In vitro Examination of the Bond Strength of PEEK and PEKK Materials Used as Substructure Materials with the Superstructure Composite before and after Artificial Aging and Examination of Scanning Electron Microscope Images

Abstract

Objective This study aims to evaluate the bond strength of polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) materials when used as substructures with composite superstructures, before and after artificial aging. Surface treatments, including sanding and sandblasting, were examined to determine their impact on bond strength and surface topography.

Materials and Methods Sixty samples, divided equally between PEEK and PEKK, were prepared. Each group was further divided into three subgroups (n = 10): control, 10,000 thermal cycles, and 30,000 thermal cycles. The samples were subjected to surface treatments using 600-, 800-, and 1,200-grit silicon carbide wet sandpaper, followed by air abrasion with 110 µm alumina particles. Bond strength tests were conducted using an INSTRON-3345 universal testing machine, and failure types were analyzed under a stereomicroscope.

Results Sandblasting with 110 µm aluminum oxide (Al₂O₃) significantly increased the bond strength of both PEEK and PEKK materials compared with sanding. PEEK samples treated with sandblasting showed the highest bond strength (mean = 1.296 MPa), while PEKK samples treated with sanding had the lowest (mean = 0.056 MPa). Thermal cycling reduced bond strength in both materials, with a more pronounced decrease observed in the 30,000 cycle groups. analysis of variance results indicated significant differences in bond strength based on the material, surface treatment, and the interaction between these factors.

Conclusion Sandblasting with 110 µm Al₂O₃ is an effective method for enhancing the bond strength of PEEK and PEKK substructures with composite materials. These findings support the continued use of PEEK and PEKK polymers in dental applications, suggesting that appropriate surface treatments can significantly improve clinical outcomes.

Keywords

bond strength - dental adhesive - dentin bonding agent - sandblasting - composite material

Introduction

Polymers belonging to the polyaryletherketone family are used in restorations, in various removable appliances, and denture base materials by providing superior physical and mechanical properties along with biocompatibility.[1] [2] The most commonly used groups of these polymers are polyetherketoneketone (PEKK) and polyetheretherketone (PEEK).[3] The second ketone group of PEKK increases polarity and backbone rigidity, thereby providing higher glass transition and melting temperatures. Furthermore, it also confers strong polymer chains and enhanced physical and mechanical properties.[4] In contrast, PEEK materials are commonly preferred in dentistry because of their excellent thermomechanical properties, biocompatibility, chemical stability, and broad chemical resistance spectrum.[5] PEEK materials are used as frame materials in various dental applications such as metal-free fixed dental prostheses, removable dental prostheses, implant-supported fixed prostheses, implant-supported overdentures, endocrowns, and resin-bonded fixed dental prostheses.[6] Furthermore, PEEK and PEKK are used in various dental applications such as abutments, prostheses, partial frames, attachments, and crowns.[7]

Artificial aging is a laboratory procedure used to simulate long-term intraoral conditions. Its objective is to evaluate the durability, esthetic properties, and biocompatibility of dental materials.[8] This process is designed to anticipate the long-term efficacy of restorations and enhance overall clinical success rates. Various artificial aging techniques are employed to simulate the deterioration of materials, including thermal cycling, mechanical loading, chemical degradation, and ultraviolet (UV) irradiation. In thermal cycling experiments, materials are subjected to temperature variations ranging from 5°C to 55°C to simulate thermal stress.[8] Mechanical loading is the application of continuous or repetitive mechanical forces to restorations. Chemical degradation is the testing of materials' chemical resistance by exposing them to chemical agents such as saliva or food.[9] UV irradiation is a method of evaluating the photodegradation and color stability of materials by exposing them to UV rays.

