Monostotic lesions generally occur in the proximal femur, proximal tibia, mandible, and ribs. Polyostotic disease, which usually presents earlier, may be unilateral or widespread, affecting long bones, hands, feet, facial bones, and pelvis. Extensive involvement of the proximal femur results in the distinctive “shepherd’s crook” deformity that is characteristic of fibrous dysplasia.

The result of this dysplastic process is a weakened bone that becomes deformed by normal stress or sustains frequent pathologic fractures. Painful stress fractures are especially common in the proximal femur. Although dysplastic bone heals at a normal rate after fracture, the resulting callus is also dysplastic, and the disease persists.

Diagnostic Studies. The classic radiographic feature of fibrous dysplasia is a hazy, radiolucent, or “ground-glass” pattern resulting from the defective mineralization of immature dysplastic bone; it is usually strikingly different from the radiographic appearance of normal bone, calcified cartilage, or soft tissue. A small monostotic lesion may be difficult to distinguish from other benign lesions, but extensive polyostotic involvement is likely to produce the characteristic ground-glass density and significant deformity. The lesion is primarily diaphyseal or metaphyseal and has been described as a “long lesion in a long bone.”

Bone scans demonstrate intense radioisotope uptake, and CT scans help visualize the ground-glass density of the lesion. MRI can delineate the extent of involvement and show the characteristic fat or cystic nature of some of the lesions. Fat in the fibrous dysplasia lesions may be present on MRI and is reassuring that the lesion is benign.

The typical histologic pattern is an irregular collection of small pieces of immature bone within a matrix of fibrous tissue. The overall histologic appearance has been likened to that of alphabet soup. The immature trabeculae are not lined with osteoblasts (as in ossifying fibroma), do not contain cement lines, and are obviously not aligned according to stress. The fibrous stroma is loosely arranged and immature, replacing the normal marrow. A variable degree of capillary vasculature is seen within the stroma.

The histologic differential diagnosis includes osteoblastoma (see Plate 6-3), osteosarcoma (see Plate 6-15), ossifying fibroma, hyperparathyroidism, and Paget disease of bone. Specific GNAS activating (gain-of-function) mutations have been described in fibrous dysplasia and McCune-Albright syndrome.

Treatment. Because more dysplastic bone usually forms and pain does not predictably resolve after curettage, the goal of management should be conservative and prevention of deformity and fracture. This is best accomplished using cortical bone allografts (taken from the fibula), which minimally remodel after incorporation. Treatment methods for severe bony involvement are reconstruction with joint replacements or internal metallic fixation.