Introduction

The coracoclavicular (CC) ligament serves as the primary support for acromioclavicular (AC) ligament. Together, these stabilize the acromioclavicular joint, which is one of the major shoulder joints. To better understand the role of the coracoclavicular ligament, it is necessary to understand the anatomy of the AC joint. The AC joint serves a vital role as a synovial plane joint, which aids in the stability of the shoulder girdle [1]. The primary support of the synovial plane joint derives from two ligaments, the acromioclavicular ligament, and the coracoclavicular ligament. These ligaments further subdivide into intrinsic and extrinsic ligaments, the coracoclavicular ligament being the extrinsic ligament. Although the coracoclavicular ligament does not directly cross the AC joint, it helps to maintain the proper anatomical relationship of the acromion to the clavicle.

Structure and Function

The coracoclavicular ligament may be referred to as a complex because it is composed of two parts, the conoid and trapezoid ligaments. The conoid and trapezoid ligaments are continuous inferiorly at the coracoid process attachment but separate at an angle before attaching to the inferior aspect of the clavicle superiorly.[2] These two parts of the coracoclavicular ligament are often separated either by a bursa or by fat.

The conoid ligament attaches to the clavicle at the conoid tubercle, which is posterior medial to the trapezoid tubercle. From superior to inferior, the conoid ligament appears as an inferior pointing cone. Thus, the superior attachment at the clavicle is wide, while the inferior attachment is narrow, wrapping around the posteromedial aspect and root of the coracoid process. 

The other part of the coracoclavicular ligament, the trapezoid ligament, is typically anterior-lateral to the conoid ligament. It is quadrilateral in shape, as its name implies, and is thinner than the conoid ligament. Inferiorly the trapezoid ligament attaches to the posterior-superior aspect of the coracoid process, coming in contact with the anterior part of the conoid ligament. Considering the spacial relationship between the trapezoid and the conoid ligaments, moving from the inferior attachment to the superior attachment, one may see that the trapezoid ligament angles anterolaterally away from the conoid ligament, while the conoid ligament is nearly vertical. Finally, the trapezoid ligament attaches at the trapezoid line on the inferior surface of the clavicle. A bursa separates the trapezoid ligament and the conoid ligament.

The coracoclavicular ligament, as described above, serves to connect the clavicle and the coracoid process of the scapula. Its two-component structure allows for proper apposition of the acromion and the clavicle while preventing vertical displacement of the scapula with respect to the clavicle. The angled space between the trapezoid and conoid ligaments allows for some rotation of the scapula with respect to the clavicle. Although not an intrinsic component of the AC joint, it adds stability to the AC joint.

Some authors mention the medial coracoclavicular ligament (MCCL) or Caldani bicorne ligament, considering coracoclavicular as a lateral ligament. The MCCL was first described in 1802 and is between the posterosuperior portion of the coracoid process, the first rib (medial border) and the lower area of the clavicle (middle third). Its description is as a pearlescent yellow bundle of a fibrous structure. The MCCL has a length of about 59.5 mm. Its role is to help stabilize the acromioclavicular joint and act as a last brake, in the presence of cranial and posterior tractions.

Embryology

The embryological origin and development of the ligament system are still under analysis. Given its integral association with musculoskeletal anatomy, the belief is that precursor cells within the ligament primordia initially develop independently but later integrate to form a single functioning joint.[3] Multiple markers for joint structure development have been the topic of study in mouse and chick models, including BMP, Wnt14, Gdf5/Gdf6, and alpha-5-beta-1 integrin.[4][5][6] Other studies have also pointed to Scleraxis to be a marker of ligament development, though further analysis of its expression requires examination.[6]

The embryological leaflet involved is the mesoderm.

Blood Supply and Lymphatics

The primary structures of the coracoclavicular ligament receive their blood supply from two sources: the suprascapular artery, which arises from the subclavian artery at the level of the thyrocervical trunk and the thoracoacromial artery off the axillary artery.

The suprascapular vein, tributary of the external jugular vein, is related to the coracoclavicular ligament.

In the area of the coracoclavicular ligament, one can find lymphatic ganglia coming from the axilla and the cervical tract.

Nerves

The innervation of joints is in accordance with Hilton's Law. It states joints receive innervation from the articular branches of nerves which supply muscles acting on the joint. The region encompassing the coracoclavicular ligament receives nerve supply from branches of the brachial plexus, specifically articular branches of the suprascapular, axillary and lateral pectoral nerves.

Muscles

There are many muscles involved in shoulder girdle movement and stability. The function of the coracoclavicular ligament is to allow complex shoulder movement without separation of the scapula from the clavicle. Major muscles that cause movement around these structures include the serratus anterior, trapezius, teres major, rhomboid major, rhomboid minor, and triceps brachii (long head). Although other muscles may be included in this list, each muscle listed attaches directly to the scapula and obtains additional mechanical stability, restriction in movement, from the coracoclavicular ligament.

The tendon of the pectoralis minor crosses the fascial arch created by the MCCL at the level of the coracoid process.

