A bone spur in the shoulder is extra bone growth on the normal bone of the shoulder. When a bone spur grows in the shoulder, it rubs on muscles, tendons or the rotator cuff, which can cause pain and immobility in that region, according to Mayo Clinic.
In the shoulder, tendons travel through a narrow passageway between the top of the shoulder and the upper arm. It is common for bone spurs to form in this space, causing the tendon undo wear and tear as it moves. This condition commonly occurs with age, or it can be a result of ongoing repetitive use of the shoulder, according to WebMD.