There are many conditions both acquired and degenerative that affect the shoulder.  

Arthritis affects the shoulder joint commonly and may be due to osteoarthritis or subsequent to tears in the rotator cuff tendons. Occassionally patients with inflammatoy arthirits such as rheumatoid arthritis will require joint replacement surgery.  

Shoulder Injuries are a common cause of presentation for medical care. Most shoulder injuries involve muscles, ligaments, and tendons rather than bones. The shoulder is highly specialized to allow extreme ranges of motion and is prone to sporting injuries as well as degenerative changes.