Acromioclavicular joint disc: Acromioclavicular joint is composed of acromion end of clavicle and acromion articular surface, belonging to plane joint. The acromioclavicular ligament is strengthened above the joint, which is the fulcrum of scapula movement. The articular disc in the joint capsule is located above, partially (rarely completely) separating the joint, and the joint mobility is very small.

The answer is obvious ~ ~ ~