Definition: Periosteomedullitis is a type of osteoarthritis that occurs in the periosteum, which covers the bone surfaces. The periosteum is the thin layer of connective tissue that lines the ends of bones and protects them from damage or infection. In this condition, the connective tissues surrounding the bone are inflamed and swollen. The inflammation can cause pain, stiffness, and difficulty in movement in affected joints, especially those located near the spine. The swelling may also affect