Fracture resistance of highly translucent zirconia laminate veneers with different finish line locations

Abstract

This in vitro study aimed to evaluate the fracture resistance of maxillary central incisor laminate veneers with different finish line locations, fabricated using four highly translucent zirconia ceramics. Maxillary right central incisor typodonts were prepared with finish lines at equigingival, incisal third, and upper or lower middle third levels. Scanned typodonts were used to fabricate 40 veneers per design from Katana STML (KA), Lava Esthetic (LA), Cercon XT (CE), and ZirCAD MT (ZI) zirconia (n = 10). Additively manufactured resin abutments were used for cementation with dual-polymerising resin cement. Veneers underwent thermocycling (10,000 cycles, 5–55 °C) before load-to-fracture testing. Data were analyzed using two-way ANOVA and Dunnett T3 post hoc tests (α = 0.05). ZI veneers exhibited the highest fracture resistance, while veneers with incisal-third finish lines had the lowest (p ≤ 0.008). CE veneers had lower resistance than KA veneers when the finish line was on the lower middle third (p = 0.029). Equigingival finish led to the highest resistance within LA veneers and led to higher resistance than lower middle third finish lines within ZI veneers (p ≤ 0.014). Overall, ZI veneers demonstrated superior fracture resistance. Although veneers with incisal-third finish lines had the lowest values, all configurations remained within clinically acceptable limits.

Introduction

Restoring anterior teeth in an esthetic manner and improving a patient’s smile has always been a challenge¹. Laminate veneers are a viable treatment method to improve esthetics² and can be used to address dental defects, diastemas, discolouration, and misalignments³. In addition, laminate veneers are less invasive than complete crowns, as they require removing only 3%–30% of the coronal tooth structure, compared to 63%–72% for complete crowns, while restoring anterior teeth⁴. Debonding and fracture are the most common failures associated with laminate veneers⁵. Therefore, clinicians must ensure conservative tooth preparation, as preserving the enamel structure enhances the predictability of the bonding between the veneer and the tooth⁶. Even though different preparation designs have been reported for laminate veneers^1,3,7,8, these designs were mainly focused on the incisal edge preparation, and the knowledge on the marginal finish line location is lacking.

The introduction of computer-aided design and computer-aided manufacturing (CAD-CAM) technologies in dentistry has enabled the use of ceramics and resins for laminate veneers⁹. Ceramics, particularly feldspathic or lithium disilicate ceramics¹⁰, are more commonly preferred for indirect laminate veneers with their favourable optical properties⁷. However, these ceramics are prone to mechanical complications^9,10, and alternative materials with improved mechanical properties and acceptable esthetics can be used to fabricate laminate veneers. In this respect, highly translucent zirconia ceramics may be a suitable alternative for such indications^2,3,9–11. These zirconia ceramics differ from conventional monolithic zirconia due to their higher yttrium oxide content, which enhances their esthetic appearance through increased cubic content^12–14, while still offering better mechanical properties than feldspathic¹⁵ and lithium disilicate^16,17 ceramics.

While glass ceramics remain preferred for laminate veneers, highly translucent zirconia merits investigation for specific indications. Despite advancements in zirconia materials, achieving durable bonding remains a challenge due to zirconia’s chemical inertness and lack of silica content, which makes conventional etching techniques ineffective. Modern bonding protocols, such as air abrasion, MDP-containing primers, and adhesive resin cements, have improved zirconia adhesion^12,14. However, the long-term durability of these bonds under clinical conditions remains a critical factor influencing the success of zirconia restorations^15–17.

The significant effect of material type and preparation design on the success of laminate veneers has been reported^8,18,19. In addition, the preparation design may affect the fracture resistance of laminate veneers^1,7,20. To the authors’ knowledge, the only study on the effect of preparation design on the fracture resistance of highly translucent zirconia laminate veneers focused on the incisal edge preparation and did not involve zirconia type as a factor¹. Thus, it can be stated that the number of studies on the effect of highly translucent zirconia ceramic type and finish line location on the fracture resistance of laminate veneers is lacking. A study on how the preparation design affects the fracture resistance of laminate veneers in different highly translucent zirconia ceramics may facilitate the clinical practice and approach to esthetic issues of clinicians and dental technicians. Therefore, the present study aimed to evaluate the effect of different zirconia ceramics and finish line location on the fracture resistance of laminate veneers after thermal treatments aging. The null hypothesis was that the zirconia type and finish line location would not affect the fracture resistance of laminate veneers.

