Although metaphyseal fibrous defects are uncommon in a surgical practice, their true incidence is much greater because most patients with these defects never undergo surgery. There were a total of 147 cases in the Mayo Clinic files (Fig. 26.13).

There was a slight male predominance.

In its classic form, metaphyseal fibrous defect is almost exclusively a disease of childhood and adolescence. The oldest patient in this series was 37 years old. Sixteen patients were older than 20 years, and approximately 73% were in the second decade of life.

Every lesion in this series was in a long bone except for three involving the clavicle and one involving the rib. In the long bones, almost all lesions were in the metaphysis. Seven lesions involved the diaphysis: three in the femur, two in the humerus, and one each in the tibia and fibula. This probably is related to the growth of the bone, which pulls away from the stationary lesion.

Four patients had two lesions each. Two patients each had lesions of the tibia and fibula, and two patients had lesions of the tibia and femur. Four patients had polyostotic involvement with metaphyseal fibrous defects. One of these patients had skin pigmentation similar to that described in the Jaffe-Campanacci syndrome. Another patient had hemangiomas of soft tissue in addition to the skin pigmentation.

Metaphyseal fibrous defect of bone is commonly silent clinically, and it is discovered incidentally when a region is studied radiographically for unrelated reasons. Local pain, usually of short duration, is sometimes produced and may be related to small pathologic fractures. Occasionally, a patient presents with a pathologic fracture.

Physical examination is of little value in diagnosing metaphyseal fibrous defect of bone. In rare instances, slight swelling may be observed if the affected bone is near the surface of the body. Occasionally, the fracture may be a compound one. As mentioned previously, two patients showed skin pigmentation.

Most metaphyseal fibrous defects have a characteristic radiographic appearance that is virtually diagnostic. When a large tubular bone is affected, the lesion is practically always located eccentrically and often produces some bulging of the cortical outline, which is usually thin over the defect (Figs. 26.14, 26.15, 26.16 and 26.17).

The lucency begins in the metaphysis, near or at the epiphyseal line, and appears to migrate toward the center of the bone as the epiphyseal region grows away from it. The inner boundary of the lesion is demarcated by a thin or prominent scalloped line of sclerosis (Fig. 26.18). Trabeculae frequently appear to traverse a defect and give it a multilocular appearance; however, these trabeculae are nearly always incomplete and the appearance is actually produced by the shadows of corrugations on the inner surface of the cavity that houses the defect. Occasionally, the entire width of the bone may be affected. The radiographic features are so characteristic that usually only chondromyxoid fibroma is in the differential diagnosis.

It is unusual to see intact specimens of metaphyseal fibrous defect. Curetted fragments show a granular lesion that is predominantly brown but has foci of yellow discoloration. If the gross specimen is intact, it will have the characteristic lobulated appearance expected from the radiographic features. The lesion attenuates the cortex but does not breach it (Fig. 26.19).

Metaphyseal fibrous defects characteristically show a spindle cell proliferation, with a loose storiform arrangement of the cells (Fig. 26.20). The arrangement of the spindled cells is much less compact than in true fibrohistiocytic neoplasms. The cells are plump but show no hyperchromasia of the nuclei. Mitotic figures may be found (Fig. 26.21). Very characteristically, a yellow to brown pigment, which special stains show to be iron, is present within the spindle cells (Fig. 26.22). Benign giant cells are always found. These are usually in clusters but focally may be very prominent and, out of context, may suggest the diagnosis of a giant cell tumor. Foam cells containing lipid are almost always found in metaphyseal fibrous defect and produce the yellow appearance grossly (Fig. 26.23).

Typically, metaphyseal fibrous defects do not contain bone. However, small foci of reactive new bone formation may be seen, especially in association with a pathologic fracture (Fig. 26.24). Spontaneous necrosis is very unusual unless pathologic fracture has occurred. With pathologic fracture, the lesion may undergo complete infarctlike necrosis (Fig. 26.25).

Because of the presence of giant cells, the lesion may be mistaken for a giant cell tumor. However, the giant cells usually are arranged in clusters, unlike that seen in a true giant cell tumor. The occurrence in the second decade of life and the characteristic location in the metaphysis practically rule out the diagnosis of giant cell tumor. The presence of foam cells, giant cells, and spindle cells in a storiform arrangement may suggest a diagnosis of fibrohistiocytic neoplasm. Indeed, at least some of the so-called fibrous histiocytomas of bone reported in the literature probably represent metaphyseal fibrous defects in unusual locations.

If one is confident of the radiographic diagnosis and the structural integrity of the bone is not in question, no treatment is needed and the progress of the lesion
can be followed by repeat radiographs. If the diagnosis is uncertain, diagnosis and therapy can be accomplished with one surgical procedure. The lesion is readily eradicated by conservative surgical means, ordinarily curettage.

Most metaphyseal fibrous defects are thought to undergo spontaneous regression. Patients with multiple lesions are at risk for developing multiple pathologic fractures. However, these tend to regress as the skeleton matures.

There were a total of 671 fibrous dysplasias in the Mayo Clinic files (Fig. 26.30). This probably does not represent the true incidence because many patients with fibrous dysplasia are asymptomatic.

In the overall group of patients with fibrous dysplasia, females predominated slightly. However, there were distinct differences in sex distribution when different sites of involvement with fibrous dysplasia were considered. There was a distinct female predominance in the group of patients with involvement of the jaws and long and flat bones, whereas there was male predominance in the group with rib and skull disease.

The age distribution of patients with fibrous dysplasia involving different sites is given in Figure 26.31. The majority of patients with fibrous dysplasia were in the second and third decades of life. However, patients with fibrous dysplasia of the ribs tend to be older. The explanation of this discrepancy probably is that older patients are more likely to have chest radiography, which shows these incidental findings.

