in general , endodontically treated teeth are considered to have increased susceptibility to fracture than vital teeth . the reasons attributed to this are the dehydration and loss of dentin caused during the endodontic therapy along with the pressure applied during obturation of the filling material as in lateral compaction technique . the structural durability of the tooth following endodontic therapy is directly proportional to the remaining tooth structure present . aggressive instrumentation of the root canal leads to loss of dentin which may structurally weaken the tooth . bender and freedland have suggested that , the highest incidence of vertical root fracture occurs in endodontically treated teeth . non - surgical retreatment techniques are the preferred choice over surgical management to reduce or eliminate the bacterial load , to contain infection and alleviate symptoms . the goal of retreatment is to completely remove the filling material to enable thorough disinfection of the endodontic system prior to refilling . various techniques have been advocated to remove obturating materials from the root canal system during retreatment . the protaper universal retreatment system consists of three retreatment files , d1 , d2 and d3 , which are designed to be used in the coronal , middle and apical third of the root canal respectively . the triangular cross section of these instruments is similar to the protaper shaping and finishing files . the use of a solvent has been recommended to aid in removal of gutta - percha by softening it . previous studies have suggested that bonded composite restorations can be considered as the first choice for coronal restorations as they play a vital role in increasing the fracture resistance of endodontically treated teeth . furthermore , it has been shown that direct and indirect cusp coverage adhesive restorations can significantly increase the fracture resistance of endodontically treated teeth . to the best of our knowledge no study has been carried out to assess the fracture resistance of teeth following endodontic retreatment . thus , the purpose of this in vitro study was to assess the fracture resistance of teeth after endodontic retreatment with protaper universal retreatment system and measure the remaining dentin thickness with spiral computed tomography ( sct ) and to correlate the loss of remaining dentinal thickness with susceptibility to fracture . a total of 30 extracted mandibular premolars with fully formed apices were used in this study . the teeth were decoronated at the cemento enamel junctions to obtain standard root lengths of 14 mm using a diamond disk ( brassler dental products , savannah , ga . ) . in group i , the working length was established by placing a size 15 k file into the canal until it was observed at the apical foramen , then decreasing the file length by 1 mm . the canals were prepared using protaper universal niti rotary shaping and finishing instruments following manufacturer 's instructions in sequence from sx to f2 to obtain a final apical size of 25 with 8% taper using crown down technique . along with this instrumentation , canals were irrigated between the use of each succeeding file , by introducing 10 ml of 3% sodium hypochlorite ( prime dental products , india ) , with a 27-gauge needle . after complete instrumentation , all specimens received final irrigation with 10 ml of 17% ethylenediaminetetraacetic acid ( edta ) ( prime dental products , india ) followed by 10 ml of sodium hypochlorite to remove the smear layer . following this , the canals were dried with sterile paper points . following the manufacturer 's instructions , the ah plus sealer ( dentsplydetrey gmbh , konstanz , germany ) was mixed and applied within the root canal . a size 25% , 2% master cone ( diadent group international , korea ) was placed at the appropriate working length and apical tuckback was confirmed . a spreader was used to laterally compact the 2% gutta - percha accessory cones coated with ah plus sealer . excess gutta - percha was sheared off and condensed with a plugger 1 mm below the canal opening . the opening of the canal was sealed with coltosol ( coltene - whaledent , cuyahoga falls , oh ) and the teeth were stored at 37c in 100% humidity for 2 weeks . in group ii , cleaning and shaping was performed in the same manner as for group i following which the teeth were subjected to sct ( light speed plus computed tomography scanner [ ge electricals , milwaukee , usa ] ) to assess the teeth at different sections of the coronal , middle and apical thirds respectively . the remaining dentin thickness was then measured from the central canal to the outer periphery in buccal , lingual , mesial and distal directions . following this , the teeth were obturated as stated in group i. in group ii , retreatment was performed after 48 h following obturation to allow for adequate time for the sealer to set with protaper universal niti rotary retreatment ( dentsply maillefer , ballaigues , switzerland ) instruments , which were used at 300 rpm and 3 n / cm torque . the retreatment files were used in the recommended sequence - d1 file ( size 30 , 0.09 taper ) for removal of root filling in the coronal third , followed by d2 file ( size 25 , 0.08 taper ) for the middle - third and finally , the d3 file ( size 20 , 0.07 taper ) was used at working length . final shaping of the apical third was performed with protaper instruments f2 ( size 25 , 0.08 taper ) and f3 ( size 30 , 0.09 taper ) . after complete instrumentation , all specimens received final irrigation with 10 ml of 17% edta followed by 10 ml of sodium hypochlorite to remove the smear layer . following this , 10 after completion of cleaning and shaping procedures , the specimens were again assessed with sct for the remaining dentin thickness as mentioned earlier . lateral compaction was done using a size 30% , 2% gutta - percha master point coated with ah plus sealer . following the manufacturer 