Impact of Access Cavity Design and Root Canal Taper on Fracture Resistance of Permanent Mandibular Molars: An In-vitro Study
Correspondence Address :
Nandini T Niranjan,
Professor, Department of Conservatve Dentistry and Endodontics, Room No. 6, Bapuji Dental College and Hospital, Davangere, Karnataka, India.
E-mail: nanduendo@gmail.com
Introduction: Minimal invasive endodontics in the form of conservative access designs and minimal root canal taper preparation have been devised to preserve tooth structure and increase fracture resistance.
Aim: To assess the influence of two different access cavity designs and two different final preparation tapers on fracture resistance of mandibular molars.
Materials and Methods: In this in-vitro study, performed over a period of two months, a total of 54 extracted human permanent mandibular first and second molar teeth with completely formed apices were selected and for infection control, the teeth were stored in 10% buffered formalin solution for two weeks before the experiment. The teeth were then randomly allocated into three groups, Group C where no tooth preparation was carried out, Group TAC in which Traditional Access Cavity design was performed and Group TREC where Truss Access Cavity was performed. The groups TAC and TREC were further subdivided into subgroup A and B with two different tapers 0.04 and 0.06 taper preparations. Mesial canals of the teeth were chosen for testing the minimal root canal taper preparations. After apical gauging the distal canals, obturation and postendodontic restoration was carried out in all teeth of both the test groups. The teeth in all three groups were subjected to fracture testing in a universal testing machine. The data were recorded. One-way Analysis of Variance (ANOVA) and Tukey’s Post-hoc test were used for statistical analysis.
Results: The fracture resistance between group C and groups TAC and TREC and their subgroups were found to be statistically significant (p<0.05). However, there was no statistically significant difference (p>0.05) observed between the two test groups TAC and TREC and their subgroups.
Conclusion: The conservative truss access design in combination with a reduced root canal taper preparation has shown to have produced better fracture resistance values in comparison to other groups and their subgroups although the results were not statistically significant.
Conservative access preparation, Minimal invasive endodontics, Truss cavity
Teeth managed endodontically are known to be weak due to caries removal, access cavity preparation and excessive use of rotary instrumentation. Vertical root fracture is a serious clinical concern and has multiple precipitating factors. Hence, in an effort to reduce such complications minimal enlargement and flare preparation of root canal space has been recommended. Since, increased cavity sizes and access cavities increase cuspal deflection, the extent of cuspal flexure after endodontic procedures also becomes a factor to be considered for potential failure (1). A study by Clark D and Khademi J states that molar fracturing can be described as retrograde vertical root fracture and that the ultimate purpose of access must be to avoid the fracturing potential of endodontically treated teeth. Since, the traditional access design focuses more on operator needs and less on restorative needs the newer trends which lays emphasis on biologic and structural aspects for teeth in adapting to the concepts of minimally invasive dentistry have been widely acclaimed in recent times (2).
In order to maintain optimal strength and fracture resistance, the Pericervical Dentine (PCD), undermined dentine, Dentinoenamel Junction (DEJ), axial wall of DEJ, cervical enamel in physiologic young teeth which have been considered of high value with respect to tissue type becomes important. The PCD is the dentine near the alveolar crest and the critical zone identified to be roughly 4 mm above the crestal bone and 4 mm below the crestal bone is important when it comes to ferrule, fracturing and dentine tubule proximity (2).
In the endodontic domain the essence of MID could be attained by shifting to access opening designs that are crafted to preserve sound tooth structure especially cervically as loss of tooth structure in this area of the teeth could make them more susceptible to fracture and by the use of minimally tapered rotary instruments in the root canal space as an attempt to avoid straightening the canals, causing irreparable defects like cracks and stripping of the root walls (3). Although undermined enamel does not aid in reinforcing the tooth with regard to fracture potential but naturally occurring undermined dentine in the form of soffit aids in adding mechanical strength and value to the teeth (4). Since, the fracture of teeth often results in extraction it can ultimately leave the dentist and patients to question the prognosis of such endodontically treated teeth.
Although the primary objective of these newer designs is ‘directed dentine conservation’ (2) several approaches to the Contracted Endodontic Cavities (CEC) technique have been discussed and demonstrated. The ‘Ninja’ and ‘Truss’ endodontic cavities (NEC) and (TREC) designs are inclusive of such demonstrations (5). The TREC is more a strategic design where cavities are prepared over each canal orifice from occlusal surface leaving a dentine truss between the cavities. The approach also proves to be more conservative in that the entire pulp chamber deroofing is avoided (5),(6).