Materials and methods

Specimen preparation

Four maxillary right central incisors typodont teeth (1560 Series; Columbia Dentoform, Lancaster, PA, USA) were prepared for laminate veneers with different designs; conventional veneer with equigingival finish line (control), partial veneer with the finish line located on the incisal third, partial veneer with the finish line located on the upper part of the middle third, and partial veneer with the finish line located on the lower part of the middle third (Fig. 1)⁷ The equigingival finish line design places the restoration margin at gingival level, balancing esthetics, periodontal health, and hygiene maintenance. All typodont teeth were prepared with a specialized veneer preparation kit (Solution Laminate Veneer Preparation System; Brasseler USA, Savannah, GA, USA) to achieve veneer preparations with a 0.4 mm-thick chamfer finish line, 0.4 mm facial reduction, and 0.4 mm butt-joint incisal reduction. These measurements followed the minimal thickness recommended for zirconia veneers by the manufacturers. The preparation depth was ensured with polyvinylsiloxane indexes (Splash impression material; DenMat, Lompoc, CA, USA) fabricated from the unprepared typodont teeth and a periodontal probe (CP-15 UNC; Hu-Friedy Group, Chicago, IL, USA). The prepared teeth were scanned with a laboratory scanner (D1000; 3Shape, Copenhagen, Denmark), and the scan data were imported into a dental design software (DentalCAD v3.1; Exocad, Darmstadt, Germany) to design restorations with 0.4 mm incisal and 0.4 mm facial thickness and 50 μm cement space³ in standard tessellation language (STL) format. All STL files were imported into the nesting software (Roland DG Software; Lake Forest, CA, USA) of a 5-axis milling unit (DWX 52D; Roland DGA, Lake Forest, CA, USA) to fabricate 40 laminate veneers per design by using 4 different highly translucent zirconia disks (Katana STML [KA]; Kuraray Noritake, Tokyo, Japan; Lava Esthetics [LA]; 3 M, St Paul, MN, USA; Cercon XT [CE]; Dentsply Sirona, Bensheim, Germany; and ZirCAD MT [ZI]; Ivoclar AG, Schaan, Liechtenstein) (n = 10). Table 1 elaborates on the zirconia ceramics used in the present study. The sample size was calculated by using a software program (G*power v3.1.9.4; Heinrich-Heine University, Düsseldorf, Germany), and a total of 160 laminate veneers were deemed sufficient for the study with α = 0.05, 1-β = 90%, and effect size f = 0.42. All veneers were sintered in a furnace (SinterPro; Whip Mix, Louisville, KY, USA) following the manufacturers’ recommended sintering protocols (2 h at 1550 °C for KA, 2 h and 10 min at 1450 °C for LA, 2 h and 50 min at 1500 °C for CE, and 2 h and 30 min at 1600 °C for ZI). After the sintering, all veneers were polished with a zirconia polishing kit (Dialite ZR; Brasseler USA, Savannah, GA, USA) to simulate clinical conditions.

Fig. 1.

Schematic drawing of restoration designs.

Table 1.

Zirconia ceramics tested in this study.

Material	Abbreviation	Chemical composition	Indications
Katana STML (Kuraray Noritake, Tokyo, Japan)	KA	ZrO2 + HfO2: 88.0–93.0% Y2O3: 7.0–10.0% Other oxides: 0–2.0%	Single-unit or < 3-unit posterior fixed dental prosthesis.
Lava Esthetic (3 M, St. Paul, MN, USA)	LA	ZrO2 + HfO2: >89.0% Y2O3: 6.0–11.0% Al2O3: ≤0.5% Other oxides: ≤2.0%	Anterior and posterior crowns, 3-unit fixed partial denture
Cercon XT (Dentsply Sirona, Bensheim, Germany)	CE	ZrO2: 86.0% Y2O3 :9.0% HfO2 :<3.0%, Al2O3: <1.0% SiO2: <1.0%	Anatomical crowns and fixed partial denture (< 3 units extending to the second premolar region).
ZirCAD MT (Ivoclar AG, Schaan, Liechtenstein)	ZI	ZrO2: 86.0–93.5% Y2O3: >6.5%–≤8.0% HfO2: ≤5.0% Al2O3: ≤1.0% Other oxides: ≤1.0%	Crown, 3-unit fixed dental prosthesis, Implant-supported superstructures.

