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Smear layer production of K3 and ProFile Ni-Ti rotary instruments in

curved root canals: A comparative SEM study

Kee-Yeon Kum, DDS, PhD,a Reza B. Kazemi, DMD,b Bruce Y. Cha, DMD,c

and Qiang Zhu, DDS, PhD,d Seoul, South Korea, and Farmington, ConnYONSEI UNIVERSITY AND UNIVERSITY OF CONNECTICUT

Objective. The aim of this scanning electron microscopic study was to compare the quality and amounts of smear layer

generated by 2 brands of nickel-titanium rotary instruments during canal preparation in the apical thirds of curved root canals.

Study design. Forty mandibular mesial root canals with intact apex and mean curvature between 30 and 35 degrees were

selected for this study. The root canals were randomly divided into 2 instrumentation groups of 15 each. Automated preparation

was performed with ProFile (Dentsply Maillefer, Ballaigues, Switzerland; n = 15) and K3 (SybronEndo, Orange, Calif; n = 15)

instrumentss using a crown-down technique. As a control group, barbed broaches (Mani; Matsutani Seisakusho, Takanezawa-

Machi Tochibi-Ken, Japan; n = 10) were used to extirpate the necrotic pulp tissue from the root canals. All root canals were

prepared to size #35. Glyde (File Prep, Dentsply Maillefer) was used as lubricant and 1% sodium hypochlorite solution as

irrigant. At the conclusion of the experiments, all roots were split longitudinally and the root canal walls were examined at the

apical third from 2 different perspectives using a scanning electron microscope. A 4-category scoring system for assessing the

smear layer accumulation was used, and the resulting scores were statistically analyzed.

Results. Less smear layer was obtained in the K3 group at the selected apical third of curved root canals (P\ .05). However, all

instruments left a smear layer. The surface texture of the smear layer, in addition to the depth and the frequency of packed

materials into the dentinal tubules, varied with instrument type.

Conclusions. This finding may imply that, compared to ProFile, compression of the remaining smear layer is minimized when

using the K3 rotary nickel-titanium system.

(Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2005;j:j-j)

The removal of the smear layer has been the focus ofmany studies. However, the common findings are thatnone of the instrumentation techniques or devices cancompletely clean root canals without leaving debris.1-4

Nonetheless, the removal of smear layer seems desirablebecause the removal will increase dentin permeability,thus allowing better disinfection of deeper layers of theinfected root canal dentin.5 A clean surface may also bebeneficial for the final seal of the root canal.6 During thepast few years, nickel-titanium (Ni-Ti) rotary instru-ments with advanced blade designs have been developedto improve the cleaning efficiency during root canalpreparation. Rake angle of the cutting blade may be oneof the blade designs that may affect the cutting and

aAssociate Professor, Department of Endodontics, College of Den-

tistry, Yonsei University.bAssociate Professor, Department of Endodontology, School of

Dental Medicine, University of Connecticut Health Center.cAssistant Clinical Professor, Department of Endodontology, School

of Dental Medicine, University of Connecticut Health Center.dAssistant Professor, Department of Endodontology, School of

Dental Medicine, University of Connecticut Health Center.

Received for publication Jun 23, 2004; returned for revision Jul 12,

2004; accepted for publication Feb 4, 2005.

1079-2104/$ - see front matter

� 2005 Mosby, Inc. All rights reserved.

doi:10.1016/j.tripleo.2005.02.079

cleaning efficiency of endodontic instruments. Positiverake angles will cut more efficiently than neutral ornegative rake angles, which scrape the inside of the rootcanal.7 ProFile rotary instruments (Dentsply Maillefer,Ballaigues, Switzerland) have a negative rake anglewithfull radial lands. Some previous studies investigated thecleaning ability of the ProFile instruments in root canalbut showed inconsistent results.2,3,8-11

Variable helix angles and pitch is another feature thatcan improve the removal of the cutting debris formed byinstrumentation.12 Once the instrument has made its cutinto the dentin, the debris needs to move out of the way.Compression occurs when debris is caught between thecanal wall and the instrument flutes. If the instrumentbecomes clogged, there will be no more space left fordebris to be transported out of the root canal. Instrumentswith consistent helix angle and pitchmay allow debris toaccumulate, particularly in the coronal part of the file,blocking the escape way of cutting debris.12

More recently a third generation of rotary files (K3;SybronEndo, Orange, Calif) was introduced. It hasmodified radial lands with a slightly positive rake angle.The helix flute angle increases from the tip to the handle.Additionally, it has a variable pitch throughout thecutting shank. The manufacturer claims that this designwill effectively cut the dentin surface and the dentinaldebris can easily be irrigated away.13 However, there is

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still a lack of scientific data regarding the debridement/cleaning ability of this new rotary system to removesmear layer that is produced during instrumentation.14

Some previous studies have shown that scanningelectron microscopy (SEM) is a valuable method forassessment of the ability of the endodontic procedures toremove smear layer from root canals, thus enablingcomparison of the treatment results when using variousinstruments and techniques.1-4,8-11,14 Therefore, the aimof this SEM study was to compare the quality andamounts of smear layer generated by 2 brands of Ni-Tirotary instruments during canal preparation in the apicalthird of curved root canals.

