G. Gambarini & J. Laszkiewicz
Department of Periodontics-Endodontics, University of Rome, “La Sapienza”, Rome, Italy.
Department of Conservative Dentistry, Institute of Dentistry, Medical University, Lodz, Poland.

Introduction.
Although thorough cleaning and shaping of the root canal system are considered as key requirements for success in root canal treatment, numerous investigations have demonstrated the limitation of manual and automated root canal instrumentation regarding the overall quality of preparation (Weine et al. 1976, Lehman & Gerstein 1982, Turek & Langeland 1982, Bolanos et al. 1988, Hülsmann & Stryga 1993, Hülsmann et al. 1997, Bertrand et al. 1999). These problems have resulted in a wide search for innovative materials, instruments, and techniques to obtain a clean, disinfected, debris-free canal for obturation (Yang et al. 1996).
Since most hand preparation techniques are time consuming, technically demanding and may lead to iatrogenic errors (ledging, zipping, canal transportation and apical blockage), attention has been directed toward nickel–titanium rotary instruments. Numerous studies have reported they could efficiently create a smooth, predetermined funnel-form shape, with minimal risk of ledging and transporting the canals (Esposito & Cunningham 1995, Glosson et al. 1995, Short et al. 1997, Thompson & Dummer 1997b). Shaping procedures can be completed more easily, quickly and predictably, but effective cleansing of the entire root canal system using Ni–Ti rotary instruments has not yet been demonstrated (Siqueira et al. 1997).
The purpose of the present study was to investigate the efficiency of GT rotary instruments in removing debris and smear layer from root canal walls.

Materials and methods.
Sixteen freshly extracted single-rooted mandibular premolar teeth with straight canals (radius of curvature less than 5) and closed apices were used. None of the teeth had received restorative or endodontic therapy. Following extraction, the teeth were stored at 4C in isotonic saline solution to avoid any effect that fixative might have on the dissolution of organic tissue.
Conventional endodontic access cavities were prepared (Endo Access Bur, Dentsply Maillefer, Ballaigues, Switzerland) in a high-speed handpiece. If initial instrumentation to the apical foramen could not be performed with a size 10 K-file the teeth were excluded from the study. To determine working length a size 10 K-file was inserted until it reached the apical foramen and one-half millimetre subtracted from this length. A small amount of wax was placed on the tip of each root to prevent irrigating solutions from passing through the apical foramen.

Canal instrumentation.
All canals were prepared using nickel–titanium rotary instruments (GT™ Rotary Files, Dentsply Maillefer, Ballaigues, Switzerland) and a crown-down canal preparation technique. Preliminary coronal enlargement was achieved with the sequential use of GT instruments with .12, .10, .08, and .06 taper. Instruments were advanced slowly into the canal exerting only a light force (passive instrumentation). Apical stop preparation was completed with .04 tapered GT™ rotary instruments sizes 20–35 to ensure adequate apical enlargement. Instruments were used in a controlled, slow speed, high torque motor at a speed of 250 r.p.m. All teeth were prepared by the same operator.
Root canals were irrigated with 2 mL of 5% NaOCl (Niclor, Ogna, Italy) between each instrument and kept flooded with irrigant during the instrumentation phase. The irrigant was delivered with an endodontic syringe with a 27-gauge blunt needle that had been placed down the canal until slight resistance was felt. At the end of instrumentation, the following final irrigation sequence was repeated two times: 2 mL of EDTA + Cetrimide for 1 min (Largal Ultra, Septodont, France) and 2 mL of 5% NaOCl for 5 min. A final physiological solution rinse was then used to neutralize the action of the irrigating agents (Gambarini 1999). Canals were dried with sterile standardized paper points.
Two other uninstrumented and unirrigated teeth served as a control. The teeth were stored in isotonic saline solution until they were prepared for SEM examination.

Figure 1. Standardized score of the the debris for specimen evaluation:
(a) score 1; (b) score 2; (c) score 3; (d) score 4; (e) score 5.
Original magnification x200.

SEM examination.
The crowns of the teeth were removed at the cemento– enamel junction. To facilitate fracture into two halves for SEM examination, all roots were grooved longitudinally on the external surfaces with a diamond disk, avoiding penetration of the root canals. The teeth were then carefully split with a hammer and chisel, and prepared for scanning electron microscopic evaluation. The two halves were dehydrated in a graded series of ethanol solutions, critical point dried, attached to coded stubs, coated with gold, and viewed with a scanning electron microscope ( JSM-JEOL Co., Ltd., Tokyo, Japan). Photomicrographs at x200 (for debris score) and x1000 (for the smear layer) were taken in the apical, middle and coronal thirds of the canals.
Separate blind evaluations were undertaken by two trained observers for debris and smear layer with a five score index for each, using reference photographs. Three microscopic fields at x200 and six microscopic fields at x1000 were randomly assessed in each third of each half of the root. Each field was graded from 1 to 5 according to the scoring system and the mean value was calculated for each region of each half of the root. The rating system used was proposed by Hülsmann et al. (1997) and criteria for the scoring were the following:

