If your shoulder hurts or feels stiff when you reach up to take an item from a shelf, you may have shoulder, or AC arthritis. This is arthritis that occurs in your acromioclavicular, or AC, joint. Your AC joint is between your collar bone, or clavicle, and a part of your shoulder blade called the acromion. Your other shoulder joint is the glenohumeral joint, which is located between your arm and shoulder blade. Arthritis can occur in either joint, but it’s more common in the AC joint.