Pre-lab Exercise: Surface Anatomy of the Forearm and Hand
Primary Lab Objectives:
- Remove the skin of the anterior forearm.
- Trace the radial, median, and ulnar nerves as they cross the elbow and travel down the forearm and into the hand. Understand the role of each nerve in innervating specific parts of the forearm and hand to understand the results of damage.
- Identify the muscles of the anterior (flexor) compartment of the forearm. Understand where they originate and how they work together.
- Trace the ulnar and radial arteries through the forearm and into the hand. Locate the common, anterior and posterior interosseous branches of the ulnar artery.
- Transect and reflect the deep flexors to expose the pronator quadratus muscle. Understand its role along with pronator teres muscle.
- Skin the palmar surface of the hand and identify the palmaris brevis muscle.
- Identify the thenar and hypothenar muscles and find the superficial palmar arterial arch. Understand the innervation of these muscles and the results of damage to the nerves that innervate them.
- Identify the lumbrical muscles and the long flexor tendons and understand their functional relationship.
- Examine the deep muscles and structures of the hand.
1. SUPERFICIAL FASCIA OF THE FOREARM
- Make an incision from the elbow to just below the wrist.
- Using your fingers or a blunt probe, separate the skin and superficial fascia from the muscles, and reflect the skin to either side
- Inspect the skin of the forearm and superficial fascia noting any cutaneous veins and nerves.
2a. NERVES OF THE FOREARM-Median and Ulnar Nerve
- Trace the median nerve from the arm to the cubital fossa.
- Locate the ulnar nerve passing into the posterior (extensor) compartment of the arm.
- Continue tracing the ulnar nerve around the medial epicondyle of the humerus and into the ventral-medial portion of the forearm.
2b. NERVES OF THE FOREARM-Radial Nerve
NOTE: The radial nerve -a posterior division nerve- also wanders out of the dorsal compartment, briefly entering the flexor compartment at the elbow to supply the brachioradialis muscle. This muscle is located at the ventral edge of the extensor group originating laterally from the distal humerus.
- Probe between the brachialis muscle (a depressor) and brachioradialis muscle (an elevator).
- Find the radial nerve as it passes around into the anterior compartment.
- Follow the radial nerve past the elbow (under the brachioradialis) until you see it dividing into the deep and superficial branches.
3. SUPERFICIAL AND DEEP FOREARM FLEXORS
NOTE: There are eight muscles in the flexor compartment of the forearm:
- Flexor carpi ulnaris
- Palmaris longus
- Flexor carpi radialis
- Flexor digitorum superficialis
- Pronator teres
- Fexor digitorum profundus
- Flexor pollicis longus
- Pronator quadratus
These muscles are divisible into two layers (but some texts will refer to three or four layers): a superficial layer arising from the flexor (or ulnar, or medial) epicondyle of the humerus (I-V); and a deep layer arising from the forearm bones (radius and ulna) and the interosseous membrane (VI-VIII). The tough interosseous membrane gives origin to the deep flexors on its ventral surface and to the deep extensors on its dorsal surface.
- Begin cleaning and separating the muscles of the superficial layer that originate from the flexor epicondyle.
- Note the bicipital aponeurosis, a fibrous band through which some of the fibers of biceps brachii insert into the deep fascia of the forearm.
- Remove the fibrous band to expose the muscles underneath.
FUNCTIONAL ANATOMY: The deep fascia of the forearm is thickened into a dense fibrous band at the wrist. This band is the flexor retinaculum. It attaches to the scaphoid and trapezium laterally, and the pisiform and hamate medially creating the carpal tunnel. The tendons of the long flexors pass through this carpal tunnel (which is of clinical relevance), under the retinaculum, preventing them from bowstringing at the wrist when their muscle bellies contract.
NOTE – the flexor retinaculum is deep in the wrist, and it is very short in a mediolateral direction (2-3cm).
- Leave the flexor retinaculum intact as you break through the fascia enclosing the forearm flexors.
- Identify the four flexors that you now see fanning out from the medial epicondyle of the humerus: pronator teres, flexor carpi radialis, palmaris longus (sometimes absent), and flexor carpi ulnaris.
- Trace the origin and insertion of the flexor carpi radialis and ulnaris muscles in the cadaver using the descriptions below to guide your identification.
FUNCTIONAL ANATOMY: The pronator teres inserts on the radius. It pulls the radius over toward the flexor epicondyle and pronates the forearm. This is the only muscle of the superficial flexor layers that does not cross the wrist.
The flexor carpi radialis is the radial flexor of the wrist. It lies to the ulnar side of the pronator teres muscle. Its tendon inserts on the base of the second and third metacarpal.
