Chapter 9 The Upper Limb
In the embryo, the limbs develop as outgrowths of the axial skeleton. The upper limb develops from body wall segments of the lower four cervical and first thoracic levels. Similarly, the lower limb develops from segments at the lumbosacral levels. As the limbs develop, they maintain the nerve and blood vessels of these levels, and this accounts for the axial source of nerve and blood supply to and from the limbs.
Functionally, the upper limb is designed for freedom of movement and prehension and is only loosely anchored to the axial skeleton. The lower limb, on the other hand, is engineered to bear the weight of the body during locomotion and standing and is thus rigidly attached to the axial skeleton. The upper limb comprises four components: the shoulder girdle, arm, forearm, and the hand.
The shoulder girdle, or pectoral girdle, consists of two bones: the scapula (shoulder blade) and the clavicle (collar bone) (Figure 9-1). The scapula is anchored to the posterosuperior surface of the thoracic cage by muscles. The clavicle is attached firmly to the manubrium of the sternum by the strong but movable sternoclavicular joint and to the scapula at the weaker acromioclavicular joint. The clavicle acts as a strut to keep the shoulders pointed laterally.
The scapula is a thin, triangular bone (Figure 9-2). Its concave anterior surface, is anchored by muscles to the posterior aspects of ribs 2 to 7. As a triangle, it possesses three sides: (1) a vertebral (medial) border that parallels the vertebral column, (2) an axillary (lateral) border that faces the axilla, and (3) a suprascapular (superior) border. The inferior angle is the apex of the triangle.
The clavicle is an elongated S-shaped bone that is subcutaneous and may be palpated over its entire length (Figure 9-3). The clavicle functions as a strut to prop the upper limb away from the body, affording the hand a greater range of positions, and transmits a portion of the weight of the upper limb through its sole articulation with the axial skeleton at the sternoclavicular joint.
The clavicle is the most commonly fractured bone in individuals less than 45 years old. A fall on the outstretched hand or a blow to the shoulder ultimately imparts its force to the clavicle. The sternoclavicular joint is generally strong enough to withstand the force, but the junction of the two curves of the clavicle is the weakest point and most vulnerable fracture site.
There are two long bones of the forearm, the radius and the ulna. In the anatomical position (palms facing forward or supine position), the radius is lateral and the ulna is medial. In the pronated position (palms facing posteriorly), the distal ends of the bones cross over, reversing their positions, and swing the distal radius to the medial side and the distal ulna to the lateral side.
Eight carpal bones make up the bones of the wrist (Figure 9-6). They are short, cuboidal bones and are arranged in two rows of four bones each to give a degree of flexibility of movement to the wrist. The bones are named rather fancifully by their shape.
The proximal row of bones articulates directly or indirectly with the distal ends of the radius and the ulna. From lateral to medial, they are the scaphoid (canoe-shaped), lunate (moon-shaped), triquetral (three-sided), and pisiform (pea-shaped) bones. The pisiform bone does not participate in the wrist joint.
Five miniature long bones, or metacarpals, form the skeleton of the palm of the hand. Each metacarpal features an expanded base that articulates superiorly with the distal row of carpals and provides attachments for muscles. A rounded head on the distal end of the shaft articulates inferiorly with the phalanges of the fingers.
The sternoclavicular joint is an articulation between the spherical medial end of the clavicle and the manubrium of the sternum. It is a relatively strong joint, reinforced by extracapsular ligaments. In addition, an intracapsular disc is interposed between the bones. The disc divides the joint space into two compartments with movements unique to each compartment. This not only adds to the range of movements of the joint, but also increases the strength of the joint by preventing the clavicle from being driven medially by excessive medial force.
Although there may be some independent movements of the pectoral girdle, such as shrugging of the shoulders, most occur as coordinated movements with those of the arm. In addition, the muscles of the pectoral girdle act to stabilize the girdle for some upper limb functions such as lifting heavy objects. For this reason, the pectoral girdle joints will be considered as a unit participating in the following movements. There are six movements possible at the pectoral girdle.
