6b. Acromioclavicular Articulation

(Articulatio Acromioclavicularis; Scapuloclavicular Articulation) (Fig. 326)

The acromioclavicular articulation is an arthrodial joint between the acromial end of the clavicle and the medial margin of the acromion of the scapula. Its ligaments are: 1

The Articular Capsule.	The Articular Disk.
The Superior Acromioclavicular.	The Coracoclavicular	{Trapezoid and Conoid.
The Inferior Acromioclavicular.

The Articular Capsule (capsula articularis; capsular ligament).—The articular capsule completely surrounds the articular margins, and is strengthened above and below by the superior and inferior acromioclavicular ligaments. 2 The Superior Acromioclavicular Ligament (ligamentum acromioclaviculare).—This ligament is a quadrilateral band, covering the superior part of the articulation, and extending between the upper part of the acromial end of the clavicle and the adjoining part of the upper surface of the acromion. It is composed of parallel fibers, which interlace with the aponeuroses of the Trapezius and Deltoideus; below, it is in contact with the articular disk when this is present. 3 The Inferior Acromioclavicular Ligament.—This ligament is somewhat thinner than the preceding; it covers the under part of the articulation, and is attached to the adjoining surfaces of the two bones. It is in relation, above, in rare cases with the articular disk; below, with the tendon of the Supraspinatus. 4 The Articular Disk (discus articularis).—The articular disk is frequently absent in this articulation. When present, it generally only partially separates the articular surfaces, and occupies the upper part of the articulation. More rarely, it completely divides the joint into two cavities. 5 The Synovial Membrane.—There is usually only one synovial membrane in this articulation, but when a complete articular disk is present, there are two. 6 The Coracoclavicular Ligament (ligamentum coracoclaviculare) (Fig. 326).—This ligament serves to connect the clavicle with the coracoid process of the scapula. It does not properly belong to this articulation, but is usually described with it, since it forms a most efficient means of retaining the clavicle in contact with the acromion. It consists of two fasciculi, called the trapezoid and conoid ligaments. 7 The Trapezoid Ligament (ligamentum trapezoideum), the anterior and lateral fasciculus, is broad, thin, and quadrilateral: it is placed obliquely between the coracoid process and the clavicle. It is attached, below, to the upper surface of the coracoid process; above, to the oblique ridge on the under surface of the clavicle. Its anterior border is free; its posterior border is joined with the conoid ligament, the two forming, by their junction, an angle projecting backward. 8 The Conoid Ligament (ligamentum conoideum), the posterior and medial fasciculus, is a dense band of fibers, conical in form, with its base directed upward. It is attached by its apex to a rough impression at the base of the coracoid process, medial to the trapezoid ligament; above, by its expanded base, to the coracoid tuberosity on the under surface of the clavicle, and to a line proceeding medialward from it for 1.25 cm. These ligaments are in relation, in front, with the Subclavius and Deltoideus; behind, with the Trapezius. 9

FIG. 326– The left shoulder and acromioclavicular joints, and the proper ligaments of the scapula. (See enlarged image) Movements.—The movements of this articulation are of two kinds: (1) a gliding motion of the articular end of the clavicle on the acromion; (2) rotation of the scapula forward and backward upon the clavicle. The extent of this rotation is limited by the two portions of the coracoclavicular ligament, the trapezoid limiting rotation forward, and the conoid backward. 10

The acromioclavicular joint has important functions in the movements of the upper extremity. It has been well pointed out by Humphry, that if there had been no joint between the clavicle and scapula, the circular movement of the scapula on the ribs (as in throwing the shoulders backward or forward) would have been attended with a greater alteration in the direction of the shoulder than is consistent with the free use of the arm in such positions, and it would have been impossible to give a blow straight forward with the full force of the arm; that is to say, with the combined force of the scapula, arm, and forearm. “This joint,” as he happily says, “is so adjusted as to enable either bone to turn in a hinge-like manner upon a vertical axis drawn through the other, and it permits the surfaces of the scapula, like the baskets in a roundabout swing, to look the same way in every position, or nearly so.” Again, when the whole arch formed by the clavicle and scapula rises and falls (in elevation or depression of the shoulder), the joint between these two bones enables the scapula still to maintain its lower part in contact with the ribs. 11 The Ligaments of the Scapula—The ligaments of the scapula (Fig. 326) are: 12 Coracoacromial, Superior and Inferior Transverse. The Coracoacromial Ligament (ligamentum coracoaromiale).—This ligament is a strong triangular band, extending between the coracoid process and the acromion. It is attached, by its apex, to the summit of the acromion just in front of the articular surface for the clavicle; and by its broad base to the whole length of the lateral border of the coracoid process. This ligament, together with the coracoid process and the acromion, forms a vault for the protection of the head of the humerus. It is in relation, above, with the clavicle and under surface of the Deltoideus; below, with the tendon of the Supraspinatus, a bursa being interposed. Its lateral border is continuous with a dense lamina that passes beneath the Deltoideus upon the tendons of the Supraspinatus and Infraspinatus. The ligament is sometimes described as consisting of two marginal bands and a thinner intervening portion, the two bands being attached respectively to the apex and the base of the coracoid process, and joining together at the acromion. When the Pectoralis minor is inserted, as occasionally is the case, into the capsule of the shoulder-joint instead of into the coracoid process, it passes between these two bands, and the intervening portion of the ligament is then deficient. 13 The Superior Transverse Ligament (ligamentum transversum scapulæ superius; transverse or suprascapular ligament).—This ligament converts the scapular notch into a foramen. It is a thin and flat fasciculus, narrower at the middle than at the extremities, attached by one end to the base of the coracoid process, and by the other to the medial end of the scapular notch. The suprascapular nerve runs through the foramen; the transverse scapular vessels cross over the ligament. The ligament is sometimes ossified. 14 The Inferior Transverse Ligament (ligamentum transversum scapulæ inferius; spinoglenoid ligament).—This ligament is a weak membranous band, situated behind the neck of the scapula and stretching from the lateral border of the spine to the margin of the glenoid cavity. It forms an arch under which the transverse scapular vessels and suprascapular nerve enter the infraspinatous fossa. 15

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