Stress Distribution and Fatigue Behavior of Simplified Glass Ceramic Restorations: Effects of Curvature Ratio of Composite Resin Substrate and Restoration Assembly

Abstract

Objectives

This study investigated the effect of preparation curvature and leucite-reinforced glass-ceramic restorations on the fatigue behavior and stress distribution of the bonded ceramic restorations.

Materials and Methods

Finite element analysis (FEA) was performed for the three experimental groups: flat preparation (Flat), preparation with a 2 mm radius of curvature (Curv1), and preparation with a 10 mm radius of curvature (Curv2). The Maximum Principal Stress (MPS) criterion was used to assess stress distribution. Then, resin composite discs were prepared to simulate the different curvatures (Flat, Curv1, and Curv2; n = 10), and leucite-reinforced glass-ceramic restorations with matching curvatures were bonded to them. Fatigue behavior was assessed using an accelerated fatigue test (10,000 cycles/step; 25 N increments; 20 Hz; initial load = 200 N). Fatigue failure load (FFL) and number of cycles for failure (CFF) were analyzed using Kaplan Meier analysis with post-hoc Mantel-Cox (log-rank) test (α = 0.05).

Result

The Flat group exhibited the lowest stress concentration (MPS = 4.5 MPa) on FEA, followed by Curv1 (MPS = 17.6 MPa) and Curv2 (MPS = 24 MPa). The Flat group also demonstrated the highest fatigue results (FFL = 632.50 MPa; CFF = 180,666 cycles). No statistically significant difference was found between Curv1 and Curv2 for fatigue values (FFL and CFF; Curv1 = 370.00 MPa and 72,510 cycles/Curv2 = 340.00 MPa and 61,000 cycles).

Conclusion

Flat preparations improve the mechanical performance of bonded leucite-reinforced glass-ceramic restorations compared to curved preparations, which exhibit higher stress concentrations and reduced fatigue resistance.

Keywords

ceramics - curvature radius - fatigue - finite element stress analysis - stress distribution

Introduction

The use of dental ceramic materials as monolithic prosthetic restorations is well described as an option that evolves in both esthetic and mechanical properties. Among ceramic materials, glass ceramics reinforced by particles such as leucite and lithium disilicate assumed protagonism due to their versatility, being able to be applied for partial and total restorations, and performing well mechanically.[1] [2] [3] [4] Furthermore, the advent of adhesive dentistry enables dentists to perform more conservative restorative techniques especially due to the ability of these materials to be conditioned by acids, promoting good adhesion to the substrate.[4] [5]

In addition, the use of computer-aided design and manufacturing (CAD/CAM) technologies are well spread on the restorative dentistry routine, due to its predictability and agility to the daily restorative process.[1] The design stage must consider aspects such as the restoration's final thickness, the occlusal design of the restoration, and the substrate's geometric configuration so that the rigidity of the assembly is correctly established.[6] Restorations with thinner thickness are naturally less capable of withstanding occlusal loads and, from the perspective of occlusal anatomy, different patterns may interfere with the way the antagonist contacts the restoration surface and consequently the outcome observed over time.[7]

Regarding the substrate design, it is important to know the complexity of the geometries obtained after prosthetic preparations, which can influence the behavior of these restorations.[7] [8] Initial studies were performed using flat glass/polycarbonate bilayer models for simplicity to analyze the behavior under loads, the damage, and the fracture mechanism.[7] [8] [9] [10] The disc-disc simplified methodology (flat substrate and flat restorations) was developed to mimic axial load under ceramic restorations on in vitro simulations,[11] and was well recognized throughout the last decade to test ceramic under fatigue load application.[12] However, to bring a geometric shape closer to the reality of a dental crown, curved structures in the form of glass shells filled with a dome-like polymer were also considered, showing that the proper path to survey the propagation of radial cracks in brittle coating structures is a normal indentation loading with simplified geometry.[7]

