ORIGINAL RESEARCH |
https://doi.org/10.5005/jp-journals-10015-2485 |
Evaluation of the Effect of Various Surface Treatments on Bond Strength of Polyetheretherketone to Veneering Composite: An In Vitro Study
1,2,4–6Department of Prosthodontics and Crown & Bridge, Meenakshi Ammal Dental College and Hospital, Meenakshi Academy of Higher Education and Research (Deemed to be University), Chennai, Tamil Nadu, India
3Department of Prosthodontics and Crown & Bridge, SRM Dental College, SRM University, Ramapuram, Chennai, Tamil Nadu, India
Corresponding Author: Lambodaran Ganesan, Department of Prosthodontics and Crown & Bridge, Meenakshi Ammal Dental College and Hospital, Meenakshi Academy of Higher Education and Research (Deemed to be University), Chennai, Tamil Nadu, India, Phone: +91 9894946334, e-mail: drlambo.prostho@madch.edu.in
Received: 15 September 2024; Accepted: 24 October 2024; Published on: 13 March 2025
ABSTRACT
Aim: The present study aims to evaluate the efficacy of five different surface modification techniques on the surface roughness and bond strength of polyetheretherketone (PEEK) crowns.
Materials and methods: Sixty PEEK samples were made from BioHPP using the computer-aided design and computer-aided manufacturing (CAD-CAM) technique and were divided into six groups (n = 10): group A, control group (untreated samples); group B, samples treated with sandblasting; group C, samples treated with sulfuric acid; group D, samples treated with laser; group E, samples treated with a combination of sandblasting and sulfuric acid; and group F, samples treated with a combination of sandblasting and laser. The samples were then evaluated for surface roughness using a scanning electron microscope, after which composite veneering was done, and the shear bond strength was evaluated using a universal testing machine. Data were recorded and statistically analyzed.
Results: Surface roughness of untreated samples was 0.04 ± 0.02 µm. Values for groups B to F were 0.40, 0.66, 1.45, 1.25, and 2.72 µm, respectively. For shear bond strength, values for groups A to F were 3.94, 9.61, 12.56, 16.80, 16.35, and 23.95 MPa, respectively. Surface roughness of the PEEK material was found to be directly related to shear bond strength, and results suggest a synergistic effect when combining mechanical and chemical or laser treatments.
Conclusion: The untreated control had the least surface roughness and exhibited the lowest shear bond strength. The combination of sandblasted and laser-treated samples had the roughest surface and the highest shear bond strength.
Clinical significance: According to the results of this study, more aggressive and combination surface treatments, like sandblasting and laser treatment, provide the best results in terms of bond strength from a clinical standpoint. The effective interface between the PEEK core material and the veneering composite is a significant factor in determining the long-term successful clinical performance of PEEK-based restorations.
Keywords: Acid etching, Laser, Polyetheretherketone, Sandblasting, Shear bond strength, Surface roughness
How to cite this article: Ganesan L, Shivasubramanian L, Rengasamy V, et al. Evaluation of the Effect of Various Surface Treatments on Bond Strength of Polyetheretherketone to Veneering Composite: An In Vitro Study. World J Dent 2025;16(1):40–44.
Source of support: Nil
Conflict of interest: None
INTRODUCTION
The historical dominance of metals in dental crown restoration is attributed to their superior mechanical properties.1 However, the increasing demand for esthetic solutions over the past century has led to the adoption of tooth-colored materials, such as ceramics and polymers, particularly in the form of veneers.2 This shift toward esthetics has facilitated the rise of all-ceramic restorations, marking a significant transition to metal-free options that prioritize both appearance and biocompatibility.3 Despite these advantages, full-ceramic restorations are not without challenges, including concerns regarding mechanical and tribological properties, as well as the high costs associated with their fabrication.4 Consequently, there remains a need for the development of better materials to address these shortcomings in dental restoration.