MATERIAL AND METHODSSelection of teeth and root canal preparation

Extracted human mandibular molars with fullyformed apices were stored in 0.5% sodium azidesolution at 48C until use. The occlusal aspects of theteeth were ground flat on a model trimmer to provide areliable reference for root canal instrument measure-ments. A conventional access cavity was then preparedwith a tapered fissure bur in a high-speed handpiece, andthe distal roots were removed. A #15 K-flexo file(Dentsply Maillefer) was placed into each mesial canalto the apical foramen. Radiographs were taken in bothmesiodistal and buccolingual direction. Root canalcurvatures were measured on both buccolingual andproximal views according to Schneider.15 Forty mesialcanals were finally selected using the following criteria:(a) roots with Weine’s type III canal configuration; (b)curvature of the canal in the mesiodistal plane between30 and 35 degrees; (c) canals displaying unidirectionalcurvature in both buccolingual and proximal viewradiographs; (d) canal length from the occlusal refer-ence point to anatomical foramen between 15 and 16mm; (e) canals having a snug fit with either #10 or #15K-flexo file after coronal flaring; and (f) canals withoutinternal calcification and apical root resorption. The rootcanals were randomly divided into 2 instrumentationgroups of 15 canals each. As a control group, a barbedbroach (Mani; Matsutani Seisakusho, Takanezawa-Machi Tochibi-Ken, Japan) was used to extirpate thenecrotic pulp tissue from the root canals (n = 10). Noadditional instrumentation was attempted for the controlgroup. The Ni-Ti rotary instruments used in this studywere K3 (SybronEndo, Orange, Calif), Quantec Flare(SybronEndo), ProFile (Dentsply Maillefer), andProFile GT (Dentsply Maillefer).

Root canal instrumentationOne investigator performed all root canal instrumen-

tation. All the canals included in the instrumentationgroups were preflared by Gates Glidden drills (#2 and

#3; Moyco Union Broach, York, Pa) to remove allconstrictions in the coronal third before obtainingworking length. The working length was establishedbymeasuring the length of the initial instrument (#15 K-flexo file) at the apical foramen and subtracting 1 mm.

All root canals were instrumented to apical size #30/.04 taper, using a new series of instruments for eachcanal. Each Ni-Ti rotary instrument was used withcrown-down technique using a reduction handpiece(16:1; W & H Dentalwerk Burmoos, Burmoos, Austria)and electric motor (TCM 3000; Nouvag, Goldach,Switzerland). All Ni-Ti rotary instruments were used inthe canal with a continuous, slight pecking movementand were never forced apically. Copious irrigation wasdone between each instrument using a 10-mL Maxi-probe syringe (Dentsply Maillefer), delivering 3 mL of1% NaOCl solution via a 27-gauge needle. The needlewas inserted as deep as possible into the root canalwithout binding. Thus the depth varied depending on thestage of instrumentation. As a lubricant, a small amountof Glyde (File Prep; Dentsply Maillefer) was coated onthe flute of every Ni-Ti file and instrumentation wascompleted throughout the entire root canal length. Theroot canals were kept floodedwith the irrigation solutionthroughout the entire instrumentation procedure. Thefollowing instrumentation sequences were used in thisstudy.

In the ProFile group, for shaping the coronal third ofthe working length of the root canals, the specimenswere instrumented using serial numbers #20/.12, #20/.10, #20/.08, and #20/.06 of the ProFile GT files at 150rpm. This was followed by the sequential use of theProFile #15/.04, #15/.06, #20/.04, #20/.06, #25/.04, #25/.06, #30/.04, #30/.06, and #35/.04 at 280 rpm to theultimate working length using a gentle pressure.

In the K3 group, similar to the ProFile group, forshaping the coronal third of the working length of theroot canals, the specimens were instrumented usingserial numbers #20/.12, #20/.10, #20/.08, and #20/.06 ofthe Quantec Flare files at 150 rpm. This was followed bythe sequential use of the K3 #15/.04, #15/.06, #20/.04,#20/.06, #25/.04, #25/.06, #30/.04, #30/.06, and #35/.04at 280 rpm to the ultimate working length using a gentlepressure.

