Comparative evaluation of dentinal crack formation after root canal preparation using ProTaper Next, OneShape, and Hyflex EDM.

INTRODUCTION: The purpose of this study was to compare the incidence of dentinal crack formation after root canal preparation using ProTaper Next, OneShape, and Hyflex electrodischarge machining (HEDM).
MATERIALS AND METHODS: A total of 75 extracted mandibular premolars were selected. The root canals were instrumented using ProTaper Next, OneShape, and HEDM rotary files. All roots were horizontally sectioned at 3, 6, and 9 mm from apex with slow-speed saw under water cooling. The sections were observed under a stereomicroscope at ×25 to determine the absence or presence of crack. Data were analyzed using post hoc test and one-way ANOVA.
RESULTS: ProTaper Next and HEDM produced significantly less cracks than OneShape.
CONCLUSION: Within the limitation of this in vitro study, it can be concluded that nickel-titanium instruments may cause cracks on the root surface. ProTaper Next and HEDM tend to produce less number of cracks as compared to OneShape.

INTRODUCTION

Technological advancement in rotary nickel–titanium (NiTi) instruments has led to new design, concepts, and easier, faster, and better root canal shaping. Root canal shaping procedures and rotary instrumentation with NiTi instruments can induce crack formation.[1] Crack is defined as a defect with complete crack lines extending from inner root canal space up to the outer surface of the root.[23]

Dentinal cracks or root fracture occur when the tensile stress in the root canal wall exceeds the tensile stress of dentin.[14] Rotary NiTi files with large tapers can produce increased friction and stresses on the canal wall and cause dentinal cracks in root dentin.[4] ProTaper Next (Dentsply Maillefer) is a set of rotary instruments that are designed with variable tapers and an off-centered rectangular cross-section which is made from M-wire technology.[5] OneShape (MICRO-MEGA) is a rotary NiTi single file system with variable cross-section along the entire length of instrument.

Recently, Hyflex electrodischarge machining (HEDM) (Coltene/Whaledent) files have been introduced that is manufactured by EDM process. EDM can be used to manufacture all types of conductive materials (e.g., metals, alloys, graphite, and ceramics) of any hardness with high precision. Spark initiated in this process is melting and vaporizing the material of the workpiece in its top layer.[67] This innovative manufacturing process (EDM) is a noncontact machining procedure used in engineering for the manufacturing of parts that would be difficult to machine with conventional technique. The removal of material is performed by pulsating electric current discharges that flow between an electrode and the workpiece and that are immersed in a dielectric medium. The electric current partially melts and evaporates small portions of the material in a well-controlled and repeatable way. The material is therefore superficially removed leaving an isotropic surface, characterized by regularly distributed craters, resulting in superior fracture resistance and improved cutting efficiency.[89] It includes three sets of instruments orifice Opner (25/0.12), glide path file (10/0.05), and shaping file (25/0.08), and three optional finishing files (40/0.04, 50/0.03, and 60/0.02). Like Hyflex CM files, HEDM files have controlled memory effect and regenerative properties.[6]

To the best our knowledge, there are no studies regarding the incidence of dentinal microcracks resulting from the use of HEDM system. Thus, the purpose of the study was to compare the incidence of cracks in root dentin after root canal instrumentation with HEDM, OneShape, and ProTaper Next system.

MATERIALS AND METHODS

A total of 75 extracted human mandibular premolars with mature apices and straight root canals (<5°) were selected and kept in distilled water. The root surfaces were examined under stereomicroscope to exclude external defects and cracks. Then, the teeth were decoronated with a slow speed saw under water coolant to obtain a standardized root length of 16 mm. As suggested previously,[3] a single layer of aluminum foil was used to cover the roots of the teeth, and each root was embedded into acrylic resin set in an acrylic tube. Root was removed, from tube, and the aluminum foil was removed from the root. A light-body silicon-based material was used to replace space created by aluminum foil and simulate periodontal ligament, and the root was immediately inserted into impression material. Apical 3 mm of root was exposed and immersed in water during instrumentation to prevent dehydration. Seventy-five teeth were divided into three groups of 25 teeth each. Canal length was measured by inserting a size 10 k-file into canal terminus and subtracting 1 mm from this measurement. A glide path preparation was done by 15 k-type files. Apical preparation was completed with size 25 instrument of each system. 1% sodium hypochlorite solution was used as an irrigant during instrumentation.

ProTaper Next

The root canals were instrumented using ProTaper Next files at a speed of 300 rpm and 200 g/cm torque with a torque controlled endodontic motor. The sequences were Sx, X1, and X2 in a brushing motion.

OneShape

Root canal orifices were enlarged with orifice Opner endoflare (25/0.12). Complete canal preparation was done by OneShape rotary file (25/0.06) at a speed of 400 rpm.

