2. Introduction
Three-dimensional (3D) printing technology has revolutionized restorative dentistry by enabling the precise and customized fabrication of complex dental restorations. Among the available materials, 3D-printed composite resins have gained increasing attention due to their favorable mechanical strength, aesthetic properties, and adaptability. However, the clinical success of these materials is highly dependent on their ability to achieve durable adhesion to underlying tooth substrates [
1,
2]. Evidence suggests that both the mechanical performance and adhesive characteristics of composite resins are key determinants of their long-term functionality in dental applications [
2,
3]. Furthermore, the correlation between fracture toughness and bond strength underscores the need to optimize these parameters to enhance the reliability and longevity of 3D-printed restorations [
4].
Ongoing advances in dentistry, along with improvements in oral hygiene practices, have led to an increased demand for durable fixed prosthetic restorations, such as crowns and bridges. This shift reflects a growing emphasis on preserving natural dentition for extended periods [
5]. Innovations in ceramic materials and adhesive technologies have further supported this trend by enabling minimally invasive preparations that conserve healthy tooth structure while achieving excellent clinical performance and esthetic outcomes [
6].
Three-dimensional (3D) printing, also referred to as additive manufacturing, has emerged as a cutting-edge technique in modern dentistry, driven by advances in computer-aided design (CAD) technologies. This method enables the fabrication of highly precise and customized dental restorations, including surgical guides, splints, crowns, and bridges [
2]. The commercial potential of 3D printing dates back to 1986, when Charles W. Hull filed a patent for stereolithography (SLA), a photopolymerization-based technology [
7]. The production of the first printed object—an eye-wash cup—marked the inception of this transformative technology, which was further developed and commercialized by 3D Systems Corporation [
8].
Today, 3D printing technology has become integral to nearly all specialized branches of dentistry, offering transformative applications in both patient care and the education of dental professionals. It is widely utilized to fabricate surgical guides, occlusal splints, diagnostic models, removable partial denture frameworks, complete dentures, and provisional restorations such as crowns and bridges using polymer-based resins [
9]. The incorporation of ceramic fillers into resin-based printing materials has significantly improved the esthetics, mechanical durability, and biocompatibility of 3D-printed restorations. This advancement has enabled their use in permanent restorations, including crowns, bridges, veneers, inlays, and onlays. As new cost-effective resin formulations continue to emerge—engineered to meet specific clinical and material demands—3D printing continues to evolve as a powerful tool for delivering personalized and high-quality dental solutions [
10].
Establishing a strong and durable bond between dental restorations and tooth structures is critical for ensuring long-term clinical success. Resin-based adhesive systems are commonly employed in restorative dentistry to achieve reliable adhesion [
11]. These systems typically comprise multiple components, including primers, bonding agents, and, in some cases, filler-reinforced adhesives that enhance mechanical strength. Common fillers used in these adhesive materials include ytterbium trifluoride and barium aluminum fluorosilicate, which not only improve mechanical strength but also release fluoride ions, providing a potential cariostatic effect. The selection of an appropriate adhesive system depends on several factors, such as the restorative material—porcelain, ceramic, or composite—and the specific characteristics of the tooth preparation [
12]. When correctly applied, adhesive resin cements exhibit superior bond strength and lower solubility compared to conventional non-adhesive cements. However, meticulous isolation during clinical procedures is essential to maximize the performance and longevity of adhesive restorations [
13].
Surface treatment plays a critical role in enhancing the bond strength between dental substrates and restorative materials. Techniques such as air abrasion, sandblasting with aluminum oxide particles, and hydrofluoric acid etching are commonly employed to modify surface topography and chemistry, thereby promoting micromechanical retention and chemical adhesion [
14,
15]. The effectiveness of these treatments largely depends on the composition and surface characteristics of the substrate material [
15]. Pretreatment of ceramic surfaces has been shown to significantly improve the bonding performance of resin cements [
15]. Among these techniques, hydrofluoric acid etching is particularly effective, as it creates microporosities that increase surface area, improve wettability, and facilitate micromechanical interlocking at the adhesive interface [
15,
16].
