Factorial analysis of variables influencing mechanical characteristics of a post used to restore a root filled premolar using the finite element stress analysis combined with the Taguchi method

Minimum residual root dentin thickness of mandibular premolars restored with a post: A finite element analysis study

STATEMENT OF PROBLEM

Thin root dentin after post space preparation will increase the risk of root fracture. However, the minimum residual root dentin thickness to be preserved after post space preparation is unclear.

PURPOSE

The purpose of this finite element analysis (FEA) study was to measure the residual root dentin thicknesses and analyze the stress distributions of post-restored mandibular premolars.

MATERIAL AND METHODS

The cone beam computed tomography (CBCT) data of 90 first and second mandibular premolars from Chinese participants (44 men, 46 women; aged between 20 and 79 years) were analyzed. Cross-sections 5 to 9 mm from the radiologic apex were used to measure the buccolingual and mesiodistal root diameters. The probability that the residual thickness of the buccolingual and mesiodistal root walls would be no less than 0.5, 0.6, 0.7, 0.8, 0.9, and 1 mm after post space preparation with a #2 Peeso reamer was calculated. Six 3-dimensional finite element models of cast post-restored mandibular premolars with 0.5, 0.6, 0.7, 0.8, 0.9, and 1 mm mesial and distal root dentin thicknesses were established. A static force of 100 N was applied to the buccal cusp tip at 45, 60, 75, and 90 degrees to the long axis of the tooth, and the maximum tensile stress and von Mises stress were analyzed. The Bonferroni post hoc test (αcorrected=.003) was used for multiple comparisons.

RESULTS

The buccolingual root diameter of mandibular premolars was wider than the mesiodistal root diameter. The probabilities that the mesiodistal residual root dentin thickness of mandibular first and second premolars at 5 mm from the apex would be no less than 1 mm after post space preparation with a #2 Peeso reamer were only 10% and 28%, respectively. The maximum tensile stress was at a minimum when the mesial and distal residual root dentin thickness was 0.6 mm in the external cervical dentin adjacent to the crestal bone and 1 mm in the apical dentin corresponding to the apex of the post. The maximum tensile stress of the mandibular premolar model with 0.6-mm mesial and distal residual root dentin thicknesses was lower than that with 0.9 mm and 1.0 mm thicknesses (Pcorrected<.001).

CONCLUSIONS

Short posts or smaller instruments for post space preparation are recommended to obtain a 1-mm residual root dentin thickness in the mesiodistal direction of mandibular premolars. If a 1-mm thickness cannot be preserved, a minimum residual root dentin thickness of 0.6 to 0.9 mm in the mesiodistal direction should be retained.

Finite element analysis of endodontically treated premolars without ferrule restored with one-piece glass fiber post and core in combination with different inner shoulder retention form systems

OBJECTIVE

The current study was performed to explore the impact of post materials as well as the inner shoulder retention form (ISRF) design on the biomechanical behavior of endodontically treated premolars without ferrule restoration using a method of mathematical three-dimensional (3D) finite element analysis (FEA).

METHODS

Based on the tooth anatomy and our previous research, eight mandibular second premolar FEA models representing different restorative situations were built: teeth with (a) 2.0 mm height ferrule (DF), (b) no ferrule (NF), (c) 0.5 mm width and 0.5 mm depth ISRF (ISRFW0.5D0.5), (d) 0.5 mm width and 1.0 mm depth ISRF (ISRFW0.5D1.0), (e) 0.5 mm width and 1.5 mm depth ISRF (ISRFW0.5D1.5), (f) 1.0 mm width and 0.5 mm depth ISRF (ISRFW1.0D0.5), (g) 1.0 mm width and 1.0 mm depth ISRF (ISRFW1.0D1.0), (h) 1.0 mm width and 1.5 mm depth ISRF (ISRFW1.0D1.5). All groups were restored with prefabricated glass fiber post and resin composite core (PGF), one-piece glass fiber post-and-core (OGF) and cast Co-Cr alloy (Co-Cr) respectively, and the zirconia crown was restored. Load (180N) was subjected to the buccal cusp at 45° to the tooth's long axis. Stress pattern, maximum principal stress values (MPS), and maximum displacement values on root, post and core, cement layer were calculated for each model.

