Dentinal defects before and after rotary root canal instrumentation with three different obturation techniques and two obturating materials.

AIM: To evaluate the role of rotary root canal instrumentation followed by obturation with three different techniques and two different materials on the incidence of dentinal defects.
MATERIALS AND METHODS: One hundred and sixty mandibular premolars were divided into eight groups (n = 20). Group I was left untreated and served as control. The other seven groups were prepared with profile rotary instruments till #40.06 taper. After preparation, group II was left unfilled, groups III, IV, and V were obturated with Gutta-percha and AH Plus sealer using passive technique, lateral compaction and warm vertical compaction, respectively. Groups VI, VII, and VIII were obturated with Resilon and Realseal sealer using passive technique, lateral compaction, and warm vertical compaction, respectively. Roots were then sectioned at 3, 6, and 9 mm from the apex and inspected under a stereomicroscope (50×) for dentinal defects. Chi-square test was performed to compare the incidence of dentinal defects between the groups (P < 0.05).
RESULTS: The unprepared control group had no dentinal defects. The instrumentation group (group II) and the obturation group (groups III-VIII) showed significantly more defects than the uninstrumented control group (group I) (P < 0.001). There was no significant difference between the root canal obturating techniques (group III-VIII) when compared with the instrumentation group (group II). On inter group comparison among the obturation groups the number of defects after lateral compaction with Gutta-percha (group IV) was significantly larger than passive Gutta-percha obturation (group III) (P < 0.05).
CONCLUSIONS: The results suggest that root canal instrumentation significantly influenced the incidence of dentinal defects or fracture. Dentinal defects were more significantly attributed to the role of root canal instrumentation rather than the type of obturation technique or material. Lateral compaction with Gutta-percha significantly produces more defects than passive Gutta-percha obturation.

INTRODUCTION

Vertical root fracture (VRF) is defined as a complete or incomplete fracture initiated from the root at any level, usually directed bucco-lingually.[1] It is a catastrophic complication during or after root canal treatment[2] presenting a significant clinical problem, which is difficult to diagnose and treat.[3] It is also well known that most vertically fractured teeth, with no history of trauma have been root filled.[2]

It is generally accepted that the strength of an endodontically treated tooth is directly related to the amount of remaining sound tooth structure.[4] However, preparation procedures and obturation techniques could also damage root dentin resulting in fracture or craze lines.[5]

A crack or craze line can propagate into VRF if the tooth is subjected to repeated stress. There is evidence that VRFs are probably caused by the formation and propagation of smaller, less pronounced defects, and not by the force practiced during preparation or obturation.[6]

Lateral compaction of Gutta-percha is widely used to fill the root canal system and was previously reported to be associated with an increased risk of VRF.[2] Spreader design and applied forces were suggested as contributing factors to the appearance of VRF.[3]

The potential for VRF is also present with warm vertical compaction[7] as the forces used may be equal to the forces of lateral compaction.

Techniques where no compaction forces (passive obturation) were used have been proposed and shown to produce an apical seal similar to that of lateral compaction when used in conjunction with dimensionally stable sealers.[8]

As an alternative to Gutta-percha, adhesive dental materials like Resilon are now available that may offer an opportunity to reinforce the root filled tooth through the use of bonded sealers in the root canal system.[9]

So far, no study has compared the effect of obturation with Gutta-percha and Resilon with three different obturation techniques in the formation of dentin defects. In the present study, the aim was to evaluate the incidence of defects in root dentine before and after rotary root canal instrumentation (ProFiles) and three different obturation techniques (passive, lateral, and warm vertical compaction) using two obturating materials (Gutta-percha and Resilon).

