BONY ANATOMY AND LANDMARKS
Anteriorly, the contour of the biceps muscle is seen starting in the upper arm and extending distally into the cubital fossa, which is the inverted triangular depression on the anterior aspect of the elbow. The flexion crease along the anterior elbow is in line with the medial and lateral epicondyles and is 1 to 2 cm proximal to the joint line when the elbow is extended. The superficial cephalic and basilic veins are the most prominent super-ficial major contributions of the anterior venous system and communicate by way of the median cephalic and median basilic veins to form a pattern resembling an M over the cubital fossa. Laterally, the tip of the olecranon, the lateral epicondyle, and the radial head form a palpable triangle called the posterolateral soft spot that can show evidence of an effusion from the elbow joint and serves as an important landmark for joint aspiration and for elbow arthroscopy. The proximal extensor forearm musculature originates from the lateral epicondyle and forms the lateral margin of the cubital fossa and the lateral contour of the forearm and comprises the brachioradialis and the extensor carpi radialis longus and brevis muscles. The proximal flexor pronator musculature forms the contour of the medial anterior forearm extending from the medial epicondyle and includes the pronator teres, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris. The relationship of these muscles can be approximated by placing the opposing thumb and the index, long, and ring fingers over the anterior medial forearm. Posteriorly, the contour of the triceps muscle is seen in the upper arm extending to the tip of the olecranon. More distally, the proximal forearm is contoured dorsally by the lateral extensor musculature, consisting of the anconeus, extensor carpi ulnaris, extensor digitorum quinti, and extensor digitorum communis.
The main bone of the upper arm is the humerus. It is a long bone composed of a shaft and two articular extremities. The proximal humerus, which includes the humeral head, greater and lesser tuberosity, and surgical neck, is discussed in detail in Section I, Shoulder.
The body, or shaft, of the humerus begins just below the surgical neck. It is somewhat rounded above and prismatic in its lower portion. Above and medially, the coracobrachialis muscle is received near the middle of the shaft; about opposite laterally is the prominent deltoid tuberosity. This is continued upward in a V-shaped roughening for the insertion of the deltoid muscle. Just below the deltoid tuberosity, a groove for the radial nerve indents the bone posteriorly, spiraling lateralward as it descends. Sharp lateral and medial supracondylar ridges spring from the respective borders inferiorly and continue into the lateral and medial epicondyles of the humerus. The inferior extremity of the bone is flattened anteroposteriorly and mediolaterally, and it is widened by the medial and lateral epicondyles. The lateral epicondyle is not conspicuous, but the medial epicondyle forms a marked medial projection above the elbow. Projecting somewhat backward, it is grooved behind for the ulnar nerve.
The articular surfaces for the radius, ulna, capitellum, and trochlea are directed somewhat forward; consequently, the inferior extremity of the humerus appears to curve anteriorly. The capitellum is roughly globular. Smaller than the trochlea, it articulates with the cupped upper surface of the radius. Above it is a shallow fossa, the radial fossa, for the reception of the edge of that bone during full flexion of the elbow. The trochlea is shaped like a spool, with a deep depression between two well-marked margins. The depression is slightly spiral and receives the central ridge of the trochlear notch of the ulna. The medial rim of the trochlea is the more prominent; the lateral rim is only a small elevation separating the trochlea from the capitellum. Above the trochlea is the coronoid fossa for the reception of the coronoid process of the ulna in front and the olecranon fossa for the olecranon behind.
The humerus ossifies from eight centers of ossification: one for the shaft and seven for the processes-head, greater and lesser tuberosities, trochlea, capitellum, lateral epicondyle, and medial epicondyle.
The shaft appears near the middle of the bone in the eighth week of fetal life and then extends toward its extremities. At birth, the humerus is ossified in nearly its whole length; only its extremities remain cartilaginous. Shortly after birth, ossification begins in the head of the bone, followed by the appearance of the centers in the greater and lesser tuberosities at 3 to 5 years of age, respectively. By age 6, all these centers have merged into one large epiphysis. In the distal humerus, secondary centers appear for the capitellum at age 2, for the trochlea at age 9 or 10, and in the lateral epicondyle at ages 13 to 14. These centers unite and fuse with the shaft at about age 13 in females and age 15 in males. The separate center for the medial epicondyle appears a ages 6 to 8 and fuses with the shaft at ages 14 to 16.