Assessment of accumulated smear layerAt the completion of the instrumentation, each canal

was flushed with 5 mL of 1% NaOCl solution followedby 5 mL of sterile saline solution. Canals werethoroughly dried with sterile paper points. The crownsof the teeth were removed at the cementoenameljunction using a low speed diamond saw (Isomet;Buehler, Lake Bluff, Ill). The mesial roots were split inhalf after 2 parallel longitudinal grooves were made

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with slow-speed carborundum disks on the outer surfaceof the roots.

To avoid contamination of the canals with smearduring the separation process, the root was split with achisel and a wooden hammer. One half of each rootwas used to look ‘‘down onto’’ the smear layer toevaluate the extent of this layer on the canal wall surfacein the apical third area. The samples were dehydratedusing a series of graded ethanol solutions (70%, 80%,90%, 100%), coated with a thin (20 nm) palladium-goldfilm, and viewed with an SEM (JSM-6320F; JapaneseElectron Optics Laboratory, Tokyo, Japan). After theSEM beam was adjusted to the apical third area of theroot canal wall under 453 magnification, 10 micro-scopic areas of the canal wall region appearing on thescreen were randomly selected. In each selected area,the magnification was increased to 7003 and a trans-

Fig 1. Control group: scanning electron micrograph of canalwall taken after the necrotic pulp tissue was removed withbarbed broaches. No root canal instrumentation was attemp-ted for this group. No visible smear layer is observed insidethe dentinal tubules, and all are shown with open orifices andhealthy dental structure. A, Surface view (score 0, 7003). B,Side view (30003).

parent grid was placed on the SEM screen. Thecleanliness of the root canal walls was evaluated on 10randomly preselected squares of the grid using thefollowing 4-score system16: 0, no smear layer/all tubulesclean and open; 1, slight superficial smear layer/tubuleopenings visible, but some contain debris plug or softtissue remnants; 2, moderate smear layer/some tubulesopen and others closed; and 3, heavy smear layer andmost/all tubule openings obscured. The final result ofthe smear layer analysis of each root canal specimenwasobtained by calculating the mean of the scores of 10selected areas of each specimen on the screen. Areas ofspecial interest were photographed at 7003 magnifica-tion. The data established for scoring the smear layerwere analyzed using the Wilcoxon ranked sum test tocompare the significance between the 2 instrumentationgroups.

The other half of each root was split perpendicular tothe long axis 2 to 5 mm from the apex and processed forSEM observation as just described. The depth and thefrequency of smear layer penetration at the junction offractured dentinal tubules and the surface of the canalwall was observed (side view). The areas of specialinterest were photographed at 30003 magnification.

RESULTSThirty-eight of the 40 roots were available for SEM

observation. Two samples were lost, 1 owing tospecimen fracture during the splitting process and1 owing to instrument fracture. No smear layer wasclearly observed in any of the control group specimens.There were no packed materials in the dentinal tubulesin any of the control specimens (Fig 1).

Table I. The smear layer scores in each instrumenta-tion group and the statistical results

Tooth # ProFile (n = 14) K3 (n = 14)

1 2.62 1.22

2 2.31 1.35

3 2.42 1.43

4 2.53 1.82

5 2.49 1.34

6 2.34 2.31

7 2.69 1.22

8 2.81 1.26

9 2.43 2.24

10 1.84 1.42

11 2.72 1.66

12 2.65 1.33

13 1.72 1.71

14 2.81 1.23

Mean 6 S.D. 2.47 6 0.69 1.49 6 0.51*

One tooth in each instrumentation group was lost during the experimental

process.

The smear layer scores in all specimens of the control group were 0.

*Statistical significance at P # .05 (Wilcoxon ranked sum test).

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Therewere, however, differences between the generalsurface textures of the smear layer on the root canalwalls, as well as relative differences between the depthand the frequency of packed materials into the dentinaltubules of the 2 instrumentation groups. Table I showsmean scores of smear layer for each group. The scoreswere 1.49 for the K3 group, 2.47 for the ProFile group,and 0 for the control group, respectively. The K3 groupresulted in significantly less smear layer than the ProFilegroup (Table I,P\.05). Completely cleaned root canalsin the selected apical third area were not found in any ofthe Ni-Ti rotary groups.

Fig 2. K3 group: scanning electron micrograph of canal wallprepared with the K3 rotary instruments shows slight smearlayer production with mostly open dentinal tubules. There areno visible tubular packed materials into the dentinal tubules.(Score 1, magnification 7003.)

Fig 3. ProFile group: scanning electron micrograph of canalwall prepared with the ProFile rotary instruments. The canalwall is totally covered by a thick and nonhomogeneous layeras a tree-bark pattern, which shows shiny and burnishedappearances with almost no open dentinal tubules. (Score 3,magnification 7003.)

