Root canal therapy (RCT) is a procedure that uses biologically acceptable mechanical and chemical treatments in the root canal system to eliminate pulpal and periradicular disease and to promote healing and repair of the periradicular tissue (1). The potential for an optimal outcome of endodontic treatment reaches up to 90%� 95% of the cases when teeth are treated under controlled clinical conditions (2). However, cross-sectional studies have demonstrated that the reality for the overall population might be somewhat different, with only 35%�60% of the root canal�treated teeth exhibiting no periapical disease (3-5).
Successful outcomes of endodontic treatment depends on the identification of all root canals which in turn guarantees complete extirpation of pulp tissue, proper chemo-mechanical cleaning and shaping and three dimensional obturation of the root canal system with an inert filling material (2). Failure of at least one of these stages entails high risk of unsuccessful root canal treatment of the tooth with a subsequent development or persistence of periapical lesion (6).
The sequence of interdependent steps characteristic of RCT may be interrupted, or even fail, at any time or stage of the process due to iatrogenic complications. These procedural errors, such as broken instruments, perforations, overfilling, underfilling and ledges may jeopardize the outcome of endodontic treatment. A procedural accident often impedes or makes it impossible to accomplish appropriate intracanal procedures and thus prevents the control of intracanal endodontic infection (7). Perforations, for example, are associated with infection of the periodontal ligament and the alveolar bone and consequently impair the healing process (8).
Although it is agreed that the radiographic quality of the root fillings is in no way indicative that the root canal was well sealed (6), assessment of the quality of the root canal filling is mainly based on the length and homogeneity of the root canal filling by means of radiographs. Root canal fillings ending 0-2 mm from the radiographic apex had been reported to affect the outcome of RCT with 87-94% healing rates. Lower healing rates were associated with �short� root fillings ending more than 2 mm from the radiographic apex (68�77%) and with over-fillings extruding beyond the apex (75�76%) (2, 9).
In addition, root fillings with a homogenous mass of filling material and with no voids are strongly correlated with lower risk of post-treatment disease. Several studies had found that a root filling that is less dense and non-homogenous will have a negative impact on the treatment outcome (10, 11).
The technical quality of root canal fillings undertaken by general practitioners in different populations has been extensively investigated(3-5, 12). The results from these studies showed that inadequate root canal treatments performed by the dentists is not the exception but the rule. While the reasons for this might be complicated, it could be strongly correlated to the endodontic teaching received at the dental schools. Dummer (13) found that some of the problems in endodontic teaching in the UK was the limited time devoted to preclinical endodontics within the curriculum and that few dental schools provided additional teaching material for undergraduates, compared with schools in Europe and the United States. He concluded that it would be reasonable to formulate curriculum guidelines on a national level and that increasing the time devoted to the practice of root canal therapy should be seen as a priority.
In their investigation, Qualtrough and Dummer (14) found that the number and scope of the endodontic lectures remained well below that reported in other European and US dental schools and suggested that yet more time should be found to cover the range of subjects necessary to provide a comprehensive education. They also found that the staff: student ratio was inadequate and that endodontic staff�s university appointment was not fully devoted to endodontics. They concluded that these factors might have some impact on the quality of root canal treatment provided by undergraduate students.
In Arab American University, School of Dentistry, the Palestinian undergraduate students are taught endodontics in three levels. A preclinical course is given in the third year which includes 16 hours of lectures and 32 hours of practicical training. Students are trained to perform root canal treatments on extracted teeth using step-back technique. They are expected to complete RCT for at least four anterior teeth, two bicuspids and two molar teeth. The other two courses are clinical and students are supervised while doing RCT for patients. In the first semester of the fourth year, students are mainly performing single canal treatments while in the second semester , students perform treatments for multi-rooted teeth. In their fifth year, students perform root canal treatments for mainly molars. Complicated cases that need non-surgical endodontic retreatment or cases with procedural errors are referred to the specialty clinics.
