1. Callus —can be profuse, e.g. after an avulsion fracture, if bones misaligned or in malunion.
2. Osteochondroma —well-defined exophytic bony mass (sessile or pedunculated), usually arising from metaphysis and pointing away from the joint. The cortex and trabeculae within the lesion should be continuous with those in the metaphysis. Can transform to chondrosarcoma: worrying features include continued growth or change in morphology after physeal closure, bone destruction, soft tissue mass and cartilage cap >1 cm thick (on ultrasound/MRI).
3. Heterotopic ossification and myositis ossificans — well-defined with dense ossification in the periphery and less density centrally. May mimic surface osteosarcoma in the early stage; follow up can help differentiate by showing maturation of ossification.
4. Surface osteosarcoma —three types:
(a) Parosteal —low grade, arises from outer periosteum, usually metaphyseal. Pedunculated ‘cauliflower’ appearance with a narrow stalk + partial cleft between the mass and underlying bone. Mature osteoid matrix within the mass which is more dense centrally (in contrast to myositis ossificans).
(b) Periosteal —intermediate grade, arises from inner periosteum, usually diaphyseal. Broad-based with cortical erosion and spiculated periosteal reaction. Less organized osteoid matrix.
(c) High grade —amorphous osteoid matrix + larger soft tissue mass ± underlying cortical destruction. Usually diaphyseal.
5. Periosteal chondroma/chondrosarcoma — both typically arise on the metaphyseal surface as a soft tissue mass ± chondroid matrix. The underlying cortex is mildly scalloped. Difficult to differentiate the two on imaging; size >3 cm is more suggestive of chondrosarcoma.
7. Parosteal lipoma —juxtacortical radiolucent mass with an associated irregular bony excrescence arising perpendicularly from the periosteum.
8. Bizarre parosteal osteochondromatous proliferation (BPOP) —typically arises from the periosteum of bones in the hands and feet. Mimics an osteochondroma but does not show medullary continuity with the underlying bone.
9. Trevor’s disease— developmental disorder, presents in children with an osteochondroma-like mass arising from an epiphysis, usually at the ankle. May involve >1 joint in the same limb, e.g. knee + ankle. Overgrowth of nearby bones is usually also seen.
10. Melorheostosis— lobulated cortical hyperostosis (both periosteal and endosteal) with a ‘flowing candle wax’ appearance in a sclerotomal distribution, often crossing joints.
11. Osteoma —usually located within the paranasal sinuses but can arise from the surface of the skull, mandible or long bones in Gardner syndrome. Small, well-defined, uniformly dense osseous nodule on the cortical surface; typically sessile in the long bones with no periosteal reaction. Can be multiple and lobulated, mimicking melorheostosis, but no endosteal involvement or sclerotomal distribution.
12. Subperiosteal abscess— usually in children; lamellated periosteal reaction ± erosion of cortex. Can progress to osteomyelitis.
13. Subperiosteal haemorrhage —e.g. due to scurvy or haemophilic pseudotumour.
14. Periarticular lesions— e.g. ganglion cysts, PVNS. These can scallop the cortex.
15. Other rare lesions not specific to this site— e.g. metastasis, subperiosteal osteoid osteoma, ABC.