would restrict pronation.
  3. Confirm that the extensor retinaculum does not attach to the ulna.
  4. Identify the extensor retinaculum on your cadaver. Then cut the extensor retinaculum and trace the extensor tendons onto the back of the hand.

Atlas Images:

The deep fascia over the wrist is specialized to form an extensor retinaculum. Under this retinaculum passes the tendons of the extensor digitorum, extensor indicis, and extensor digiti minimi (among others inserting on the thumb) on their way to the digits. The extensor digitorum passes one tendon to each of the found fingers (excluding the thumb) while the extensor digiti minimi passes a single tendon to digit V (pinky) and the extensor indicis passes a single tendon to digit II (index finger). However, in the dorsum of the hand, the tendinous bands of the extensor digitorum communicate between adjacent digits and with the tendon of the extensor digiti minimi, while the extensor indicis tendon remains independent. Make a loose fist and prress your intermediate (middle) phalanges flush onto a tabletop. Now attempt to extend each digit individually. You will notice that while you are capable of fully extending digits II and V, you will not be able to independently extend digits III and IV. This is because digits II and V have extensor muscles exclusively dedicated to them, while digits III and IV are extended only with extensor digitorum whose action can be restricted by pinning down some or all of the non-target digits.


  1. Dissect the dorsum of the hand to expose the dorsal interosseous muscles.
  2. Note the course of the radial artery. It traverses through the anatomical snuffbox, into the dorsum of the hand, where it then pierces through the first dorsal interosseus muscle, to anatomose with the deep palmar arch on the palm of the hand.
  3. Dissect the extensor hood (expansion) on the back of the third digit. Note that the lumbricals, which are on the palmar surface of the hand, insert into this extensor expansion.

Atlas Images:


Clinical Correlate

Tennis elbow is a condition in which overuse or strain of the extensor musculature causes pain and weakness from the lateral epicondyle. Symptoms include pain on the lateral epicondyle and weakness in the wrist when moving the wrist into extension, especially against resistance. The most common muscle to be affected by this is the extensor carpi radialis brevis muscle.




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Updated 11/04/18 -Zeininger