Diamond–coated ultrasonic tip decreases debris and uninstrumented surface after preparation of curved canals with isthmus

Abstract The aim of this study was to evaluate root canal preparation with nickel titanium rotary instruments and complementary preparation with ultrasonic tip in curved canals of mandibular molars with isthmus. Twenty–eight mesial roots of mandibular molars with curvature between 20° and 40° and presence of isthmus throughout the entire extension of the root canals were prepared using ProDesign Logic CM (PDL) up to size 40.05, or HyFlex EDM (HFEDM) up to size 40.04. Complementary preparation was performed in the isthmus region using the ultrasonic insert E18D (Helse, Istmo Diamantada). The root canals were scanned using micro–CT (SkyScan 1176) at 9 µm voxel size before and after each preparation step. Transportation, percentage of increase in volume, debris and uninstrumented surface (UNS) were evaluated. Mann Whitney, Wilcoxon, paired and non–paired t–tests were used for statistical analysis (α = 0.05). The canals prepared with PDL and HFEDM obtained similar results for all the variables assessed before using E18D (p > 0.05). E18D significantly decreased the percentage of debris and UNS values in both Groups (p < 0.05). The complementary preparations with E18D caused a smaller quantity of debris in the isthmus of the canals previously prepared with PDL in comparison with HFEDM (p < 0.05). PDL and HFEDM provided similar root canal preparation. PDL promoted a smaller quantity of Debris in the isthmus than HFEDM after using E18D. E18D significantly improved cleaning, and reduced Debris and UNS.


Introduction
During root canal preparation, the use of mechanized instruments promotes a smear layer that can accumulate, particularly in the anatomic complexities of the root canal. 1,2The presence of complexities in the root canal system (RCS), such as isthmus and curvatures, make it difficult to perform cleaning and disinfection. 3The isthmus area is capable of maintaining microorganisms, 4 accumulating debris after root canal preparation 5,6 with different cleaning protocols. 7,8The presence of isthmus occurs with high frequency in the mesial roots of mandibular first molars, 9,10 particularly in the middle third. 9,11,12reparation of root canals with Nickel Titanium (NiTi) instruments maintains their original anatomy. 13,14NiTi instruments with heat treatment have high flexibility and cyclic fatigue resistance, making it possible to perform greater apical enlargement [15][16][17] with centralized preparation in root canals with curvature 18,19 .Apical enlargement improves the cleaning of root canal systems 19 , by diminishing the area of uninstrumented surface 8 and favoring irrigation. 2,20roDesign Logic (PDL) (Easy Equipamentos Odontológicos, Belo Horizonte, Brazil) is a rotary system manufactured with NiTi alloy with heat treatment Control Memory, (CM). 21The HyFlex EDM (HFEDM) System (Coltene/Whaledent, Altstätten, Switzerland) is manufactured with NiTi alloy with heat treatment CM and an additional process of electro erosion, that favors mechanical strength, and increases both the flexibility and cyclic fatigue resistance. 22,23PDL and HFEDM are capable of maintaining the preparation centralized in curved canals. 24][27] The use of ultrasonic activation in the root canal preparation of flat and oval canals promoted greater reduction of uninstrumented surface. 28he ultrasonic tip E18D-Istmo Diamantada (Helse Ultrasonic, Santa Rosa de Viterbo, Brazil) has been proposed for preparing areas of isthmus.It has a tip diameter 33, active part of 12 mm with conicity of 0.02, and cutting action for areas of isthmus.There are no studies that have evaluated the action of the E18D tip.
The aim of the present study was to compare the root canal preparation performed by using NiTi rotary systems up to size 40, and the efficacy of complementary preparation using ultrasonic E18D tip in canals with the presence of isthmus and curvature.The first null hypothesis was that PDL and HFEDM would have similar preparation ability.The second null hypothesis was that the E18D insert would have no influence on the percentage of debris and uninstrumented surface in root canals with the presence of isthmus and curvature.

