The humerus is the bone that forms the upper arm, and joins it to the shoulder and forearm.
The humerus acts as an attachment site for many muscles and ligaments, resulting in various raised roughening on the bony surface.
In this article, we are going to look at the osteology of the humerus, its regional anatomy, and its clinical correlations.
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.
Clinical Relevance: 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.
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
Clinical Relevance: 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.
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.
The proximal region of the humerus articulates with the scapula to form the glenohumeral joint (shoulder joint). The distal part of the humerus articulates with the ulna and radius at the elbow joint, at the trochlea and capitulum respectively. Here, the bone adopts a flattened, almost 2-D shape.
Clinical Relevance: 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.