Fibrous Dysplasia of the Spine

Fibrous dysplasia is a benign, nonhereditary bone tumor-like disease that can involve one (monostotic) or multiple (polyostotic) bones, affecting the entire skeleton. It is caused by a mutation of the gene GNAS1 resulting in a maturation defect of bone-forming mesenchymal tissue, leading to poorly mineralized woven bone trabeculae surrounded by fibrous tissue. Involvement of the spine is uncommon and may involve any vertebral element at every vertebral level, more frequently at the lumbar spine. Most monostotic lesions are asymptomatic and incidentally found. Expansile lesions tend to clinically manifest with pain, fractures, and deformities. Scoliosis is an important manifestation in polyostotic FD with multilevel involvement. Imaging plays a pivotal role in the diagnosis, identifying typical features, in the surveillance and follow-up, and therapeutic management. Additionally, it helps to guide the biopsy procedures required in case of atypical non-diagnostic radiologic findings. The radiographic presentation varies based on the histopathologic features. On plain radiograph and CT, FD may appear as pure osteolytic, or ground glass lesions surrounded by a sclerotic rim. CT allows a better assessment of lesion extension, cortical boundaries, and endosteal scalloping. On MRI, signal intensity is heterogeneous, mostly hypo/isointense on T1-weighted images (WI) and hyperintense on T2-WI. Contrast enhancement is usually present in variable intensity and distribution. PET-CT and SPECT are used in the evaluation of FD lesion burden, especially in the polyostotic form, and may be helpful in monitoring bisphosphonate therapy. Treatment options range from conservative approach and observation in asymptomatic monostotic lesions, bisphosphonate administration in painful polyostotic form, to surgical augmentation procedures in the case of vertebral fractures and surgical fixation for severe scoliosis deformities.