Acromioclavicular Joint Anatomy

Introduction

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  The most widely used classification is , which is based on severity of injury.

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  Clinical evaluation is especially important in assessing posterior dislocations and scapulothoracic dyskinesia.

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  Imaging should always include standard shoulder views, including an axillary view in addition to bilateral Zanca views.

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  Nonoperative treatment is the best approach for most acromioclavicular (AC) injuries and should include short-term immobilization and active physical therapy.

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  Type III injuries are the most controversial in the literature, but there is a recent trend toward conservative treatment as well.

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  Surgical treatment should be considered in late-presenting patients, high-level athletes, and overhead workers.

Articulating Surfaces

The AC joint consists of an articulation between the lateral end of the clavicle and the acromion of the scapula. Being a diarthrodial joint, the AC joint is surrounded by a capsule and has an intraarticular synovium and an articular cartilage interface. The distal clavicle articulates with the acromion via the medial facet, which is oriented posteriorly and laterally; the articular surface of the acromion is directed medially and anteriorly ( ). The average size of the joint surfaces in the adult AC joint has been found to be 9 mm (vertical) by 19 mm (anteroposterior [AP]) ( ). The average width of the AC joint ranges from 1 mm to 3 mm and decreases with advancing age, regardless of gender. It has two atypical features:

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  The articular surfaces of the joint are lined with fibrocartilage (as opposed to hyaline cartilage). The hyaline articular cartilage becomes fibrocartilage on the acromial side of the joint by the age of 17 years and on the clavicular side by the age of 24 years.

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  The joint cavity is partially divided by an articular disk, a wedge of fibrocartilage suspended from the upper part of the capsule. This disk varies in size and shape and is of mainly two types: a complete disk (very rare) and a meniscoid-like disk that only partially separates the articular surfaces and occupies the upper part of the articulation ( ). The thickness of this disk varies, ranging from 1.5 to 4 mm. Beginning in the second decade of life, this structure undergoes rapid degeneration; by the fourth decade, this degeneration is significant.

Ligaments

The AC joint is supported by multiple ligaments, including the joint capsular ligaments, the coracoacromial (CC) ligament, and the coracoclavicular ligament. The AC joint capsule, although thin, provides considerable ligamentous support. The capsule, especially its superior and posterior parts, has been identified as the primary restraint to posterior translation of the clavicle ( ). There are four AC ligaments: superior, inferior, anterior, and posterior.

The superior AC ligament is a quadrilateral band composed of parallel fibers that interlace with the trapezius and deltoid aponeuroses, covering the superior part of the articulation and extending between the upper part of the lateral end of the clavicle and the adjoining part of the upper surface of the acromion. This ligament is thicker (2.0–5.5 mm) than the inferior and has a more defined insertion into the distal clavicle. Inferiorly, it is in contact with the articular disk when this is present.

The inferior AC ligament is the thinner of the two and covers the under part of the articulation and is attached to the adjoining surfaces of the two bones. Below this ligament lies the tendon of the supraspinatus. The CC ligament, which runs from the coracoid process to the acromion, 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. Superior to the ligament are the clavicle and undersurface of the deltoid and inferiorly is related to the tendon of the supraspinatus, a bursa being interposed. 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 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.

Two other major ligaments supporting the AC joint are:

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  Conoid: runs vertically from the coracoid process of the scapula to the conoid tubercle of the clavicle

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  Trapezoid: runs from the coracoid process of the scapula to the trapezoid line of the clavicle

Collectively, the conoid and trapezoid ligaments are known as the coracoclavicular ligaments. They are a very strong structure, effectively suspending the weight of the upper limb from the clavicle ( Fig. 11.1 ).

The conoid tubercle is located at the most posterior aspect of the clavicle, at the point where the middle third of the shaft curves into the lateral third. The trapezoid ridge extends anteriorly and laterally across the inferior surface of the lateral third of the clavicle. These landmarks represent the insertions of the corresponding ligaments ( ). The conoid ligament is the posteromedial portion and the trapezoid ligament is the anterolateral portion of the CC ligament complex. Bursae can exist between these ligaments. The clavicular insertion of the conoid ligament is approximately twice as wide (medial to lateral) and thick (anterior to posterior) as its coracoid insertion, giving rise to its inverted cone shape. The trapezoid ligament is three times thicker at its clavicular end than at its coracoid end, but it shows less narrowing of its width compared with the conoid ligament. The coracoid origin of the trapezoid covered the posterior half of the coracoid dorsum; the conoid origin is more posterior on the base of the coracoid, limited anteriorly by the trapezoid insertion. The trapezoid ligament varies from 0.8 cm to 2.5 cm both in length and in width, and the conoid ligament varies from 0.7 cm to 2.5 cm in length and from 0.4 cm to 0.95 cm in width. Several studies have shown the center of the trapezoid and the conoid ligament insertion to be located 2.5 cm and 4.6 cm from the lateral edge of the clavicle, respectively ( ).

Neurovascular Supply