All is true about Fibrous dysplasia except

Fibrous dysplasia is a 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 and pain.Morphology Fibrous dysplasia is characterized by well circumscribed intramedullary lesions of varying sizes, large masses expand and disto the bone. Robbins basic pathology 9th edition page no 780,box=