Physiologic Variants

A 1975 study reported that the coracoclavicular ligament might be replaceable with a coracoclavicular bone bridge.[7] That same study cited a 1941 radiographic study describing the presence of a coracoclavicular joint, instead of a coracoclavicular ligament, in approximately 1.2% of 1000 individuals. A more recent radiographic study, comparing the prevalence of coracoclavicular joints in the French population with skeletal remains from medieval times, reported the prevalence of a coracoclavicular joint to be 0.82%.[8]

In a different cadaveric study, 24 coracoclavicular ligaments were analyzed to determine minor variations relating specifically to the conoid ligament. Researchers found three types of variations. Although 24 structures were analyzed, they reported that 50% (9/18) of the conoid ligaments analyzed ended at the root of the coracoid process, 33% (6/18) of the conoid ligaments were confluent with the superior transverse scapular ligament, and 15% (3/18) of the conoid ligaments had a distinct fascicle that originated at the inferior attachment of the conoid ligament and attached superiorly at the lateral attachment of the trapezoid ligament.[9]

Surgical Considerations

Historically, there have been multiple methods for treatment of acromioclavicular and coracoclavicular ligament injuries.[10][11][12] Treatment recommendations have their basis on the Rockwood Classification of Acromioclavicular Joint Separation.[13] Those with Type I and II injuries generally receive nonoperative treatment. Management with sling immobilization, rest, ice, and physical therapy is typical. Currently, Type III injuries are managed on an individual basis, as there are no consensuses for operative management. Those with IV-VI injuries are managed surgically with one of the methods discussed below. 

In recent years there has been a trend towards anatomic coracoclavicular reconstruction for Type IV-VI AC joint dislocations.[14][15] Anatomic reconstruction necessitates treating both components, the conoid and trapezoid ligaments, as separate entities and restoring their attachment sites near the physiologic origin. In anatomic reconstruction, patients are at increased risk for fracture at lower force due to the formation of bone tunnels for anchoring.[16][17] Other limitations include decreased range of motion, revision failure, pain with rotation, and pain or deformity at the incision site.[18] Methods of anatomic reconstruction include allographic reconstruction or fixation with a suture button.[13] Prior methods to stabilize the acromioclavicular and coracoclavicular joint included screws (Bosworth Technique), pins (Phemister Technique), cerclage wires or lag screws from the clavicle to the coracoid. While these methods demonstrated good outcomes, hardware failure, pin migration, and loss of reduction have correlated with these procedures. Studies have shown anatomic coracoclavicular reconstruction, with either suture button or native reconstruction, provides good to excellent outcomes.[16][19][20] Additionally, follow up of 23 patients over 58 months who underwent suture button coracoclavicular fixation showed that 96% were satisfied or very satisfied.[21] Though, a multicenter study of 119 cases by Calvert et al. reported an overall 27.1% complication rate, of which 11 were due to hardware failure.[22]

Clinical Significance

Symptomatic Coracoclavicular Joint

A symptomatic coracoclavicular joint is a condition associated with the actual presence of a coracoclavicular joint. While the prevalence is of the coracoclavicular joint morphology is around 1%, the literature suggests it is typically asymptomatic.[23][8] In the event of diagnosis, conservative treatment is advocated before surgical correction.[24]

Trauma

AC joint injuries commonly result from a direct fall onto the shoulder. Trauma, whether direct or indirect, may lead to dislocation or failure of the AC joint. The CC ligament helps to oppose the separation of the joint and maintain the approximation of the acromion and clavicle. If one component of the coracoclavicular joint suffers disruption, the other component acts as a fulcrum for the coracoid process to rotate under the clavicle. In the setting of injury to the AC joint, these sites of injury act for the basis of the multiple acromioclavicular joint dislocations that may occur. Rupture of the CCL occurs in grade III to VI injuries according to Rockwood Classification.[25]

• Type I—Sprain of acromioclavicular (AC) ligament, joint space intact, no CC involvement.
• Type II— Tear of AC ligament. Widened joint space; coracoclavicular distance is maintained with intact CC ligament.
• Type III—Complete disruption of AC. Coracoclavicular ligaments disrupted up to 100%, though are intact and remain attached to the periosteal sleeve — a high probability of muscle detachment. The clavicle appears mildly is displaced superiorly.
• Type IV— AC joint rupture with posterior joint dislocation into the trapezius. AC ligament complete disruption. Evident widened joint space: partial or complete coracoclavicular disruption.
• Type V— Demonstrates complete disruption of acromioclavicular ligament, coracoclavicular ligament, and muscle attachments. The clavicle dislocates grossly superiorly.
• Type VI—Complete disruption of acromioclavicular ligament. Coracoclavicular ligament completely torn, the coracoclavicular interval does not exist - partially intact or absent muscle attachments. The clavicle is displaced inferiorly relative to the acromion process.

Other Issues

Behavioral and Structural Properties 

The viscoelastic properties of the coracoclavicular ligaments varied from that of other ligaments within the shoulder capsule.[26] While there was no difference between the structural properties of the conoid and trapezoid ligaments, they were shown to be stiffer than the lateral band of the coracoacromial ligament and the glenohumeral ligaments.[27][28] Further, the anatomical orientation of the ligamentous fibers which compose the coracoclavicular ligament alter its loading dynamics and its ability to respond to external loads and subsequent risk of rupture during injury.[26] Therefore, the structural properties of each component are essential to supporting the AC joint under various internal and external loading forces.