What do you think of when you think of a joint? Now forget the drugs: what do you think of when you think of a joint in an anatomical context? You most probably think of some useful piece of skeletal wizardry like your knee or your elbow: a place where serious, respectable bones are joined sturdily together in such a way as to move back and forth when pulled by the equally serious and respectable muscles attached to those very same bones, muscles which have evolved over millions of years to provide precisely that motion. Or maybe you think of the mechanical wonder that is your hip joint: strong enough to bear many times your own weight, yet mobile in so many different directions, capable of serving as a fulcrum for forces strong enough to shoot an athlete metres up into the air yet also of yielding so far as to allow a gymnast or dancer to perform the splits with no fear of injury.
The joint of which I would speak is not such a joint as these. Neither a hinge joint like your elbow, nor a ball and socket joint like your hip. More just a couple of bones stuck together at one end with pieces of string, with no muscles devoted to its motion, it is a joint you are unlikely to notice unless something goes wrong with it or you abuse it. But then you will notice it far more than you would have wished.
The acromio-clavicular joint, as its name suggests, is the articulation between the acromion and the clavicle. The clavicle is your collarbone, known and indeed legendary in saga and song for its tendency to break in riding accidents. The acromion is a spur of bone sticking up from the shoulderbone, apparently provided for no other purpose than to meet the clavicle and to share a joint with it. Where they meet their surfaces are covered with a thick layer of cartilage to enable them to slide against each other without wear, and a capsule enclosing them contains synovial fluid for lubrication. Ligaments hold the two bones in place. The connection to the collarbone keeps the shoulder stable enough to provide a fulcrum for the motion of the arm, while the mobility of the joint enables the shoulder itself to move relative to the thorax, providing for a yet greater range of movements.
As I mentioned, this is not a joint that you want to malfunction. Arthritis of the acromio-clavicular joint can be painful and lead to reduced upper-body mobility. Fractures of the bones or sprains and tears of the ligaments joining them are also painful and lead to a loss of shoulder function. For this reason it is advisable to ride your horse or bicycle with due care and attention, particularly when in the vicinity of tram lines.