Joint · Shoulders

Acromioclavicular joint

Articulatio acromioclavicularis

The small joint on the top of the shoulder where the outer end of the collarbone meets the acromion — the bony shelf of the shoulder blade. You can usually feel it as a step under the skin. It is a plane joint that permits only small gliding movements, but those movements let the shoulder blade rotate and tilt on the collarbone as the arm goes overhead. It sits directly under the skin at the point of the shoulder with little to protect it, which is why a fall onto the shoulder loads it so directly.