The AC joint, also known as the acromioclavicular joint, is located at the top of the shoulder complex and is formed by the outer edge of the clavicle (collarbone) and the acromion process of the scapula (shoulder blade). Ligaments or soft tissue holds these two bones together.
Injury or separation of the AC joint is fairly common in sports like football, hockey, wrestling, rugby, or falling off a bicycle. It can occur after a fall or sharp blow to the top of the shoulder. This causes the collarbone to be forced away from the acromion, which can separate the joint by tearing the ligaments.
When an athlete injures their AC joint it is common to experience pain at the end of the collarbone with tenderness, swelling, and bruising. In more serious injuries there may be an obvious upward pointing lump at the top of the shoulder. This is the collarbone separated from the acromion.
Your doctor can make the diagnosis through an exam and X-rays. The AC joint separations are graded from 1 - 6 based on the extent of ligament injury and space between the clavicle and acromion. Grade 1 is a sprain of the joint, Grade 2 involves a tear of the ligament without displacement of the clavicle, Grades 3 - 6 all involve complete tears of the ligaments with displacement of the clavicle. Treatment is dependent on the grade of injury. With low-grade AC joint injuries you will rest the joint with a sling and ice. Higher-grade injuries may require surgical stabilization of the joint.