In part 1 of this two-part feature, Chris Mallac outlines the relevant anatomy and biomechanics of the accromio-clavicular joint, how they are injured, how to clinically assess injury and the relevant radiological requirements in determining the extent of injury.
Injuries to the acromioclavicular joint (ACJ) are reasonably common injury in the athlete and recreational pursuits such as cycling. It is a common complaint in contact sport athletes such as rugby, AFL, NFL and combat sports such as judo and MMA. In fact, Headey et al. (2005) show that in elite-level rugby union in the United Kingdom, ACJ injuries account for 32% of shoulder injuries, being the most frequent structure to be injured playing Rugby Union.
How these injuries are managed remains controversial. Low-grade ACJ sprains are managed conservatively; however, the more serious ACJ separations can be managed surgically or non-surgically. Bosworth, in 1941 offered the first surgical intervention into serious ACJ dislocations. Today numerous surgical techniques are available to repair a dislocated ACJ.
The joint between the clavicle and the acromion (AC joint) and the sternum and the clavicle (SC joint) are the only connections the appendicular skeleton (arms) has to the axial skeleton (trunk). Therefore the acromioclavicular joint (ACJ) may be subject to high forces due to this unique anatomical arrangement.
The ACJ is a diarthrodial joint with four planes of movement, anterio-posterior (forwards and backwards) and superoinferior (up and down). It is also able to spin on its axis during shoulder movements. The joint is surrounded by a capsule and has an intra-articular synovium making this a synovial joint. Both the clavicular and acromial bones are covered with cartilage (hyaline in late teens and early twenties that matures into fibrocartilage in the 20s).
The ACJ also has an intervening meniscoid disc between the two bones. The exact function of the meniscoid disc is poorly understood and the disc is not as well defined as the meniscoid disc that is housed within the sternoclavicular joint (SCJ). This may explain why the disc degenerates quickly with age. By the age of 40, it may be non-existent (Peterssen 1983). Due to the small nature of this disc and the high compressive forces encountered at the joint due to muscle contraction of the powerful shoulder muscles such as the pectoralis major and latissimus dorsi, this disc is thought to be prone to early breakdown along with the distal end of the clavicle. Both the disc and distal clavicle are prone to compressive failure, evidenced by the high rate of osteolysis in the distal clavicle, especially in athletes who impose huge forces on the joint such as weightlifters (Richards 1993).
The ACJ is supported by four AC ligaments – superior, inferior, anterior and posterior. These ACJ ligaments prevent excessive antero-posterior movement. Furthermore, ligaments join the coracoid process to the clavicle (the coracoacromial ligaments – CC ligaments) and these are the trapezoid and conoid ligaments. These ligaments provide supero-inferior support (up and down) as well as anterior translation support.
When the acromion separates from the clavicle (fall onto the shoulder), the ACJ ligaments are the first ligaments to stretch and withstand force (in particular, the superior ACJ ligaments), followed then by the conoid ligament and trapezoid ligament. Therefore, injuries to only the AC ligaments may still be considered a stable injury whereas injury to the conoid/ trapezoid ligaments will result in there being disruption of the ACJ as well as part of the CC ligament complex.
In 1986, Fukuda et al performed a study on cadaver specimens that investigated the contribution these ligaments play in ACJ stability. What they found can be summarised below:
Furthermore, Rockwood (1998) states that the superior AC fibers blend with the fibers of the upper trapezius and deltoid which attach to the superior aspect of the clavicle and acromion, and as a result, he argues these muscles may be important in providing active support to the ACJ.
Injuries to the ACJ are far more common in men in their early twenties, highlighting behavior as a pre-determining factor in ACJ injury. Young men are more commonly involved in sports and pastimes that will potentially lead to a traumatic ACJ injury. Sports such as football, rugby, motocross riding, mountain bike riding and combat fighting (judo, MMA, jujitsu) are sports where there exists a potential for traumatic ACJ injury.
The most common mechanisms of injury are direct falls onto the shoulder whereby the acromion is driven into the ground and the clavicle separates from the acromial attachment. This can happen in contact situations in football sports or falls off a bike whereby the rider lands on the shoulder.
When the acromion contacts the ground, a downward displacement of the clavicle is primarily resisted through an interlocking of the sternoclavicular ligaments (Bearn 1967). The clavicle remains in its normal anatomic position, and the scapula and shoulder girdle are driven inferiorly. The result, then, of a downward force being applied to the superior aspect of the acromion is either give-way of the AC and CC ligaments or clavicle fracture. There may be an additional anteroposterior direction to the force. If the downward force continues, tears of the deltoid and trapezius muscle attachments occur from the clavicle, as well as ruptures of the CC ligament. With severe force, the skin overlying the AC joint can also be disrupted. In the rare type VI injury to the AC joint, a different mechanism of injury is responsible. A severe direct force onto the superior surface of the distal clavicle, along with abduction of the arm and retraction of the scapula, has been described. The clavicle is driven inferiorly, where it lodges beneath either the acromion or the coracoid.
Indirectly, a fall onto an outstretched arm may force the humeral head to push up against the acromion, thus resulting in an ACJ separation. This is known as an indirect injury. These types of injuries only affect the AC ligaments and the CC ligaments remain intact.
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