Although studies (4),(5) have been conducted on fracture resistance of conservative cavity designs and root canal instrumentation of increasing tapers (6) no study till date has been conducted in combination of the two in the same experimental teeth. Hence, the aim of this study was to assess the influence of two different access cavity designs and two different final preparation tapers on fracture resistance of mandibular molars. The null hypothesis is that, there is no difference in the fracture resistance of teeth with two different access cavity designs and two different root canal taper preparation.
In this in-vitro study, performed over a period of two months between September and October 2020 in the Department of Conservative Dentistry and Endodontics, Bapuji Dental College and Hospital, Davangere, Karnataka, India. A total of 54 extracted human permanent mandibular first and second molar teeth with completely formed apices were selected and the teeth were stored in 10% buffered formalin solution for two weeks before the experiment. At no stage in the procedure, were the teeth allowed to dehydrate. Ethical committee clearance was obtained from the Research Development and Sustenance Committee, Bapuji Dental College and Hospital, Davangere (Ref.No.BDC/Exam/467/20018-2019).
Inclusion and Exclusion criteria: Non carious teeth with mature apices, teeth with no visible fracture lines or cracks and free of any developmental defects, teeth with similar morphology and relative coronal dimensions were included in the study. However, teeth with previous restoration or endodontic manipulation, short thin or curved roots, fused roots, fused mesial canals and canal calcifications, internal or external resorption were excluded.
The teeth where then randomly allocated into three groups. Standardised radiographs (Paralleling Technique) of each tooth in both the buccal-lingual and mesial-distal directions was taken. The anatomic crown height of the 54 teeth was measured from the occlusal surface to the CEJ on all four sides of the teeth; buccolingual and mesiodistal (MD) dimensions were measured at the occlusal surface. Tooth measurements were taken with a digital caliper (Digimatic 500). Teeth with similar dimensions were selected. Therefore, homogenous groups were created based on the averages of tooth dimensions in order to minimise the influence of size and shape variations. The specimens were randomly divided into two test groups (TAC and TREC) and one control group containing 18 teeth each. The two test groups were divided into two subgroups, Subgroup A (n=9): 0.04 taper and Subgroup B (n=9): 0.06 taper.
Study Procedure
1. A Access cavity preparation: Access cavities were performed with a size 856 diamond point and Endo z bur in a high-speed air rotor with water cooling.
In the intact group, no treatment was performed on teeth, and they remained intact until the fracture resistance test. In the TAC group, traditional endodontic access cavities were prepared following conventional guidelines (7). TRECs (Table/Fig 1) was performed by 46keeping part of the pulp chamber roof intact. Then, a single access to the mesial canals was created in the buccal-lingual direction, and another circular one was made to reach the distal canal orifice. The single oval access to the mesial canals was determined by joining the two access slots created following the perpendicular projection to the occlusal surface of the mesial canals and enlarging it up to 1.2 mm for the oval minimum diameter; the circular access over the distal canal was started with one access slot created following the perpendicular projection to the occlusal surface of the distal canal, and it was enlarged circularly to a 1.2 mm diameter. The diameters were measured and checked with a digital caliper. The two accesses on the same occlusal surface were separated by an enamel/dentine bridge (5),(8).
2. Canal instrumentation: In mesial canals of all specimens of group TAC and TREC, working length was determined by advancing a size 10 K-file into the canal until it was just visible at the foramen and then 1 mm was subtracted from this measurement. The size of the minor constriction was standardised, and any tooth where the size 15 K-file extruded beyond the apical foramen was excluded (9).
Group TAC and TREC: Subgroup A: The mesial canals of teeth were shaped with rotary instruments reaching a final continuous 0.04 taper up to tip size 25 using crown down technique, in the order of files sequence according to manufacturer instructions.
Group TAC and TREC: Subgroup B: The mesial canals of teeth were shaped with rotary instruments reaching a final continuous 0.06 taper up to tip size 25 using crown down technique, in the order of files sequence according to manufacturer instructions.
The final apical file size and taper of the distal canals of all teeth in the two test groups were determined by apical gauging. Also, instruments were used with an endodontic motor (X-Smart, Dentsply Maillefer) following the manufacturer’s instruction.
During the shaping p rocedure, a #10 K-file was taken to the working length to check patency, and intermittent irrigation with 5.25% NaOCl was performed with disposable syringes of 5 mL with 27 G needles.The final flush was done using 17% EDTA and saline. The root canals were then dried using paper points.