Among recent advancements in metal-free restorative materials, polyetheretherketone (PEEK), a high-performance polymer with exceptional mechanical properties, biocompatibility, and chemical resistance, has emerged as a promising option for metal-free restorations.5 These characteristics have pushed PEEK into the spotlight in the field of materials for crown and bridge prosthetics. On the other hand, one limitation of PEEK is its lower translucency, which restricts its use in regions of high esthetic demand.6 To overcome this limitation, PEEK has been veneered with more esthetically acceptable materials, such as resin composites, to achieve the desired esthetic outcome. While PEEK has excellent biocompatibility, this property is largely due to its chemically inert nature. However, this inertness inhibits effective adhesion with veneering materials, making various surface modifications necessary to enhance the bond strength between PEEK and veneering materials. Consequently, various surface treatment modalities have been reported in the literature to modify the surface of PEEK, aiming to improve its bond with veneers.7,8 These modifications include mechanical treatments, such as sandblasting; chemical treatments, involving acids like sulfuric acid; and advanced techniques like laser surface treatment.
Each of these surface modification techniques produces specific changes in the surface topography of PEEK, which, in turn, enhances its bond with veneers. While sandblasting increases surface roughness, leading to the production of microretentive areas, laser treatment enhances surface energy, creating micro- and nanoscale patterns.9 Chemical treatments, especially with sulfuric acid, cause etching on the surface and also alter the surface chemistry by introducing hydrophilic sulfonyl groups, depending on the duration of the etching.10 In summary, the effectiveness of physical and chemical surface modifications in enhancing the bond between PEEK and veneering materials depends on the resulting surface physics and chemistry.
While quite a bit of literature is available on surface modifications of PEEK, their comparative efficacy for bonding with veneering materials is still less reported in the literature. Hence, this paper aims to describe the impact of five different surface modification techniques on the surface roughness and bond strength of PEEK crowns.
MATERIALS AND METHODS
This in vitro study was performed at Meenakshi Ammal Dental College from April 2024 to June 2024 after obtaining approval from the Institutional Ethics Committee (Ref no.: MADC/IEC/II/106/2024). The study was conducted in accordance with the guidelines of the CRIS criteria.
The sample size for this study was calculated from Zhou et al.11 using G*Power software version 3.1.9.4. In this study, 10 samples per group were considered, making a total of 60 samples. The samples were fabricated based on the Schmitz–Schulmeyer method (Fig. 1).12 The PEEK cores (Bredent BioHPP) were designed to dimensions of 10 mm in length, 5 mm in height, and 5 mm in width using 3D Builder software and saved as an STL file. The file was transferred to exocad DentalCAD software, and 60 samples of PEEK core were fabricated by milling a Bredent BioHPP blank. The samples were then hand-polished for 45 seconds using 600-grit abrasive paper (silicon carbide) under tap water, as suggested by Shabib.13
Fig. 1: Samples fabricated based on the Schmitz–Schulmeyer method
Samples were randomly divided into six equal groups (n = 10) as follows: group A, untreated samples; group B, samples treated with sandblasting; group C, samples treated with sulfuric acid; group D, samples treated with laser; group E, samples treated with a combination of sandblasting and sulfuric acid; and group F, samples treated with a combination of sandblasting and laser.
