Lab 12 Pre-Lab Exercise:
Note: If possible you should have the material from your bone box or that available in the model room with you when you use this pre-lab material.
1. Radius and ulna
There are two primary bones of the forearm (remember the arm is the section between the shoulder and elbow and the forearm is the section between the elbow and wrist): The radius (on the lateral side that articulates with the wrist on the thumb side of the hand) and the ulna (on the medial side).
The radius lies on the lateral (thumb) side of the forearm. Proximally, the radial head articulates with the capitulum of the humerus. Distally, the radius articulates with the scaphoid and lunate (two carpal bones discussed later).The radius is distinctive in having a concave head at the proximal end that allows a wide angle of rotation at the humeroradial joint, where its fossa articulates with the capitulum.
- Identify the following features of the radius:
- radial head
- radial tuberosity
- styloid process
The ulna lies on the medial side of the forearm. Proximally, it articulates with the trochlea of the humerus. The proximal ulna is marked by the olecranon process (where the triceps muscles insert and what you use to "elbow" an opponent in basketball) and by the trochlear notch (where the ulna articulates with the trochlea of the humerus in a hinge joint that, unlike the radius, allows almost no rotation). Therefore the elbow joint, which consists of the humeroradial and humeroulnar joints, flexes and extends at its articulation with the ulna and rotates at its articulation with the radius, the latter providing the mechanism for pronation in which the hand is flipped palm downward. The pointed projection at the distal end is called the styloid process and is attached to the radius by a band of fibrocartilage.
- Identify the following features on an ulna:
- olecranon process
- trochlear notch
- radial notch
- coronoid process
- styloid process
- Observe the medial (flexor) and lateral (extensor) epicondyles of your own humerus. Feel the stringy ulnar nerve as it passes superficially between the medial epicondyle and olecranon process. The tingly sensation of hitting your "funny bone" occurs as a result of banging your ulnar nerve here. At the wrist, palpate the ulnar styloid process and the distal end of the radius. Locate the pisiform bone, a sesamoid carpal bone protecting the ulnar artery and nerve as they enter the hand.
- When you rotate your hand from palm ventrally (as in anatomical position) to palm facing dorsally you have moved your hand from a supinated position to a pronated one. This movement of supination to pronation occurs when the distal head of the radius rotates on the capitulum , the bone rotates along its long axis, and the proximal head of the radius flips over the ulna. With the upper limb in anatomical position (i.e., forearm fully supinated), feel the positions of the shafts of the radius (along the lateral surface of the forearm) and the ulna (along its medial surface). Now pronate the forearm while following the movements of the radius. Notice how the radius rotates across the ulna (realize that this rotation takes place between the heads of the radius and ulna). Look at an articulated skeleton and examine the movements of the radius during supination and pronation.
- Surface anatomy: Near the cubital fossa of your arm, attempt to locate the basilic vein, the cephalic vein, and the median cubital vein (commonly used for venipuncture). Realize that superficial veins are commonly variable - the median cubital vein may form a more direct connection between the cephalic and basilic veins.
There are 8 carpal bones (small irregular bones) arranged in two rows: a proximal row and a distal row, with four bones each. Thus there is a radiocarpal and an ulnocarpal joint, midcarpal joints (between the carpals in each) , and carpometacarpal joints (between the carpals and the palm).
2a. Proximal Row: The carpal bones are named for their shapes. From lateral to medial, the bones which make up the proximal carpal row are:
- the scaphoid (resembles a boat),
- the lunate (resembles a crescent moon),
- the triquetral (with three articular facets),
- the pisiform (pea-shaped), a very unassuming little round bone which is unique in that it articulates with only one other carpal bone (the triquetral).
2b. Distal row: The bones of the distal carpal row are (again from lateral to medial):
- the trapezium (four-sided), which has a large saddle-shaped facet on its distal surface for articulation with the first metacarpal,
- the trapezoid (also four-sided), slightly smaller than the trapezium and basically irregular in shape,
- the capitate (with a rounded head), the largest carpal bone, which occupies a central position in the distal carpal row,
- the hamate (hook-shaped), with its distinctive hamulus process.
