a thorough knowledge of the external and internal anatomy of teeth is a very important factor in root canal treatment . in many cases , dentists have to deal with various morphological variations .
if the dentist fails to detect the morphological variations , it would be a major cause of failure .
when a preoperative radiograph shows an atypical tooth shape , further radiographic examinations should be considered in order to detect unusual anatomical differences . in maxillary
first molars , morphological variations , such as abnormal numbers of roots , canals , fusion and germination and the existence of c - shaped root canals have been widely known .
a few cases of c - shaped root canals in maxillary molars have been reported , though c - shaped canals are most frequently found in mandibular second molars .
some authors have reported that c - shaped canals result from the fusion of mesiobuccal ( mb ) and palatal ( p ) roots of maxillary molars , while others have reported that the distobuccal ( db ) and p roots of maxillary molars were fused , and even a case of fusion of the mb and db roots of maxillary molars was reported . the incidence of c - shaped canals in maxillary first
molar has been reported to be as low as 0.091% based on radiographic examination . in case of anatomical abnormalities , periapical surgery , intentional replantation and even extraction
intentional replantation has been performed for more than a thousand years and this technique consists of intentional tooth extraction , cleaning of the apical part of the tooth and reinsertion of the extracted tooth into its own socket immediately .
many authors agree that this technique should be the  last option ' after all the other procedures have failed or when endodontic periradicular surgery can not be performed .
the purpose of this report is to present a morphological variation of c - shaped canal in a maxillary first molar in which the mb , db and p roots were fused to mimic the letter  o ' .
a 39-year - old male was referred by a private practitioner to the department of conservative dentistry at yonsei university dental hospital .
the reason for referral was high possibility of fracture of the maxillary left first molar while trying to remove a pre - existing old post in the palatal canal ( figure 1 ) .
the tooth had been treated endodontically and restored with a post and core 10 years ago .
however , he had no symptoms at that point in time . on the clinical examinations , percussion and mobility tests were within normal limits and probing depth was also normal .
based on clinical and radiographic findings , the diagnosis of chronic apical periodontitis was established .
the possibility of root perforation by the post and sinus involvement by the roots could not be ignored as the cause of the symptom . for further examination cone - beam computed tomography ( cbct , rayscan symphony ; ray co. , ltd , seoul ,
cbct examination revealed a single - rooted maxillary first molar , and all the roots seemed to be fused together into one o - shaped root .
the sinus wall seemed to be intact ( figure 3 ) , but the possibility of perforation by the post or root fracture could not be excluded because it was presumed that the existence of an o - shaped root was unlikely at that time and the overlapping of root images were persistent . it was concluded that the conventional root canal retreatment was not possible because of difficulty in negotiating all canals and the possibility of root fracture during removal of the post .
hence , intentional replantation was planned . on the day of surgery , patient received a preoperative regimen of antibiotics and anti - inflammatory drugs . with delicate luxation using a root elevator , the tooth was extracted without fracture .
the inflamed granulation tissue in the center of the fused roots was removed meticulously , and one root with an o - shape was observed . on a side view , the root was rectangular in shape , and on an apical view all roots were fully connected and no perforation by the post was observed ( figure 4 ) . when the tooth was examined with a surgical operating microscope ( carl zeiss opmi pico ; carl zeiss , oberkochen , germany ) , more than five or six small foramina were observed .
it was decided to resect the apical end of the root for removing the unnoticed small foramina .
the apical 3  mm of the root were trimmed . on the prepared apical o - shaped root surface
there were 56 root canals with connecting fins , and hence , a 360 circular root end cavity was made with an ultrasonic tip and it was checked by methylene blue ( figure 5 ) . during intentional replantation , the tooth was kept under wet gauze for maintaining the pdl cells of the root surface vital .
the root canal was re - cleaned and filled with retrograde root filling material ( resin - modified glass ionomer ; fuji ii ; gc , tokyo , japan ) to cover the long root end cavity .
and the tooth was re - implanted into its own socket . at the 9 months recall visit ,
the tooth was asymptomatic and a progressive healing of the lesion was evident ( figure 6 ) .
we herein present the case of a patient with unusual root morphology of the maxillary first molar which has not been reported up to now .
we named this morphological variation as an o - shaped root following the concept of c - shaped roots . at first
, we suspected that this tooth had a c - shaped root because a blunt apex and an unclear root shape were seen on the periapical radiographs .
the axial and cross - sectional view of the maxillary arch showed the symmetric morphology of the maxillary first molars with an o - shaped root , but we could not exclude the possibility of intimate proximity of roots or simple fusion between the c - shaped root and the other root . at first , periapical microsurgery was considered for establishing the diagnosis and management of the unusual root morphology .
however , the tooth also had difficulties with surgical approaches and the possibility of maxillary sinus perforation during microsurgery .
as a result , intentional replantation was planned carefully because the possibility of tooth fracture could not be excluded .
the extraction and replantation procedure was also expected to be difficult because of the rectangular shape of the root .
extraction of the tooth from its socket was done successfully without causing root fracture or alveolar bone fracture , while trying to preserve the periodontal ligament and not exerting too much pressure on the tooth and socket walls .
after removal of the granulation tissue that covered the root from the apical concave area up to the normal furcation area , an o - shaped root was observed .
no visible perforation by the post was detected on the root surface , and also no sinus involvement was detected .
because of this inner granulation tissue , the possibility of perforation by the post could not be excluded . in this case , the extraoral time needed was almost 17  min for meticulous extraction and management of the unusual root morphology .
reported that the initial ankylosis did not show when experimental group tooth was treated with the extraoral ( complete dry ) time of 15  min .
, mineral trioxide aggregate has been accepted as the material of choice for root - end filling in endodontic surgery , but mineral trioxide aggregate is a technique sensitive material of root end filling for handling in comparison with other materials . in this case ,
resin - modified glass ionomer was selected as a retrograde filling material because it had marginal sealing ability in narrow root end cavity , though it was less tissue - tolerant .
unusual root anatomy of the maxillary molars that has been reported previously includes the fusion of buccal roots , the fusion of mb and p roots , and the fusion of db and p roots , but to the best of our knowledge this is the first case report of all roots being fused together forming an o - shape with a normal furcation . in this case ,
cbct was useful for diagnosing the unusual root morphology because of its ability to display the serial sections of the tooth .
meticulous examination and recognition of an o - shaped root morphology using periapical radiographs and cbct could be helpful to diagnose the rare  o - shaped root ' .
the value of this case report was to present maxillary first molar with a very unusual o - shaped root canal system , as such case is seldom mentioned in textbooks . during endodontic therapy , even though the incidence of an o - shaped root is very rare , the recognition with periapical radiographs and cbct should not be underestimated .
 this case report is to present a maxillary first molar with one o - shaped root , which is an extended c - shaped canal system . 
 patient with chronic apical periodontitis in maxillary left first molar underwent replantation because of difficulty in negotiating all canals . 
 periapical radiographs and cone - beam computed tomography ( cbct ) were taken . 
 all roots were connected and fused to one root , and all canals seemed to be connected to form an o - shape . 
 the apical 3  mm of the root were resected and retrograde filled with resin - modified glass ionomer . 
 intentional replantation as an alternative treatment could be considered in a maxillary first molar having an unusual o - shaped root . 
INTRODUCTION
CASE REPORT
DISCUSSION
CONCLUSION