ARM, CUBITAL FOSSA, EXTENSOR FOREARM, AND DORSUM OF HAND (Grant's Dissector (16th Ed.) pp. 39-44; 59-65) TODAY’S GOALS: 1. Identify the main superficial veins (cephalic, basilic, median cubital) and cutaneous nerves (lateral and medial cutaneous nerves of the forearm and superficial radial nerve) of the arm and forearm. 2. Identify the compartments, muscles, nerves, and vessels in the arm. 3. Define the boundaries and identify the contents of the cubital fossa. 4. Identify the relevant skeletal anatomy of the distal humerus and proximal radius and ulna (Dissector p. 40, Fig. 2.17). 5. Review the dorsal venous arch on the dorsum of the hand and the origins of the cephalic and basilic veins. 6. Identify the extensor retinaculum and dissect the muscles and tendons of the extensor compartment of the forearm. 7. Trace the course of the radial nerve and its branches in the forearm and hand. 8. Expose the blood supply to the extensor compartment of the forearm. 9. Define and dissect the “anatomical snuff box” and recognize structures in its floor.
DISSECTION NOTES: Reflect skin from the arm to the hand (Dissector pp. 29, 30; Figs. 2.5B and 2.6). Before removing superficial fascia, identify the superficial veins, beginning at the dorsum of the hand: dorsal venous arch, cephalic vein, and basilic vein. In the cubital fossa, locate the median cubital vein. Also, locate cutaneous branches of the lateral cutaneous nerve of the forearm and the superficial branch of the radial nerve. Observe the brachial fascia (deep fascia of the arm) and the medial and lateral intermuscular septa that attach to the respective sides of the humerus to create anterior and posterior compartments of the arm (Dissector p. 39; Fig. 2.16).
Arm Posterior (extensor) compartment of the arm. (Dissector p. 26, Fig. 2.3). This should be a review from the Shoulder and Scapular Region lab. Identify: 1. Triceps brachii. The single extensor muscle in the arm. It consists of three heads: lateral head, long head, and medial head that unite to form a strong tendon that inserts into the olecranon of the ulna.
2. Radial nerve and profunda (deep) brachial artery descending in the triangular interval between the lateral and long heads and within the spiral or radial groove of the humerus. (Q: What is the nerve supply to muscles of the posterior compartment of the arm?)
Anterior (flexor) compartment of the arm. (Dissector p. 41, Fig. 2.18). 1. Coracobrachialis. Observe the musculocutaneous nerve piercing this muscle. Find it after it emerges from this muscle and follow its diagonal course in a connective tissue plane between the biceps brachii and brachialis muscles to the cubital fossa where it emerges along the lateral border of the biceps brachii. Here it continues into the forearm as the lateral cutaneous nerve of the forearm (in company with the cephalic vein). (Q: What are the attachment sites and actions of the coracobrachialis?)
2. Biceps brachii. Identify the two heads of the biceps brachii; long head (originates at supraglenoid tubercle), short head (originates at coracoid process). (Q: What two other muscles attach to the coracoid process?) Identify the strong, cord-like tendon of the biceps, which inserts into the radial tuberosity. (Q: What are the actions of the biceps brachii?) Observe the bicipital aponeurosis, a broad sheet of connective tissue that leaves the medial aspect of the biceps tendon and blends into the deep fascia of the forearm (antebrachial fascia).
3. Brachialis. This broad, flat muscle lies deep to the biceps brachii. It arises from the anterior surface of the distal half of the humerus and crosses the elbow to insert into the coronoid process of the ulnar. It has a singular action: flexes the forearm. (Q: What is the nerve supply to muscles of the anterior compartment of the arm?)
4. Follow the musculocutaneous nerve from the lateral cord, through the coracobrachialis muscle, and through the anterior compartment of the arm. Note that the musculocutaneous nerve terminates as the lateral cutaneous nerve of the forearm (at this point sensory only).
5. Follow the medial cutaneous nerve of the forearm from its origin off the medial cord of the brachial plexus into the forearm. Note that it accompanies the basilic vein near the elbow. Next, follow the median nerve into the cubital fossa. It courses closely with the brachial artery and provides no branches in the arm. Identify the ulnar nerve where it originates from the medial cord of the brachial plexus and follow it into the arm. During its descent it passes through the medial intermuscular septum near the elbow and enters a groove on the posterior surface of the medial epicondyle of the humerus before entering the forearm.
6. Follow the brachial artery, the continuation of the axillary artery inferior to the lower border of teres major, to its bifurcation into the ulnar and radial arteries in the cubital fossa (Dissector p. 42, Fig. 2.19). Important branches of the brachial artery include the: profunda (deep) brachial artery, seen with the radial nerve in the triangular interval (Dissector p. 26, Fig. 2.3), and superior and inferior ulnar collateral arteries. Brachial veins (venae commitantes) accompany the brachial artery and its branches. Remove these as necessary.
Cubital Fossa (Dissector p. 42, Fig. 2.20) The triangular area on the anterior surface of the elbow bounded by: • An imaginary line between the medial and lateral epicondyles – superiorly • Pronator teres – medially • Brachioradialis – laterally Two muscles, the brachialis and supinator, form the floor of the cubital fossa, and its “roof” consists of deep fascia and the tent-like bicipital aponeurosis. The median cubital vein typically lies superficial to the deep fascia and is available for venipuncture. Divide the bicipital aponeurosis and reflect it medially to reveal the contents of the fossa.