RESULTS

Stress distributions were similar while the values were different among groups. Regardless of restorative approaches, roots restored with PGF showed the highest MPS values, followed by OGF and Co-Cr groups. Regardless of post materials, NF groups resulted in the highest MPS values and maximum displacement values, while ISRF and DF groups exhibited similar results. Compared with PGF groups in association with ISRF, except for OGF with ISRFW0.5D0.5, the remaining OGF groups with ISRF and all Co-Cr groups in association with ISRF presented lower values than that of DF groups. And among different ISRF systems, roots restored with ISRFW1.0D1.0 presented the lowest stress (PGF: 32.96 MPa, OGF: 31.69 MPa, Co-Cr: 29.66 MPa).

CONCLUSIONS

For endodontically treated premolars without ferrule, restored with OGF in combination with ISRF preparation could effectively enhanced its load-bearing capacity. Furthermore, the ISRF with a depth and width of 1.0 mm is recommended.

Structural Integrity of Anterior Ceramic Resin-Bonded Fixed Partial Denture: A Finite Element Analysis Study

This study was conducted as a means to evaluate the stress distribution patterns of anterior ceramic resin-bonded fixed partial dentures derived from different materials and numerous connector designs that had various loading conditions imposed onto them through the utilization of the finite element method. A finite element model was established on the basis of the cone beam computed tomography image of a cantilevered resin-bonded fixed partial denture with a central incisor as an abutment and a lateral incisor as a pontic. Sixteen finite element models representing different conditions were simulated with lithium disilicate and zirconia. Connector height, width, and shape were set as the geometric parameters. Static loads of 100 N, 150 N, and 200 N were applied at 45 degrees to the pontic. The maximum equivalent stress values obtained for all finite element models were compared with the ultimate strengths of their materials. Higher load exhibited greater maximum equivalent stress in both materials, regardless of the connector width and shape. Loadings of 200 N and 150 N that were correspondingly simulated on lithium disilicate prostheses of all shapes and dimensions resulted in connector fractures. On the contrary, loadings of 200 N, 150 N, and 100 N with rectangular-shaped connectors correspondingly simulated on zirconia were able to withstand the loads. However, two of the trapezoidal-shaped zirconia connectors were unable to withstand the loads and resulted in fractures. It can be deduced that material type, shape, and connector dimensions concurrently influenced the integrity of the bridge.

Probability of survival and failure mode of endodontically treated incisors without ferrule restored with CAD/CAM fiber-reinforced composite (FRC) post-cores

PURPOSE

This study evaluated the probability of survival and failure mode of endodontically treated incisors without ferrule restored with CAD/CAM FRC post-cores.

METHODS

Root canals of bovine incisors were treated, leaving post preparations of ∼10 mm. Teeth were allocated into three groups: (i) cast metal post-core, (ii) FRC prefabricated post with a direct resin core build-up, and (iii) CAD/CAM FRC post-core. Posts and zirconia crowns were cemented using resin cement. Specimens were subjected to step-stress accelerated-life fatigue testing in water. Use level probability Weibull curves, probability of survival for a mission of 100,000 cycles at 25, 50, and 100 N, Weibull modulus, and characteristic strength were calculated and plotted. Failure mode was examined under a stereomicroscope.

RESULTS

Restored incisors demonstrated high probability of survival (93-100%) for missions estimated at 25 and 50 N, irrespective of post-core foundation. At 100 N, incisors restored with metal posts presented significantly higher probability of survival (99%) relative to CAD/CAM posts (79%), whereas FRC groups demonstrated no significant difference. Weibull analysis indicated no significant difference on the Weibull modulus (m = 3.38-5.92). Incisors reconstructed with metal post-cores (431 N) presented significantly higher characteristic strength relative to prefabricated (200 N) and CAD/CAM (202 N) FRC post-cores. While post fracture was the chief failure mode for prefabricated and CAD/CAM FRC post-cores, post and/or root fracture were the main event for metal post-cores.

CONCLUSION

Endodontically treated incisors without ferrule restored with CAD/CAM FRC post-cores presented promising probability of survival for loads compatible with anterior masticatory forces and favorable failure modes.

