Fibrous dysplasia and other fibrous and cystic bone lesions
- Lesion
Common tumour-like lesions of young bones: a fibrous defect in the cortex, bone replaced by fibrous tissue, and two kinds of cysts.
What it is
Several benign lesions of children and young adults are often found by chance or after a fracture. Nonossifying fibroma (fibrous cortical defect when small) is an eccentric metaphyseal lesion of spindle cells that usually heals by itself. Fibrous dysplasia is a developmental lesion in which bone is replaced by fibrous tissue with curved trabeculae of woven bone, caused by a postzygotic activating GNAS mutation; it can be monostotic, polyostotic, part of McCune–Albright syndrome or of Mazabraud syndrome. Aneurysmal bone cyst is a blood-filled multicystic tumour driven by USP6 rearrangements, and the simple (unicameral) bone cyst is a fluid-filled cavity of the proximal humerus or femur.
Structure
- Nonossifying fibroma: cellular fibroblastic tissue in a whorled pattern with scattered giant cells, haemosiderin and foamy macrophages.
- Fibrous dysplasia: bland fibrous tissue with curvilinear trabeculae of woven bone that lack an osteoblast rim; cartilage nodules sometimes.
- Aneurysmal bone cyst: blood-filled spaces separated by septa containing fibroblasts, osteoclast-type giant cells and reactive woven bone.
- Simple bone cyst: single cavity with clear fluid, a thin fibrous lining and sometimes a 'fallen fragment' after fracture.
Step by step
- In fibrous dysplasia, an activating GNAS mutation arises in an early embryonic cell after conception (mosaicism).
- The mutant Gsα keeps adenylyl cyclase on, so cAMP stays high in osteoprogenitor cells.
- These cells proliferate but fail to mature, laying down fibrous tissue and irregular woven bone.
- The earlier the mutation arises in development, the more tissues carry it: bone only, or bone, skin and endocrine glands (McCune–Albright).
- The weak bone bends and fractures, and lesions stop growing in adulthood.
Fibrous and cystic lesions compared
| Lesion | Age and site | Genetics | Key features |
|---|---|---|---|
| Nonossifying fibroma | Children; metaphysis of distal femur or tibia, cortical | Often KRAS or FGFR1 mutations | Eccentric lucency with sclerotic rim; regresses |
| Fibrous dysplasia | Children and young adults; ribs, femur, skull and jaw | GNAS activating (mosaic) | Ground-glass, Chinese-letter trabeculae; McCune–Albright |
| Aneurysmal bone cyst | Under 20; metaphysis, vertebrae | USP6 rearrangement | Expansile blood-filled cysts, fluid levels |
| Simple bone cyst | Children; proximal humerus and femur | Not a clonal tumour | Single fluid-filled cavity, fallen fragment |
Clinical notes
- Many lesions are found by chance on X-rays taken for injury; typical features allow observation without biopsy.
- Fibrous dysplasia of the skull and jaw causes facial asymmetry and can compress cranial nerves.
- Girl with precocious puberty, café-au-lait spots and limping: think of McCune–Albright syndrome.
Test yourself
Think of the answer, then tap the question to see it.
Which mutation causes fibrous dysplasia?
A postzygotic activating mutation of GNAS, the gene for the Gsα protein.
What are the features of McCune–Albright syndrome?
Polyostotic fibrous dysplasia, café-au-lait spots with irregular borders and endocrine hyperfunction such as precocious puberty.
Which gene is rearranged in primary aneurysmal bone cyst?
USP6.
What is the natural history of nonossifying fibroma?
It usually heals and fills with bone on its own as the child grows.
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: McCune-Albright syndrome: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2025); Primary Bone Cancer - NCI (National Cancer Institute, 2025).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.