The application of MTA as apical plug for root canal obturation – in vitro study

Summary Introduction Prognosis of endodontically treated tooth is directly correlated to the quality of apical obturation. Modern concept of endodontics in particular way points out the quality of apical filling. The aim of this study was to assess the quality of root canal obturation with MTA apical plug using the method of gas (argon) penetration. Material and Methods Sixty-six freshly extracted single rooted (single canal) teeth were divided into the two experimental (30 teeth) and one control group (6). All canals were instrumented using hand and rotary files in step-back technique and copious irrigation of 1%NaOCl. In the first experimental group teeth were obturated using different sealers: Gutta Flow (Roeko), AH Plus (DeTrey), Acroseal (Septodont) and mono gutta-percha cone (10 canals each). In the second experimental group obturation involved 3 mm of MTA-Angelus apical plug while the remaining canal space was filled with the same three sealers as in the first group. The rate of gas permeability by Leak detector-Edwards LD 416 was measured in all teeth. Results The best seal was found in teeth obturated with Gutta Flow and MTA plug with average diffusion rate of 264.4 sec while the worst quality of obturation was found with Acroseal (178.5 sec-the highest gas permeability). All samples with MTA plug exhibited significantly lower leakage than the samples filled without MTA apical plug. Conclusion Root canals filled with MTA apical plug exhibited statistically significant lower gas permeability in comparison to the ones filled with sealer and guttapercha cones only.


INTRODUCTION
Adequately done endodontic treatment supports and stimulates reparative processes in apical periodontal tissues. Up to date doctrine of endodontic treatment is based on biomechanical canal instrumentation and irrigation [1]. The idea about optimal apical hermetic sealing with biologic properties forced many investigators to find ideal obturation material that could also prevent overfilling [2,3]. This material needed to be biocompatible, adhesive and physico-chemically stable to perform obturation [4,5]. Mineral trioxide aggregate (MTA) material fulfills many of required characteristics [5]. Torabinejad and Asgary were the first ones to use MTA in vitro and then in vivo studies in restorative and endodontic procedures in the nineties of the past century [5]. It was invented to improve and satisfy appropriate hardness, low solubility and short setting time as obturation material. The first dark-gray variant of MTA was mostly based on hydroxyapatite particles [4], similar to the original Portland cement (PC) formulation. It has been examined through the series of physicochemical and biological tests for use in dentistry as cheaper bioactive dentine replacement material besides expensive MTA [5][6][7][8]. Up to nowadays, many variants of CS cements showed impressive results as endodontic sealers, pulp-capping agents, apical retrograde fillings [9,10] apical obturation sealers [11,12], perforation healing liners [13], as well as propelling agent during apexogenesis/apexification [12,13]. One of CS preparations, an improved MTA cement, iRoot cement (iR) as root canal sealer is confirmed to kill E. faecalis in bacterial medium [14]. It is considered bioactive material but with high toxicity and certain antibacterial properties [15,16]. Methods for evaluation of apical hermeticity are numerous but diffusion of dye is the one most frequently used, whether by vacuum system or without. Similar methods were used at the end of past century, but with more precision, displaying apical permeability through the function of time [17,18].
The aim of this study was to assess the canal filling hermeticity of MTA apical plug using the method of gas diffusion.

MATERIAL AND METHODS
The study involved sixty-six extracted single-rooted teeth grouped into the two experimental groups of 30 teeth each and one control group of six teeth. The used endodontic sealers were: Gutta Flow (Colthane /Whaledent, Germany), AH Plus (DeTrey, Germany) and Acroseal (Septodont, France). Standardized gutta-percha cones of 2% taper were used for obturation (Pearl Endopia, Pearl Dent Co.). In the first group (A) canals were obturated with single gutta-percha cone and sealer. The second group (B) had canals obturated with apical MTA plug and rest of the canal with gutta-percha and sealer. The roots of teeth had similar diameter and volume of apical portion without any defect along the root. The coronal part of the teeth was removed from the root at the cement-enamel junction using high-speed hand-piece and fissure diamond bur enabling direct access to the canal. After the initial patency (Kerr files #10 and #15), preoperative radiograms were performed. Pulp tissue was removed using barbed broach. Working length was determined by insertion of hand file into the canal until it goes out through the apex and then subtraction for 1 mm and taking xray. Canals were prepared to the WL (#15 do #40) and then by step-back technique up to the coronal third of canal using copious irrigation (1% NaOCl) and lubricant (Canal +, Septodont, France). Instrumentation was completed with hand Ni-Ti reamers and files (Sybron, Germany).

Experimental groups
Upon canal preparation (apical matrix of #40 size) root canals were obturated with three sealers and single gutta-percha cone (groups A1, B1 and C1) or with previously inserted MTA plug by pluggers for vertical condensation (Maillefer, Swiss) and then sealer and gutta-percha cone in the groups A2, B2 and C2. Obturation was performed using Lentulo spiral and guttapercha cones in combination with endodontic sealers: Gutta Flow, Acroseal and AH Plus (DeTray, Germany). Gutta Flow was inserted using dispensing gun (Coltene/Whaledent, Swiss).
Apical plug was 3 mm long where periapical radiographs from two directions confirmed the quality of apical MTA filling ( Figure 1). Access cavities were temporary filled with Cavit (Premier, Dental Products, Germany).
Control group involved six canals that were obturated only with gutta-percha cones to show that argon (Ar) is capable of going only through the apical foramen and root canal filling material. The teeth of experimental and control group were kept for 7 days at 37°C in the area of absolute humidity to allow definite hardening of sealers.

