The Effect of Surface Treatments on the Shear Bond Strength of Polymer-Based Material for Pediatric Crown Alternatives

Jihyun Kim; Jieun Han; Gimin Kim; Jaesik Lee

doi:10.5933/JKAPD.2025.52.1.89

J Korean Acad Pediatr Dent > Volume 52(1); 2025 > Article

Kim, Han, Kim, and Lee: The Effect of Surface Treatments on the Shear Bond Strength of Polymer-Based Material for Pediatric Crown Alternatives

Original Article

J Korean Acad Pediatr Dent 2025;52(1): 89-101.

Published online: February 21, 2025

DOI: https://doi.org/10.5933/JKAPD.2025.52.1.89

The Effect of Surface Treatments on the Shear Bond Strength of Polymer-Based Material for Pediatric Crown Alternatives

Jihyun Kim¹

, Jieun Han¹

, Gimin Kim¹

, Jaesik Lee^1,²

¹Department of Pediatric Dentistry, School of Dentistry, Kyungpook National University, Daegu, Republic of Korea

²Institue for Dental Research, Kyungpook National Dental Hospital, Daegu, Republic of Korea

Corresponding author: Jaesik Lee Department of Pediatric Dentistry, Institute for Dental Research, Kyungpook National University, 2177, Dalgubeol-daero, Jung-gu, Daegu, 41940, Republic of Korea
Tel: +82-53-600-7201 / Fax: +82-53-426-6608 / E-mail: leejs@knu.ac.kr

Received October 21, 2024 Revised November 23, 2024 Accepted November 27, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Polyetheretherketone (PEEK) crowns offer a provisional option for esthetic restorations in pediatric patients. PEEK has mechanical properties similar to dentin but exhibits low bond strength with luting cements. This study aimed to evaluate the effect of different surface treatments on the bond strength between PEEK and four luting cements by measuring shear bond strength (SBS). Sixty specimens were divided into three surface treatment groups: control, sandblast, and 98% sulfuric acid, and further divided into four cement subgroups: methyl methacrylate (MMA)-based resin cement, composite-based resin cement, resin-modified glass ionomer cement (RMGI), and glass ionomer cement (GI). Failure modes and treated surfaces were examined using scanning electron microscopy (SEM), and wettability was assessed through water contact angle. Both surface treatment methods showed significantly higher SBS than the control. The combination of sulfuric acid treatment with MMA-based resin cement demonstrated the highest bond strength (11.25 ± 1.86 MPa), while the second highest value was observed with sandblasting combined with MMA-based resin cement. These values were significantly higher than other groups and exceeded the clinically acceptable SBS threshold of 10 MPa. SEM analysis revealed that sandblasting created irregular fissures with large grooves and cracks for improved micro-retention, while sulfuric acid treatment produced a complex fiber network with sponge-like porosities. Within the limits of this in vitro study, MMA- and composite-based resin cement application with sandblast or sulfuric acid surface treatment proved effective bonding methods.

Keywords: Polymer-based material, Polyetheretherketone, Pediatric crown restoration, Shear bond strength, Surface treatment

Introduction

Stainless steel crowns are the most commonly used pediatric crowns; however, their metallic appearance has led to dissatisfaction among some parents [1]. In recent times, parents place greater value on the aesthetic importance of primary teeth, even though these teeth will eventually exfoliate, and the focus on aesthetics in pediatric dental care continues to grow [2,3]. Although zirconia crowns present a more visually pleasing alternative, they come with drawbacks like the risk of wearing down opposing teeth and their higher costs [4]. As the search for the ideal aesthetic crown continues, polyetheretherketone (PEEK) crowns are becoming a promising alternative.

Polyetheretherketone (PEEK), a linear thermoplastic, aromatic, and semicrystalline polymer, has gained interest for applications in dentistry due to its excellent thermomechanical characteristics, chemical stability, biological inertness, stability with almost all organic and inorganic chemicals, good strength, stiffness, toughness, and fatigue properties [5]. PEEK is currently used in dental clasps, healing abutments, transitional abutments, and as an alternative rigid material in removable partial dentures and fixed dental prostheses. It has also shown considerable potential for use in metal‑free and ceramic‑free fixed prostheses due to its ability to withstand masticatory forces in the posterior region [6]. Another advantage is that it does not cause wear of the opposing natural dentition when used as a crown or a bridge due to its low modulus of elasticity (3 - 4 Gpa), similar to dentin [7]. PEEK can be contoured or trimmed with regular burs without complexities[8].

