The Upper Limb

The upper limb is related to the lateral aspect of the lower portion of the neck and with the thoracic wall. It’s suspended from the trunk by muscles and a small skeletal articulation between the clavicle and the sternum – the sternoclavicular joint. Based upon the position of it’s major joints and component bones, the upper limb is split into shoulder, arm, forearm, and hand.

The shoulder is the area of upper limb connection to the trunk. The arm is the part of the upper limb between the shoulder and the elbow joint; the forearm is between the elbow joint and the wrist joint; and the hand is distal to the wrist joint.

The axilla, cubital fossa, and carpal tunnel are significant regions of transition between the different parts of the limb. Essential structures go through, or are related to, every of these regions.

The axilla is an irregularly shaped pyramidal area created by muscles and bones of the shoulder and the lateral surface of the thoracic wall. The apex or inlet opens directly into the lower portion of the neck. The skin of the armpit creates the floor. All major structures that pass between the neck and arm go through the axilla.

The cubital fossa is a triangularly shaped depression created by muscles anterior to the elbow joint. The major artery, the brachial artery, going from the arm to the forearm goes through this fossa, so does one of the major nerves of the upper limb, the median nerve.

The carpal tunnel is the entrance to the palm of the hand. Its posterior, lateral, and medial walls create an arch, that is created from small carpal bones in the proximal region of the hand. A thick band of connective tissue, the flexor retinaculum, covers the distance between every side of the arch and creates the anterior wall of the tunnel. The median nerve and all the long flexor tendons going from the forearm to the digits of the hand go through the carpal tunnel.

Functions

Positioning the Hand

Unlike the lower limb, that is utilized for support, stability, and locomotion, the upper limb is highly mobile for positioning the hand in space.

The shoulder is suspended from the trunk predominantly by muscles and hence moves relative to the body. Sliding (protraction and retraction) and rotating the scapula on the thoracic wall changes the position of the glenohumeral joint (shoulder joint) and extends the reach of the hand. The glenohumeral joint enables the arm to move around three axes with a broad range of motion. Movements of the arm at this joint are of the radius over the ulna. This movement called pronation, takes place solely in the forearm. Supination returns the hand to the anatomical position.

At the wrist joint, the hand can be abducted, adducted, flexed, extended, and circumducted. These movements, combined with those of the shoulder, arm, and forearm, allow the hand to put in a broad range of positions relative to the body.

The Hand as a Mechanical Tool

One of the major functions of the hand is to grip and manipulate objects. Gripping objects usually involves flexing the fingers against the thumb. Depending on the type of grip, muscles in the hand act to:

• Modify the actions of long tendons that emerge from the forearm and insert into the digits of the hand.
• Produce combinations of joint movements inside every digit that can’t be generated by the long flexor and extensor tendons alone coming from the forearm.

The Hand as a Sensory Tool

The hand is utilized to discriminate between objects on the basis of touch. The pads on the palmar aspect of the fingers. Include a high density of somatic sensory receptors. Also, the sensory cortex of the brain devoted to interpreting info from the hand, particularly from the thumb, is disproportionately large relative to that for many other regions of skin.

Component Parts

Bones and Joints

• The bones of the shoulder contain the scapula, clavicle, and proximal end of the humerus.
• The clavicle articulates medially with the manubrium of the sternum and laterally with the acromion of the scapula, which arches over the joint between the glenoid cavity of the scapula and the head of the humerus (the glenohumeral joint).
• The humerus is the bone of the arm. The distal end of the humerus articulates with the bones of the forearm in the elbow joint, that is a hinge joint that enables flexion and extension of the forearm.
• The forearm includes two bones:
  • The lateral bone is the radius.
  • The medial bone is the ulna.
• At the elbow joint, the proximal ends of the radius and ulna articulate with every other as well as with the humerus.
• Along with flexing and extending the forearm, the elbow joint enables the radius to spin on the humerus while sliding against the head of the ulna during pronation and supination of the hand.
• The distal portions of the radius and the ulna also articulate with each other. This joint enables the end of the radius to flip from the lateral side to the medial side of the ulna during pronation of the hand.
• The wrist joint is composed between the radius and carpal bones of the hand and between an articular disc, distal to the ulna, and carpal bones.
• The bones of the hand contain the carpal bones, the metacarpals, and the phalanges.
• The 5 digits in the hand are the thumb and the index, middle, ring, and little fingers.
• Joints between the 8 small carpal bones enable only limited amounts of movement; as a result, the bones work together as a unit.
• The 5 metacarpals, 1 for every digit, are the primary skeletal foundation of the palm.
• The joint between the metacarpal of the thumb (metacarpal I) and one of the carpal bones enables greater mobility than the limited sliding movement that takes place at the carpometacarpal joints of the fingers.
• Distally, the heads of metacarpals II to V (i.e., with the exception of that of the thumb) are interconnected by strong ligaments.
• Lack of the ligamentous connection between the metacarpal bones of the thumb and index finger along with the biaxial saddle joint between the metacarpal bone of the thumb and the carpus supply the thumb with greater freedom of movement in relation to the other digits of the hand.
• The bones of the digits are the phalanges. The thumb has 2 phalanges, while every of the other digits has 3.
• The metacarpophalangeal joints are biaxial condylar joints (ellipsoid joints) that enable abduction, adduction, flexion, extension, and circumduction. Abduction and adduction of the fingers is defined in reference to an axis going through the center of the middle finger in the anatomical position. The middle finger hence abduct both medially and laterally and adduct back to the central axis from either side. The interphalangeal joints are primarily hinge joints that enable only flexion and extension.