Score of the debris (Fig. 1 a– e):

• Score 1: Clean root canal wall, only few small debris particles.
• Score 2: Few small agglomerations of debris.
• Score 3: Many agglomerations of debris covering less than 50% of the root canal wall.
• Score 4: More than 50% of the root canal wall covered by debris.
• Score 5: Complete or nearly complete root canal wall covered by debris.

Score of the smear layer (Fig. 2 a– e):

• Score 1: No smear layer, dentinal tubules open.
• Score 2: Small amount of smear layer, some dentinal tubules open.
• Score 3: Homogenous smear layer covering the root canal wall, only few dentinal tubules open.
• Score 4: Complete root canal wall covered by a homogenous smear layer, no open dentinal tubules.
• Score 5: Heavy, non-homogenous smear layer covering the complete root canal wall.

Data were recorded and analysed statistically. Because of the ordinal nature of the scores, the parametric chisquared test was used.

Results.
Mean canal preparation time was 7.20 min (SD 0.5).
Mean scores for debris removal in the coronal, middle and apical thirds were 1.06, 1.38 and 2.25, respectively. Although the best results were observed in the coronal sections, statistical analysis showed that there was no significant difference (P > 0.05) in debris between the three regions of the root canals.
Mean scores for smear layer removal in the coronal, middle and apical thirds were 1.50, 2.00 and 3.38, respectively. Comparison of the removal of smear layer between the three regions showed that there was a statistically significant difference (P < 0.001) between all parts, especially between the coronal and apical thirds (Table 1).
The uninstrumented canals showed walls completely covered with tissue, confirming that specimen preparation alone did not remove tissue.

Table 1. Comparison of the removal of the smear layer between the three regions of the canal.

Discussion.
The ability to clean effectively the endodontic space is dependent on both instrumentation and irrigation. Endodontic instruments may, in themselves, vary in their debris removal efficacy and in their smear layer production, due to their specific flute design (Bertrand et al. 1999). Irrigation plays a key role in successful debridement and disinfection. Sodium hypochlorite is an irrigant solution widely used in root canal treatment because of its bactericidal properties and ability to dissolve organic tissue, however, it is not effective in removing the inorganic smear layer. Therefore, a combination of NaOCl and EDTA has been reported to be suitable for removing both the organic tissues and inorganic smear layer (Baumgartner & Mader 1987). More recently, it has been shown (Gambarini 1999) that cleaning can be significantly improved once the shaping procedure has been completed (the ‘shaping and cleaning’ concept). At the end of instrumentation, root canal diameters have been adequately enlarged to a funnel-form shape that provides easier and superior penetration of the irrigants in the apical portions. At this stage, no further instrumentation is required and, consequently no more smear layer is produced. This allows the irrigating solutions, which are left undisturbed for an adequate period of time, to efficiently remove the remaining debris. The results of the present study showed that GT™ rotary instrumentation followed by a specific final irrigation sequence could produce good canal cleanliness. In most cases, canal surfaces were smooth and free of pulpal remnants.

Figure 2. Standardized score of the smear layer for specimen evaluation:
(a) score 1; (b) score 2; (c) score 3; (d) score 4; (e) score 5.
Original magnification x1000

The new file design seemed to be effective in debris removal. However, it is important to note that the GT rotary instrumentation sequence used in our study is the one recommended by the manufacturer, which consisted of eight rotary instruments. No Accessory GT rotary instruments were used. It means that after initial use of the four standard GT rotary instruments, apical preparation was completed with .04 tapered GT rotary instruments sizes 20–35. Those instruments are essentially ProFile .04 instruments. Cleaning in the apical third is therefore related to the combined action of two different rotary instruments (GT and ProFile) and the large apical stop produced.
Use of the rotary instrumentation resulted in a substantial amount of smear layer. This smear layer consists of dentine particles and pulp tissue closely compacted against the root canal wall and extending into the dentinal tubules (McComb & Smith 1975, Mader et al. 1984, Bechelli et al. 1999). The smear layer produced by instrumentation should be removed, because it could contain bacteria and increase leakage of the canal filling (Yamada et al. 1983, Aktener et al. 1989). Following this hypothesis, additional irrigation with antibacterial solutions or chelating agents has been recommended by many authors to remove debris as well as the smear layer, however, this did not produce the expected smear-free surfaces in the apical third of the canal.

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