Superstructure composite materials are those that form the superstructure of implant-supported dentures. These materials offer high durability and esthetic qualities that closely resemble the esthetic and functional characteristics of natural teeth. These materials are biocompatible, thereby reducing the risk of allergic reactions and exhibiting resistance to masticatory forces. Their use is conducive to long-term dental health, as it enhances the adhesion and durability of restorative materials. Classification according to filler particle size is as follows: macrofilled, that is, traditional composites; microfilled composites; hybrid composites; and nanofilled composites.[10]

Despite their exceptional properties, the low surface energies of these materials and their weak bond strength due to their inert hydrophobic surfaces have made them an important focus of research.[11] [12] Various studies have applied different roughening procedures to increase the surface energies and adhesion capabilities of these polymers. These procedures include, but are not limited to, air abrasion methods, applications of sulfuric acid at various particle sizes, piranha solution, silica-coating processes, and plasma and laser technologies.[11] [12] [13] [14] Enhancements in bonding through surface treatments have become a center of focus in many studies. The technique of air abrasion entails the application of roughened, fine-grade abrasive particles, such as aluminum oxide (Al₂O₃), to the surface of the tooth via compressed air.[15] The procedure is less invasive, preserves more tooth tissue, and produces no heat, sound, or vibration, thus increasing patient comfort. Sulfuric acid chemically erodes the tooth surface, dissolving minerals and creating a rough texture, which is used to increase bond strength.[11] It should be used with caution due to its corrosive nature. Piranha solution, a mixture of sulfuric acid and hydrogen peroxide, is a highly corrosive agent that cleans and corrodes surfaces by oxidizing organic residues and dissolving minerals. It is a highly dangerous chemical and can cause severe chemical burns. The silica coating is applied to the tooth surface to provide a rough surface that allows restorative materials to adhere. This is achieved by applying a layer of silicon dioxide to the tooth surface by sol-gel, chemical vapor deposition, or physical vapor deposition methods.[16] However, it requires specialized equipment. Plasma technology provides homogeneous roughening without contact using ionized gases for surface modification and disinfection but requires advanced technology and expertise.[14] [17] Laser technology employs focused beams of light to remove and roughen enamel or dentin from the tooth surface. This process is precise and can be performed without damaging surrounding tissues. However, it requires costly equipment and specialized training.[18]

The aim of this study was to measure the roughness values of surfaces treated with 600-, 800-, and 1,200-grit silicon carbide wet sandpaper, as well as surfaces subjected to 110 µm Al₂O₃ sandblasting, using a profilometer, and to examine the surface topography with a scanning electron microscope. Additionally, the impact of the rough surface on adhesion and its effect on bond strength during thermal cycling was investigated.

Materials and Methods

This study was designed at Department of Prosthodontics, Faculty of Dentistry, Istanbul Aydın University. Our study was conducted as a laboratory study within the framework of a doctoral thesis. The laboratory research was performed in the laboratory of Yeditepe University Dental Hospital. Since the study was performed on laboratory materials (no animal, patient, or patient group data were used), ethics committee approval or individual consent was not required.

Study Protocol

The samples were randomly divided into three groups before the composite PEEK and PEKK application. A total of 60 samples, 30 in the PEEK group and 30 in the PEKK group, were included in the study. These samples were divided into three subgroups (n = 10/group). A total of 30 samples in the PEEK group were manually cut from KETRON 1000 PEEK rods obtained from Mitsubishi Chemical Advanced Materials (Belgium). For the cutting process, a diamond cutting wheel with a diameter of 10 cm and a thickness of 3.5 cm (BUEHLER, IsoMet 1000, Germany) was used. For the PEKK group, 30 samples were milled from CAD-CAM Pekkton blocks (Pekkton). For milling, a cutting bit (imes-icore, Germany) with a diameter of 10 mm and a thickness of 3.5 mm was used. The bonding surface of each disk was polished using 600-, 800-, and 1,200-grit silicon carbide abrasive papers with a Phoenix Beta Twin Wheel polishing device (Buehler, Germany). During the polishing process, abrasive sanding and water cooling were utilized. Surface roughness (Ra) values were determined using a surface profilometer (Perthometer M1, MAHR, Germany), and these measurements were conducted before surface treatments and experiments.

In accordance with the surface treatment protocol, 30 PEEK and 30 PEKK samples were divided into three groups. Within each group, there were 10 control samples, 10 samples subjected to 10,000 thermal cycles, and 10 samples subjected to 30,000 thermal cycles. The surfaces of the PEEK and Pekkton disks were subjected to air abrasion with 110 μm alumina particles (Renfert GmbH, Germany) at 2 bar pressure for 10 seconds. Before the application of the adhesive material, the samples were cleaned with distilled water and dried at room temperature. Before and after the surface treatment, the surface roughness values of the samples were measured twice. The first measurement was conducted on stabilized surfaces using 600-, 800-, and 1,200-grit wet sandpaper. The second measurement was performed on surfaces subjected to air abrasion with 110 μm alumina particles.

