Evaluation of multimode adhesion promoters with functional monomers without and with silica-coating for resin composite repair

Abstract

This study aimed to investigate the effect of universal primers on resin composite repair with and without silica-coating while assessing adhesion durability under various aging methods. Specimens were fabricated using multiple increments of composite resin (Clearfil Majesty Esthetic) placed into cylindrical forms (diameter: 3.2 mm, height: 4 mm) and photopolymerized. Specimens (N = 300, n = 15 per group) were randomly assigned to three groups for aging (control: fresh-dry, 6 months water storage and thermocycling ×5000 cycles, 5–55 °C). These three main groups were further randomly assigned to two groups according to the surface conditioning procedures; (a) Control: No air-abrasion, and (b) Air abrasion with silica-coating (Co-Jet, 30 µm, 5 s, 2.5 bar). All subgroups were then randomly divided into five subgroups to be treated with adhesion promoters (a) All Bond Universal: AB (Bisco), (b) Clearfil Universal Bond Quick: SKB (Kuraray), (c) Monobond Plus: MP (Ivoclar Vivadent), (d) G-Premio Bond: GP (GC), (e) Clearfil Universal Bond: CU (Kuraray). After bonding according to the manufacturer’s instructions to new composite blocks, shear bond strength tests were conducted using a Universal testing machine (1 mm/min). Obtained Data (MPa) were analyzed by 3-way ANOVA and Tukey`s post-hoc tests (alpha = 0.05). Bond strength results were significantly affected by the adhesive type (p < 0.05), silica-coating (p < 0.05), and aging (p < 0.001) but the interaction terms were not significant (p > 0.05). In non-aged groups (24 h), no significant effect on bond strength was observed using silica-coating (min; 8.41; max:12.98) but after 6 months, silica-coating increased the results for AB, GP, and CU ranging between 9.56 and 11.25 (p < 0.05) and even more after thermocycling for SKB, MP, GP, CU (10.23–13.95) (p < 0.05) compared to non-conditioned groups (6.83–10.37 for 6 months, 8.96–11.18 for thermocycling, respectively). Failure types were mainly adhesive for non-air abraded groups but exclusively cohesive after silica-coating. For immediate or short-term resin composite repairs, silica-coating may not improve adhesion but shear bond strength could be secured by this conditioning method after aging.

Keywords:

• Adhesion
• aging
• air-abrasion
• bond strength
• repair
• silane
• universal adhesives

Introduction

Today, many restorative material options are available for clinicians for restoring or replacing dental restorations. Unfortunately, in the aggressive oral environment, all restorative materials have a finite lifespan, and failures due to secondary caries, marginal gap formation, or fracture of an already existing restoration can be observed over a period of clinical function [1–3]. One restorative material alternative is resin composite which is commonly used in restorative dentistry and according to some research the most clinically preferred material of choice [4]. The successful use of direct resin composite restorations can be explained by its advantages, such as reduced cost and chair time compared to those of indirect restorative materials, the opportunity for non-invasive preparation design, acceptable aesthetics, the potential for minimizing microleakage through adhesive bonding to dentin and enamel and the possibility of repair [5,6].

Advances in dental adhesive technologies had an impact on the practice of modern restorative dentistry and also on the boarded implementation of the concept of minimally-invasive dentistry [7]. Since the approach of ‘extension for prevention’, proposed by GV Black in 1917, can no longer be justified, it has been replaced by the minimally-invasive treatment procedures in dentistry [3,8–11]. Adhesive systems have been further advanced to enable minimal invasive treatments in dentistry. To avoid a decrease in bonding ability throughout technique sensitivity, materials with less steps and higher chemical and mechanical properties were developed.