The palm of the hand is made up of 5 metacarpals. Each metacarpal bone consists of a base, a shaft, and a head. Proximally, each articulates with the distal carpal row. Distally, each articulates with a proximal phalanx. The proximal ends of the metacarpals tend to have a concave or flat articular surface, while the distal ends have a convex articular surface. They are numbered I through V, starting with the lateral bone.
There are two phalanges in the thumb and three phalanges in digits II-V for a total of 14 phalanges on each hand.
4a. Proximal phalanges: The first row of five phalanges, the proximal row, articulates proximally with the distal ends of the metacarpals. The proximal articular surface of a proximal phalanx is concave, and fits with the convex head of the metacarpal. Distally, the proximal phalanges articulate with the middle phalanges (except in the thumb, where there is no middle phalanx). The distal articular surface is flattened mediolaterally, with a slight depression in the center. This gives the distal articular surface the appearance of having two "lobes".
4b. Middle phalanges: The middle four phalanges (the thumb has no middle phalanx) articulate proximally with the proximal phalanges, and distally with the distal phalanges. The proximal
articular surface is generally concave, with the concavity "divided" to articulate with the two lobed distal articular surface of the proximal phalanges. The distal articular surfaces are very similar in appearance to those of the proximal phalanges.
4c. Distal phalanges: The five distal phalanges articulate proximally with the middle phalanges.
The articular surface is similar to the proximal articular surfaces of the middle phalanges. Distally, the distal phalanges are marked by apical tufts.
5. Bony landmarks of the forearm and hand
Humerus: the medial epicondyle projects posteriorly and medially as a hard knob-like structure in the region of the elbow. The lateral epicondyle is less prominent than the medial epicondyle. When the elbow is fully extended it can be felt and when the elbow is partially flexed it comes into view.
Ulna: proximally the olecranon process forms a prominence on the dorsum of the elbow between the two epicondyles. The medial and lateral epicondyles and the tip of the olecranon process lie upon the same horizontal plane when the elbow joint is fully extended. When the elbow is flexed to a right angle, the 3 bony points mark the positions of the angles of an equilateral triangle. The posterior border of the ulna can be palpated throughout the length of the bone. Both the head of the ulna and the styloid process can be palpated on the posteromedial aspect of the wrist: the head is a rounded subcutaneous prominence and the styloid process projects distally from the head of the ulna.
Radius: the head of the radius is found proximally and rotates during pronation and supination. It can be palpated posteriorly just below the lateral epicondyle. The styloid process can be palpated on the lateral aspect of the wrist. Note that the tip of the radial styloid process extends more distally and lies on a more anterior place than the ulnar styloid (therefore more ulnar deviation than radial deviation is possible at the wrist joint).
Carpals: the tuberosity of the scaphoid is felt at the base of the thenar eminence at a point which marks the junction of the lateral 1/3 with the medial 2/3 of the distal flexor crease at the wrist. Pressure in the floor of the anatomical snuffbox is used to detect the tenderness caused by a fracture of the scaphoid. The pisiform is felt at the base of the hypothenar eminence immediately deep to the medial end of the distal flexor crease of the wrist. The hook of the hamate is felt on deep pressure immediately distal and lateral to the pisiform. The lunate occupies the mid position in the proximal row of carpals and lies between proximal and distal flexor skin creases. Immediately anterior to the lunate is the median nerve.
Metacarpals: the metacarpals heads stand out as the prominent knuckles when the fingers are flexed. The metacarpal bases are felt as an indistinct transverse row of bony knobs, 2-3 cm distal to the distal end of the radius. The posterior border of the metacarpals can be palpated throughout the length of the bones.
Phalanges: the proximal and intermediate phalanges can be palpated dorsally throughout the length of the entire bones. The proximal and distal ends of the phalanges can be palpated ventrally (palm).
