<unk>  new bone formation or<unk>  can result from any of a large number of causes. The morphological appearance of periostitis (figure below) can be helpful, but is usually not sufficiently specific to determine its etiology. Establishing whether<unk>  new bone formation is solitary versus multi-focal (or generalized) is useful in narrowing the diagnostic possibilities. Common causes of solitary<unk>  new bone formation would include healing fractures, chronic stress injuries, subperiosteal hematomas (e.g., cephalohematoma), osteomyelitis, and bone tumors. The appearance of the adjacent bone will gives clues as to which of these is the most likely etiology.
Fig. 1 Schematic of different morphologies of<unk>  (after Resnick). (Upper left) single layer of new bone may be seen in benign or malignant tumors, infection, and secondary hypertrophic osteoarthropathy. (Upper mid) multiple layers - "onion skinning" - of new bone may be seen in hypertrophic osteoarthropathy, osteomyelitis, and tumors. (Upper right) irregular excrescences may be seen in hypertrophic osteoarthropathy, venous stasis, and thyroid acropachy. (Lower left) linear spicules are often associated with Ewing's sarcoma. (Lower mid) linear spiculates in a radial configuration - "sunburst" - are associated with osteosarcomas. (Lower right) linear new bone in a triangular configuration - "Codman triangle" - is suggestive of malignancy.
Generalized<unk>  new bone formation is less frequently encountered and has its own differential diagnosis.
Primary Hypertrophic Osteoarthropathy (Pachydermoperiostosis)
Secondary Hypertrophic Osteoarthropathy
Pulmonary - neoplasm, abscess, cystic fibrosis, mesothelioma
Cardiac - cyanotic congenital heart disease (e.g., Tetrology of Fallot)
Abdominal - Ulcerative colitis, Crohn's disease, cirrhosis, polyposis
Chronic Venous Stasis
Thyroid Acropachy
Multi-focal Osteomyelitis
Infantile Cortical Hyperostosis (Caffrey's Disease)
Child Abuse
Periosteal reaction
Heterotopic ossification
Capsular contracture
Osteolysis
Exostosis
Intramembranous ossification
Bone spur
Bone infarction