The elbow-joint includes two articulations : (1) humero-ulnar, between the trochlea of the humerus and tlae trochlear notch (semilunar notch) of the ulna, and (2) humeroradial, between the capitulum of the humerus and the facet on the head of the radius. The joint-cavity and the articular capsule of the elbowjoint are continuous with the corresponding parts of the superior radio-ulnar joint, and these articulations are sometimes grouped together as the cubical articulation.
The humero-ulnar and humeroradial articulations together form a ginglymus or hinge-joint, the ligaments of which are: Capsular, Anterior, Medial, Posterior and Lateral.
The capsular ligament (figs. 535 to 537).–The anterior part of the capsular ligament is a broad and thin fibrous layer, and constitutes the anterior ligament of the elbow-joint. It is attached, above, to the front of the medial epicondyle and to the front of the humerus immediately above the coronoid and radial fossa; below, to the anterior surface of the coronoid process of the ulna and to the annular ligament, being continuous at the sides with the medial and lateral ligaments. Its superficial fibers pass obliquely from the medial epicondyle of the hulnerus to the annular ligament. The middle fibers, vertical in direction, pass from the upper part of the coronoid depression and become partly blended with the preceding, but are inserted mainly into the anterior surface of the coronoid process. The deep or transverse set intersects these at right. angles. It is in relation, in front, with the brachialis, except at ifs most lateral part. The posterior part of the capsular ligament is thin and membranous; and consists of transverse and oblique fibers, which constitute the posterior ligament of the elbow-joint. Above, it is attached to the humerus immediately behind the capitulum and close to the medial margin of the trochlea, to the margins of the olecranon fossa, and to the back of the lateral epicondyle some little distance from the trochlea. Below, it is fixed to the upper and lateral margins of the olecranon, to the posterior part of the annular ligament, and to the ulna behind the radial notch. The transverse fibers form a fasciculus which bridges the olecranon fossa; under cover of this fasciculus a pouch. of synovial membrane and a pad of fat are displaced into the upper part of the fossa when the joint is extended. In the fat a few scattered fibrous bundles pass from the deep surface of the transverse band to the upper margin of the fossa. It is in relation, behind, with the tendon of the triceps axxd the anconeus muscle.
Projecting into the joint-cavity between the radius and uhna from behind there is a crescentic fold of the synovial membrane, suggesting the division of the joint into two : one the humeroradial, the other the humero-ulnar. This fold is irregularly triangular in outline and contains a variable quantity of extrasynovial fat.
Between the capsular ligament and the synovial membrane there are three other pads of fat. The largest, over the olecranon fossa, is pressed into the fossa by the triceps during flexion of the joint; the second, over the coronoid fossa, and the third, over the radial fossa, are pressed by the brachialis into their respective fossm during extension. In addition, smaller tags of fat covered with synovial membrane project into the joint-cavity opposite to the constrictions on each side of the trochlear notch, and cover the small non-articular areas of the bone in these situations.
The nerves of the joint consist of a twig from the ulnar nerve; one from the branch of the musculocutaneous nerve to the brachialis; one from the radial nerve; and two from the median nerve.
Movements.–The elbow is a hinge-joint and therefore its movements consist of flexion and extension, the ulna moving on the trochlea, and the head of the radius on the capitulum of the humerus; as the capitulum is limited to the anterior and inferior surfaces of the lower end of the humerus, the posterior edge of the head of the radius can be felt projecting at the back of the joint when the forearm is fully extended. The movement of extension is limited by the tension of the ligaments and muscles on the front of the joint; that of flexion chiefly by the tension of the structures on the back of the joint.
When the forearm is fully extended and the hand supinated, the upper arm and forearm are not in the same line; the forearm is directed somewhat laterally, and forms with the upper arm an angle of about 167 degrees in the female and 173 in the male. This ‘carrying angle’ is caused partly by the medial edge of the trochlea of the humerus which projects about 6 mm. below the lateral edge, and partly by the obliquity of the superior articular surface of the coron oid process, which is not set at right angles to the shaft of the ulna. The angles which the articular surfaces of the humerus and the ulna make with the long axes of the bones are approximately equal, and as a result the carrying angle disappears on full flexion of the forearm and the two bones come to lie in the same plane. When this movement is carried out with the arm by the side the u1nar border of the little finger lies over the clavicle on account of the position of the resting humerus. If the humerus is rotated laterally during the movement the hand is carried upwards in front of the shoulder. The carrying angle is masked also in pronation of the extended forearm, and this has the effect of bringing the upper arm, the semipronated forearm and the hand into the same straight line. This arrangement increases the precision with which the hand, and any instrument or weapon held in the hand, can be controlled in full extension of the elbow or while the elbow is being extended.
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