For sandblasting, samples were held at a distance of 10 mm from the nozzle and at 90° to the nozzle. They were held using customized wooden holders fabricated for this purpose. Sandblasting was performed using 110 μm aluminum oxide for 10 seconds (2.5 bar pressure) with an airblasting machine (Bego Air Blaster, Germany). Final cleaning was done with oil-free dry air for 10 seconds, as described by Rosentritt et al.14
Acid etching was done using 98% sulfuric acid to evenly cover the entire front surface for 60 seconds using glass pipettes. Subsequently, the samples were rinsed under tap water for 60 seconds using tweezers and then dried with oil-free dry air for 20 seconds. This procedure was adopted from Sproesser et al.15
The laser treatment for the PEEK samples was performed with the Er:YAG laser (2940 D Plus, DEKA, Italy). The wavelength used was 2940 nm with an energy of 150 mJ, an application time of 20 seconds, 1.5 W output power, and 119.42 J/cm energy density with a 700 μs pulse duration at a 10 mm distance using a 4 mm diameter arm irradiation system (spot size: 4 mm). The treated surface was cooled with a water spray at a rate of 5 mL/minute during the surface treatment process.9
The surface-treated samples were then evaluated for surface roughness using a scanning electron microscope (FEI Company, Hillsboro, Oregon, United States) at 100× magnification using the gold sputtering method. The recorded surface images were analyzed by ImageJ software version 1.53 for surface roughness measurement, as described by BrugésMartelo et al.16
On each sample, Visio.link primer (Bredent) was applied and polymerized using light for 90 seconds prior to the application of the veneering composite. The veneering composite (Crea.lign) was then bonded onto the PEEK core with the help of a customized split mold of 4 mm length, 4 mm height, and 5 mm width. The polymerization was performed using light for 180 seconds.
The samples were mounted in an acrylic resin mold (Fig. 2) attached to the fixed lower section of the universal testing machine (Instron model 3345, England). A 0.5 mm wide uni-beveled chisel blade was secured to the movable upper section of the machine. Compression force was applied at the PEEK/composite interface using the chisel blade, with a crosshead speed of 1.0 mm/minute, continuing until the samples failed. Shear bond strength (SBS) was recorded in MPa by dividing the force necessary for failure (Newtons) by the surface area (mm²) through BlueHill software (Instron, England), according to Aboushelib.17
Fig. 2: The samples were mounted in an acrylic resin mold attached to the fixed lower section of the universal testing machine, and shear bond strength values were evaluated
The data were tabulated in Microsoft Excel 2010, and statistical analysis was performed using the R statistical package version 4.1.2. Descriptive statistics were given by mean, standard deviation, minimum, and maximum values. The test for normality was conducted using the Shapiro–Wilk test. Since values deviated significantly from normal distribution, inferential statistics were performed using the Kruskal–Wallis test. Correlation between surface roughness and shear bond strength (SBS) was calculated using Pearson correlation. p-values < 0.05 were considered significant.
RESULTS
Surface roughness of group A is 0.04 ± 0.02 µm, group B is 0.40 ± 0.06 µm, group C is 0.66 ± 0.10 µm, group D is 1.45 ± 0.07 µm, group E is 1.25 ± 0.05 µm, and group F is 2.72 ± 0.19 µm. Group F showed the maximum surface roughness, while group A showed the least (Table 1).
| Groups | Group A | Group B | Group C | Group D | Group E | Group F |
|---|---|---|---|---|---|---|
| Mean | 0.04 | 0.40 | 0.66 | 1.45 | 1.25 | 2.72 |
| SD | 0.02 | 0.06 | 0.10 | 0.07 | 0.05 | 0.19 |
| Min | 0.01 | 0.29 | 0.51 | 1.36 | 1.19 | 2.52 |
| Max | 0.06 | 0.45 | 0.76 | 1.55 | 1.32 | 2.97 |
| Normality (Shapiro–Wilk test) | 0.003 | (Significant deviation from normal distribution) | ||||
| Kruskal–Wallis p-value | 0.00003288 | (Significant) |
Shear bond strength of group A is 3.94 ± 0.48 MPa, for group B is 9.61 ± 0.64 MPa, group C is 12.56 ± 0.79 MPa, group D is 16.80 ± 0.92 MPa, for group E is 16.35 ± 1.01 MPa, and for group F is 23.95 ± 0.94 MPa. Group F showed the maximum shear bond strength, while group A showed the least (Table 2).