Despite significant advancements in adhesive technology, achieving consistent and durable bonding between 3D-printed composite resins and tooth structures remains a clinical challenge. A key factor contributing to this issue is the limited research on appropriate surface treatment protocols for 3D-printed composites, resulting in a gap in understanding how such treatments influence shear bond strength. Successful bonding is influenced by several variables, including effective surface preparation, the selection of suitable resin cement, and precise curing techniques. While contemporary adhesive systems and meticulous clinical protocols help address these challenges, evidence regarding the optimal surface conditioning methods for 3D-printed composites remains sparse [
15,
17].
Therefore, this study aims to evaluate the effect of various surface treatments—including untreated (control), air abrasion, hydrofluoric acid etching, and their combination—on the shear bond strength of 3D-printed composite resin discs bonded to feldspathic ceramic rods using Panavia V5 resin cement. This investigation seeks to contribute to the evolving knowledge base surrounding surface treatment protocols for emerging 3D-printed dental materials.
Null Hypothesis: Surface treatments do not significantly affect the shear bond strength of 3D-printed composite resin compared to untreated control surfaces.
3. Materials and Methods
This in-vitro study is reported in accordance with the CRIS guidelines [
18]. The 3D-printed composite resin discs were fabricated using RODIN SCULPTURE 2 (Shade A3, Lot #312065; Pac-Dent, Inc., Brea, CA, USA). Feldspathic ceramic rods were prepared from VITA Mark II CAD/CAM blocks (Lot #91260; VITA Zahnfabrik, Bad Säckingen, Germany) and served as the bonding substrate. Panavia V5 resin cement (Lot #000149; Kuraray Noritake Dental Inc., Tokyo, Japan) was used as the luting agent for all specimens. Surface conditioning of the 3D-printed composite discs was performed using 9.6% hydrofluoric acid etching gel (Porcelain Etch Gel; PULPDENT Corporation, Watertown, MA, USA, Lot #161115). Details of all materials used are provided in
Table 1.
A total of 48 rectangular composite resin discs (2 mm thickness × 14 mm length × 14 mm width) were fabricated using ASIGA design software (Asiga Composer 2.0, Asiga, Sydney, Australia) (
Figure 1A) and printed with the ASIGA MAX UV (Asiga, Sydney, Australia) 3D printer (
Figure 1B). The discs were produced using RODIN SCULPTURE 2 composite resin (Shade A3, Lot #312065), known for its favorable esthetic and mechanical properties. Post-printing, all specimens were thoroughly cleaned in isopropanol to eliminate residual uncured resin, ensuring a clean surface for optimal bonding and mechanical performance (
Figure 1C). Final polymerization was performed using a Gas N2 4500 curing unit Otoflash G171 (NK-optiK, Baierbrunn, Germany). in a nitrogen atmosphere to prevent oxygen inhibition and promote complete curing of the material (
Figure 1D).
Following fabrication, the specimens were randomly divided into four groups (n = 12 per group) according to the surface treatment protocol. The control group consisted of specimens with untreated surfaces. In the air abrasion group, the discs were sandblasted with 50 µm aluminum oxide particles at a pressure of 2.5 bar from a distance of 10 mm. In the hydrofluoric acid (HF) etching group, the discs were treated with 9.6% HF gel (Porcelain Etch Gel; PULPDENT, Lot #161115) for 60 s, rinsed with water for 30 s, and air-dried for 20 s. The combination group received sequential treatment with air abrasion followed by HF etching as described above [
19].
The Vita Mark II ceramic blocks (Lot #91260) (
Figure 2A) were embedded in Buehler EpoxiCure 2 resin and mounted on a Palmgren 12″ drill press, Model 80150A (Palmgren, Naperville, IL, USA). to maintain stability during specimen preparation. Using a 3.5 mm diamond-coated drill bit (McMaster-Carr, Cat. 2868A23; Elmhurst, IL, USA) (
Figure 2B), cylindrical rods were drilled from the ceramic blocks at a controlled rotational speed under continuous water irrigation (
Figure 2C) to minimize heat generation and prevent structural damage. A total of 48 rods were prepared, each with a diameter of 3.5 mm and a length of 8 mm, ensuring uniform bonding interfaces and reproducible testing conditions. The rods were subsequently smoothed using 800-grit sandpaper, followed by 1200-grit fine polishing, to eliminate surface irregularities and enhance bonding reliability (
Figure 2D).
Each ceramic rod was bonded to the surface of a 3D-printed composite resin disc using Panavia V5 resin cement (Lot #000149). Prior to cementation, a ceramic primer (Clearfil Ceramic Primer Plus, Kuraray Noritake, Tokyo, Japan) was applied to both the ceramic rods and the composite resin discs, following the manufacturer’s protocol. This bonding procedure was standardized across all groups—untreated, air abrasion, hydrofluoric acid, and combination treatment groups—by applying a 5 N load (approximately 0.51 kg) for 5 min to ensure consistent cement thickness. Light curing was then performed for 20 s on each side using an LED curing unit (Bluephase G2, Ivoclar Vivadent, Schaan, Liechtenstein) with a light intensity of 1200 mW/cm
2, ensuring optimal polymerization of the resin cement [
19].
After preparation, all specimens were stored in distilled water at 37 °C for 24 h to simulate oral conditions before shear bond strength testing. All mechanical testing was performed at room temperature. Two specimens from each group, including the Vita Mark II rods, were examined for bonding integrity and cement layer thickness using a VHX-7000 digital optical stereomicroscope (Keyence Corp., Osaka, Japan). This analysis assessed variations in bonding performance and cement layer uniformity across the different surface treatments.
Two specimens from each surface treatment group underwent microscopic evaluation using a digital optical stereomicroscope (25–40× magnification). This assessment focused on cement layer uniformity, presence of voids, and bonding interface integrity.
Shear bond strength was evaluated for all groups, each consisting of 10 specimens, using an Instron universal testing machine (Model 5566A, Instron Corp., Norwood, MA, USA) with a 1-kN load cell. A half-round stainless-steel blade (4 mm diameter, 2 mm edge thickness) was centrally positioned on each specimen to ensure uniform loading (
Figure 3). The blade was aligned perpendicularly to the bonded interface between the Vita Mark II ceramic rods and the 3D-printed composite resin discs, and the load was applied parallel to this interface at a crosshead speed of 0.5 mm/min until failure occurred. Shear bond strength (MPa) was calculated using the formula: shear bond strength = F / (π × r
2), where F is the load at failure (in newtons) and r is the radius of the bonded area (in millimeters) [
20].
After shear bond strength testing, the failure mode of each specimen was examined under a stereomicroscope at 25–40× magnification. Failure modes were classified into three categories: adhesive failure at the interface, cohesive failure within the composite or ceramic, or mixed failure involving both interface and material fractures. This analysis provided insights into the nature of the bond failures and the effectiveness of the different surface treatments.
Differences in shear bond strength among the four surface treatment groups were analyzed using one-way analysis of variance (ANOVA) to determine whether the type of surface conditioning had a statistically significant effect on bond strength. When ANOVA indicated significance, pairwise comparisons were performed using Tukey’s Honestly Significant Difference (HSD) post-hoc test. The significance threshold for all analyses was set at p < 0.05. Statistical evaluations were conducted using IBM SPSS Statistics software version 29.0.2.0 (20) (IBM Corp., Armonk, NY, USA).
4. Results
The shear bond strength of 3D-printed composite resin discs was evaluated following four distinct surface treatment protocols: untreated (control), air abrasion, hydrofluoric acid etching, and a combination of air abrasion and hydrofluoric acid etching. Significant differences in bond strength were observed among the treatment groups.
In the untreated group (control), the mean shear bond strength was 24.35 ± 6.77 MPa (range: 15.54–33.77 MPa). The air abrasion group exhibited a slightly lower mean bond strength of 19.07 ± 8.14 MPa (range: 8.56–31.55 MPa). In contrast, the hydrofluoric acid etching group demonstrated a substantial increase in bond strength, with a mean of 37.79 ± 9.25 MPa (range: 28.01–54.87 MPa). The combination treatment group recorded the highest bond strength, with a mean of 40.73 ± 11.53 MPa (range: 28.29–55.21 MPa) (
Table 2).
