Fatigue resistance and 3D finite element analysis of machine-milled ceramic occlusal veneers with new preparation designs versus conventional design: an in vitro study

Ethical approval

This study was approved by the Research Ethics Committee of the Faculty of Dentistry, Cairo University. Approval number: 1587.

Fatigue resistance test

Sample size estimation. To compare the fatigue resistance of machine milled ceramic occlusal veneers with new preparation designs versus conventional design, one-way analysis of variance (ANOVA) was done. Based on a previous study by Stappert et al.⁸, the outcome variable is normally distributed and an expected effect size of approximately 0.44 is estimated, therefore a total sample size of 54 (18 in each group) would be sufficient to detect large effect size (f=0.44) with power 80% and 5% significance level. (G*Power program).

Sample fabrication

Teeth collection, periodontal ligament simulation and occlusal veneer preparation

A total of 54 human mandibular molars, obtained from the Outpatient Clinic of the Department of Oral Surgery, Faculty of Dentistry, Cairo University, which were free of any caries, defects and cracks were chosen. Teeth mean dimensions of the anatomical crown bucco-lingual width (distance between the maximum convexities on buccal and lingual surfaces) (11±1mm), mesio-distal widths of (10±2 mm) at the level of the cementoenamel junction (CEJ). An ultrasonic scaler was used to remove calculus deposits, debris and soft tissue remnants. The teeth were stored until use in distilled water.

Root surfaces were plunged into blue inlay melted wax (Bego crown and bridge wax) 2.0 mm below the CEJ, resulting in a 0.2- to 0.3-mm thick wax layer⁹. Teeth were then mounted in Epoxy resin (CMB, Egypt) using a plastic ring (2.5 cm in diameter and 2 cm in length) and a custom-made paralleling device was used to allow precise vertical centralization of the long axis of each tooth parallel to the long axis of the plastic ring.

The wax was separated from the root surface and the resin cylinder “alveolus” after resin polymerization. Silicone elastomeric material (Elite Light Body Zhermack GmbH Deutschland) was then injected in the resin cylinders, the tooth was repositioned in the epoxy cylinder and the surplus elastomeric material was removed with a scalpel blade.

Randomization and grouping. The mounted teeth were randomly distributed into three equal groups of 18 teeth each according to the occlusal veneer preparation design (Table 1). Randomisation was performed using the random.org random number generator.

Table 1. Sample grouping for different preparation designs.

Areas of reduction		Planar occlusal veneer preparation group I (conventional)	Modified Occlusal veneer preparation with circumferential finish line group II (design 1)	Modified occlusal veneer preparation with intracoronal cavity group III (design 2)
Occlusal anatomical preparation depth	At cusp tips	1.5 mm	1.5 mm	1.5 mm
	At central fossae	1 mm	1 mm	1 mm
Intra coronal cavity depth				2 mm
Axial walls preparation			1mm
Margin preparation			1 mm deep chamfer	_
Taper (intra coronal box occlusal divergence)				8°
Taper (axial walls)			8°

Tooth preparation. A special milling machine (AF30 Nouvag, Switzerland) was used for tooth preparation. The assembly incorporates a conventional-speed straight hand-piece perpendicular to the surveyor platform. A customized base was constructed to accurately fit the epoxy resin block, which was held stable in place on the magnetic table of the milling machine. Then the researcher operates the machine to prepare all the teeth in a standardized manner (Figure 1).

Figure 1. Preparation designs.

Group I (left), Group II (middle) and Group III (right).

Construction of occlusal veneers. CAD/CAM (CEREC AC, Sirona, Bensheim, Germany) was used for the fabrication of all samples in this research. Each prepared tooth was scanned using the CEREC Omnicam and designing was carried out using CEREC Premium 4.4 software. Cusp height thickness was adjusted at 1.5 mm in all groups while thickness of the restorations at the central groups was adjusted as follows: Group I and Group II at 1 mm and Group III at 2.5 mm. Milling of the Occlusal veneers was done from Zirconium Lithium Silicate (ZLS) blocks (VITA Suprinity, VITA Zahnfabrik H. Rauter GmbH & Co.KG) in 4-axis milling machine CEREC MC XL). This process was repeated 18 times to end up with 18 milled VITA Suprinity occlusal veneers for each of the three groups. Finally, occlusal veneers were fully crystallized and glazed in Programat P510 furnace (Ivoclar Vivadent AG. Principality of Liechtenstein) (Figure 2).

Figure 2. Sliced restorations thickness as viewed in the software (top) and milled restorations (bottom).

Group I, red; Group II (yellow), Group III (blue).

