The scapula is connected with the clavicle by a synovial joint with its ligaments at the acromio-clavicular articulation; and also by a set of ligaments passing between the coracoid process and the clavicle. So, that we have to consider: the acromio-clavicular articulation, the coraco-clavicular ligaments, the proper scapular ligaments are also best described in this section - viz., the coraco-acromial and transverse.

The Acromio-clavicular Joint

Class. - Diarthrosis. Subdivision. - Arthrodia.

The acromio-clavicular joint is surrounded by an articular capsule and frequently contains an articular disc.

The articular capsule completely surrounds the articular margins, and is composed of strong, coarse fibers arranged in parallel fasciculi, of fairly uniform thickness, which are attached to the borders as well as the surfaces of the bones. It is somewhat lax in all positions of the joint, so that the clavicle is not tightly braced to the acromion. The fibers extend three-quarters of an inch (2 cm.) along the clavicle posteriorly, but only a quarter of an inch (6 mm.) anteriorly. Superiorly, they are attached to an oblique line joining these two points, while inferiorly they reach to the ridge for the trapezoid ligament with which they blend.

At the acromion, they extend half way across the upper and lower surfaces, but at the anterior and posterior limits of the joint they are attached close to the articular facet. The anterior fibers become blended with the insertion of the Coraco-acromial ligament. The fibers are strengthened above by the aponeuroses of the trapezius and deltoid muscles; and all run from the acromion to the clavicle medially and backward.

The articular disc is occasionally present, but is usually imperfect, only occupying the upper part of the joint; it may completely divide the joint into two cavities, or be perforated in the center. It is usually thicker at the edge than in the center, and some of the fibers of the articular capsule are blended with its edges.

The synovial membrane lining the joint is occasionally either partially or entirely divided into two by the articular disc.

Relations

Superiorly skin and fascia and the tendinous intersection between the deltoid and the trapezius. Inferiorly, the coraco-acromial ligament and supraspinatus. Anteriorly, part of the origin of the deltoid. Posteriorly, part of the insertion of the trapezius.

Movements

A certain amount of gliding movement occurs at this joint, but the most important movement is a rotation of the scapula whereby the glenoid cavity is turned forward and upward, or downward. As these movements occur the inferior angle of the scapula moves forward as the glenoid cavity turns upward and the superior angle recedes.

The forward movement of the inferior angle is produced mainly by the inferior fibers of the serratus anterior (magnus), aided by the inferior fibers of the trapezius, and it is by this movement that the arm is raised above the level of the shoulder forward.

The reverse movement is produced mainly by the rhomboideus major aided by the latissimus dorsi.

The Coraco-clavicular Union

The coraco-clavicular ligament consists of two parts, the conoid and the trapezoid ligaments.

The conoid ligament

The conoid ligament is the medial and posterior portion, and passes upward and laterally from the coracoid process to the clavicle.

It is a very strong and coarsely fasciculated band of triangular shape, the apex being fixed to the medial and posterior edge of the root of the coracoid process just in front of the scapular notch, some fibers joining the transverse ligament. Its base is at the clavicle, where it widens out, to be attached to the posterior edge of the inferior surface, as well as to the coracoid tubercle. It is easily separated from the trapezoid, without being absolutely distinct. A small bursa often exists between it and the coracoid process; medially, some of the fibers of the subclavius muscle are often attached to it.

The trapezoid ligament

The trapezoid ligament is the anterior and lateral portion of the coraco-clavicular ligament. It is a strong, flat, quadrilateral plane of closely woven fibers, the surfaces of which look upward and medially toward the clavicle, and downward and laterally over the upper surface of the coracoid process.

At the coracoid it is attached for about an inch (2.5 cm.) to a rough ridge which runs forward from the angle, along the anterior border of the process. At the clavicle, it is attached to the oblique ridge which runs laterally and forward from the coracoid tubercle, reaching as far as, and blending with the inferior part of the acromio-clavicular ligament. Its anterior edge is free, and overlies the coraco-acromial ligament; the posterior edge is shorter than the anterior, and is in contact with the posterior and lateral portion of the conoid ligament.

Vessels and nerves

The arterial supply is derived from the transverse scapular (suprascapular), acromial branches of the thoraco-acromial, and the anterior circumflex.