To statistically evaluate the effects of the surface treatments on bond strength, one-way ANOVA was performed. A one-way ANOVA revealed significant differences in shear bond strength among the surface treatment groups (F = 13.159,
p < 0.001) (
Table 3). Post-hoc Tukey’s Honestly Significant Difference (HSD) tests indicated no significant difference between the untreated and air abrasion groups (mean difference = 5.28 MPa,
p = 0.570), suggesting that air abrasion alone did not enhance bond strength. In contrast, hydrofluoric acid etching significantly increased bond strength compared to both the untreated group (mean difference = −13.44 MPa,
p = 0.011) and the air abrasion group (mean difference = −18.72 MPa,
p < 0.001). The combination treatment also yielded a significant improvement over the untreated group (mean difference = −16.38 MPa,
p = 0.002) and the air abrasion group (mean difference = −21.66 MPa,
p < 0.001). However, no significant difference was observed between the hydrofluoric acid etching and combination treatment groups (mean difference = −2.95 MPa,
p = 0.887) (
Table 4).
Failure mode analysis, performed using a stereomicroscope, revealed that adhesive failure at the interface between the ceramic rods and the 3D-printed composite resin was the predominant type across all groups. In most specimens, the resin cement remained adhered to the ceramic rods, with only minor residual cement on the composite surfaces. This indicates that failure consistently occurred at the bonded interface, with occasional cohesive fractures within the cement layer. While surface topography analysis (e.g., SEM) was beyond this study’s scope, the failure mode evaluation provides indirect yet clinically relevant insight into the bond interface, complementing the mechanical resistance results.
Microscopic examination revealed no noticeable differences in bonding characteristics—such as continuity of the cement layer, presence of voids, or interface separation—among the 3D-printed composite discs, Vita Mark II ceramic rods, and Panavia V5 resin cement across all surface treatment groups. Slight variations in cement layer thickness were observed within individual specimens and between groups, likely due to minor inconsistencies in sample preparation or inherent surface irregularities of the ceramic rods (
Figure 4).
5. Discussion
Strong adhesion between restorative materials and tooth structures is crucial for the long-term success of dental restorations. This study emphasizes the pivotal role of surface treatments in enhancing adhesion, particularly in 3D-printed composite resins, a rapidly evolving material in dental technology. Our findings provide clear evidence that hydrofluoric acid etching, either alone or in combination with air abrasion, significantly improves the shear bond strength of 3D-printed composite resin discs. These findings underscore the effectiveness of chemical surface treatments in enhancing the bonding performance of emerging 3D-printed dental materials. In this discussion, we will review the results and analyze the influence of each surface treatment on bond strength.
The results of this study demonstrated that surface treatments significantly influenced the shear bond strength of 3D-printed composite resin discs. Hydrofluoric acid etching, both alone and in combination with air abrasion, resulted in significantly higher bond strengths compared to no surface treatment and air abrasion alone. The improved bond strength associated with hydrofluoric acid etching can be attributed to its ability to create a micro-retentive surface, thereby increasing surface roughness, facilitating mechanical interlocking, and enhancing the adhesion of resin cement to the composite material [
21].
The lack of a notable improvement in bond strength with air abrasion alone for 3D-printed composite resin suggests that, although mechanical roughening increases surface texture, it is insufficient to significantly enhance adhesion without chemical modification. Air abrasion provides physical roughness but does not chemically alter the composite surface, thereby limiting its contribution to bonding effectiveness [
21,
22]. The slightly reduced bond strength observed in the air abrasion group, although not statistically significant, further indicates that mechanical roughening alone is of limited benefit. In contrast, hydrofluoric acid (HF) etching significantly improved bond strength by generating a micro-retentive surface and introducing chemical alterations that enhance surface energy and facilitate stronger resin–substrate interactions. While direct evidence for 3D-printed composites is limited, findings from silica-based ceramics consistently highlight the superior efficacy of HF etching, supporting the extension of these principles to 3D-printed composites [
23,
24,
25].