Cementation of Occlusal veneers. For surface treatment of occlusal veneers, etching of the intaglio surfaces of the veneers was conducted using Bisco Porcelain Etch (BISCO Dental Products, Illinois, U.S.A.) for 20 seconds, then rinsed with water and dried with air, then silanized using Bisco Silane (BISCO Dental Products, Illinois, U.S.A.) for 60 seconds then air dried for 5 seconds.

For surface treatment of the prepared natural tooth, the prepared surfaces of the tooth were acid-etched using 35% phosphoric acid (ScotchBond Universal Etchant, 3M ESPE AG. ESPE Platz Seefeld. Germany) then rinsed for 10 seconds. Two successive coats of Adper Single Bond 2 Adhesive (3M ESPE AG. ESPE Platz Seefeld. Germany) were applied with a micro-brush and gently air-dried for 5 seconds, then light polymerized for 10 seconds.

Rely X Ultimate (3M ESPE AG. Seefeld, Germany) Dual cure, adhesive resin cement was used to cement 54 Occlusal Veneers. The occlusal veneer was seated on the corresponding tooth and held in place with light pressure. A gel state was achieved by tack-curing excess with light curing (Elipar DeepCure-S LED Curing Light) (3M ESPE AG. Seefeld, Germany) for approximately 2 seconds and the excess cement was removed with an explorer, then a glycerin-based gel (K-Y Jelly; Johnson & Johnson, USA) was applied at the margins of the occlusal veneer to prevent the formation of the oxygen inhibiting layer. During cementation, a custom-made loading device was used to apply a steady load of 3 kg parallel to each tooth’s long axis¹⁰ for 5 minutes to allow cement to self-cure as recommended by the manufacturer^11,12. All surfaces (axial and occlusal) were light cured for 20 seconds. (Figure 3)

Figure 3. Cemented occlusal veneers.

Left, Group I; middle, Group II; right, Group III.

Fatigue resistance test

Step-stress accelerated life testing (SSALT)

Cyclic failure loads were established in an electric machine (Instron, Model 3345, Instron, USA) using a compressive mode of load applied occlusally, dynamically loaded under water in a chamber at 35-37°C to simulate the intraoral conditions at a frequency of 2 Hz, where the sample is subjected to a prescribed number of cycles at each of a sequence of increasing stress levels, until failure of the sample. Starting with a stress level below the anticipated material’s fatigue limit (30–60% of single load to failure)¹³ is chosen.

The sample is then tested at the mild profile stress level of (698 N) (30% of single load to failure mean value recorded for zirconia-reinforced lithium silicate)¹⁴ until either failure occurs or the run-out at a previously set number of cycles (5000 cycles) is achieved. If failure occurs, the stress level and the number of cycles is recorded in Newtons. The failure was manifested by an audible crack and confirmed by a sharp drop in the load-deflection curve. If run-out occurs, the stress level is increased by a preselected stress increment and the same sample is run again at the new stress level¹⁵. This procedure is continued until the specimen does fail. Where load is stepped at (100 N per 5000 cycles) run as 700, 800, 900, 1000, 1100, 1200, 1300, 1400, 1500, 1600 and 1700 N at a maximum of 5000 cycles each.

The maximum fatigue strength supported by each specimen was calculated according to the equation¹⁶

Where 𝜎_e is the maximum fatigue strength corresponding to N_Life cycles, 𝜎₀ is the previous maximum fatigue stress that did not result in failure, Δ𝜎 is the step increase in maximum fatigue stress, N_Fail the cycles to failure at the fatigue stress and the N_Life defined cyclic fatigue life.

Examination of fractured samples. Fractographic analysis was performed to detect the modes of failure of the samples and the retrievability of remaining tooth structure. This analysis was done for all samples using high-performance Leica MZ6 Stereomicroscope (Meyer instruments. USA) with 8:1 zoom indicating areas of interest for further examination under a scanning electron microscope (SEM; Model Quanta 250 Field Emission Gun, FEI company., the Netherlands) attached with EDX Unit (Energy Dispersive X-ray Analyses), with accelerating voltage 30 K.V., 50X and 200X magnification and resolution of 1 nm. The samples were clean and conductive, and no coating was required since the LowVac mode was used. The specimens were attached to the specimen holder using any suitable SEM vacuum-quality adhesive.

The 3D designing of models

3D scanning. Three samples of the fifty-four samples that were constructed for the fatigue resistance test (one for each group) and their corresponding Occlusal veneers were scanned before cementation to produce models with real geometrical measures. 3D reconstruction from Cone beam computed tomography (CBCT) data was found to be accomplished with a high linear, volumetric, and geometric accuracy¹⁷. A CBCT scanner (Next Generation iCAT scanner; Kaltenbach & Vaigt Gmb, Germany) was used to obtain CBCT images in this research.

