Fibrous cortical defects and nonossifying (nonosteogenic) fibromas are the most common fibrous lesions of bone and are predominantly seen in children and adolescents. More common in boys than in girls, they have a predilection for the long bones, particularly the femur and tibia (Fig. 19.1). Some authors prefer the term fibroxanthoma for both lesions, whereas Schajowicz prefers the term histiocytic xanthogranuloma. These lesions are not true neoplasms and are considered by many investigators developmental defects.

Fibrous cortical defect (metaphyseal fibrous defect) is a small asymptomatic lesion found in 30% of normal individuals in the first and second decades of life. The radiolucent lesion is elliptical and confined to the cortex of a long bone near the growth plate; it is demarcated by a thin margin of sclerosis (Figs. 19.2 and 19.3). Most of these lesions disappear spontaneously, but a few may continue to enlarge. When they encroach on the medullary region of a bone, they are designated nonossifying fibroma (Fig. 19.4). With continued growth, these lesions, which are typically located eccentrically in the bone, display a characteristic scalloped sclerotic border (Fig. 19.5).

Occasionally, nonossifying fibroma may involve several bones, in which case the condition is called disseminated nonossifying fibromatosis. Some of the patients with this presentation may exhibit on the skin café-au-lait spots with smooth (“coast of California”) borders, similar to those seen in neurofibromatosis. Furthermore, they may develop neurofibromas affecting various nerves (see Chapter 33). This association is known as Jaffe-Campanacci syndrome (Fig. 19.6).

Skeletal scintigraphy shows a minimal to mild increase in activity. During the healing phase, mild hyperemia may be seen on the blood pool image, and the positive delayed scan reflects the osteoblastic activity. Computed tomography (CT) may demonstrate to better advantage the cortical thinning and medullary involvement (Fig. 19.7) and may delineate early pathologic fracture more precisely. Hounsfield attenuation values for nonossifying fibroma are higher than for normal bone marrow. Magnetic resonance imaging (MRI), usually performed for another reason, shows intermediate to low signal intensity on T1-weighted and intermediate to high signal on T2-weighted sequences (Fig. 19.8). After gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA) injection, both fibrous cortical defects and nonossifying fibromas invariably exhibit a hyperintense border and signal enhancement (Fig. 19.9). Mineralization of the lesion during healing appears predominantly as low signal intensity on MR images.

Histologically identical, regardless of size, fibrous cortical defect and nonossifying fibroma are composed of spindle and histiocytic cells that have a clear, foamy cytoplasm. In addition, osteoclast-like multinucleated giant cells are present, and varying numbers of inflammatory cells (lymphocytes) and plasma cells are scattered in the background. The cells are often arranged in a storiform pattern, typifying fibrohistiocytic lesions. Some lesions contain an excessive amount of fat within the foam cells, and the term xanthoma or fibroxanthoma may be applied to such lesions.

The term benign fibrous histiocytoma, although it may be controversial, is useful to subclassify lesions with histologic features similar to those of nonossifying fibroma but having an atypical clinical presentation and an atypical radiographic pattern. This lesion frequently has radiographic features very similar to those of nonossifying fibroma; it is radiolucent, with sharply defined and frequently sclerotic borders, without any mineralization of the matrix (Figs. 19.12 and 19.13). Its differentiation from nonossifying fibroma is made on purely clinical grounds because the histologic features of both lesions are almost identical. Patients presenting with benign fibrous histiocytoma are older (usually older than 25 years) than those with nonossifying fibroma; unlike the latter lesion, benign fibrous histiocytomas may produce symptoms such as pain or discomfort in the involved bone. These lesions also seem to run a more aggressive clinical course and may recur after treatment, which consists of curettage and bone grafting.

The periosteal desmoid is a tumor-like fibrous proliferation of the periosteum. It occurs in patients between the ages of 12 and 20 years and has a striking predilection for the posteromedial cortex of the medial femoral condyle. Many patients have a history of injury, although trauma is not necessarily a predisposing factor. The lesion simulates a fibrous cortical defect, except in the specificity of its location. Occasionally, it may simulate an aggressive and even malignant tumor. Radiographically, the hallmarks of a periosteal desmoid are its radiolucent saucer-shaped appearance, with sclerosis at the base eroding the cortex or producing cortical irregularity (Fig. 19.14). The radionuclide bone scan is usually normal but sometimes may show a focal increase in activity. CT shows a well-defined lesion, commonly with a sclerotic border (Fig. 19.15). On MRI, the lesion appears hypointense on T1-weighted and hyperintense on T2-weighted images, with a dark rim on both sequences at or near the sites of the bony attachment of the medial head of the gastrocnemius muscle. Periosteal desmoid belongs to the “don’t touch” lesions (see Table 16.10), so it should not undergo biopsy. Most lesions disappear spontaneously by the time the patient reaches age 20 years.

The histologic appearance of the lesion demonstrates fibroblastic spindle cells that produce large amounts of collagen. Large areas of hyalinization and fibrocartilage and small fragments of bone may be scattered within the fibrous tissue.

Fibrous dysplasia, occasionally termed fibrous osteodystrophy, osteodystrophia fibrosa, or osteitis fibrosa disseminata, is a fibroosseous lesion that some authorities classify among the group of developmental dysplasias. The term fibrous dysplasia was coined in 1938 by L. Lichtenstein to describe the aberrant development of fibroosseous tissue replacing normal cancellous bone. At present time, this condition is considered to be a genetically based sporadic disorder due to mutation in the GNAS1 gene, the defect that prevents
osteoblasts to form a normal lamellar bone. There are two common GNAS1 mutations associated with fibrous dysplasia, both occurring at codon 201, with arginine being substituted for either cysteine or histidine, R201C and R201H, respectively. Most recently reported, the third GNAS1 gene mutation (Q227L) represents only about 5% of the GNAS1 mutation in this condition. Clonal chromosomal alterations have also been reported, with structural recurrent aberrations of chromosome 12 (12p13).

Fibrous dysplasia may affect one bone (monostotic form) or several bones (polyostotic form). It is characterized by the replacement of normal lamellar cancellous bone by an abnormal fibrous tissue that contains small, abnormally arranged trabeculae of immature woven bone formed by metaplasia of the fibrous stroma.