The present study was conducted in the Department of Pedodontics and Preventive Dentistry, Faculty of Dental Sciences, Sri Ramachandra University, Chennai in 2009. Seventy-five extracted human mandibular primary second molars stored in 10% formalin. The inclusion criteria for the study are human mandibular primary second molars with a minimum of 2/3^rd of its root length and no evident defects or anomalous morphology. The following were considered as exclusion criteria, teeth with inadequate root length, teeth showing radiographic evidence of internal resorption or root canal obliteration, grossly decayed teeth and canals showing radiographic evidence of calcification.

Coronal access was prepared using a diamond bur (BR 40, MANI, INC, Japan) and the distal root canals were controlled for apical patency with a file of size 10. Distal roots of the primary mandibular second molar were selected, since they generally have a curved, large, single root canal with a uniform canal outline and relatively less intracanal ramifications compared with the mesial roots [11]. The 15 K size file was inserted into the canal until its tip was flushed with root surface at the apical foramen. The working length was calculated as 1mm short of this length. The collected teeth were then separated into three equal groups (n=25 each) according to the instrument utilized.

The distal canals were randomly scattered and instrumented by hand and rotary files. Copious irrigation with 2mL of saline and 5.25% NaOCl solution after the use of each file using a 27-G irrigation needle (Unident) was done. The files were lubricated with EDTA gel each time they were used. All root canal instrumentation was consummated by one operator and all types of rotary files were utilized with 128:1 reduction contra-angle handpiece (NiTi contra-angle, Anthogyr). Each instrument was transmuted after five canals. Instrumentation time including time for irrigation and filing for each root canal was also recorded. The same operator did the instrumentation for the entire specimen.

Instrumentation (Step-back technique) was performed with .02 taper stainless steel K-files (Mani). A size 20 file was positioned to length, and a combination of a filing and a reciprocal reaming action was used until it fitted loosely in the canal and reached the working length. This method was repeated until the apical portion was prepared to a size of 35 file. Studies suggest that instrumentation should be halted at size 30 [12], whereas others recommend shaping up to 35 or 40 [13].

Specimens in this group were instrumented with K₃ Rotary NiTi Endodontic File (.02 taper) (Sybron Dental, Westcollins, CA, USA). Canals were instrumented at a rotational speed of 250 rpm. The files were advanced while rotating slowly towards the apex and withdrawn as soon as the working length was reached, rotating until the file appeared outside the canal. Instrumentation was done up to 35 size file.

The 25 canals in this group were prepared using K₃ Rotary NiTi Endodontic File (.04 taper) (Sybron Dental, westcollins, CA, USA) at a rotational of speed 250 rpm using a similar technique as described for K₃ (.02 taper) group. Instrumentation was performed up to 25 size file.

Using a light speed plus CT scanner (GE Electricals, Wilwaukee, USA), the specimen were scanned before and after instrumentation. Both cross sectional and longitudinal views were taken. A constant thickness of 0.65mm per slice and a constant spiral or table speed of 0.75 and 120 KVP was used. The scanned images were measured using defined guidelines. A line was drawn connecting the cementoenamel junction mesiodistally in the centremost slices after longitudinal section, which was considered as the reference line [14]. Cross Sectional slices extend from the reference line till the apex of the apex of the tooth. The coronal section was referred as a third slice from the reference line. The centremost cross sectional slice forms the midcoronal section where it extends from the reference line to the apex of the tooth. The third slice from the apical end of the tooth was used to assess the apical section [Table/Fig-1a]. For the purpose of standardization, four reference points were taken on the outer walls of the distal root as buccal (A), lingual (B), mesial (C) and distal (D) points [10].

[Table/Fig-1a,b]:

Pictorial representation: a) the reference lines taken for Coronal, Midcoronal and Apical region, (b) references used for calculation of amount of dentin removal

The amount of dentine removal was measured by subtracting the area of the uninstrumented measurement taken between the inner wall of the buccal and lingual reference point (Z₁) to the instrumented canal of the buccal and lingual reference point (Z₂) [Table/Fig-1b].

Lateral Perforation was assessed on both pre instrumentation and post instrumentation cross sectional slices. Any discontinuity on the border of the section was considered as perforation.

Mean and standard deviation of the amount of dentine removed were estimated from the sample from each study group. The mean values of different study groups were compared using Kruskal Wallis One-way ANOVA. Mann-Whitney U Test was used, followed by the Bonferroni Correction method to list out the significant groups at the 5% level. Lateral perforations were estimated for each study group by using Pearson’s Chi-square Test was used to calculate the p-value. One-way ANOVA was used to compare the working time between the hand and rotary, as well as to calculate the P-value. The Tukey-HSD procedure was employed to identify the significant groups at the 5% level. In the present study, p<0.05 was considered as the level of significance.

The mean and standard deviation of the amount of dentine removed according to the type of instrumentation are summarized in [Table/Fig-2]. Stainless steel K file (.02 taper) removed a larger amount of dentine at the coronal level than K₃ (.02 taper) and K₃ (.04 taper). K₃ (.04 taper) removed a larger amount of dentine at midcoronal and apical level than other two groups [Table/Fig-3a,b].

