Polyostotic Fibrous dysplasia of the Zygoma and the Pubic bone

Three month history of pain in the left side of face and gradual development of a swelling at the same site. Pain in the right thigh was already present but revealed a year later.

The boy presented to the GP with a three month history of intermittent pain in the left side of the face. He was treated with antibiotics for sinusitis which did not relieve his symptoms. He was referred to the maxillofacial surgeons as a swelling was noticed in the left cheek. An orthopantomogram performed initially showed no abnormality. A CT scan which was performed revealed expansion and sclerosis of the left zygoma including the lateral walls of the adjacent orbit and maxillary antrum. There was a permeative destructive process involving the Zygomatic bone. There was soft tissue thickening of the wall of the left maxillary sinus and the lateral wall of the left globe. The full blood count was normal. A bone biopsy that was performed showed features consistent with Fibrous Dysplasia. He was managed conservatively and reviewed regularly. A year later he presented with a history of pain in the right thigh that was unrelated to movement or exercise. Plain x-ray revealed an area of sclerosis in the right superior pubic ramus. Bone scintigraphy showed increased tracer uptake in the same region. CT scan revealed sclerosis and expansion of the right superior pubic ramus consistent with Fibrous Dysplasia. He is currently being managed conservatively.

Fibrous Dysplasia (FD) is a non inherited, sporadic skeletal developmental anomaly of the bone-forming mesenchyme in which osteoblasts fail to undergo normal morphologic differentiation and maturation. Abnormal fibrous tissue is seen replacing the spongiosa and filling in the medullary cavity of the affected bone. FD presents in two forms, monostotic (70%-80%) and polyostotic (20%- 30%). Most cases are diagnosed in the second and third decades of life.
The polyostotic form tends to present early because of its more severe manifestations that lead to early clinical and radiological recognition. Monostotic FD most frequently affects the ribs, femur, tibia, mandible, skull and humerus. In the polyostotic form the skull and facial bones (craniofacial FD) are most commonly involved, followed by the pelvis, vertebral column and shoulder girdle. The common loci in craniofacial FD are the frontal followed by sphenoid, maxillary and ethmoid bones. In our case the Zygomatic bone was involved. The monostotic form is often asymptomatic until a pathological fracture occurs. It is often found incidentally. The polyostotic form usually presents with pain and deformity.
Radiographic features vary depending on the location. In the skull, hazy radiolucent lesions, with widened diploic spaces are typical. The expansion is in an outward direction with both tables of the skull vault being intact and usually thickened. In facial bones there is increased density, often with gross deformity (leontiasis ossei). In the long bones, the lesions are usually intramedullary and predominantly diaphyseal. They have a ground glass appearance and are well defined by a zone of reactive sclerosis or a thickened, hypertrophied neighbouring cortex. CT findings are usually diagnostic and are helpful in defining the full extent and nature of skeletal involvement. On CT, the lesions are expansile with well defined margins, with areas of sclerosis and lucencies. They have a hazy ground glass appearance that helps to differentiate them from other lytic lesions.
In our case, there was a permeative destructive process involving the zygoma which made us consider other causes such as Osteomyelitis and Eosinophilic Granuloma (EG) which can be considered as differential diagnoses in children, especially presenting with a solitary lesion. Osteomyelitis is usually accompanied by clinical and haematological evidence of infection. EG can appear as expansile, osteolytic lesions but, the lytic areas have a lower CT density and appear ‘black’ in comparison to the ground glass appearance in FD. Biopsy is the only definitive test in EG. In our case histological examination helped to confirm the diagnosis of FD and hence rule out EG.
Bone scintigraphy shows increased uptake of the radiopharmaceutical but this is non specific for the diagnosis of FD and must be correlated with other imaging data and clinical findings. In MR imaging, the lesions are of low intensity on T1 and vary from low to high on T2 weighted images.
Although our patient had suffered from right thigh pain during the initial presentation, he did not reveal it until a year later. This signifies the importance of a thorough clinical history and examination in a patient with craniofacial FD, bearing in mind that polyostotic FD commonly affects the skull and facial bones, presenting usually with pain and deformity. Monostotic involment does not usually convert to a polyostotic disease and in most cases the size and the number of lesions do not increase. The lesions generally become quiescent at puberty and malignant degeneration is rare (0.4%-1%).