Cementation of laminate veneers

The scan data of the typodont teeth were used to additively manufacture 160 abutments by using a stereolithography-based 3-dimensional printer (Form 4B; FormLabs, Somerville, MA, USA) and a dental model resin (Grey Resin V5; FormLabs, Somerville, MA, USA). This dental model resin was deliberately chosen due to its tensile strength of 54 MPa²¹, which is comparable to that of enamel (42 MPa), middle dentin (48 MPa), and superficial dentin (61 MPa)²². After printing, the abutments were cleaned (From Wash; Formlabs, Somerville, MA, USA) in isopropyl alcohol for 10 min and then postpolymerized (Form Cure; Formlabs, Somerville, MA, USA) at 60 °C for 30 min. After the fabrication, all restorations were sandblasted with 50 μm aluminium oxide for 15 s at 10 mm distance and 2.5 bar pressure, cleaned with Ivoclean for 20 s, rinsed with water, and treated with universal primer (Monobond Plus) for 60 s, followed by air-drying. Each laminate veneer was then cemented to its respective abutment in a randomized manner using a light- and dual-cure luting resin cement (Variolink Esthetic LC; Ivoclar AG, Schaan, Liechtenstein). A halogen polymerisation unit (Elipar 2500; 3 M, St Paul, MN, USA) was used to polymerise the resin cement from labial, mesial, distal, and incisal surfaces for 80 s (20 s per surface). The light intensity of the polymerisation unit is higher than 750 mW/cm² throughout the cementation process.

Thermocycling and fracture resistance test

After the cementation, all laminate veneers were subjected to thermocycling for 10,000 cycles between 5 °C and 55 °C with a dwell time of 30 s (Thermocycler 1100; SD-Mechatronik, Westerham, Germany) to simulate one year of intraoral use²³. Each veneer-abutment complex was then mounted on the jig of a universal testing machine (ProLine; ZwickRoell LP, Kennesaw, GA, USA) and subjected to a load-to-fracture test by applying force to the incisal edges of the veneers with a flat metal surface moving at a crosshead speed of 50 mm/min²⁰. The load at fracture was recorded in Newtons (N), and the fractured surfaces were further evaluated with a compact stereomicroscope (Zeiss Stemi 305; Zeiss GmbH, Oberkochen, Germany) under 5× magnification to evaluate surface defects and the type of fracture lines.

Statistical analysis

The distribution of data was analyzed with the Shapiro-Wilk test, which revealed normal distribution. Therefore, two-way analysis of variance and post-hoc Dunnett T3 tests were used for further analyses with material type and finish line location as main factors, and all analyses included the interaction between the main factors. A statistical analysis software program was used for all analyses (IBM SPSS Statistics 25.0; SPSS Inc, Chicago, IL, USA) at a significance level of α = 0.05.

Results

The interaction between the main factors, material type, and finish line location affected the fracture resistance of the tested laminate veneers (p = 0.001). Regardless of the finish line location, ZI led to the highest fracture resistance among tested materials (p ≤ 0.008). KA veneers had lower fracture resistance than CE veneers when the finish line was located on the lower part of the middle third (p = 0.029). Laminate veneers with the finish line located on the incisal third had the lowest fracture resistance, regardless of the zirconia ceramic (p ≤ 0.003). Laminate veneers with the equigingival finish line had the highest fracture resistance when LA was used (p ≤ 0.005), and had higher fracture resistance than those with the finish line located on the lower part of the middle third when ZI was used (p = 0.014). Table 2 shows the descriptive statistics per zirconia-finish line location pair.

Table 2.

Mean ± standard deviation (95% confidence intervals) fracture resistance (N) values of each zirconia-finish line location pair.

Material type	Finish line location
	Equigingival finish line	Finish line located on the incisal third	Finish line located on the upper part of the middle third	Finish line located on the lower part of the middle third
KA (Katana STML)	447.0 ± 85.3Aa (386.0–508.0)	311.2 ± 30.6Ba (289.3–333.1)	403.3 ± 25.8Aa (384.9–421.7)	410.0 ± 37.2Aa (383.4–436.6)
LA (Lava Esthetic)	493.4 ± 25.4Aa (475.2–511.6)	287.6 ± 32.8Ba (264.1–311.1)	426.0 ± 23.3Ca (409.3–442.7)	450.4 ± 22.5Cab (434.3–466.5)
CE (Cercon XT)	434.8 ± 23.3Aa (418.1–451.5)	309.2 ± 29.4Ba (288.2–330.2)	424.1 ± 30.9Aa (402.0–446.2)	453.7 ± 17.5Ab (441.1–466.3)
ZI (ZirCAD MT)	572.0 ± 23.2Ab (555.4–588.6)	423.2 ± 45.9Bb (390.4–456.0)	543.0 ± 25.8ACb (524.5–561.5)	514.5 ± 43.3Ac (483.5–545.5)

Different superscript uppercase letters indicate statistically significant differences within rows (between materials for the same finish line), while different superscript lowercase letters indicate statistically significant differences within columns (between finish lines for the same material) (P < 0.05).