The modern adhesive systems with fewer application steps are available in a single bottle, known as self-etching adhesives, and cause substrate demineralization and monomer infiltration simultaneously [12,13]. However, the longevity of dental restorative materials is still limited and failures in the form of fractures, wear, or discoloration can still be encountered in clinical practice. Therefore, dental restorations require constant retreatment procedures [14,15]. In such failure situations, clinicians can either replace the failed restoration completely or repair it. Repairing allows a more conservative approach to the tooth structure, takes less time, and maybe less distressing for patients when compared to complete replacement of the restoration. Previous research has also reported that a complete replacement of the failed restoration is not considered an appropriate treatment in the majority of the clinical cases [16,17] due to possible trauma to the healthy hard dental tissues as the old restoration has to be removed by drilling [16,17].

When there are defects in the restoration, resin composite materials allow for minimally-invasive repair measures, whereby large parts of the original restoration are retained. Such repairs are minimally-invasive options and they can prolong tooth retention in the long term [18]. Especially in student clinics due to lack of experience, shade and form corrections are performed which is also considered some form of early or immediate repair. On the other hand, in repair procedures of an old resin composite, it is often the case, that the resin composite has been exposed to the oral environment for some time. During this time, the composite absorbs water by diffusion through the resin phase and also through the interfaces between the organic resin matrix and the inorganic filler particles [19]. This is likely to decrease the adhesion of the new composite to the aged composite in the oral environment. In case of a failure and repair of a composite restoration, the new placed layer exhibits a high unreacted C=C double bonds population as a result of an oxygen inhibition layer facilitating cross-polymerization while the lack of unpolymerized surface layer reduces the next layers bond strength [8,11,20]. Since an unpolymerized surface layer is missing in aged restorations, different conditioning methods have been proposed for repairing procedures for a durable bond of aged and non-aged resin composite. These methods are either mechanical or chemical in nature or a combination of them both, which serve for maximizing the adherence of repair composite material to the existing composite restoration [17,20–22]. A literature review revealed that abrasion by airborne particles and etching of composite surfaces increases roughness, thereby affecting adhesion between composite layers [23–25].

A great variety of aging methods have been suggested for resin-based materials, such as water storage [26], boiling in water [27], and thermocycling at alternating temperatures [28]. However, as it has been reported previously, water is still being used extensively as an aging media for different periods of time varying between 24 h to 6 months (i.e. 3 weeks [20]; 20 days [24]; 24 h [29]; 21 days [30]; 6 weeks [31]; 24 h [32]; Kamel et al., 1 day, 3 months, 6 months [33]. The lack of standardization in aging conditions also creates variation in the reported data on resin composite repair. According to previous research, long-term aging in water aims for simulating the hydrolytic degradation occurring in the resin matrix in the intraoral conditions [34]. The bonding of adhesives and the type of tooth structure play a big role when it comes to the durability of adhesive restorations [35,36]. Water storage briefly leads to hydrolysis and a release of filler particles along with water uptake in the matrix of the resin composite material [37]. One other approach for practicing accelerated aging in adhesion research is thermocycling where thermal expansion causes stress in restorative materials as a result of different thermal expansion coefficients which eventually yield to stress and bond failures [38]. Some studies suggest the use of adhesive resin to increase bond strength in case of failure [39,40] while studies favor airborne particle abrasion that significantly increases the bond strength [41–43]. Developments in the universal primers containing functional monomers are expected to eliminate the implementation of rather cumbersome air-abrasion protocols at the chairside but few are known about their longevity after substrate aging processes [44].

The goal of this study was the investigation of the multimode primers' effect on adhesion of resin composite in repair (immediate and aged) with and without silica-coating. The null hypothesis was: (a) there would be no significant difference in the substrate aging process on the bond strength, (b) air abrasion would have no effect on the shear adhesion of the repaired resin composites, and (c) there would be no significant effect in terms of bond strength between the different selected primer systems.

Materials and methods

The experimental flow chart is presented in Figure 1.

Figure 1. Flow-chart showing distribution of the experimental groups based on the aging methods, adhesive types, surface conditioning methods.

Table 1 shows the materials, brands, manufacturers, and chemical composition.

Table 1. Brands, abbreviations, manufacturers, and chemical compositions of the materials used in this study.

