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Fig. 4.18. Odontogenic myxoma is composed of poorly cellular Fig. 4.19. Odontogenic fibroma consists of fibrous areas contain-myxoid material that surrounds pre-existing jaw bone ing epithelial odontogenic nests

4.4.2 Odontogenic Tumours -Mesenchymal

Mesenchymal odontogenic tumours are derived from the ectomesenchymal part of the tissues that participate in the development of teeth and periodontal tissues. Odontogenic epithelial rests may be part of the histo-logic picture they show, but only fortuitously by tumour tissue-engulfed structures. They have no neoplastic or inductive potential.

4.4.2.1 Odontogenic Myxoma

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Odontogenic myxomas usually occur in the 2nd or 3rd decade of life, although cases occurring at a very young or old age have been reported. It is one of the more common odontogenic tumours [8].

Myxomas consist of rather monotonous cells with multipolar or bipolar slender cytoplasmic extensions that lie in a myxoid stroma. Nuclei vary from round to fusiform in appearance. Binucleated cells and mitotic figures are present, but scarce (Fig. 4.18). Occasionally, the lesion contains odontogenic epithelial rests. They are a fortuitous finding without any diagnostic or prognostic significance. Myxoma cells are positive for vimentin and muscle-specific actin, whereas posi-tivity for S-100 is controversial [82, 102, 169].

Myxoma may be mimicked by dental follicle and dental papilla. Both contain myxoid areas [71, 101, 165]. Dental papilla tissue can be distinguished from myxoma by the presence of a peripheral layer of columnar odontoblasts. For both dental papilla and dental follicle clinical and radiographic data are decisive in avoiding misinterpretation of myxomatous tissue in jaw specimens: in the first case, a tooth germ lies in the jaw area from which the submitted tissue has been tak en whereas in the second case, the tissue sample covered the crown area of an impacted tooth.

Odontogenic myxomas occur in the maxilla as well as the mandible and in both anterior and posterior parts. Swelling may be the presenting sign as well as disturbances in tooth eruption or changes in position of teeth already erupted. In maxillary cases, nasal stuffiness may be the presenting sign due to tumour growth in nasal and paranasal cavities. Radiographically, lesions show a unilocular or soap-bubble appearance.

As the lesion lacks encapsulation, treatment usually consists of excision with a margin of uninvolved tissue [161]. Incidentally, cases with extremely aggressive local growth have been reported [35].

4.4.2.2 Odontogenic Fibroma

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Odontogenic fibroma is a controversial entity. Uncertainty exists about the broadness of histologic spectrum that these lesions may show, and about its distinction from other fibrous jaw lesions [73, 181].

Odontogenic fibroma has an age distribution of 980 years and occurs predominantly in females [55]. The lesion is seen within the jaw as well as in the gingiva [32].

Odontogenic fibroma consists of fibroblasts lying in a background of myxoid material intermingled with collagen fibres that may vary from delicate to coarse. Odontogenic epithelium, either scarce or abundant, may occur (Fig. 4.19). Only rarely is the epithelial component so conspicuous that differentiation between odontogenic fibroma and ameloblastoma may be difficult [66]. This histologic spectrum may expand to include cell-rich myxoid areas, a greater epithelial component and varying amounts of amorphous calcified globules or mineralised collagenous matrix. Tu mours with this more variegated histology have been referred to as complex odontogenic fibroma or WHO-type odontogenic fibroma [73, 181]. Odontogenic fibroma may also contain granular cells. These lesions have been called granular cell odontogenic fibromas or, alternatively, granular cell ameloblastic fibromas (see Sect. 4.4.3.1). This tumour, however, could also represent a unique entity: central odontogenic granular cell tumour [15, 170]. The granular cells are negative for epithelial markers and S-100 whereas posi-tivity for CD68 suggests a histiocytic nature [15, 23]. Rarely, this tumour may show atypical histologic features including mitotic activity and aggressive behaviour [121].

There are also lesions that combine the histologic features of giant cell granuloma and central odontogenic fibroma [108]. Their aggressive nature suggests that the giant cell granuloma (see Sect. 4.6.1) component determines the clinical behaviour.

When odontogenic fibromas show a preponderance of myxoid material, distinguishing them from odonto-genic myxomas may become problematic. It is probably best to consider such cases to be myxomas and to treat them accordingly.

Another significant diagnostic problem is the distinction between odontogenic fibroma and desmoplas-tic fibroma, which is clinically very important, as the former is benign whereas the latter shows aggressive behaviour [160]. Lesions with features of both odontogenic fibroma and desmoplastic fibroma may occur in patients with tuberous sclerosis [10].

Peripheral odontogenic fibroma should not be confused with peripheral ossifying fibroma, a gingival soft tissue lesion characterised by the presence of mineralised material of various appearances, but lacking odontogenic epithelium (see Chap. 3) [69]. Peripheral odontogenic fibroma has also to be distinguished from peripheral ameloblastoma, the former lesion lacking ameloblasto-matous epithelium. Cases of peripheral odontogenic fibroma with an extensive epithelial component have in the past been reported with the designation odontogenic epithelial hamartoma [176].

All histologic features shown by odontogenic fibroma may also be displayed by the dental follicle [37, 71, 85, 165]. In these cases, the radiographic appearance of the lesion, a small radiolucent rim surrounding the crown of a tooth buried within the jaw, will make the distinction.

Central odontogenic fibromas may present as local bony expansions of the involved jaw area. Quite often, they are incidental findings on radiographs performed for other diagnostic purposes: demarcated unilocular radiolucencies located adjacent to the roots of the neighbouring teeth or surrounding an impacted tooth. Peripheral odontogenic fibromas are firm-elastic gingival swellings.

Fig. 4.20. A hard tissue mass firmly connected to the root surface of the tooth involved is diagnostic for cementoblastoma

Treatment consists of enucleation. Peripheral cases, however, may recur after excision [32].

4.4.2.3 Cementoblastoma

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Cementoblastomas are heavily mineralised cementum masses connected to the apical root part of a tooth (Fig. 4.20) [73, 181]. These tumours are most often seen in young adults and show a predilection for males [14].

They are composed of a vascular, loose-textured fibrous tissue that surrounds coarse trabeculae of baso-philic mineralised material bordered by plump cells with ample cytoplasm and large but not atypical nuclei. Mitotic figures are rare. At the periphery, the mineralised material may form radiating spikes. Also, osteo-clastic giant cells form part of the histologic spectrum. The hard tissue component is connected with the root of the involved tooth, which usually shows signs of external resorption. The sharp border between the tubular dentin of the root and the hard tissue component forms the hallmark of cementoblastomas (Fig. 4.21).

All features of the cementoblastoma may also be shown by the osteoblastoma, except the connection with the tooth root. Therefore, cases in which this connection cannot be demonstrated should be diagnosed as osteo-blastoma and not cementoblastoma [156].

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