Influence of thickness and surface conditioning on fracture resistance of occlusal veneer

Tooth surface loss (TSL) is a complex system that has a multi-etiology which is a combination of mechanical (attrition, abrasion, abfraction) or chemical (erosion) [1]. Mostly TSL patients presented at the clinic complaining of functional impairment, loss of vertical dimensions, esthetic impairment or sensitivity [2]. Because of the biological risk of full-crown preparation, such as vitality loss and the need for endodontic treatment over time, full-crown preparation is not recommended [3]. These concepts nowadays is replaced by minimal intervention strategies which integrates prevention, remineralization and minimal invasion for the placement and replacement of restorations [4]. Due to this, occlusal veneers have been developed for the prosthetic restoration of eroded teeth using minimally invasive techniques and conservative preparation [1, 5‐7].

Lithium disilicate glass ceramics have a wide range of clinical applications due to its optical properties, mechanical behavior, ease of fabrication, etchable and can be adhesively cemented which enabled the minimal invasive concept [8, 9]. One of the factors that positively affecting the fracture resistance of the ceramic restoration is its increased thickness, But the modern minimally invasive concepts of restorative and prosthetic dentistry often support the use of thin occlusal restorations [10‐17]. Many previous studies concerned various thicknesses of lithium disilicate with a promising fracture resistance values [10, 13‐16]. However 0.5 mm thickness was debatable among many studies [11, 12, 17]. Hence, the minimum thickness allowed for lithium disilicate occlusal veneers is still in short supply and need more investigation.

Effective bonding of the ceramic surface is a critical step for the clinical success of indirect ceramic-bonded restorations [18]. The lithium disilicate surface changed mechanically by acid etching in order to encourage the roughness of ceramic surface so that luting resin cement can get through and penetrate into the pores, in addition to chemical bonding by a silane coupling agent. Acids that used for silicate etching include hydrofluoric acid (HF), acidulated phosphate fluoride (APF), ammonium hydrogen difluoride [19].

Hydrofluoric acid (HF) etching creates a deep porous structure by removing and dissolving the glassy phase matrix. Pore size rely upon the etching time and etching concentration [19]. Previous studies revealed that HF acid etching might negatively affect the lithium disilicate strength and that the average surface roughness was negatively correlated to the mechanical strength [20]. However, the weakening effect of HF acid etching is time dependent [20]. HF acid is a potentially dangerous substance. It was found that it can cause serious tissue damage with low concentration. A 5% HF acid can cause dermal absorption and superficial skin damage when exposed for 3 min. With higher concentration it can penetrate into deeper tissue and cause severe damage so other etching materials are used [21].

Due to its low hydrofluoric acid content and few fluoride ions, acidulated phosphate fluoride (APF) works more superficially on the ceramic surface [22]. While HF acid is not appropriate for intra oral use, APF is used in orthodontics bracket bonding on porcelain crown and intraoral ceramic repair [23].

Silanization enhanced the ceramic surface bond strength between the ceramic and the resin. It was reported that the application of silane and a layer of resin luting agent improve the strength of glass ceramic due to crack bridging [24]. Silane molecules incorporating the cracks, and polymerization shrinkage of the resin cement enhance the apparent strength of cemented ceramics by putting the molecules together, rather than away from each other, as a consequence, cracks would not be able to open freely [25].

The former bonding method for silicate ceramics requires more steps and takes longer time. HF has been substituted in a new product with ammonium polyfluoride, which also contains silane, combining the two surface treatments and simplifying the bonding process [26]. Ammonium polyfluoride has been proven to be more biocompatible, secure, and less aggressive than HF acid [27].

The overall load-bearing capacities of all-ceramic crowns could decrease after combined cyclic stress, moist and thermal cycling [28]. It has been stated that water storage softens the polymer of resin matrix and induce hydrolysis of the interfacial silane coupling agent [29, 30]. Furthermore, mechanical failure of dental restorations occurs after several years of use, reflecting fatigue rather than acute overload [31].

Comparative studies on the effect of thickness and surface treatment and their interaction on the fracture resistance of machinable lithium disilicate are scarce. Therefore, the present study was carried out to evaluate the influence of different thickness and surface treatment and their interaction on fracture resistance of occlusal veneer. The null hypothesis of the present study was that neither surface treatment nor veneer thickness could influence fracture load of CAD/CAM fabricated lithium disilicate occlusal veneers.

The hypothesis of the present study was that neither surface treatment nor veneer thickness could influence fracture load of CAD/CAM fabricated lithium disilicate occlusal veneers. However, the results of the present study rejected both hypotheses. Fracture load of 1 mm thickness occlusal veneer was significantly higher than 0.5 mm thickness occlusal veneers regardless of surface treatment. Also, surface treatment affected fracture load of occlusal veneers at the same thickness.

In order to simulate intraoral conditions, specimens were stored in water for 75 days followed by cyclic loading fatigue for 240,000 cycles with opposing natural molars to simulate one year clinically. The aging procedures of the specimens did not lead to any failure of the restorations. Ioannidis et al. (2019) [15] and Maeder et al. (2019) [36] applied load of 49 N at 1.7 Hz loading frequency for 1’200’000 cycles of dynamic loading, which was reported in literatures to simulate 5 years of clinical service. All specimens survived the cyclic loading and thermal cycling. In accordance with this, 0.5 mm thickness occlusal veneers fabricated from CAD/CAM lithium disilicate seems to be promising for mouth rehabilitation in case of TSL in the molar area. However, normal masticatory forces can reach higher values than 49 N. In the posterior region, they can range from 400 to 800 N [37]. Taking this into consideration, clinicians should be aware of the values which are achieved by fracture resistance test.

