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The effect of different endodontic access cavity designs in combination with WaveOne Gold and TruNatomy on the fracture resistance of mandibular first molars: A nonlinear finite element analysis
1 Department of Odontology, School of Dentistry, Faculty of Health Sciences, University of Pretoria
Martin Vorster
Correspondence
Corresponding author: Prof Martin Vorster, University of Pretoria Oral Health Centre, 31 Bophelo Road, Prinshof Campus, Riviera, Pretoria, 0002, South Africa. , Telephone: 0834156116
1 Department of Odontology, School of Dentistry, Faculty of Health Sciences, University of Pretoria 2 Department of Civil Engineering, University of Pretoria, South Africa
When evaluating longevity and retention of endodontically treated teeth, fracture and subsequent loss after endodontic treatment remains a major concern
. Access cavity preparation and canal instrumentation are therefore aimed at preserving tooth structure as much as possible without compromising visibility and access.
Some studies highlight the importance of preserving dental hard tissue
Comparative evaluation of the impact of minimally invasive preparation vs. conventional straight‐line preparation on tooth biomechanics: a finite element analysis.
The effect of access cavity designs and sizes of root canal preparations on the biomechanical behavior of an endodontically treated mandibular first molar: a finite element analysis.
The influence of endodontic access cavity design on the efficacy of canal instrumentation, microbial reduction, root canal filling and fracture resistance in mandibular molars.
A laboratory study of the impact of ultraconservative access cavities and minimal root canal tapers on the ability to shape canals in extracted mandibular molars and their fracture resistance.
. The reduction of fracture resistance has also been reported to be associated more with the loss of a marginal ridge or cusp rather than the design of the endodontic access cavity
. Research also seems to suggest no significant difference between conservative and ultra-conservative access cavity designs in their effect on fracture resistance
A laboratory study of the impact of ultraconservative access cavities and minimal root canal tapers on the ability to shape canals in extracted mandibular molars and their fracture resistance.
Influence of minimally invasive endodontic access cavities on root canal shaping and filling ability, pulp chamber cleaning and fracture resistance of extracted human mandibular incisors.
The effect of access cavity designs and sizes of root canal preparations on the biomechanical behavior of an endodontically treated mandibular first molar: a finite element analysis.
. There is sufficient research validating the use of conservative access cavities in an attempt to preserve the structural integrity and longevity of endodontically treated teeth, but little evidence exists on the role pericervical dentin and its preservation play in the fracture resistance of endodontically treated teeth
The effect of access cavity designs and sizes of root canal preparations on the biomechanical behavior of an endodontically treated mandibular first molar: a finite element analysis.
Traditional access cavities (TAC) are prepared by deroofing the entire pulp chamber to obtain straight-line access to the coronal and middle third of the canal. Canal orifices should be visible without the need to change angulation
. Conservative access cavity preparation (CAC) aims at dentin preservation and partial deroofing of the pulp roof to locate canal orifices without necessarily achieving straight line access. Canal orifices are located by accessing the pulpal chamber from the central fossa in such a manner that dentin is preserved as far as possible. Access cavity walls can be either slightly convergent or divergent
Advances in endodontics like magnification and increased flexibility of endodontic instrumentation systems have diminished the need for TAC preparation in certain cases. Minimally invasive preparation and dentin preservation should be considered, taking the possible increased risk of instrument separation into account
. However these CAC preparations might also affect the cleaning, shaping and obturation of canals and increase the risk of iatrogenic complications during endodontic procedures
The Effect of Different Molar Access Cavity Designs on Root Canal Shaping Times Using Rotation and Reciprocation Instruments in Mandibular First Molars.
showed greater preparation time when using WaveOne Gold (Dentsply Sirona, Ballaigues, Switzerland) and TruNatomy (Dentsply Sirona) in combination with CAC than when using TAC designs.
