Fibrous dysplasia.

Fibrous dysplasia is a skeletal developmental anomaly of the bone-forming mesenchyme that manifests as a defect in osteoblastic differentiation and maturation.

In fibrous dysplasia, the medullary bone is replaced by fibrous tissue, which appears radiolucent on radiographs, with the classically described ground-glass appearance. Trabeculae of woven bone contain fluid-filled cysts that are embedded largely in collagenous fibrous matrix, which contributes to the generalized hazy appearance of the bone.

The following 4 disease patterns are recognized: 
(1) monostotic 
(2) polyostotic 
(3) craniofacial 
(4) cherubism

Skull and facial bones - x-ray features

The frontal bone is involved more frequently than the sphenoid, with obliteration of the sphenoid and frontal sinuses. The skull base may be sclerotic. Single or multiple, symmetric or asymmetric, radiolucent or sclerotic lesions in the skull or facial bones may be present. The external occipital protuberance may be prominent.Most commonly, maxillary and mandibular involvement has a mixed radiolucent and radiopaque pattern, with displacement of the teeth and distortion of the nasal cavities. The diploic space is widened, with displacement of the outer table. The inner table of the skull is spared in fibrous dysplasia, unlike Paget disease. Cystic calvarial lucencies, which commonly cross the sutures with sclerotic margins, may have a doughnut configuration.