The nerve-supply is derived from the suprascapular and axillary (circumflex) nerves.

Movements

In the movements of the shoulder girdle, the scapula moves upon the lateral end of the clavicle, and the clavicle, in turn, carried by the uniting ligaments, moves upon the sternum; so, that the entire scapula moves in the arc of a circle whose center is at the sterno-clavicular joint, and whose radius is the clavicle. The scapula, in moving upon the clavicle, also moves upon the thorax forward and backward, upward and downward, and also in a rotatory direction upon an axis drawn at right angles to the center of the bone. Throughout these movements the inferior angle and base of the scapula are kept in contact with the ribs by the latissimus dorsi, which straps down the former, and the rhomboids and serratus anterior (magnus), which brace down the latter. The glenoid cavity could not have preserved its obliquely forward direction had there been no acromio-clavicular joint, but would have shifted round a vertical axis, and thus the shoulder would have pointed medialward when the scapula was advanced, and lateralward when it was drawn backward. By means of the acromio-clavicular joint, the scapula can be forcibly advanced upon the thorax, the glenoid cavity all the time keeping its face duly forward. Thus, the muscles of the shoulder and forearm can be with advantage combined, as, for example, in giving a direct blow. The acromio-clavicular joint also permits the lower angle of the scapula to be retained in contact with the chest wall during the rising and falling of the shoulder, the scapula turning in a hinge-like manner round the horizontal axis of the joint.

There are no actions in which the scapula moves on a fixed clavicle, or the clavicle on a fixed scapula; the two bones, bound together by their connecting ligament, must move in unison.

The Proper Scapular Ligaments

There are three proper ligaments of the scapula, which pass between different portions of the bone, viz.

Coraco-acromial.

Superior transverse.

Inferior transverse.

The coraco-acromial ligament

The coraco-acromial ligament is a flat, triangular band with a broad base, attached to the lateral border of the coracoid process, and a blunt apex which is fixed to the tip of the acromion. It is made up of two broad marginal bands, and a smaller and thinner intervening portion. The anterior band, which arises from the anterior portion of the coracoid process, is the stronger, and some of its marginal fibers can often be traced into the short head of the biceps, which can then make tense this edge of the ligament. The posterior band, coming from the posterior part of the coracoid process, is also strong.

The intermediate part, of variable extent, is thin and membranous, containing but few ligamentous fibers; it is often incomplete near the coracoid process, leaving a small gap.

The superior surface of the ligament looks upward and a little forward, and is covered by the deltoid muscle; the inferior looks downward and a little backward, and is separated from the capsule of the shoulder-joint by a bursa and the tendons of the supraspinatus and sub- scapularis muscles. At the coracoid process, it overlies the coraco-humeral ligament. It is barely one-third of an inch (8 mm.) above the capsule of the shoulder, and in the undissected state there is scarcely a quarter of an inch (6 mm.) interval. The anterior band projects over the center of the head of the humerus, and is continued into a tough fascia under the deltoid; the posterior band is continuous with the fascia over the supraspinatus muscle. It binds the two processes firmly together, and so strengthens each; it holds the deltoid off the capsule of the shoulder, and protects the joint from slight injuries directed downward and backward against it.

The superior transverse (coracoid, or suprascapular) ligament

The superior transverse (coracoid, or suprascapular) ligament is a small triangular band of fibrous tissue, the surfaces of which look forward and backward; and its edges, which are thin and sharp, are turned upward and downward. It continues the superior border of the scapula, bridging over the scapular notch.

It is broader medially, where it springs from the upper border of the scapula on its dorsal surface; and narrow laterally, where it is attached to the base of the coracoid process; some of its fibers are inserted under the edge of the trapezoid ligament, and others pass upward with the conoid to reach the clavicle. The transverse scapular {suprascapular) artery passes over it, and the suprascapular nerve beneath it. Medially, some fibers of the omo-hyoid muscle arise from it.

The inferior transverse (spino-glenoid) ligament

The inferior transverse (spino-glenoid) ligament reaches from the lateral border of the spine of the scapula to the margin of the glenoid cavity, and so forms a foramen under which the transverse scapular (suprascapular) vessels and suprascapular nerve gain the infraspinous fossa. It is usually a weak membranous structure with but few ligamentous fibers.