The WaveOne Gold reciprocating preparation system with its unique parallelogram-shaped cross section is a popular file cutting in a counterclockwise motion. WaveOne Gold files are designed with a progressively decreasing taper to preserve coronal dentin
. The TruNatomy instrumentation system, which is intended to aid in minimally invasive endodontic preparation, is used in rotation motion. The instruments are manufactured using a smaller initial wire blank (0.8mm diameter) than the 1.1mm diameter of similar instruments. The post-grinding thermal treatments further results in a product with increased flexibility
FEA is an accurate, non-invasive method used to predict the fracture resistance and the effect of different access cavity designs and instrumentation systems on the biomechanical properties of different biological material
. The objective of this study was to investigate the effect of different access cavity designs and instrumentation systems on the fracture resistance of endodontically treated first mandibular molars. TruNatomy is marketed as a minimally invasive preparation system, preserving pericervical dentin and ultimately aiming to increase the fracture resistance of endodontically treated teeth. A study by Vorster et al
reported less pericervical dentin (volume) loss during preparation with TruNatomy compared to that of WaveOne Gold. This can be explained by the difference in taper design between these two instruments. WaveOne Gold Primary (25/07) will result in a larger final preparation size compared to TruNatomy Prime (26/04). In this current study the authors investigated if these findings translated to an increase in fracture resistance as conflicting results on the preservation of specifically pericervical dentin in mandibular molars and its role in fracture resistance exist in literature. The null hypothesis was that fracture resistance is not influenced by the endodontic access design or preparation system.
METHOD AND MATERIALS
Ethics approval (reference 484/2020) was obtained from the Research Ethics Committee, Faulty of Health Science, University of Pretoria before the study commenced. Four extracted, human, permanent mandibular first molars meeting strict inclusion requirements were selected to ensure sample standardization. Teeth had to be intact, caries-free, unrestored with minimal wear facets, without cracks or resorption, previously untreated, with only three canals in the root canal system (mesiobuccal, mesiolingual and one distal canal) clearly visible on pre-preparation radiographs. Reason for extraction was mainly due to periodontal disease and orthodontic reasons. Only first mandibular molars with similar dentin volumes (between 620 and 640 mm3) and mesiobuccal root canals with moderate canal curvatures between 25° and 35° and radii < 10mm were selected to standardise samples
. To simulate clinical scenarios and for standardization, specimens were mounted in an FKG vice (FKG Dentaire, La Chaux-de-Fonds, Switzerland). Access cavity preparation was performed with an Endo-Access bur (Dentsply Sirona) and refined with an EndoZ bur (Dentsply Sirona) in the TAC design samples, using a Dental Operating Microscope (Zumax Medical Co. Ltd). TAC (Figure 1A) and CAC (Figure 1B) design principles as described in the introduction were used to prepare the endodontic access cavities. Working length was determined by subtracting 0.5mm from the length of the canal measured to the major apical terminus under 10x magnification after exploring and negotiating canals to patency using a size 8 K-Flex file (Kerr, Romulus, MI).
Figure 1Traditional access cavity (A) and Conservative access cavity (B) preparation from occlusal view.
WaveOne Gold Primary samples (in combination with both TAC and CAC)
After access cavity preparation, a pre-curved stainless steel size 10 K-file was negotiated to working length after which the WaveOne Gold Glider (Dentsply Sirona) was used to enlarge each canal in this sample. Final preparation was done with the Primary WaveOne Gold (Dentsply Sirona) instrument.
TruNatomy Prime sample (in combination with both TAC and CAC)
After access cavity preparation, a pre-curved stainless steel size 10 K-file was negotiated to working length after which the TruNatomy Orifice Modifier (Dentsply Sirona) and Glider (Dentsply Sirona) was used to enlarge each canal in this sample. Final preparation was done with the TruNatomy Prime (Dentsply Sirona) instrument.
Canal preparation
Glide path and final canal preparation of all three canals of the root canal system were performed by a single experienced operator in strict accordance with the manufacturer’s recommendations for each system and only used once. The WaveOne Gold Glider and WaveOne Gold Primary instrument were used in reciprocating motion (150° counterclockwise and 30° clockwise), completing 360° in three cycles. The TruNatomy Orifice Modifier, Glider and TruNatomy Prime were used in rotation motion at 500rpm and 1.5Ncm torque. Files were allowed to progress apically upon activation, using three easy amplitudes in a pass until working length was reached. Instruments were removed and cleaned for another three-amplitude pass if deemed necessary until working length was reached. Preparation was confirmed by satisfactory fit of corresponding master gutta percha cones.
Geometry acquisitions and finite element analysis
The four teeth were scanned using the XTH 225 ST (Nikon Metrology, Leuven, Belgium) micro-focus X-ray CT system with a spatial resolution of 1-6μm at the South Africa Nuclear Energy Corporation (SOC) Limited (NECSA).
