Teaching Case

A 35-year old female presented to the emergency-room department with severe pain in the left hip and impaired walking. The patient had previous history of two traumatic left femoral fractures at the ages of 8 and 9, which were surgically treated. There was no other relevant medical or surgical history.

Conventional radiography (CR) of the pelvis (Fig.1) revealed a heterogeneous, mixed lytic and sclerotic, expansile lesion involving the neck and proximal diaphysis of the left femur. Similar lesions were found in the left acetabulum and ischiopubic ramus. A shepherd’s crook deformity with bowing and varus angulation of the proximal femur was also apparent.

Computed Tomography (CT) of the hips (Fig.2) depicted several well defined, slightly expansile lesions with a ground-glass matrix in both femurs and iliac bones.

Magnetic Resonance Imaging (MRI) of the hips (Figs.3,4) confirmed the presence of multiple centromedullary, expansile and well-defined lesions.

Bone scintigraphy (Fig.5) revealed multifocal tracer uptake in the appendicular skeleton and skull.

CR (Fig.6) and CT (Fig.7) of the skull revealed an expansile, mixed ground-glass/ sclerotic lesion of the occipital bone and predominantly sclerotic involvement of the left sphenoid bone. CR of the forearms (Fig.8) and legs (Fig.9) also depicted classic ground-glass expansile lesions.

Background

Fibrous Dysplasia (FD) is a genetic skeletal disorder caused by a sporadic mutation in the a-subunit of the Gs-stimulatory-protein, leading to replacement of normal bone and marrow by poorly organized fibro-osseous tissue.[1] It accounts for approximately 7% of all benign bone tumors.[2] The disease may be localized to a single bone – monostotic FD – or involve multiple bones – polyostotic FD. McCune-Albright and Mazabraud syndromes are two rare forms of disease, respectively associated with endocrine disorders and intramuscular myxomas.[1]

Clinical Perspective

The majority of FD cases are diagnosed in the first three decades of life and gender distribution is uniform.[2]

Monostotic FD is commonly asymptomatic and diagnosed incidentally on radiographs.

Polyostotic FD is more frequently symptomatic and can be associated with pain, swelling, disability and pathologic fractures.[3]

Malignant transformation is uncommon, occurring in less than 5% of cases.[3]

Imaging Perspective

Polyostotic FD most commonly affects the skull, mandible, pelvic bones and femur.[1] Classical lesions are intramedullary, expansile, well-defined with a narrow transition zone and a ground-glass matrix appearance. However, they can also appear as completely radiolucent or densely sclerotic lesions.[4] Cortical thinning or endosteal scalloping may be seen, but a smooth outer cortical contour is always preserved. FD lesions typically do not involve the adjacent soft tissues and periosteal reaction is uncommon unless there is an associated fracture.[1] In polyostotic FD severe skeletal deformities can be seen, the most characteristic being the shepherd’s crook deformity, in which there is bowing and varus angulation of the proximal femur.[4]

CT can define the extent of the disease and assess compression of adjacent structures, particularly in complex anatomical locations, such as the craniofacial bones.

MRI findings are variable, with intermediate/low signal on T1-weighted images, intermediate/high signal on T2-weighted images and heterogeneous enhancement.[5] Therefore, MRI should be reserved for complex cases, such as suspected pathological fractures or malignant transformation.[1]

Bone scans are essential to evaluate the overall extent of the disease and detect clinically silent lesions.[6]

Outcome

Treatment varies greatly depending on the degree of bone involvement and patient symptomatology. In polyostotic FD, since surgical treatment is often not an option, bisphosphonates are commonly used with positive effects on bone density and patient’s pain.[7]

Teaching Points

• FD can present not only as of the classical homogeneous, ground-glass matrix lesion but also as lytic, sclerotic or heterogenous density lesions.
• Polyostotic FD is more aggressive clinically and radiographically and can be associated with severe skeletal deformities.

Written informed patient consent for publication has been obtained.