Introduction
There are two types of ossifying fibroma, central and peripheral. The central type arises from the endosteum or periodontal ligament adjacent to the root apex and expands within the medullary cavity of the bone. The peripheral type occurs solely on the soft tissue overlying the alveolar process and is a ‘non-neoplastic’, reactive, fibrous proliferation of gingiva (Neville et al., 2002; Moon et al., 2007). The pathogenesis of this lesion is uncertain and is thought to arise from the periosteal or the periodontal membrane (Kendrick and Waggoner, 1996). It has also been reported that it represents a maturation of a pre-existing pyogenic granuloma or a peripheral giant cell granuloma (Bhaskar and Jacoway, 1966). Clinically, it resembles a peripheral fibroma, but histopathological analysis always reveals immature bone and osteoid within the lesion. Peripheral Ossifying Fibroma (POF) is found exclusively on the gingiva and does not arise in other oral mucosal locations. It varies from pale pink to cherry red in color, can be either pedunculated or sessile and is typically located in the interdental papilla region (Singh et al., 2010). It is more commonly seen in the 1^st and 2^nd decades of life and has a female preponderance. There is a slight predilection for the maxillary arch and in the incisor cuspid region (Jain and Deepa, 2010). Herein, we are reporting a rare case of a peripheral ossifying fibroma of posterior mandibular gingiva in a 60 year old male patient.

Case Report
A 60 years old male patient came to our department with complaint of a growth in the lower left back tooth region since 2 years (Figure1). Patient was asymptomatic 2 years back; initially the growth was smaller in size, which gradually progressed to the present size. The growth was not associated with pain since its onset although the patient had difficulty in chewing and swallowing food along with difficulty in speech. The medical history was not contributory. On intra-oral examination, a solitary, well-defined, pedu- nculated, exophytic growth was seen on the posterior aspect of left side of the mandible, arising from edentulous region of 38 and marginal gingiva of 37 on the lingual aspect, measuring approximately 4x8 cm in its greatest dimensions (Figure 2, 3). It extended antero-posteriorly from the interdental region of 36 and 37 to the retromolar area and inferiorly, 0.5 cm away from lingual sulcus to superiorly till the occlusal level of 36 and 37. Medially, the growth was extending on to the tongue. The growth was reddish in color. On palpation, the growth was non-tender, firm in consistency, did not blanch on pressure, and was non-compressible and non-reducible. Based upon the clinical examination, a provisional diagnosis of Pyogenic granuloma was given and the list of clinical differential diagnosis considered included Peripheral fibroma, Peripheral ossifying fibroma, Peripheral giant cell granuloma, Haemangioma, and Carcinoma of gingiva. The investigatory work-up included Complete Haemogram which was found to be within normal limits. Mandibular Occlusal Radiograph revealed a soft tissue shadow of the lesion along the lingual aspect of the involved teeth (Figure 4) with radiopaque foci distal to 37 (Figure 5). Excisional biopsy was done under general anesthesia (Figure 6) and the excised specimen (Figure 7) was sent for histopathological examination which revealed parakeratinized stratified squamous epithelium with blunt rete-ridges with fibrous connective tissue stroma with irregular osteoid containing osteocytes and osteoblasts (Figure 8). Based upon the clinical, radiological and histopathological features, the final diagnosis was given as Peripheral Ossifying Fibroma. The extraction of 37 was carried-out to prevent recurrence and post-treatment follow-up was done for 2 years although no recurrence was reported.

Discussion
Peripheral ossifying fibroma (POF) is a common solitary gingival growth thought to arise from the periodontal ligament (Jain and Deepa, 2010). The commonly used synonyms for POF include peripheral cementifying fibroma, peripheral fibroma with cementogenesis, peripheral fibroma with osteogenesis, peripheral fibroma with calcification, calcifying or ossifying fibrous epulis and calcifying fibroblastic granuloma (Singh et al., 2010). Bhasker et al. in 1984 described this lesion as peripheral fibroma with calcification while the term POF was coined by Eversol and Robin (Mesquita et al., 1998; Keluskar et al., 2008). The aetiology and pathogenesis of POF remains unknown. Some investigators consider it a neoplastic process, while others argue it to be a reactive phenomenon; in either case, the lesion is thought to arise from cells in the periodontal ligament. Trauma or local irritants, such as dental plaque, calculus, microorganisms, masticatory forces, ill-fitting dentures and poor quality restorations have been implicated in the aetiology of POF (Mesquita et al., 1998; Singh et al., 2010). Chronic irritation of periosteal and periodontal membrane is thought to lead to metaplastic changes in the connective tissue initiating the formation of bone or dystrophic calcifications (Kendrick and Waggoner, 1996). Hormonal influences may also play a role, given the higher incidence of POF among females (Kenney et al., 1989). The peripheral ossifying fibroma (POF) is a common gingival growth usually arising from the interdental papilla. Females are more commonly affected than males (5:1) (Nazareth et al., 2011). The lesion occurs in any age group, predominating in the second decade of life and with a declining incidence after the third decade. Only 0.5% cases are reported in the older age group, (Keluskar et al., 2008) as in our case POF occurred in a 60 year old male. Approximately 60% of POFs occur in the maxilla and they are found more often in anterior region, (Farquhar et al., 2008) the lesion in the present case was however present in mandibular posterior region. POF usually measures <1.5 cm in diameter and rarely reaches more than 3 cm, but patients with lesions of 6 cm and 9 cm diameter have also been reported (Singh et al., 2010). The growth in the above case was significantly larger in size than the average lesion. Basically, the lesion represents varying stages of a fibroma with ossification as the name POF implies; however, ossification or calcification may not be evident in all cases, particularly in the earlier stages of the lesion. POF can produce a mild cupping defect of adjacent alveolar bone. Though radiographic changes are not always seen with POF, occasionally foci of radiopaque material may be seen; particularly in larger lesions (Kumar et al., 2009) The present case demonstrated marked dystrophic calcification within the lesion (Figure 4). Clinical findings alone are not sufficient for a diagnosis of POF because there are other conditions that may have similar clinical appearances and clinical course, such as pyogenic granuloma or peripheral giant cell granuloma. Therefore, biopsy and histopathological examination is required for a definitive diagnosis (Singh et al., 2010). Histologically, POF appears to be a non-encapsulated mass of cellular fibroblastic connective tissue of mesenchymal origin with central areas of calcification covered with a stratified squamous epithelium, which gets ulcerated in 23%-66% of cases. Mineralization can vary between cementum-like material, bone (woven and lamellar) and dystrophic calcification (Singh et al., 2010). Immunohistochemical profile studies indicate that the proliferating cells are of myofibroblastic origin. Studies show CD68 positive histiocytic component intermingling with lymphocytes and plasma cells suggesting the existence of a reactive phenomenon (Chhina et al., 2011). Treatment requires a thorough/wide base surgical excision including the involved periodontal ligament and periosteum to prevent the recurrence of the lesion, as a high recurrence rate of 7-20% has been reported (Nazareth et al., 2011) thereby, mandating a regular follow-up. Recurrence probably occurs due to the incomplete removal of lesion, repeated injury and/or persistence of local irritants. The average time interval for the first recurrence is 12 months (Jain and Deepa, 2010). 

Conclusion
POF is a slowly progressing lesion, with limited growth. Many cases progress for prolonged periods before patients seek treatment because of the lack of symptoms associated with the lesion. Thorough radiographic and histological examination must be done, for a proper diagnosis and management of the lesion, as it is difficult clinically to differentiate between the various gingival lesions that are bound to have varying aetiologies and require different treatment modalities for their cure.

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