After acquisition, data were exported and transferred in DICOM format. Mimics software (version 17; materialize, Belgium) was used for segmenting the scanned objects into separate elements. Definitive threshold was assigned to select certain tissue and its volumetric model was automatically calculated. The produced STL files were viewed and assembled on Meshmixer software version 3.3.15 (Autodesk, Inc, USA) where mesh improvement and refinement was carried out.

Reverse engineering and assembly. Reverse-engineering was performed using NX software v10 (Siemens) (FreeCAD is an open access alternative). The generated 3D CAD geometry of each element was then assembled using Solidworks software 2017 (Dassault Systems SolidWorks Corporation) (FreeCAD is an open access alternative) to produce three models one for each group (Figure 4).

Figure 4. Vertical cross section of 3D computer-aided design model showing the assembled elements subjected to load.

Finite element analysis

The process of finite element analysis was carried out by ANSYS R16.2 software (ANSYS, Canonsburg, PA, USA) (OpenFOAM is an open access alternative). It included three phases: the pre-processing, processing and post-processing phases.

In the pre-processing phase, the type of element was defined as Solid 10 node 187. It was presumed that the characteristics of all materials were isotropic, homogenous, and linear elastic. Each material's characteristics (elasticity module and Poisson ratio) were gathered and uploaded to ANSYS R16.2 software (Table 2). A perfect bonding (rigid) condition with no slip interface conditions between the components. During this process each model was divided into small parts called elements, connected at points called nodes to form a mesh structure. Parabolic tetrahedral solid elements were used to form a fine solid mesh. The overall number of elements and nodes was recorded (Table 3).

In the processing phase, following the creation of the 3D meshes, a zero-displacement boundary condition was set at all nodes of the cortical bone that were confined in X, Y, and Z directions. The occlusal surface of the occlusal veneer was loaded by a 500 N 3D ball model at the inner inclines of the mesiobuccal, distobuccal and mesiolingual cusps¹⁸.

In the post-processing phase, the output of the processing phase was displayed as graphical and numeric outputs in the form of von Mises Stress^25,26 The von Mises criterion is a formula for calculating whether the stress combination at a given point will cause failure. If the von Mises Stress exceeds the yield stress, then the material is considered to be at the failure condition. Outputs were also displayed as total deformation²⁵, physical deformations that can be calculated on and inside a part or an assembly, and Maximum Equivalent Stress Safety Tool²⁵, a particular combination of principal stresses cause failure if the maximum equivalent stress in a structure equals or exceeds a specific stress limit.

Statistical analysis

Data from the three groups were gathered, arranged and analysed using IBM SPSS, version 21 (SPSS, Chicago, IL, USA). One-way ANOVA was used to determine whether there was statistical difference between the three groups. Survival rates (%) were compared by chi square test. Statistical analysis was performed using Graph-Pad Instat statistics software (version 3.06) for Windows. The data obtained using the accelerated fatigue test were visualised using a Kaplan Meier survival curve to cycles and life table for load to compare fatigue resistance of the groups. P values ≤0.05 were considered to be statistically significant in all tests.

Fatigue resistance

It was discovered that the largest mean±SD values were registered for Group II (circumferential preparation); (890.57±211.53 N) followed by Group I (conventional preparation) mean±SD values; (883.54±135.91 N) whereas, the smallest mean±SD values were recorded for Group III (intracoronal extension) (875.57±143.52 N). As indicated by ANOVA test the difference among groups was statistically non-significant (p=0.9814). Raw fatigue resistance data, alongside all other raw results, are available as Underlying data²⁷.

Descriptive statistics

Descriptive statistics of fatigue failure load test results; mean values, standard deviation (SD) and confidence intervals (low and high) for all groups are summarized in Table 4 and depicted graphically in Figure 5.

Figure 5. Column chart showing fatigue failure load mean values for all groups.

Survival rate. Survival rate of samples recorded for all experimental groups as function of step load was tabulated and graphically drawn (Table 5, Figure 6).

Figure 6. Linear curves showing survival rate (%) of samples recorded for all experimental groups.

Kaplan-Meier survival curve. Analysis using Kaplan-Meier survival curves, through the log-rank test, showed non-significant differences between groups (Figure 7). The survival performance was best for Group II and worst for Group III.

Figure 7. Survival linear curve showing fraction survival of samples recorded at each load step.

Fractographic analysis of fractured samples

Mode of failure

Fracture behaviour of the samples was classified in to three modes and described in Table 6 as observed by stereomicroscope (Figure 8) and SEM (Figure 9), where retrievable survival of the remaining tooth structure is classified as modes I and II and irretrievable survival of remaining tooth structure: mode III. Results of each group are tabulated (Table 7) and classified as retrievable and irretrievable (Table 8).

Figure 8. Descriptive stereomicroscopic images.

Left, Mode I; middle, Mode II; right, Mode III.

Figure 9. Scanning electron microscope images.

Left, retrievable survival (crack limited to veneer); right, irretrievable survival (crack beyond the veneer).

