What is the AC Joint in the shoulder?

The shoulder is made up of three bones: the scapula (shoulder blade), the humerus (upper arm bone) and the clavicle (collarbone). The part of the scapula that makes up the roof of the shoulder is called the acromion. The joint where the acromion and the clavicle join is known as the acromioclavicular (AC) joint.

In some ways, this joint is like any other joint. It has two bones that need to attach together, but needs to be flexible as well. The ends of the bones are covered with cartilage, a slick, rubbery surface that allows movement and protects the bones. The acromioclavicular (AC) joint is different from joints like the knee or ankle, because it doesn't need to move near as much as these joints. The acromioclavicular (AC) joint just needs a little movement so that the shoulder can move freely but still be supported by the clavicle - the joint just shifts a bit as the movement occurs.

Usually there is a protuberance or bump in this area, which can be quite large in some people normally. 

This joint, like the knee joint, has a cartilage disk or meniscus inside and the ends of the bones are covered with cartilage. 

The joint is held together by a capsule, and the clavicle is held in the proper position by two heavy ligaments called coracoclavicular ligaments.
 

Anatomy of the AC Joint

Diagrammatic  (click on picture to enlarge)

The inside of the joint  (click on picture to enlarge)

Acromioclavicular Joint Separation

Acromioclavicular Joint Dislocation X-rays

Classification of injuries

Reconstruction of the acromio-clavicular joint.

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