Brands	Abbreviations	Manufacturers	Chemical compositions
All Bond Universal	AB	Bisco, Schaumburg, IL, USA	Bis-GMA, HEMA, MDP, initiators, ethanol, water
Clearfil Universal Bond Quick	SKB	Kuraray Noritake Dental, Tokyo, Japan	Bis-GMA (10–25%), ethanol (10–25%), HEMA (2.5–10%), 10-MDP, hydrophilic amide monomer, colloidal silica, silane coupling agent, sodium fluoride, camphorquinone, water
Monobond Plus	MP	Ivoclar Vivadent, Schaan, Liechtenstein	MPTMS, 10-MDP, Disulfide dimethacrylate, Ethanol
G-Premio Bond	GP	GC Corp., Tokyo, Japan	4-MET, 10-MDP, MDTP, phosphoric acid ester monomer, thiophosphate monomer, dimethacrylate, butylated hydroxytoluene, acetone, water, photoinitiator, silicon dioxide
Clearfil Universal Bond	CB	Kuraray Noritake Dental, Tokyo, Japan	Bis-GMA, HEMA, MDP, camforoquinone, colloidal silica, silane coupling agent, ethanol, water, hydrophilic aliphatic dimethacrylate
Clearfil Majesty Esthetic	CM	Kuraray Noritake Dental, Tokyo, Japan	Bis-GMA, hydrophobic aromatic dimethacrylate, hydrophobic aliphatic dimethacrylate, di-camphorquinone, initiators, accelerators, pigments silanated barium glass filler, prepolymerized organic fillers

bis-GMA: bisphenol-A glycidyl methacylate; HEMA: 2-hydroksyethylmethacrylate; MDP, 10-MDP: 10-Methacryloyloxydecyl dihydrogen phosphate; 4-MET: 4-Methacryloyloxyethyl trimellitic acid; MDTP: Methacryloyloxydecyl dihydrogen thiophosphate.

Specimen preparation

Standardized disc-shaped resin composite substrate specimens (Clearfil Majesty Esthetic, Kuraray, Japan) of 2 mm height and 4 mm diameter were prepared using cylindrical hollow stainless steel. To maintain a flat surface Teflon molds were set on a glass plate of the specimens after polymerization and resin composites were condensed into a mold to a height of 4 mm. Each layer was photo-polymerized with a light-emitting diode (LED) polymerization device (LED Elipar 2, 3 M ESPE, St. Paul, USA) for 20 s. Light intensity was assured to be higher than 800 mW/cm², which was verified by a radiometer (Optilux Model 100, SDS Kerr Danbury, USA) after every 20 specimens. Thereafter, specimens were randomly assigned to three aging groups.

Substrate aging and surface conditioning

In the first group, surface conditioning and adhesive procedures were performed immediately on the substrate surfaces which acted as the control group (fresh-Dry). While in the second group the specimens were stored in distilled water at 37 °C for 6 months, the specimens in the third group were subjected to hydrothermal aging (5000 cycles, 5–55 °C, dwell time: 20 s, transfer time: 10 s; Haake DC 10, Sigma-Aldrich, St. Louis in Missouri, USA) before conditioning procedures.

Specimens in each of these groups were further randomly assigned into two groups according to the surface conditioning procedures; (a) Control: No air-abrasion, and (b) Air abrasion with silica-coating in circling motions using a chairside air-abrasion device (Dento-Prep™, RØNVIG A/S, Daugaard, Denmark) (30 µm, 5 s, 2.5 bar; Co-Jet Sand, St. Paul, USA). The sand particle remnants were removed by air-blowing after the air-abrasion treatment.

Application of adhesive systems

The bonding procedures with all of the adhesive systems tested in this study were carried out in accordance with each manufacturer’s instructions regarding the repair of resin composites by the same operator throughout the experiments. The aged resin composites specimens were conditioned with a corresponding adhesive system and the same resin composite (height: 3 mm; inner diameter: 3 mm) as the substrate resin was then polymerized using a Teflon matrix in two increments for 20 s each. One operator completed all adhesive procedures.

