Monostotic fibrous dysplasia of the mandible

Clinical History

An 11-year-old healthy girl presented with some discomfort over the right mandible. There was no history of trauma, constitutional symptoms or recent surgery. Clinical examination and blood tests were unremarkable. Plain radiograph was requested by the maxillofacial surgeon as he was concerned regarding a bone tumour.

Imaging Findings

Plain radiograph (Fig. 1) showed subtle loss of cortico-medullary differentiation and increased lucency affecting the right ramus of the mandible. Subsequent CT (Fig. 2) showed ground-glass change and expansion of the right ramus of the mandible extending to the mandibular condyle. No aggressive periosteal reaction was seen and appearances were in keeping with a benign bone lesion. Follow-up CT (Fig. 3) was performed in 12 months time on the request of surgeons which showed similar ground-glass appearance and no associated soft tissue mass. The patient remained symptoms free during this period, therefore biopsy was not considered.

Discussion

Fibrous dysplasia (FD) is a benign fibro-osseous developmental anomaly of the mesenchymal precursor of bone, predominantly seen in 1st and 2nd decades of life with 75% of patients present before the age of 30 [1, 2] with about equal incidence in males and females [3]. FD becomes dormant by the third decade. Skull base and face is involved in 25% of cases, most commonly affecting maxilla and mandible, followed by frontal bone.
Histopathologically, FD is composed of fibrous tissue containing bone trabeculae [3].

Mandibular FD may be asymptomatic, or patients present with non-specific symptoms including pain, swelling, tenderness, and stress or overt fracture [1, 2].

FD on plain radiograph has a ground-glass or smoky matrix with altered and remodelled normal bone architecture. The lesions are mostly medulla based, may be eccentric within the affected bone, but not cortical. Ultimately, the affected bone undergoes an expansile remodelling secondary to the enlarging mass of fibro-osseous tissue. On CT, lesions typically have ground-glass appearance with sclerotic well-defined borders and no aggressive periosteal reaction. MRI is rather nonspecific and lesions appear homogeneous/ mildly heterogeneous marrow lesions, mainly hyperintense to fat on T2W, and hypointense to muscle on T1W images.
FD may undergo malignant transformation in 0.5% cases, most commonly osteosarcoma, followed by fibrosarcoma and chondrosarcoma [4].
Differentials include ossifying fibroma (OF), chronic recurrent multifocal osteomyelitis (CRMO) and Paget's disease. OF typically has thick, bony rim and lower density centre and occurs mostly in the third and fourth decades of life. OF tends to occur more commonly in the anterior mandible and is smaller in size, whereas fibrous dysplasia is more common in the posterior mandible and the lesions tend to be larger. CRMO manifest as chronic mandibular pain and/or swelling. Initial stages demonstrate a lytic process with associated variable amounts of sclerosis and associated soft-tissue inflammation, without evidence of discrete abscess formation. Mandibular disease has been shown to be associated with lesions in other skull or facial bones such as the maxilla and zygoma [5].

Treatment of FD involves bony recontouring for cosmesis and function however, the lesions can show surprising growth potential during their active growth phase if treated surgically [6]. Considering this fact and self-limiting nature of the lesion, the patient was treated conservatively.

Radiological features of FD of the mandible have been described. Familiarity with these features would help to reassure patients and alleviate their anxiety.

Differential Diagnosis List

Final Diagnosis

Fibrous dysplasia of the mandible

URL:	https://www.eurorad.org/case/10050
DOI:	10.1594/EURORAD/CASE.10050
ISSN:	1563-4086