General features ~The ulna is the medial bone of the forearm and is parallel to the radius when the forearm is supine. The upper end is thick, strong and hook like (fig. 419), the concavity of the hook being directed forwards. The lateral border of the shaft forms a thin, sharp crest. This information is sufficient to enable the student to refer a given ulna to its correct side of the body. The bone diminishes in size from its upper to its lower end, which bears a small, rounded enlargement, termed the head of the ulna. The shaft is triangular on section, and no difficulty will be experienced in determining its three surfaces and their limiting borders.
The upper end of the ulna (fig. 419) includes two substantial processes, named the olecranon and the coronoid process, and two articular areas, termed the trochlear (semilunar) and radial notches, which articulate, respectively, with the humerus and the radius.
The olecranon is the uppermost part of the bone. It is bent forwards at its summit to form a prominent beak, which is received into the olecranon fossa of the humerus when the forearm is extended. Its posterior aspect, smooth and triangular in outline, can easily be felt through the skin, and its upper border constitutes the point of the elbow. The anterior surface is smooth and articular, and forms the upper part of the trochlear (semilunar) notch. The base of the olecranon is constricted where it joins the shaft, and this is the narrowest part of the upper end of the ulna.
The trochlear (semilunar) notch articulates with the trochlea of the humerus and is shaped accordingly. It is formed by the anterior surface of the olecranon and the superior surface of the coronoid process. The junction. between these two areas is constricted, and a narrow, rough strip may separate them completely. A smooth ridge, which corresponds to the groove of the trochlea, divides the notch into a larger, medial, and a smaller, lateral part. The larger, medial part conforms to the large medial flange of the trochlea of the humerus.
The radial notch (fig. 419) is an oblong, articular depression on the upper part of the lateral aspect of the coronoid process. It articulates with the articular circumference of the head of the radius, and is separated from the lateral part of the trochlear notch by a smooth ridge.
The lower end of the ulna is slightly expanded and includes the rounded head and the styloid process. The head presents a convex, somewhat semilunar articular surface on its lateral side for articulation with the ulnar notch of the radius. Its inferior surface (fig. 421) is smooth and is separated from the carpus by the articular disc of the inferior radio-ulnar joint, which is attached by its apex to the small rough area interposed between the articular surface and the styloid process. The styloid process is a short rounded projection, which springs from the posteromedial aspect of the lower end of the ulna. Its tip can be felt through the skin on the posteromedial aspect of the wrist, where it lies about 1 cm. above the level of the tip of the styloid process of the radius. On the dorsal aspect of the lower end a shallow groove intervenes between the head of the ulna and the styloid process.
The shaft of the ulna is triangular in section (fig. 420) in its upper three-fourths, but is almost cylindrical in its lower fourth. The bone is not perfectly straight, and shows a slight but appreciable double curve. Throughout its whole length it forms a gentle curve, the convexity of which is directed backwards. In addition, the upper half or more shows a slight curvature to the lateral side, and the lower half or less a similar curvature in the opposite direction. The surfaces of the shaft are anterior, posterior and medial ; the borders, interosseous, posterior and anterior.
Particular features.-The upper surface of the olecranon gives attachment, in front, to the capsular ligament of the elbow-joint ; in its posterior two-thirds, which are roughened, it provides insertion for the tendon of the triceps muscle. Occasionally these two areas are separated by a smooth bursal area. The medial aspect of the process is marked in its upper part by a rough elevation, which gives attachment to the posterior and oblique bands of the medial ligament of the elbow-joint and the ulnar head of the flexor carpi ulnaris. Its lower part is smooth and rives origin to the uppermost fibers of the flexor digitoruin profundus. The lateral aspect of the process, and the adjoining part of the posterior surface of the shaft down to the oblique line already mentioned, gives insertion to the anconeus muscle. Its posterior aspect is separated from the skin by a bursa.
The anterior surface of the coronoid process, including the tuberosity of the ulna, receives the insertion of the brachialis muscle. Its medial border is sharp, and a small, rounded tubercle is situated at its upper end. This tubercle gives attachment to the oblique and anterior bands of the medial ligament of the elbow-joint and to the lowest part of the hntnero-uhiar head of the flexor digitorum sublimis muscle. Below the tubercle the margin gives origin to the small coronoid head of the pronator teres, and below that to an occasional coronoid head of the flexor pollicis longus. The medial aspect of the process .is concave and gives origin to fibers of the flexor digitoruin profundus. The anterior and posterior borders of the radial notch provide attachment, for the annular ligament of the radius, and the depressed area: below the notch is limited behind by the supinator crest. The supinator muscle arises from the crest and from the adjoining part of the depression.