MATERIALS AND METHODS

One hundred and sixty single rooted mandibular premolar teeth[10] extracted atraumatically for periodontal reasons were collected. Teeth with external cracks or defects, deep caries, with previous restorations, immature apices, and teeth with more than one canal were excluded. The teeth were placed in 0.5% sodium hypochlorite for half an hour, ultrasonically cleaned and stored in purified filtered water.[11] The root surfaces were observed under 7.5× magnification in a stereomicroscope using an external light source to exclude cracks. All the teeth were decoronated using a diamond disc in a micromotor under water spray leaving a root length of approximately 13+/- 1 mm.[12] The roots were divided into eight groups (n = 20).

Group I

It served as control where no instrumentation and obturation procedures were done.

Group II

Shaping and cleaning was standardized for Groups II, III, IV, V, VI, VII, and VIII.

Shaping and cleaning

Initial working length was determined with a #10 K-File (Dentsply Maillefer, Ballaigues, Switzerland) inserted into the root canal until the tip of the file was observed at the apical foramen. Final working length was established by decreasing the file length by 1 mm from the initial working length. To avoid dehydration all the roots were kept in distilled water throughout the experimental procedure. All samples were hand instrumented to ISO #20 K file (Dentsply-Maillefer Ballaigues, Switzerland), and then prepared to #40/.06 taper with ProFile instruments (Dentsply-Maillefer, Ballaigues, Switzerland). Initially the coronal third of the root canals were prepared by Orifice Shaper. Canal preparation was done in a crown down sequence with ProFile instruments 40/.06, 35/.06, 30/.06, 40/.04, and 35/.04 to resistance and then with 35/.06 and 40/.06 to working length.[13]

Canal patency was maintained by using #10 file reaching to working length and irrigation was done with 2% sodium hypochlorite after each change of instrument. Flushing of the canal with normal saline was carried out after every three instruments and a final rinse with 5 ml of 17% EDTA — SmearClear (SybronEndo) was done prior to the obturation. A total of 12 ml of sodium hypochlorite and saline was employed per tooth. The roots were surrounded in polyvinyl siloxane (Aquasil, Dentsply DeTrey, Germany) during obturation.

Group III passive obturation gutta-percha + AH plus sealer

A master Gutta-percha point #40 with 0.02 taper (Dentsply-Maillefer, Ballaigues, Switzerland) was verified for tug back, coated with epoxy AH Plus sealer (Dentsply DeTrey Gmbh, Konstanz, Germany) and placed till the determined working length. Additional #25 (0.02 taper) cones were placed to a depth where resistance was met without use of a spreader.[14]

Group IV lateral compaction gutta-percha + AH plus sealer

Master cone #40 Gutta-percha with 0.02 taper was coated with AH Plus sealer to the estimated working length. Lateral compaction was done with #25 stainless steel spreader with a stopper placed at 1 mm short of working length initially,[15] and the exerted force not exceeding 2 kg, which was measured using a digital scale.[10] Additional cones of #20 (0.02 taper) (Dentsply-Maillefer, Ballaigues, Switzerland) were used. An average of 7-8 cones were placed in each canal.

Group V: Warm vertical compaction gutta-percha + AH plus sealer

Master cone #40 Gutta-percha with 0.06 taper (Dentsply-Maillefer, Ballaigues, Switzerland) coated with AH-Plus sealer till determined working length and downpack was done by continuous wave compaction with medium light plugger up to 5 mm short of working length with exerted force not exceeding 2 kg, which was measured using a digital scale[10] using downpack instrument E and Q Plus heat source (Meta Biomed Co. Ltd., Korea) with temperature set at 160°C. The canal was again coated with sealer, and backfilling was carried out with thermoplasticized Gutta-percha using E and Q plus gun (Meta Biomed Co Ltd, Korea) with the temperature set at 160°C.

Group VI: Passive obturation resilon + real seal sealer

Teeth were obturated similar to group III with Resilon and Real seal (SybronEndo, Orange, CA) according to manufacturer's instruction.

Group VII: Lateral compaction resilon + real seal sealer

Teeth were obturated similar to group IV with Resilon and Real seal according to manufacturer's instruction.

