| ObjectiveThis study aimed to investigate the three-dimensional morphologic classifications and the analysis of canal isthmuses in permanent molars using micro-computed tomography(micro-CT).Constructing 3D printed models of each type of isthmus to explore the influence of different types of isthmuses on root canal irrigation.This study consists of three parts:PartⅠ Three-dimensional morphologic classifications and analysis of canal isthmuses in permanent molarsMaterials and Methods1.Micro-CT reconstructed images of 248 molars were evaluated.2.The tooth positions,the number of roots and canals,tooth and roots with isthmuses,different types of isthmuses,the number and locations of the isthmuses in roots,and canals configurations were recorded.3.The maximum of the major diameter of all canals cross-sections in one isthmus(dmax),the minor diameter of canals in same cross-section(dmin),the distance between the dmax crosssection and apex(Dm-a),isthmus length(Li),and distance from the isthmus ending crosssection to apex(De-a)were measured and analyzed with a significance threshold which was 5%.Results1.Isthmuses were classified into four types based on the boundary characteristics:TypeⅠ,isthmuses with roof,Type Ⅱ,isthmuses with floor,Type Ⅲ,band-shaped isthmuses,and Type Ⅳ,isthmuses without boundary.2.In the 248 isolated teeth,isthmuses were found in 187(75.4%)teeth.Among 255 nonsingle canal roots,216 roots(84.7%)have isthmuses.3.In maxillary molars,95.8%of isthmuses were found in the mesial buccal roots.TypeⅢ isthmuses(38.2%,)were the most common in maxillary first molars,and Type Ⅱ isthmuses occurred more frequently in maxillary second molars(50%).Type Ⅰ isthmuses happened more commonly in the mesial root of mandibular first molars(37.3%).Type Ⅳ isthmuses were mainly found in distal roots(53.8%).In mesial roots of mandibular second molars,the occurrence of Type Ⅱ isthmuses was more than that of other types(31.1%),and Type Ⅳisthmuses were still dominated in the distal roots(52.9%).The dmax and Li of mandibular molars were significantly longer than those of maxillary molars.There was no statistical difference between dmin and De-a for each tooth.4.Type Ⅰ isthmuses appeared more often in the apical region or from the middle third to the apical region(75.9%)and they all happened in the roots with merged or intersected canals.57.1%of Type Ⅱ isthmuses occurred in the coronal third of roots,and 87.0%of them were found in roots with canals which were divided or intersected.32%of Type Ⅲ isthmuses occurred in the middle third of the roots,26%of them were located in the apical third of roots,and 44%of them were found in roots with parallel canals.43.1%of Type Ⅳ isthmuses went through the entire root canal,and 83.3%were found in the converged canals.The median of dmin for Type Ⅰ to Ⅲ isthmuses were 0.054 mm,0.069 mm and 0.064 mm,respectively.The medians of Li were ranged from 1.511 mm to 2.144 mm.While the median dmin of type Ⅳisthmus was 0.134 mm,and the median Li was 6.217 mm.Part Ⅱ Construction of 3D printing models of root canal isthmuses Materials and Methods1.According to the characteristics of type of isthmuses,the data of micro-CT were selected,and the surface morphological data of root canals and isthmuses were obtained by Mimics software.And then the main root canals were marked and widened according to the prepared root canals diameter data.The surface morphological data of the widened main root canals were obtained.The Geomagic software was used to combine and encapsulate two data to construct a digital model of the isthmus.Objet30 Pro 3D resin printer was used to print the models.If the model isthmus was blocked,Geomagic software was used to adjust the narrow diameter of isthmus which would be printed again.2.The isthmus narrow diameter of the digital models and the printed models(the printed model was reconstructed by micro-CT scan)were measured,and 3D deviation analysis was performed on the 3D images of the two models.3.Hydrogel was used as root canal contents and gel forming time was measured.The curvature of main root canals and hydrogel fillings of the isthmus were measured and observed with optical microscope.Results1.Four types of 3D canal isthmuses models were constructed and printed by micro-CT data of selected molars which were accorded to the types of isthmuses and measurements.2.The isthmus narrow diameter of the Type Ⅰ to Ⅳ digital models were 0.071 mm,0.049 mm,0.098 mm and 0.115 mm,respectively.And those of the Type Ⅰ to Ⅳ printed models were 0.050 mm,0.027mm,0.109 mm and 0.066 mm,respectively.The mean 3D deviation distance between digital model and print model of Type Ⅰ to Ⅳ were(-0.006±0.045)mm,(-0.010 ±0.033)mm,(0.003±0.026)mm,and(0.025±0.055)mm.3.The sol-gel transformation of hydrogel was completed within 26 to 28 minutes,and the fulfilling effect of hydrogel in isthmus models were satisfying.Part Ⅲ The influence of different type of isthmus on root canals irrigation Materials and Methods1.The effects of different irrigants(deionized water and 2.5%sodium hypochlorite,NaClO)on the hydrogel blocks were observed.2.The prepared four-type isthmus models were divided into six groups:Deionized water+lateral opening needle group(G1),2.5%NaClO+lateral opening needle group(G2),deionized water+passive ultrasonic irrigation(PUI)group(G3),2.5%NaClO+PUI group(G4),deionized water+photon-induced photoacoustic streaming(PIPS)group(G5),and 2.5%NaClO+PIPS group(G6),with 6 samples in each group.The removing effect of hydrogels of different types of isthmuses with different irrigation protocols were recorded by surgical microscope.The pixel value of isthmus hydrogel residue at each time in the irrigation steps were recorded and analyzed.And the completely removing time of isthmus hydrogel were also recorded and analyzed.Results1.Within 20 minutes,the hydrogel droplets did not change into deionized water,but the time of complete decomposition was different in 2.5%NaClO due to the size and contacted surface of the hydrogel blocks or droplets.2.Different irrigation protocols can be used to remove hydrogels from all types of isthmuses except G1 in type Ⅰ,G1 in type Ⅲ and G5 in type Ⅲ isthmus.The different irrigation protocols and different types of isthmuses were the influencing factors respectively,which could significantly affect the removal effect of hydrogel,and the interaction of the above two factors also had a significant impact on the removal effect of hydrogel(Scheirer-Ray-Hare test,P<0.001).After G4 and G6 protocols,there were almost no hydrogel left in each type of isthmuses,and there was no statistical difference between the two groups(Dunn’s test with Bonferroni adjustment,P=1).There was no significant difference in type Ⅱ and type Ⅳisthmuses with different irrigation protocols(Dunn’s test with Bonferroni adjustment,P=0.0552).The isthmuses hydrogel was completely removed in 66 models,and the mean time was 144.7±44.4 s.ConclusionsIn this study,3D classification of root canal isthmus was proposed based on the locations and number of boundaries,which provided a new perspective way for the morphological study about root canal isthmus.Each type of isthmus has its own characteristics according to tooth positions,root positions and canal configurations,and the measurements of isthmus which improve description of the morphological characteristics of each type.This study not only provided a reference for clinicians to predict the existence of the isthmus,but also provided data supports for laboratory design and manufacture of simulation isthmus models.Based on the data of micro-CT of different types of isthmuses,3D printing technology was used to design and construct visual and homogeneous models of the root canals and isthmus systems,which were similar to the real ones in morphology and measurements.The influence of different type of isthmus on hydrogel removal from 3D printed isthmuses models with different irrigation protocols was performed.It was found that different irrigation protocols and different types of isthmuses could result different the effects of removing isthmus contents.The type of isthmus,that is,the number and position of isthmus boundaries,is one of the factors that affect the irrigation effect,which provides a basis for future research on root canal isthmus. |
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