Primary Lab Objectives:
- Skin extensor surface of forearm and hand. Identify brachioradialis and the superficial group of extensors.
- 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.
- Identify the medial muscles in the superficial group.
- Identify the extensor retinaculum at the wrist and note how it keeps the tendons of the extensor muscles from “bowstringing”. Consider this is in respect to the flexor retinaculum.
- 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."
- Trace the course of the posterior interosseus artery through the extensor compartment and supinator muscle.
- 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 feed into it.
There are 11 muscles in the extensor compartment of the forearm:
- "Superficial" group:
- Extensor carpi radialis longus (ECR longus)
- Extensor carpi radialis brevis (ECR brevis)
- Extensor carpi ulnaris (ECU)
- Extensor digitorum
- Extensor digiti minimi
- "Deep" group:
- Abductor pollicis longus
- Extensor pollicis longus
- Extensor pollicis brevis
- Extensor indicis
The distinction of these extensor muscles (forearm elevators) as to "superficial" (1-6) and "deep" (7-11) 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 superficial group is further divisible into lateral and medial groups. Between these two groups, the deep layer sends tendons of insertion out toward the thumb. The radial nerve supplies the extensor compartments of both the arm and forearm. Recall that in the previous lab, you located the radial nerve at the elbow and found its deep and superficial branches.
1. SUPERFICIAL LATERAL EXTENSORS OF THE FOREARM
- Skin the dorsal surface of the forearm and hand.
- Break through the deep fascia and separate the superficial layer, beginning with the lateral group.
- From the lateral edge of the 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 compartment.
- Compare their attachments and actions with those of the single radial flexor of the wrist (flexor carpi radialis).
2. SUPERFICIAL AND DEEP BRANCHES OF THE RADIAL NERVE
- Lift brachioradialis up at the elbow and again expose the radial nerve splitting into superficial and deep branches.
- Follow the deep branch until it disappears into a muscle. This is the supinator muscle, which will be considered later along with the deep extensors.
- Follow the superficial branch of the radial nerve to the wrist.
Atlas Images (note, the first image is as viewed from the flexor surface):
3. SUPERFICIAL MEDIAL EXTENSORS OF THE FOREARM
- Separate the superficial medial extensor group.
- Locate the most medial of the three muscles in this group: the ulnar extensor of the wrist, extensor carpi ulnaris.
- Verify that its attachment mirrors that of the ulnar flexor.
- Identify the most lateral muscle in this group, extensor digitorum.
- In between these two muscles is a separate slip known as the extensor digiti minimi.
4. EXTENSOR RETINACULUM
- Identify the extensor retinaculum.
The deep fascia over the wrist is specialized to form an extensor retinaculum. Under this retinaculum passes the extensor tendons on their way to the digits. One tendon passes to each of the four fingers (excluding the thumb). As you just saw, there is an additional tendon to the little finger, with its own independent belly, the extensor digiti minimi. However, tendinous bands unite these five tendons so they cannot work entirely independently.
5. EXTRINSIC MUSCLES OF THE THUMB and the ANATOMICAL SNUFFBOX
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.
- Locate the abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus. These muscles are found in the mid-forearm between the lateral and medial groups of superficial extensors.
- Follow these muscles down to their insertions on the thumb.
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.
- Locate your “anatomical snuffbox” by extending and abducting your thumb.
- With your forefinger, feel for the radial pulse here.
- Now trace the radial artery through the cadaver's anatomical snuffbox.
- Lift the medial group of superficial extensors up and locate the extensor indicis and supinator. The muscle belly of extensor indicis will be found medial to extensor pollicis longus. It gives the index finger added independence from the medial three fingers. Supinator will be found closer to the elbow with the muscle belly lateral to abductor pollicis longus.
6. POSTERIOR INTEROSSEOUS ARTERY
- Find the origin of the posterior interosseous artery and trace it back to where it entered the extensor compartment (i.e., around the superior free edge of the interosseous membrane).
- Look for the branches of the posterior interosseous vessels and nerve piercing the supinator to supply the extensor compartment.
In the flexor compartment of the forearm, we saw that the flexor digitorum superficialis combined elements of the superficial and deep layers. The same is true of the supinator in the extensor compartment. Part of this muscle is a superficial bundle coming off the humerus (the equivalent of pronator teres) and part is a deep sheet stretching between ulna and radius (vaguely resembling pronator quadratus). The two origins of the supinator are fused at their insertion.
Between the superficial and deep parts of the supinator, the radial nerve passes back into the extensor compartment of the forearm. It sends motor fibers to the lateral group of superficial extensors before it pierces the supinator, and then to the other extensors after it reaches the posterior compartment (deep branch or posterior interosseous). The superficial branch of the radial nerve running down the forearm under the brachioradialis is entirely a cutaneous sensory nerve to the hand.
The medial elements of the superficial layer of the elevators 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.
7. EXTENSOR RETINACULUM REVISTED and DORSUM OF THE HAND
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).
- Consult a skeleton and note the grooves in the back of the radius, which correspond to various extensor tendons.
- Demonstrate on an articulated skeleton that a tight extensor retinaculum would restrict pronation.
- Confirm that the extensor retinaculum does not attach to the ulna.
- Sever the extensor retinaculum and trace the extensor tendons onto the back of the hand.
8. DORSAL INTEROSSEOUS MUSCLES
- Dissect the dorsum of the hand to expose the dorsal interosseous muscles.
- 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.
- Dissect the extensor 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.
- Students with orthopedic ambitions may attempt to distinguish the retinacular ligament (part of the lateral band). This ligament ensures that when the proximal interphalangeal joint is extended (by extensor digitorum), the distal joint extends, and it also produces flexion in the proximal joint when the distal joint is flexed (by flexor digitorum profundus).