Comparative Evaluation of TruNatomy, XP Endoshaper and Hand K File Systems on Dentinal Microcrack Formation and Vertical Root Fracture Resistance – An In Vitro Study

Introduction

Root canal treatment is the mainstay of treatment for pulpal and periapical infections where infected tissue and microbes are eliminated from root canal system. Chemo-mechanical preparation, one of the important steps of endodontic treatment involves the instrumentation of root canal system in combination with endodontic irrigants.^1-3 Evolution has taken place in endodontic file systems from traditional hand file systems to recent Nickel Titanium (NiTi) rotary and reciprocating file systems. Files have been designed with different taper, cross-section and metallurgical properties to eliminate iatrogenic errors caused by stiffer file systems when used in curved and narrow canals.^4,5

Unfortunately, instrumentation with NiTi rotary file systems exert force on root dentinal wall causing momentary stress concentration. This leads to formation of dentinal microcrack formation which is challenging in terms of diagnosis and restoration.⁶ Over a period of time, cracks propagate under occlusal force causing Vertical root fracture (VRF). VRF accounts for 11% of failures in endodontically treated teeth. It is a detrimental event which often has poor prognosis with the ultimate treatment option being extraction of the tooth.⁷

Instrumentation technique is the root cause for dentinal microcrack formation and VRF, which is influenced by the degree of taper, force exerted, number of rotations, metallurgical properties and cross section of file systems. Studies report Protaper Universal sequential files with higher cutting efficiency and larger tapers to induce greater percentage of dentinal defects.^8,9

Recently, focus has been drawn to introduce newer file systems with smaller taper and usage of single file systems to preserve the root canal dentin and to reduce the number of rotations within the canal. Minimally invasive endodontics accentuate the significance of preservation of tooth structure, thereby leaving it stronger.¹⁰ TruNatomy (TRN) and XP Endoshaper (XPS) are newer file systems introduced with the aim of minimal root canal preparation.

TRN is heat treated, slim shaping instrument with less memory and super elastic properties made of 0.8mm NiTi wire. XPS is a snake shaped single file system made of MaxWire alloy with an initial taper of .01 which expands within the canal to a taper of .04 reaching intracanal areas.¹¹

Therefore the present study aimed to assess the dentinal microcrack formation and VRF resistance of roots following instrumentation with TruNatomy, XP Endoshaper hand K File systems.

Materials and Methods

This in vitro study was approved by the Institutional Ethical Committee of GDCRI No. GDCRI/IEC-ACM (2)/10/2020-21. One hundred twenty-five freshly extracted mandibular premolars were used in the study. Teeth were kept in 0.1% thymol solution for 24 hours and then were stored in saline. Decoronation of tooth samples was done using diamond-coated bur under water cooling to standardize the root length to 13 mm. The working length was established as 1 mm short. A stereomicroscopic examination of root samples was carried out at 16X magnification to eliminate the presence of defects like fractures, cracks, or craze lines. One hundred twenty-five root samples were divided into three experimental groups and a control group based on the type of instrument used.

1. Control (n=5): Not instrumented

2. Group TRN (n=40): Prepared using TruNatomy file system

3. Group XPS (n=40): Prepared using XP Endoshaper file system

4. Group HF (n=40): Prepared using Stainless Steel Hand K file system

Samples within each experimental group were further subdivided where 20 samples were left unobturated after instrumentation for examination of the presence of dentinal microcracks and the other 20 samples were obturated using respective obturation techniques, followed by which samples were subjected to universal testing machine for analysis of VRF resistance.

Biomechanical Preparation of Samples

Canal patency was obtained following negotiation using #15 K file followed by instrumentation with respective file systems according to the manufacturer’s instructions.

Group TRN (n=40)

The TruNatomy Orifice Modifier (Dentsply Sirona, Switzerland) with the tip size #20 and variable taper was used with two to three gentle apical movements with amplitudes of 2-5 mm into the canal. Then, the TRN Prime shaping file (tip size 25, average .04 regressive taper) was used set at 500 rpm and torque 1.5 N/cm² .

Group XPS (n=40)

The XPS instrument (FKG, La Chaux-de Fonds, Switzerland) was used in rotation motion at 800 rpm and 1-Ncm torque for one minute in each canal.

In groups TRN & XPS, the instruments were advanced apically using an in-and-out pecking motion with an amplitude of approximately 3 mm. Gentle apical pressure was applied with a brushing motion against the lateral walls. After three pecking motions, the instrument was removed from the canal and cleaned with sterile gauze.

Group HF (n=40)

The root samples were instrumented with stainless steel hand K-files (Dentsply Malliefer, Ballaigues, Switzerland) up to file #40, which served as the master apical file, and then the step-back technique was used for flaring where instrumentation was done with progressively larger files at 2 mm increment up to size #80.

During instrumentation, all the roots were covered with moist gauze, and 2.5% NaOCl was used to irrigate the canal after each instrument. The final rinse was performed using 2.5% NaOCl followed by distilled water.

After instrumentation, 20 samples from each of the experimental groups were obturated. Canals in Groups TRN and XPS were obturated using single cone technique with AH plus sealer, whereas canals instrumented in Group HF were obturated using lateral compaction with AH plus sealer.

Crack Evaluation

The unobturated roots were sectioned at 3, 6, and 9 mm from the apex with a low-speed saw using water coolant. Sections were examined under stereomicroscope at a magnification of 16X. The presence of cracks and their location were noted for all sections, and pictures were taken. The photographs were scored according to Yoldas et al. criteria for scoring. Each sample was checked by two operators for the presence of dentinal defects (Micro cracks). The term ‘‘No defect’’ was given if root dentin was devoid of any craze lines or micro-cracks either at the external surface of the root or at the internal surface of the root canal wall. The term ‘‘Defect’’ was given to root sections with micro-cracks, lines, or fractures. A total of 60 sections were examined in each group. The results were expressed as the Number (n) and Percentage (%) of roots in each group.