Methodology Sample calculation
The sample calculation was performed using G* Power software (3.1.7 for Windows, Heinrich Heine, Universitat Dusseldorf, Germany).An alpha-type error 0.05 and power beta 0.80 was established for all the variables.For t-test in two independent groups, previous studies with similar methodology were used to calculate the effect size of each variable: Debris, 1.3782; 2 percentage of increase in volume, 4.0249; uninstrumented area, 1.1773; and transportation, 4.0249. 18For t-test in dependent means, a previous study 29 was used to determine the effect size for debris, 2.7110 and uninstrumented surface, 2.0064.A total of 13 specimens for each group was indicated as being the ideal size required.A sample of 14 root canals per group was determined.

Sample selection
After approval of the Institutional Ethics Committee (CEP No. 02733618.9.0000.5416),28 mesial roots of extracted human mandibular molars with a curvature between 20° and 40° (Schneider, 1971)  30 and presence of isthmus throughout extension of the root canal were selected.For specimen selection, the teeth were radiographed (Kodak RVG 6100), and the degree of curvature was evaluated using the Image J program (National Institutes of Health, Bethesda, MD, USA).All teeth selected were scanned, at 35 µm of voxel size, using the micro-CT device SkyScan 1176 (Skyscan 1176, Bruker-microCT, Konttich, Belgium), with the following parameters: copper and aluminum filter, exposure time of 87 milliseconds, frame averaging of 3, rotation of 180°, rotation step of 0.5, 80 kV, and 300 μA.The three-dimensional images were used for evaluating the presence of isthmus and initial volumetric analyses.The roots were divided into two groups (n = 14), by simple random stratified sampling, taking into consideration the initial volume of the canals.The access cavities were performed in the conventional manner, and the working length was defined at 1 mm short of the apical foramen, by using a type K#10 file (Dentsply, Maillefer, Switzerland).The roots were placed in acrylic resin appliances, surrounded with condensation silicone (Oranwash, Zhermack SpA, Badia Polesine, Italy) to simulate the periodontal ligament, as was done in a previous study. 19All the preparation and complementary procedures were performed by a trained and calibrated operator, under a surgical microscope at 13x magnification (D.F.Vasconcellos, Valença, Brazil).

Root canal preparation
PDL: The Instruments with size 25.01 were activated by means of an electric motor (VDW.SILVER, VDW GmbH, Munich, Germany) at 350 rpm and 1 Ncm in accordance with the manufacturer's specifications, using in-and-out movements up to the working length.After this, the instrument PDL size 25.06 and PDL size 40.05 were used at 600 rpm and 3 Ncm, with in-and-out movements up to the working length, in accordance with the manufacturer's specifications for curved canals.
HFEDM: The Instruments with size 10.05 were used, activated by means of an electric motor (VDW.SILVER, VDW GmbH, Munich, Germany) at 300 rpm and 1.8 Ncm in accordance with the manufacturer's specifications, with in-and-out movements up to the working length.After this, the instrument HFEDM size 25.08 and HFEDM size 40.04 were used at 500 rpm and 2.5 Ncm, with in-and-out movements up to the working length.
Root canal irrigation was performed with 5 mL of 2.5% sodium hypochlorite (NaOCl) for each instrument, using a 30G side-vented needle (NaviTip, Ultradent Products, South Jordan, UT) adapted to a 5 mL syringe (Ultradent Products).As final irrigation, 5 mL of 2.5% NaOCl was used, followed by agitation of 17% EDTA for 3 minutes and application of 5 mL of physiological solution.After the irrigation procedures all canals were dried with absorbent paper cones, corresponding to the preparation (Tanari Indústria Ltda., São Paulo, Brazil).

Complementary preparation with ultrasonic tip
Preparation of the isthmus region was performed with the E18D tip (Figure 1) coupled to the Ultrawave XS ultrasonic device (Ultradent, South Jordan, USA) at a frequency of 50/60 Hz and 20% power.The tip was activated in the buccolingual direction up to 4 mm of the working length.Three cycles of 30 seconds were performed, 15 s towards the mesiobuccal canal and 15 s towards the mesiolingual canal.The tip was positioned in contact with the isthmus, then used with in-and-out movements, thus being activated for 10 seconds without irrigation and 5 seconds with irrigation with water.For each cycle, 3 mL of 2.5% NaOCl were used (1.5 mL before and 1.5 mL after the cycle).

