Localization: Diaphysis of the long bones. The tibia is affected in 80 % of cases. Followed by the fibula, femur, humerus, ulna, and radius. Exceptional in the soft tissues of the leg.

Clinical: Pain and swelling. Usually, trauma is recorded in clinical history.

Imaging: Eccentric osteolysis, mostly involving the anterior cortex of the tibia, with expansion and frequent cancellation of the cortex, with no or minimal periosteal reaction, frequent sclerotic borders, and soft tissue mass. CT and MRI indicate a solid fibrous tissue (low T1, high T2 signal), sharply demarcated from the surrounding tissues. Sometimes (mainly in younger patients), osteolyses are multifocal and extended in the diaphysis and associated with anterior bowing of the tibia, so that the imaging is, at least initially, identical to that of osteofibrous dysplasia.

Histopathology: Grossly, the tissue is solid, whitish, and fibrous-firm, with occasional hemorrhagic or cystic areas. Histologically, it is composed by epithelial-like aggregates, surrounded by a fibrous or fibro-osseous stroma. Different histological patterns can be observed. The more frequent and typical (basaloid) is represented by cords and islands of epithelial cells, similar to a basal cell carcinoma. Cells may be dispersed and spindling in the center of the islands, cubic, and palisading in the periphery, recalling the ameloblastoma of jaw bones. Reticulin fibers surround the islands, but are lacking between cells. Sometimes, the tumor is composed by spindle sarcomatous cells, in intersecting fascicles or in whorls, with reticulin fibers between cells. Otherwise, small ramified tubules or alveolar cavities are seen, matted by cubic or flattened cells, in one or more layers. This aspect may resemble an adenocarcinoma of the breast or a hemangioendothelioma. Rarely and focally is a squamocellular aspect seen, with prickles and pearls of squamous epithelium. The cells of adamantinoma, whatever their shape and aggregation, are of small to medium size, with rather hyperchromatic nuclei, small or absent nucleoli, but usually with scarce pleomorphism and rare mitosis. They regularly stain with keratin antibodies and, at the electron microscope, show tonofibrils, desmosomes, microfilaments, and basal membrane. All the aforementioned cellular aggregates are surrounded by a fibrous stroma, usually abundant and dense, whose cells are positive for vimentin, not for keratins. Frequently, and especially at the periphery of the tumor, the stroma is similar to the tissue of osteofibrous dysplasia.

Course and Staging: Adamantinoma has an indolent course. The interval between the first symptoms and surgical treatment may be of several years, up to 20. Mets occur in about 15 % of cases, to the lungs, lymph nodes, and skeleton. The stage of the tumor at presentation is I A or B.