Humerus

The humerus is the largest of the bones in the upper limb. It is a typical long bone, having a shaft(body) and two extremities. Proximally, the humerus articulates with the glenoid fossa of the scapula forming the shoulder joint, and distally with the radius and ulna forming the elbow joint.

The major feature proximally, is the head of the humerus with its smooth, rounded articular surface facing upwards, medially and backwards. It is almost hemispherical, being siderably larger than the socket formed by the glenoid fossa. The head is joined to the upper end by the anatomical neck, a slightly constricted region encircling the bone at the articular margin, separating it from the tubercles.

The greater tubercle is a prominence on the upper lateral part of the bone, next to the head. It merges with the shaft below and is marked by three distinct impressions for muscular attachment. The greater tubercle projects laterally past the margin of the acromion and is the most lateral bony point at the shoulder.

The smaller lesser tubercle is a distinct prominence on the anterior aspect below the anatomical neck. It has a well-marked impression on its medial side for muscular attachment. Between these two tubercles, and passing onto the shaft of the humerus, is the deep intertubercular groove(sulcus). The crests of the greater and lesser tubercles continue down from the anterior borders of the tubercles to form the lateral and medial lips of the groove. Between the two lips is the floor of the groove.

Below the head and tubercles, where they join the shaft, there is a definite constriction. This region is termed the surgical neck because fractures often occur here, particularly in the elderly.

The shaft of the humerus is almost cylindrical above, becoming triangular in its lower part with distinct medial and lateral borders. It presents three borders and surfaces, although the borders are frequently rounded and indistinct. They are described as anterior, medial and lateral. Between the three borders are the three surfaces of the shaft. The intertubercular groove is continuous with the anteromedial surface, the medial border beginning as the crest of the lesser tubercle and ending by curving towards the medial epicondyle. The smooth anterolateral surface is marked about its middle by the deltoid tuberosity. The posterior surface is crossed obliquely from superomedial to inferolateral by the spinal groove(radial groove). It reaches the lateral border below the deltoid tuberosity, but is often poorly marked.

The lower end of the humerus is expanded laterally, flattened anteroposteriorly, and bent slightly forwards. It presents two articular surfaces separated by a ridge. The lateral articular surface, the capitulum, is situated on the anteroinferior aspect and is a rounded, convex surface, being less than a hemisphere in size. The capitulum articulates with the radius, making its greatest contact with the radius when the elbow is fully flexed.

Medial to the capitulum is the trochlea, the articular surface for the ulna. The trochlea is a grooved surface rather like a pulley, the medial edge projecting further distally and anteriorly than the lateral. This causes the ulna also to project laterally and results in a carrying angle between the humerus and ulna(figure b).

On the medial side of the trochlea is the large medial epicondyle. Its posterior surface is smooth and has a shallow groove for the ulnar nerve. The sharp medial supracondylar ridge, comprising the lower third of the medial border, runs upwards onto the shaft. On the lateral side of the capitulum is the lateral epicondyle with the lateral supracondylar ridge, comprising the lower third of the lateral border, running upwards onto the shaft.

Just above the articular surfaces, the lower end of the humerus presents three fossae for the bony processes of the radius and ulna. Situated posteriorly is the deep olecranon fossa which receives the olecranon process of the ulna when the elbow is extended. Anteriorly, there are two fossae: the lateral radial and medial coronoid fossae, which receive the head of the radius and coronoid process of the ulna respectively on full flexion of the elbow. Many of the bony features can be seen in the figure below.

Palpation

At the upper end of the humerus the most lateral bony point at the shoulder is the greater tubercle, whose quadrilateral superior, anterior and posterior surfaces can be felt. Further differentiation can be made by palpating the lateral margin of the acromion and then running the fingers off its edge onto the greater tubercle. The rounded lesser tubercle can be left through deltoid, and is just lateral to the hip of the coracoid process. To the lateral side of the lesser tubercle the impression of the intertubercular sulcus can usually be felt. The shaft of the humerus is covered with thick muscle, but can be palpated on its medial and lateral sides. At the lower end, the prominent medial epicondyle is the most obvious bony landmark. The ulnar nerve can be rolled in the groove behind it(the “funny bone”). Running upwards from the medial epicondyle the sharp medial supracondylar ridge can be palpated. The lateral epicondyle can be palpated at the base of a dimple on the lateral aspect of the elbow, as can the lateral supracondylar ridge running upwards from it. Posteriorly, the olecranon fossa can be felt through the triceps tendon, if the relaxed elbow is flexed.

Ossification

A primary ossification centre appears in the shaft in the eighth week in utero and spreads until, at birth, only the ends are cartilaginous. Secondary centres appear in the head early in the first year, in the greater tubercle at about 3 years and in the lesser tubercle at about 5 years. These fuse to form a single cap of bone between the ages of 6 and 8 years, finally fusing with the shaft between 18 and 20 in females and 20 and 22 years in males.

At the lower end of the humerus, secondary centres appear for the capitulum during the second year, for the trochlea between 9 and 10 years, and for the lateral epicondyle between 12 and 14 years. These join together at about 14 years, fusing with the shaft at 15 years in females and 18 years in males. A separate centre for the medial epicondyle appears between 15 and 18 years with a spicule of bone projecting down from the shaft medial to the trochlea. This latter ossification centre lies entirely outside the joint capsule. Most of the growth in length of the humerus occurs at its upper end.