The Posterior Or Extensor Muscles

These comprise the triceps and anconeus muscles (Fig. 298).

The triceps arises by its long head from the lower part of the rim of the glenoid cavity and adjoining border of the scapula; by its external or lateral head from the upper outer portion of the humerus from the greater tuberosity above to the radial (musculospiral) groove below; by its internal or medial head from the posterior surface of the humerus below the radial groove. It inserts into the posterior part of the upper surface of the olecranon. Just above its insertion it is separated from the bone by a bursa. It is continuous from the outer edge of the olecranon as a firm fascia which passes down over the anconeus to be attached to the upper fourth of the ulna and the deep fascia. This is an important structure in fractures of the olecranon.

Fig. 298.   Extensor muscles of the back of the elbow.

Fig. 298. - Extensor muscles of the back of the elbow.

The anconeus passes downward and backward from the lateral (external) condyle to insert into the side of the olecranon and upper fourth of the ulna. Its fibres are practically continuous with the lower fibres of the triceps and it may be considered as a fourth head of that muscle. It covers the posterior portion of the head of the radius and overlaps somewhat the supinator (brevis) muscle.

Surface Anatomy

Having become acquainted with the bones and muscles, one will be better able to appreciate the surface markings and understand their significance (Fig. 299).

When the elbow is fully extended the bony projections are obscured by the soft tissues, hence in examining an elbow it should be flexed at approximately a right angle. The first object to strike the eye is the prominent olecranon process. It is subcutaneous and the bony ulna beneath can be felt and followed down the back of the forearm. From the tip upward for a couple of centimetres can be felt the upper surface of the olecranon into which the triceps inserts. To feel this distinctly the forearm should be slightly extended to relax the triceps; the outline of the upper portion of the olecranon then becomes perfectly distinct.

Projecting on each side of the elbow are the two condyles of the humerus. These bony projections do not belong to the forearm. The two condyles are nearly on the same level. The medial (internal) is much more prominent and has the appearance of being a trifle higher and slightly anterior. A line joining them crosses the long axis of the humerus at an angle of 90 degrees, but makes an angle of only 80 degrees with the forearm. By deep pressure the lateral (external) supracondylar ridge can readily be felt running up the arm somewhat posteriorly from the lateral (external) condyle. The medial (internal) supracondylar ridge is much less easily felt though the intermuscular septum is more evident on this side. When the elbow is flexed at a right angle a line drawn parallel with the humerus and prolonged through the two condyles will cut the tip of the olecranon. If the forearm is extended the olecranon passes slightly posterior to this line; if the forearm is flexed, the olecranon passes somewhat in front of it. Hence in examining the elbow for injury it is desirable to determine the relation of these points when the elbow is bent at a right angle. The coronoid process lies anteriorly, deep in the flexure of the elbow, and cannot be distinctly felt.

Fig. 299.   Surface anatomy of the back of the elbow.

Fig. 299. - Surface anatomy of the back of the elbow.

If, now, the elbow is extended, the tip of the olecranon can still be felt with the medial (inner) condyle to its inner side. Between the two is a deep groove in which lies the ulnar nerve. To the outer side of the olecranon is a deep pit or short groove; the bone marking its outer edge is the lateral (external) condyle. In the bottom of this pit at its lower portion, about 2.5 cm. (1 in.) below the tip of the olecranon, can be felt the head of the radius. If the thumb is placed on it and the hand rotated, the head of the radius can be felt turning beneath. Immediately above the head of the radius, lying to the outer side of the olecranon, if the elbow be again flexed to a right angle, can be seen and felt the bony projection of the capitellum covered by the strong expansion from the triceps. By careful palpation a groove can be felt between the lower edge of the capitellum and the head of the radius which marks the limits and point of articulation of the two bones.

On the posterior aspect of the joint the ulnar nerve is the most important structure; there is, however, a bursa between the upper or posterior surface of the olecranon and the skin and also another on its inferior surface, extending downward, which from its exposed position is frequently injured and enlarged. Such an enlargement occurs from chronic irritation in certain occupations, hence the name " miners' elbow".

With the elbow flexed at a right angle there is seen on its anterior surface a crease which runs from one condyle across to the other. If a knife were held parallel with the forearm and entered at this crease, it would strike the humerus above the level of the joint line, that is, the line of contact of the bones. This joint line runs from 1.25 cm. (1/2 in.) below the lateral (external) condyle to 2.5 cm. (1 in.) below the medial (internal) condyle.

Anteriorly the muscular masses form prominent landmarks. In the middle of the crease can be felt the tendon of the biceps muscle. The muscular swell above the crease is formed by the biceps muscle with the brachialis anticus beneath. The sharp upper edge of the bicipital fascia can be distinctly felt when the muscle contracts. The limits of the biceps can be felt as two lines, radiating like the letter V from the biceps tendon upward. These are the commencing bicipital furrows or grooves.

The outer branch marks the depression between the outer edge of the biceps and the swell forming the supinator group of muscles. The inner branch marks the inner edge of the biceps, and between it and the medial condyle can be felt a muscular mass which is formed by the inner portion of the brachialis anticus. In the middle of the flexure of the elbow below the crease is a depression called the antecubital fossa. To its outer side is the muscular prominence of the extensors and supinator. To its inner side is the muscular prominence of the flexors and pronator. The inner muscular swell ends at the medial (internal) condyle, but the external one passes well up on the arm. The muscles so prolonged upward are the extensor carpi radialis longior for about 5 cm. (2 in.) above the lateral (external) condyle, and the brachioradialis (supinator longus) for 10 cm. (4 in.) higher. The outer limit of the antecubital fossa is formed by the inner edge of the brachioradialis. The inner side is formed by the pronator radii teres muscle.

To the inner side of the biceps tendon lies the brachial artery, which bifurcates opposite the neck of the radius, approximately 2 cm. or a finger's breadth below the crease of the elbow. Still farther to the inner side lies the median nerve. In the groove between the biceps and brachialis anticus on the inner side and brachioradialis (supinator longus) and extensor carpi radialis longior on the outer side lies the radial (musculospiral) nerve; it divides above or opposite the lateral (external) condyle into the superficial branch and posterior interosseous nerve