Post Placement and Restoration of Endodontically Treated Canines: A Finite Element Analysis Study

The purpose of this study was to show the improved outcomes of restoring endodontically treated teeth with fiberglass posts compared to restorations using metal posts. In our study, we used the Finite Element Method (FEM), which is based on the principle that a physical model that supports a given load distributes the stress throughout its volume. We sought to assess what stress results in a tooth when it is restored using a fiberglass post compared to restoration using a metal post. The finite element analysis showed that a system consisting of a tooth with a fiberglass post is more stable in terms of the maximum stress than a system consisting of a tooth with a metal post. The maximum displacements and deformations were obtained in the case of a canine restored with a fiberglass post, which showed that this system had a high elasticity, therefore, higher strength than a canine restored with a metal post, which had high rigidity.

A critical analysis of research methods and experimental models to study the load capacity and clinical behavior of the root filled teeth

The prognosis of root‐filled teeth depends not only on a successful root canal treatment but also on the restorative prognosis. This critical review discusses the advantages and limitations of various methodologies used to assess the load capacity or clinical survivability of root‐filled teeth and restorations. These methods include static loading, cyclic loading, finite element analysis and randomized clinical trials. In vitro research is valuable for preclinical screening of new dental materials or restorative modalities. It also can assist investigators or industry to decide whether further clinical trials are justified. It is important that these models present high precision and accuracy, be reproducible, and present adequate outcomes. Although in vitro models can reduce confounding by controlling important variables, the lack of clinical validation (accuracy) is a downside that has not been properly addressed. Most importantly, many in vitro studies did not explore the mechanisms of failure and their results are limited to rank different materials or treatment modalities according to the maximum load capacity. An extensive number of randomized clinical trials have also been published in the last years. These trials have provided valuable insight on the survivability of the root‐filled tooth answering numerous clinical questions. However, trials can also be affected by the selected outcome and by intrinsic and extrinsic biases. For example, selection bias, loss to follow‐up and confounding. In the clinical scenario, hypothesis‐based studies are preferred over observational and retrospective studies. It is recommended that hypothesis‐based studies minimize error and bias during the design phase.

Fatigue failure of anterior teeth without ferrule restored with individualized fiber-reinforced post-core foundations

OBJECTIVES

The aim was to explore the survival of extensively damaged anterior teeth without ferrule restored with different fiber-reinforced composite (FRC) post-core foundations and composite crowns.

MATERIALS AND METHODS

Sixty extracted upper central incisors were decoronated and randomly divided into four groups (n = 15). After endodontic treatment, the specimens were restored with different individualized fiber-reinforced post-core foundations as follows: control group (CTRL): multiple unidirectional FRC-post + dual-cure composite-core, PFC: multiple unidirectional FRC-post + packable short fiber-reinforced composite (SFRC), BPFC: Bioblock technique with only packable SFRC, BFFC: Bioblock technique with only flowable SFRC. After core build-up, the teeth were finalized with adhesively luted CAD/CAM composite crowns. Cyclic isometric loading (5 Hz) was applied at 100 N for 5000 cycles, and then 200 N and 300 N for 15,000 cycles each in a fluid chamber. The specimens were loaded until fracture occurred or when a total of 35,000 cycles were reached. Kaplan-Meyer survival analysis was conducted, followed by pairwise log-rank post hoc comparisons (Mantel-Cox).

RESULTS

The survival rates of the control (8279 cycles) and PFC (6161 cycles) were significantly higher compared to BPFC (3223 cycles) and BFFC (2271 cycles) (p < 0.05). Regarding fracture pattern, nearly all specimens fractured in a restorable manner.

CONCLUSIONS

For restoring extensively damaged anterior teeth, multiple unidirectional FRC posts are recommended.

CLINICAL RELEVANCE

Although different FRC post/core systems are available for the restoration of damaged root canal treated anterior teeth, multiple unidirectional FRC posts tend to be a good option when the ferrule is missing.

Effect of different design of abutment and implant on stress distribution in 2 implants and peripheral bone: A finite element analysis study

STATEMENT OF PROBLEM

How adjacent dental implants with different sizes, designs, and abutment connection shapes affect stress on the prosthetic structure is unclear.