The adhesive application was performed using an adhesive composed of distilled water and photoinitiators. Polymerization was performed for each sample surface for 90 seconds using an Optilux 501 light-curing device.

For composite resin application, a cylindrical acrylic mold with an inner diameter of 4 mm and a height of 4 mm was used. The samples were placed in the acrylic mold before the application of composite resin, and after polymerization, they were immersed in distilled water at 37°C in an incubator for 24 hours. The samples kept in the incubator were divided into the following groups: control groups (PEEK = 10, Pekkton = 10), 10,000 thermal cycles group (PEEK = 10, Pekkton = 10), and 30,000 thermal cycles group (PEEK = 10, Pekkton = 10). Cycles were performed at water temperatures between 5°C and 55°C, with each bath lasting for 30 seconds.

The shear bond strength was measured using an INSTRON 3345 universal testing machine. The samples were positioned parallel to the bonding surface and subjected to a shear force at a crosshead speed of 1 mm/min using a chisel-shaped indenter. The shear bond strength was calculated in megapascal using the ratio of the measured load on the area of the sample surface in contact with it.

For failure analysis, the terms “adhesive failure,” “cohesive failure,” and “mixed failure” were used. Adhesive failure is defined as the separation of the restoration due to a weak bond between the adhesive and the adhered surface. Cohesive failure is defined as a fracture in the internal structure of the adhesive or in the bonded material. Adhesive failure is typically attributed to inadequate surface preparation or the selection of an inappropriate adhesive. Cohesive failure indicates that the bond between the adhesive and the substrate is strong, but the internal strength of the adhesive is insufficient.[19] Mixed failure is a phenomenon that exhibits both adhesive and cohesive failure properties. The schematic view of fracture types is shown in [Fig. 1]. The fracture types identified as adhesive failure, cohesive failure, and mixed failure were examined under a Leica MZ10F stereomicroscope (Germany) at ×40 magnification. An example stereomicroscope image of PEEK and PEKK materials is presented in [Fig. 2].

Fig. 1 The schematic view of fracture types. (Adapted from von Fraunhofer (2012).[19])

Fig. 2 Electron microscope images of the materials after 110 µm air etching. (A) PEEK material ×200 magnification, (B) PEEK material ×2,000 magnification, (C) PEKK material ×200 magnification, and (D) Pekkton material ×2,000 magnification. PEEK, polyetheretherketone; PEKK, polyetherketoneketone.

Statistical Analysis

Quantitative variables were presented as mean, standard deviation (SD), minimum, and maximum values. Two-way analysis of variance (ANOVA) and post hoc Tukey's test were used in the analysis. In the comparison of measurements across groups, treatments, and processes and for analyzing the interaction between groups, treatments, and processes, a two-way ANOVA test was used. Tukey's test was employed for pairwise comparisons within groups. SPSS v27.0 (IBM, Chicago, Illinois, United States) was used for statistical analysis. Results are expressed at a significance level of p < 0.005 and 95% confidence interval.

Sample Size

The power analysis conducted using G*Power software resulted in a required sample size of n = 10 for each subgroup, with a minimum of 30 samples within each group. This was determined for an effect size (d) of 0.399, SD of 2, power of 0.80, and significance level (α) of 0.5. With the two groups evaluated, 60 samples were included in the study.

Results

Surface Data

The descriptive statistics of the bond strength measurements for each material and treatment are presented in [Table 1]. The results showed that the bond strength was higher in the PEEK material group treated with sandblasting (1.136–1.362; 1.296 ± 0.048). In contrast, bond strength was higher in the Pekkton material group treated with sanding (0.047–0.067; 0.056 ± 0.006). Significant differences were observed in the analysis of material × treatment interaction. Significant differences were observed within and between the groups ([Table 1]).