The distinction between HF etching and air abrasion lies in their different modes of surface modification. HF etching creates nano-micro porosity, irregular layers, and unsaturated oxygen bonds that chemically interact with phosphate monomers in resin cement, enabling both micromechanical and chemical adhesion. Air abrasion, in contrast, primarily produces grooves that enhance mechanical interlocking but lack chemical conditioning. This explains why the bond strength values of HF alone were comparable to those of the combined treatment (air abrasion + HF). When HF was applied after sandblasting, the improvement was largely attributable to the chemical effects of etching, with air abrasion providing minimal additional benefit. Collectively, these findings reaffirm that HF conditioning is the most effective method for optimizing interfacial adhesion in such materials because it combines micromechanical retention with chemical bonding. To fully elucidate these mechanisms, future studies should incorporate detailed surface characterization techniques—such as SEM, AFM, or EDS—to clarify the microstructural and chemical changes underlying these adhesion improvements [
21,
22,
23,
24,
25].
The variation in cement thickness observed during the interface analysis provides a clear explanation for the wide range of shear bond strength values and large standard deviations noted in this study. Thicker cement layers can lead to void formation and uneven stress distribution. The variation in cement thickness observed during interface analysis offers a clear explanation for the wide range of shear bond strength values and the large standard deviations reported in this study. Thicker cement layers can lead to void formation and uneven stress distribution, which weakens overall bond strength. In contrast, thinner cement layers allow for more uniform stress distribution, thereby enhancing bonding efficiency. This observation aligns with previous studies suggesting that thinner cement layers—typically between 50 and 150 µm—are associated with higher bond strengths [
26,
27]. Additionally, the irregular bonding surfaces of the Mark II rods may have contributed to the observed variability. Surface unevenness can disrupt consistent cement flow and distribution during bonding, further exacerbating inconsistencies in bond strength [
28]. These factors—variation in cement thickness and the irregularity of the rod surfaces—underscore the importance of controlling both cement application and bonding surface preparation. Ensuring uniform cement thickness and addressing surface irregularities are essential for achieving reliable adhesion, especially when working with advanced materials like 3D-printed composites. By standardizing these aspects, variability in bond strength can be minimized, resulting in more predictable and robust bonding outcomes [
28].
One of the strengths of this study is the use of standardized feldspathic ceramic rods (Vita Mark II) as bonding substrates. This approach provided a controlled and reproducible testing environment, reducing variability inherent in natural tooth structures and allowing for a precise evaluation of the surface treatment effects. Although natural teeth offer greater clinical relevance, they introduce variability related to composition, age, and preparation technique. The use of a uniform ceramic substrate minimized these confounding factors, ensuring that differences in bond strength could be more confidently attributed to surface treatment protocols. However, this also presents a limitation, as the results may not fully replicate clinical conditions. Future research should incorporate natural tooth substrates to validate these findings under more realistic intraoral environments.
In this study, shear bond strength values ranged from 19.07 to 40.73 MPa, with all groups surpassing the ISO 10477 minimum requirement of 5 MPa for fixed restorations, confirming the reliable bonding potential of 3D-printed composite resins when properly conditioned. The biaxial flexural strength (103.29–113.60 MPa) also complied with ISO 10477 standards for anterior crowns (≥100 MPa), though it fell below the ISO 6872 thresholds for posterior crowns (≥300 MPa) and fixed partial dentures (≥500 MPa). These findings suggest that 3D-printed composites are currently most suitable for anterior or provisional restorations, whereas zirconia and lithium disilicate remain superior choices for high-stress, long-term applications. Looking forward, advancements in resin formulation, filler technology, and hybrid surface treatment protocols may improve their mechanical resilience and expand their use in more demanding restorative scenarios [
29,
30,
31].
In summary, this study shows that hydrofluoric acid etching is the most effective surface treatment for enhancing the bond strength of 3D-printed composite resins. Air abrasion alone offered little benefit, while variations in cement thickness and substrate irregularities contributed to performance variability. These results highlight the importance of chemical surface modification and controlled cement application in achieving predictable bonding. Future studies should validate these findings using natural teeth and assess long-term durability under clinical conditions.