The deltoid is the shoulder muscle. Anything 2ml and less can be appropriately administered in the deltoid. It would be highly unusual if not impossible for them to enter into the joint space with the needles used for a IM injection. Additionally, even if they did it would have no effect on your rotator cuff, which is just a group of 4 muscles. Its not one physical structure like most people tend to assume. You need at least a 1.5 inch needle to enter into the glenohumeral space, the needles in IM injections are usually 1 inch or less and they are not going all the way through l. I'm 6'5" and I've never had an issue getting an IM shoulder injection. I also do joint space injections all the time and you have to be very specific in where you aim and how to even enter into the space as the acromion blocks most of the joint off.