Pre-lab Exercise: Surface Anatomy of the Forearm and Hand
Primary Lab Objectives:
- Remove the skin of the anterior forearm. and hand to understand the results of damage to any of these nerves.
- Identify the muscles of the anterior (flexor) compartment of the forearm. Understand where they originate and how they work together. Trace the median and ulnar nerves as they cross the elbow and travel down the forearm.
- 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.
- Skin the palmar surface of the hand and identify the palmaris brevis muscle and the palmar aponeurosis.
- Identify the thenar and hypothenar muscles. 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.
2. 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 three layers: a superficial layer arising from the flexor (or ulnar, or medial) epicondyle of the humerus (numbers 1-4 above); an intermediate layer arising from the medial epicondyle and the radius (number 5 above); and a deep layer arising from the radius, ulna, or interosseous membrane (numbers 6-8 above). The tough interosseous membrane gives origin to the deep flexors on its ventral surface and to the deep extensors on its dorsal surface.
- Superficially, 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 bicipital aponeurosis to expose the muscles beneath it.
- Using a blunt probe, carefully break through the fascia enclosing the forearm flexors.
- Clean and separate the muscles of the superficial layer that originate from the medial epicondyle of the humerus. From lateral to medial, identify the pronator teres, flexor carpi radialis, palmaris longus (if present), and flexor carpi ulnaris.
- Trace the origin and insertion of these four muscles.
FUNCTIONAL ANATOMY: The pronator teres inserts on the radius. It pulls the radius toward the ulna 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 medial side of pronator teres. Its tendon inserts on the base of the second and third metacarpal.
The flexor carpi ulnaris is the ulnar flexor of the wrist and is the most medial muscle of the superficial flexor layer. 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 sesamoid bone embedded in the tendon of the muscle. The flexor carpi ulnaris has an aponeurotic secondary origin from the medial edge of the ulna.
- Oppose your thumb and pinky 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 between flexor carpi ulnaris and flexor carpi radialis. 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.
- Cut 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 reconstruct the muscles later). Alternatively, if you possible, you can just push the superficial flexors to the side to see the deeper layers.
- Reflect the cut superficial flexors.
- The muscle exposed beneath the superficial flexors is the flexor digitorum superficialis, the one muscle in the intermediate layer.
- Explore the origins of the flexor digitorum superficialis from the humerus, ulna, and radius.
- Review the course of the ulnar nerve from the brachial plexus to the elbow. At the elbow, the ulnar nerve passes behind the medial epicondyle, then can be found medial to the flexor digitorum superficialis.
- Extend your dissection of this nerve down to the wrist.
- As you follow the ulnar nerve from the brachial plexus to the wrist, note any motor branches.
FUNCTIONAL ANATOMY: The ulnar nerve innervates only the flexor carpi ulnaris muscle and the medial half of flexor digitorum profundus in the forearm. Its main function is innervation of the muscles of the hand. The median nerve innervates the majority of the flexors in the forearm.
- Trace the median nerve from the brachial plexus, through the cubital fossa into the forearm near pronator teres. After passing through this muscle, follow it between the two heads of the flexor digitorum superficialis.
- Transect the humeroulnar and radial heads of flexor digitorum superficialis. Be careful not to cut the underlying nerves or vessels.
- Reflect the flexor digitorum superficialis 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.
- Cut the flexor digitorum profundus and flexor pollicis longus muscles at different levels above the wrist or push them to the side 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.
3. ULNAR, RADIAL, 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.
- Find the division of the brachial artery into the radial and ulnar arteries.
- Follow the ulnar artery distal to the point at which it crosses under the median nerve.
- Locate the common interosseous branch of the brachial artery.
- Continue to follow the ulnar artery down towards the wrist on the medial aspect of the forearm.
- Locate the radial artery where it branches off of the brachial artery, and follow that down on the lateral aspect of the forearm, running alongside the brachioradialis muscle.
4. PALMAR SURFACE AND PALMAR APONEUROSIS
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).
- Remove the skin of the wrist, palm, thumb, and 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. You should tie the hand to one of the boards provided. You definitely need to use a sharp scalpel for this task.
- As you remove the 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 medial aspect of the palm near the wrist.
- On either side of the long flexor tendons, notice that the palmar aponeurosis fuses with the fascia enclosing the thenar muscles and hypothenar muscles.
- Make a cut in the palmar aponeurosis near the base of the fingers.
- Reflect the palmar aponeurosis superiorly then then make another cut just inferior to the flexor retinaculum so that you can 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.
- Deep to the superficial palmar arch, locate the fibrous flexor tendon sheaths that wrap around the the long flexor tendons.
FUNCTIONAL ANATOMY: The arterial supply to the hand comes from the radial and ulnar arteries. These two arteries enter the palm and form two palmar arches. The superficial palmar arch is formed mainly by the ulnar artery with contributions from the radial artery and lies just under the palmar aponeurosis. The radial artery primarily supplies the deep palmar arch with contributions from the ulnar artery.
5. 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.
- Trace the flexor digitorum profundus and superficialis tendons to their respective insertions on the distal and middle phalanges of the finger.
- Examine how the superficial tendon splits at its insertion to allow the deep tendon to pass through it to the distal phalanx.
FUNCTIONAL ANATOMY: From each of the flexor digitorum profundus tendons, a lumbrical muscle originates. The lumbrical muscles are associated with the long flexor tendons, and insert onto the extensor hood of the finger. There are also 8 interosseous muscles belonging to the deep intrinsic musculature of the hand, 3 on the palmar side and 5 on the dorsal side. 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 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, flexor digiti minimi, 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.
6. DEEP PALM
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.
- Cut the flexor retinaculum.
- Carefully move the deep flexor tendons off to the side so that you can see the underlying structures. If you'd like, you can cut one or two of the deep flexor tendons, along with the associated lumbricals. The deep palmar arch is now exposed.
- Using a blunt probe, establish continuity of the deep palmar arch 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.
- Identify the adductor pollicis and the palmar and dorsal interossei muscles.
- 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).
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.