3. Obturation: Master cone was selected and obturation was then carried out using cold lateral compaction technique and AH Plus sealer with all canals. The orifices were sealed using flowable resin composite and postendodontic core build up was done using resin composite.
4. Fracture resistance testing: The 54 specimens were mounted in self-curing resin (SR Ivolen; Ivoclar Vivadent, Schaan, Lichtenstein) with the roots embedded up to 2 mm apical to the CEJ as reported in a previous study (4).The specimens were then placed in the Hounsfield universal testing machine (Table/Fig 2) equipped with a 500 N cell load that applied a continuous compressive strength force at a crosshead speed of 1 mm/min. The teeth were positioned vertically and a cylindrical hardened steel rod attached to the upper crosshead was lowered until the cone shaped point of the rod rested on the teeth. The universal load-testing machine was then connected to a microsoft based Qmat Pro that collected all the information and indicated the load at which each mandibular molar tooth fractured. The load at which the fracture occurred was then measured in kilogram force.
Statistical Analysis
The values obtained from samples were analysed using R software version 4.0.2. The descriptive statistics, including Mean and Standard Deviation (SD) were calculated for each group tested. The data was normally distributed hence, One-way ANOVA (Analysis of Variance) was used for intergroup and repeated ANOVA was used for intragroup analysis data for significant differences. Pair-wise comparison between the groups were done using Tukey’s Post-hoc test. A p<0.05 was considered statistically significant.
The descriptive statistics between the three groups have been shown in (Table/Fig 3) and the mean and SD of intact tooth group was found to be higher than the two test groups. (Table/Fig 4) reveals that The intact teeth have maximum fracture resistance with a Mean±SD of 262.9±58.75 kilograms force.
(Table/Fig 5) reveals that intact teeth group in comparison to TAC and Truss group showed statistical significance (p<0.05). However, 47TAC group was found to show statistically significant difference in comparison to intact teeth group (p<0.05) but showed statistically non significant difference in comparison to Truss group. Similarly, the Truss group showed statistically significant difference (p<0.05) in comparison to intact teeth. However, it did not show statistically significant difference when compared to TAC group.
(Table/Fig 6) shows multiple comparisons between the groups and subgroup. The intact teeth group was found to produce statistically significant difference when compared to TAC (Subgroup A), TAC (Subgroup B), Truss (Subgroup A) and Truss (Subgroup B) with (p<0.05).
The traditional (Subgroup A) showed statistically significant difference when compared to intact teeth with (p<0.05). It did not show statistically significant difference when compared to Traditional (Subgroup B), Truss (Subgroup A), and Truss (Subgroup B).
Also, Traditional (Subgroup B) was found to give a statistically significant difference with intact teeth group with (p<0.05) and did not show statistically significant difference with the remaining group.
Similarly, Truss (Subgroup A) and Truss (Subgroup B) groups also showed statistically significant difference only with intact teeth group (p<0.05) and not with other groups.
The present study was aimed to understand if minimal invasive access design in combination with reduced taper root canal preparation 0.04 and 0.06 taper has increased the fracture resistance of teeth in comparison to traditional access design with similar taper root canal preparation and intact control group.
The minimal invasive endodontics approach has gained popularity in current times as it aids in maintaining a balance in functional, biological, adhesive, mechanical and aesthetic parameters through maximum preservation and conservation of tooth structure (3),(10).
In the present study, a statistically significant result was obtained between the intact teeth group and the two test groups TAC and Truss endodontic access design. It is important to understand, however, that restoring teeth after access cavity preparation has been shown to enable teeth to regain 72% of their fracture resistance (11),(12) and that, it is not the cavity design per se. The traditional access design based on GV Black’s extention for prevention and the Truss design based on the concept of directed dentine as described by Clark D and Khademi J (2) that highlights preserving PCD and a portion of coronal pulp chamber, the soffit are not the main reason for reduced fracture resistance but rather the loss of mesial and distal ridges as observed by Corsentino G et al., and Silva AA et al., (5),(13).
In this study irrespective of tapers, Truss access design has performed better than the traditional access design, however, a statistically significant difference could not be obtained and this could be attributed to smaller samples in each of the groups and their subgroups. The better performance can to a certain extent attributed to dentine preservation as claimed by Clark D and Khademi J, Plotino G et al., (2),(14). However, several studies that followed to test this proposition as tabulated in (Table/Fig 7) did not report any statistically significant difference in fracture resistance of contracted/conservative cavities in comparison to TAC. The results of the present study is in accordance with these studies (5),(6),(12),(15),(16),(17),(18).