Group VIII: Warm vertical compaction resilon + real seal sealer

Teeth were obturated similar to group V with Resilon and Real seal according to manufacturer's instruction.

Storage of specimens

Roots were stored for a period of 1 week at 37°C and 100% humidity to allow the sealer to set.[16]

Examination of roots

All roots were sectioned horizontally at 3, 6, and 9 mm from apex[17] with a slow-speed saw (Isomet, Buehler, Lake Bluff, IL) under water cooling.

Sections were then viewed through a stereomicroscope (Model Meiji Techno, EMT 52145) using an external light source. Pictures were taken with a camera (Nikon) at a magnification of 50× [Figure 1]. The root surfaces were inspected and defects were noted.

Figure 1

Stereo microscopic sections (50×): Arrows point at dentinal defects. a) No Defects. b) Fractures. c) Multiple Defects. d) Craze lines

Defects were classified as suggested by Wilcox et al.[5] and categorized as follows:

A. No defect

B. Fracture

C. All other defects

A. No defect

‘No defects’ is defined as root dentine devoid of any lines or cracks where both the external surface of the root and the internal root canal had no defects.

B. Fracture

‘Fracture’ is defined as a line extending from the root canal space to the outer surface of the root.

C. Other defects

‘Other defects’ is defined as all other craze lines observed that did not reach the canal lumen or a partial crack extending from the canal wall into the dentine without reaching the outer surface of the root.

Statistical analysis

Chi-square test was performed to compare the incidence of fractures and other defects between the eight groups using the SPSS/PC version 17.0. The level of significance was set at P < 0.05.

RESULTS

Tables 1 and 2 summarizes the results.

Table 1

Distribution of defects at three levels (Coronal, Middle, Apical) among the test groups

Table 2

Statistical analysis among tested groups at three levels (Coronal, Middle, Apical)

No defects were found in the control group. Defects were found in all test groups (groups II-VIII) and were statistically significant than the unprepared control group (group I) (P≤0.001). There was no statistically significant difference between instrumentation group (group II) and obturation groups and between obturation groups lateral compaction Gutta-percha (group IV) was significant than the passive Gutta-percha obturation (group III) (P < 0.05). Intragroup comparison between the groups at coronal, middle, and apical sections showed no statistically significant difference.

DISCUSSION

Resistance to fracture is frequently measured to assess the weakening of the root after different procedures. This method applies an external force until the root fractures.[18] In the current experiment, the influence of various procedures were directly observed and no external forces were applied.

The sectioning method used in the current study allowed the evaluation of the effect of root canal treatment procedures on root dentine by direct inspection of the root canal dentinal wall, observing not only root fracture but also dentinal defects such as craze lines and incomplete cracks, which predispose teeth to VRF.

The roots were embedded in polyvinyl siloxane in an attempt to mimic periodontal conditions that may change the force distribution pattern around the tooth when external forces are used in obturation.[1019] However, the actual clinical situation may be more complex.

Apical preparations were standardized with ProFile 40/.06 taper according to Singla et al.[13] who concluded that ProFile 40/.06 taper instruments offered the advantage of maximum debridement without significant reduction in root fracture resistance.

In the current study, craze lines were not seen in unprepared teeth (group I) proving that craze lines in the other groups were caused by the preparation and obturation procedures. There was a statistically significant increase in the incidence of dentinal defects between the uninstrumented control group (group I) and all the test groups.

Many of the defects did not connect with the pulp space, and were located in places away from direct contact with intracanal instruments, which is in accordance with a previous study by Wilcox et al.,[5] who speculated that the stresses generated from inside the root canal are transmitted through the root to the surface where they overcome bonds holding dentine together.

