Pacific Arthritis Care Center is a group rheumatology practice based in Los Angeles and Santa Monica. A rheumatologist is an internist who has undergone additional training in the diagnosis and treatment of arthritis and other diseases of the joints, muscles and bones. A rheumatologist treats arthritis, certain autoimmune diseases, musculoskeletal pain disorders and osteoporosis. There are more than 100 types of these diseases, including rheumatoid arthritis, osteoarthritis, gout, lupus, back pain, osteoporosis, fibromyalgia and tendonitis. Some of these are very serious diseases that can be difficult to diagnose and treat.