Patients with fibrous dysplasia were divided into four distinct groups: those with involvement of the jaws, those with involvement of the skull bones, those with involvement of the ribs, and those with involvement of all other bones. The largest single group was the last, and of this group, the proximal femur was by far the most common site. The jawbones accounted for the second largest group, and of these, the maxilla was involved much more commonly than the mandible.

Sixty-four patients had polyostotic fibrous dysplasia. Several patients with involvement of the jawbones had lesions of both the mandible and maxilla; these were not counted as examples of polyostotic fibrous dysplasia.

Many lesions of fibrous dysplasia are asymptomatic and are incidental findings on radiographs. This is especially true of the lesions of the ribs. Fibrous dysplasia may present with a pathologic fracture, especially in the femoral neck region. Some examples of fibrous dysplasia may produce swelling, especially in the region of the jaw and the skull. Occasionally, fibrous dysplasia in other locations can also become massive.

Involvement of the skull and the jawbones with fibrous dysplasia may show deformities. Patients with polyostotic fibrous dysplasia may have characteristic café-au-lait pigmentation of the skin.

Some patients with fibrous dysplasia have associated intramuscular myxomas, as described by Mazabraud. There are four cases of such patients in the Mayo Clinic files. However, fibrous dysplasia was confirmed histologically in only two of these patients; in the other two, radiographs supported the diagnosis of fibrous dysplasia and there was histologic confirmation of soft-tissue myxomas.

The defects of fibrous dysplasia are usually well-defined zones of rarefaction. The rarefied zone is often surrounded by a narrow rim of relatively sclerotic bone (Figs. 26.32, 26.33, 26.34, 26.35, 26.36 and 26.37). Expansion with thinning of the cortex is especially likely to occur in narrow bones such as the ribs. Occasionally, the lesion produces a large expansile mass that bulges into soft tissues. Lesions with a large osseous component, such as are commonly seen around the base of the skull and maxilla, are likely to be relatively radiopaque. This characteristic is accentuated if the lesion bulges into an air-containing sinus. Some examples of fibrous dysplasia contain large amounts of cartilage, which may be evident on radiographs as ringlike or dotlike calcification. This is especially common in the femoral neck region. Rarely, an example of fibrous dysplasia has a superimposed aneurysmal bone cyst. This may give rise to an aggressive-looking radiographic appearance that suggests the diagnosis of sarcoma.

Examination shows considerable variation in the lesions of fibrous dysplasia, but the average lesion is well defined and composed of dense fibrous tissue (Figs. 26.38 & 26.39). Usually embedded in this fibrous tissue are enough small trabeculae of bone to impart a distinctly gritty quality. However, rarely is the ossification sufficient to require decalcification before a section is made. Slight to extensive cyst formation may be present, whereas lesions with pronounced ossification may resemble osteoma. Lesions containing cartilage may have small nodules of well-defined islands of cartilage or the cartilaginous component may dominate the appearance.

The major feature is a proliferation of fibroblasts that produce a dense collagenous matrix. The fibroblasts tend to be plump; they show no cytologic atypia. Mitotic figures are extremely uncommon. In an otherwise typical fibrous dysplasia, one may find large areas without bone production. In these areas, the spindle cells may form a storiform pattern. Characteristic metaplastic bone formation is seen in fibrous dysplasia. Typically, the bony trabeculae show no osteoblastic rimming. However, the presence of osteoblastic rimming does not rule out the possibility of fibrous dysplasia. The bony trabeculae form a woven bone pattern unlike the architecture in mature bone. The bony trabeculae are arranged in a meaningless fashion and may have peculiar shapes characterized as “Chinese characters” (Figs. 26.40, 26.41, 26.42, 26.43, 26.44, 26.45 and 26.46). Occasionally, fibrous dysplasia shows trabecular bone formation, especially in the jawbones. The bony trabeculae may form rounded ossicle-like structures resembling psammoma bodies (Fig. 26.47). This pattern is especially prominent in the base of the skull and may lead to a mistaken diagnosis of meningioma. However, this pattern may also be seen in other locations.

Collections of foam cells almost always occur in fibrous dysplasia and may be mistaken for metastatic clear cell carcinoma, especially in limited biopsy samples. Clusters of giant cells are also commonly seen in fibrous dysplasia (Fig. 26.48). Occasionally, one may find large collections of giant cells simulating the appearance of a true giant cell tumor.

Some examples of fibrous dysplasia show marked myxoid change of the matrix. The lesion is extremely hypocellular, and characteristic areas are seen only
at the periphery (Figs. 26.49 & 26.50). Many of the fibromyxomas reported in the literature are undoubtedly examples of myxoid fibrous dysplasia.

Large islands of cartilage may dominate the histologic appearance of fibrous dysplasia. The cartilage forms rounded nodules or, occasionally, plate-like structures that resemble epiphyseal plates (Fig. 26.51). Secondary aneurysmal bone cyst areas may be seen engrafted upon fibrous dysplasia.

Treatment should be conservative. The lesions commonly stop growing when the patient reaches puberty. Therapy should be directed at restoring the normal configuration when the skull or jawbones are affected. In large bones, deformity caused by the disease may require correction. Radiation therapy is probably of no value and introduces a hazard of sarcomatous change.

The prognosis in fibrous dysplasia is good. Deforming lesions of the jawbones or skull may sometimes recur, but these lesions ordinarily respond favorably to additional conservative surgical therapy. Some of the large lesions in weight-bearing bones require curettage and bone grafting for maintenance of function. Patients with extensive disease of the ribs may experience respiratory problems. In our files, one patient with polyostotic fibrous dysplasia of the ribs died of respiratory complications.