Dental restorations, such as crowns, are structures which tend to show cracks, making them susceptible to failures in the chewing function, so it is also important to study and understand the different cracking modes that cause their failures.[13] The disc-disc simplified methodology described by Chen et al[11] induces radial cracks, with failure initiation and propagation from the adhesive interface between restoration and substrate due to the bending of the ceramic disc. On the other hand, a recent fatigue study with monolithic ceramic crowns showed a higher prevalence of radial crack type failures, that is, from the contact point.[14] From the clinical standpoint, dental preparation and ceramic restoration present curved and flat faces simultaneously, which might induce distinct stress distribution and risk for failure when subjected to periodic cyclical loading until fatigue failure from variable points of origin. In this regard, understanding that the degree of curvature of the restoration and substrate in the load application region influences the stress distribution and mechanical behavior of the restoration appears to be an aspect yet to be understood.

Based on these premises, it is essential to study the mechanical behavior under fatigue in bonded samples while considering different variables to better simulate possible clinical outcomes. Evaluating the curvature degree of both the substrate and the restoration provides valuable scientific insights, as the adhesive bond enables efficient stress transfer from the restoration to the supporting tooth in clinical service, thereby enhancing the fracture resistance of the restorative material.[15] Moreover, the finite element analysis (FEA) method is a powerful technique for assessing stress and strain distribution in three-dimensional (3D) models and predicting the behavior of mechanical structures under mechanical loading.[16] Thus, the present research aims to assess the effect of distinct curvature radii in a resin composite dental preparation and a leucite-reinforced glass ceramic restoration on its fatigue behavior, as well as its impact on stress distribution in the restored setup. The null hypothesis states that the curvature radius of the preparation and restoration does not influence the ceramic fatigue behavior or stress distribution throughout the restoration.

Materials and Methods

Materials and Study Design

[Table 1] describes the materials used, their commercial representations, manufacturers and batch numbers, and their compositions. The samples simulate an occlusal restoration for a posterior tooth[11] and are composed of leucite-reinforced glass ceramic discs (IPS Empress CAD Multi, Ivoclar, Schaan, Liechtenstein) with a final diameter of 10 mm and thickness of 1 mm. These ceramic discs are adhesively cemented onto resin composite substrate discs (Opallis, FGM, Joinville, Brazil). The specimens were divided into three groups (n = 10) according to the curvature radius of the upper surface of substrate and restoration discs as follows:

Table 1 Trade market, manufacturer, batch number, and composition of the materials used in this in vitro study

Material	Trade market, manufacturer (Batch number)	Composition
Leucite-reinforced glass ceramic	IPS Empress CAD Multi, Ivoclar (X49765)	SiO2, Al2O3, K2O, Na2O, CaO, and other oxides and pigments
Resin composite	Opallis, FGM (240516)	Bis-GMA monomers, BisEMA, TEGDMA, UDMA, camphorquinone, co-initiator, silane, silanized barium-aluminum silicate glass, pigments
5% hydrofluoric acid	Condac Porcelana, FGM (030518)	10% hydrofluoric acid, water, thickener, surfactant and coloring
Silane coupling agent	Monobond N, Ivoclar (W90329)	Alcohol solution of silane methacrylate, phosphoric acid methacrylate, and sulfide methacrylate
Primer	Multilink N Primer A and B, Ivoclar (Y25800 and Y31808)	Primer A: water, initiators; primer B: phosphonic acid acrylate, hydroxyethyl methacrylate, methacrylate mod. polyacrylic acid, stabilizer
Dual cure resin cement	Multilink N, Ivoclar (Y16283)	Dimethacrylates, HEMA, barium glass filler, Ba-Al-Fluoro-Silicate glass, ytterbium trifluoride, highly dispersed silica, catalysts and stabilizer, pigments

• Flat – control group, having flat resin composite discs on which the flat ceramic pieces were bonded.

• Curv1–resin composite discs with a 2-mm curvature radius on which ceramic (equal curvature) was bonded.

