an important parameter necessary to achieve a proper endodontic treatment is the quality of root canal filling . the success rate of root canal treatments ( rcts ) is reported to be > 90% . the success of this treatment depends on many factors , and the technical quality of rct is one of the most important . on the other hand , numerous studies have been reported in several countries on the high prevalence of poor fillings in association with periapical radiolucency , leading to increasing number of public health problems . amongst the several risk factors corresponding to progressive periapical radiolucency , the quality of root filling was one of the most important factors . therefore , efforts are needed for improving the quality of rcts . to improve the quality of clinical performance , some factors such as knowledge , training , ability and utilization of technology are necessary . of these gates - glidden ( gg ) drills are low - speed rotary instruments that have been used for over one hundred years without major changes in design . some advantages of such instruments include safety , ease of use , reduction in working time and low cost . in spite of the advantages of rotary instruments the decision as to whether new techniques and materials should be incorporated into the undergraduate curriculum can pose problems for teachers of endodontology . the study of quality of rcts and prevalence of endodontic procedural accidents can help improve educational programs , thus leading to improvement in oral health - related quality of life in society . therefore , for improving the quality of endodontic performance of practitioners in clinical practice , their basic , preclinical performance and knowledge must be taken into consideration . until date , very few studies have focused on the preclinical abilities of students . the aim of this study was to evaluate the technical quality of molar rcts performed by undergraduate practitioners at a dental teaching school during the preclinical course . in addition , the effect of gg drills use on the final quality of the performed rcts was evaluated . in this retrospective cross - sectional study , 315 roots of 105 endodontically treated molar teeth in preclinical practice in 2010 were obtained from the undergraduate program in endodontics of the school of dentistry , isfahan university of medical sciences , isfahan , iran . all the undergraduate students underwent extensive laboratory training on the instruments and filling techniques on all groups of teeth . the obtained teeth consisted of 64 maxillary and 41 mandibular permanent molar teeth . for mounting the teeth , the students had used different materials with different concentrations ; therefore prior to evaluation , the teeth were removed from the mounts and then assessed . there were two equal groups of endodontically treated teeth : one group had been prepared using k - files along with gg drills ( gg group ) and the other using only hand instruments ( k - files ) ( no gg group ) . all the rcts were performed by fourth grade undergraduate dental students , who possessed the same level of knowledge and practical experience of rct , using the step - back technique . they were taught by the instructors to use at least files number 15 for initial length determining via conventional radiographs . after determination of working length , the master apical file was set at minimum of number 30 . in the gg group , gg drills number 1 - 3 were used after the root canals were instrumented by files number 20 - 25 . using patency file ( # 10 or 15 ) and copious canal lavage was recommended to the students . all the teeth were obturated using the cold lateral condensation filling technique using the gutta - percha ( ariadent , tehran , iran ) and ah-26 sealer ( dentsply , detrey , konstanz , germany ) . although the students had taken conventional radiographs of completely treated teeth to pass their educational course , the radiation environments were different . therefore , to calibrate the images and prevent bias , new digital images were obtained . in addition , all the teeth were held in a plaster block containing a very thin layer of wax to ensure immobility . the distances from the x - ray tube to the teeth and the teeth to the receptor were 2 and 0.5 cm , respectively . the radiographs of maxillary and mandibular molar teeth were obtained in the buccolingual direction using paralleling technique . moreover , as recommended , a 20 horizontal mesial angulation of x - ray tube was used for the mandibular molars to prevent the superimposition of roots . the radiographic procedure was performed using a dental x - ray unit ( planmeca intra planmeca oy , helsinki , finland ) at 50 kvp , 8 ma and 0.01 s exposure time , and digital sensor ( mps , progeny dental , buffalo grove , usa ) . the selected exposure environments ( i.e. kvp , ma and exposure time ) were based on a pilot exposure prior to the final imaging confirmation of an endodontist and a maxillofacial radiologist . in the present study , a total of 146 images