[Table/Fig-3]:

Cross-sectional spiral computed tomography view of dentin removal: a) before instrumentation; b) after instrumentation

The mean time with the K₃ rotary files (.04 taper) instrument was 3.51 minutes, whereas with the K₃ rotary files (.02 taper) instrument was 3.59 minutes. No significant differences were found among the two systems. The mean time with stainless steel K file (.02 taper) was 4.45 minutes that were statistically significant [Table/Fig-4].

Results regarding root perforation are summarized in [Table/Fig-5]. The perforation rate was higher in the K₃ (.04 taper) group than other two groups at apical level [Table/Fig-6].

[Table/Fig-6]:

Cross-sectional spiral computed tomography view of lateral perforation: a) before instrumentation; b) after instrumentation

There were no hand files or rotary instrument breakage during preparation.

While performing endodontic therapy in primary teeth, due to the factors like ovular-shaped canals with discontinuous and unpredictable constrictions, curvature of the primary tooth root canal system, endo-perio communications and possible damage to the permanent tooth bud, it is not always possible to effects an adequately, done shape and fill of the root canal system, like in permanent teeth [15].

A comprehensive understanding of root canal morphology is important for successful endodontic treatment. Unfortunately, canal preparation is adversely influenced by the unpredictable root canal anatomy and the relative incapability of the operator to visualise this anatomy from radiographs [16]. Procedural faults such as ledge development or transportation of the apical foramen from zipping to apical perforation, or stripping can occur [17].

Various methods have been utilized to assess endodontic instrumentation including plastic models, histologic sections, serial sectioning, scanning electron microscopic studies, radiographic comparison and silicone impression of uninstrumented canals [16]. CT has been put to good use in endodontics in recent times as it allows for measuring the amount of dentin removed in a non destructive manner [18].

Based on the results of this study, suggest that dentine removal was significantly less in K₃ (.02 taper) group in all the three slices. In the coronal slice stainless steel K file (.02 taper) showed the highest amount of dentine removal whereas in the midcoronal and apical slice, dentine removal was highest in K₃ (.04 taper). The findings of the present study are consistent with Schafer and Schlingemann which compared the cleaning efficacy of the K₃ system versus K- flexofile and observed K- flexofile was more effective in cleaning the coronal and middle thirds of canals [19]. Kummer et al., proved the manual instrumentation removed more dentine when compared with rotary instruments in primary teeth [20]. Azar et al., reported no significant differences in cleaning efficiency between Manual K-files and the Mtwo and ProTaper rotary systems in primary teeth [21]. Other deciduous molars studies comparing manual files and rotary instrumentation found no significant difference in the amount of dentine removal [11,22].

In this study, K₃ with .04 tapers were found to remove more amount of dentine in the midcoronal and apical sections compared to the other two instruments. This might be due to increasing the taper of the instrument compared to the other two groups. Rosa et al., reported two perforations using rotary systems in primary teeth [23]. Versumer et al., reported that with the greater instrument size the amount of instrumented canal walls will increase [24].

The present study shows a higher lateral perforation rate in K₃ rotary files (.04 taper) (32%) followed by a stainless steel K file (.02 taper) (16%). K₃ rotary files (.02 taper) showed the least perforation (8%) among all three groups. K₃ rotary files.04 tapers produced more perforation compared to the other two groups, because of the increase in the taper of the instruments and the lesser thickness of the dentinal wall of primary teeth that resulted in lateral perforation of the canal. Michael Hulsmann stated that the incidence of perforations in clinical treatment as well as in experimental studies has been reported as ranging from 2.5 to 10% [25].

The mean time for canal preparation was significantly reduced with a rotary system when compared to stainless steel K files. Rosa et al., reported that mean time for root canal preparation was significantly shorter using the rotary system than using the manual system [23]. Bahrololoomi et al., stated that Working time was significantly shorter when using the rotary system [26]. The findings of the present study are consistent with Sergio Luiz Pinheiro et al., which compared the instrumentation time and cleaning between manual and rotary techniques in deciduous molars reported the Protaper system presented shorter instrumentation time compared to manual instrumentation [27].

The present study emphasizes the usage of rotary K₃ (.02 taper) over manual instrumentation in primary teeth as it requires a lesser instrumentation time and provides considerable dentin removal thereby reducing the risk of lateral perforation however ensuring adequate canal cleaning. The K₃ rotary files are shorter than the competitors, so utilization of K₃ rotary files is well indicated in paediatric dentistry, especially treating uncooperative children.

The limitation of the present study is that the distal canals have only been used to assess all the parameters. Though In-vitro studies have shown the K₃ NiTi rotary files are better than stainless-steel hand instruments, randomized clinical trials are required to assess clinical outcomes.

According to the results obtained from the present study, the K₃ rotary files (.02 taper) were more effective for root canal instrumentation in primary teeth, presenting shorter working time than a stainless steel K file. Further investigation should be conducted to determine the effectiveness of K₃ rotary files in primary teeth.