Cysts in the arm may be formed due to the following reasons: 1. It is possible that the arm is located close to the joint position, and frequent activities lead to frequent stretching and stimulation of the local joint capsule, ligaments or other soft tissues, resulting in local tissue degeneration and thus cyst formation. 2. 3, there may be hereditary reasons or personal physical problems, usually the father and grandfather have multiple cysts, thus inherited to the next generation and appear.