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Fig. 4.29. Ossifying fibroma contains both cell-rich and cell-poor areas as well as well-structured bone and amorphous calcified material

Fig. 4.31. Juvenile trabecular ossifying fibroma shows slender bony trabeculae rimmed with osteoblasts that merge with an extremely cellular stroma

Fig. 4.30. Ossifying fibroma may also contain more smoothly contoured bony elements, formerly thought to represent cemen-tum
Fig. 4.32. At higher magnification, the plump osteoblasts that line the bony trabeculae in juvenile trabecular ossifying fibroma are shown to be a prominent feature

Ossifying fibroma is composed of fibrous tissue that may vary in cellularity from areas with closely packed cells displaying mitotic figures to almost acellular scleros-ing parts within one and the same lesion. The mineralised component may consist of plexiform bone, lamellar bone and acellular mineralised material, sometimes all occurring together in one single lesion (Figs 4.29, 4.30).

Juvenile psammomatoid and juvenile trabecular ossifying fibroma are subtypes [40]. Juvenile trabecu-lar ossifying fibroma consists of cell-rich fibrous tissue with bands of cellular osteoid together with slender trabeculae of plexiform bone lined by a dense rim of enlarged osteoblasts (Figs. 4.31, 4.32). Sometimes these trabeculae may anastomose to form a lattice. Mitoses are present, especially in the cell-rich areas. Also, multinucleated giant cells, pseudocystic stromal degeneration and haemorrhages may be present. Due to its cellularity and mitotic activity, the lesion may be confused with osteosarcoma. However, atypical cellular features or abnormal mitotic figures are not seen.

Moreover, the lesion is demarcated from its surroundings [73, 158, 181].

Juvenile psammomatoid ossifying fibroma is characterised by a fibroblastic stroma containing small ossicles resembling psammoma bodies, hence its name. The stroma varies from loose and fibroblastic to intensely cellular. The spherical or curved ossicles are acellu-lar or include sparsely distributed cells (Fig. 4.33). They should not be confused with the cementum-like deposits that are present in conventional ossifying fibroma. These particles have a smooth contour whereas the ossicles in juvenile psammomatoid ossifying fibroma has a peripheral radiating fringe of collagen fibres. Ossicles may coalesce to form trabeculae. Sometimes, juvenile psammomatoid ossifying fibroma contains basophilic, concentrically lamellated particles, as well as irregular thread-like or thorn-like calcified strands in a hyalin-ised background (Fig. 4.34). Other features such as trabeculae of woven bone as well as lamellar bone, pseudo-cystic stromal degeneration and haemorrhages resulting

Fig. 4.33. Psammomatoid ossicles in a cellular stroma characterise juvenile psammomatoid ossifying fibroma. A comparison with Fig. 4.30 shows that these particles are not as smoothly outlined as those occurring in conventional ossifying fibromas
Fig. 4.34. Thread-like calcifications in an eosinophilic matrix are also often present in juvenile psammomatoid ossifying fibroma

in areas similar to an aneurysmal bone cyst, multinu-cleate giant cells, and mitotic figures can also be observed. Juvenile psammomatoid ossifying fibroma has the bony walls of the paranasal sinuses as site of predilection [179].

Ossifying fibroma may be confused with fibrous dysplasia. The most important distinguishing feature is the presence of demarcation and/or encapsulation in ossifying fibroma as opposed to the merging with its surroundings as shown by fibrous dysplasia. In addition, the variation in cellularity as well as in appearance of mineralised material distinguishes ossifying fibroma from fibrous dysplasia. To distinguish ossifying fibroma from osseous dysplasia, data on clinical presentation and radiographic appearance are indispensable (see Sect. 4.5.3).

Juvenile psammomatoid ossifying fibroma has to be differentiated from meningioma with psammoma bodies; immunohistochemistry positive for EMA rules out juvenile psammomatoid ossifying fibroma. Moreover,

Fig. 4.35. Osseous dysplasia typically lies at the root tip. It consists of fibrous tissue containing mineralised material of varying appearance

the psammomatoid ossicles in juvenile psammomatoid ossifying fibroma are clearly different from the acellular spherical real psammoma bodies [172].

Clinically, ossifying fibroma causes expansion of the bone involved leading to a palpable swelling. Radio-graphically, a demarcated lesion is seen that may have radiodense as well as radiolucent areas depending on the various contributions of soft and hard tissue components to an individual lesion [13].

Excision of ossifying fibromas usually yields a permanent cure.

4.5.3. Osseous Dysplasia

ICD-O:9272/0

Osseous dysplasia is a pathologic process of unknown aetiology located in the tooth-bearing jaw areas in the vicinity of the tooth apices and is thought to arise from the proliferation of periodontal ligament fibroblasts that may deposit bone as well as cementum. The condition occurs in various clinical forms that bear different names. However, all have the same histomorphology: cellular fibrous tissue, trabeculae of woven as well as lamellar bone and spherules of cementum-like material (Fig. 4.35). The ratio of fibrous tissue to mineralised material may vary and it has been shown that these lesions are initially fibroblastic, but over the course of several years may show increasing degrees of calcification. This variation in ratio of soft tissue to hard tissue is reflected in the radiographic appearance; lesions are predominantly radiolucent, predominantly radiodense or mixed.

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