The acromioclavicular joint, or ACJ for short, is a small but important joint that connects the collar bone (clavicle) to the top of the shoulder blade (acromion). The joint is there to provide stability but also as a pivoting point around which the shoulder blade can articulate when we reach up, reach across or when we shrug our shoulders. This particular joint is separate to the main call and socket joint of the shoulder but works in partnership.