Table 1 Descriptive statistics of bond strength measurements

Material	Treatment	Minimum	Maximum	Mean	SD
PEEK	Sandblasting	1.136	1.362	1.296	0.048
	Sanding	0.041	0.060	0.050	0.005
	Total	0.041	1.362	0.673	0.629
Pekkton	Sandblasting	1.235	1.272	1.256	0.008
	Sanding	0.047	0.067	0.056	0.006
	Total	0.047	1.272	0.656	0.605
Total	Sandblasting	1.136	1.362	1.276	0.040
	Sanding	0.041	0.067	0.053	0.006
	Total	0.041	1.362	0.664	0.615

Abbreviations: PEEK, polyetheretherketone; SD, standard deviation.

According to the results of the two-way ANOVA analysis, a statistically significant difference was found between the material groups in terms of bond strength (F = 14.538, p < 0.05). Furthermore, a significant difference was found in bond strength within the material groups with respect to treatments applied (F = 26.334, p < 0.05). Bond strength shows a difference with respect to all analyzed factors ([Table 2]).

Table 2 Bond strength analysis

Source	Type III squares total	SD	Mean square	F	p-Value
Adjusted model	44.898	3	14.966	24,233.038	0.000[a]
Intercept	52.955	1	52.955	85,744.965	0.000[a]
Material	0.009	1	0.009	14.538	0.000[a]
Treatment	44.873	1	44.873	72,658.241	0.000[a]
Material × treatment	0.016	1	0.016	26.334	0.000[a]
Error	0.072	116	0.001
Total	97.925	120
Adjusted total	44.970	119
R 2 = 0.998

Abbreviation: SD, standard deviation.

^a p < 0.05 significant difference, p > 0.05 no significant difference; two-way analysis of variancetest.

Instron Data

The descriptive statistics of bond strength measurements for each material and process are presented in [Table 3].

Table 3 Descriptive statistics for bond strength measurements

Material	Procedure	Bond strength N				Bond strength MPa
		Minimum	Maximum	Mean	SD	Minimum	Maximum	Mean	SD
PEEK	Control	161.530	215.280	187.441	16.087	12.850	17.130	14.912	1.281
	10,000 cycles	46.640	76.120	60.891	10.192	3.710	6.050	4.840	0.810
	30,000 cycles	22.390	34.850	28.774	4.135	1.780	2.770	2.286	0.329
	Total	22.390	215.280	92.369	70.505	1.780	17.130	7.346	5.611
Pekkton	Control	122.580	242.020	182.437	36.757	9.750	19.250	14.513	2.924
	10,000 cycles	40.550	63.020	49.733	7.198	3.220	5.010	3.952	0.573
	30,000 cycles	18.830	29.550	24.299	4.268	1.490	2.350	1.929	0.340
	Total	18.830	242.020	85.490	73.580	1.490	19.250	6.798	5.855

Abbreviations: PEEK, polyetheretherketone; SD, standard deviation.

Descriptive statistics of bond strength measurements for each material and process are presented in [Table 3]. The results demonstrated a decrease in bond strength measurements in control, 10,000 cycles, and 30,000 cycles. A decrease was also observed in the PEEK and Pekkton groups; however, the degree of this decrease/change was not statistically different between the groups ([Table 3]) ([Fig. 3]).

Fig. 3 Bond strength of PEKK and PEEK materials during the thermocycling process. PEEK, polyetheretherketone; PEKK, polyetherketoneketone.

According to the two-way ANOVA analysis results, there was no significant difference between the material groups in terms of bond strength N measurements (F = 2.365, p > 0.05). A significant difference was found in bond strength values with respect to the processes performed (F = 474.457, p < 0.05). Tukey's post hoc analysis results demonstrated that all processes exhibited significant differences within themselves (p < 0.05). However, no statistically significant difference was found between the processes for the materials analyzed (F = 0.230, p > 0.05). These results show that the difference between the processes was not significant for the materials. The change in bond strength N values observed after the cycles was not significant ([Table 4]).