There was no statistically significant difference between the instrumentation group (group II) and three different obturation groups using either of the two obturating materials (groups III-VIII). The results suggest that root canal instrumentation alone (group II) significantly increases the incidence of dentinal defects when compared with the uninstrumented control (group I). Topcuoglu et al.[20] assessed the fracture resistance of three different obturation materials namely Gutta-percha, BeeFill, and Thermafil and found no significant difference among the three obturation techniques. Shemesh et al.[10] assessed the effects of canal preparation and passive obturation on the incidence of dentinal defects. They found that lateral compaction of Gutta-percha produced significantly more defects than passive obturation.

Our study is the first to compare the effect of Resilon employed in three different techniques namely passive, lateral, and warm vertical on the incidence of dentinal defects. The results of our study indicate two important clinical findings. The primary finding is that both the obturation materials tested (Gutta-percha and Resilon) in three different techniques (passive, lateral, warm vertical) showed no significant difference in the incidence of dentinal defects when compared with the instrumentation group (group II). This is in concurrence with other studies that show dentinal defects being primarily attributed to rotary instruments rather than to the obturating techniques.[1020]

The secondary finding of our study in relevance to the inter group comparison among the obturation techniques and materials shows that lateral compaction with Gutta-percha created more dentinal defects than the passive obturation. This is in concurrence with the study done by Shemesh et al.[10] who showed that lateral compaction of Gutta-percha results in significantly more dentinal defects than Gutta-percha obturated with a passive technique.

Resilon is a thermoplastic synthetic polymer because of the incorporation of polycaprolactone, a biodegradable aliphatic polyester that has a low glass transition temperature of 62°C. It is bondable to methacrylate-based resins as it contains dimethacrylate resins. This highly filled, radio-opaque root filling material can couple to a variety of dentin adhesives and resin type sealers, including Ephiphany (Pentron Clinical Technologies, Wallinford, CT), Real Seal (SybronEndo, Orange, CA), and Next (HeraeusKulzer, Armonk, NY).[21]

Primary components of Resilon are (1) Resilon core material containing bioactive glass, Bismuth oxychloride, and Barium sulfate. (2) Resin sealer is a dual cure, resin-based composite sealer. The resin matrix is composed of bisphenol-A- glycidyldimethacrylate (BisGMA), Urethane dimethacrylate (UDMA) and hydrophilic difunctional dimethacrylates. It contains fillers of calcium hydroxide, barium sulfate, barium glass, and silica. (3) Self-etch primer that contains sulfonic acid — terminated fuctional monomer, 2-hydroxyethyl methacrylate (HEMA), water, and a polymerization initiator. HEMA enhances the bonding of resin to dentin.[22]

Studies have shown that the combined use of self etch adhesives and methacrylate-based resin sealers with Resilon are more resistant to bacterial leakage and root fracture.[2324]

In a study by Baba et al.,[25] Resilon demonstrated higher fracture resistance as compared with Gutta-percha when lateral compaction was used while Sagsen et al.[12] and Ribeiro et al.[18] showed no statistically significant difference in terms of fracture resistance between teeth obturated with Gutta-percha and Resilon. The results of our study show no significant difference among the three obturation techniques employed with Resilon on the incidence of dentinal defects. The probable reason that the adhesive systems do not influence dentinal defects could be due to the effect of oxygen molecules present in dentine tubules on the polymerization of the sealer/dentin interface. The low modulus of elasticity of Resilon and Gutta-percha cones when compared with dentin could play a role in this. Very high C-factor of root canals during polymerization of resinous endodontic sealers may also cause gaps along dentin/filling material interface.[18]

Our results indicate that the instrumentation of root canals can have a significant effect on dentin defect formation and therefore on fracture resistance. However, it is not clear whether all micro cracks would lead to VRF and this need to be studied further.

CONCLUSION

Within the limitation of this in vitro study, we can conclude that NiTi instrumentation induces the formation of dentinal defects during canal preparation. Dentinal defects are more significantly attributed to the role of root canal instrumentation rather than the type of obturation technique or material. Among the tested obturation techniques; lateral compaction with Gutta-percha significantly produces more defects than passive Gutta-percha obturation.