Despite its numerous advantages, the use of PEEK has been limited by its hydrophobic, chemically inert nature and low surface energy. As adhesion between luting cements and crown materials is an essential property of dental restorative materials, various surface treatment methods and luting cements have been investigated to address these challenges [9]. However, there is no established protocol for crown cementation and surface treatment methods. Therefore, this study aimed to assess the impact of various surface treatment methods on the shear bond strength (SBS) of PEEK, as well as to determine combinations of surface treatments and luting cements that provide optimal bond strength for pediatric crown restorations.

Materials and Methods

1. Sample preparation

Sixty rectangular PEEK specimens (25.0 × 20.0 × 50.0 mm) were fabricated by pressure molding PEEK rods (Vestakeep^®, Daical-Evonik, Tokyo, Japan) and autoclaved at 1500°C. The specimens were positioned centrally and embedded in self-curing acrylic resin (Vertex, Vertex Dental, Soesterberg, Netherlands). Following this, each specimen underwent a 10-minute wash with distilled water using an ultrasonic cleaning machine (Twin Tornado, Medifive, Seoul, Republic of Korea).

2. Surface treatments

The samples were randomly divided into 3 subgroups of 20 specimens each based on surface treatment: control, sandblast, and sulfuric acid. The control group did not receive any surface treatment, whereas the sandblast group was exposed to 50 μm Al₂O₃ particles (Cobra, Renfert GmbH, Hilzingen, Germany) for 10 seconds at a pressure of 2 bar, keeping a 10 mm distance perpendicular to the bonding surface with a dental sandblaster (Basic Classic, Renfert GmbH). In the sulfuric acid group, surfaces were treated with 98% sulfuric acid applied to the PEEK surface using a microbrush for 60 seconds, followed by rinsing with an air-water spray for 30 seconds. After surface treatment, all specimens were rinsed with distilled water in an ultrasonic cleaner for 60 seconds and dried with oil-free air.

3. Luting of specimens with cement and storage

The PEEK specimens of the 3 surface treatment groups were further divided into 4 subgroups (n = 5 for each group), according to cement type (Table 1): methyl methacrylate (MMA)-based resin cement (Superbond EX, Sunmedical, Moriyama, Japan), composite-based resin cement (Rely X U200, 3M ESPE, Seefeld, Germany), resin-modified glass ionomer (RMGI) cement (GC FujiCem 2, GC, Tokyo, Japan), and glass ionomer (GI) cement (GC Fuji I, GC, Tokyo, Japan). Each subgroup consisted of five specimens with two cement cylinders applied to each specimen, resulting in a total of 10 measurements (n = 10) per subgroup. Two cement cylinders were built on each specimen, using a clear PVC Tygon tube (Saint Gobain Corp., Aurora, OH, USA) with an internal diameter of 2.38 mm and a height of 3 mm (Fig. 1). The cementation procedure for the four types of cement followed the manufacturer’s guidelines. Any excess cement was removed from the bonding edge using a disposable brush. The MMA-based resin, composite-based resin, and RMGI cements were light-cured using a Bluephase^® 20i (Ivoclar Vivadent, Schaan, Liechtenstein) light-curing device at an intensity of 1000 mW/cm² for 20 seconds, while the GI cement was left to self-cure. Once the luting cement was applied, the specimens were stored in water at 37℃, and the Tygon tubes and tape were carefully removed with a sharp blade. Specimens that detached during this step were recorded as having a bond strength of 0 MPa. Prior to debonding, all specimens were immersed in distilled water in a 37℃ incubator for 24 hours to replicate oral conditions.

4. Evaluation of the SBS between the luting cement and the specimen

After incubation, the SBS for each specimen and the luting cement was determined using an Instron 3366 universal testing machine (Instron^®, Norwood, MA, USA), operating at a crosshead speed of 1 mm/min, with the bonding surface aligned parallel to the loading piston. The maximum load was recorded prior to debonding. The bond failure force was measured in Newtons (N) for each sample. Subsequently, SBS was calculated in megapascals (MPa) using the formula: SBS = force/adhesion surface area. Once debonding occurred, the fractured surfaces were examined visually to categorize the failure mode for each specimen. The failure modes were classified as follows: (i) adhesive failure (where no remnants of luting cement were observed on the specimen surface), (ii) cohesive failure (where failure occurred within the PEEK or cement materials), and (iii) mixed failure (characterized by both luting cement remnants and exposed specimen surface).

