Acromioclavicular (AC) joint

Articular surfaces

The AC joint is the articulation between the acromial end of clavicle and the clavicular facet of the acromion of scapula. The clavicular articular surface is oval and slightly convex while the acromial surface is accordingly concave. Both articular surfaces are covered with fibrocartilage.

Normally, this joint does not contain an articular disc since the articular surfaces are fully congruent i.e. they comformly oppose each other. However, in some individuals where the congruity is incomplete due to normal bony anatomical variations, an incomplete disc (meniscus) is attached to the superior acromioclavicular ligament and interposed between the articular surfaces. Very rarely, a complete articular disc is found within the joint, attaching to the superior and inferior acromioclavicular ligaments. Besides its fully congruent arrangement, the stability of the AC joint is enhanced by a joint capsule and two sets of strong ligaments.

Ligaments and joint capsule

The acromioclavicular joint is completely enclosed by a fibrous capsule. Its inner surface is lined with a synovial membrane, bounding the synovial cavity of the joint. The capsule is externally supported by the acromioclavicular ligament and trapezius muscle on its superior aspect. The posterior capsular surface is strengthened by the trapezius muscle solely, while the anterior surface is supported by the deltoid muscle. The inferior part of the capsule has no additional musculo-ligamentous support and may become incomplete later in life.

The AC joint is stabilized by the two sets of ligaments;

• A pair of intrinsic ligaments found within the articular capsule; the superior and inferior acromioclavicular ligaments
• A single extrinsic ligament found outside the joint capsule; the coracoclavicular ligament.

The superior acromioclavicular ligament connects the superior surfaces of the acromion and acromial end of clavicle. A part of the trapezius muscle fibers fuse with this ligament, supporting the joint capsule from its superior side. Similarly, the inferior acromioclavicular ligament connects the inferior surfaces of the acromion and the acromial end of clavicle. It is thinner than its superior counterpart and may perforate with age. It is particularly important in individuals with non congruent articular surfaces of AC joint, as it provides an attachment point for the articular disc if it’s present.

The coracoclavicular ligament extends between the acromial end of clavicle and coracoid process of scapula. It consists of both trapezoid and conoid parts. Fused at their coracoid attachments, these parts extend through different planes towards their respective clavicular attachments.

The trapezoid part forms the quadrangular anterolateral part of the ligament, spanning between the superior surface of coracoid process to the trapezoid line on the inferior surface of clavicle.

The conoid part comprises the conical posteromedial part and extends between the conoid tubercle of clavicle to the posterior surface and root of coracoid process. Its coracoid attachment lies lateral to the scapular notch and is continuous with the transverse scapular ligament that bridges the notch. With this arrangement, the conoid and trapezoid parts enclose a triangular space oriented posteriorly and superiorly, filled either with a pad of adipose tissue or with a bursa.

Understand the anatomy and function of synovial joint by exploring this study unit.
 

Innervation

The acromioclavicular joint is innervated by the lateral pectoral and suprascapular nerves. The lateral pectoral nerve is a branch of the lateral cord of brachial plexus, while the suprascapular nerves originate from the superior trunk of the plexus.

Blood supply

Blood supply to the AC joint comes from the thoracoacromial and suprascapular arteries. The former is a branch of the axillary artery, while the latter stems from the subclavian artery via thyrocervical trunk.

Movements

The acromioclavicular joint comprises the scapulothoracic articulation, together with the sternoclavicular joint and the soft tissue connection of the scapula and posterior chest wall. The function of this articulation is to enable the movements of the scapula that will position the glenohumeral joint in space.

Having the clavicle as a bone in common, the movements within acromioclavicular and sternoclavicular joints are closely related to each other. More specifically, the most important function of the AC joint is to enable a wider range of movements (RoM) within the pectoral girdle, namely from the point at which the sternoclavicular joint has reached its own maximal RoM.

The AC joint is a multiaxial joint which has three degrees of freedom;

1. Protraction - retraction
2. Elevation - depression (total range of motion 15°)
3. Lateral rotation - medial rotation (total range of motion 30°)

Protraction and retraction of the acromion occur around a vertical axis that goes through the lateral end of clavicle, between the AC joint and coracoclavicular ligament. Protraction is limited by the posterior part of the joint capsule and conoid part of acromioclavicular ligament. Whereas the anterior part of capsule and the trapezoid part of the acromioclavicular ligament limit retraction.

Elevation and depression of the acromion, on the other hand, happen in a sagittal axis. Elevation is sustained by tension in both parts of the coracoclavicular ligament, while  depression is eventually limited by contact between the coracoid process of scapula and  inferior surface of clavicle.

Rotation of the acromion occurs around a transverse axis that passes through the AC joint and conoid ligament. It is important for integrating the shoulder movements with those of pectoral girdle, as it enables the flexed arm to be fully elevated. The movements are limited by both parts of coracoclavicular ligament.

The AC joint takes a closed packed position when the arm is abducted at 90°, while it is in a loose pack position when the arm is resting by the side of the body. The capsular pattern of the acromioclavicular joint refers to pain at extreme range of movement, especially in relation to horizontal adduction and full elevation of the arm. The joint allows accessory movements in two axes (biaxial); protraction-retraction and elevation-depression of lateral end of clavicle.

Learn more about the mechanics of the human body movements with our articles, quizzes, videos and illustrated diagrams.

Muscles acting on the acromioclavicular joint

The movements within the acromioclavicular joint are completely passive, and depend on the movements of the scapula and clavicle within the scapulothoracic and glenohumeral joints.

The elevation and lateral rotation of the scapula on the AC joint are primarily produced by the descending and transverse parts of the trapezius muscle, which pull the acromion, spine of scapula and lateral third of clavicle superomedially. If the levator scapulae muscle contracts at the same time, it will neutralize the rotation, and the end result will be the elevation of scapula.

These movements are followed by the depression of the medial end of clavicle on the sternoclavicular joint.

The depression and medial rotation of scapula happen passively, due to relaxation of elevators combined with the force of gravity. If necessary, the pectoralis minor and the inferiormost fibers of the serratus anterior muscle forcefully depress it, while the levator scapulae, rhomboids and pectoralis minor are active medial rotators. These movements are followed by the elevation of the sternal end of the clavicle on the sternoclavicular joint.

Protraction of scapula on the AC joint is primarily produced by the serratus anterior and pectoralis minor muscles, while the retraction occurs due to contraction of the horizontal and ascending parts of the trapezius muscle, and rhomboid minor and major muscles. In protraction and retraction, the sternal end of clavicle translates posteriorly and anteriorly on the sternoclavicular joint, respectively.

AC joint separation

Acromioclavicular joint separation is a traumatic injury in which the clavicle separates from the scapula, compromising the acromioclavicular joint. It is a common injury within physically active people and is sustained by direct knocks to the shoulder (e.g. in contact sports such as rugby), or falling on an outstretched arm (FOOSH injuries). There are three grades of ACJ separation, based on the severity of the joint damage (Tossy classification);

1. Grade I - The ligaments are intact, but stretched
2. Grade II - Subluxation of clavicle due to rupture of acromioclavicular ligament
3. Grade III - Complete luxation (dislocation) of the joint, due to rupture of all ligaments

The displacement of clavicle happens in upward direction and is noticeable in the injured patient. Applying pressure to the clavicle pushes it down, however will  it lift back up when the pressure is released (“piano key sign”). Most grade I injuries do not require operative treatment. Instead, rest, pain killers, anti-inflammatory drugs combined with physical therapy leads to healing in two to three months. Higher grade injuries require reconstructive surgical methods, after which the joint usually heals in 2-3 months.