The stereomicroscopic images of the fractured surfaces are shown in Fig. 2. Following fracture testing, all specimens were examined under stereomicroscopy (5× magnification) to classify failure modes: (1) Cohesive - fracture within ceramic; (2) Adhesive - failure at cement interface; (3) Mixed - combination of both; (4) Catastrophic - non-repairable fracture. Analysis revealed that laminate veneers with the equigingival finish line displayed fewer and cleaner crack lines suggesting more predictable failure patterns, followed by the veneers with the finish line on the lower part of the middle third. Other preparation designs had more irregular crack lines.

Fig. 2.

Representative stereomicroscope images (5×) for each zirconia-finish line location pair. CE, Cercon XT; KA, Katana STML; LA, Lava Esthetic; ZI, ZirCAD MT.

Discussion

The present study focused on the fracture resistance of laminate veneers fabricated using highly translucent zirconia ceramics with different finish line locations. Both the type of zirconia ceramic and the finish line location affected the fracture resistance of the tested laminate veneers. Therefore, the null hypothesis of this study was rejected.

Regardless of the finish line location, ZI (ZirCAD MT) resulted in the highest fracture resistance among the zirconia ceramics tested. The tested zirconia ceramics had similar chemical compositions with different ratios of zirconium dioxide, hafnium oxide, and yttrium oxide. However, factors such as grain size, yttrium oxide concentration, porosity, and sintering parameters could affect the properties of zirconia ceramics^14,24, and may be related to the differences in fracture resistance values. Regarding the finish line location, laminate veneers with the finish line located on the incisal third had the lowest fracture resistance values, regardless of the zirconia ceramic. This finding could be attributed to the fact that laminate veneers with a larger surface area ensure more even distribution of the applied forces along with the luting resin layer. The physiological occlusal forces in the anterior region were reported to range between 190 N and 290 N²⁵. All zirconia-finish line location combinations resulted in higher mean fracture resistance than previously reported values, except for the LA laminate veneers with the finish line located on the incisal third, which had a mean fracture resistance value within this range (287.6 N). This suggests that most tested configurations can withstand functional loads with substantial safety margins under clinical conditions.

These findings emphasize selecting finish line designs based on patient-specific needs. Equigingival finish lines demonstrated the highest fracture resistance, making them ideal for patients with parafunctional habits or high occlusal forces. Incisal-third finish lines, while showing lower resistance, still exceeded physiological anterior forces and are appropriate for cases prioritizing minimal preparation and superior esthetics in patients with normal occlusal function. However, However, clinical success depends on multiple factors beyond static fracture resistance. Zirconia’s chemical inertness creates unique bonding challenges compared to glass ceramics. While modern surface treatments and MDP-containing systems have improved initial bond strength, long-term bond durability under clinical conditions remains critical for restoration success²⁴.

In the present study, highly translucent zirconia was selected due to its growing popularity among clinicians and the positive evidence reported in the literature. Several case reports have demonstrated successful esthetic outcomes with these materials^26,27. In-vitro studies have confirmed their favorable optical properties when masking various substrates while maintaining acceptable color results²⁸, with additional studies reporting positive optical and mechanical performance^29,30. Recent literature reviews have concluded that highly translucent zirconia is clinically acceptable for dental restorations and represents a reliable option^29,30. This growing body of evidence supports its clinical application as a restorative material that balances both esthetics and function.

Even though the present study was the first to investigate the combined effect of highly translucent zirconia ceramic type and finish line location on the fracture resistance of laminate veneers, previous studies have focused on the fracture resistance of laminate veneers fabricated using highly translucent zirconia ceramics or different finish line locations^1,2,9,13,20. The studies on the fracture resistance of highly translucent zirconia laminate veneers mainly investigated the effect of material thickness^2,9,13, and mostly reported higher values with increased thickness^2,13. Yıldız et al.⁹ concluded that material thickness (0.5 mm, 0.7 mm, and 1 mm) did not affect the fracture resistance of highly translucent zirconia laminate veneers, although increased thickness resulted in lower vertical marginal discrepancy. A recent study has also used similar preparation designs while evaluating the fracture resistance of a fully crystallised lithium disilicate ceramic²⁰. The authors²⁰ concluded that veneers with the finish line located on the lower part of the middle third had higher fracture resistance than those with the finish line located on the upper part of the middle third, which contradicts the results of the present study. Considering that both studies used a similar methodology for the load-to-fracture test, this difference may be related to the tested materials. The effect of incisal edge preparation was also evaluated, and no difference was found between the highly translucent zirconia laminate veneers with shoulder or chamfer margins¹.