5. Scanning electron microscopic evaluation

Failed specimens representing each surface treatment were analyzed using scanning electron microscopy (SEM; Hitachi E-1045, Tokyo, Japan), which provides greater depth of field and higher resolution compared to a standard light microscope. To further investigate surface morphology, additional discs were prepared for each group. The specimens were mounted using carbon adhesive and coated with a layer of gold palladium. The fractured surfaces were imaged at 30× magnification, followed by a detailed surface morphology analysis conducted at 2000× magnification.

6. Water contact angle measurements

To assess the water contact angle, additional specimens from each surface treatment group (n = 3 per group) were prepared. The water contact angle was measured under controlled conditions using a sessile drop method with a contact angle measuring device (OCA 15 plus; DataPhysics, Filderstadt, Germany). A droplet of ultrapure water was dispensed onto the PEEK sample surfaces at a rate of 2.0 µl/s using a precision syringe equipped with a 25-gauge needle. A video camera captured the droplet images over a 15-second interval, and the average water contact angle was calculated based on measurements from three different points on each sample.

7. Statistical analysis

The SPSS version 29.0 (IBM SPSS Inc., Chicago, IL, USA) program was utilized for statistical analysis. Group differences were analyzed with the Kruskal-Wallis test (α = 0.05), followed by a Bonferroni correction for multiple comparisons, adjusting the significance level to α = 0.0083. The Kruskal-Wallis test also assessed water contact angle differences (α = 0.05). Fisher’s exact test evaluated failure modes, with significance set at α = 0.05.

Results

1. Shear bond strength

According to the ISO 10477 standard, which requires a minimum shear bond strength of 5 MPa, four combinations of surface treatments and luting cements met the required threshold. When the bond strength was ranked from highest to lowest, the order was sulfuric acid with MMA-based resin cement, composite-based resin cement, sandblast with MMA-based resin cement, and composite-based resin cement. In contrast, all cements in the control group demonstrated values below this threshold, with only the MMA-based resin cement showing a significantly higher value of 1.19 MPa. In the sandblast group, MMA-based resin cement showed the highest value with a mean value of 10.12 MPa, statistically significantly higher than that of composite-based resin cement, RMGI, and GI cement (p < 0.017; Fig. 2). In the sulfuric acid group, the highest SBS was observed in each cement group: MMA-based resin cement with 11.25 ± 1.86 MPa, composite-based resin cement with 10.29 ± 2.26 MPa, RMGI with 1.49 ± 0.98 MPa, and GI with 1.37 ± 0.69 MPa. However, there was no statistically significant difference between the MMA-based resin cement and composite-based resin cement (p < 0.017; Table 2, Fig. 3)

For MMA- and composite-based resin cements, shear bond strength increased significantly following sandblast and sulfuric acid surface treatments compared to the control group (p < 0.001; Table 3, 4). However, there was no significant difference between the sandblast and sulfuric acid treatments in the MMA-based resin cement group (p = 0.123; Table 3, Fig. 4). In contrast, bond strength was significantly higher with sulfuric acid treatment compared to sandblast treatment in the composite-based resin cement group (p < 0.001; Table 4, Fig. 5).

2. Failure mode analysis

The analysis revealed distinct differences in failure modes among the different groups of surface treatments based on the distribution of failure types. The SEM images illustrate the surface characteristics associated with these failure modes (Fig. 6, 7). In the control group, the most frequent failure mode was adhesive failure, characterized by flat surfaces with a constant texture (Fig. 7A). Conversely, the sandblast and sulfuric acid-treated groups showed a significant increase in mixed and cohesive failures compared to the control group, with mixed failures being significantly higher in both groups (p < 0.001). Specifically, the sandblast group exhibited a substantial portion of mixed failures, indicating a combination of adhesive and cohesive characteristics (Fig. 7B, 7C). The sulfuric acid group also showed a notable increase in cohesive failures, prominently featuring mixed failure modes. The specimens treated with sulfuric acid showed a significant proportion of mixed failures and cohesive fractures within the resin, characterized by irregular surfaces with partially flat and circular areas (Fig. 7D, 7E).