including 64 of maxillary molars and 82 of mandibular molars ( from two directions ) the results were then compared , and a final consensus was reached . in the case of disagreement , a third observer with more experience was asked to interpret the images for final agreement . the observers used the same monitor ( lg flatron l1755s 17-inch , manufactured by madiran , iran , under license of lg electronics inc . ) . the observers were free to use and perform all the options in the software , such as brightness , contrast adjustment and magnification . the observers were asked to evaluate the radiographic quality of the fillings according to a criteria defined in table 1 . as in other similar studies , the filling quality was assessed on the basis of three parameters : filling length , density ( homogeneity ) of filling and taper of root canals [ table 1 ] . apart from the above characteristics , overextension and coke bottle shaped root canals root filling beyond the apex and also the canal that has not been filled adequately within its confines was diagnosed as overextension . the coke bottle shape was diagnosed when a bottle - like widening produced by the over action of gg drills was observed in the root canals . criteria followed for evaluation of root fillings as shown in table 1 , a specific score was assigned to each parameter , which was similar to that in the study of santos et al . the root canal was considered as a sampling unit , and the quality of the root filling was evaluated according to the following parameters : 
 length , density andtaper , which were scored ( s ) as s0 , s1 and s2 . s2 corresponded to an adequate standard whereas s1 and s0 referred to slight and considerable deviation from the adequate standard , respectively table 1 . a root canal filling was considered acceptable ( total score : 6 ) only when the length , density and tapering of the filling were adequate ( three scores of 2 ) . fillings with total scores of 0 - 2 and 3 - 5 were considered to be poor and moderate , respectively . descriptive analysis was performed separately for each type of root canal , and the sum of all endodontic errors was calculated . chi - square test was used to compare the gg drill ( gg group ) with hand instrument ( no gg group ) with respect to taper of root canal ( adequate , under - shaped and over - shaped ) , length of fillings ( adequate , under - fill , over - fill and tip - to - tip ) and scoring ( poor , moderate and acceptable ) . to compare the coke bottle shaped errors between the groups , whitney test was used for comparison of frequency of voids within each root canal and the scores of the two groups were obtained . all the aforementioned analyses used for comparing the gg and no gg groups were also used to compare maxillary and mandibular root canals . in this retrospective cross - sectional study , 315 roots of 105 endodontically treated molar teeth in preclinical practice in 2010 were obtained from the undergraduate program in endodontics of the school of dentistry , isfahan university of medical sciences , isfahan , iran . all the undergraduate students underwent extensive laboratory training on the instruments and filling techniques on all groups of teeth . the obtained teeth consisted of 64 maxillary and 41 mandibular permanent molar teeth . for mounting the teeth , the students had used different materials with different concentrations ; therefore prior to evaluation , the teeth were removed from the mounts and then assessed . there were two equal groups of endodontically treated teeth : one group had been prepared using k - files along with gg drills ( gg group ) and the other using only hand instruments ( k - files ) ( no gg group ) . all the rcts were performed by fourth grade undergraduate dental students , who possessed the same level of knowledge and practical experience of rct , using the step - back technique . they were taught by the instructors to use at least files number 15 for initial length determining via conventional radiographs . after determination of working length , the master apical file was set at minimum of number 30 . in the gg group , gg drills number 1 - 3 were used after the root canals were instrumented by files number 20 - 25 . using patency file ( # 10 or 15 ) and copious canal lavage was recommended to the students . all the teeth were obturated using the cold lateral condensation filling technique using the gutta - percha ( ariadent , tehran , iran ) and ah-26 sealer ( dentsply , detrey , konstanz , germany ) . although the students had taken conventional radiographs of completely treated teeth to pass their educational course , the radiation environments were different . therefore , to calibrate the images and prevent bias , new digital images were obtained . in addition , all the teeth were held in a plaster block containing a very thin layer of wax to ensure immobility . the distances from the x - ray tube to the teeth and the teeth to the receptor were 2 and 0.5 cm , respectively . the radiographs of maxillary and mandibular molar teeth