Muscles

Some muscles of the shoulder, like the trapezius, levator scapulae, and rhomboids, attach the scapula and clavicle to the trunk. Other muscles attach the clavicle, scapula, and body wall to the proximal end of the humerus. These contain the pectoralis major, pectoralis minor, latissimus dorsi, teres major, and deltoid. The most significant of these muscles are the 4 rotator cuff muscles—the subscapularis, supraspinatus, infraspinatus, and teres minor muscles, which attach the scapula to the humerus and give support for the glenohumeral joint.

• Muscles in the arm and forearm are divided into anterior (flexor) and posterior (extensor) compartments by layers of fascia, bones, and ligaments.
• The anterior compartment of the arm is located anteriorly in position and is divided from muscles of the posterior compartment by the humerus and by medial and lateral intermuscular septa. These intermuscular septa are continuous with the deep fascia enclosing the arm and connect to the sides of the humerus.
• In the forearm, the anterior and posterior compartments are divided by a lateral intermuscular septum, the radius, the ulna, and an interosseous membrane, which joins adjacent sides of the radius and ulna.
• Muscles in the arm act primarily to move the forearm in the elbow joint, while those in the forearm function predominantly to move the hand in the wrist joint and the fingers and thumb.
• Muscles seen entirely in the hand, the intrinsic muscles, generate delicate movements of the digits of the hand and modify the forces produced by tendons entering the fingers and thumb from the forearm. Included among the intrinsic muscles of the hand are 3 small thenar muscles, which create a soft tissue mound, termed the thenar eminence, over the palmar aspect of metacarpal I. The thenar muscles enable the thumb to move freely relative to the other fingers.

Relationship to other regions

Neck

The upper limb is directly related to the neck. Being located on every side of the superior thoracic aperture in the base of the neck is an axillary inlet, that is created by:

• the lateral margin of rib 1,
• the posterior surface of the clavicle,
• the superior margin of the scapula, and.
• the medial surface of the coracoid process of the scapula.

The major artery and vein of the upper limb pass between the thorax and the limb by passing over rib 1 and via the axillary inlet. Nerves, predominantly originated from the cervical portion of the spinal cord, also go through the axillary inlet and the axilla to supply the upper limb.

Back and Thoracic Wall

Muscles that connect the bones of the shoulder to the trunk are related to the back and the thoracic wall and contain the trapezius, levator scapulae, rhomboid major, rhomboid minor, and latissimus dorsi.

The breast on the anterior thoracic wall has a number of significant relationships with the axilla and upper limb. It overlies the pectoralis major muscle, which creates majority of the anterior wall of the axilla and attaches the humerus to the chest wall. Frequently, part of the breast called the axillary process extends around the lateral margin of the pectoralis major into the axilla.

Lymphatic drainage from lateral and superior parts of the breast is predominantly into lymph nodes in the axilla. Several arteries and veins that supply or drain the gland also starts from, or drain into, major axillary vessels.

Key Points

Innervation by Cervical and Upper Thoracic Nerves

Innervation of the upper limb is by the brachial plexus, that is created by the anterior rami of cervical spinal nerves C5 to C8, and T1. This plexus is initially created in the neck and after that continues via the axillary inlet into the axilla. Major nerves that ultimately innervate the arm, forearm, and hand starts from the brachial plexus in the axilla.

As a consequence of the innervation pattern, clinical testing of lower cervical and T1 nerves is performed by examining dermatomes, myotomes, and tendon reflexes in the upper limb. Another consequence is that the clinical signs of problems related to lower cervical nerves-pain; pins-and-needles sensations, or paresthesia; and muscle twitching—appear in the upper limb.

Dermatomes of the upper limb are frequently tested for sensation. Regions where overlap of dermatomes is minimal, contains:

• upper lateral region of the arm for spinal cord level C5,
• palmar pad of the thumb for spinal cord level C6,
• pad of the index finger for spinal cord level C7,
• pad of the little finger for spinal cord level C8, and.
• skin on the medial aspect of the elbow for spinal cord level Tl.

