Sclerosing Bone Lesions

Ivory Exostosis

Ivory exostoses represent a type of osteoma that affects bones formed by membranous ossification, chiefly affecting calvarial and mandibular surfaces ( Fig. 21-2 ). Multiple osteomas of this type are frequently associated with colonic polyposis and fibrous soft tissue tumors, including mesenteric fibromatosis and epidermal inclusion cysts in the familial disorder known as Gardner's syndrome. Some patients also had distinct hyperplastic changes of the retinal pigmented epithelium. The development of ocular lesions may precede the development of colonic and osseous manifestations. The polyps of the gastrointestinal tract are adenomatous and typically involve the colon but can also be present in the duodenum and stomach. The adenomatous polyps of the gastrointestinal tract have a high propensity for malignant transformation. In fact, colon cancer develops in all affected individuals unless prophylactic colectomy is performed. The disorder is transmitted by an autosomal dominant pattern of inheritance. Similar to other major familial colonic polyposis syndromes, such as flat adenoma and Turcot's syndrome (colorectal polyposis with brain tumor), Gardner's syndrome is linked to a malfunctioning adenomatous polyposis coli ( APC ) gene mapped to chromosome 5q21. Mutations involving different regions of APC are associated with the severity of the clinical syndrome and the extent of extracolonic manifestations. It has been shown that mutations involving the 3′ end around codon 1444 of the APC gene are associated with particularly severe osseous manifestations. Patients with this type of APC alteration also have a higher incidence of soft tissue fibromatosis. The soft tissue fibrous lesions range from aggressive fibromatosis at one end of the spectrum to a lesion which is histologically similar to nuchal-type fibroma. These fibromas of low cellularity have been designated as Gardner fibromas, and their significance as early indicators of the development of other manifestations of Gardner's syndrome such as desmoid-type fibromatosis has been proposed. Unlike typical nuchal-type fibromas, which most often affect patients in the third through fifth decades of life, Gardner-associated fibromas arise in younger patients and the majority of them present in the first decade of life. The most common location is in the back and paraspinal region followed by the head and neck and extremities. Immunohistochemically, these lesions stain positively for CD34 and CD99. Many of these lesions show positive nuclear immune-reactivity for β-catenin. Approximately 50% of patients with Gardner-associated fibromas eventually develop a desmoid-type fibromatosis at the same site. Overall, 10% to 15% of patients with Gardner's syndrome develop aggressive desmoid-type fibromatosis. They typically develop intraabdominally spontaneously or more often after surgery. Similar to Gardner-associated fibromas, desmoid fibromatoses associated with this syndrome harbor the inactivating mutations of the APC gene. However, similar to sporadic fibromatoses, they are positive immunohistochemically for nuclear β-catenin. In addition to osteomas, which are often multiple, dental malformation or more diffuse sclerotic changes in the craniofacial region may develop in patients with Gardner's syndrome. Osteomas in Gardner's syndrome may increase in size over time; that is, progressively more severe malformation of the craniofacial bones may develop. Osseous lesions in Gardner's syndrome do not behave as true neoplasms, and there is no evidence of their evolution into malignant lesions. Moreover, the presence of osteoblastoma-like areas, predominantly seen in sporadic sinoorbital osteomas, is not a feature of osseous lesions in Gardner's syndrome.

Microscopically, ivory exostoses are button-like excrescences of mature lamellar bone limited to the cortical surfaces and usually continuous with the outer table of the skull or the mandibular cortex. The dense cortical lamellar bone making up these exostoses contains osteons with haversian canals. Rare cases of compact surface osteoma have been reported in other flat bones as well as on long bones.