• Curv2–resin composite discs with a curvature radius of 10 mm, having the ceramic (equal curvature) bonded on it ([Fig. 1]).

Fig. 1 Specimens' configuration.

Finite Element Analysis

FEA was performed to simulate the mechanical test and evaluate stress distribution using representative 3D models of the experimental groups within the test assembly. The models were designed using specialized software (Rhinoceros version 5.0 SR8, McNeel North America, Seattle, Washington, United States) and consisted of ceramic discs, a resin composite substrate, a cement layer, and a 40-mm diameter stainless steel sphere piston used for the mechanical test.

The analysis was performed using the ANSYS CAE software (ANSYS 19.3, ANSYS Inc., Houston, Texas, United States). All materials were considered isotropic, linear, and homogeneous. Young's moduli (GPa) and Poisson's ratio for leucite-reinforced glass ceramic (E = 40.78 GPa; v = 0.23), resin composite (E = 11 GPa; v = 0.28), resin cement (E = 7.5 GPa, v = 0.3), and stainless steel sphere (E = 190 GPa; v = 0.27) were obtained from previous studies.[17] [18] [19] The name and composition of each material are presented in [Table 1]. All the samples connections were considered perfectly bonded and frictional (0.12) to the piston and restoration, according to previous studies using the same methodology.[20] The models were loaded with 100 N at the top of the piston and constrained at the bottom surface of the base, following a methodology similar to that described by Benazzi et al and Dal Piva et al.[18] [21] Finite element meshes were generated for complex 3D geometries. The meshes were controlled using quadratic tetrahedral elements, characterized by triangular-based pyramids with a node at each vertex and another at the center of each edge. After the coherence and mesh convergence test, the maximum principal stress (MPS) was used as failure criteria to compare the groups. The FEA processing steps are illustrated in [Fig. 2].

Fig. 2 Three-dimensional (3D) modeling and finite element analysis (FEA) processing steps. (A) Solid model with piston. (B) Design of separate structures for modeling. (C) Mesh generation for FEA. (D) Load application (red arrow). (E) Fixed support (blue arrow).

Specimen Preparation and Mechanical Fatigue Test

Specimen Preparation

Ceramic Restoration and Resin Composite Substrate Discs

The sample designs defined through FEA for each group were exported as STL files (Standard Tessellation Language) and imported into the 3D printer software (W3D Print, Wilcos, Photon S, Anycubic, Shenzhen, China) to generate 3D-printed resin models (Resilab Premium, Wilcos) ([Fig. 3A]).

Fig. 3 Computer-aided design and manufacturing (CAD/CAM) processing for ceramic disc specimen fabrication. (A) Three-dimensional (3D)-printed substrates and ceramic discs from each group. (B) Scanned 3D model of the flat substrate. (C) Designed flat ceramic disc. (D) Scanned model of the curved substrate. (E) Designed curved ceramic disc. Both flat and curved samples were designed with the same thickness (1.0 mm) (red arrows), following the substrate curvature.

The printed models of each group were scanned and disc-shaped ceramic samples (Ø = 10 mm; thickness = 1.0 mm), with the respective substrate curvature (according to corresponding group) were obtained through a CAD/CAM system (CEREC inLab MC XL, Sirona, Bensheim, Germany) using leucite-reinforced glass ceramic blocks. Thus, the printed model of each group was placed in a silicone matrix with two adjacent gypsum teeth on each side of the printed model. The set was scanned (CEREC inLab Scanner InEOS Blue, Sirona Dental Systems Gmbh, Germany), 3D images were processed (CAD CEREC inLab 18.0 3D software; Sirona Dental Systems Gmbh), and the final restorations were created following the same parameters analyzed by FEA ([Fig. 3]).

The printed models of each group were copied with a 1-mm-thick acetate plate in a vacuum pressing machine to obtain matrices which were used to standardize the resin composite discs preparations. Then, the resin composite (Opallis, FGM) was compacted and filled on the acetate plate using a resin spatula and then light-cured for 30 seconds (1200 mW/cm², Bluephase N, Ivoclar).

