Humerus is a typical long bone present in the arm that articulates proximally with the scapula and distally with the bones of the forearm, namely, radius and ulna. Its proximal end consists of the head, greater tubercle and lesser tubercle. Its body, which is longer than any of the other bones of the upper limb, is cylindrical proximally and somewhat flattened distally. At the distal end, which is expanded to form the epicondyles on either side, it supports the trochlear and capitular articular surfaces for the ulna and radius respectively. The humerus has three main parts;
- The proximal end
- The shaft
- The distal end
Proximal end of Humerus:
The proximal end is the thickest and stoutest part of the bone. It consists of head, greater tubercle and lesser tubercle. The head forms about one-third of a sphere and is covered with articular cartilage. It is directed proximally, medially, and slightly dorsally, and rests in the glenoid cavity of the scapula.
Separating the head from the tubercles laterally is a shallow groove, which fades away on the surface of the bone inferiorly. This is named the anatomical neck and serves for the attachment of the capsule of the shoulder-joint. The articular edge of the groove opposite the lesser tubercle is usually notched for the attachment of the superior gleno-humeral ligament.
Greater tubercle abuts on the lateral side of the head and becomes continuous with the body distally. Its proximal surface forms a quadrant, which is subdivided into three more or less smooth areas of unequal size. The lateral surface of this tubercle, which bulges beyond the line of the shaft, is rough and pierced by numerous vascular foramina.
Anteriorly the greater tubercle is separated from the lesser tubercle by a well-defined furrow, called the intertubercular groove or bicipital groove. The transverse humeral ligament stretches across the groove between the two tubercles, thus converting the groove into a canal in which the tendon of the long head of the biceps and the ascending articular branch of the anterior circumflex artery of the humerus are lodged.
The lesser tubercle lies in front of the lateral half of deltoid tuberosity. It forms a pronounced elevation, which fades into the shaft distally. Laterally it forms the prominent medial lip of the inter-tubercular groove. Distal to the head and tubercles the shaft of the bone rapidly contracts, and is here named the surgical neck owing to its liability to fracture at this spot.
Shaft of Humerus:
The body or shaft is cylindrical in its proximal half. Inter-tubercular groove continues downward and slightly medially on the anterior potion of the shaft. The edges of the groove, which are termed its lips, are confluent proximally with the greater and lesser tubercles, respectively. Here they are prominent, and form the crests of the greater and lesser tubercles. Distally the lips of the intertubercular groove gradually fade away.
The deltoid tuberosity to which the powerful deltoid muscle is attached, is a rough, slightly elevated V-shaped surface, placed on the lateral anterior surface of the body about its middle. The anterior limb of the V is parallel to the axis of the body, and is continuous proximally with the lateral lip of the inter-tubercular groove
The medial epicondyle is the more prominent of the two, and furnishes a surface for the origin of the pronator teres, and the superficial flexor muscles of the forearm. The lateral epicondyle serves for the attachment of the common tendon of origin of the extensor muscles.
The posterior surface of the distal half of the body is smooth and rounded from side to side and somewhat flattened distally, where the whole body tends to incline forwards. It becomes continuous on either side with the posterior surfaces of the epicondyles, the medial of which is grooved for the passage of the ulnar nerve, whilst the lateral supplies an origin for the anconeus muscle.
Distal end of Humerus:
The distal end of the humerus is furnished with two articular surfaces. The lateral of these is a rounded eminence, placed on the anterior surface and distal border, but not extending on to the posterior surface called the capitulum. It articulates with the proximal surface of the head of the radius, Proximal to it, in front, there is a shallow depression (radial fossa), into which the margin of the head of the radius sinks when the elbow is strongly flexed. The medial of the two articular surfaces is called trochlea which is a grooved articular surface, with prominent edges winding spirally round the distal end of the body. A shallow groove separates the capitulum medially from the trochlea. On the anterior surface of the bone, immediately proximal to the trochlea, is a depression the coronoid fossa into which the coronoid process of the ulna slips in flexion of the joint. In a corresponding position on the posterior aspect of the distal end of the body there is a hollow, called the olecranon. Into this the olecranon process of ulna sinks when the elbow is extended.
Nutrient foramina of Humerus:
They are two fairly constant points where nutrient foramina exist; one at or near the surface for the insertion of the coracobrachialis muscle, and the other usually close to the posterior border of the deltoid tuberosity. Numerous vascular foramina are scattered along the line of the anatomical neck, the larger ones being situated near the proximal end of the inter-tubercular groove. The vascularity of the bone is here intimately associated with the activity of its growth.
Ossification of Humerus:
At birth the body of the humerus is usually the only part of the bone ossified. The centre of ossification for the body makes its appearance early in the second month of intra-uterine life. Within the first six months after birth a centre usually appears for the head; this is succeeded by one for the greater tubercle during the second or third year. These soon coalesce; and a third centre for the lesser tubercle begins to appear about the end of the third year, or may be delayed till the fourth or fifth year. These three centers are all blended by the seventh year, and form an epiphysis, which ultimately unites with the body about the age of twenty-five.