Dental caries remains one of the most common preventable diseases worldwide, although current methods like fluoride applications and fissure sealants are well known to be effective in prevention to a large extent.^[1] Non-prevented and treated caries lesions’ rapid progression causes pain and tooth loss, which adversely affects the quality of life. Conventional treatment of dental caries is performed by removing the decayed tissue and placing a restoration whose longevity depends on its size and sealing.^[2]

Removing noncarious tooth area to achieve an adequate bonding surface leads to fracture and failure of the restorations.^[3] If the lesion requires restoration of over one-third of the intercuspal distance, the cusp fracture susceptibility increases.^[3]

In recent years, the most accepted minimally invasive dentistry approach is to choose the treatment procedures that consider the natural tissues. One of the crucial principles of the minimally invasive approach is minimizing the risk of recurrent disease.^[4]

Given the growing tendency for minimally invasive dentistry, partial restorations have become an alternative to conventional treatment procedures. These treatment options have enabled minimal invasive tooth preparation and consequently greater preservation of the dental tissues.‌^[4] Partial restorations are classified as inlays (not covering the cusps), onlays (covering at least one cusp), and overlays (covering all cusps) according to the area to be restored. The use of CAD/CAM is important for reasons such as the difficulty of taking measurements in the conventional workflow, short treatment time (chair-side), accurate output profile with multiple axis cutting, and aesthetic formation.^[5]

There is unequivocal evidence that secondary caries is the most common cause of restoration failure. It is essential to prepare the pathogenic non-bacteria cavity prior to restoration to avoid any secondary caries formation.^[6]

A significant decrease in the prevalence and severity of dental caries has been reported with the introduction of fluoride-containing oral products.^[7] However, it is also known that using oral products containing fluoride alone is not sufficient. In addition, economic status and diet are also known important factors of dental caries. With the increase in the consumption of free sugar and processed carbonated foods, inequalities in the prevalence of caries were observed, especially among groups with socioeconomic differences.^[8–10]

It is known that silver compounds have been used in medicine for many years to control infections.^[11] While silver has antibacterial effects, fluoride has a mineralizing effect. Silver and fluoride are combined to form a clear liquid, silver diamine fluoride (SDF), which has both antibacterial and remineralizing properties.^[12–14] This compound inhibits the propagation of caries by inhibiting the Colony Forming Units (CFU) proliferation, especially of mono-species strains of S. mutans and Actinomyces naeslundii.‌^[15] However, SDF increases the mineral content of dental hard tissues and promotes calcium absorption. Therefore, since SDF increases the mineral content of the teeth and supports calcium absorption, it is very effective in the treatment of caries lesions and causes high surface microhardness.^{[16, 17]}

SDF [Ag(NH₃)₂F] is a topical agent that has recently gained popularity due to its effectiveness in stopping and preventing dental caries.^[18] It allows for a more conservative tooth preparation because it is minimally invasive, arresting remaining decay and remineralizing the affected dentin.^[19]SDF can also be used as a cavity disinfectant prior to restoration with the antimicrobial effect of silver and the remineralization effect of fluoride.^[20]

Because SDF is an effective way of treating active lesions, its application has also been recommended during the SARS-Cov-2 pandemic period and has been useful in reducing the aerosol-generating procedures.^[21]SDF has been approved by the US Food and Drug Administration (FDA) for use in the United States^[22] and has been widely used for the prevention of primary tooth decay in children, the prevention of cavities and fissures of molars in adults, and the prevention of caries in the roots of teeth in geriatric patients^{[16, 23, 24]}.

Mineralization is a process that continues from birth to death. In this process, an inorganic substance is accumulated on organic matrix. Although the accumulation process of minerals is still not known, there are serious studies on this subject in the literature. Understanding the accumulation process of minerals will enable the prevention of mineralization-related disturbances.^[25] When teeth are investigated, their anatomical structure consists of enamel, pulp-dentin complex, and cementum. Of these sections, the dentin section is a 70% mineralized section, and is also the largest section of the tooth.^[26]

It is important to prevent tooth demineralization and saliva, fluoride therapy, diet and probiotic bacteria are included in the process. In particular, the neutralizing effect of saliva has been used to protect the tooth exposed to acid. It has been reported in studies that the pathogenesis of dental erosion is closely related to the buffering capacity and secretion rate of saliva.^[27]

One of the most effective methods of preventing tooth decay is fluoridation of the teeth. Fluoridation of the teeth is carried out with the use of topical fluoride, such as toothpaste.^[28] As a mineral, florapatite (FAP) is known to be the most common phosphate mineral. In the formation of FAP, the calcium in hydroxyapatite is replaced with fluorine and forms FAP. FAP requires ten calcium ions and six phosphate ions for every two fluoride ions. Therefore, if there is insufficient calcium and phosphate, then the remineralization process may not occur. FAP has much lower soluble properties compared to hydroxyapatite.^{[29, 30]} To prevent demineralization, fluoride acts as a catalyst that helps remineralize enamel with phosphate ions dissolved in saliva.‌^{[31, 32]} Also, the replacement of hydroxide with fluoride eliminates the vulnerability to lactic acid in hydroxyapatite, so that FAP [Ca₁₀(PO₄)₆F₂] is not soluble in the mouth.^[33] It is known how important mineralization is for fracture resistance, and tests have been developed to measure fracture resistance.^[34–37]

Fracture resistance test was performed because we thought that it would change the dentinal tubules, elasticity, and fracture strength on the surfaces where SDF was applied. It is known that the bond strength between the dentin-applied cement with SDF and the dentin surface is higher than the bond strength between the cement without SDF and the dentin surface. Multiple previous studies showed the effect of SDF on the shear bond strength and micro-tensile bond strength of different restorative materials on sound dentin.^[38] However, there is a research gap in the effects of SDF application on the fracture resistance of restorations in the available literature. Therefore, the null hypothesis of the present study is that SDF is not effective on fracture resistance of the feldspathic glass ceramic inlay restorations.

