The AC joint is made of the ends of two bones, the Acromion from the shoulder blade, and the Clavicle, or collarbone. This joint is not critical to our functioning. The collarbone functions as a strut between our breast bone (sternum) and shoulder joint. This length gives our shoulders and associated muscles the mechanical advantage needed to function at a high level.

AC JOINT ARTHRITIS AND DISTAL CLAVICULAR OSTEOLYSIS

Two conditions exist where we may have a painful AC joint. The first is AC joint arthritis. This refers to the erosion of cartilage at the end of the collarbone and/or acromion. The bone subsequently swells and gives us pain. The second condition occurs with a process called distal clavicular osteolysis. The end of the collarbone becomes very inflamed typically as a result of aggressive weight lifing.
Treatment of both conditions is the same. Initially activity avoidance and ice are used to calm down the bone inflammation. If this doesn't work, oral anti-inflammatories may be used to calm down the pain and swelling. A corticosteroid injection may be used as well.
If pain persists, surgery may be used to eliminate the pain. A distal clavicle resection is curative. For the operation, a small wedge of the end portion of the collarbone is removed. Typically no more than 5-10 millimeters of bone is resected. The clavicle still functions as a strut without the bones rubbing against each other and giving pain. Immediate motion followed by strengthening allow people to return to aggressive activities six to eight weeks following surgery.

AC SEPARATIONS (SEPARATED SHOULDERS)

Two critical sets of ligaments help stabilize the AC joint. The first pair is the AC ligaments, connecting the end collarbone to the acromion. The other pair is the CC, or coracoclavicular, ligaments.

A separation refers to an injury where one or both sets of ligaments have been injured.
Grade I injury-AC ligaments stretched
Grade II injury-AC ligaments torn, CC ligaments stretched
Grade III injury-both AC and CC ligaments torn
Grade IV-VI injury-both AC and CC ligaments torn; collarbone buttonholed through other tissue surrounding the shoulder

The treatment for Grades I and II injuries involve a brief period of rest in a sling. As pain allows, full return to sport is allowed. There is a small risk of premature arthritis necessitating distal clavicle excision with these injuries. The treatment of grade III injuries is controversial. An element of weakness or pain may persist in a small percentage of people. While there is a prominent bump over the AC joint, most individuals are able to return to all activities with excellent strength and function without pain without surgery. Surgery typically is reserved for individuals experiencing persistent pain and/or weakness. Grades IV, V, and VI generally are treated operatively because the buttonholing of the clavicle is painful and cosmetically unappealing.

Surgery for grades III, IV, V, and VI injuries involved resecting a small portion of the clavicle to prevent the late development of arthritis. The coracoclavicular lligaments then are reconstructed. This may be done either with allograft (donor) tissue or hamstring tissue from one of our knees. A sling is used for five months followed by another four to five months of physical therapy.

A gentleman separated his shoulder skiing, and sustained a grade III AC
separation."

Despite time and physical therapy, he continued to have pain and weakness
in the shoulder. He underwent coracoclavicular ligament reconstruction. He
has returned to full sport with minimal complaints."