The elbow-joint [articulatio cubiti] Class. - Diarthrosis. Subdivision. - Ginglymus.is a complete hinge, and, unlike the knee, depends for its security and strength upon the configuration of its bones rather than on the number, strength, or arrangement of its ligaments.
The bones composing it are the lower end of the humerus above, and the upper ends of the radius and ulna below; the articular surface of the humerus being received partly within the semilunar notch (great sigmoid cavity) of the ulna, and partly upon the cup-shaped area (fovea) of the radial head. The ligaments form one large and capacious capsule [capsula articularis], which, by blending with the annular ligament, and then passing on to be attached to the neck of the radius, embraces the elbow and the superior radio-ulnar joints, uniting them into one. Laterally, it is considerably strengthened by superadded fibers arising from the epicondyles of the humerus and inseparably connected with the capsule. For convenience of description it will be spoken of as consisting of four portions:
The anterior portion is attached to the front of the humerus above the articular surface and coronoid fossa, in an inverted V-shaped manner, to two very faintly marked ridges which start from the front of the medial and lateral epicondyles, and meet a variable distance above the coronoid fossa. Below, it is fixed, just beyond the articular margin, to the front of the coronoid process and it is intimately blended with the front of the annular ligament, a few fibers passing on to the neck of the radius.
It varies in strength and thickness, being sometimes so thin as barely to cover the synovial membrane; at others, thick and strong, and formed of coarse decussating fibers, the majority of which descend from the medial side laterally to the radius.
The posterior portion, thin and membranous, is attached superiorly to the humerus, in much the same inverted V-shaped way as the anterior; ascending from the medial epicondyle, along the medial side of the olecranon fossa nearly to the top; then, crossing the bottom of the fossa, it descends on the lateral side, skirting the lateral margin of the trochlear surface, and turns laterally along the posterior edge of the capitulum. Inferiorly, it is attached to a slight groove along the superior and lateral surfaces of the olecranon, and the rough surface of the ulna just beyond the radial notch, and to the annular ligament, a few fibers passing on to the neck of the radius.
It is composed of decussating fibers, most of which pass vertically or obliquely downward, a few taking a transverse course at the summit of the olecranon fossa where the ligament is usually thinnest.
The medial portion, the ulnar collateral ligament, is thicker, stronger, and denser than either the anterior or posterior portions. It is triangular in form, its apex being attached to the anterior and under aspect of the medial epicondyle, and to the condyloid edge of the groove between the trochlea and the condyle. The fibers radiate downward from this attachment, the anterior passing forward to be fixed to the rough overhanging medial edge of the coronoid process ; the middle descend less obliquely to a ridge running between the coronoid and olecranon processes, while the posterior pass obliquely backward to the medial edge of the olecranon just beyond the articular margin.
An oblique band (the oblique ligament of Sir Astley Cooper) connects the margin of the olecranon process with the margin of the coronoid process. It lies superficial to the posterior fibers of the ulnar collateral ligament. The anterior fibers are the thickest, strongest, and most pronounced.
The lateral portion, the radial collateral ligament, is attached above to the lower part of the lateral epicondyle, and from this the fibers radiate to their attachment into the lateral side of the annular ligament, a few fibers being prolonged to reach the neck of the radius. The anterior fibers reach further forward than the posterior does behind. It is strong and well-marked, but less so than the medial portion.
The synovial membrane lines the whole of the capsule, and extends into the superior radio-ulnar joint, lining the annular ligament.
Outside the synovial membrane, but inside the capsule, are often seen some pads of fatty tissue; one is situated on the medial side at the base of the olecranon, another is seen on the lateral side projecting into the cavity between the radius and ulna; this latter, with a fold of synovial membrane opposite the front of the lateral lip of the trochlea, suggests the division of the joint into two parts - one medially for the ulna, and another laterally for the radius. There are also pads of fatty tissue at the bottom of the olecranon and coronoid fossae, and at the tip of the olecranon process.
Vessels and nerves of the elbow-joint
The arterial supply is derived from each of the vessels forming the free anastomosis around the elbow, and there is also a special branch to the front and lateral side of the joint, from the brachial artery, and the arterial branch to the brachialis also feeds the front of the joint.
The nerve-supply comes chiefly from the musculo-cutaneous; the ulnar, median, and radial (musculo-spiral) also give filaments to the joint.
Relations of the elbow-joint
In front of the joint, and in immediate relation with the capsule, are the brachialis, the superficial and deep branches of the radial (musculo-spiral) nerve, the radial re- current artery, and the brachio-radiahs. The brachial artery, the median nerve, and the pronator teres are separated from the capsule by the brachialis. Directly behind the capsule are the triceps, the anconeus, and the posterior interosseous recurrent artery. On the medial side are the ulnar nerve, the superior ulnar collateral (posterior ulnar recurrent) artery, and the upper parts of the flexor carpi ulnaris and flexor digitorum sublimis. On the lateral side lie the extensor carpi radialis longus and the upper part of the common tendon of origin of the superficial extensors of the wrist and fingers.
The movements of the elbow-joint
The movements permitted at the elbow are those of a true hinge joint, viz., flexion and extension. These movements are oblique, so that the forearm is inclined medially in flexion, and laterally in extension; they are limited by the contact respectively of the coronoid and olecranon processes of the ulna with their corresponding fossae on the humerus, and their extent is determined by the relative proportion between the length of the processes and depth of the fossae which receive them, rather than by the tension of the ligaments, or the bulk of the soft parts over them. The anterior and posterior portions of the capsule, together with the corresponding portions of the collateral ligament, are not put on the stretch during flexion and extension; but, although they may assist in checking the velocity, and thus prevent undue force of impact, they do not control or determine the extent of these movements. The limit of extension is reached when the ulna is nearly in a straight line with the humerus; and the limit of flexion when the ulna describes an angle of from 30° to 40° with the humerus.
The obliquity of these movements is due to the lateral inclination of the upper and back part of the trochlear surface, and the greater prominence of the medial lip of the trochlea below; thus, the plane of motion is directed from behind forward and medially, and carries the hand toward the middle third of the clavicle. The obliquity of the joint, the twist of the shaft of the humerus, and the backward direction of its head, all tend to bring the hand toward the mid- line of the body, under the immediate observation of the eye, whether for defense, employment, or nourishment. This is in striking contrast to the lower limb, where the direction of the foot diverges from the median axis of the trunk, thus preventing awkwardness in locomotion. In flexion and extension, the cup-like depression of the radial head glides upon the capitulum, and the medial margin of the radial head travels in the groove between the capitulum and the trochlea. This allows the radius to rotate upon the humerus while following the ulna in all its movements. In full extension and supination, the head of the radius is barely in contact with the inferior surface of the capitulum, and projects so much backward that its posterior margin can be felt as a prominence at the back of the elbow. In full flexion the anterior edge of the radial head is received into, and checked against, the depression above the capitulum; while in mid-flexion the cup-like depression is fairly received upon the capitulum, and in this position, the radius being more completely steadied by the humerus than in any other, pro- nation and supination take place most perfectly.
Muscles which act upon the elbow-joint
Brachio-radialis, Pronator teres,
Flexor carpi radialis,
Flexor digitorum sublimis,
Flexor carpi ulnaris.
Anconeus, and the muscles which spring from the lateral epicondyl
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