1a. Not cracked: Absence of lines or cracks in root dentin.

1b. Cracked: Presence of cracks that extend either from the outer root surface into the dentin or from the root canal lumen to the dentin.

Fracture Resistance

The obturated samples were embedded in acrylic resin exposing 8 mm of the root coronally. The specimens were secured in the lower plate of the Universal testing machine. A steel tip was attached to the upper plate and lowered to contact the circumference of the orifice of each specimen. A vertical loading force was applied at a crosshead speed of 1 mm/min until fracture occurred. The force was recorded in Newtons (N).

Statistical Analysis

SPSS version 20 (IBM SPASS statistics) was used to perform the statistical analysis. Descriptive statistics of the explanatory and outcome variables were calculated by Mean, Standard deviation for quantitative variables, Frequency and Proportions for qualitative variables. Inferential statistics like Chi-square test was applied for qualitative variables to find the association. Kruskal-Wallis test was applied to compare the mean vertical root fracture resistance among the groups with Post-hoc Bonferroni for inter-group comparison. The level of significance was set at 5%.

Results

Crack Incidence

Table 1 and Figure 1 represent the distribution of dentinal microcrack formation following instrumentation with different file systems at the level of 3 mm, 6 mm, and 9 mm from the apex. Hand K files presented 0% cracks at 3 mm, 6 mm, and 9 mm from the apex. TruNatomy file system induced microcracks in 10% of samples at apical third and in 5% of samples at coronal third. However, Hand K file and TruNatomy groups showed no significant difference between them (p >0.05). XP Endoshaper induced more number of cracks with 40% of cracks in apical sections, 30% in middle third, and 35% in coronal third.

Fracture Resistance

Table 2 and Figure 2 represent the mean load at which fracture occurred in different groups. Table 3 represents the inter-group comparison between the groups. Mean vertical root fracture resistance was higher for Hand K file group. Statistically significant difference was seen between Hand K file and XP EndoShaper; TruNatomy Vs XP Endoshaper (p <0.05) (Table 3).

Discussion

Biomechanical preparation of root canals causes 30% reduction in root fracture resistance. This majorly occurs due to the stress on the dentinal walls caused by rotary/reciprocating file systems. In order to overcome the drawback of excessive tooth structure removal and stress concentration on dentinal wall, improvement in metallurgical properties and use of single files with less degree of taper were introduced. It is considered that VRF is not an immediate phenomenon but rather a consequence of crack propagation.^12,13 Thus, the present study was done to investigate the formation of dentinal microcracks and VRF following instrumentation with conventional Hand K files and newer file systems TruNatomy and XP Endoshaper.

The two newer file systems used in the study were TruNatomy and XP Endoshaper. TruNatomy file system, as the name suggests is claimed to provide slim shaping instrumentation respecting the anatomy of root canal, preserving the root dentin. Single file systems were introduced with the idea of reduced number of rotations within the canal, thereby reducing the stress concentration in the root dentin.¹⁴ XP Endoshaper is one such single file system.

Stereomicroscopic examination revealed no cracks in control group proving sectioning to be a safe way for obtaining root sections. Statistical analysis revealed Hand file system (0%) and TruNatomy (5%) produced significantly less percentage of microcracks than XP Endoshaper (35%) single file system. Absence of cracks in Hand K file group could be related to its unaggressive movement, no continuous rotations and its lesser degree of taper (0.02). This was in accordance to the study done by Bier et al., which reported Hand file instrumentation to cause 0% cracks.¹⁵ Only 5% of samples presented with cracks in TruNatomy group. According to the manufacturer, TruNatomy file system presents with regressive taper and is said to preserve the original shape of canal during preparation with less removal of tooth structure.¹⁶ XP Endoshaper presented higher percentage of cracks (35%). Many studies reported XP Endoshaper to produce less microcrack formation. However, our results correlate with the results obtained by Sarah et al., which reported XP Endoshaper to produce cracks similar to Protaper Universal. The author stated the reason to be its higher rotational speed (800 rpm) and the nature of movement of the file inside the canal.¹⁷ In addition to this, Single file system though introduced with the idea of reducing stress on dentinal wall, is said to cause aggressive cutting as it is the only file shaping the canal in contrast to conventional file system, where the canal is enlarged using sequence of files. This exerts more stress on dentin resulting in more cracks.¹⁸

Secondary objective of the study was related to the clinical relevance of the incidence of cracks. Crack initiation is considered to be a stress concentrator that might result in VRF over a period of time. Hence, the fracture resistance of roots prepared with the three file systems was tested. The results showed the force required to fracture root was significantly less in teeth prepared with XP Endoshaper when compared to those prepared with Hand K files and TruNatomy. This reveals the direct correlation between the presence of cracks and root fracture resistance. Peet et al., reported TRN file system maintains the structural integrity by preserving the pericervical dentin with its regressive taper and slim design.¹⁶ Though the manufacturer claims XP Endoshaper to be extremely flexible and minimally invasive, studies report XP Endoshaper changes the canal morphology to a more conical shape thereby causing stress on dentin leading to less resistance to root fracture.¹⁹

Conclusion

Within the limitations of the study, it can be concluded that irrespective of the same taper shared by TruNatomy and XP Endoshaper, TruNatomy file system is minimally invasive causing less incidence of dentinal cracks. XP Endoshaper exerts more force on the dentinal walls due to higher rotational speed and the nature of file movement within the canal. Thus, TruNatomy can be a safer file system of choice for root canal instrumentation.

Conflicts of interest

Nil