The aim of the study was to investigate whether the 38% SDF solution applied prior to under ceramic inlay restorations affects the fracture resistance of the restoration.

Fig. 2 shows the fracture force between the groups with and without SDF. The 2-way ANOVA revealed a nonsignificant difference between the two groups [fracture force (N)±standard deviation: without SDF 1138.19±581.65 and with SDF 1067.93±555.65; p=0.712] (Fig. 2).

Dental fracture is a common clinical problem in posterior teeth due to high bite forces or inappropriate contact with opposing teeth.^[42] Also, it is well known that enlarging in the tooth structure loss increases the susceptibility to fracture.^{[3, 42]} To avoid extended tooth structure loss, minimally invasive dentistry (MID) and an approach focusing primarily on preventing and controlling oral diseases have been accepted.^[43] Application of SDF has been recommended as part of this approach since it arrests and prevents the formation of new caries, either solely or under restorations.^{[1, 19]}

Another beneficial approach for caries arrest is Silver Modified Atraumatic Restorative Technique (SMART), which allows the SDF application on the same appointment before restoration. Alvear et al. reported that SMART helps to eradicate cariogenic bacteria and promotes remineralization.^[44]

The effects of applying SDF under restorations on new caries development and the bond strength of restorative materials are well documented.^[1] However, there is a research gap on its effect on fracture resistance of restored teeth. Therefore, the present study aimed to examine the effect of SDF use immediately before restoration on the fracture resistance of restored teeth. We applied SDF solution before the cementation procedure of the CAD-CAM ceramic restorations. During the cementation procedure, 37% phosphoric acid gel was applied only on the enamel surface for 20 seconds to increase adhesive strength and then washed with water for 20 seconds. SDF solution was still effective on the tooth surface after this procedure because normally, in SDF applications, it is applied in open cavities on the surface of the tooth and is waited for 1 minute. Excess SDF is removed with cotton. There is no need to take care that the material does not come into contact with water and contact with water does not affect the material.

From the perspective of MID, if the prevention has failed, then restorative procedures respecting the patient’s natural tissues should be chosen.^[43] Depending on the loss extent, various direct and indirect treatment options with composite resin and ceramic materials are available for caries of the posterior teeth.^[45] It is worth noting that a restorative material while replacing the lost tooth structure should increase fracture resistance and provide marginal sealing.^[46] However, polymerization shrinkage stress of the direct composite resins causes a gap between the cavity walls and the filling material. Consequently, bacterial infiltrations through the passage of fluids from this gap result in secondary caries.^[46] Although this complication seems to be solved by lower polymerization shrinkage bulk-fill composites, they have inferior to conventional resins in terms of wear resistance and fracture toughness.^[46] Indirect inlay/onlay techniques have been developed to overcome the mentioned problems of direct restorative procedures, enabling lesser tooth preparation and better preserving the dental tissues.^[3] In this in vitro study, all-ceramic (leucite-reinforced glass-ceramic, Gulsa) restorative material was used because it is considered an excellent option for esthetic restorations.

Thermal cycling is an aging procedure that exposes the sample to extreme temperatures to simulate thermal stresses typically occurring in the mouth.^[47] According to ISO TRY 11450 (1994) standards, immersing the specimens in water baths at least 20 seconds of 5°C and 55°C for 500 times and transfer time between baths of 5-10 seconds is a suitable accelerated aging method. However, this cycle number is insufficient to imitate the tooth bonding efficiency of the restoration.^[48] Therefore, the specimens, in the present study, underwent 5000 thermo-cycles between 5°C and 55°C with a dwell time of 30 seconds and a transfer time of 15 seconds.

Shimizu and Kawagoe reported that recurrent caries did not develop after 26 months in the SDF-pretreated primary teeth.^[49] Mei et al. found a reduction in secondary caries under the composite resin and glass ionomer cement restorations following conditioning with SDF.^[16] Teeth roots were embedded in acrylic cylinders to mimic alveolar bone, preventing stress-intensification. The compressive load fracture test is beneficial for assessing the ultimate resistance to fracture of restored posterior teeth. The method enables quantifying the efficacy of different factors of restoration procedures.^[42] The null hypothesis was accepted since the application of SDF did not affect the fracture resistance of restored teeth.

This in vitro study has some limitations in that it simulates clinical conditions. In fact, clinical fracture is due to fatigue caused by multidirectional cyclic loading. In this study, samples were subjected to thermal cycling aging protocol and then compression loads. In addition, adhesive resin cement is indicated for ceramic restorations. However, we used G-CEM LinkAce, a self-adhesive resin cement. It can be considered a limitation by affecting the integrity of the tooth-restoration complex. Therefore, we pre-treated the enamel before bonding to increase the bond strength of self-adhesive resin cements. It can be used as a fast cementation material of choice for patients who do not have coperitis in pediatric dentistry.

SDF did not have a significant, either negative or positive, effect in terms of fracture force following restoration and application in cavities in teeth tested in both groups. Long-term clinical studies with different mechanical tests are needed for the safe use of SDF in pre-restoration cavities with CAD/CAM blocks.