Microscopically, the surface of the canal wall in theK3 group displayed a small amount of smear layers withfew scattered debris. Many dentinal tubules were openin most specimens (Fig 2). In the ProFile group, thetexture of the smear layer was thicker and finer than thatof K3 group. The surface of the root canal wall wastotally covered by a thick nonhomogeneous smear layerin a tree-bark pattern (Fig 3). The smear layer showed ashiny and burnished appearance with very few openingsof dentinal tubules.

The side view perspective revealed that the frequencyof the packed materials and the depth of penetrationinto the dentinal tubules differed. At the margins of thefractured wall the smear layer covered just the orificeof dentinal tubules in most specimens of the K3 group(Fig 4). The ProFile group showed a thick regular tightly

Fig 4. Root canal wall prepared with the K3 rotary instrumentat the 2 mm apical level. No visible smear packed in thedentinal tubules. (Original magnification 30003.)

Fig 5. Root canal wall prepared with the ProFile rotaryinstruments at the apical 2 mm level. Fine particulate materialis densely packed into the dentinal tubules. The smear layershows ‘‘muddy’’ appearance. (Original magnification 30003.)

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attached smear layer on the canal wall, and the packedsmear was generally penetrating deeper into the dentinaltubules when compared to the K3 group. In somespecimens, fine particulate materials penetrated into thedentinal tubule as far as 10-15 mm (Fig 5).

DISCUSSIONThe present study showed that the thickness, fre-

quency, and dentin tubular penetration of the remainingsmear layer formed by the 2 Ni-Ti rotary instrumentsvaried in the selected apical third area of curved rootcanals. The design of the cutting blade of the 2 differentinstruments may have affected the outcome of theinstrumentation.

In general, the smear layer produced by the K3instrument was thinner and less frequently occurringthan in the ProFile group. Thismay be due to the positivecutting edges for optimum cutting efficiency. In addi-tion, the chips resulting from the cutting action can beeasily dislodged from the working area and carried offvia the variable helix and pitch of the cutting blade of theK3 rotary instrument.

In contrast, the SEM observations of the ProFileinstruments displayed a dense, thick, and nonhomoge-neous smear layer with almost no dentinal tubuleopenings seen on the canal walls. A more detailed SEMobservation in most specimens revealed the appearanceof 2 or more different layers of smear superimposed onthe dentinal walls in a tree-bark configuration.17 Theconstant helix angles and pitch, and the burnishingaction of the lands of the ProFile instruments may beresponsible for this.2

In agreement with previous findings,18,19 the presentstudy also showed 2 distinct and separate smear layeraccumulations: a surface smear layer, as describedearlier, and a smear layer penetrated into the dentinaltubules. Despite the use of a chelator paste (Glyde) and1%NaOCl solution, the cleanliness in the apical portionwas poor. This may be due to the fact that serial rotaryinstrumentation may recreate the smear layer in the rootcanal after each instrument. The distribution of thechelator paste over the curved root-canal walls may alsobe more difficult to control than in the straight canals. Interms of a clinical application, our findings suggest thatGlyde did not effectively help in removing the smearlayer during Ni-Ti rotary instrumentation.

The side-view perspective revealed that the incidenceof tubular packed materials was more frequent anddeeper into the dentinal tubules in the ProFile groups.Clinically, this finding may be important because thesmear layer formed by instrumentation in the infectedroot canal may include necrotic debris and bacteria.18

This is undesirable because microorganisms, which

remain in the apical portion of root canals at the time ofroot filling, are considered a major cause of failure.20

The present study compared the cleaning ability of 2Ni-Ti rotary systems in the apical third of the curvedroot canals and demonstrated that the K3 instrumentsproduced less smear layer than the ProFile group.However, it remains unclear whether the shaping abilityof K3 instrument system is an advantage in curved rootcanals. Further study is necessary to evaluate themaintenance of root canal curvature, cutting efficiency,incidence of procedural errors such as file separations,as well as root perforations that may occur in the curvedroot canal.

In conclusion, within the constraints of this study, theuse of the K3 rotary system generated less formation ofsmear on the root canal walls compared to the ProFilesystem in the selected apical third area of curved rootcanals. This finding may imply that compression of theremaining smear layer is reduced.

The authors acknowledge special thanks to Dr JohnAghajanian for his expertise and valuable guidance regardingthe SEM observations.

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Reprint requests:

Dr Reza B. Kazemi, DMD

School of Dental Medicine

University of Connecticut

263 Farmington Avenue

Farmington, CT 06030-1615

kazemi@nso2.uchc.edu