Shear bond strength test

Specimens were mounted in a jig of the Universal Testing Machine (Zwick ROELL Z2.5 MA 18-1-3/7, Ulm, Germany) and a shear force was applied using a shearing blade to the adhesive interface until debonding occurred. The adhesive interface of the bonded specimens was loaded at a crosshead speed of 1 mm/min. The applied load was set as close as possible to the surface of the substrate. Maximum load (N) was divided by the bonding surface area of the resin composite specimens to obtain the megapascal (MPa) values.

Statistical analysis

The statistical analysis was conducted using the software (SPSS software V.20, Chicago, IL, USA). Data were submitted to 3-way analysis of variance (ANOVA), Bonferroni and Tamhan’s T2 test with bond strength values and aging types (three levels: dry, 6 months, thermocycling), conditioning method (no coating vs. silica-coating), adhesive systems (five levels: AB, SKB, MP, GP, CU) as independent variables (p < 0.05).

Results

Bond strength results (MPa) were significantly affected by the adhesive type (p < 0.05), silica-coating (p < 0.05), and aging (p < 0.001) but the interaction terms were not significant (p > 0.05) (Table 2).

Table 2. The mean shear bond strength values (MPa) of all experimental groups.

Aging			Dry	6 months	Thermocycled
Adhesive promoters		Surface conditioning
All Bond	No silica-coating		12.98 ± 3.23^a,A	8.10 ± 2.99^b,A	11.08 ± 1.07^a,A
	Silica-coating		11.91 ± 1.99^a,A	11.25 ± 2.22^a,B	12.84 ± 2.88^a,B
Clearfil Universal Bond Quick	No silica-coating		10.40 ± 1.04^a,A	10.37 ± 2.48^a,A	11.18 ± 1.61^a,A
	Silica-coating		8.41 ± 2.56^a,B	11.79 ± 1.58^b,B	13.95 ± 3.66^c,B
Monobond Plus	No silica-coating		9.20 ± 1.81^a,B	6.83 ± 2.29^b,A	6.47 ± 2.34^b,A
	Silica-coating		9.74 ± 1.40^a,B	6.33 ± 3.50^b,A	12.54 ± 2.60^c,B
G-Premio	No silica-coating		10.64 ± 1.75^a,A	7.34 ± 2.74^b,A	8.96 ± 2.35^b,A
	Silica-coating		10.22 ± 1.64^a,A	9.56 ± 1.36^b,B	12.62 ± 2.88^a,B
Clearfil Universal Bond	No silica-coating		10.42 ± 2.48^a,A	8.30 ± 2.30^b,A	10.23 ± 2.16^a,A
	Silica-coating		10.58 ± 1.83^a,A	11.06 ± 2.63^a,B	13.94 ± 2.69^b,B

The same superscript uppercase letters in the same column and lowercase letters in one row indicate no significant differences between non-silica-coated and silica-coated conditions for each adhesive promoter (p < 0.05).

In the fresh-dry group, silica-coating did not show a significant effect on bond strength (8.41–12.98) (p > 0.05) but after 6 months, silica-coating increased the results for AB, GP, CU (9.56–11.25) (p < 0.05) and even more after thermocycling for SKB, MP, GP, CU (10.23–13.95) (p < 0.05) compared to the non-conditioned groups (6.83–10.37 for 6 months, 8.96–11.18 for thermocycling, respectively) (Figure 2).

Figure 2. The mean shear bond strength values (MPa) of all repaired resin composite groups as a function of substrate aging, adhesive systems and application of silica-coating. For group abbreviations see Table 1.

Regardless of the adhesive type, silica-coating increased the bond strength values compared to those that were not coated (p < 0.0001). This effect was more evident for SKB, MP, GP, and CU.