3. SEM micrographs

Specimens after the different surface treatments are shown by SEM (Fig. 8). The SEM analysis revealed that the PEEK materials exhibited minor surface scratches, while maintaining a smooth and homogenous structure. The sandblasted samples, treated with 50 μm Al₂O₃ particles, showed numerous convex sediments on their surfaces, resulting in a more rugged and rough texture, characterized by irregular and fissured regions embedded with polygonal alumina oxide particles. The specimen treated with 98% sulfuric acid developed a spongelike porous fiber network, along with signs of subsurface corrosion.

4. Water contact angle

The contact angle is a key indicator that reflects the wettability of a material and its interaction with functional groups on the surface. The sandblasted samples displayed a higher contact angle, whereas those treated with sulfuric acid had a lower contact angle compared to the control group. Nonetheless, no significant differences were observed between the untreated, sandblasted, and sulfuric acid-treated groups (p > 0.05; Fig. 9).

Discussion

Polyetheretherketone (PEEK) is widely used in industries such as electronics, aerospace, automotive, and medical devices due to its biocompatibility, aesthetic benefits, and durable mechanical properties like wear resistance and high fatigue strength. To utilize PEEK effectively as a pediatric crown material, it is essential to evaluate both its mechanical properties and bonding capabilities for restorative purposes. This study aimed to determine whether PEEK, an aromatic and semicrystalline polymer, has clinically acceptable bonding potential by measuring shear bond strength (SBS) across various surface treatments and luting cements. Although no specific guidelines exist for pediatric crown restorations, ISO 10477 suggests a minimum SBS of 5 MPa, while other studies propose a clinical threshold of 10 ‒ 12 MPa under oral conditions [10-13]. According to these standards, three combinations in this study met the clinically acceptable SBS range (exceeding 10 MPa): MMA-based resin cement and composite-based resin cement in the sulfuric acid-treated group, and MMA-based resin cement in the sandblast group.

Among them, surface treatment with 98% sulfuric acid and the MMA-based resin cement exhibited the highest strength of 11.25 MPa. The corrosive effect of the acid created microporosities by breaking down the PEEK matrix via sulfonation, which facilitated micromechanical bonding with the luting agent. SEM images verified that the resin cement penetrated deeper and bonded strongly and effectively by creating a complex fiber network with porosities and a blistered subsurface from a plain homogenous pattern after treatment with 98% sulfuric acid. The increased mixed failure mode also supported the improved SBS in the 98% sulfuric acid-treated group. In addition to achieving high SBS values, the failure mode analysis showed distinct differences across groups. Surface treatments such as sandblasting and sulfuric acid not only increased SBS but also promoted mixed failures, combining adhesive and cohesive characteristics. A high frequency of adhesive failures was observed in the control group, indicating insufficient bond strength, consistent with previous studies [14]. Mixed failures were more common in sandblast and sulfuric acid-treated groups, likely due to shear force distribution at the resin/PEEK interface [14,15]. Sandblasting created a rough texture that facilitated mechanical interlocking, reducing adhesive failure, while sulfuric acid treatment enhanced mechanical and chemical bonding through deeper resin penetration. These findings align with prior research linking surface roughness and microstructural changes to improved bonding and mixed failure modes.

