I have supraspinatus tendonosis and am told biceps tendon and bursa implicated as well. For years I have slept on my stomach hugging one pillow in each arm and angling my head down. Now, with this injury I try to sleep on my back but can’t. I assume that it is bad to sleep on my stomach with my arms up, but honestly, its more comfortable than on my “good” side which hurts my ‘bad’ shoulder – guess I need a pillow under that arm. And oddly, snuggling up on my ‘bad’ side, with shoulder against the underneath-pillow, isn’t bad at all.
Shoulder instability exists on a continuum, and on the extreme end is complete dislocation, or when the ball of the joint becomes so loose it actually comes out of the socket. It's also possible to partially dislocate your shoulder, called subluxation, which occurs when the shoulder almost slips completely out of the socket, but then pops back in. With subluxation, people often notice that their shoulder feels loose and slips in certain positions—often when their arm is raised over their head. Both subluxation and dislocation can also result from trauma, such as a fall or blow that occurs with enough force to overpower the strength of the muscles and ligaments of the shoulder.