9: The Upper Limb

Chapter 9 imageThe 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.

1 Skeleton

SHOULDER (PECTORAL) GIRDLE

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.

Scapula

Description

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.

ARM

Humerus

Features

FOREARM

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.

THE WRIST AND HAND

2 Joints, Movements, and Muscles

JOINTS OF THE PECTORAL GIRDLE

Movements

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.

Elevation is the drawing upward of the scapula, as in shrugging of the shoulders. The main muscles of elevation include the levator scapulae and the trapezius (upper fibers) muscles.

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.

In upward rotation, the scapula rotates about a midscapular axis in which the glenoid fossa points upward. The muscles responsible for upward rotation are the trapezius and serratus anterior 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.

GLENOHUMERAL JOINT

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.

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.

Movements

The glenohumeral joint is capable of six movements (Figure 9-9):

Circumduction is a combination of flexion, extension, abduction, and adduction in a wide conical arc about the shoulder.

THE ELBOW (HUMEROULNAR) JOINT

Features

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.

Movements

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 RADIOULNAR JOINTS

Movements

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:

WRIST JOINT (RADIOCARPAL AND MIDCARPAL JOINTS)

Features

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

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Jan 5, 2015 | Posted by in General Dentistry | Comments Off on 9: The Upper Limb
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