The flexor carpi ulnaris is the ulnar flexor of the wrist and lies on the ulnar side of the superficial flexor sheet. It attaches to the base of the fifth metacarpal. It does so indirectly by inserting on the pisiform and pulling on the pisometacarpal ligament (a ligament connecting the pisiform to the base of the fifth metacarpal). The pisiform is a large sesamoid bone embedded in the tendon of the muscle. The flexor carpi ulnaris has a largely aponeurotic secondary origin from the medial edge of the ulna.
- Oppose your thumb and ring finger while slightly flexing your wrist.
- Look for the tendon of palmaris longus popping up in the center of your wrist.
- Examine the wrists of your group members for presence or absence of palmaris longus.
NOTE: The palmaris longus muscle inserts into palmar apponeurosis. It runs along between the two marginal muscles that flex the wrist. It is absent in about 10-15% of all people, and is variable in form. The tendon of palmaris longus (if present) is typically visible in the living body when the wrist is flexed against resistance.
- Transect the four superficial flexors that you identified earlier each at different levels (i.e. stagger the level at which each muscle is cut --this will make it easier to tell the cut ends apart).
- Reflect the cut superficial flexors.
- The muscle exposed is the flexor digitorum superficialis.
- Explore its origins from the humerus, ulna, and radius.
- Review the course of the ulnar nerve from the brachial plexus to the elbow.
- Extend your dissection of this nerve down to the wrist.
- As you follow the ulnar nerve from the plexus to the wrist, note its motor branches.
FUNCTIONAL ANATOMY: Ordinarily, the ulnar nerve innervates only the flexor carpi ulnaris muscle and the ulnar edge of flexor digitorum profundus in the forearm. Its main function is in the hand. The median nerve innervates the rest of the flexors in the forearm.
- Trace the median nerve down to the proximal edge of the flexor digitorum superficialis, where you will see it passing between the heads of that muscle.
- Transect the humeroulnar and radial heads of flexor digitorum superficialis. BE CAREFUL TO NOT CUT UNDERLYING NERVES AND ARTERIES!
- Reflect it distally to expose the flexor digitorum profundus and flexor pollicis longus.
- Locate the median nerve and the ulnar artery traversing the space between the deep and superficial digital flexors.
4. ULNAR AND INTEROSSEOUS ARTERIES
NOTE: Near the apex of the cubital fossa the brachial artery divides into the radial and ulnar arteries. Shortly after this division, the ulnar artery gives off the common interosseous artery
- Locate the brachial artery in the cubital fossa (between the biceps brachii tendon and the median nerve) and follow it down into the forearm.
- Follow the ulnar artery distal to the point at which it crosses under the median nerve.
- Locate here the common interosseous branch of the brachial artery.
HINT: You will be able to trace the common interosseous only a very short distance because it splits almost immediately into anterior and posterior interosseous arteries. The anterior interosseous nerve and artery supply the deep flexor sheet. They are branches of the median nerve and common interosseous branch of the ulnar artery, respectively.
- Study a diagram of the arterial anastomoses around the elbow.
5. PRONATOR QUADRATUS
- Identify the flexor digitorum profundus and flexor pollicis longus muscles.
- Transect them at different levels above the wrist.
- Reflect them to expose the pronator quadratus muscle.
- Examine the interosseous membrane where it lies exposed between the pronator quadratus and the deep digital flexor origins.
FUNCTIONAL ANATOMY: The final member of the deep flexor group, pronator quadratus is the most used pronator of the forearm. The more superficial pronator teres is brought into play against resistance, as when using a screwdriver.
6. PALMAR SURFACE
- Remove the skin of the palm, the thumb, and the second and third digits.
HINT: In the palm, the skin is attached to the deep fascia by fibrous bands, the superficial fascia is reduced to pockets of fat, and the skin is relatively immobile. This makes it quite difficult to skin. Therefore, you should tie the hand to one of the boards provided. You definitely need to use a sharp scalpel for this task.
- Cut the palmar digital tendons above the wrist to mobilize the digits.
- As you skin, note the cutaneous nerves and vessels running along each side of the flexor surface of every digit.
- Note also the small cutaneous muscle, palmaris brevis, which overlies the ball of the little finger.
7. PALMAR APONEUROSIS
- On either side of the long flexor tendons, notice that the palmar aponeurosis fuses with the fascias enclosing the thenar muscles and hypothenar muscles.
- Make a slit in the palmar aponeurosis near the base of the fingers between the central two long flexor tendons and push them apart.
FUNCTIONAL ANATOMY: Nine intrinsic muscles of the hand (8 interossei muscles and the adductor pollicis muscle) lie between the deep fascia and the dorsal surface of the hand. These short intrinsic muscles are principally concerned with abduction and adduction of the digits.