Sandblast, regarded as a relatively simple surface treatment method, enhances surface roughness, removes organic contaminants, and creates a fresh, active surface layer, thereby improving the micromechanical interlocking of polymer-based dental materials [16,17]. Similar to prior findings, which highlighted the effectiveness of sandblasting in enhancing shear bond strength (SBS) due to increased surface roughness and micromechanical interlocking, this study also observed significant improvements in SBS values with sandblasting treatments [18-20]. Sandblasted specimens treated with 50 μm Al₂O₃ in previous research demonstrated high SBS values, which were attributed to the creation of a rough, irregular surface that allowed better flow and adhesion of cement. Consistent with these observations, the increased SBS in this study further supports the critical role of sandblasting in optimizing bonding performance. Previous research has demonstrated a significant increase in SBS values following sandblasting compared to untreated PEEK due to changes in surface topography. In alignment with these studies, the sandblasting group in this study exhibited statistically significantly higher SBS values than the control group. Moreover, the combination of MMA-based resin cement exceeded 10 MPa, surpassing the minimum clinical SBS requirement. In addition, several studies have reported that MMA-based adhesives, like Visio. link, yield higher bond strengths on PEEK surfaces compared to other systems, primarily due to the chemical composition of the adhesive and the increased micromechanical retention created by surface treatments [21-23]. This aligns with our findings, as the combination of sandblasting and an MMA-based resin cement exceeded clinical SBS requirements, reinforcing the significance of specific surface treatments and adhesive systems for achieving robust bonding. High SBS values in the sandblast group are attributed to the altered surface topography created by sandblasting, as the irregularities in the bonding area increased the material’s contact area for adhesion. SEM images verified that the irregular fissure patterns with larger grooves and cracks improved microretention by creating a rough, textured surface with numerous convex features, facilitating resin cement flow.

Notably, the water contact angles of the three surface groups (control, sandblast, and sulfuric acid) were not statistically different, suggesting that the surface treatments did not markedly alter the wettability or induce significant chemical modification, maintaining similar levels of hydrophobicity across all groups. While bond strength is influenced by surface modifications, surface chemical groups can also affect the water contact angle, altering wettability and impacting the bond strength. According to previous studies, peaks and valleys formed by airborne particle abrasion during sandblasting can act as barriers to liquid spreading, thus increasing contact angles [24]. The sandblasting treatment promoted surface stripping and increased surface roughness without increasing reactive groups on the PEEK surface, as there was no chemical reaction with the aromatic rings of the polymer chain. In this study, although the contact angle in the sandblasting group slightly increased compared to the control group, it was not statistically significant. It can be attributable that the main bonding factor of sandblasting is physical modifications on the surface morphology, specifically the mechanical interlocking between materials caused by the irregularities formed after the treatment.

In contrast, while prolonged chemical treatments with sulfuric acid at higher concentrations have been shown in previous studies to affect hydrophobicity by increasing the number of functional groups and subsequently altering the water contact angle, this effect was not observed in the present study [25]. Although the water contact angle in the sulfuric acid group decreased slightly compared to the control group, the change was not statistically significant. These findings suggest that, in this study, the surface treatments of sandblasting and sulfuric acid primarily influenced topographic alterations rather than chemical interactions, indicating that mechanical modifications played a more substantial role in affecting bond strength.

Different cements yield varying bond strengths for stainless steel crowns (SSCs) and zirconia crowns commonly used in pediatric dentistry. SSCs primarily rely on undercuts for retention, making them user-friendly and advantageous for fluoride release in children [26-28]. In contrast, zirconia crowns require a passive fit, with retention heavily dependent on the type of cement used. While GI cement can offer sufficient retention, superior outcomes are typically observed with resin or RMGI cements [29]. In this study, neither GI nor RMGI cement reached clinical standards for PEEK, even after surface treatment. However, MMA-based and composite-based resin cements met the ISO 10477 standard of 5 MPa after surface treatment. Among them, the combination of sulfuric acid treatment and MMA-based resin cement achieved the highest SBS of 11.3 MPa. This value, although acceptable, is somewhat lower compared to the 16.4 MPa bond strength between pediatric zirconia crowns and resin cement reported in the study by Ishii et al. [30].

When comparing the bonding mechanism of PEEK to zirconia, PEEK exhibited higher flexibility and a low elastic modulus of 3 ‒ 4 GPa due to its polymer structure [31,32]. PEEK can be trimmed and shaped before cementation, allowing for a custom fit and improved retention [33]. Thus, PEEK crowns can benefit from mechanical and chemical retention mechanisms, unlike zirconia, which relies heavily on chemical bonding, the type of cement. Further studies should investigate how undercut geometry influences retention strength when combined with surface treatment and cement types, maximizing mechanical retention. By understanding the versatile nature of PEEK, which possesses mechanical and chemical retention properties, clinicians can have a broader range of choices by positioning PEEK as an alternative material for pediatric restorations.