Gas diffusion marking
Quantification of visual information was done using VegaCam device (Edwards LD, 416, UK) and detector device by added software program (Argon Leak Detector, Edwards, UK). The quality of root canal obturation was assessed using the technique of gas permeability system [18]. Ar concentration was measured using argon-leak detector (Edwards LD 416, UK) [18]. Obturated teeth were fixed in prospecting detectors for stability. Argon gas was introduced by a special micro-cannula placed in the coronal part of canal when the measurement chamber was vacuumed. The time (sec) needed for gas penetration through the apex was measured using the timer [19]. The quality of obturation was expressed as the time interval needed for Ar to penetrate through the apex whereby longer time meant better obturation.
Statistical analysis was performed using ANOVA test for differences between the means of experimental groups using confidentiality level of p=0.01. Newman-Keuls test was used to compare differences between the groups.

RESULTS
In the group of teeth that were obturated with apical MTA plug, the lowest gas permeability was shown by combined obturation with Gutta Flow and MTA apical plug (269.4 sec), then AH Plus and MTA plug (258.5 sec) while the highest gas permeability was with Acroseal and MTA plug (255.2 sec). The differences among them were statistically insignificant (p > 0.01) ( Table  1, Graph 1).

Graph 1.
Comparative values of the time (sec) required for argon diffusion from coronal part of restoration to the apex (groups of teeth with sealer and gutta-percha without MTA plug and groups with sealer and gutta-percha and MTA plug) Grafikon 1. Uporedne vrednosti vremena (sek.) potrebne za difuziju argona od kruničnog dela ispuna do apeksa kod grupe A1, B1 i C1 prema grupama A2, B2 i C2 In the group of teeth that were obturated with no apical MTA plug, the lowest gas permeability was recorded in Gutta Flow samples (186.7 sec) then AH Plus (179.9sek) and the highest permeability in the Acroseal paste samples (178.5 sec). The differences among them were statistically insignificant (p > 0.01). The MTA samples showed statistically significant lower argon leakage in comparison to the ones obturated only with sealer and gutta-percha cones without MTA plug (p < 0.05).

DISCUSSION
MTA has characteristics of bioactive material and it is known for its use as an apical plug with good physico-chemical properties exhibiting good adherence to dentin, compressive strength and push-out forces [4,7,8,20,21]. Moreover, its positive biological properties such as biostimulative, osteoconductive and antimicrobial effects have been confirmed either on animal model or in clinical conditions (pulp capping, pulpotomy, root perforations) [4,6,7,8,19,20].
Several methods have been used for the evaluation of apical hermeticity such as bacterial penetration [21], electric currency [22], dye diffusion [2] and radioactive agents [23]. Gas permeability method has good value in assessing the quality of canal obturation. Due to the use of different gases in this method (Ar, O 2 , N 2 , CO 2 , noble gases, organic gases), comparison of the results with different gas nature or different experimental models may not be possible [18,19]. Argon as the noble element was chosen for our study due to its inactivity to endodontic sealers [18,19]. However, this method only allows assessing overall permeability without showing the path of its diffusion. In addition, combined argon diffusion with optical analysis (XRD and spectrophotometry) could possibly explain physical phenomenon of leakage events [18,24].
MTA was chosen as most commonly used cement in several investigations [4-9, 15, 25, 26] and due to its biocompatibility, high alkalinity and low solubility [2,27]. Our satisfying results obtained by MTA-plug could be explained by its low solubility due to water absorption and minimal expansion that was confirmed in several studies [8,9,12,23,27,28].
The use of iR as sealer or filler and for reparation of canal-wall perforation presented satisfactory results [27] due to missing alumina particles and resemblance to white MTA. iR is considered appropriate for root canal filling material due to its viscosity [28] and could be used as sealer-filler with or without gutta-percha cones [29].
The advantage of combination of MTA apical plug and sealer over sealer only samples in the current study may be explained by its expansion in the first several hours during setting and water absorption when MTA exposed weight increase around 12% upon submerging for 24 h [30].
Low solubility and good adherence to the canal walls of Gutta Flow and AH Plus paste has been shown in several studies [17,18,31,32]. Our results are consistent with the results of the study of Bracket et al. [33] and Bouillaguet et al. after 12 months follow-up [34]. Bouillaguet et al. explained that in single-cone obturation technique the volume of sealer is higher than the volume of gutta-percha, and this ratio promotes void formation and reduces the quality of seal due to the contraction phenomenon. Use of single-cone technique also allowed comparison of the performance of all materials under relatively standardized conditions. Leakage of AH-Plus may have resulted from inadequate bonding between sealer and gutta-percha cone allowing fluid to flow at the interface. This finding is in agreement with the results from Sagsen et al. [29]. Bouillaguet et al. also considered that gun pressure for sealer injection created better adhesion of Guta Flow than AH Plus to the canal walls and consequently lower micro-leakage. Martin et al. based of bacterial canal penetration confirmed better sealability in AH Plus cases than when Gutta Flow paste was used as obturation material [35].
Acroseal as calcium-hydroxide based sealer showed inadequate (poor) hermeticity and that finding is also in accordance with several studies [34,36]. As per Mc Michen et al. AH Plus was more stable than Acroseal that was also confirmed in the current study [37].
The explanation for better apical hermeticity in canals obturated by MTA apical plug than in samples without MTA plug was demonstrated in the study of Guven et al. [38]. They explained this finding by the nanosphere structure of MTA particles that allows the material to penetrate into dentinal tubules and interact with moisture inside the tubules for final setting. This creates mechanical bond with dentin upon setting and renders the material with exceptional dimensional stability. In addition, many MTA brands provided either water or hydrosoluble gel to improve its washout resistance.

CONCLUSIONS
Root canal obturation with different sealers, gutta-percha and MTA apical plug demonstrated significantly lower gas penetration compared to standard technique that included guttapercha and sealer without MTA plug.