6. Muscles and tendons of the forearm and the hand
Brachioradialis: is visible a muscular bulge on the lateral aspect of the arm and forearm. It is especially apparent when the elbow is flexed to 90° against resistance with the forearm in mid position between supination and pronation. The radial nerve in the distal arm emerges from behind the humerus to lie deep to this muscle. The brachioradialis muscle forms the lateral border of the cubital fossa.
Pronator teres: from the medial epicondyle to the radius. It forms the medial border of the cubital fossa. Pronating the forearm against resistance can test this muscle.
Tendons that pass from the forearm into the hand are readily visible in the distal forearm and can be used as landmarks to locate major vessels and nerves.
Flexor tendons: can be found on the anterior aspect of the distal forearm. The tendon of the flexor carpi radialis can be felt and seen laterally on the forearm when the wrist is abducted and flexed against resistance. The radial artery is immediately lateral to this tendon. The palmaris longus tendon (if present) can be felt and seen at the midline of the forearm when the wrist is flexed against resistance. The flexor carpi ulnaris tendon (which insert on the pisiform) can be seen and felt medially on the forearm when the wrist is adducted and flexed against resistance. The ulnar artery and nerve travel under the lateral lip of this tendon and lateral to the pisiform. If the fingers are alternatively flexed and extended rapidly, the tendons of flexor digitorum superficialis can be seen and felt deep to and on either side of the tendon of palmaris longus. The median nerve lies under the palmaris longus and superficial to the flexor digitorum superficialis muscle.
Extensor tendons: can be found on the posterior aspect of the distal forearm. If the hand is made into a fist and the wrist extended against resistance, the tendons of extensor carpi radialis longus, extensor carpi radialis brevis and extensor carpi ulnaris can be felt just proximal to the bases of the 2nd, 3rd, and 5th metacarpals respectively. Extensor carpi radialis longus and brevis are on the lateral side of the wrist. Extensor carpi ulnaris is on the medial side between the wrist and the distal end of the ulna. If the fingers are extended, the tendons of the extensor digitorum become very prominent on the dorsum of the hand.
Adductor pollicis: can be palpated in the 1st intermetacarpal space when the thumb is adducted against resistance.
When the thumb is abducted and extended, palpate the tendons of the abductor pollicis longus and extensor pollicis brevis. Note that these tendons form the lateral boundary of the anatomical snuffbox. Locate and palpate the tendon of the extensor pollicis longus and note that it forms the medial boundary of the anatomical snuffbox. The radial artery passes through the anatomical snuffbox and the cephalic vein crosses the roof of the anatomical snuffbox.
First dorsal interosseous: is easily palpates in the 1st intermetacarpal space between the 1st and 2nd metacarpals when the index finger is abducted against resistance.
Thenar and hypothenar eminences: a thenar eminence occurs at the base of the thumb and is formed by the underlying thenar muscles. A similar hypothenar eminence occurs along the medial margin of the palm at the base of the little finger. Palpate the thenar eminence laterally and the hypothenar eminence medially.
7. Visualizing the cubital fossa and its content
The base of the cubital fossa is an imaginary line between the medial and lateral epicondyles of the humerus. The brachioradialis muscle forms the lateral border while the pronator teres muscle forms the medial border. The margin of the brachioradialis can be found by flexing a semi-pronated forearm against resistance. The margin of the pronator teres can be estimated by an oblique line extending between the medial epicondyle and the midpoint along the length of the lateral surface of the forearm. The approximate apex of the cubital fossa is where this line meets the margin of the brachioradialis (see image above). Contents from the cubital fossa from lateral to medial are the tendon of biceps brachii, the brachial artery, and the median nerve. The tendon of the biceps brachii is easily palpable.
Often the cephalic, basilic, and median cubital veins are visible in the subcutaneous fascia overlying the cubital fossa. The ulnar nerve passes behind the medial epicondyle of the humerus. The radial nerve travels into the forearm deep to the margin of the brachioradialis muscle, anterior to the elbow joint.