MMA-based resin cement exhibited the highest bond strength, with significantly higher shear bond strength values for the control and sandblast groups compared to composite-based resin cement. Previous studies have indicated that MMA-based resin cement exhibits higher bond strength than composite-based resin cement due to several key factors. First, MMA-based resin cements form a semi-interpenetrating polymer network (semi-IPN) at the cement/PEEK interface, which significantly improves bond strength by allowing small MMA molecules to penetrate the polymer chains, resulting in mechanical interlocking. Moreover, the lower viscosity of MMA-based resin cement facilitates infiltration into the narrow grooves on the PEEK surface created by sandblasting, further enhancing mechanical retention [34]. In contrast, composite-based resin cements, due to their larger molecular size, are less effective at penetrating the spaces between PEEK molecules, which hinders the formation of an IPN structure and results in lower bond strength.

While this study found no statistically significant difference in bond strength between MMA-based and composite resin cements in the sulfuric acid-treated group, the sandblast-treated group exhibited a significant difference, with MMA-based resin cement showing higher shear bond strength. The sulfuric acid treatment enhances the PEEK surface by introducing sulfonic acid groups that increase hydrophilicity and surface roughness, improving chemical and mechanical adhesion, and thus reducing the bond strength difference between the two types of cement. Although sandblasting and sulfuric acid surface treatments enhance adhesion, the underlying mechanisms differ, leading to varying results depending on the type of cement used.

This study has some limitations. First, it was an in vitro study with a short duration, which may not accurately reflect the long-term changes in bond strength, although the bond strength between the PEEK and cement may degrade over time in the oral environment. Second, light curing was applied to the cement directly; however, only light passing through the crown is used for cement polymerization in clinical settings. The actual amount of light available for cement polymerization may differ even with the same amount of light exposure. Therefore, additional research replicating these clinical conditions is necessary. Furthermore, comparative studies with existing restorative materials, such as SSC and zirconia crowns, are needed to expand the variety and reliability of treatment options available for pediatric patients.

While this study focused on shear bond strength (SBS), it is also essential to evaluate how surface treatments impact other physical properties of PEEK, such as wear resistance and durability. Prolonged sulfuric acid exposure can alter PEEK’s topography and potentially weaken it, whereas shorter exposure may improve bonding without compromising strength [14,24,35]. However, the effects of treatment and rinse duration on PEEK’s degradation remain unclear. Future studies should investigate these factors, as well as resin penetration and bond strength, to better understand PEEK’s clinical performance, particularly in pediatric applications. Additional properties like occlusal wear, tooth reduction, color stability, and plaque adhesion should also be examined, as they influence the long-term success of restorations and patient oral health.

Conclusion

Sandblasting and 98% sulfuric acid surface treatments significantly enhanced the SBS of PEEK composites with cement by improving surface morphology, shape, and texture. Sulfuric acid-treated PEEK with MMA and composite-based resin cement achieved clinically acceptable SBS due to microporosity and deeper cement penetration. Sandblasting also effectively improved SBS by creating micro-mechanical interlocks with irregular fissure patterns. These findings highlight the importance of appropriate surface treatments in enhancing the adhesive strength and clinical applicability of PEEK crowns in pediatric dental restorations, making them a viable alternative to traditional materials.

NOTES

Acknowledgments

This study was supported by Kyungpook National University Dental Hospital Institute for Dental Research, 2024.

Conflicts of Interest

The authors have no potential conflicts of interest to disclose.

Funding information

This study was supported by Kyungpook National University Dental Hospital Institute for Dental Research, 2024.

Fig 1.

Schematic diagram of the sample used in the study.

PEEK: Polyetheretherketone.

Fig 2.

Boxplots showing the shear bond strength (MPa) of various luting cements with sandblast surface treatment. Bars with the same letter did not differ significantly. Vertical bars refer to the standard deviation.

MMA: methyl methacrylate; RMGI: resin-modif ied glass ionomer; GI: glass ionomer.

Fig 3.

Boxplots showing the shear bond strength (MPa) of various luting cements with sulfuric acid surface treatment. Bars with the same letter did not differ significantly. Vertical bars refer to the standard deviation.

MMA: methyl methacrylate; RMGI: resin-modif ied glass ionomer; GI: glass ionomer.

Fig 4.

Boxplots showing the shear bond strength (MPa) between MMA-based resin cements and specimens according to different surface treatments. Bars with the same letter did not differ significantly. Vertical bars refer to the standard deviation.