Contents of the cubital fossa, from medial to lateral (Dissector p. 42, Fig. 2.20): Median nerve, brachial artery, tendon of the biceps brachii, and radial nerve (Median nerve- Brachial ArteryTendon-Radial nerve [MATR]) or TAN going lateral to medial without the radial nerve.
DISSECTION NOTES: Extensor Forearm Review the dorsal venous arch on the dorsum of the hand and the origins of the cephalic and basilic veins (Dissector p. 30, Fig. 2.6). Identify the antebrachial fascia covering the extensor compartment of the forearm and a special thickening of this fascia, the extensor retinaculum, at the distal forearm. Incise and remove the antebrachial fascia to identify the musculature. Tendons of the extensor compartment pass beneath the extensor retinaculum within synovial sheath-lined compartments (e.g., fibroosseous tunnels) enroute to bones of the hand. The tendons normally slide smoothly within these compartments and are prevented from “bowstringing” during muscle contraction.
The extensor muscles of the forearm can be divided into superficial and deep layers. Superficial Layer: Each muscle in this group has full or partial origin from the lateral epicondyle (via a common extensor tendon) and supracondylar ridge of humerus (Dissector p. 61, Fig. 2.37). These muscles generally extend the wrist and fingers. From lateral to medial they are the: 1. Brachioradialis. Attaches from lateral supracondylar ridge to lateral surface of distal radius. It functions as a flexor of the forearm in the mid-prone position. 2. Extensor carpi radialis longus. Attaches from lateral supracondylar ridge to the base of the 2nd metacarpal. 3. Extensor carpi radialis brevis. Attaches from lateral epicondyle to the base of the 3rd metacarpal. 4. Extensor digitorum. Attaches from the lateral epicondyle to the extensor expansions of the medial four digits (2-5) (Dissector pp. 61, 62; Figs. 2.37, 2.38). 5. Extensor digiti minimi. Attaches from the lateral epicondyle to the extensor expansion of the 5th digit. 6. Extensor carpi ulnaris. Attaches from the lateral epicondyle to the base of the 5th metacarpal.
Deep Layer: This group of muscles arise from the posterior surfaces of the radius, ulna, and interosseous membrane. These muscles generally supinate the hand, extend the index finger, and abduct and extend the thumb. From lateral to medial they are the: 1. Supinator. Attaches from proximal ulna to lateral surface of upper 1/3 of radius. Observe the deep radial nerve entering this muscle and exiting near its distal edge as the posterior interosseous nerve. 2. Abductor pollicis longus. Attaches from posterior surface of radius and interosseous membrane to base of 1st metacarpal. One of the “outcropping” muscles. It also serves as the anterior boundary of the anatomic snuff box. 3. Extensor pollicis brevis. Attaches from posterior surface of radius and interosseous membrane to the base of the proximal phalanx of the thumb. One of the “outcropping” muscles. It also serves as the anterior boundary of the anatomic snuff box. 4. Extensor pollicis longus. Attaches from posterior surface of ulna and interosseous membrane to the base of the distal phalanx of the thumb. One of the “outcropping” muscles. It also serves as the posterior boundary of the anatomic snuff box. 5. Extensor indicis. Attaches from the posterior surface of ulna and interosseous membrane to the extensor expansion of the index finger.
The anatomic “snuff box” is bounded by the tendons of abductor pollicis longus, extensor pollicis brevis and extensor pollicis longus. At the “floor” of this depression is the scaphoid bone and a portion of the radial artery (Dissector pp. 63, 64; Figs. 2.39, 2.40) from which a pulse can be felt.
The nerve and blood supply to the extensor compartment course in a connective tissue plane between the superficial and deep layers of muscle (Dissector p. 63, Fig. 2.39). Locate these by creating a space between the extensor carpi radialis brevis and extensor digitorum. Trace the radial nerve from its location in the cubital fossa. Identify its superficial (sensory) and deep branches (motor). The deep branches are motor to all the muscles of the extensor compartment, except the brachioradialis and extensor carpi radialis longus muscles. The deep branch changes its name to the posterior interosseous nerve once it emerges from the supinator. The posterior interosseous artery courses with the nerve. Review its origin and course from the common interosseous artery.
Dorsum of the Hand On the dorsum of the hand, observe the dorsal interosseous muscles (Dissector Fig. 2.40) filling the spaces between the metacarpal bones. There are four dorsal interosseous muscles, responsible for abducting the fingers. (The thumb and little finger have their own abductors). Note the large first dorsal interosseous muscle. The radial artery leaves the anatomic snuff box and passes between the two heads of this muscle to enter the deep palm of the hand where it forms the deep palmar arch.
PEER TEACHING GUIDE: Organize your narrative to minimally include: 1. A review of skin reflections, major superficial veins and cutaneous nerves of the arm and forearm. 2. A review of the relevant osteology of the distal humerus and proximal radius and ulna. 3. Demonstration of the compartments, muscles, nerves, and vessels in the arm. 4. Demonstration of the boundaries and contents of the cubital fossa. 5. Review of the dorsal venous arch and origins of the cephalic and basilic veins and distribution of superficial branches of the radial nerve. 6. Demonstration of the muscles of the extensor forearm and description of their distal attachments and actions. 7. Demonstration of the course of the radial nerve and its branches in the forearm and hand. 8. Demonstration of the course of the radial artery through the anatomic snuff box into the palm of the hand. 9. Demonstration of the dorsal interosseous muscles.