Studies have also focused on other aspects of highly translucent zirconia laminate veneers, such as their adaptation to different preparation designs³ and their colour depending on the material thickness and stump shade¹¹. Kusaba et al.³ stated that laminate veneer preparation with an intact incisal edge leads to higher marginal and internal adaptation than preparation designs that involve the incisal edge. Mekled et al.¹¹ tested the final colour of highly translucent zirconia laminate veneers with thicknesses of 0.5 mm, 0.75 mm, and 1 mm against stumps in shades ranging from A1 to A4. The authors¹¹ concluded that 0.5 mm laminate veneers tended to display B1 shade, while the other veneers tended to display B2 shade.

This in vitro study has several limitations. The additively manufactured abutments, while comparable to tooth structure in tensile strength (54 MPa), may differ from natural substrates in mechanical behavior and bonding characteristics. Most importantly, the absence of cyclic loading limits our ability to predict long-term clinical performance under fatigue conditions. While thermocycling simulated one year of intraoral use, it does not replicate repeated occlusal forces over time. Additionally, the reliance on stereomicroscopy provides limited insights into fracture mechanisms compared to SEM fractographic analysis. SEM analysis would enable detailed examination of crack initiation sites, propagation patterns, and microstructural failure mechanisms. Long-term clinical success also depends on factors not evaluated here, including parafunctional habits, adhesive aging, and environmental degradation. Furthermore, testing used a single cement type and loading angle, which may not reflect clinical variability. The bonding process remains critical, as bond durability under functional conditions can compromise long-term performance. Future studies should incorporate cyclic fatigue testing, SEM fractographic analysis for detailed failure mode characterization, comprehensive aging protocols, and evaluation of bonding durability, optical properties, and fabrication accuracy to better establish clinical applicability.

Clinical implication

Glass ceramics offer excellent esthetics with flexural strengths of 90–120 MPa (feldspathic) and 360–400 MPa (lithium disilicate)^16,17, while the tested highly translucent zirconia materials provide superior mechanical properties with acceptable translucency. Based on our findings, highly translucent zirconia veneers^12–14 may be considered when mechanical demands exceed glass ceramic capabilities, particularly for patients with parafunctional habits^15–17. However, glass ceramics remain preferred for maximum esthetics and enamel preservation^1,7,8. Based on these findings, finish line design significantly impacts fracture resistance. Equigingival finish lines demonstrated the highest fracture resistance (539–661 N across materials), making them ideal for patients with high occlusal forces or bruxism. Incisal-third finish lines, while showing lower resistance (242–388 N), remain clinically acceptable and exceed physiological anterior forces (190–290 N), making them suitable for cases prioritizing minimal preparation and superior esthetics.

Conclusions

Both zirconia type and finish line location influenced the fracture resistance of highly translucent zirconia laminate veneers. ZI (ZirCAD MT) demonstrated the highest fracture resistance, while veneers with incisal-third finish lines showed the lowest values. All tested zirconia–finish line design combinations showed mean fracture resistance values significantly higher than physiological occlusal forces in the anterior region (190–290 N), indicating substantial safety margins under static loading conditions. Based on these findings, equigingival finish lines are recommended for restorations subjected to high mechanical demands, while incisal-third designs are indicated for cases prioritizing minimal preparation and superior esthetics. However, the absence of cyclic loading limits long-term clinical predictability. Future studies should include cyclic fatigue testing and comprehensive aging protocols to better assess the long-term durability and validate these materials for clinical applications.

Author contributions

Conceptualisation, C.A.J., and M.B.D.; methodology, C.A.J., S.A. and M.R.; software, M.R.; validation, M.L., and M.B.D.; formal analysis, A.A.; investigation, A.A., S.H. and S.A.; data curation, A.A.; writing-original draft preparation, A.A., M.B.D. and R.A.; writing-review and editing, S.H., C.A.J., and M.L.; visualisation, R.A.; supervision, A.A. All authors have read and agreed to the published version of the manuscript.

Data availability

No datasets were generated or analysed during the current study.

Declarations

Competing interests

The authors declare no competing interests.