Hyflex electrodischarge machining

HEDM files were used at 500 rpm and at a torque of up to 2.5 Ncm. After using orifice Opner, single file 25/0.12 was used.

Sectioning and microscopic examination

Roots were sectioned perpendicular to long axis at 3, 6, and 9 mm from the apex using a low speed saw under water cooling. Digital images of each section were captured at ×25 magnification using a digital camera attached to a stereomicroscope. Each specimen was checked by two operators for the presence of dentinal defects. No crack was defined as root dentin devoid of any microcracks or craze lines either at the external surface of root or at the internal surface of root canal wall. Crack was defined as any lines, microcracks, or fracture observed on the slice that either extended from root canal lumen to the dentin or from outer root surface into the dentin. Data were analyzed by SPSS version 15 Software (IBM, Chicago, IL, USA) [Table 2].

Table 2

Data analysis in SPSS software

RESULTS

The numbers of cracks in each group are shown in Table 1. Table 3 summarizes statistically significant difference among the experimental groups (P < 0.05). As shown in Table 4, ProTaper Next and HEDM produced significantly less cracks than OneShape (P = 0.008 and 0.002, respectively). Between ProTaper Next group and HEDM group the difference is statistically insignificant.

Table 1

Dentin cracks for different instrumentation

Table 3

One-way ANOVA for testing the difference of mean cracks

Table 4

Post hoc test for pairwise comparison

DISCUSSION

The primary aim of chemomechanical root canal preparation includes the preservation of original course of the canal and cleaning of the entire root canal system. One common complication associated with mechanical canal preparation is vertical root fracture (VRF), which usually leads to tooth loss.[10] Various NiTi instruments with different design have been introduced, but all of them result in incomplete cracks or even VRF. Hence, such defects should be prevented. Bier et al. suggested that craze lines occurred in 4% to 16% which may develop into fractures during retreatment or after long-term functional stresses such as chewing. This proves that root canal preparation with NiTi rotary systems and every following additional procedure in endodontics as obturation and retreatment with rotary system can create fractures or craze lines.[1]

According to our study, incidence of crack observed in root dentin was greater after instrumentation with OneShape as compared to HEDM and ProTaper Next. Similar results were found by Liu et al.;[11] they reported cracks in 35% of roots instrumented with OneShape.

The design of file may affect shaping forces on root dentin; these forces may cause root fracture.[1213] ProTaper Next has a rectangular cross-section design, increased and decreased tapering over entire length. Off-centered rectangular design of ProTaper Next may have contributed to less number of cracks than OneShape. This design generates a swaggering motion, which decreases screw effect, dangerous taper lock, and torque on the file.[3514] OneShape has asymmetrical cross-section over entire length and variable pitch, noncutting safety tip.[1516]

HEDM files are produced by control memory treatment just like Hyflex CM file. EDM process created a rough and hard surface that could improve cutting efficiency of these files. HEDM files have tip size of 25 (0.08) taper contrast to OneShape and ProTaper Next 25 (0.06). The taper is constant in apical 4 mm of instrument but reduces progressively up to 0.04 in coronal portion. This new file has three different cross-sections over the entire length of working part (rectangular) in apical part, trapezoidal cross-section in middle part, triangular in coronal part to increase fracture resistance, and cutting efficiency.[6] Different speed and torque were used for the three-file system which may be a limitation of the study. According to Peter et al.,[17] increased rotational speed is associated with increased cutting efficiency. HEDM files are more resistant to cyclic fatigue,[6] so recommended speed is 500 rpm which is higher than other two files tested in this study. Thus, HEDM file could result in less cracks than other two files. Previous studies stated that the tip design of rotary instrument, cross-sectional geometry, constant or variable pitch, and taper, flute form could be related to crack formation.[11318] In contrast, HEDM having more taper (0.08) than other two files results in less crack. This may be due to the manufacturing process.

All the three tested instruments have variable pitch and noncutting tip. ProTaper Next and HEDM have rectangular cross-sectional design.[6] OneShape has almost triangular cross-sectional design.[15] Thus, this difference in design could be attributed to more cracks in OneShape. Previous studies suggested that instruments manufactured from M-wire alloy and controlled memory NiTi wire have more flexibility than conventional NiTi wire.[171920] Hence, ProTaper Next and HEDM manufactured from these wires would have contributed to less number of cracks than OneShape.

One of the limitations of this study was application of elastomeric material to simulate the periodontal ligament. Adorno et al.[21] suggested that elastomeric material may collapse and permit direct tooth to acrylic contact; moreover, clinical situation is more complex because the presence of periodontal ligament influences the distribution of stresses.

CONCLUSION

Within the limitation of this in vitro study, it can be concluded that NiTi instruments may cause cracks on the root surface. ProTaper Next and HEDM tend to produce less number of cracks as compared to OneShape. Further, research is required to assess the effect of these instruments on crack formation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.