's instructions , the master cone was placed at the appropriate working length and apical tuckback confirmed and a spreader was used to laterally compact the gutta - percha accessory cones coated with ah plus sealer . excess gutta - percha was sheared off and condensed with a plugger 1 mm below the canal opening . the opening of the canal was sealed with coltosol f and the teeth were stored at 37c in 100% humidity for 2 weeks . all the teeth in both groups were mounted on a custom made acrylic blocks such that 9 mm of the root surface remained exposed while the rest was embedded in the acrylic resin ( digital products international products , india ) . the temporary material was removed using a bur and the root canal access was shaped to accept the loading fixture . the acrylic blocks were mounted with the roots aligned vertically on the instron testing machine one at a time . the mode of application of the vertical loading force required to fracture the root specimens was similar to the technique proposed by sedgley and messer to test the brittleness of endodontically treated teeth . the specimens were subjected to a constant load at a crosshead speed of 1.0 mm / min until the roots fractured . in this study ,  fracture  was defined as the point at which a sharp and instantaneous drop was observed . this point was verified by a computer attached to the instron testing machine ( model 3382 , instron , uk ) . for most specimens , the test was terminated at this point and the force applied was recorded and measured in newtons . statistical analysis was performed using independent samples t - test for analysis of remaining dentin thickness using sct within group ii and paired samples t - test was used for assessment of fracture resistance between group i and ii ( p < 0.05 ) . a total of 30 extracted mandibular premolars with fully formed apices were used in this study . the teeth were decoronated at the cemento enamel junctions to obtain standard root lengths of 14 mm using a diamond disk ( brassler dental products , savannah , ga . ) . in group i , the working length was established by placing a size 15 k file into the canal until it was observed at the apical foramen , then decreasing the file length by 1 mm . the canals were prepared using protaper universal niti rotary shaping and finishing instruments following manufacturer 's instructions in sequence from sx to f2 to obtain a final apical size of 25 with 8% taper using crown down technique . along with this instrumentation , canals were irrigated between the use of each succeeding file , by introducing 10 ml of 3% sodium hypochlorite ( prime dental products , india ) , with a 27-gauge needle . after complete instrumentation , all specimens received final irrigation with 10 ml of 17% ethylenediaminetetraacetic acid ( edta ) ( prime dental products , india ) followed by 10 ml of sodium hypochlorite to remove the smear layer . following this , the canals were dried with sterile paper points . following the manufacturer 's instructions , the ah plus sealer ( dentsplydetrey gmbh , konstanz , germany ) was mixed and applied within the root canal . a size 25% , 2% master cone ( diadent group international , korea ) a spreader was used to laterally compact the 2% gutta - percha accessory cones coated with ah plus sealer . excess gutta - percha was sheared off and condensed with a plugger 1 mm below the canal opening . the opening of the canal was sealed with coltosol ( coltene - whaledent , cuyahoga falls , oh ) and the teeth were stored at 37c in 100% humidity for 2 weeks . in group ii , cleaning and shaping was performed in the same manner as for group i following which the teeth were subjected to sct ( light speed plus computed tomography scanner [ ge electricals , milwaukee , usa ] ) to assess the teeth at different sections of the coronal , middle and apical thirds respectively . the remaining dentin thickness was then measured from the central canal to the outer periphery in buccal , lingual , mesial and distal directions . in group ii , retreatment was performed after 48 h following obturation to allow for adequate time for the sealer to set with protaper universal niti rotary retreatment ( dentsply maillefer , ballaigues , switzerland ) instruments , which were used at 300 rpm and 3 n / cm torque . the retreatment files were used in the recommended sequence - d1 file ( size 30 , 0.09 taper ) for removal of root filling in the coronal third , followed by d2 file ( size 25 , 0.08 taper ) for the middle - third and finally , the d3 file ( size 20 , 0.07 taper ) was used at working length . final shaping of the apical third was performed with protaper instruments f2 ( size 25 , 0.08 taper ) and f3 ( size 30 , 0.09 taper ) . after complete instrumentation , all specimens received final irrigation with 10 ml of 17% edta followed by 10 ml of sodium hypochlorite to remove the smear layer . following this , 10 after completion of cleaning and shaping procedures , the specimens were again assessed with sct for the remaining dentin thickness as mentioned earlier . lateral compaction was done using a size 30% , 2% gutta - percha master point coated with ah plus sealer . following the manufacturer 's instructions , the master cone was placed at the appropriate working length and apical tuckback confirmed and a spreader was used to laterally compact the gutta - percha accessory cones coated with ah plus sealer . excess gutta - percha was sheared off and condensed with a plugger 1 mm below the canal opening . the opening of the canal was sealed with coltosol f and the teeth were stored at 37c in 100% humidity for 2 weeks . all the teeth in both groups were mounted on a custom made acrylic blocks such that 9 mm of the root surface remained exposed while the rest was embedded in the acrylic resin ( digital products international products , india ) . the temporary material was removed using a bur and the root canal access was shaped to accept the loading fixture . the