There are many possible injuries to the shoulder. However there are a few which make up the majority of the problems we see. The commonest is simply an over work of the shoulder often caused by unusual activity like gardening or decorating. When we tend to over use our shoulders in a very repetitive way. This produces swelling normally of the shoulder tendons and leads to protective muscle tightness of all the shoulder muscles. This causes a characteristic pain in the outside of the upper arm and frequently loads the neck causing secondary problems, bringing out any underlying problems here. The second type of shoulder problem is a capsulitis. The swelling this time is of the bag surrounding the joint which keeps the lubricating fluid in the right place. This problem is normally more painful and is characterised by pain at the end of every direction of movement.
The third type is the frozen shoulder ( adhesive capsulitis) similar to the capsulitis but with an added element damage is caused to the capsule possibly by the bodies immune system , this causes the capsule to shrink so as well as pain in all directions the range of movement reduces sometimes dramatically so you cant reach out to the side or above your shoulder height.
The last common problem is a rotator cuff tear, this is a muscle / tendon tear of an important group of muscles controlling shoulder stabilisation . The shoulder is a ball an socket joint like the hip but with the the socket reduced in size. This causes problems for control as the ball has to keep in the correct alignment with the socket. In extreme situations if this does not happen the shoulder can dislocate. The muscles that produce this balancing act are the rotator cuff, they can become torn by heavy lifting or in accidents. The ball will often ride up in the socket and produce what is call impingement as it rubs against a shelf of bone above the joint which is formed from the end of the shoulder blade. An important tendon runs in the space between these two bones and this becomes squeezed between them causing pain.