The part of the trochlear notch, which is formed by the olecranon is, typically, divided into three areas. Of these the most medial faces forwards and slightly medially and is hollowed out, to fit the medial flange of the trochlea : the intermediate area is flattened and, fits the lateral flange of the trochlea ; the most lateral area, which, forms a narrow strip directed to the radial side, comes into contact with the trochlea only when the elbow is extended. The constriction of the articular surface is more pronounced than the constric tion of the base of the olecranon. The resulting small non-articular parts of the anterior aspect of the olecranon are covered in the fresh specimen and in life by tag-like processes of the synovial membrane, which contain a little fat. The coronoid part of the trochlear notch is divided into medial and lateral parts, which correspond, respectively, to the medial and intermediate parts of the olecranon area. Of these the medial is hollowed out much more than the lateral, in order to conform to the convexity of the medial flange of the trochlea. The medial and anterior borders of this axes give attachment to the medial and anterior portions of the capsular ligament of the elbow-joint.
The subcutaneous posterior border of the ulna gives attachment to the deep fascia of the forearm, which acts, in its upper three-fifths, as an additional origin for the flexor carpi ulnaris, and in its middle third as an additional origin for the extensor carpi ulnaris. Both of these muscles are therefore connected to the posterior border. The interosseous border is usually continuous above with the supinator crest. Except at its upper end it gives attachment to the interosseous membrane of the. forearm. The rounded anterior border is covered in its upper three-fourths by the flexor digitorum profundus, to which it gives origins The anterior surface, gives origin in its upper three-fourths to the flexor digitorum profundus. In the same extent the muscle arises also from the anterior border and the medial surface, extending upwards on to the medial sides of the coronoid process and the olecranon. The rough strip which crosses the lower fourth of this surface provides origin for the pronator quadratus. The anconeus is inserted into the posterior surface above the oblique line already mentioned, and extends upwards on to the lateral aspect of the olecranon. The narrow strip between the interosseous border and the vertical ridge gives part origin to three of the deep muscles of the forearm. The abductor pollicis longus arises from its upper fourth, and a ridge may separate this area from the succeeding fourth, which gives attachment to the extensor pollicis longus. The extensor indicis is attached to the third fourth of this area. The broad strip to the medial side of the vertical ridge is devoid of muscular attachments but is covered by the extensor carpi ulnaris, the tendon of which occupies the groove on the posterior aspect of the lower end of the bone. The medial ligament of the wrist-joint is attached to the tip of the styloid process. The articular disc separates the head of the ulna, from the medial part of the lunate bone and in ulnar deviation of the hand, from the triquetral bone.
Structure.–The structure of the ulna is similar to that of the other long bones.
Ossification (figs. 422, 423).–The ulna is ossified from three centers : one each for the shaft, the lower end, and the top of the olecranon. Ossification begins near the middle of the shaft, about the eighth week of fetal life, and soon extends through its greater part. About the fourth year, the center for the lower end appears in the middle of the head, and extends into the styloid process. About the tenth year, a center appears in the olecranon and forms a thin scale for the top of the process, the chief part of the process being formed by an upward extension of the shaft ; sometimes the upper part of the olecranon is ossified from two centers. The upper epiphysis joins the shaft about the sixteenth, the lower about the twentieth year. The former may take part in the formation of the semilunar notch although it does not usually do so.
Applied Anatomy-When indirect force is applied to the forearm the radius as a rule gives way, though both bones may suffer. Fractures from indirect force generally take place somewhere about the middle of the bones, while those from direct violence may occur at any part, but are most frequent in the lower half of the bones. A point of interest in connection with these fractures is the tendency for the two bones to unite across the interosseous membrane : the limb should therefore be put up in a position midway between supination and pronation, which is not only the most comfortable position, but also separates the bones most widely from each other.
Fractures of the radius may consist of : (1) Fracture of the head of the bone ; this for the most part takes place in conjunction with some other lesion, but may occur as an uncomplicated injury. (2) Fracture of the neck also may occur, but is usually complicated with other injury. (3) Fractures of the body of the radius are very common, and may take place at any part of the bone. In fracture of the upper one-third of the body- that is to say, above the insertion of the pronator teres-the displacement is very great. The upper fragment is strongly supinated by the biceps and supinator, and flexed by the biceps; while the lower fragment is pronated and drawn towards the ulna by the two pronators. If such a fracture be put up in the ordinary position, midway between supination and pronation, the bone will unite with the upper fragment in a position of supiuation, and the lower one in the mid-position and thus considerable impairment of the movement of supination will result ; the limb should therefore be put up with the forearm supinated. (4) The most important fracture of the radius is that of the lower end (Colles’s fracture). The fracture is transverse, and generally takes place about 2.5 cm. from the lower end. It is caused by falls on the palm of the hand, and is an injury of advanced life, occurring more frequently in the female than in the male. Separation of the lower epiphysis of the radius may take place in the young. This injury and Colles’s fracture may be distinguished from other injuries in this neighborhood -especially- dislocation of the wrist, with which they are liable to be confounded- by observing the relative positions of the styloid processes of the ulna and radius. In the natural conditions of parts, with the arm hanging by the side, the styloid process of the radius is on a lower level than that of the ulna. After fracture or separation of the epiphysis the styloid process of the radius is on the same level as, or on a higher level than, that of the ulna, whereas it would be unaltered in position in dislocation.
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