PURPOSE

The purpose of this finite element analysis (FEA) study was to analyze stress distribution around bone and around 2 implants with different sizes, diameters, shapes, and loading directions placed next to each other in splinted and unsplinted prostheses.

MATERIAL AND METHODS

On 3D FEA models representing the posterior right lateral segment of the mandible, 1 implant (Ø3.5×12 mm) and 1 implant (Ø5.5×8 mm) were placed adjacent. Three different contemporary implant models were created with different teeth, pitch, spiral numbers, and self-taping features, and different abutments for them were modeled in 3D. The implant-abutment connection was internal hexagonal (MIH), stepped conical (MSC), and internal conical (MIC). Vertical and oblique loads of 365 N for molar teeth and of 200 N for premolar teeth were applied as boundary conditions to the cusp ridges and grooves in a nonlinear FEA.

RESULTS

The MIH implants resulted in improved stress conditions. According to the von Mises stresses occurring on the screw, abutment, and implant, especially under oblique loads, MIH was exposed to less stress than MSC, and MSC was exposed to less stress than MIC.

CONCLUSIONS

When a standard implant and a short implant were placed adjacent and splinted by crowns, the implants, abutments, and screws had unfavorable stress levels; therefore, adjacent splinted implants should be of similar size. The form of the implant-abutment junction is also an important factor affecting stress.

Validated Finite Element Models of Premolars: A Scoping Review

Finite element (FE) models are widely used to investigate the biomechanics of reconstructed premolars. However, parameter identification is a complex step because experimental validation cannot always be conducted. The aim of this study was to collect the experimentally validated FE models of premolars, extract their parameters, and discuss trends. A systematic review was performed following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Records were identified in three electronic databases (MEDLINE [PubMed], Scopus, The Cochrane Library) by two independent reviewers. Twenty-seven parameters dealing with failure criteria, model construction, material laws, boundary conditions, and model validation were extracted from the included articles. From 1306 records, 214 were selected for eligibility and entirely read. Among them, 19 studies were included. A heterogeneity was observed for several parameters associated with failure criteria and model construction. Elasticity, linearity, and isotropy were more often chosen for dental and periodontal tissues with a Young’s modulus mostly set at 18–18.6 GPa for dentine. Loading was mainly simulated by an axial force, and FE models were mostly validated by in vitro tests evaluating tooth strains, but different conditions about experiment type, sample size, and tooth status (intact or restored) were reported. In conclusion, material laws identified herein could be applied to future premolar FE models. However, further investigations such as sensitivity analysis are required for several parameters to clarify their indication.

Influence of restorative procedures on endodontically treated premolars: Finite element analysis of a CT-scan based three-dimensional model

An endodontically treated tooth with mesial-occlusal-distal (MOD) cavity is often restored with composite resin. Palatal and buccal cusp reduction (MODP, MODPB), and/or fiber-reinforced composite posts (P), are used in an attempt to improve the longevity of the restoration. The aim of this study was to determine the effects of these procedures on von Mises stress values and distribution in dental tissues and restorative materials using finite element analysis. Based on CT scans of an extracted second upper premolar, six 3D endodontically treated tooth models (MOD, MODP, MODPB, MOD+P, MODP+P, MODPB+P) were created. Each model was subjected to a summary force of 150 N on the occlusal surface simulating the normal biting pattern and maximal von Mises stresses were calculated. MODP seems to reduce von Mises stress values in dental tissues and P seems to transfer some of the stresses from dental tissues to the composite filling.

Multi-Fiber-Reinforced Composites for the Coronoradicular Reconstruction of Premolar Teeth: A Finite Element Analysis

A coronoradicular reconstruction (CRR) has conventionally used a metallic inlay core (MIC) or a single-fiber-reinforced composite (sFRC) but extensive dentin removal can lead to root fracture. We propose herein a multi-fiber-reinforced composite (mFRC) based on a bundle of thin flexible fibers that can be adapted to the root anatomy without removing additional dentin. The aim of this study was to compare the mechanical behavior of the root reconstructed with mFRC, MIC, or sFRC using a finite element analysis (FEA). Models with or without a ferrule effect were created using Autodesk© software and divided into four parts: root, post, bonding composite or cement, and zirconia crown. For both models, extreme stress values (ESV), stress distribution, and risk of fracture were calculated for an oblique force (45°) of 100 N applied to the top of the buccal cusp. Results indicated that mFRC and mFRCG present a lower risk of fracture of the root and of the CRR without ferrule and thus could be valuable alternatives for premolar CRR. Further studies are necessary to evaluate the clinical success of these CRR.