When the interaction between the factors ‘substrate aging’ and ‘adhesive systems’ was observed, the lowest mean bond strength was observed for MP applied on incubation in 37 °C distilled water for 6 months. Based on the ‘surface conditioning’ and ‘adhesive systems’ interaction, SKB and CB applied on air-abraded and thermo-cycled groups showed the highest bond strength. When MP was applied to non-treated and thermo-cycled specimens showed the least bond strength, as well as the air-abraded specimens incubated at 37 °C for 6 months (Figures 3(a–c), Figure 4(a-c)).

Figure 3. Percentage difference in bond strength without silica-coating between (a) dry-6 months, (b) dry-thermocycle, (c) 6-months-thermocyle aging for all adhesive promoters.

Figure 4. Percentage difference in bond strength with silica-coating between (a) dry-6 months, (b) dry-thermocycle, (c) 6-months-thermocyle aging for all adhesive promoters.

Failure types were mainly adhesive (70%) for non-air abraded groups but exclusively cohesive (100%) after silica-coating.

Discussion

Even if there have been huge developments made in reparative dentistry, failures in resin-based composite materials are still reported in the form of microleakage, fracture, wear, and secondary caries [20]. Complete replacement might result in further complications, such as loss of healthy substrates and pulpal trauma. Thus, priority should be given to restoration repair, as it is a more conservative approach than restoration renewal [45–48,50]. This study evaluated therefore the effect of adhesive promoters upon resin composite-composite adhesion for repair (immediate and aged) with and without silica-coating. According to the results, the null hypothesis could be rejected as adhesive type, silica-coating and aging significantly influenced the results.

Further, reports noted that the shear bond strength test can be used to ensure measurements of the maximum stress applied at the bonding interface during mastication as chewing movements are often exposing a combination of shear and tensile forces to the adhesive interface [51]. Shear tests may also develop non-homogeneous stress in the bonding interface and according to previous studies, the origin of failures is often the substrate itself and not the adhesive zone. Thus, these factors can cause an underestimation or a misinterpretation of the results [52]. Nevertheless, since the shear bond test presents less operator-related errors, this test was chosen in this study.

The null hypothesis, that there would be no significant difference between the aging groups was rejected, as significant differences in shear bond strength values could be observed. Some factors influence the repair process between aged and newly polymerized composite restorations. Such factors include chemical adhesion to fillers on the surface of the polymerized composite, micromechanical adhesion caused by monomer infiltration into the repaired composite, and the amounts of unreacted monomers in previously polymerized layers [44,53,54]. Specimens incubated at 37 °C in distilled water showed similar shear bond strength for SKB and lower one for AB, MP, GP, and CB compared to new dry specimens.

Specimens conditioned with primers AB, SKB, and CB remained the same after thermocycling, while MP and GP had lower shear adhesion values. Specimens conditioned and thermocycled with primers SKB, MP, and GP remained the same as specimens in the group stored in water for 6 months. The groups AB and CB sowed higher values after thermocycling compared to 6 months of storage. Shear adhesion value increase despite aging could be attributed to unreacted monomer in polymerized resin composite as a consequence of an oxygen inhibition layer. This layer might impact the chemical and mechanical properties of the bonding surfaces, as unreacted monomers tend to copolymerize with freshly applied layered composite resin. The fresh-dry group in this study represented the immediate repair situation, where corrections are made by relayering when the color or form of the resin composite restoration is not ideal.

