Dr. Hiren Patel M.D.S (Oral Surgery) (Contact Author)
Dr. Jigar Purani M.D.S (Oral Pathology)
Dr. Chintan Thakkar B.D.S
Professor & Head, Oral & Maxillofacial Surgery Department,
Reader,  Oral Pathology Department,
Tutor, Oral & Maxillofacial Surgery Department,
Faculty of Dental Science

Abstract

Ameloblastoma is a neoplasm of odontogenic epithelial origin. The tumor cells are derived from caricatures of enamel organ and some may resemble ameloblast without enamel matrix formation. It is the 2nd most odontogenic tumor after odontoma. Although its clinical behavior with high recurrence rate makes it the most significant odontogenic neoplasm. Unicystic ameloblastoma occurs in 5-15% of all intraosseous ameloblastoma of jaw. It is a cystic lesion that shows clinical, radiographic or gross picture of a cyst but histological examination revels typical ameloblastic epithelium. Here we present a case of 30 year old female patient with unicystic ameloblastoma associated with lower right canine with review of literature.

Introduction
Benign tumors of the oral cavity can be divided into odontogenic and non- odontogenic in origin. These may include odontoma, ameloblastoma, central giant cell tumour, fibro-osseous lesion etc.11 Ameloblastoma represents 10% of all odontogenic tumors and 1% of cysts & tumors of the jaw. It develops from epithelial cellular elements or from dental tissue in the various developmental stages. The concept of this tumour was first introduced by Robinson and Martinez in 1977.1 It is slow growing, anatomically benign, clinically persistent and locally aggressive lesion. Although it can occur at any age; peak incidence is in 3rd or 4th decade of life with equal sex distribution .3

The vast majority of ameloblastomas occur in the mandibular molar / ramus region. The ameloblastomas mostly have three forms, namely multicystic, peripheral, and unicystic 3. The most common variety is multilocular or multicystic which represents almost 86% of cases. Peripheral odontogenic tumours with the histological characteristics of intraosseous ameloblastoma occur solely in the soft tissues covering the tooth-bearing parts of the jaws. Unicystic tumour occurs in 10-15% of total intraosseous ameloblastoma cases. These include those that have been variously referred to as mural ameloblastomas, luminal ameloblastomas, and cystic ameloblastomas arising from dentigerous cysts 4. The goal of treatment of ameloblastoma therefore ranges from the radical resection reserved for solid ameloblastomas to simple enucleation generally considered option for a dentigerous cyst. Unicystic ameloblastoma is less aggressive & hence gives better result to enucleation than solid ameloblastoma. Mural histological subtype had greater recurrence than other unicystic ameloblastoma. However, recently there has been a resurgence of interest in the management of unicystic ameloblastomais to achieve complete surgical excision of the lesion and appropriate reconstruction. Plexiform unicystic ameloblastoma shows pattern of epithelial proliferation in cystic lesion of the jaw. 15 to 30 % of all ameloblastomas form in the wall of a dentigerous cyst. 5 It is not clear whether they arise de novo or from a neoplastic transformation of cells of dentigerous epithelium.2 Lesions that experienced recurrences are identical histologically to the original lesions, making them unlikely to develop from dentigerous cysts. The pathogenesis of unicystic ameloblastoma and cystic degeneration are not clear yet. However, it has been suggested that epithelial dysadhesion (e.g. defective desmosomes) or, more likely, to the intrinsic proteinases production (e.g. metalloproteinases, serine proteinases); enzymes that normally responsible for degradation of the central zone of the enamel organ after tooth development leads to unicystic ameloblastoma formation. 6