A gentle preparation was performed on each sample using a fine-grained diamond bur (PM 852F HP 023, conical with a rounded end, 44 mm – Jota). All resin composite substrates were thermocycled for 5,000 cycles between 5°C and 55°C, with an immersion time of 30 seconds and a transfer time of 2 seconds (Nova Ética, São Paulo, Brazil) before receiving the ceramic discs for bonding.

Bonding Procedures

The ceramic discs were cleaned in an ultrasonic bath in isopropyl alcohol for 5 minutes and dried by free-oil air jet. The restoration's intaglio surfaces were etched by hydrofluoric acid 5% (HF) for 60 seconds, as recommended by the ceramic manufacturer. Then, it was washed with water for 60 seconds and gently air-dried for 15 seconds. Next, the silane agent (Monobond N, Ivoclar) was applied using a microbrush, allowing it to act for 60 seconds.

One drop of each self-etching primer A and B of the luting system (Multinlink N, Ivoclar) were mixed and actively applied using a microbrush on the resin composite surface for 20 seconds and dried for 5 seconds.

After the surface treatments, the base and catalyst pastes (Multilink N, Ivoclar) were mixed at a 1:1 ratio and applied on the cementation surface of the ceramic disc and on the counterpart resin composite disc. The discs were bonded together and a static load of 7.5 N was applied to the occlusal surface of the ceramic using an adapted device. The excess cement was removed, followed by light-activation (1200 mW/cm², Bluephase N, Ivoclar) for five exposures of 20 seconds each (0°, 90°, 180°, 270° on the occlusal surface of the ceramic).

Mechanical Fatigue Test

The samples were submitted to an accelerated fatigue test (Instron ElectroPuls E3000, Instron Corporation, Norwood, Massachusetts, United States). The load was applied with a 40-mm diameter stainless steel sphere in the center of the “occlusal” ceramic surface supported by a flat steel base under water. An adhesive tape (110 μm) was placed on the occlusal side between specimen and piston to improve stress distribution. This test setup was carried on to obtain radial cracks failure pattern, corresponding to that found clinically as observed in preceding articles.[20] [22] [23] [24]

Cyclic loading was applied at a frequency of 20 Hz, starting with an initial load of 200 N for 5,000 cycles, followed by progressive increments of 25 N (10,000 cycles per step: 225 N, 250 N, 275 N, and so on) until failure detection. Specimens were visually inspected at the end of each load step for the presence of failure (radial cracks) using light oblique transillumination.[25] If failure was detected, the test was terminated and the collected data were recorded for statistical analysis, including fatigue failure load (FFL) and number of cycles for failure (CFF). If the specimen remained intact, the load was increased, and the test continued until failure occurred.

Failure Analysis

Representative samples from each group were selected for scanning electron microscope (SEM) analysis. After initial inspection using stereomicroscopy (Discovery V20, Carl Zeiss, Göttingen, Germany), some specimens were sectioned perpendicularly to the fracture line using a low-speed diamond saw (Labcut 1010, Extec, Enfield, United States) under abundant water cooling. The prepared specimens were then mounted on aluminum stubs, gold-sputtered (Emitech SC7620 Sputter Coater, Emitech, Montigny-le-Bretonneux, France), and analyzed using a SEM (Inspect S50, FEI, Hillsboro, United States) operating at 15 to 20 kV, with backscattered electron imaging performed at 30 kV.

Data Analysis

A statistical software program (IBM SPSS Software; IBM, Armonk, New York, United States) was used with a significance level of 0.05. The FFL and CFF data were subjected to a Kaplan–Meier analysis with post-hoc Mantel–Cox (log-rank) test (α = 0.05). The failure analyses were qualitatively evaluated.