Impact of simulated reduced alveolar bone support, increased tooth mobility, and distal post-supported, root-treated abutment tooth on load capability of all-ceramic zirconia-supported cantilever FDP

OBJECTIVES

The aim of this in vitro study was an analysis of the impact of simulated reduced alveolar bone support and post-restored, endodontically treated distal abutment tooth on load capability of all-ceramic zirconia-based cantilever-fixed dental prosthesis (CFDP).

MATERIAL AND METHODS

The roots of human lower sound premolars (n = 80) were divided into five experimental groups to be restored with all-ceramic zirconia-supported three-unit CFDP regarding bone loss (BL) relative to the cement-enamel junction (CEJ): 2 mm below CEJ = 0% BL (control group), group 25% distal BL, group 50% distal BL, group 50% mesial and distal BL, and group 50% distal BL and adhesive post-supported restoration. Specimens were exposed to simulated clinical function by thermo-mechanical loading (6.000 cycles 5°-55°; 1.2 × 10⁶ cycles 0-50 N) and subsequent linear loading until failure.

RESULTS

Tooth mobility increased significantly for groups with simulated bone loss (p < 0.001). Four specimens failed during thermal cycling and mechanical loading (TCML). The maximum load capability ranged from 350 to 569 N, and did not differ significantly between experimental groups (p = 0.095). Groups with simulated bone loss revealed more tooth fractures at distal abutment teeth, whereas technical failures were more frequent in the control group (p = 0.024).

CONCLUSIONS

Differences of alveolar bone support and respectively increased tooth mobility between mesial and distal abutments did not influence load capability. A distal adhesively post-and-core-supported, root-treated abutment tooth did not increase risk of three-unit CFDP failure.

CLINICAL RELEVANCE

CFDPs are a treatment option used with caution when reduced alveolar bone support, increased tooth mobility, and distal post-supported, root-treated abutment teeth are involved.

Biomechanical behavior of endodontically treated premolars using different preparation designs and CAD/CAM materials

OBJECTIVES

To evaluate the effect of restoration design ('2.5-mm deep endocrown', '5-mm deep endocrown' or '5-mm deep post&crown') and CAD/CAM material type (composite or lithium disilicate glass-ceramic) on the load-to-failure of endodontically treated premolars in absence of any ferrule.

METHODS

The crowns of 48 single-rooted premolars were cut and the roots were endodontically treated. Teeth were randomly divided into six groups (n=8); teeth in each group were restored using one of the two tested materials with standardized CAD/CAM fabricated endocrowns (with either 2.5-mm or 5-mm deep intra-radicular extension) or conventional crowns (5-mm deep post&crown). After cementation using luting composite, the specimens were immersed in distilled water and subjected to 1,200,000 chewing cycles with a load of 50N applied parallel to the long axis of the tooth (0°). After cyclic loading, a compressive load was applied at 45° to the tooth's long axis using a universal testing machine until failure. Load-to-failure was recorded (N) and the specimens were examined under a stereomicroscope with 3.5x magnification to determine the mode of failure.

RESULTS

All specimens survived the 1,200,000 chewing cycles. A significant interaction between restoration design and CAD/CAM material was found using two-way ANOVA. In the '2.5-mm deep endocrown' groups, the composite achieved a significantly higher load-to-failure than the lithium disilicate glass-ceramic, while no differences between materials were found in the '5-mm deep endocrown' and '5-mm deep post&crown' groups. More unfavorable failures (root fractures) were observed for higher load-to-failure values.

CONCLUSIONS

Only following a '2.5-mm deep endocrown' design, composite appeared more favorable than lithium disilicate glass-ceramic as crown material; this may be explained by their difference in elastic modulus.

CLINICAL SIGNIFICANCE

Shallow endocrown preparations on premolars present less surface for adhesive luting and a difference in crown material becomes apparent in terms of load-to-failure. The use of a more flexible composite crown material appeared then a better option.