Sinoorbital Osteoma

Osteomas involving paranasal sinuses, orbit, and facial bones tend to project into the sinus cavity from a broad attachment to the affected bone ( Figs. 21-3 and 21-4 ). Some osteomas involving this region, especially those that are multifocal, are associated with Gardner's syndrome. Microscopically, they are indistinguishable from ivory exostosis. In contrast, the so-called sporadic sinoorbital osteomas (not associated with Gardner's syndrome) are usually composed of dense, immature bone and may have osteoblastic rimming as well as signs of bone resorption ( Fig. 21-5 ). These lesions may contain central or peripheral foci of spongy or cancellous appearance ( Figs. 21-6 and 21-7 ). Sometimes the latter areas show active remodeling, with cellular fibrous tissue filling the interstices, osteoblastic activity, and osteoclastic resorption. In ethmoid and frontal sinus osteomas, these lesions may occasionally contain radiolucent areas that are histologically indistinguishable from conventional osteoblastoma and even aggressive osteoblastoma ( Fig. 21-8 ). The latter cases sometimes resemble multifocal osteoblastomas arising within osteomas. The clinical behavior of these lesions is similar to that seen in osteoblastomas and aggressive osteoblastoma groups; that is, they exhibit a local destructive growth pattern and recurrences. For this reason, we prefer to regard the sporadic osteomas that arise in the orbit, paranasal sinuses, and facial bones, especially those that contain osteoblastoma-like foci or features of active bone production and remodeling, as benign bone-forming neoplasms rather than as hamartomas or dysplasias. Osteomas in this region typically do not exceed 2 cm in diameter. Occasionally, they attain a larger size (giant osteoma) that compresses adjacent structures and cause a disfiguring deformity.

Bone Island (Enostosis)

Bone islands are foci of dense, compact bone within the medullary cavity (i.e., within cancellous bone) ( Fig. 21-9 ). Occasionally, they may be attached to the inner surface of the cortex. They are ovoid, with the long axis parallel to the cortex of the affected bone. Most bone islands are small (less than 1 cm in diameter), and the majority of lesions measure from 0.1 to 2.0 cm. Extremely rare examples of giant bone islands that measure more than 2.0 cm in diameter have been described ( Figs. 21-10 and 21-11 ). The characteristic feature is the presence of so-called thorny spicules or pseudopodia at the periphery of the bone island.

Microscopically, bone islands are composed of compact (cortical) lamellar bone. The thornlike projections represent thickened trabeculae radiating from the main mass and blending with the surrounding bone ( Fig. 21-12 ). The lesions show prominent haversian canals that may contain osteoblasts and osteoclasts within Howship's lacunae. Features of active bone deposition and remodeling are more often seen in larger lesions ( Fig. 21-13 ). This explains the increased amount of bone turnover in bone islands, which can be demonstrated by skeletal scintigraphy. In fact, some bone islands may slowly increase in size (growing bone islands).

Biopsy specimens from small bone islands are rarely obtained, but larger lesions may raise the suspicion of a neoplasm, such as a sclerotic variant of osteosarcoma. We also have seen a giant bone island that was misinterpreted microscopically as a low-grade intraosseous fibroblastic osteosarcoma. Although some osteoblastic rimming may be present within dilated haversian channels of giant bone islands, the spindle-cell stroma and infiltrative pattern of growth characteristic of low-grade intraosseous fibroblastic osteosarcoma are not present in these lesions.

Surface Osteoma of Long Bones

Surface osteoma that involves the clavicle, pelvic, or long bones of the extremities is the rarest form of osteoma. On radiographs, it appears to resemble ivory exostosis (surface osteoma) of the craniofacial bones ( Fig. 21-14 ). Similar to other osteomas, this lesion is composed of mature lamellar bone organized into osteons and haversian canals. The outer surface is covered by a thin layer of fibrous tissue. There is no cartilaginous cap covering the lesion. The underlying cortex may be thickened but is otherwise intact. No continuity between the lesion and the underlying medullary cavity can be seen grossly or on radiographs. Osteoma that involves the surface of a long bone must be radiographically and microscopically differentiated from parosteal osteosarcoma, melorheostosis, and osteochondroma. The spindle-cell fibroblastic stroma that is characteristic of parosteal osteosarcoma is not present in osteoma. Moreover, the bone in osteoma is mature and forms solid areas with recognizable haversian canals, which are typically not present in parosteal osteosarcoma. The distinction from melorheostosis is based mainly on the radiographic pattern of the bone surface lesion. Multifocality and the so-called wax-dripping patterns are not present in surface osteoma. In contrast to osteochondroma, parosteal osteoma is not covered by a cartilaginous cap, and there is no continuation between the lesion and the underlying medullary cavity.