Table 4 Bond strength N analysis results

Source	Type III squares total	SD	Mean square	F	p-Value
Adjusted model	285,664.770	5	57,132.954	190.348	0.000[a]
Intercept	474,503.801	1	474,503.801	1,580.885	0.000[a]
Material	709.810	1	709.810	2.365	0.130
Procedure	284,816.937	2	142,408.468	474.457	0.000[a]
Material × process	138.023	2	69.012	0.230	0.795
Error	16,208.139	54	300.151
Total	776,376.710	60
Adjusted total	301,872.908	59
R 2 = 0.942

Abbreviation: SD, standard deviation.

^a p < 0.05 significant difference, p > 0.05 no significant difference; two-way analysis of variance test.

According to the results of the two-way ANOVA analysis, there was no significant difference between the material groups in terms of bond strength MPa values (F = 2.371, p > 0.05). However, a statistically significant difference was found in bond strength MPa values with respect to the processes performed (F = 474.608, p < 0.05). Tukey's post hoc analysis results demonstrated that all processes exhibited statistically significant differences within themselves (p < 0.05). No significant difference was found between the processes for the materials analyzed (F = 0.229, p > 0.05). These results show that the difference between the processes was not significant for the materials. The change in bond strength MPa values observed after the cycles was not statistically significant ([Table 5]).

Table 5 Bond strength MPa analysis results

Source	Type III squares total	SD	Mean square	F	p-Value
Adjusted model	1,809.065	5	361.813	190.409	0.000[a]
Intercept	3,000.791	1	3,000.791	1,579.206	0.000[a]
Material	4.505	1	4.505	2.371	0.129
Procedure	1,803.690	2	901.845	474.608	0.000[a]
Material × process	0.871	2	0.436	0.229	0.796
Error	102.610	54	1.900
Total	4,912.467	60
Adjusted total	1,911.676	59
R 2 = 0.946

Abbreviation: SD, standard deviation.

^a p < 0.05 significant difference, p > 0.05 no significant difference; two-way analysis of variance test.

The results demonstrated a decrease in bond strength MPa measurements in control, 10,000 cycles, and 30,000 cycles. A decrease was also observed in PEEK and Pekkton groups, but the degree of this decrease/change was not statistically different between the groups ([Table 4]).

Discussion

One of the most significant limitations to the utilization of strong polymers, such as PEEK and PEKK, is the low bond strength resulting from their hydrophobic surface.[11] [12] The objective of our study was to assess the bond strength of PEEK and PEKK material surfaces following an etching procedure. We sandblasted the surfaces with 110 µm Al₂O₃. Then we examined the roughness values of these surfaces, the effect on adhesion, and the bond strength during thermocycling. The present study demonstrates that sandblasting with 110 µm Al₂O₃ is an effective method to increase the bond strength of PEEK and PEKK materials. Our findings indicate that PEEK and PEKK polymers can continue to be used in dental applications and that appropriate surface treatments can significantly improve clinical outcomes.

Roughening methods are important to create a surface for the adhesion of restorative materials. The low surface energy of PEEK and PEKK necessitates surface treatments such as nonthermal plasma, silicoating, acetone, sulfuric acid, and air abrasion with Al₂O₃.[14] [20] Lee et al reported that air abrasion with alumina and silica coating provided higher PEKK bond strength than sulfuric acid.[21] Zhou et al performed sulfuric acid, hydrofluoric acid, argon plasma, and sandblast with 50 μm Al₂O₃ particles on PEEK materials. As a result of the study, it was shown that 98% sulfuric acid and argon plasma treatment achieved higher bond strength than other methods.[22] Schmidlin et al reported that sandblasting PEEK surfaces with different particle sizes of Al₂O₃ did not create a significant difference between the two processes.[23] However, in a study by Stawarczyk et al, the sandblasting process with 110 µm Al₂O₃ had a positive effect on surface roughness and shear bond strength.[16] Gouveia et al reported that pretreatment of 110-μm Al₂O₃ airborne-particle abrasion increased the bond strength of PEEK and PEKK compared with the control group.[24] Silthampitag et al reported that etched with 98% sulfuric acid achieved higher PEEK bond strength than etched with piranha solution and sandblasting with 50 µm alumina.[25] Stawarczyk et al tested the effects of 98% sulfuric acid, air abrasion for with either 50 or 110 μm alumina on PEEK material and showed that 110 μm air-abraded and silica-coated PEEK surfaces had the highest roughness and lowest contact angles.[16] In a study investigating the effects of different adhesives on bond strength of PEEK material safter plasma pretreatment, it was reported that plasma treatment had no effect on bonding to resin cements.[17] In the same study, the highest shear bond strength was obtained in the acid-etched group. In a study by Keul et al with 680 PEEK specimens, the air abrasion method was shown to provide the highest surface roughness, surface free energy, and tensile bond strength.[26] As can be seen in the literature, different pretreatments have been tried on PEEK and PEKK materials with different results. In our study, we found an increase in roughness and bond strength on PEEK and PEKK surfaces sandblasted with 110 µm Al₂O₃. This shows that surface treatments can improve composite bond strength.