Selected joint movements are utilized to test myotomes:

• Abduction of the arm a t the glenohumeral joint is controlled predominantly by C5.
• Flexion of the forearm in the elbow joint is controlled primarily by C6.
• Extension of the forearm in the elbow joint is controlled primarily by C7.
• Flexion of the fingers is controlled primarily by C8.
• Abduction and adduction of the index, middle, and ring fingers is controlled predominantly by Tl.

In an unconscious patient, both somatic sensory and motor functions of spinal cord levels can be tested using tendon reflexes:

• A tap on the tendon of the biceps in the cubital fossa tests primarily for spinal cord level C6.
• A tap on the tendon of the triceps posterior to the elbow tests primarily for C7.

The major spinal cord level related to innervation of the diaphragm, C4, is immediately above the spinal cord levels related to the upper limb.

Evaluation of dermatomes and myotomes in the upper limb can give essential info about potential breathing problems that may develop as complications of damage to the spinal cord in regions just below the C4 spinal level.

Every of the major muscle compartments in the arm and forearm and every of the intrinsic muscles of the hand is innervated predominantly by one of the major nerves that starts from the brachial plexus in the axilla:

All muscles in the anterior compartment of the arm are innervated by the musculocutaneous nerve.

• The median nerve innervates the muscles in the anterior compartment of the forearm, with two exceptions – one flexor of the wrist (the flexor carpi ulnaris muscle) and part of one flexor of the fingers (the medial half of the flexor digitorum profundus muscle) are innervated by the ulnar nerve.
• Most intrinsic muscles of the hand are innervated by the ulnar nerve, with the exception of the thenar muscles and 2 lateral lumbrical muscles, that are innervated by the median nerve.
• All muscles in the posterior compartments of the arm and forearm are innervated by the radial nerve.
• In addition to innervating major muscle groups, every of the major peripheral nerves originating from the brachial plexus carries somatic sensory info from patches of skin quite different from dermatomes. Sensation in these type of regions can be utilized to test for peripheral nerve lesions.
• The musculocutaneous nerve innervates skin on the anterolateral side of the forearm.
• The median nerve innervates the palmar surface of the lateral three and one -half digits, and the ulnar nerve innervates the medial one and one-half digits.
• The radial nerve supplies skin on the posterior surface of the forearm and the dorsolateral surface of the hand.

Nerves Linked to Bone

Three essential nerves are directly related to parts of the humerus:

• The axillary nerve, which supplies the deltoid muscle, a major abductor of the humerus in the glenohumeral joint, enters around the posterior aspect of the upper part of the humerus (the surgical neck).
• The radial nerve, which supplies all of the extensor muscles of the upper limb, enters diagonally around the posterior surface of the middle of the humerus in the radial groove.
• The ulnar nerve, that is ultimately destined for the hand, enters posteriorly to a bony protrusion, the medial epicondyle, on the medial side of the distal end of the humerus.

Fractures of the humerus in any one of these three regions can endanger the related nerve.

Superficial Veins

Large veins embedded in the superficial fascia of the upper limb are frequently utilized to access a patient’s vascular system and to withdraw blood. The most significant of these veins are the cephalic, basilic, and median cubital veins.

The cephalic and basilic veins starts from the dorsal venous network on the back of the hand.

The cephalic vein comes from over the anatomical snuffbox at the base of the thumb, enters laterally around the distal forearm to reach the anterolateral surface of the limb, and after that continues proximally. It crosses the elbow, then enters up the arm into a triangular depression—the clavipectoral triangle.

Clavipectoral Triangle (Deltopectoral Triangle)

Between the pectoralis major muscle, deltoid muscle, and clavicle. In this depression, the vein enters into the axilla by penetrating deep fascia just inferior to the clavicle.

The basilic vein comes from the medial side of the dorsal venous network of the hand and enters proximally up the posteromedial surface of the forearm. It enters into the anterior surface of the limb just inferior to the elbow and after that continues proximally to enter deep fascia about midway up the arm.

At the elbow, the cephalic and basilic veins are connected by the median cubital vein, which crosses the roof of the cubital fossa.

Orientation of The Thumb

The thumb is positioned at right angles to the orientation of the index, middle, ring, and little fingers. Because of this, movements of the thumb take place at right angles to those of the other digits. For instance, flexion brings the thumb across the palm, on the other hand abduction moves it far from the fingers at right angles to the palm.

Importantly, with the thumb positioned at right angles to the palm, only a slight rotation of metacarpal I on the wrist brings the pad of the thumb into a position directly facing the pads of the other fingers. This opposition of the thumb is crucial for normal hand function.