were obtained in the buccolingual direction using paralleling technique . moreover , as recommended , a 20 horizontal mesial angulation of x - ray tube was used for the mandibular molars to prevent the superimposition of roots . the radiographic procedure was performed using a dental x - ray unit ( planmeca intra planmeca oy , helsinki , finland ) at 50 kvp , 8 ma and 0.01 s exposure time , and digital sensor ( mps , progeny dental , buffalo grove , usa ) . the selected exposure environments ( i.e. kvp , ma and exposure time ) were based on a pilot exposure prior to the final imaging confirmation of an endodontist and a maxillofacial radiologist . in the present study , a total of 146 images including 64 of maxillary molars and 82 of mandibular molars ( from two directions ) were evaluated . the results were then compared , and a final consensus was reached . in the case of disagreement , a third observer with more experience was asked to interpret the images for final agreement . the observers used the same monitor ( lg flatron l1755s 17-inch , manufactured by madiran , iran , under license of lg electronics inc . ) . the observers were free to use and perform all the options in the software , such as brightness , contrast adjustment and magnification . the observers were asked to evaluate the radiographic quality of the fillings according to a criteria defined in table 1 . as in other similar studies , the filling quality was assessed on the basis of three parameters : filling length , density ( homogeneity ) of filling and taper of root canals [ table 1 ] . apart from the above characteristics , overextension and coke bottle shaped root canals were detected . root filling beyond the apex and also the canal that has not been filled adequately within its confines was diagnosed as overextension . the coke bottle shape was diagnosed when a bottle - like widening produced by the over action of gg drills was observed in the root canals . criteria followed for evaluation of root fillings as shown in table 1 , a specific score was assigned to each parameter , which was similar to that in the study of santos et al . the root canal was considered as a sampling unit , and the quality of the root filling was evaluated according to the following parameters : 
 length , density andtaper , which were scored ( s ) as s0 , s1 and s2 . 
  taper , which were scored ( s ) as s0 , s1 and s2 . s2 corresponded to an adequate standard whereas s1 and s0 referred to slight and considerable deviation from the adequate standard , respectively table 1 . a root canal filling was considered acceptable ( total score : 6 ) only when the length , density and tapering of the filling were adequate ( three scores of 2 ) . fillings with total scores of 0 - 2 and 3 - 5 were considered to be poor and moderate , respectively . descriptive analysis was performed separately for each type of root canal , and the sum of all endodontic errors was calculated . chi - square test was used to compare the gg drill ( gg group ) with hand instrument ( no gg group ) with respect to taper of root canal ( adequate , under - shaped and over - shaped ) , length of fillings ( adequate , under - fill , over - fill and tip - to - tip ) and scoring ( poor , moderate and acceptable ) . to compare the coke bottle shaped errors between the groups , whitney test was used for comparison of frequency of voids within each root canal and the scores of the two groups were obtained . all the aforementioned analyses used for comparing the gg and no gg groups were also used to compare maxillary and mandibular root canals . the distributions of the evaluated parameters , that is , length , density and taper of rcts are shown in table 2 . a significant relationship was found between the type of teeth ( maxillary or mandibular ) and the length of fillings , with more number of under - fillings in mandibular root canals ( p = 0.001 ) . a significant more number of voids ( lower density ) were seen in maxillary root canals ( p < 0.001 ) . the chi - square test showed a significant number of under - shapings in mandibular root canals ( p = 0.007 ) . only one ( 0.3% ) palatal root of the maxillary molar was over - shaped [ table 2 ] . length , density and taper of rcts in different root canals fisher 's exact test did not reveal a significant relation between coke bottle shape mishap and type of the teeth ( p = 0.212 ) . the exact distribution of this mishap is demonstrated in table 2 . table 3 shows the length , density and taper of rcts based on the type of instrument used ( whether gg drills were used ) . no relationship was found between the type of instrument used and the length ( p = 0.499 ) and taper of fillings ( p = 0.238 ) . on the other hand , the mann whitney test showed that the root instrumented with gg drills had a significantly higher density when compared with those for which gg drills were not used ( p = 0.004 ) . moreover , gg drills did not increase the rate of the coke bottle shaped canals ( p = 0.467 ) . length , density and taper of rcts based on the types