Since surface treatments alone do not provide sufficient bond strength, it is necessary to use a variety of adhesive materials in addition. For example, adhesives containing methyl methacrylate (MMA) monomers were effective in increasing bond strength. Similarly, it was observed that phosphoric acid ester could modify the surface of PEKK polymers.[17] [27] [28] Labriaga et al used Visio.link to compare the effect of nonthermal plasma on PEKK materials with other surface pretreatments.[14] Gouveia et al used Visio.link to bond all specimens in their study with PEEK and PEKK.[24] Caglar et al evaluated the effect of several different pretreatments on PEEK materials. In this study, Visio.link, and Signum PEEK bond were used, and it was reported that adhesive systems increased bond strength.[28] In the study by Stawarczyk et al, PEEK materials were adhered with Signum PEEK bond, Visio.link, Ambarino P60 after plasma pretreatment. In this study, the effects of different adhesives on bond strength were investigated, and it was reported that the use of MMA-based adhesives enabled bonding between PEEK and self-adhesive resin cements.[17] We also used Visio.link adhesive agent containing MMA in our study and showed that the adhesive could increase the bond strength.

Material quality, lifetime, biocompatibility, and durability are important considerations in the selection of dental materials and restorations. In vitro testing to determine the physical and chemical durabilities of the dental materials is essential to ensure patient satisfaction and comfort. For this purpose, artificial aging tests are performed to simulate long-term intraoral conditions. In the study by Labriaga et al, 10,000 cycles of thermocycling were used, and it was shown that the bond strength values of the thermal cycling groups were lower than those of the control group.[14] In the thermocycling application, various cycle values can be utilized. In the studies by Caglar et al and Keul et al, 5,000 thermocycles were used.[26] [28] In a study by Sakihara et al, investigating the effect of sulfuric acid and vinyl sulfonic acid on bond strength, 20,000 thermal cycles were utilized.[27] In our study, we investigated the effect of thermocycling on composite bond strength at two levels of cycles: 10,000 and 30,000. The higher level of cycles was selected because it more closely resembles the long-term aging that prosthetic materials will be exposed to in daily life. However, future studies should investigate the effects of different cycles and explore methods that can mimic the oral environment in a better way to further understand the effects of thermal cycling.

Limitations

There are certain limitations of this study. First, the sample size was relatively small. A larger sample size can enhance the generalizability of the results. Second, only certain surface treatments were used in this study. Different surface treatments may produce different results. Third, only Visio.link containing MMA was used as the adhesive agent. Comparing the effects of different adhesives may lead to more comprehensive results. The thermal cycling time used in this study was determined to provide flexibility in selection rather than adhering to a specific standard procedure. However, the lack of detailed investigation into the effects of different cycling times could be a limitation. No advanced methods were used to fully simulate the oral environment. Therefore, the results obtained may not fully align with those obtained under real clinical conditions. The aging period used to assess the long-term effects of the thermal cycling process is limited. A longer aging period may provide better insights into the long-term effects.

Conclusion

This study demonstrated that sandblasting with 110 µm Al₂O₃ is an effective method for increasing the bond strength of PEEK and Pekkton polymers with composite materials. These results support the use of PEEK and Pekkton polymers in dental applications. Factors such as the choice of adhesive materials and the long-term effects of thermal cycling have an impact on the outcomes of the procedures and should be considered. Additionally, further studies are needed to compare these materials with similar structures.

Conflict of Interest

None declared.

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Artikel online veröffentlicht:
26. September 2024