Results

[Fig. 4] presents the FEA images and [Table 2] the MPS values. The flat group exhibited the lowest MPS (4.5 MPa), whereas the curv1 (17.64 MPa) and curv2 (24 MPa) groups showed significantly higher MPS values, indicating a substantial increase in tensile stress with curvature. Additionally, the flat group displayed a larger compression area (dark blue), suggesting greater contact between the ceramic and the piston.

Table 2 Results for maximum principal stress (MPS) values in MPa obtained by Finite element analysis, and the fatigue failure load (FFL) in Newtons and number of cycles for failure (CFF) obtained by fatigue mechanical test

Groups' codes	Ratio	MPS (MPa)	Fatigue mechanical test
			n	FFL (mean – 95% CI)[a]	CFF (mean – 95% CI)[a]
Flat	Absent	4.5	10	632.50 (595.22–669.78) A	180,666 (162,652–197,347) A
Curv1	2.0 mm	17.6	10	370.00 (347.13–392.86) B	72,501 (64,075–80,927) B
Curv2	10.0 mm	24	10	340.00 (312.47–367.52) B	61,000 (49,989–72,010) B

Abbreviation: CI, confidence interval.

^a Different letters in each column indicate statistically significant differences (p < 0.005) using Kaplan–Meier followed by Mantel–cox post-hoc test for FFL and CFF.

Fig. 4 Finite element analysis (FEA) tensile stress distribution images. Contact surface images showing dark blue circles in the compressed area (piston contact area), indicating different contact surface sizes (Flat > Curv1 > Curv2). Inner surface images show peak tensile stress concentration in the area opposite to the contact zone, with higher stress levels in specimens with greater curvature (Curv2).

[Table 2] presents also the fatigue mechanical test results, indicating that the flat group exhibited significantly higher FFL and CFF values compared with the curv1 and curv2 groups (p < 0.05). However, no statistically significant difference was observed between curv1 and curv2 (p > 0.05). Survival curves for FFL and CFF ([Fig. 5]) demonstrate a higher cumulative survival rate for the flat group across all test levels, with curv1 showing a slight advantage over curv2. [Fig. 6] presents a representative image of the failure pattern found in all the experimental groups: radial cracks initiated at the cementation surface without reaching the load application surface.

Fig. 5 Survival graphs for fatigue failure load (FFL; left) and number of cycles for failure (CFF; right) data obtained by Kaplan–Meier analysis. Flat group presents higher FFL and cycles values.

Fig. 6 Topography micrographs showing the crack origin at the cementation interface of fracture specimens under 100× magnification (left), and 500× (right) magnifications.

Discussion

Our results indicate that the curvature of the substrate preparation and ceramic restoration design significantly influences stress distribution and the mechanical fatigue behavior of bonded leucite-reinforced glass ceramic restorations compared with flat assemblies (substrate and restoration discs without curvature). Therefore, the null hypothesis—stating that the curvature radius of the preparation and restoration does not influence the ceramic fatigue behavior or stress distribution throughout the restoration—was rejected.

Flat samples were described by Chen et al[11] as a valid in vitro model to simulate radial crack failures commonly observed in monolithic ceramic restorations, a frequent clinical outcome (Kelly et al, 1999). Based on the premise of reproducing clinical conditions, other aspects have also been extensively explored, such as the selection of pistons with appropriate shape and material to ensure that in vitro failure patterns closely resemble clinical findings (Velho et al., 2022a; Velho et al., 2022b).[26] [27] Additionally, the damage mechanisms induced in dental ceramics by intermittent loading have been considered the most representative of in situ conditions, forming the basis for studies investigating different variables and their effects on the fatigue behavior of dental ceramics.[12] Indeed, research grounded in these principles has contributed valuable insights into dental ceramics, particularly regarding the influence of substrate materials, surface treatments, and the intrinsic properties of the tested materials. However, it should be considered that anatomical variations of prosthetic crowns, including cusps, slopes, flat, and curved areas, can impact significant variabilities in the distribution of stresses throughout the materials, potentially influencing their mechanical behavior.[14]