Case Report

A 30 year old lady presented to the department of Oral Surgery, Faculty of Dental Science, Dharmsinh Desai University, with a slowly growing swelling on the right side of lower 3rd of the face since six month (Figure 1). There was no associated pain, no difficulty in opening the mouth, chewing or articulating. On physical examination, there was slight facial asymmetry, a hard non-tender mass, measuring 6-7 cm X 3-4 cm arising from the symphysis of the mandible, involving body up to the right lower 1st molar tooth. The oral mucosa was normal in texture with slight bluish discolouration. Cervical lymph nodes were not palpable. Systemic examination was normal. Lower anterior teeth are missing with history of extraction after loosening of those teeth. An orthopantomogram (OPG) showed large cystic lesion in the right side of mandible associated with right lower impacted canine tooth. The opposite canine was also impacted. There were slight resorbtion of right lower premolar teeth (Figure 2). The size of the lesion was 6X3 cm ( Figure 3). Surgical removal of lesion was done under general anaesthesia. Enucleation was done via intra oral approach and achieved primary closure. The respected specimen (Figure 4), had histopathologic features suggesting of unicystic ameloblastoma.

Figure:- 2

Figure:- 3

Figure:- 4

Discussion
Clinical picture of unicystic ameloblastoma (UCA)
Sex distribution of unicystic ameloblastoma is similar to that of multilocular lesion with male to female ratio is 1.3:1. Both occur intraosseously in molar ramus region. Unlike solid ameloblastoma, unicystic ameloblastoma occurs mostly in 2nd to 3rd decade of life with mean age of 18year 13. Unicystic ameloblastoma rarely occurs after age of 40. 2 Mandible is affected in almost 90% of cases. Posterior mandible is affected the most followed by parasymphysis mandible, anterior maxilla & lastly posterior maxilla. Unicystic ameloblastoma is most commonly associated with impacted 3rd molar; although it may occur in ramus region without any tooth relation is called non dentigerous type. Dentigerous type unicystic ameloblastoma occurs 8 year earlier in average than non dentigerous type and ratio between the two are 8:7. 8

Radiographic picture of unicystic ameloblastoma (UCA)
Unicystic ameloblastoma shows well circumscribed radiolucency around crown of the tooth with well decorticated and well delineated margins. Small lesions are found accidentally on routine radiographs taken for missing impacted teeth or unexplained loosening or crowding of teeth. 7 The minimum criteria for diagnosing a lesion as unicystic ameloblastoma histologically, is the demonstration of a single cystic sac lining of odontogenic (ameloblastomatous) epithelium often seen only in focal areas. Unicystic ameloblastoma has a higher recurrence rate than odontogenic cysts 9.
Ackermann classified unicystic ameloblastoma into the following three histological groups: Group I: Luminal unicystic ameloblastoma (tumor confined to the luminal surface of the cyst) Group II: Intraluminal / plexiform unicystic ameloblastoma (nodular proliferation into the lumen without infiltration of tumor cells into the connective tissue wall) Group III: Mural unicystic ameloblastoma (invasive islands of ameloblastomatous epithelium in the connective tissue wall not involving the entire epithelium) Another histologic subgrouping by Philipsen and Reichart has also been described:

Subgroup 1: Luminal unicystic ameloblastoma
Subgroup 1.2: Luminal and intraluminal
Subgroup 1.2.3: Luminal, intraluminal and intramural
Subgroup 1.3: Luminal and intramural

The unicystic ameloblastoma diagnosed as subgroups 1 and 1.2 can be treated conservatively (careful enucleation), whereas subgroups 1.2.3 and 1.3 showing intramural growths require radical resection, as for a solid or multicystic ameloblastoma 14. Vigorous curettage of the bone after enucleation should be avoided as it may implant foci of ameloblastoma more deeply into bone. Chemical cauterization with Carnoy's solution is also advocated for subgroups 1 and 1.2. Subgroups 1.2.3 and 1.3 have a high risk for recurrence, requiring more aggressive surgical procedures like resection of mandible. This is because the cystic wall in these cases has islands of ameloblastoma tumor cells and there may be penetration into the surrounding cancellous bone10-12,14.
The average interval for recurrence being 7 years. Recurrence is also related to histologic subtypes of unicystic ameloblastoma, with those invading the fibrous wall having a rate of 35.7%, but others have only 6.7% 12 . Recurrence rates are also related to the type of initial treatment. Lau et al 11 reported recurrence rate of 3.6% for resection, 30.5% for enucleation alone, 16% for enucleation followed by Carnoy's solution application, and 18% by marsupialization followed by enucleation (where the lesion reduced in size)

