OPBSPathologyVigorita chapter 9
Osteoma is a benign, slow growing, radiodense lesion made of mature lamellar bone. Exam interest centres on skull lesions, Gardner syndrome and its likeness to parosteal osteosarcoma.
Must Know
- Benign growth of mature lamellar, haversian type bone.
- Commonest in the skull, facial bones and paranasal sinuses.
- Sinus order of frequency is frontal, ethmoid, maxillary, then sphenoid.
- Usually a painless mass, under 5 cm.
- Sinus lesions cause sinusitis, headache, discharge or loss of smell.
- Frontoethmoid growths can erode dura, causing CSF leak, meningitis or abscess.
- Orbital lesions cause proptosis, diplopia and visual loss.
- Does not recur after excision and does not turn malignant.
- Cause unknown, with hamartoma, reactive and burnt out fibrous dysplasia theories.
Clinical Types
| Type | Features |
|---|---|
| Enostosis (bone island) | Osteoma like nodule within medullary bone |
| Skull and facial | Dome shaped ivory excrescences on calvarium, orbit or sinus |
| Parosteal (long bone) | Surface mass of cortical bone, lower limb, fourth and fifth decades |
| Gardner syndrome | Multiple osteomas with colonic polyposis |
| Systemic illness | Meningeal osteomas linked with renal disease |
Parosteal Osteoma
- Dense, smooth or lobulated mass attached to the cortical surface.
- No periosteal reaction and no lucent areas.
- Underlying cortex may thicken slightly.
- Made of cortical type bone without the cellular fibrous stroma of parosteal osteosarcoma.
- Distinction from parosteal osteosarcoma can be hard, so some advise complete excision.
Gardner Syndrome
- Autosomal dominant.
- Triad of intestinal polyposis, osteomas and epidermal inclusion cysts.
- Osteomas of mandible, skull, sinuses and long bones.
- Also dental anomalies, fibromas and mesenteric fibromatosis.
- Polyps begin in late childhood or early adulthood and often become carcinoma.
- Osteomas can be the first sign, prompting colonic screening.
Pathology
- Gross lesion is dense, hard, dome shaped or lobulated, often under a thin shiny membrane.
- Mature lamellar cortical bone with haversian canals.
- Sparse intertrabecular spaces with vessels, fibrous tissue, fat or marrow.
- Abuts host bone, often with a lobulated outline.
More on pathology
- Described subtypes are ivory, compact, spongy and mixed. The distinction has no clinical value.
- Meningeal osteomas at the dura falx junction were found in 5% of one autopsy series.
- Those meningeal lesions showed active remodelling and were linked with renal disease.
- Paranasal sinus osteomas were estimated at 3%, peaking in the fifth and sixth decades.
Osteoblastoma Like Osteoma
- Active remodelling with many osteoblasts and osteoclasts.
- Heavily ossified, solid compact bone, unlike the expansile partly calcified osteoblastoma.
- Behaves like an ordinary osteoma.
Differential Diagnosis
- Parosteal osteosarcoma. Bone within a cellular spindle cell stroma, often a cleavage plane.
- Osteochondroma. Corticomedullary continuity and a cartilage cap.
- Osteoblastoma. Expansile, partly calcified, cellular.
- Myositis ossificans. Zonal maturation, separate from cortex.
Numbers to Remember
- Size. Usually under 5 cm.
- Sinus osteoma. About 3%, fifth and sixth decades.
- Parosteal osteoma. Fourth and fifth decades.
Memory Aids
- Gardner POS. Polyps, Osteomas, Skin cysts.
- Sinus order FEMS. Frontal, ethmoid, maxillary, sphenoid.
Exam Traps
- Multiple jaw osteomas in a young adult need colonoscopy.
- A surface bony mass with cellular fibrous stroma is parosteal osteosarcoma until proven otherwise.
- Osteoma does not undergo malignant change.
Source
Vigorita VJ, Ghelman B, Mintz D. Orthopaedic Pathology. 3rd ed. Philadelphia, Wolters Kluwer, 2016. Chapter 9, Bone-Forming Bone Tumors and Tumor-like Lesions.
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 17 October 2026.