Micro-CT image reconstruction was performed using VG Studio-Max visualisation software (Volume Graphics GmbH, Heidelberg, Germany). These 3D models were exported as a .dwg in the visualization software and imported into FEMAP (Siemens PLM software, Plano, USA) where all surface elements generated through the scan were deleted.
Nodes were arranged in an .xyz file format and imported into MeshLab (Visual Computing Laboratory, Pisa, Italy) to generate and visualize the mesh (MeshLab 3D), where the nodes were reduced manually by deleting them according to the domain of the tooth needed (enamel, dentin, gutta percha and composite resin). The normals were computed for the point set for each domain after which discretization was performed using the screened Poisson method
. Close vertices were imported into Recap Photo (Autodesk, CA USA) to convert the .stl (Mesh) to a .obj (object) file. The .obj file was then imported into MicroStation Connect Edition (Terrasolid Inc., Espoo, Finland) to perform the necessary modifications to be able to generate a solid and export it in a .stp format.
A mesh was created, converted and imported into FEMAP to obtain the representative models that were used in this research (Figure 2A). The supporting alveolar bone (15mm cube) and uniform periodontal ligament space of 0.2mm around the roots, 1.5mm below the cemento-enamel junction (CEJ), were also discretized within FEMAP, which formed the support domain of the tooth as it was not included in the CT scan. The final mesh of each tooth was discretized with tetrahedral finite elements by implementing the automatic mesh tool in FEMAP software. Modifications were made to simulate the gutta percha obturation and a final restoration of access cavities with a composite resin material, a process that led to 8 numerical models. The anatomy of final restorations was designed to replicate possible realistic clinical scenarios as nearly as possible. The number of finite elements recorded in each numerical model ranged between 266,654 and 377,936 while the corresponding numbers of nodes ranged between 48,973 and 67,293.
Figure 2Representative sample (A) 3D constructed model and (B) model with red areas indicating the distribution of vertically applied forces and arrows indicating horizontal forces. Occlusal view (xy).
Based on the numerical analysis performed, FEA was based on the assumptions that each material was homogenous, isotropic with nonlinear behaviour, while between all components a perfect bond model was adopted. Material properties are summarised in Table 1
Vertical and horizontal loads were applied on the tooth where contact would occur during loading conditions, based on their unique geometry and identifying wear facets on extracted teeth (Figure 2B). This was done to replicate oral conditions that the tooth was exposed to in vivo as closely as possible. A vertical load of 0.15kN was applied at five points in close proximity to simulate one contact point. A total of five contact points in the vertical direction were used for the analysis, where a horizontal load of 0.3kN was applied at two points in close proximity to simulate one contact point that models the load developed at a specific concentrated area of the tooth when in mastication function. The two horizontal loads were applied on opposite sides of the tooth (buccal and lingual), simulating in this way the development of tension along the structure of the tooth. These loads were combined with the five vertical loads applied on the tooth’s surface to model a realistic response and load generation at the surface of each tooth. Using the full Newton-Raphson solution nonlinear algorithm, the horizontal and vertical loads were applied in relatively small load increments until structural failure of the tooth occurred. These failure loads were tabulated (see Table 2) and the stress at these ultimate load increments was recorded according to the nonlinear analyses and the respective failure load (Figure 3).
It is important to note that the solution algorithm incorporated in NX Nastran (Siemens) foresaw the use of multiple iterations and the adjustment of the load increment according to the material nonlinearities that occurred during the solution. Furthermore, it was assumed that when a material point reached its ultimate strength, the stiffness at that point was set to zero. It was deemed realistic to assume that the behaviour of the materials used foresaw that their behaviour was elastic until the ultimate stress level was reached. The tooth sample prior to access preparation and instrumentation in each sample was used as the basis of comparison in terms of quantifying the efficiency of each intervention. The failure criterion that was selected was the von Mises, whereas, when yielding was reached the strength was set to zero. This interprets that the material had no ductility. The rationale behind this approach, from a numerical analysis point of view, was to avoid the use of a numerically unstable and computationally demanding method such as X-FEM. Therefore, the push-over nonlinear analysis used in this study foresaw the use of a less numerically unstable material model that did not introduce discontinuities within the mesh, where the material properties were assigned in such a manner that the post-yielding mechanical behaviour of a material point did not retain any remaining strength. Forces were applied on the instrumented teeth and the reduction (percentage) in terms of ultimate failure load was reported and compared across the different samples.