Analysis by micro-CT
Before and after preparation and after the use of the ultrasonic tip, the specimens were scanned using  Transportation was analyzed by means of the shortest distance between the mesial edge of the root and the canal before (X1), and after preparation (X2), shortest distance between the distal edge of the root and the canal before (Y1), and after preparation (Y2).Cross-sections were determined by the arithmetic mean.Transportation was obtained by means of the equation: (X1-X2) -(Y1-Y2) as proposed by Gambill. 32Five cross-sections were measured for each third (cervical, middle and apical and determined by the arithmetic mean value.The thirds corresponded to 3, 6 and 9 mm from the anatomic apex, with each third covering 3 mm of the extension of the canal.

Statistical analysis
The data obtained were submitted to the Shapiro Wilk normality test.When data distribution was not normal, the Mann Whitney tests was used for comparisons between groups; and the Wilcoxon Paired test for comparisons between the groups before and after complementary preparation.When data distribution was normal, the non-paired t-test was used for comparisons between the groups, and the Paired t-test for comparisons between the same groups before and after complementary preparation.The level of significance adopted for all analyses was 5%.

Results
The E18D tip sig nificantly decreased the percentage of debris and uninstrumented surface in the canal and isthmus in both groups (p < 0.05).The percentage of increase in volume and transportation were similar between Groups PDL and HFEDM before and after use of the E18D tip (p > 0.05) (Tables 1 and 2).PDL showed a lower percentage of debris than HFEDM in the isthmus after use of the E18D tip (p < 0.05) (Table 3).The 3D reconstruction in the CTVox software (Bruker-microCT) demonstrated accumulated debris (Figure 2) and uninstrumented surface (Figure 3) after preparation with PDL or HFEDM and after preparation with the E18D tip.

Discussion
The fi rst null hy pot hesis was accepted; the ProDesign Logic and HyFlex EDM systems provided similar results after preparation regarding transportation, volume, uninstrumented surface and debris.Previous studies have reported that these systems have similar capacity for preparation in mesial roots of mandibular molars 24 .HFEDM and PDL provided centralized preparations in canines, maxillary molars and mandibular molars with curvature. 24,33,34n the present study, the root canals were prepared up to size 40, using a sequence of three instruments.The use of more than one instrument during instrumentation procedures reduce the stress on instruments 17,19 and favors apical enlargement and cleaning, 2,19,31 as well as the flow of irrigant solutions. 24ble 1.Median, minimum, and maximum values (% and mm 3 ) of Debris and Uninstrumented Surface (% and mm 2 ) in canals with isthmus prepared with ProDesign Logic or HyFlex EDM, before and after use of E18D tip.The presence of debris in the root canal makes it difficult to disinfect the root canal system 8 and adhesion of the endodontic sealer to the dentinal tubules. 35Apical enlargement reduces the percentage of debris in curved canals of mandibular molars, without causing deviations 24 .However, in the present study a high percentage of debris and uninstrumented surface was found after the use of the two aforementioned systems.Roots with the presence of isthmus throughout their entire extension favored the accumulation of debris because a lower effect of the instruments or irrigant solutions occurred in the region of the isthmus during preparation. 36,37Since isthmus is a narrow, tape-shaped communication between 2 canals, 9 the mechanized NiTi instruments do not have efficient mechanical action in this region.Mesial roots of mandibular molars were used in the present study due the higher incidence of isthmus along their entire length, which occurs in 16% of cases with isthmus. 38hus, the second null hypothesis was rejected since the use of the complementary preparation with the E18D tip resulted in significant reduction of debris and uninstrumented surface in the isthmus and throughout the canal.E18D tip was developed to allow physical and mechanical action in areas of isthmus and difficult access.For this propose, this ultrasonic tip has conical conformation, with tip diameter 33, conicity 02, and active part measuring 12 mm, favoring both lateral and in-depth action.The greater access gained for preparation of the isthmus region diminished the percentage of uninstrumented surface, thereby improving the flow of irrigant solutions. 37,39he mechanical action of ultrasonic tips favors greater contact with the walls of root canals.The Flatsonic ultrasonic tip, used as auxiliary instrument in preparation, demonstrated significant reduction in the percentage of uninstrumented surface and debris in oval and flattened canals. 28,29The ultrasonic activation performed with the E18D tips in the present study, also promoted physical action, potentiating the action of the irrigant solution.When agitation of the irrigant solution used is increased, as in passive ultrasonic irrigation (PUI), a larger amount of debris is removed from the isthmus region. 40,41Ultrasound activation promotes the phenomena of cavitation, which improves the action of the irrigant solution in the RCS, favoring cleaning of areas of isthmus. 42fter use of the E18D tip, a higher percentage of debris was observed in the canals prepared with HEDM in comparison with PDL.Previous studies have shown that the mechanical action of the ultrasonic tip may be influenced by the preparation previously performed. 28,29In canals that were prepared with PDL, Flatsonic diminished the percentage of debris in all the root canal thirds, however, after preparation with Reciproc Blue, this ultrasonic tip did not reduce the accumulation of debris in the middle and apical thirds. 29More extensive enlargements favor the use of ultrasonic tips, 7,28 by providing increased access to and action of the insert in both the canal and isthmus. 7herefore, the lower quantity of debris accumulated after preparation with PDL could be related to the larger taper of the instrument 40.05 in comparison with HFEDM 40.04, contributing to increased access and effect of the ultrasonic tip.In addition, HFEDM shows 3 different cross sections on the instrument 33 and PDL instruments have a modified S-shaped cross section with 2 or 3 cutting blades 24 .Root canal preparation can also be influenced by tip design and cross-sectional geometry. 13,24 alysis by micro-CT made it possible to evaluate different quantitative and qualitative variables in the root canal 17,24,27 and changes in the area of the isthmus caused by instrumentation. 5This data analysis methodology allowed evaluation of the debris and uninstrumented surface in pre-determined regions of the root canal system such as the areas of isthmus. 7n spite of having small conicity 0.02, the E18D ultrasonic insert has a tip diameter of 0.33.Therefore, adequate preparation is necessary to enable its passive access without interference in the root canals, so that it can act in the areas of isthmus.The heat treatment CM of the PDL and HFEDM instruments up to size .40,promoted centralized preparations without procedural errors.Apical dilation up to size 0.40 may have favored the action of the ultrasonic tip in areas of isthmus, in addition to allowing increased flow of the irrigant solution. 2,20Although the association of the E18D ultrasonic tip with preparation performed with NiTi rotary instruments significantly improved the cleaning of root canals with isthmus, a considerable quantity of debris and uninstrumented surface was still observed.

