Skull base fibrous dysplasia and traumatic right orbital floor fracture through infraorbital canal

Findings

Small, non-displaced right orbital floor fracture extending through the infraorbital canal. Replacement of normal trabecular bone pattern of the basi-occiput, sella turcica, planum sphenoidal, and posterior nasal septum with an expansile lesion with a ground-glass matrix.


Diagnosis: Skull base fibrous dysplasia and traumatic right orbital floor fracture through infraorbital canal


Key points

Fibrous dysplasia is a fairly common benign congenital disorder caused by a defect in osteoblastic differentiation and maturation. It results in expanding bony lesions with mixture of fibrous tissue and woven bone.

Clinical presentations of FD are:
1) Monostotic (85%) – Femur, tibia, facial bones/skull base, ribs (in descending frequency)
2) Polyostotic (15%) - Skull / Facial bones (>50% cases), pelvis, long bones, ribs.

Bones most frequently involved in craniofacial FD: maxilla > mandible > frontal bone > ethmoid and sphenoid > temporal bone.

CT is the most helpful imaging tool for evaluation and, in general, demonstrates expanded bone with "ground glass" matrix.

There are actually three very general appearances that FD may be manifest (lesion appearance is thought to correlate with level of disease activity – in descending order):
- Cystic
- Pagetoid (mixture of the other two)
- Sclerotic (homogeneous "ground glass")

Lesion location of facial / skull base FD determines clinical presentation:
- Orbital FD – optic neuropathies (compression of CN II); Temporal FD – EAC sclerosis, conductive hearing loss, CN VII palsies; Sino nasal – mucocele.
- Severe facial bone involvement has a very characteristic appearance known as leontiasis ossea ("lion facies"). McCune-Albright Syndrome involves polyostotic FD, endocrine abnormalities (precocious puberty) and cutaneous hyperpigmentation (café-au-lait spots).

Sarcomatous transformation occurs rarely (< 0.5% cases, usually osteosarcoma).

Imaging points:
- CT: preferred modality, see above.
- MR: Low T1 and T2 signal usually. T2 hyper intensity may correlate with clinical-pathological activity of lesions. T1+C may be used to evaluate cranial neuropathies or suspected malignant transformation.
- Radionuclide scintigraphy: Frequently used to assess skeletal extent of polyostotic FD