| Groups | Group A | Group B | Group C | Group D | Group E | Group F |
|---|---|---|---|---|---|---|
| Mean | 3.94 | 9.61 | 12.56 | 16.80 | 16.35 | 23.95 |
| SD | 0.48 | 0.64 | 0.79 | 0.92 | 1.01 | 0.94 |
| Minimum | 2.98 | 8.53 | 11.32 | 15.42 | 14.32 | 22.38 |
| Maximum | 4.51 | 10.18 | 13.65 | 18.32 | 17.65 | 25.23 |
| Normality (Shapiro–Wilk test) | 0.01437 | (Significant deviation from normal distribution) | ||||
| Kruskal–Wallis p-value | 8.63E-11 | (Significant) |
Results of the Pearson correlation analysis indicated that there is a significant large positive relationship between surface roughness and shear bond strength (r(4) = 0.971, p = 0.001). Surface roughness of the PEEK material was found to be directly related to shear bond strength, and results suggest a synergistic effect when combining mechanical and chemical or laser treatments. The untreated control had the least surface roughness and exhibited the lowest shear bond strength. The combination of sandblasted and laser-treated samples had the roughest surface and the highest shear bond strength.
DISCUSSION
Adhesion of polymer surfaces is a result of the physical and chemical properties of the surface. Physical properties contributing to adhesion mainly include surface roughness and surface energy. Surface roughness enhances mechanical interlocking between the surface and the veneer due to an increase in surface area for adhesion. Therefore, surface roughness directly enhances the adhesion of the veneer to PEEK surfaces. The next factor, surface energy, promotes better wettability of the adhering material to PEEK, thereby increasing adhesion. With respect to chemical properties, polar functional groups on the surface of PEEK can improve chemical bonding with adhesives. Therefore, depending on the veneering material, both chemical and physical modifications may be needed, since PEEK is specifically known to have low surface energy, which results in poor adhesion with other materials such as veneers or composites. The analysis of surface roughness and shear bond strength of PEEK crowns in this study after several surface treatments revealed significant findings that highlight a clear correlation between the degree of surface roughness and the corresponding increase in shear bond strength, emphasizing the critical role of surface treatments in enhancing the adhesive properties of PEEK.
The untreated PEEK samples (group A) exhibited low surface roughness, which is characteristic of PEEK’s inherently inert surface and contributes to the material’s poor adhesion properties, as evidenced by the correspondingly low shear bond strength. The introduction of surface treatments resulted in a marked increase in surface roughness across all groups, with the combination of sandblasting and laser treatment achieving the highest roughness. The roughened surface created by sandblasting offers micromechanical retention sites, improving adhesion compared to untreated PEEK but remaining lower than other treated groups. Sulfuric acid etching enhances surface wettability, thereby creating additional bonding sites. The chemical change is basically sulfonation of the benzene rings as a function of the duration of etching. The results indicate that chemical treatment alone can significantly improve bond strength, surpassing that of sandblasting alone. The laser effectively modifies surface energy and creates micro- and nano-scale topographical features, which substantially enhance adhesion potential. This method outperforms both sandblasting and sulfuric acid etching in terms of bond strength, underscoring the effectiveness of laser treatments in modifying PEEK surfaces.
The shear bond strength results mirrored the trend observed in surface roughness. The increase in shear bond strength across treated samples is noteworthy, particularly in group F, where the combination of sandblasting and laser treatment yielded the maximum bond strength. This is a substantial improvement compared to the untreated group, indicating that the combined effect of surface roughening and energy modification via laser treatment greatly enhances the bonding interface. These results suggest a synergistic effect when combining mechanical and chemical or laser treatments, with the combination of sandblasting and laser treatment proving to be the most effective method.
Minimum shear bond strength between core and veneering material is about 5 MPa, and values above 10 are clinically acceptable.18 Groups D, E, and F produced results comparable to such studies. Zhang et al.19 reported a direct correlation of etching time to the size of the pore, eventually to shear bond strength. They reported shear bond strengths of about 29 MPa with etching alone for up to 120 seconds. The value of unmodified PEEK is similar to that obtained in the literature. Therefore, chemical methods have been shown to have better potential for modifying the surface of PEEK for adhesion.