Conclusions
Preparations performed with ProDesign Logic and HyFlex EDM up to diameter 40 were similar, without promoting transportation in canals with curvature and the presence of isthmus.PDL promoted a smaller quantity of Debris in the isthmus than Hyflex EDM after use of the E18D tip.The E18D tip significantly improved the preparation of curved root canals with presence of isthmus, by reducing debris and uninstrumented surfaces.

Figure 1 .
Figure 1.Representative image of ultrasonic tip.(a) E18D-Diamond Isthmus (b) Representative drawing of active part of E18D-Diamond Isthmus.

Figure 2 .Figure 3 .
Figure 2. 3D Reconstructions obtained by Micro-CT of mesial roots of mandibular molars with isthmus showing Debris accumulated in canal (Black) after preparation with NiTi (Red) and after preparation with E18D (Green).
31ch parameter was evaluated for the entire root canal and in the isthmus.An approximate value of 9 mm was determined for analysis of the total length.CTAn software was used to determine the lower part of the root that corresponded to the the bottom value, and then 9 mm was added to this value to determine the top value.. Initial volume, final volume and final surface after preparation values were obtained.Based on these values, the percentage increase in volume, percentage of debris and percentage of uninstrumented surface of the canal and isthmus were calculated, using the following formulas:31 Different superscript lower case letters on same line indicate statistical difference before and after use of E18D tip in same group.Superscript capital letters in the same column indicate statistical difference between the groups (p < 0.05). *

Table 2 .
Mean and standard deviations of pre-operative volume and increase in volume (%) in canals with presence of isthmus prepared with ProDesign Logic or HyFlex EDM, before and after use of E18D tip.
*There were no statistical differences between the groups.

Table 3 .
Median, minimum, and maximum of transportation (mm) in canals with presence of isthmus prepared with ProDesign Logic or HyFlex EDM, before and after use of E18D tip.