Differential Diagnosis
Dentigerous cyst comes first but it has unicortical expansion & displacement of teeth rather than resorbtion of their roots. Other cysts like odontogenic keratocyst, residual cyst can also be put as differential diagnosis but odontogenic keratocyst rarely shows cortical expansion and residual cyst is generally associated with missing tooth that has history of extraction. Adenomatoid odontogenic tumour, giant cell tumour and some time solid ameloblastoma are put as possible differential diagnosis but adenomatoid odontogenic tumour occurs in anterior maxilla ; central giant cell tumour mostly occurs anterior to 1st molar region of mandible and solid ameloblastoma rarely occurs in the patient less than 30 year of age 9

References:-
1. Robinson L, Martinez MG. Unicystic ameloblastoma. A prognostic ally distinct entity. Cancer 1977; 40: 278-85
2. Sapp JP, Eversole LR, Wysocki GP. Contemporary oral and maxillofacial pathology. 2nd edition. St. Louis: Mosby. 134-63
3. Phillipsen HP , Reichart PA, Unicystic ameloblastoma and Classification of Odontogenic tumour. In Odontogenic tumours and allied lesions Quitessence Pub. Co. Ltd; 2004 21-23 and 77-86.
4. Phizer ME, Page DG, Svirsky JA : Thirteen year follow-up of large recurrent unicystic ameloblastoma of mandible in 15 year old boy. J Oral Maxillofacial Surg. 2002, 60; 211-5 PMID 11815925
5. Wood NK, Kuc IM. Pericoronal radiolucencies. In: Wood NK,Goaz PW,. Differential diagnosis of oral and Maxillofacial lesions. 5th edition. St. Louis: Mosby. 1997
6. Rosenstein T, Pogrel MA, Smith RA, Regezi JA. Cystic ameloblastoma-Behaviour and treatment of 21 cases. J OralMaxillofac Surg 2001; 59: 1311-16
7. Li TJ, Kitano M, Arimura K, Sugihara K: Recurrence of unicystic ameloblastoma: A case report and review of the literature. Arch Pathol Lab Med 1998, 122:371-4 PMID 9648908
8. Eversole LR, Leider AS, Strub D. Radiographic characteristics of cystogenic ameloblastoma. Oral Surg Oral Med Oral Pathol 1984; 57: 572-77
9. Stanley HR, Diehl DL. Ameloblastoma potential of follicular cysts. Oral Surg 1965; 20: 260-68
10. Li T, Wu Y, Yu S, Yu G: Clinicopathological features of unicystic ameloblastoma with special reference to its recurrence. Zhonghua Kou Qiang Yi Xue Za Zhi 2002, 37:210-2.
11. Lau SL, Samman N: Recurrence related to treatment modalities of unicystic ameloblastoma: A systematic review. Int J Oral Maxillofac Surg 2006, 35:681-90.
12. Li TJ, Wu YT, Yu SF, Yu GY. Unicystic ameloblastoma: A clinicopathologic study of 33 Chinese patients. Am J Surg Pathol 2000; 24: 1385-92
13. Roos RE, Raubenheimer EJ, van Heerden WF: Clinico-pathological study of 30 unicystic ameloblastomas.J Dent Assoc S Afr 1994, 49:559-62 PMID 9508960
14. Rakesh S Ramesh, Suraj Manjunath, Tanveer H Ustad, Saira Pais and K Shivakumar Unicystic ameloblastoma of the mandible - an unusual case report and review of literature Head & Neck Oncology 2010, 2:1doi:10.1186/1758-3284-2-1