The heads of the four medial (ulnar-side) metacarpals are bound together by specialized fibers of the deep fascia, forming a fibrous band stretching across the palm. These deep ligaments do not bind the thumb. If it were bound, the thumb would not be divergent and opposable. The dorsal fibers of these ligamentous tubes condense into palmar ligaments over each of the joints. In between these ligamentous tubes, the ventral fibers condense into fibrous digital sheaths, which attach to the bone and hold down the long tendons.
- On a skeleton, look at the ventral sides of several phalanges and identify ridges on the radial and ulnar edges left by the fibrous sheaths.
- Remove the palmar aponeurosis, but leave the flexor retinaculum intact.
- Look at the superficial palmar arch of arteries.
- Trace its medial end back to the ulnar artery at the wrist.
- Examine its digital branches.
- Remove the superficial arch. The long flexor tendons are now exposed, wrapped in the synovial sheaths that lubricate their movement inside the fibrous flexor sheaths.
FUNCTIONAL ANATOMY: The arterial supply to the hand comes principally from the radial and ulnar arteries. These two arteries enter the palm and form two palmar arches. The superficial palmar arch is formed chiefly by the ulnar artery and lies just under the palmar aponeurosis. The radial artery primarily supplies the deep palmar arch.
8. PALM MUSCLES
- Cut open the fibrous flexor sheaths and dissect out the insertions of the long flexor tendons of the thumb and one other finger.
- Pull on these tendons and experiment with their functions.
- Verify that there is no superficial flexor tendon to the thumb (the proximal interphalangeal joint flexed by this tendon is not found in the thumb).
- Trace the flexor digitorum profundus and superficialis tendons to their respective insertions on the distal and middle phalanges.
- Examine how the superficial tendon splits at its insertion to allow the deep tendon to pass to the distal phalanx.
- Reflect the deep tendon distally, so that you can examine the insertion of flexor digitorum superficialis.
FUNCTIONAL ANATOMY: From each of the digital flexor profundus tendons, a lumbrical muscle originates. The lumbrical muscles are associated with the long flexor tendons, while the 8 interosseous muscles belong to the deep intrinsic musculature of the hand. This allows for the tendons of the lumbricals and interossei to insert on opposite sides of the deep ligaments. Blood supply to the deep intrinsic musculature of the hand is via the deep palmar arch, which is supplied mainly by the radial artery.
- Trace one of the lumbrical muscles around the radial side of its digit.
- Follow it to its insertion into the extensor hood and expansion.
- Correlate these attachments with a textbook description of its action (flex metacarpophalangeal joints while extending interphalangeal joints).
- Examine the thenar and hypothenar muscles in your cadaver.
- Separate abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis in the thenar group
- Separate abductor digiti minimi, fexor digiti minimi brevis, and opponens digiti minimi in the hypothenar group.
NOTE: Adductor pollicis (one of the deep intrinsic muscles of the hand) and flexor pollicis brevis attach to the sesamoids adjoining the first metacarpal head. The tendon of flexor pollicis longus lies between the two muscles.
FUNCTIONAL ANATOMY: There are three layers of deep intrinsic hand muscles:
- The adductor pollicis, which adducts the thumb.
- The four palmar interosseous muscles, which adduct the digits.
- The four dorsal interosseous muscles, which abduct the digits. The 3rd digit needs two abductors, one for each side. The dissection of the interosseus muscles is described below.
9. DEEP PALM
- Complete the removal of the superficial palmar arch.
- Sever the flexor retinaculum.
- Reflect the long flexor tendons along with the lumbricals. The deep palmar arch is now exposed.
- Using a blunt probe, establish continuity with the radial artery in the anatomical snuffbox. The radial artery is the main contributor to the deep arch.
- Note that the radial artery passes into the deep arch through the two heads of the first dorsal interosseous muscle.
- Study a diagram of the arterial supply to the hand and understand the contributions of the deep arch.
- Identify the adductor pollicis and the palmar and dorsal interossei muscles.
- You should also be able to find the deep branch of the ulnar nerve.
- Cut the deep ligament joining the second and third metacarpals.
- Trace the second palmar and dorsal interossei to their respective insertions. Each dorsal interosseous arises by two heads, one from each of the adjoining metacarpals.
- Trace the median nerve beneath the flexor retinaculum.
- Follow the median nerve into the palm, and notice that it breaks up into cutaneous branches to the lateral three and a half digits.
- Trace its motor branches to the first and second lumbricals (digital branch) and to the thenar muscles (recurrent branch).
- Locate the ulnar nerve in the forearm and trace it to the proximal edge of the flexor retinaculum. Here it divides into a superficial (cutaneous) branch and a deep branch.
- Follow the deep branch of the ulnar nerve into and through the hypothenar muscles (it innervates them as it goes by).
- Locate the deep branch as it emerges in the deep layer of the palm.
- Follow it across the palm and beneath the long flexor tendons (this portion innervates the remaining two lumbricals and all the deeper muscles of the palm).