With regard to sandblasting, previous authors have shown improvement in bond strength with various bonding agents due to sandblasting, similar to the results of the present study. However, this could be modified by the use of a suitable bonding agent. They have also reported that surface roughness or preparation in the statistical sense reduces the standard deviation, meaning the values are more predictable. The results of the current study are comparable to previous studies that used sandblasting.20
With regard to laser modification of PEEK surface, it modifies the wettability of the surface of PEEK and results in a significant gain in SBS, as seen in the current study.21 Jahandideh et al.9 have reported significant improvement in SBS due to lasers; however, the values were less compared to the present study. This may be due to the difference in laser irradiation protocols. However, by principle, laser treatment enhances the adhesion of PEEK. In the other direction, laser treatment promoted adhesion by the formation of retentive holes; resins may not flow well into these surface irregularities, thereby negatively affecting the shear bond strength.22
Sandblasting increases surface roughness and improves the adhesion of veneers; however, it can cause surface defects that potentially weaken the material. Sulfuric acid etching enhances surface texture and wettability, promotes better bonding with biomaterials, but carries the risk of chemical residues that may affect biocompatibility. Laser surface treatment can offer precise surface modification with controlled roughness and no chemical residues. On the downside, it has a high cost due to sophisticated equipment.
Previous literature has compared various parameters of one technique but has seldom reported comparisons of multiple techniques. Hence, there is a lacuna in the literature that can be addressed by comparative studies such as the current study. While SBS is the ultimate result for analyzing bond strength, having established this positive correlation with surface roughness, there needs to be a discussion on surface roughness from various techniques. It can be seen from the results that improvement with sulfuric acid was phenomenal from untreated PEEK; its combination with sandblasting produced only an improvement of 4 MPa. However, when sandblasting was combined with laser, it can be observed that higher surface roughness contributes to better bonding. However, the role of surface chemistry modification is yet to be determined. PEEK does dissolve in concentrated sulfuric acid slowly over a period of days. Exposure to this acid for durations near a minute may result in surface dissolution of PEEK, creating defects. This raises the question of whether this minor defect formation leads to better adhesion or if it is due to the formation of sulfonate groups that make the surface hydrophilic, leading to better wetting of the surface. When comparing these groups, namely C and E, SBS was not significantly different (p = 0.08). Further, groups B and E differed statistically (p = 0.002). On the other hand, the combination of sandblasting and sulfuric acid showed much higher SBS, raising doubts about the role of sulfonation in SBS. Even when observing surface roughness, similar trends are seen. Therefore, sulfuric acid should be seen as a surface dissolving agent rather than a sulfonating agent. Previous literature has also shown similar findings; however, for a different application.23 As a concluding note, it can be stated that surface roughness contributed immensely to bonding with veneers, and physical phenomena are better in affording the SBS than the chemical ones. The synergistic effect of sandblasting and laser treatment provided the maximum bond strength. However, this study was performed in an in vitro environment, which may not exactly simulate intraoral conditions.
Further, it is also important to consider the potential impact of these treatments on the integrity and mechanical properties of PEEK, as excessive surface roughening could potentially weaken the material or affect its biocompatibility. Additionally, studies could be designed to investigate the efficacy of alternative surface treatments or combinations for improving the adhesive properties of this promising material to further optimize the bond strength and clinical performance of PEEK-based restorations.
CONCLUSION
Within the limitations of the present study, it can be concluded that the combination of sandblasted and laser-treated samples (group F) had the roughest surface and highest shear bond strength. The present study provides valuable insights into the association of surface roughness and shear bond strength in PEEK crowns, which are particularly relevant for restorative dentistry, where strong and durable bonds are essential for the long-term success of PEEK-based restorations.
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