MMA: methyl methacrylate.

Fig 5.

Boxplots showing the shear bond strength (MPa) between composite-based resin cements and specimens according to different surface treatments. Bars with the same letter did not differ significantly. Vertical bars refer to the standard deviation.

Fig 6.

Comparison of the failure modes of the polyetheretherketone (PEEK) crowns following different surface treatments.

Fig 7.

Scanning electron microscopy images after the shear bond strength test showing fracture mode on polyetheretherketone (PEEK) samples (× 30). (A) Adhesive failure on surfaces of no treatment. (B, C) Mixed failure on surfaces treated by sandblasting. (D, E) Mixed failure on surfaces treated with 98% sulfuric acid.

Fig 8.

Scanning electron microscopy images of polyetheretherketone (PEEK) samples (×2.00 k). (A) no treatment, (B) sandblast, (C) 98% sulfuric acid.

Fig 9.

Water contact angle of polyetheretherketone (PEEK) samples. (A) no treatment, (B) sandblast, (C) 98% sulfuric acid.

Table 1.

Components and manufacturers of materials used in the study

Material	Material group	Manufacturer	Composition
Super-Bond Universal	MMA-based resin	Sun medical, Moriyama, Japan	PMMA, 4-META, MMA, TBB-O, PMMA, co-activator, MMA, UDMA, HEMA, MTU-6, borate catalyst. self-cure type (bulk-mix technique)
RelyX Unicem2 Automix	Composite-based resin	3M ESPE, Sweefeld, Germany	Bis-phenol-A-bis-(2-hydroxy-3-methacryloxypropyl)ether (Bis-GMA), Triethylene glycol dimethacrylate (TEGDMA) Silanized glass and silica filler; Initiator system: sodium p-toluen sulfinate, camphorquinone;
GC Fujicem 2 Automix	RMGI	GC Corp., Tokyo, Japan	Alumino-silicate glass, silica powder, UDMA, 2-Hydroxyethylmethacrylate
Fuji I	GI	GC Corp., Tokyo, Japan	Fluoro-alumino-silicate glass 95 wt%, Polyacrylic acid powder 5 wt%, Polybasic carboxylic acid

MMA: methyl methacrylate; RMGI: resin-modified glass ionomer; GI: glass ionomer; PMMA: polymethyl methacrylate; TBB-O: partially oxidized tri-n-butyl borane; UDMA: urethane dimethacrylate; HEMA: 2-hydroxyethyl methacrylate; MTU: methacryloyloxyhexyl 2-thiouracil.

Table 2.

Mean shear bond strength between cement materials luted on specimens according to the surface treatment methods

Surface treatment	Luting agent	N	Shear bond strength (Mean ± SD)
Control	MMA-based resin^a	10	1.09 ± 0.44
	Composite-based resin^b	10	0.07 ± 0.15
	RMGI^b,c	10	0.00
	GI^b,c,d	10	0.00
Sandblast	MMA resin^e	10	10.12 ± 1.85
	Composite resin^f	10	6.01 ± 0.72
	RMGI^g	10	0.64 ± 0.53
	GI^h	10	0.14 ± 0.24
Sulfuric acid	MMA resinⁱ	10	11.25 ± 1.86
	Composite resinⁱ	10	10.29 ± 2.26
	RMGI^j	10	1.49 ± 0.98
	GI^j,k	10	1.37 ± 0.69

Kruskal-Wallis test and Bonferroni test.

a, b, c, d, e, f, g, h, I, j, k : The same character means no statistical difference by the Bonferroni test (p < 0.017).

MMA: methyl methacrylate; RMGI: resin-modified glass ionomer; GI: glass ionomer.

Table 3.

Shear bond strength of surface treatments in MMA-based resin cement groups

MMA	Sandblast	Sulfuric acid
Control	< .0001^*	< .0001^*
Sandblast		0.123
Sulfuric acid

p value from the Mann-Whitney U test (^*: p < 0.05).

MMA: methyl methacrylate.

Table 4.

Shear bond strength of surface treatments in the composite-based resin cement group

Composite	Sandblast	Sulfuric acid
Control	< .0001^*	< .0001^*
Sandblast		< .0001^*
Sulfuric acid

p value from the Mann-Whitney U test (^*: p < 0.05).

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