= ENOSTOSIS = ENDOSTEOMA = COMPACT ISLAND

= FOCAL SCLEROSIS = SCLEROTIC BONE ISLAND

= CALCIFIED MEDULLARY DEFECT

= focal lesion of densely sclerotic (compact) bone nesting within spongiosa

Age: any age (mostly 20–80 years of age); grows more rapidly in children

Histo: nest of lamellar compacted bone with haversian system embedded within medullary canal

Pathogenesis: ? misplaced cortical hamartoma, ? developmental error of endochondral ossification as a coalescence of mature bone trabeculae with failure to undergo remodeling; not inherited

• asymptomatic

Location: ilium + proximal femur (88–92%), ribs, spine (1–14%), humerus, phalanges (not in skull)

Size: usually 2–10 mm; lesion >2 cm in longest axis = giant bone island

• round / oval / oblong solitary osteoblastic lesion with abrupt transition to surrounding normal trabecular bone:
  • “brush border” = “thorny radiations” = sharply demarcated margins with feathery peripheral radiations (HALLMARK) blending with trabeculae of surrounding spongiosa
  • long axis of bone island parallels long axis of bone
• may demonstrate slow growth / decrease in size (32%)
• NO involvement of cortex / radiolucencies / periosteal reaction

MR:

• low signal intensity at T1WI and T2WI

NUC:

• may show activity on bone scan (<10%), esp. if large

Prognosis: may increase to 8–12 cm over years (40%); may decrease / disappear

DDx:

1. Osteoblastic metastasis (aggressive, break through cortex, periosteal reaction)
2. Low-grade osteosarcoma (cortical thickening, extension beyond medullary cavity)
3. Osteoid osteoma (pain relieved by aspirin, nidus)
4. Benign osteoblastoma
5. Involuted nonossifying fibroma replaced by dense bone scar
6. Eccentric focus of monostotic fibrous dysplasia
7. Osteoma (surface lesion)