Fibrous dysplasia of bone
Fibrous dysplasia is a very rare nonhereditary genetic disorder where normal bone and marrow is replaced with fibrous tissue, resulting in formation of bone that is weak and prone to expansion. As a result, most complications result from fracture, deformity, functional impairment, pain, and the impingement of nerves.
Source: Wikipedia — Fibrous dysplasia of bone (CC BY-SA 4.0)