Our study indicated higher stress concentration at the cementation interface (FEA images, [Fig. 4]) and exclusively radial crack initiation at this interface, without propagation to the top surface ([Fig. 6]). In turn, Packaeser et al[14] tested crowns with simplified anatomies using the same 40-mm piston, applying load to the buccal and lingual cusp tips, predominantly leading to failures at the contact point (named Hertzian cone cracks). The 40-mm piston and disc-shaped samples are known to concentrate stresses at the interface while minimizing surface (occlusal) damage due to the larger contact area. More than 15 years ago, Yi and Kelly[28] demonstrated that smaller contact areas in in vitro ceramic tests create contact conditions that do not accurately replicate clinical scenarios. Compared with Packaeser et al,[14] our setup exhibited a larger compression zone at the contact between the piston and the cusp area, preventing Hertzian cone cracks. Furthermore, the ceramic thickness must be considered, as the 1.0-mm thickness used in this study favors bending of the restoration and, consequently, stress concentration at the cementation interface.

The curv1 and curv2 groups exhibited a reduced contact area between the piston and the sample compared with the flat group ([Fig. 4]), which may have contributed to their inferior mechanical performance ([Table 2], [Fig. 5]). In other words, concerning fatigue behavior, we highlight that the flat group presented significantly higher FFL and CFF than the curv1 and curv2 groups, which presented statistically similar CFF and FFL results between then ([Table 2]). The fatigue resistance of the curved groups decreased by approximately 50% compared with the flat group, with a concomitant increase in stress concentration (from 4.5 MPa in the flat group to 17.64 and 24 MPa in the curv1 and curv2 groups, respectively). This difference may be attributed to the occlusal contact conditions mentioned earlier, as well as the peak stress concentration at the cementation interface, visually evident as more concentrated on curved condition ([Fig. 4]).

The accelerated failure of curved areas is associated with the broader spatial distribution of tensile stresses in more convex surfaces, resulting in a systematic decrease in failure load as convexity increases.[29] The reason curved surfaces are so vulnerable to crack propagation is the sudden spread of cracks throughout the restorations, thus suggesting that the crowns must be designed to avoid highly convex areas close to the margins in the occlusal contact areas. Taking into account that the preparation geometry has clinical implications, guidelines and clinical studies have reported that preparations for ceramics should have a geometry as simple as possible to provide better stress distribution in restorations.[30] [31] [32] [33] Regarding cementation, a flat surface reduces the polymerization stress of the cementation composite resulting in a lower C factor when compared with a preparation with a more complex geometry.[34]

Even though this study has some inherent limitations (such us the use of nonanatomic specimens, the axial load application, and a resin as substrate), it was demonstrated that curved surfaces affect the lifetime of leucite restorations. In contrast, a flat surface induces lower tensile stress concentration and lower risk for failure. Thus, special attention should be taken when prosthetic preparations are performed and restorations are planned, and making a flat surface when possible, mainly in regions under strong masticatory load. The question whether the type of material used in tabletop restorations (hybrid, lithium disilicate, or zirconia polycrystals) on enamel/dentin as substrate will present the same behavior under fatigue remains unanswered. Thus, further investigations should be conducted for more predictable clinical applications.

Conclusion

Curved preparations (curv1 and curv2) generated markedly higher MPS, concentrated at the cementation interface. This stress increase was associated with a significant decline in FFL and CFF, as the flat group showed highest FFL and CFF of leucite-reinforced glass ceramic restorations. The characteristic failure mechanism across all groups—radial cracks initiating at the cementation interface.

Highlights

• - A flat preparation results in the highest fatigue failure load of leucite-reinforced glass ceramic restorations, whereas curved preparations induce the lowest fatigue failure load.

• - The highest maximum tensile stress occurs in curved restoration preparations, while the flat preparation has the lowest maximum tensile stress.

Conflict of Interest

None declared.

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Publication History

Article published online:
16 January 2026