The results of this invitro study revealed that maximum fracture load mean value for HF-0.5 group differed from HF-1 group. Test group with 1 mm thickness showed higher fracture load. However, fracture load of HF-0.5 group yielded maximum fracture load at higher value than the masticatory force, which ranges from 400 to 800 N in the molar area [37]. Ioannidis et al. (2019) [15] and Maeder et al. (2019) [36] reported similar results. On the other hand, Schlichting et al. (2011) [11] did not recommend the use of lithium disilicate in 0.6 mm thickness in patient with excessive occlusal force. Also, Sasse at al. (2015) [12] reported 610 N fracture load of 0.3–0.6 mm group and recommended the use of lithium disilicate occlusal veneers with a thickness of 0.7 to 1 mm. Differences in results between the present study and other studies might be due to different research parameters that cannot be unified for all researches.

In this study, when the occlusal veneers were treated with Monobond etch & prime 1 mm thickness showed higher results than 0.5 mm group. Similarly, Baldissara et al. (2019) [38] studied Monobond etch & prime treated lithium disilicate occlusal veneers with different thickness (0.5, 0.8, and 1.2 mm). Survival was significantly influenced by the restoration thickness. Thicker restorations exhibited a higher survival rate. However, in the present study MON-0.5 group fracture load yielded maximum fracture load at higher value than the masticatory force in the molar area.

Similarly, there was significant difference between APF-1 and APF-0.5 groups. Unlike HF-0.5 group and MON-0.5 group, APF-0.5 mean fracture load was 962.05 ± 249.56 N. This value is close to the masticatory force in the molar area when bruxism existed (800 N). So APF treated 0.5 mm occlusal veneers fabricated from lithium disilicate is not recommended for patients with parafunctional oral habits such as bruxism and clenching.

In the current study, it was noticed that MON groups showed the highest mean fracture load for 0.5 and 1 mm thickness, followed by HF groups. The least values were reported for APF groups for both thicknesses. In the English language literature to the best of our knowledge, rare previous studies investigated the fracture load of lithium disilicate under compressive load with different surface treatments especially APF. Many previous studies declared the weakening effect of HF on lithium disilicate [21, 39]. HF etching increased ceramic roughness, even for periods as short as 20 s which is the time recommended by the manufacturer. The authors explained the decrease in flexure strength by the amount of the glass phase involving the lithium disilicate crystals. This might explain the higher mean fracture load of MON test groups than HF test groups in this study.

In terms of fracture load, the present study found that there was no significant difference between Monobond etch & prime and HF & Monobond N surface treatment at the same thickness. Also, in terms of bond strength, Maier et al. (2019) [40], Román-Rodríguez et al. (2017) [26] and Wille et al. (2017) [21] found that tensile bond strength for Monobond etch & prime was equivalent to HF and silane. However, HF acid is a potentially dangerous substance [21]. It was found that it can cause serious tissue damage with low concentration used in dental field [21]. Furthermore, it has been proven that ammonium polyfluoride is more biocompatible and secure than HF [26]. Thus, to avoid health hazards of HF in the dental office, Monobond etch & prime is recommended to be used.

It has been proven that adequate bonding between the ceramic-cement-tooth complex surely increased the ceramic strength [3]. In the present study APF treated samples showed the less fracture load mean values in 0.5 mm samples and 1 mm samples. This might be explained by the fact that APF produced shallow irregularities with deposits precipitation in a way compromising the bond strength between ceramic surface and resin, while ceramic surface gets its strength from strong bond of ceramic-cement-tooth complex. Once this bond was compromised, ceramic strength decreased. These results might be supported by the results of Canay et al. (2001) [22] and Della Bona et al. (2002) [18]. However, other research [23, 41] found that regardless the less roughness produced by APF, bond strength was not compromised. This disagreement with results of this study might be due to different etchant concentration, different etching time, and different etched materials.

Mode of failure of APF samples was mainly fracture with separation of the veneer from the underlying tooth structure unlike other samples that was mainly extensive crack formation in the restoration only or in the restoration tooth complex without debonding of occlusal veneers. SEM topography revealed catastrophic fracture of occlusal veneers in HF and MON groups, while in APF groups part of the restoration de-bonded after fracture showing etched tooth surface with remaining cement particles. Crack formation without debonding form of failure indicated that the bond of restoration-cement-tooth complex was stronger than the force applied. Meanwhile, separation of APF treated samples indicated that the bond at veneer/cement/prepared tooth interfaces was affected [42].

Regarding surface treatment, there was significant difference between APF-0.5 group and MON-0.5 group. However, there was no significant difference between other surface treatment groups at the same thickness. Hence, at 1 mm thickness any surface treatment can be used. But when it comes to 0.5 mm thickness, APF is not recommended and Monobond etch & prime is preferred.

Regarding the interaction between thickness and surface treatment, there was not statistically difference in the current study (p = 0.785). By far in the English language literature, we could not find previous studies investigating the interactive relation between thickness and surface treatment, therefore further research should be conducted.

Limitation of this in-vitro study could be the fact that bonded specimens were stored in water only. However, in the clinical situation the bonded restoration is exposed to saliva and other beverages. In addition, bonded restoration intraorally subjected to variation in temperature, therefore, thermal cycling is needed in further investigation. Also, it is an invitro study so long-term clinical study should be conducted to support the results of this invitro study.

Within the limitation of this invitro study, the following conclusions were drawn: