Evaluation of the Canal Transportation by OneShape and Neolix Rotary Systems in the Mesial Root of Mandibular First Molars by Cone Beam Computed Tomography

EXTENDED ABSTRACT

Background

Correct cleaning and shaping are fundamental stages of root canal treatment because canal preparation should facilitate debridement and subsequent filling while preserving the original position and form of the canal and apical foramen. [1,2] Curved root canals are particularly susceptible to procedural changes during instrumentation. [3,4] Canal transportation results from preferential removal of dentin from the outer aspect of a curvature as an instrument tends to return toward a straighter configuration, potentially altering canal geometry and working length. [5] Nickel-titanium (NiTi) rotary instruments have been developed to improve mechanical preparation of curved canals, and their flexibility can help maintain the original canal path. [6-9] OneShape and Neolix are continuous-rotation, single-file NiTi systems with different designs and manufacturing characteristics. [2,3,7,10,11] Cone beam computed tomography (CBCT) provides a non-destructive method for evaluating canal geometry before and after instrumentation. [5,12] Because shaping ability and the time required for preparation are important performance characteristics of rotary systems, direct comparison of these two files in curved canals is warranted. This study aimed to compare canal transportation and preparation time produced by the OneShape and Neolix rotary systems in the mesiobuccal canals of extracted mandibular first molars using CBCT.

Methods

This experimental laboratory study evaluated the mesiobuccal canals of extracted human mandibular first molars with fully formed apices. The teeth had been extracted for reasons unrelated to the study, including unfavorable periodontal prognosis. Forty-three teeth were initially examined, and 30 teeth that met the study criteria were selected; the planned sample size was 15 specimens per group based on the authors' sample-size calculation. Inclusion criteria were a mature root, a healthy root, 25-35° of mesiodistal curvature, and two canals with separate orifices. Teeth with root resorption, immature apices, canal calcification, or curvature greater than 35° or less than 25° were excluded. Specimens were examined for cracks under a dental microscope, and canal curvature was determined by the Schneider technique. [13] Access cavities were prepared, working length was established with a size 10 K-file at 0.5 mm short of the apical terminus, and crowns were shortened to standardize working length at 18 mm. Canals were initially prepared with size 10 and 15 K-files; canals larger than ISO size 15 were excluded. Pre-instrumentation CBCT scans were obtained with a Cranex 3D unit at 90 kV, 3 mA, 27 mAs, and 0.2-mm axial slice thickness. Teeth were randomly allocated to OneShape or Neolix groups (n=15 each) and instrumented by a board-certified endodontist. OneShape size 25/0.06 instruments were operated at 350-450 rpm with a maximum torque of 2.5 N.cm, whereas Neolix size 25/0.06 instruments were operated at 350-500 rpm with a maximum torque of 1.5 N.cm. Irrigation with normal saline and 5.25% sodium hypochlorite and patency checking with a size 10 K-file were performed during preparation. Preparation time was recorded by stopwatch and, as described by the authors, included active instrumentation, irrigation, and file cleaning. Post-instrumentation CBCT scans were obtained using the same positioning, with specimens stabilized in a wax arch to reproduce scan orientation. Canal transportation was calculated with the Gambill formula from mesiodistal measurements at 3 and 9 mm from the apical foramen. [14] Measurements were made by two oral and maxillofacial radiologists using NNT Viewer software. Data normality was assessed with the Shapiro-Wilk test, and between-group comparisons used independent t-tests in SPSS version 16 with a significance level of P<0.05. The study was approved by the Golestan University of Medical Sciences Ethics Committee (IR.GOUMS.REC.1395.234).

Results

All 30 selected teeth were included in the comparison, with 15 specimens in each rotary-system group. The Shapiro-Wilk test indicated that the analyzed data followed a normal distribution, supporting use of the independent t-test. At 3 mm from the apex, mean canal transportation was 0.19±0.03 mm with OneShape and 0.10±0.02 mm with Neolix. Although the mean value was numerically higher with OneShape, the between-group difference did not reach the prespecified significance threshold (P=0.055). At 9 mm from the apex, corresponding mean transportation values were 0.20±0.03 mm for OneShape and 0.10±0.02 mm for Neolix, again without a statistically significant difference between the systems (P=0.39). The numerical ordering was therefore consistent across the two evaluated levels: OneShape showed the higher mean transportation at both 3 and 9 mm, whereas the reported Neolix mean was 0.10 mm at both levels. However, neither comparison was statistically significant at α=0.05. Preparation time showed a similar pattern. The mean preparation time was 67.8±3.42 seconds in the OneShape group and 65.49±5.72 seconds in the Neolix group. OneShape therefore required a slightly longer mean preparation time, but the between-group difference was not statistically significant (P=0.73). The authors additionally reported that the measured working length was maintained in both groups. Accordingly, the primary outcome of canal transportation did not differ significantly between OneShape and Neolix at either the apical 3-mm section or the 9-mm section, and the secondary outcome of preparation time was likewise comparable. No statistically significant between-group difference was identified for any of the three reported comparisons. Confidence intervals and effect-size estimates were not reported in the article. Overall, the reported CBCT measurements support comparable shaping performance for the two systems under the experimental conditions, despite numerically greater transportation and a slightly longer preparation time with OneShape.

Table 1. Comparison of canal transportation and preparation time between the Neolix and OneShape rotary systems.

Conclusion

Under the conditions of this laboratory study, both OneShape and Neolix maintained the curvature of mesiobuccal canals of mandibular first molars without a statistically significant difference in canal transportation at 3 or 9 mm from the apex. Preparation time was also similar between the two single-file systems. Although OneShape produced numerically greater mean transportation and required slightly more time, these differences were not statistically significant. The authors concluded that both systems have suitable shaping ability for curved canals and can preserve canal curvature. Because each system uses a single file for preparation, neither demonstrated a significant preparation-time advantage over the other in this experiment.

Keywords: Cone Beam Computed Tomography, Root Canal Therapy, Rotary Files, Transportation

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