RESULTS
Table 2 indicates the predicted ultimate failure loads (kg) and corresponding percentage load decrease of each representative sample. A significantly reduced ultimate capacity for failure after endodontic treatment was noted in the TAC groups compared to the CAC derived from the numerical investigation. On the other hand, no significant difference in the reduction of failure load was observed between the two instrumentation systems within the traditional and contracted endodontic access cavity design groups. When evaluating the numerically obtained von Mises stresses and strains (Figure 3), it was found that nonlinearities, thus cracks, were concentrated largely at the surface of the tooth where the enamel domain is found, due to the direct application of the loads and the tensile deformation of the teeth that is attributed to the horizontal loads. Von Mises stress distribution was largely concentrated around the cervical region and did not propagate into the root.
DISCUSSION
The preservation of dentin specifically pericervical dentin remains a controversial topic when investigating the effect on fracture resistance, with conflicting results. In this study FEA was used to simulate and evaluate fracture resistance in CAC and TAC designs. WaveOne Gold and TruNatomy preparation instrumentation systems were also compared to evaluate their effect, in combination with different endodontic access cavity designs, on the possible fracture resistance of endodontically treated mandibular first molars. TruNatomy is marketed as a minimally invasive endodontic preparation system, while WaveOne Gold is a popular single-file reciprocating system. Four representative samples were evaluated to see the effect of dentin preservation on fracture resistance, with the unprepared tooth in each sample acting as a control. Groups prepared with minimally invasive CAC designs required almost identical loads after endodontic treatment to induce failure compared to the corresponding control tooth. In the group where traditional methods were used to prepare the access cavity, a much-reduced ultimate load was required for the sample to fail compared to the control tooth sample. It is important to note that exact figures cannot be used as an indication of a reduction in fracture resistance, as all four samples were anatomically different (although similar in size and root curvature); therefore the percentage decrease in ultimate tooth capacity is a more accurate representation of possible clinical scenarios and the overall effect that the preparation has on each tooth. In the parametric investigation it was also found that there was no significant difference in the reduction of the failure load between the two instrumentation systems. This finding could suggest that in both traditional and conservative endodontic access cavity designs the root canal instrumentation system and therefore canal taper did not influence the overall fracture resistance in this study. A study by Elkholy et al
also concluded that access and the removal of dental hard tissue was more instrumental in fracture resistance than canal taper. A study by Vorster et al
evaluating the remaining dentin thickness and volume loss of pericervical dentin after traditional and conservative access cavity preparation in combination with WaveOne Gold and TruNatomy instrumentation reported no significant difference in dentin loss within the same access cavity preparation groups. Similarly, in this study no significant difference in fracture resistance and stress distribution was reported between samples with similar access cavity designs.
The nonlinear FEA investigation revealed that the von Mises stresses were concentrated around the cervical area of the samples both pre- and post-preparation. Strain was observed to be concentrated largely in the distal roots. This is similar to a study by Zhang et al
where stress concentration was also observed around the larger palatal root in maxillary first molars. In addition, stress contours and crack formation within enamel presented in Figure 3 were similar to those described by Zhang et al
Cracks propagated in a mesiodistal plane following the central fossa.
By investigating the mechanical response of the composite resin, it was found that in all four prepared models the material did not develop any damage due to the relatively low stress level development. This mechanical response phenomenon is attributed to the fact that the composite resin is more than 10 times more flexible than the enamel and develops similar strains to those in the enamel during the nonlinear analysis. This leads to the lower stress development within the composite resin material, which can be easily determined using Hooke’s law. Furthermore, it was found that the enamel and the stiffer exterior material of the tooth act as a shell for the encapsulated dentin and gutta percha domains, leading to a failure of the enamel and then the failure of the other materials. This mechanical response was expected; it is the classical mechanical response of a structure that consists of materials of different hardness and strength, where the strongest and stiffest material is on the exterior and the more flexible and weaker materials are found towards the core. For this reason and because the strain development is continuous for all materials of the tooth, the material with the largest Young modulus (in this case the enamel) develops the largest stresses for the same strain development. Because the composite resin is designed to have a higher ultimate stress than the enamel, it is evident that the enamel will always fail first at the same strain levels; whereas, since debonding between the enamel and composite resin was not considered in these numerical models, the numerical failure is expected to occur at the surface of the enamel.