However, several other studies that followed to test this proposition such as Moore B et al., where CEC showed mean failure loads at (1703-558 N; range, 1205-3021 N) and TEC that showed failure loads at (1384-377 N; range, 966-2381 N) (12), Chlup Z et al., where mean failure loads for mandibular premolars where 1079.0±383.2 N for CEC and for TEC was 946.6±384.1 N, Ivanoff CS et al., (CEC-601.7±307.9 N and TEC- 600.9±360.3 N), Rover G et al., (18) (CEC-996.30-490.78 N and TEC 937.55-347.25 N), Corsentino G et al., (TEC 1149.8 N/mm2 and TREC-1237.1 N/mm2) and Ozyurek T et al., (CEC and TEC with class 11 cavities restored with Ever X Posterior and SDR (TEC+EverX Posterior-971.03±114.28 N, CEC+EverX Posterior -1008.25±216.83 N, TEC+SDR-1451.92±205.39 N, CEC+SDR- 1674.07±238.36 N), Sabeti M et al., (Conservative access cavity- 1705.691250 (591.51) N, Traditional access cavity-1471.113125 (435.34) N) did not report any statistically significant difference in fracture resistance of contracted/conservative cavities in comparison to TEC (5),(6),(13),(16),(17),(18),(19). The results of the present study (Mean fracture resistance values expressed as break force- {TAC - 167.4 Kg and TREC - 184.8 Kg) are in accordance with these studies.
Similarly between Traditional 0.04 and 0.06 tapers and Truss 0.04 and Truss 0.06 tapers, 0.04 taper, in both groups has performed better than 0.06 taper but then again a statistically significant result could not be obtained because of smaller sample size.
With regard to the tapers 0.04 and 0.06 that was tested in the current study, studies by Sabeti M et al., and Zogheib C et al., have shown similar results and it was concluded by Sabeti M et al., that, increasing tapers 0.06 to 0.08 files increased stress in root dentine and reduces the fracture resistance(6),(19).
Although previous studies have emphasised that the root canals are significantly weakened by instrumentation alone (20),(21),(22). A study by Zandbiglari T et al., has shown that greater taper instruments greatly weaken the teeth (23). The amount of remaining dentine thickness and its preservation impacts the resistance of prepared root canals to fracture is henceforth a serious consideration; however, the compromised efficiency of disinfection of root canals through such minimal access preparations cannot be overlooked.
Teeth like the mandibular molars are more prone to vertical root fracture (24) and severe tooth structure loss has been proposed as an important cause for tooth fracture (2),(4),(6).The conservation of tooth structure through newer access designs and minimal canal preparation with lesser tapers and maintaining smaller apical diameters had been the focus of the present study that aimed to incorporate the minimal invasive approach in endodontics (3),(10).
Based on the results of the present study the null hypothesis is partially rejected as the intact control group (Group C) has shown statistically significant difference when compared to Traditional (0.04 taper), Traditional (0.06 taper), Truss (0.04 taper) and Truss (0.06 taper). However, although the Truss group (Group TREC) and its subgroups A and B were found to have mean values of fracture resistance slightly above the Traditional group (Group TAC) and its subgroups A and B, a statistically significant difference was not obtained, this could be attributed to smaller samples in each of the groups and their subgroups.
Limitation(s)
The limitations of the present in-vitro study are that exact oral conditions could not be simulated. Also, smaller sample size could have resulted in not producing a statistically significant difference between the test groups. Further studies with larger sample sizes and its application into clinical settings are necessary and recommended.
Within the limitations of the present in-vitro study and based on the results, following conclusions can be drawn. There was a statistically significant difference between the intact teeth group and the two test groups and their subgroups. There was no statistically significant difference between the test groups and their subgroups. However, the conservative Truss access design in combination with a reduced root canal taper preparation has shown to have produced better fracture resistance values in comparison to other groups and their subgroups indicating a need for more studies to be carried out with larger sample sizes.
DOI: 10.7860/JCDR/2022/52143.16267
Date of Submission: Aug 27, 2021
Date of Peer Review: Oct 29, 2021
Date of Acceptance: Dec 13, 2021
Date of Publishing: Apr 01, 2022
AUTHOR DECLARATION:
• Financial or Other Competing Interests: None
• Was Ethics Committee Approval obtained for this study? Yes
• Was informed consent obtained from the subjects involved in the study? NA
• For any images presented appropriate consent has been obtained from the subjects. NA
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