acrylic blocks were mounted with the roots aligned vertically on the instron testing machine one at a time . the mode of application of the vertical loading force required to fracture the root specimens was similar to the technique proposed by sedgley and messer to test the brittleness of endodontically treated teeth . the specimens were subjected to a constant load at a crosshead speed of 1.0 mm / min until the roots fractured . in this study ,  fracture  was defined as the point at which a sharp and instantaneous drop was observed . this point was verified by a computer attached to the instron testing machine ( model 3382 , instron , uk ) . for most specimens , the test was terminated at this point and the force applied was recorded and measured in newtons . statistical analysis was performed using independent samples t - test for analysis of remaining dentin thickness using sct within group ii and paired samples t - test was used for assessment of fracture resistance between group i and ii ( p < 0.05 ) . in group ii , the intra group comparison of the remaining dentin thickness in the coronal third reveals statistical significance , with a significant difference noted in the apical third [ table 1 ] . remaining dentin thickness  paired sample ( t - test ) results of the fracture resistance reveal a statistically significant difference ( p < 0.05 ) between groups i and ii with the mean fracture resistance of group i being higher than that of group ii [ table 2 ] . fracture resistance ( in newtons ) the prognosis of any endodontic therapy is directly correlated to the execution of its different phases appropriately . the persistence of bacterial colonies within the intricacies of the root canal system leads to refractory apical periodontitis in endodontically treated teeth . this has led to the choice of non - surgical management by retreatment of previously treated teeth by employing techniques for removal of obturating material followed by sufficient cleaning and reshaping procedures to provide adequate disinfection by appropriate irrigants . it has already been established in the endodontic literature that the incidence of vertical root fracture is greater in root canal treated teeth , which often leads to extraction . hence during the re - treatment procedures care should be exercised to prevent loss of dentin that may occur from excessive and aggressive re - instrumentation of root that can substantially weaken the structural integrity of the tooth making it more susceptible to fracture . sct provides a means for non - invasive assessment without sample destruction and also provides a precise 3d reconstruction of the root canal system . in the present study , sct has been used to assess the remaining dentin thickness after initial cleaning and shaping and also following retreatment to evaluate the loss of root dentin for the samples in group ii . protaper universal retreatment system has been comprehensively designed to aid in removal of existing obturating material and to assist in cleaning and shaping to provide space for irrigation and disinfection and subsequent filling of the root canal . d1 is used for removal of filling material the coronal thirds and d2 and d3 from the middle and apical thirds respectively . the convex cross section of the d1 instrument along with its active tip facilitates its initial penetration into filling materials . the removal of gutta - percha from the coronal portion of the root provides space for the introduction of solvents , which aids in further instrumentation procedures . this system has been attributed to perform better due to their progressive taper , length of instrument ; specific flute design and rotary motion , which tend to cut and pull gp into file flutes and direct it toward the orifice . various studies assessing the efficacy of instruments to remove filling materials from the root canal have postulated that filling residue is minimized when the enlargement in retreatment was bigger than the enlargement performed before the canal filling . in the present study , retreatment procedure was completed with the instrument used for enlargement , increased by only one size , before obturation . the results of the present study reveal a significant difference with decreased fracture resistance in the retreatment group having a mean value of 280.6 n while the initial endodontic treatment group had a mean value of 448.6 n [ table 2 ] . this statistically significant difference ( p < 0.05 ) in the fracture resistance of the two groups could be possibly due to a substantial difference in the remaining dentin thickness pre and post retreatment in the coronal and middle - third of the root canal [ table 1 ] . this could be attributed to the use of d1 and d2 retreatment files , which has been found to cut superficial layer of dentin during root filling removal , which along with subsequent cleaning and shaping procedures may have resulted in the substantial loss of root dentin . earlier investigations have reported that the filling material traced in the canal would be reduced when the enlargement in the retreatment was bigger than the enlargement performed before the canal filling . the present study follows the protocol to increase the apical size following retreatment to one size larger than the initial treatment group , which may have accounted for the loss of root dentin from the apical thirds [ table 1 ] . the sct analysis reveals the loss of root dentin in the pre and post retreatment samples in group ii . this loss of root dentin during the course of retreatment procedure may have contributed to the reduced fracture resistance of the samples in group ii . within the limitations of this study , it can be concluded that endodontically retreated teeth have shown significantly decreased resistance to fracture and this has a positive correlation to the increased loss of root dentin during the re - treatment procedures as has been confirmed by the aid of sct .