The support of the teeth by the human bone was found to be sufficient since it never failed for the applied loads, forming a sufficient foundation to the under-study teeth and their roots. As discussed above, the root canal system and gutta percha obturation material do not control the failure type and this is attributed to the fact that the loads are applied at the surface of each tooth. This is composed of enamel, the stiffest material component of the structure. As the stiffer enamel receives the load from human mastication directly, it fails first, leading to the termination of the nonlinear analysis due to numerical instabilities. Numerical instabilities are caused by an ill-conditioned stiffness matrix: many tetrahedral elements enter the nonlinear zone leading to zero or negative diagonal terms in the stiffness matrix, creating singularities. If a stiffness matrix is singular it cannot be inverse and therefore the solution cannot continue. The physical interpretation of this numerical phenomenon is, therefore, failure of the tooth structure. The numerical investigation revealed that the decrease in tooth ultimate load within the CAC groups was significantly lower than in the two TAC groups. This finding is consistent with other recent studies on fracture resistance in molar teeth
Five static vertical and horizontal loads were applied in this study to model loads developed during a realistic mastication event. The failure of endodontically treated teeth is ostensibly caused by fatigue rather than an acute overload during mastication
. This is a limitation of this study, as fatigue-induced failure was not taken into consideration. However, static loading is considered an important step in obtaining the basic biomechanical behaviour pattern in the response to stress loading in endodontically treated teeth during mastication. The possibility of micro-fractures during access cavity preparation and final canal instrumentation are also factors not considered in this analysis; this could be the subject of future research. In all models the highest stress concentration was seen in the cervical area.
The preservation of dentin by means of conservative treatment of endodontic access cavities might lead to increased fracture resistance. In future research, other factors such as the risk of procedural errors, transportation and centering ability should be considered when deciding on an appropriate endodontic access cavity.
It is now evident that the null hypothesis is rejected, given that within the limitations of this study it was found that the size of the access cavity and the amount of dentin and enamel sacrificed during endodontic access cavity preparation can contribute significantly to fracture resistance in mandibular first molars. The researchers also concluded that the type of instrumentation system did not play a significant role in increasing fracture resistance when comparing WaveOne Gold and TruNatomy. They would like to emphasize that only fracture resistance in mandibular molars prepared with traditional and conservative access cavities was evaluated and that many other factors should also be considered when clinicians choose the type of access cavity.
ACKNOWLEDGEMENTS
The authors thank Dentsply Sirona for donating the WaveOne Gold Glider, WaveOne Gold Primary, TruNatomy Glider, Orifice Modifier and TruNatomy Prime files for this study.
REFERENCES
Reeh ES
Messer HH
Douglas WH
Reduction in tooth stiffness as a result of endodontic and restorative procedures.
Comparative evaluation of the impact of minimally invasive preparation vs. conventional straight‐line preparation on tooth biomechanics: a finite element analysis.
The effect of access cavity designs and sizes of root canal preparations on the biomechanical behavior of an endodontically treated mandibular first molar: a finite element analysis.
The influence of endodontic access cavity design on the efficacy of canal instrumentation, microbial reduction, root canal filling and fracture resistance in mandibular molars.
A laboratory study of the impact of ultraconservative access cavities and minimal root canal tapers on the ability to shape canals in extracted mandibular molars and their fracture resistance.
Influence of minimally invasive endodontic access cavities on root canal shaping and filling ability, pulp chamber cleaning and fracture resistance of extracted human mandibular incisors.
The Effect of Different Molar Access Cavity Designs on Root Canal Shaping Times Using Rotation and Reciprocation Instruments in Mandibular First Molars.
Conservative endodontic access cavity preparation reduces the overall stress concentration and increases the fracture resistance. The preservation of dental hard tissue should be considered in increasing resistance to fracture during endodontic treatment.
The authors deny any conflicts of interest related to the study or sponsors
We affirm that I/We have no financial affiliation (e.g., employment, direct payment, stock holdings, retainers, consultantships, patent licensing arrangements or honoraria), or involvement with any commercial organization with direct financial interest in the subject or materials discussed in this manuscript, nor have any such arrangements existed in the past three years. Any other potential conflict of interest is disclosed.
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