In Palestine, there are two dental schools which had been recently established. No reports on the technical quality of root canal fillings performed by Palestinian undergraduate dental students had been published. However, a single epidemiologic study had been performed on the technical quality of root canal fillings in Palestinian population (12). One reason cited for the relatively poor technical quality of root fillings performed by the general practitioners was the inadequate endodontic teaching and training received at dental schools(15). Better training of undergraduate students in the field of endodontics may be one of the possible solutions to the problem. The aim of this study was to evaluate the technical quality of root canal fillings performed by the 4th and 5th year students in the Dental School of the Arab American University, Palestine. This was undertaken by examining the radiographic quality of root fillings and the incidence of iatrogenic errors caused by the undergraduate students during their clinical practice.
Materials and Methods
The dental records of patients with complete root canal treatment performed by undergraduate student with the same pre-clinical training at the Dental School of the Arab American University In Jenin, Plaestine, between 2009 and 2012, were randomly selected for evaluation. The inclusion criteria for this selection were :
– Teeth were endodontically treated for the first time by the 4th and 5th year undergraduate students.
– At least three radiographs (pre-operative, working length determination and post-operative ) were present in the records.
– The post-operative radiograph taken with the long cone paralleling technique and showed the entire root and at least 3 mm part of the periapical area.
Records without complete root canal treatment and retreated cases were excluded. Unreadable radiographs due to developing procedures (over-exposure, over or under-developed) , improper storage conditions, superimposition of root canal fillings and superimposed anatomical structures were also excluded. As a result, 612 cases were finally evaluated. These cases included 203 anterior teeth, 238 premolars and 171 molars.
All root canal treatments were carried out by the 4th and 5th �year undergraduate students under the supervision of an endodontic specialist. The ratio of supervisor to students was 1:8 for both years. Teeth examined in this study were endodontically treated for the first time. Complex cases that involved teeth with unusual anatomy or teeth with a history of perforation, ledge formation, and broken instruments were usually referred to a specialist.
All endodontic treatments were performed using a rubber dam and an aseptic technique. Following access cavity preparation, Gates-Glidden drills (Premier Dental, Norristown, PA, USA) number 2,3 and 4 were usually used in the coronal third to facilitate straight-line access to the apical third. Working length was estimated with the use of apex locator ( NovApex, Forum Technologies, Rishon Le-Zion, Israel) and confirmed by a radiograph. Root canals were instrumented with step-back technique using stainless steel K-files (Dentsply, Maillefer, Ballaigues, Switzerland) of 0.02 taper and irrigated with 1% sodium hypochlorite solution. In cases where treatment could not be completed in one session, calcium hydroxide paste mixed with sterile water was used as an intracanal dressing, and a temporary restoration was applied between appointments. Root canal fillings were carried out with gutta-percha and Endom�thasone (Specialit�s-Septodont, Saint Maur-des-Foss�s, Cedex, France) sealer using the lateral condensation technique. The teeth were restored with a temporary filling and a post-operative radiograph was then exposed using a paralleling technique.
Two specialists in endodontics who worked as instructors for undergraduate students, served as examiners in this study. Before conducting the study, the two observers were calibrated by scoring 45 randomly selected periapical radiographs of endodontically treated incisors, premolars, and molars which were not included in the main study. Inter-examiner agreement with regard to the evaluation of the root canal filling length, presence of voids and procedural errors was determined by computing Cohen�s kappa value. The k-values were 0.84 and 0.88 and 0.80 respectively. In case of disagreement, the two observers reached a consensus. Intra-observer reproducibility was evaluated by a repeat scoring of the same 45 periapical radiographs, 2 months after the first examination and showed high agreement. The k-values for intra-examiner reproducibility were 0.90 for �length of root filling� , 0.90 for �presence of voids� and 0.82 for the presence of procedural errors
The method of viewing the radiographs was standardized. The radiographs were examined by the two observers in a darkened room using an illuminated viewing box with magnification( 2.5 x) whilst mounted in a cardboard slit to block off ambient light emanating from the viewer. A transparent ruler with 0.5 mm gradations was used to measure the distance from the radiographic apex. An evaluation form was designed to record the information gathered from the postoperative radiographs. The following data were recorded ( the tooth treated, the tooth location, position of the tooth in the arch, number of roots and canals, root canal filling length from the radiographic apex, presence of voids in root canal filling and presence of a ledge, perforation or fractured instrument in each root canal that was examined).
Using the root canal as the assessment unit, the technical quality of the root filling was evaluated according to the density of the filling and the distance between the end of the filling and radiographic apex