ARTICLE ON ANATOMY OF THE UPPER LIMB (HUMERUS)

INTRODUCTION

The humerus (plural humeri) is a long bone in the arm or forelimb that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a rounded head, a narrow neck, and two short processes (tubercles, sometimes called tuberosities). The body is cylindrical in its upper portion, and more prismatic below. The lower extremity consists of 2 epicondyles, 2 processes (trochlea & capitulum), and 3 fossae (radial fossa, coronoid fossa, and olecranon fossa). As well as its true anatomical neck, the constriction below the greater and lesser tubercles of the humerus is referred to as its surgical neck due to its tendency to fracture, thus often becoming the focus of surgeons.

STRUCTURE

Articulation:

At the shoulder, the head of the humerus articulates with the glenoid fossa of the scapula. More distally, at the elbow, the capitulum of the humerus articulates with the head of the radius, and the trochlea of the humerus articulates with the trochlear notch of the ulna.

Proximal landmarks of the humerus

The important anatomical features of the proximal humerus are the head, anatomical neck, surgical neck, greater and lesser tubercles and intertubercular sulcus. A tubercle is a round nodule, and signifies an attachment site of a muscle or ligament.

The head of the humerus is connected to the greater and lesser tubercles by the anatomical neck, which is short in width and nondescript.

The greater tubercle is located laterally on the humerus. It has an anterior and posterior face. The greater tubercle serves as attachment site for three of the rotator cuff muscles – supraspinatus, infraspinatus and teres minor.

The lesser tubercle is much smaller, and more medially located on the bone. It only has an anterior face. It is a place of attachment for the last rotator cuff muscle – subscapularis.

Separating the two tubercles is a deep depression, called the intertubercular sulcus, or groove. The tendon of the long head of biceps brachii runs through this groove. The edges of the intertubercular sulcus are known as lips. Tendons of the pectoralis major, teres major and latissimus dorsi attach to the lips of the intertubercular sulcus.

The surgical neck runs from the tubercles to the shaft of the humerus.

Shaft of the humerus 

The shaft of the humerus contains some important bony landmarks such as the deltoid tuberosity and radial groove, and is the site of attachment for various muscles.

On the lateral side of the humeral shaft is a roughened surface where the deltoid muscle attaches. This is known is as the deltoid tuberosity.

The radial groove is shallow depression that runs diagonally down the posterior surface of the humerus, parallel to the deltoid tuberosity. The radial nerve and profunda brachii artery lie in this groove. The following muscles attach to the humerus along its shaft:

• Anteriorly: Coracobrachialis, deltoid, brachialis, brachioradialis
• Posteriorly: Medial and lateral heads of the triceps 

Distal region of the humerus

The lateral and medial borders of the humerus form medial and lateral supraepicondylar ridges. The lateral supraepicondylar ridge is more roughened, as it is the site of attachment for many of the extensor muscles in the posterior forearm.

Immediately distal to the supraepicondylar ridges are the lateral and medial epicondyles – projections of bone. Both can be palpated at the elbow (the medial more so, as it is much larger). The ulnar nerve passes into the forearm along the posterior side of the medial epicondyle, and can also be palpated there.

Distally, the trochlea is located medially, and extends onto the posterior of the bone. Lateral to the trochlea is the capitulum, which articulates with the radius.

Also found on the distal portion of the humerus are three depressions, known as the coronoid, radial and olecranon fossae. They accommodate the forearm bones during movement at the elbow.

Nerves

The axillary nerve is located at the proximal end, against the shoulder girdle. Dislocation of the humerus's glenohumeral joint, has the potential to injure the axillary nerve or the axillary artery. Signs and symptoms of this dislocation include a loss of the normal shoulder contour and a palpable depression under the acromion.

The radial nerve follows the humerus closely. At the midshaft of the humerus, the radial nerve travels from the posterior to the anterior aspect of the bone in the spiral groove. A fracture of the humerus in this region can result in radial nerve injury.

The ulnar nerve lies at the distal end of the humerus near the elbow. When struck, it can cause a distinct tingling sensation, and sometimes a significant amount of pain. It is sometimes popularly referred to as 'the funny bone', possibly due to this sensation (a "funny" feeling), as well as the fact that the bone's name is a homophone of 'humorous'. It lies posterior to the medial epicondyle, and is easily damaged in elbow injuries.

FUNCTIONS

Muscular attachment

The deltoid originates on the lateral third of the clavicle, acromion and the crest of the spine of the scapula. It is inserted on the deltoid tuberosity of the humerus and has several actions including abduction, extension, and circumduction of the shoulder. The supraspinatus also originates on the spine of the scapula. It inserts on the greater tubercle of the humerus, and assists in abduction of the shoulder.

The pectoralis major, teres major, and latissimus dorsi insert at the intertubercular groove of the humerus. They work to adduct and medially, or internally, rotate the humerus.

The infraspinatus and teres minor insert on the greater tubercle, and work to laterally, or externally, rotate the humerus. In contrast, the subscapularis muscle inserts onto the lesser tubercle and works to medially, or internally, rotate the humerus.

The biceps brachii, brachialis, and brachioradialis (which attaches distally) act to flex the elbow. (The biceps do not attach to the humerus.) The triceps brachii and anconeus extend the elbow, and attach to the posterior side of the humerus.

The four muscles of supraspinatus, infraspinatus, teres minor and subscapularis form a musculo-ligamentous girdle called the rotator cuff. This cuff stabilizes the very mobile but inherently unstable glenohumeral joint. The other muscles are used as counterbalances for the actions of lifting/pulling and pressing/pushing.

CLINICAL SIGNIFICANCE AND CORRELATIONS

Surgical neck fracture:

The surgical neck of the humerus is a frequent site of fracture – usually by a direct blow to the area, or falling on an outstretched hand.

In any fracture, it is important to consider the regional anatomy; to assess any additional structures at risk of damage. In a surgical neck fracture, there are two nearby neurovascular structures – the axillary nerve and posterior circumflex artery.

Axillary nerve damage will result in paralysis to the deltoid and teres minor muscles. The patient will have difficulty performing abduction of the affected limb. The nerve also innervates the skin over the lower deltoid (regimental badge area), and sensation in this region may be impaired.

Mid-shaft fracture:

A mid-shaft fracture could easily damage the radial nerve and profunda brachii artery, as they are tightly bound in the radial groove.

The radial nerve innervates the extensors of the wrist. In the event of damage to this nerve, the extensors will be paralysed. This results in unopposed flexion of the wrist occurs, known as ‘wrist drop’.

There is also some sensory loss over the dorsal (posterior) surface of the hand, and the proximal ends of the lateral 3 and a half fingers dorsally.

Supracondylar fracture:

A supracondylar fracture is a fracture of the distal humerus that spans between the two epicondyles. The fracture is typically transverse or oblique, and the most common mechanism of injury is falling on an outstretched hand.

In this type of injury, the brachial artery can be damaged – either directly, or via swelling following the trauma. The resulting ischaemia can cause Volkmann’s ischaemic contracture – uncontrolled flexion of the hand, as flexor muscles become fibrotic and short.

There also can be damage to the anterior interosseous nerve (branch of the median nerve), ulnar nerve or radial nerve.