18 Elbow and Wrist Joint | Muscles of the Posterior Arm
Learning Objectives:
- Identify and describe the components of the elbow joint
- Identify and describe the joints of the forearm and wrist.
- Identify and describe the muscles of the posterior forearm.
Terms to Know
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Elbow Joint
Wrist and Hand Joints
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Muscles of the Posterior Forearm
Other Terms
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Introduction
In this lab, you will review the elbow and wrist joints and the muscles of the posterior forearm. The muscles of the posterior compartment of the forearm generally contain muscles that extend the wrist. Most of these extensors originate from the area of the lateral epicondyle of the humerus. Palpate your lateral epicondyle and move your fingers to the mass of muscle just distal to it. Extend your wrist, and you should feel this extensor mass contract. The posterior compartment also contains a muscle that abducts the thumb, as well as the supinator muscle. The supinator muscle works with the biceps brachii to supinate the hand. Supination makes the palm face forward in anatomical position. You can remember the position of a supinated hand by thinking that, in the supinated position, you can hold a cup of soup.
Lab Activities
Activity 1: The Elbow and Wrist Joints
Examine the elbow joint. The elbow is actually composed of three separate joints: the humeroulnar joint between the humerus and ulna, the humeroradial joint between the humerus and radius, and the proximal radioulnar joint between the radius and ulna proximally.
- Observe the annular ligament. This ligament surrounds the head of the radius, holding it tight to the ulna while still allowing it to rotate on the ulna with pronation and supination.
- Observe the radial collateral ligament and ulnar collateral ligament. They are located on the radial and ulnar sides of the humeroulnar joint, respectively, and they function to prevent abduction and adduction at the joint. Often these ligaments are blended into the surrounding capsule.
Examine the wrist joint in the videos on canvas and using atlas resources. Feel these joints as you move your own wrist.
- The distal radioulnar joint is the distal articulation between the radius and ulna. The ulna is smaller distally while the radius is larger.
- The radiocarpal joint is between the radius and carpal bones. The ulna does not articulate directly with the carpals.
- The midcarpal and intercarpal joints are between the bones of the wrist, with the midcarpal joint between the proximal and distal rows of carpals.
- The carpometacarpal joints are between the distal carpals and the metacarpals.
Activity 2: Muscles of the Posterior Forearm
Observe the muscles of the posterior forearm. The names of these muscles tell a lot about their location and/or function. They tend to extend the wrist or digits, and most originate on the lateral epicondyle of the humerus. This compartment can be divided into superficial and deep layers, each with five muscles. We won’t ask you which layer a muscle is located in. However, sometimes breaking the muscles into smaller groups can be helpful for learning.
Superficial Layer:
- On the radial side of the forearm, observe the extensor carpi radialis longus and extensor carpi radialis brevis muscles. Don’t confuse these with the brachioradialis, which sits partially superficial to these. The brachioradialis has an origin proximal to the lateral epicondyle, while these muscles originate on or near the lateral epicondyle. Both extensor carpi radialis longus and brevis cross the wrist and act to extend it. The difference between the longus and brevis muscles is in the tendon length and position. Extensor carpi radialis longus has a longer tendon and is just superficial to the extensor carpi radialis brevis. These muscles also assist with radial deviation (abduction of the wrist).
- Observe the extensor carpi ulnaris (only one on this side) on the ulnar side of the wrist. It extends the wrist and assists with ulnar deviation (adduction of the wrist).
- Observe the extensor digitorum muscle. It extends to the distal phalanx of digits 2-5 and extends the MCP, DIP, and PIP of these digits. Note that there is only one extensor digitorum muscle, so there is no other descriptor in the name (e.g., superficialis, profundus, brevis, or longus. Observe the small, angled dense connective tissue connecting the tendons of the extensor digitorum in the hand. This is called the juncturae tendinae. It functions to coordinate and distribute force across the extensor tendons. Try extending your ring finger (4th digit) while keeping the other digits in a fist. You are not able to extend that finger without moving the others because of the juncturae tendinae. However, you can extend your 5th digit much farther while the others are flexed. That is because of the extensor digiti minimi muscle.
- Observe the extensor digiti minimi. This muscle acts to extend the fifth digit. In this tissue, it appears that this muscle has blended with the extensor digitorum muscle.
Deep Layer:
- You may also notice that you can extend your 2nd digit (pointer finger) while flexing your other digits. That is because another muscle extends this digit: extensor indices. Observe the extensor indices muscle and its tendon extending to the digit 2.
- Another deep muscle of the posterior forearm is the supinator, which acts to supinate the forearm. This muscle is deep and more difficult to find, but it can be observed running from the lateral epicondyle to the shaft of the radius.
- The other muscles of the deep posterior forearm act on the thumb. Abductor pollicis longus acts to abduct the thumb, while extensor pollicis brevis and extensor pollicis longus act to extend the joints of the thumb. The extensor pollicis brevis tendon sits between the abductor pollicis longus and extensor pollicis longus tendons.
- Observe the dense connective tissue running over the extensor tendons. This is called the extensor retinaculum. This structure holds the extensor tendons in place and provides a mechanical advantage with extension.
Wrap up and clinical connection
The clinical connection for today involves looking at two unique elbow injuries. Elbow dislocations are rare. However, most elbow dislocations are posterior dislocations. What about the anatomy of the elbow makes it more likely to dislocate posteriorly? You may have also heard a lot about “Tommy John” surgeries and procedures in throwing athletes. What ligament is injured in the elbow leading to surgical repairs? Why is this ligament commonly injured in overhead and throwing athletes?