The decrease in shear bond strength values of some groups may be explained by water absorption during the substrate aging process, which reduces the adhesion of the new composite layer to the aged composite in the oral environment [8,11,20,54]. Previous investigations have reported that the temperature of the oral cavity varies between 35 and 37 °C depending on food and beverages. Therefore, resin composite restorations are exposed to different temperature conditions simulating the oral environment [55,56]. These temperature variations in the oral cavity can lead to mechanical stresses and cracks in the resin composite materials, especially because of differences in the thermal expansion of the filler in the resin matrix and the matrix itself. All these processes can result in aging and clinical failure in the restorative material [57]. In in vitro studies, hydrothermal aging through thermal cycling can be used to imitate the intraoral temperature changes in worse-case situations. Several specimens in this study were exposed to thermal cycling for 5000 cycles between 5 and 55 °C before being repaired with the multimode adhesives and the resin composite resin used in this study. Aging in a clinical context affects the mechanical, chemical, and physical properties of the resin composite materials and consequently also the repairability of these materials [58]. It has also been reported that 5000 cycles correspond to a period of 6 months of in vivo aging [59]. Overall, it has been previously reported that thermal cycling leads to a reduction in bond strength [60]. The majority of the multimode adhesives used in this study yielded to shear bond strength values, which almost remained the same or even surprisingly increased after thermocycling. Kusdemir et al. reported a similar observation in their studies and have explained this phenomenon with a higher degree of 55 °C during thermal cycling that might enhance the degree of polymerization and thereby physical properties at the repair interface [44].

While thermocycling creates hydrolysis due to temperature changes in the presence of water, which can be detrimental to adhesive interfaces representing the worst-case scenario, water aging on the other hand results in water uptake and thereby results in another type of aging. Also, settings of thermocycling at different research centres main not be comparable, whereas boiling techniques are considered as more reproduceable aging methods for such in-vitro adhesion studies [61,62].

For a sufficient repair strength of resin composite restorations, enhancement of bond strength between the new and old composite layer is crucial. Various methods can be used in the repair process of resin composite restorations which lead to mechanical retention, the increased surface roughness of the old composite layer, and also to improved mechanical interlocking [63]. Acid etching [23,24], aluminium oxide (Al₂O₃) airborne particle abrasion [44], and silica-coating using CoJet application [64] are some of the techniques which have been investigated previously. Clinicians often abrade the old composite surface with aluminium oxide (Al₂O₃) particles or roughen with diamond burs before repairing resin composite restorations as these methods are cos-effective and the latter do not require additional armamentarium [48,63,64]. While diamond burs lead to more macro-retentive and irregular features of the surface, air-abrasion forms more homogeneous surfaces with micro-retentive features. Thus, the adhesion area produced after air-abrasion was reported to be higher [65]. Therefore, the surfaces of aged composites were roughened using a chairside air-abrasion device. Silica-coating did not increase the bond strength for all adhesion promoters tested when immediate resin composite repair was performed [66,67]. However, air abrasion using alumina particles coated with silica had a significant effect on shear bond strength of repaired resin composites regardless of the materials used. Therefore, the second hypothesis was also rejected. Silica-coated specimens showed higher bond strength values than non-treated groups and the aged substrates benefitted, even more, from this conditioning effect [41,43,63,68].

Considering the interaction between the factors ‘surface conditioning’, ‘adhesive system’, and ‘aging procedure’, all primers used in this study applied on air-abraded and thermocycled specimens showed the highest shear bond strength values. AB and SKB applied on silica-coated specimens, which were stored in distilled water at 37 °C for 6 months showed increased shear bond strength values, while MP led to a decreased value. Shear bond strength values of GP and CB remained the same in this context. Considering the interaction between MP and air-abrasion, surprisingly MP showed a lower value after 6 months of aging in distilled water at 37 °C and a higher value after being thermocycled. One reason for this could be hydrolytic degradation of the silane in the composition. This multimode adhesive contains 10-Methacryloyloxydecyl dihydrogen phosphate (MDP) and methacryloyloxydecyl dihydrogen thiophosphate (MDTP) along with silane which possibly made siloxane bonds with the silica layer of the air-borne particles of CoJet sand.

