OSSIFYING FIBROMA: CONTROVERSIES, CLINICAL FEATURES, RADIOLOGY & HISTOPATHOLOGY
Author: Sanketh DS, MDS
INTRODUCTION, CONTROVERSIES & ETIOLOGY
Fibro-osseous lesions (FOL) are those bone lesions, where normal bone is replaced by numerous fibroblasts and collagen with varying degree of dispersed calcifications in the form of immature bone and cementum like tissue. Among the many fibro-osseous lesions classified in literature, ossifying fibroma is a neoplastic FOL with proliferative capabilities that exhibits bony expansion.
Before we dive into the specifics of the lesion, let us try to understand few concepts to better comprehend the controversies behind nomenclature and etiology of the lesion.
Under the microscope, ossifying fibroma shows calcification in the form of bony trabeculae and basophilic spherules which appear similar to cementum like material. These spherules were thought to be “cementum-like”, due to the fact that they appear similar to cementicles. Cementicles are basophilic spherical calcifications that appear in the periodontal ligament and attached to or sometimes embedded in the cementum. These calcifications are thought to arise from epithelial rests near the tooth root or from spicules of cementum or bone detached due to trauma. Hence, basophilic spherical calcifications were designated to be cementum or “cementum-like” tissue.
Due to the fact that, ossifying fibromas appear exclusively in the jaws and have cementum-like material, their origin was thought to be either odontogenic or from the periodontal ligament. OFs showing only “cementum-like” calcifications were termed, “cementifying fibroma” and those with a mixture of both bony trabeculae and basophilic spherules were termed “cemento-ossifying fibroma.”
However, it is known that certain bone lesions appearing in sites other than the jaw, like the skull, the facial skeleton or sometimes the long bones may also exhibit basophilic cementum-like spherules of calcification. Moreover, pathologists and investigators, in spite of numerous studies, have not been able to distinguish between bone and cemental tissue. There is no sufficient evidence to differentiate between bone and cemental tissue. In fact, most contend the basophilic spherules to be a variation of bone tissue. Pathologists, therefore, advice against using the term “cementum” to describe the spherical basophilic calcifications unless these calcifications are in direct contact or attached to the cementum. In such a scenario, the term “cementifying” fibroma or “cemento-ossifying” fibroma becomes confusing. Hence, most pathologists and researchers consider the term “ossifying fibroma” to more accurately describe the lesion.
The etiology of the lesion is unknown, although evidence points to mutation of the HRPT2 gene in select cases of ossifying fibroma. But how this mutation plays a role in causing the lesion remains not well understood!
“However, there is a general consensus that when ossifying fibroma arises in the tooth-bearing areas, it is of odontogenic origin and arises within the periodontal ligament. In 2017, the (WHO) consensus group felt that the term cemento-ossifying fibroma is suitably descriptive and indicates that these lesions are specific to the tooth-bearing areas of the jaws and can be distinguished from the two juvenile variants of ossifying fibroma. The new 4th edition therefore classifies cemento-ossifying fibroma as a benign mesenchymal odontogenic tumour. “
– WHO Consensus group (2017)
Reference: Speight PM, Takata T. New tumour entities in the 4th edition of the World Health Organization Classification of Head and Neck tumours: odontogenic and maxillofacial bone tumours. Virchows Arch. 2017;472(3):331-339.
Most ossifying fibromas are solitary lesions that are well circumscribed. OFs occur in the second through the fourth decades of life with a mean age of 32. These tumors have a female predilection (70%) and predominantly occur in the mandible (77%). They are asymptomatic, slow growing, and may cause displacement of teeth and swelling of the jaws with a lingual or buccal expansion of the cortical plates. Large lesions may also cause swelling of the inferior aspect of the mandible. They may also at times be small, not causing expansions and can be diagnosed only through a radiograph.
Radiographically, OF is a well circumscribed lesion with a sclerotic border. It usually shows an unilocular radiolucency or a mixed radiolucent-radiopaque pattern. End stages of the tumor may also show a complete radiopaque lesion. The tumor causes expansion of cortical plates and characteristically may also cause expansion of the inferior border of the mandible. Resorption or displacement of tooth roots may be seen.
Microscopically, ossifying fibroma shows calcifications in the form of bony trabeculae and basophilic spherules in a hypercellular connective tissue with plenty of fibroblasts and dense fibrosis. The stroma is relatively avascular. Bony trabeculae are of woven bone type and are lined by osteoblasts. The basophilic spherules are usually lined by an eosinophilic rim.
VARIANTS OF OSSIFYING FIBROMA
Juvenile ossifying fibroma (JOF) is considered to be a subset in the spectrum of ossifying fibroma although most pathologists consider it to be a separate clinic-pathologic entity distinct from ossifying fibroma. This is believed due to the fact that juvenile ossifying fibromas a) do not show the HRPT2 gene mutation shown by certain ossifying fibromas and b) are very aggressive lesions unlike ossifying fibromas. There are 2 types of JOFs, them being the trabecular type and the psammomatoid type.
JOFs, as the name implies, occur in the young population and are more rapidly growing and aggressive. The trabecular type occurs more often in the maxilla while the psammatoid type occurs more often in extra-gnathic sites like the orbit and the paranasal sinuses.
TREATMENT & PROGNOSIS
Complete surgical excision is recommended for ossifying fibroma and is done easily due to the lesion being well circumscribed. Recurrence is rare and the prognosis for ossifying fibroma is very good.
Neville BW, Damm DD, Allen CM, Chi A. Oral and Maxillofacial Pathology. South Asian ed. Elsevier; 2016.
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