Injuries in and around the shoulder are common in today’s athletic society.
Acromioclavicular (AC) joint injuries are common and often seen after bicycle wrecks, contact sports, and car accidents.
The acromioclavicular joint is located at the top of the shoulder where the acromion process and the clavicle meet to form a joint.
Several ligaments surround this joint, and depending on the severity of the injury, a person may tear one or all of the ligaments. Torn ligaments lead to acromioclavicular joint sprains and separations
Functional Anatomy :
The normal width of the acromioclavicula joint is 1-3 mm in younger individuals; it narrows to 0.5 mm or less in individuals older than 60 years.
The acromioclavicular joint is made up of 2 bones (the clavicle and the acromion), 4 ligaments, and a meniscus inside the joint.
The acromioclavicular joint is surrounded by a thin joint capsule and 4 small ligaments. These ligaments mostly give joint stability to anterior and posterior translation, as well as provide horizontal stability to the joint.
Another set of ligaments also provides vertical stability to the acromioclavicular joint. These ligaments are called the coracoclavicular ligaments, which are found medial to the acromioclavicular joint and go from the coracoid process on the scapula to the clavicle.
Different injuries result in different tears of the 2 coracoclavicular ligaments (the conoid and the trapezoid). Torn acromioclavicular joint ligaments and/or torn coracoclavicular ligaments are seen in acromioclavicular joint sprains. The meniscus that lies in the joint may also be injured during sprains or fractures around the acromioclavicular joint. The acromioclavicular capsular ligaments provide most of the joint stability in the anteroposterior (AP) direction. The conoid and trapezoid ligaments aid in providing superior-inferior stability to the joint. Compression of the joint is restrained mainly by the trapezoid ligament.
Acromioclavicular joint sprains have been classified according to their severity.
In a type I sprain, a mild force applied to these ligaments does not tear them. The injury simply results in a sprain, which hurts, but the shoulder does not show any gross evidence of an acromioclavicular joint dislocation.
Type II sprains are seen when a heavier force is applied to the shoulder, disrupting the acromioclavicular ligaments but leaving the coracoclavicular ligaments intact. When these injuries occur, the lateral clavicle becomes a little more prominent.
In type III sprains, the force completely disrupts the acromioclavicular and coracoclavicular ligaments. This leads to complete separation of the clavicle and obvious changes in appearance. The lateral clavicle is very prominent.