Several studies examined the influence of adhesive systems and also the effects of mechanical roughening methods on resin composite surfaces to find out whether a sufficient joint between old and new resin composite surfaces is possible to achieve. Wendler et al. reported several mechanisms to achieve an adequate joint, namely micromechanical retention by penetration of the freshly applied monomers into the irregularities of the roughened surface, chemical bonding of these monomers to the matrix, and/or to the exposed filler particles. [65]. Furthermore, the research group also reported that air-abrasion of the surface can contribute to create a micro-retentive surface, which enables mechanical interlocking of the new adhesive material. According to the investigations of Özcan et al. silica-coated particles instead of pure Al₂O₃ have shown to enhance the chemical bonding by the coating of the silica content on the surface, especially if a silane coupling agent is used before the application of an adhesive system. It also has to be noted that the silica-coated groups showed exclusively cohesive failures indicating the reliability of the adhesive joint. The reason for adhesive failures in the non-treated group could be due to the lack of reactive free monomers on the substrate surface. Hence, several studies suggest air-abrasion as an effective pretreatment as air-abrasion coats the surface with silane and make it more reactive for the methacrylate groups of the repair composite resin [34,69]. Since silanes are functional monomers, they act as a binding agent between the inorganic filler particles and the organic matrix and are therefore preferred in repair processes. By modifying the surface energy, silanes can also increase the wettability of the surface [49]. In this study, universal primers, such as CB and SKB contain silane coupling agents and therefore the use of these primers in combination with air-abrasion activates the methacrylate groups in repaired resin composite and may enhance the adhesion at the repaired joint even after being aged by thermocycling.

Among the adhesive systems, there were significant differences but some performed similar results in terms of bond strength. The primers SKB and CB in the present study showed the highest shear bond strength values after being thermocycled. These two primers also contain silane, which could have reacted with the silica and presented improved repair bond strength. These findings are also in parallel with the results of other studies by Kusdemir et al. [44] and Yao et al. [70]. Both research groups could observe that silane molecules can be hydrolyzed to silanol, adhere to the resin composite surface, and react with the monomers in resin composite which lead to an increase in bond strength values.

It also has to be noted that the duration may certainly affect the silica amount deposited at the surface. Yet, prolonged duration of air-abrasion results in material loss from the resin composite surface. The concavity on the surface can then impair the adhesion results. Therefore, different from the zirconia or metallic surfaces polymeric surfaces cannot be air-abraded too long. One option could be to decrease the pressure but increase the duration of air-abrasion which needs to be investigated to optimize conditioning composite surfaces.

In this study cohesive strength of the resin composite was not tested which is a limitation of the study but the cohesive strength of composite resins was previously reported to be in the range of 15 to 17 MPa [71,72].

The results obtained in this study did not exceed these values indicating that repair strength is still weaker than the cohesive strength of the material tested. Nevertheless, the results of this study showed that air-abrasion could not be eliminated from the repair protocol. Thus, future adhesives with more effective functional monomers merit further research.

Conclusions

From this study, the following could be concluded:

1. Silica-coating did not increase the bond strength for all adhesion promoters tested when immediate resin composite repair was performed.

2. Repair bond strength on 6 months or hydrothermally aged composite substrates increased when the substrate surfaces were silica-coated with 30 µm particles.

3. For the repair of resin composite, silane primers can be used effectively, but silica-coating particularly enhanced the adhesion to aged substrates.

4. Failure types were exclusively cohesive in silica-coated groups.

Clinical relevance

Surface conditioning with alumina particles coated with silica improves composite-to-composite adhesion during repairs. Although immediate repair of plastic composites cannot benefit from silica coating, for aged plastic composites it increases the bond strength of all adhesion promoters tested.

Author contributions

KNK and MÖ: conceptualization, methodology, validation, formal analysis, and investigation. MÖ: resources. KNK, NA, and MÖ: data curation, writing—original draft preparation, writing—review and editing, and visualization. MÖ: supervision, project administration, and funding acquisition.

Acknowledgements

The authors acknowledge Kuraray Europe GmbH, Hattersheim am Main, Germany, for the generous provision of the materials used in this study and Mr. Trottman, University of Zurich, Center of Dental and Oral Medicine, Zürich, Switzerland, for his assistance with the experiments.

Disclosure statement

No potential conflict of interest was reported by the author(s).