Depression is the lowering of the scapula and shoulder to the resting position through the pull of gravity. Forced depression is performed by the pectoralis major, pectoralis minor, latissimus dorsi, and subclavius muscles.
During protraction, the scapula is drawn upward and forward over the rib cage, placing the shoulders in a forward position. Muscles active in protraction include the levator scapulae, pectoralis major, pectoralis minor, and serratus anterior muscles.
Retraction is the resumption of the anatomical from the protracted position with the shoulders squarely back. The muscles that help retract the shoulders are the trapezius, the latissimus dorsi, and both rhomboid muscles.
Downward rotation produces the opposite effect. The glenoid fossa and the tip of the shoulder are pulled downward by the actions of the levator scapulae, rhomboids, latissimus dorsi, pectoralis major, and pectoralis minor muscles.
The glenohumeral, or shoulder, joint is an articulation between the spherical head of the humerus and the shallow depression of the glenoid fossa of the scapula (Figure 9-8). The shallow fossa is deepened only slightly by the glenoid labrum, a short fibrocartilage ring that encircles the rim of the fossa. The tendon of the long head of biceps passes through the cavity of the joint en route to the intertubercular sulcus of the humerus.
Figure 9-8 Anterior view of the right glenohumeral (shoulder) joint. The anterior capsule has been removed and the head of humerus displaced laterally to display the joint cavity. Note the shallow glenoid fossa.
The joint is a ball-and-socket configuration allowing three degrees of freedom and a considerable range of movement. Two factors contribute to this increased range: the shallow cuplike glenoid fossa and a fibrous articular capsule that is far more lax than those of other joints.
The shallow glenoid cavity and lax ligaments contribute to a greater range of shoulder movement but increase the risk of injury and dislocation. The head of the humerus is usually dislocated inferiorly below the glenoid fossa, where muscle support is weakest. The dislocated head may impinge on the axillary nerve, resulting in loss of sensation on the lateral aspect of the shoulder and atrophy of the deltoid muscle.
The elbow is a compound articulation between the distal end of the humerus and the proximal ends of the ulna and radius (Figures 9-10 and 9-11). The two sites of articulation take place between (1) the trochlea of the humerus and the trochlear notch of the ulna and (2) the capitulum of the humerus and the radial head of the radius. The joint capsule is lined by synovium and reinforced externally by radial (lateral) and ulnar (medial) collateral ligaments.
The elbow joint is basically a hinge-type joint allowing only two movements (Figure 9-12): flexion, or bending, is a movement that decreases the angle between the arm and forearm, and extension is a straightening movement that moves the limb back toward the anatomical position.
The head of the radius is shaped like a thick, bony disc. Its superior border has already been described as articulating with the capitulum of the humerus in the elbow joint. The side of the disc revolves within the radial notch of the ulna, a shallow depression that seems to have been carved out for it on the lateral aspect of the coronoid process of the ulna. The head is held in place by an annular ligament that surrounds the head of the radius and holds it within the notch.
The distal joint appears to be a reversal of the proximal joint. Here the distal end of the ulna is roughly disc-shaped, and it seems to have worn a facet for itself on the adjacent ulnar notch of the radius.
The forearm is capable of twisting around its long axis (Figures 9-13 and 9-14). The head of the radius spins within its annular ligament while the distal end of the ulna curves around the distal ulna from a lateral position to a medial position and back again. This results in two possible movements at these joints:
Supination is a more powerful action than pronation. For this reason, screws and bottle caps are tightened in a clockwise direction, taking advantage of the stronger supinators of the forearm. This is designed as an advantage for the majority who are right-handed but not for the left-handed minority who are consigned to use the weaker pronators of their dominant left hands for these tasks. Left-handers, on the other hand, have an advantage in unscrewing bottle caps.
The wrist joint is the articulation between the forearm and the carpal bones of the wrist (see Figure 9-10). The ulna is separated from these bones by an articular disc so only the radius makes contact with the distal row of carpals. The main joint of the wrist is the biaxial, condyloid radiocarpal joint. The midcarpal joint