3D finite element analysis

The FEA revealed stresses at every node in each model. These results were displayed as stress contours overlaid on the original model. The calculated numeric data of stress, deformation and safety factor in the models were transformed into color graphics.

Equivalent (von Mises) stress. For each group, the von- Mises stress values of different areas were calculated and compared (Table 9). The stress distribution values were generally found to be low and within the safe range except at certain areas.

Total deformation. The maximum value of total deformation in Group I was 0.035 mm, located on the lingual half of the occlusal veneer tooth complex. The maximum value in Group II was 0.03785 mm, equally distributed over the occlusal veneer tooth complex. In Group III, the maximum value was 0.04 mm located on the distal half of the occlusal veneer tooth complex as well as distal half of root (Figure 10).

Figure 10. Colour graphics showing total deformation.

Top, Group I; Middle, Group II; Bottom, Group III.

Safety factor. In the three groups there was high safety factor where the maximum equivalent stress was less than the stress limit. Where the lowest safety factor recorded (Figure 11, denoted by the red color) for Group I (0.2135) was at the buccal neck of the tooth, for Group II (0.25126) it was at the neck of the tooth and for Group III (0.19275) it was at the lingual neck of the tooth.

Figure 11. Colour graphics showing safety factors.

Top, Group I; Middle, Group II; Bottom, Group III.

The stress distribution among different layers of the model differed as well as areas of total deformation. This could be correlated with the fracture behavior of samples of the three groups as follows:

Group I (planar occlusal veneer preparation design)

This preparation design delivers almost equivalent amounts of stresses to the occlusal veneer and to axial walls of the tooth. Where stresses are concentrated towards lingual, mesial and distal areas of occlusal veneer tooth complex, with maximum stresses concentrated at lingual half of occlusal veneer and tooth. The maximum total deformation was observed at lingual and mesial areas of occlusal veneer- tooth complex.

Upon observation of fracture behavior of Group I samples of fatigue resistance, the failure in most of the samples occurred at lingual and mesial regions of restoration which fails early upon dynamic loading causing enamel fracture at underlying axial wall (Figure 12).

Figure 12. Group I: total deformation and fracture behaviour.

Group II (circumferential occlusal veneer preparation design)

This preparation design equally distributed stresses over the tooth where maximum stresses are observed at the occlusal veneer center and the lingual half of the occlusal veneer and tooth structure. The maximum total deformation was equally absorbed by the entire occlusal veneer shielding the underlying tooth structure.

Upon observation of fracture behavior of most of group II samples of fatigue resistance, the failure in most of the samples occurred at the lingual half of the occlusal veneer not including the tooth in any form of fracture or cracking (Figure 13).

Figure 13. Group II: total deformation and fracture behaviour.

Group III (intracoronal cavity occlusal veneer preparation design)

This preparation design delivers less stress to the occlusal veneer and transfers larger stresses to the center of the tooth and cervical area, concentrated towards the lingual and distal areas, with maximum stresses at the center of occlusal veneer. The maximum total deformation was observed at the distal and lingual portions of the occlusal veneer-tooth complex reaching as far as cervical area and the root.

Upon observation of fracture behavior of group III samples of fatigue resistance, the failure of the majority of the samples occurred along the lingual and distal portions of the occlusal veneer- tooth complex extending to the tooth cervical area (Figure 14).

Figure 14. Group III: total deformation and fracture behaviour.

Clinical significance

Choosing the occlusal veneer preparation design to restore worn molars may influence the stresses within the crown-tooth system. It is essential to guarantee that the occlusal veneer preparation design offers the already worn tooth sufficiently conservative and non-invasive preparation to be able to withstand occlusal forces and, in case of fracture, the remaining tooth structure is not placed at risk and is considered restorable.

This research disclosed that planar and circumferential finish line occlusal veneer preparation designs had elevated incidences of restorable fractures relative to intracoronal extension occlusal veneer preparation design. Therefore, clinicians may choose the planar and the circumferential designs to improve the clinical performance and longevity of the restored worn molars.

Recommendations

1. In vivo studies should be conducted to compare the behavior of the three tested designs.

2. A research project to measure the difference in retention between the planar, circumferential finish line and intracoronal extension occlusal veneer preparation designs should be conducted.

3. Other preparation designs that include a missing cusp with differently leveled finish line should be designed.

Underlying data

Open Science Framework: Fatigue Resistance and 3D Finite Element Analysis of Machine Milled Ceramic Occlusal Veneers with New Preparation Designs Versus Conventional Design. “In-Vitro Study”. https://doi.org/10.17605/OSF.IO/57RQY²⁷.

This project contains the following underlying data:

Data are available under the terms of the Creative Commons Zero "No rights reserved" data waiver (CC0 1.0 Public domain dedication).