of instruments used by students table 4 shows the technical quality scores of rcts performed in different root canals . a root canal filling was considered acceptable only when the length , density and tapering of the filling were adequate . the percentages of rcts with acceptable , moderate and poor quality were 35.6 , 39 and 24.1% , respectively . mandibular root canals had significantly better quality mean score ( p = 0.024 ) when compared with maxillary root canals . technical quality scores of performed rcts in different root canals the number of poor , moderate and acceptable fillings with gg drills were 27 ( 17.3% ) , 64 ( 41% ) and 65 ( 41.7% ) , respectively . on the other hand , the distributions of poor , moderate and acceptable fillings performed using only k - files were 49 ( 31.4% ) , 59 ( 37.8% ) and 47 ( 30.1% ) , respectively . the mann - whitney test showed that the mean score of root fillings quality was significantly higher for the gg group ( 4.15  0.12 ) than the no gg group ( 3.61  0.14 ) ( p = 0.008 ) . in the present study , the technical quality of root fillings in a preclinical course as well as the influence of using gg on occurrence of endodontic mishaps was evaluated . it should be noted that although the radiographic technical quality of fillings was evaluated in several studies , only in a few studies the quality of preclinical course was considered and that was via questionnaires filled by students . recently , one study radiographically assessed the quality of rcts performed in preclinical setting . although it is difficult to compare the results of the present work with other studies because of the differences in the design of the studies , other similar works looked into the quality of rcts performed by undergraduate dental students . in most of the studies , over - filling was considered to be equal to tip - to - tip filling . however , similar to the study of santos et al . , we considered them to be different and assigned a better score to tip - to - tip filling . in this study , the root fillings that were adequate , short , tip - to - tip and over - filled were 52.1 , 18.7 , 14.3 and 13.7% , respectively . the number of fillings with adequate length was more than those of the lupi - pegurier et al . the percentages of teeth with adequate , short and over - fillings were reported to be 61.3 , 34.5 and 4.2% , respectively . peak et al . evaluated the technical quality of fillings performed by royal air force dental students and found the percentages of adequate , short and over - filled root canals to be 50 , 32 and 18% , respectively . these aberrations can be due to several reasons , the most important being the radiographic technique used . in real clinical practice , the radiographs are commonly obtained using bisecting - angle technique , whereas the paralleling technique was used in the current study . the results of the present study showed that there was a significant under - filling in mandibular molars . it is reported that most of the under - filled root canals are observed in mandibular molars . this can be explained by the anatomy of multi - canalled curved roots in these teeth . in the current study , a significant percentage of adequately tapered root canals ( 85.3% ) were reported in barrieshi - nusair et al . study . as shown in table 2 , most of the poorly tapered root canals were , in fact , under - shaped ( 41% ) , and only a few were over - shaped ( 0.3% ) . this may be due to the inadequate consideration of undergraduate students to completely clean and shape the canals . further , 64.4% of root fillings had an adequate density , and 35.6% had at least one visible void . the number of root fillings with adequate density was somewhat less than those reported in the studies of barrieshi - nusair et al . did not find a significant difference between the density of maxillary and mandibular root fillings . in contrast , in our study , most of the fillings with poor density were observed in maxillary molars when compared with the mandibular molars . preclinical students were divided into two groups : gg group ( used k - files along with gg drills ) and no gg group ( used only k - files ) . no significant difference was observed between the two groups with respect to length of fillings and taper of root canals ; however , usage of gg drills significantly increased the final density of fillings . in addition , the technical quality scores of root canals were significantly higher in the gg group . this may be attributed to the fact that gg drills provide a better coronal enlargement and shaping of the root canals , leading to a straight - line access to the root apex . the better straight - line access in the properly shaped teeth may lead to a further depth of insertion of spreaders during gutta - percha administration , thus resulting in a better homogeneity of the filling . coronal enlargement , adequate shaping and straight - line access are the factors responsible for the greater number of acceptable fillings in the gg group in this study . unexpectedly , coke bottle shape incidence was not significantly related to usage of gg drill and only occurred in 18 ( 5.7% ) of the root canals . this may be because the students , who were taught by preclinical instructors , did not apply too much force when using gg drills . as can be clearly observed from the results , gg drills not only reduced practitioners errors but also significantly increased the quality mean score of rcts when compared with the no gg group . however , wu et al . concluded that gg drills can increase the risk of perforation ; therefore , they must be used with caution . in this study , the percentage of acceptable root fillings reported in this study was nearly in accordance with the studies of er et al . this percentage is much higher than those of the studies of hayes et al . and tarim ertas et al . the low percentage of acceptable root fillings found in the current study could be attributed to several reasons , including the design of the study , criteria followed , rct techniques used and most importantly , the endodontic curriculum of the school . according to the european society of endodontology , an important criterion is the time allocated to the subject matter ; however , in this faculty , 55 h are devoted to preclinical endodontic teaching , which seems to be insufficient . the staff to student ratio was 1:15 , which should be improved when compared with some other studies . in summary , to improve the technical quality of rcts performed by the undergraduate dental students , the endodontics curriculum can be revised in some aspects . furthermore , students must be taught new techniques and instruments to ensure that they are up - to - date . the clinical training course must be designed in such a way that the students are provided with proper skills in endodontics , starting with the basic principles of real clinical endodontics . the teeth radiographs taken by students were not used because of the different radiation angles used , and image recapturing was performed by one examiner in order to calibrate the data . digital radiographs were used to develop the images and to ensure that small endodontic errors were not missed . in addition , other accurate tools such as scanning electron microscopy , stereomicroscopy , cone beam computed tomography , etc . could be used instead of digital imaging . however , another limitation of this study was that the survey was done in a preclinical setting , and some procedural accidents such as strip perforation was not assessed because the students do not deliver these cases to their evaluators . this fact can lead to a study bias when evaluating the role of gg drills in the final quality of fillings . the technical quality of preclinical molar rcts performed by undergraduate dental students using step - back technique and cold lateral condensation was considered to be acceptable in 35.6% of the cases . when gg drills were used along with k - files , the technical quality of rcts was enhanced .
background : for improving the quality of endodontic performance of practitioners in clinical practice , their basic , preclinical performance and knowledge must be taken into consideration . 
 this study aimed to radiographically evaluate the technical quality of preclinical molar root canal treatments ( rcts ) performed by undergraduate dental students at a dental school in iran . 
 further , the effect of using gates - glidden ( gg ) drills on the final quality of rcts was evaluated.materials and methods : in this retrospective cross - sectional study , 315 roots of 105 endodontically treated teeth in preclinical practice were evaluated radiographically . the analyzed quality parameters included length , taper and density of fillings , which were scored as s2 ( adequate standard ) , the s1 ( slight deviation ) , or s0 ( considerable deviation ) . 
 for all the parameters , acceptable , moderate and poor fillings received total scores of 6 , 3 - 5 and 0 - 2 , respectively . 
 there were two groups of students : one group had used only k - files , and the other had used k - files along with gg drills . 
 the quality of rcts between these groups was evaluated using the aforementioned scoring protocol . 
 the results were analyzed using chi - square , mann  whitney and fisher 's exact tests (  = 0.05).results : under - fillings ( p = 0.001 ) and under - shapings ( p = 0.007 ) occurred mostly in mandibular root fillings . a lower density was found in maxillary fillings ( p < 0.001 ) . no relationship was observed between the technique used ( irrespective of gg drills usage ) and length ( p = 0.499 ) and taper of fillings ( p = 0.238 ) . 
 the roots instrumented with gg drills had a higher filling density ( p = 0.004 ) . 
 the quality mean score of rcts was improved when gg drills were used ( p = 0.008).conclusion : the technical quality of preclinical molar rcts performed by undergraduate dental students was considered acceptable in 35.6% of the cases . 
 when gg drills were used along with k - files , the technical quality of rcts was enhanced .