The acromioclavicular joint

The synovial acromioclavicular joint connects the clavicle with the shoulder blade. The role that this joint plays in the movement of the pectoral girdle is considered by some to be greater than that of the sternoclavicular joint, particularly for movements in or close to the sagittal plane.

Articular surfaces

The articulation is between an oval flat or slightly convex facet on the lateral end of the clavicle, and a similarly shaped flat or slightly concave facet on the anteromedial border of the acromion process. Both joint surfaces are covered with fibrocartilage. The major axis of both facets runs from anterolateral to posteromedial, so that the clavicular facet faces laterally and posteriorly, and that on the acromion faces medially and anteriorly. Consequently, the lateral end of the clavicle tends to over-ride the acromion, which together with the slope of their articulating surfaces favours displacement of the acromion downwards and under the clavicle in dislocations.

Joint capsule and synovial membrane

A relatively loose fibrous capsule surrounds the joint attaching to the articular margins. Its strong coarse fibres run in parallel fasciculi from one bone to the other. The capsule is thickest and strongest above where it is reinforced by the fibres of trapezius. Some authorities contend that the joint capsule is reinforced by two strong ligaments, the superior and inferior acromioclavicular ligaments, passing between the adjacent surfaces of the two bones. In reality these are no more than capsular thickenings, which will show varying degrees of thickening in different individuals.

Synovial membrane lines the inner surface of the capsule attaching to the margins of the articular surfaces. 

Intra-articular surfaces

A wedge-shaped, fibrocartilaginous articular disc partially divides the cavity in most joints. When present, the disc is attached to the upper inner part of the capsule and dips down between the two articulating surfaces. Only rarely does the disc form a complete partition within the joint. The presence of the articular disc partially compensates for the small degree of incongruity between the two joint surfaces.

Ligaments

Apart from the capsular thickening alluded to above, the strength of the acromioclavicular joint is provided by an extracapsular accessory ligament, the coracoclavicular ligament.

Coracoclavicular ligament

The coracoclavicular ligament is extremely powerful, anchoring the lateral end of the clavicle to the coracoid process. The ligament, which is medial to the acromioclavicular joint, stabilizes the clavicle with respect to the acromion. It is in two parts which are named according to their shapes, these being the posteromedial conoid and anterolateral trapezoid ligaments. The two parts tend to be continuous with each other posteriorly but are separated anteriorly by a small gap in which is found a synovial bursa.

The apex of the fan-shaped conoid ligament is attached posteromedially to the “elbow” of the angulated coracoid process. From here the ligament broadens as it passes upwards, more or less in the frontal plane, to attach to the conoid tubercle on the under surface of the clavicle.

The stronger and more powerful trapezoid ligament is a flat quadrilateral band. It is attached inferiorly to a roughened ridge on the upper surface of the coracoid process. The wider superior surface of the ligament is attached to the trapezoid line on the under surface of the clavicle, which runs anterolaterally from the conoid tubercle. Although the two surfaces of the trapezoid ligament are set obliquely, the ligament lies more or less in the sagittal plane, being more nearly horizontal than vertical.

Because the conoid and trapezoid ligaments lie in different planes, which are more or less at right angles to each other, and because the posterior edge of the trapezoid ligament is usually in contact with the lateral edge of the conoid ligament, a solid angle facing anteromedially is formed between them.

The two parts of the coracoclavicular ligaments are set so as to restrain opposite movements of the scapula with respect to the clavicle. The conoid ligament limits forward movement of the scapula, while the trapezoid limits backward movements. The importance of these limiting movements will be discussed more fully in the section on movements of the pectoral girdle. Both ligaments, but especially the trapezoid ligament, prevent the acromion being carried medially under the lateral end of the clavicle when laterally directed forces are applied to the shoulder. 

Blood and nerve supply

The arterial supply to the joint is by branches from the suprascapular branch of the subclavian and acromial branch of the thoracoacromial trunk. Venous drainage is to the external jugular and axillary veins. Lymphatic drainage will be to the apical group of axillary nodes.

The nerve supply to the joint is by twigs from the lateral supraclavicular, lateral pectoral, suprascapular and axillary nerves, from roots C4, 5 and 6.

Relations

The attachments of trapezius and deltoid cover the posterosuperior and anterosuperior aspects of the joint respectively. Medial to the coracoclavicular ligament the transverse superior scapular ligament converts the scapula notch into a foramen. The suprascapular vessels pass above the ligament, while beneath it and through the foramen runs the suprascapular nerve. The lateral supraclavicular nerve crosses the clavicle medial to the acromioclavicular joint.

Although not directly associated with the joint, the coracoacromial ligament, as its name suggests, is attached to both the coracoid and acromion process.

Stability

The stability of the joint is essentially provided by the coracoclavicular ligament. Trapezius and deltoid by virtue of their crossing the joint will also provide a certain amount of stability during movement of the joint.

Movement

The movements of the joint are entirely passive as there are no muscles connecting the bones which could cause one to move with respect to the other. Muscles which move the shoulder blade cause it to move on the clavicle. Indeed, all movements of the shoulder blade involve movement at both the acromioclavicular and sternoclavicular joints. All movements at the acromioclavicular joint, except that of axial rotation, are gliding movements with the coracoclavicular ligament acting so as to limit these movements.

The acromioclavicular joint has three degrees of freedom of motion about three axes. These movements are probably best described in terms of their relation to the shoulder blade rather than with respect to the cardinal axes of the body, since the joint constantly changes its relation to the trunk. The most important function of the joint is that it provides an additional range of movement for the pectoral girdle after the range of movement at the sternoclavicular joint has been exhausted.

Movement about a vertical axis(a)

This movement is associated with protraction and retraction of the shoulder blade. The axis of movement passes vertically through the lateral end of the clavicle midway between the joint and the coracoclavicular ligament. As the acromion glides backwards with respect to the clavicle, the angle between the clavicle and shoulder blade increases: similarly as the acromion glides forwards this angle decreases. Backward movement of the acromion is checked by the anterior joint capsule and actively limited by the trapezoid ligament as it becomes stretched. Forward movement will be checked by the posterior joint capsule and limited by the stretching of the conoid ligament. Towards the end of forward movement of the acromion, the trapezoid ligament may also be put under tension and therefore help to limit the movement. Compensatory movements of the clavicle at the sternoclavicular joint accompany these actions at the acromioclavicular joint.

Movement about a sagital axis(b)

Movement about this axis occurs when the shoulder blade is elevated or depressed. It has been estimated that the total range of movement about this axis is no more than 15°. Elevation is limited by tension developed in both parts of the coracoclavicular ligament, with the conoid ligament becoming stretched first; depression is checked by the coracoid process coming into contact with the under surface of the clavicle.

Axial rotation(c)

Axial rotation at the acromioclavicular joint is associated with medial and lateral rotation of the shoulder blade, that is when the glenoid fossa faces inferiorly or superiorly respectively. The range of rotation of the shoulder blade with respect to the clavicle is of the order of 30°, and occurs about an axis that passes through the conoid ligament and the acromioclavicular joint. The movement allows the flexed arm to be fully elevated. Restraints to rotation are provided by both parts of the coracoclavicular ligament.

Accessory movements

With the subject lying supine, downward pressure applied with the thumb on the lateral end of the clavicle causes it to glide backward against the acromion process.

Palpation

The line of the acromioclavicular joint can be palpated from above by applying a downward pressure to the lateral end of the clavicle.