Standardized Protocol to Evaluate the Effect of Surface Treatments, Luting Cements, And Thermocycling On PEEK&#8211;Composite Bond Strength

Kilinc Zeynep Cigdem; Dikbas Idil; Selen Erkul

doi:10.3791/71208

Method Article

Standardized Protocol to Evaluate the Effect of Surface Treatments, Luting Cements, And Thermocycling On PEEK–Composite Bond Strength

DOI:

10.3791/71208

⸱

June 9th, 2026

Kilinc Zeynep Cigdem¹ , Dikbas Idil² , Selen Erkul²

¹Private practice, ²Department of Prosthodontics, Faculty of Dentistry, Yeditepe University

Summary

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This protocol aims to evaluate the effects of surface treatment protocols, luting cement selection, and thermocycling on the shear bond strength between PEEK and an indirect composite resin.

Abstract

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Polyetheretherketone (PEEK) has gained increasing attention as a biomaterial in prosthodontics; however, achieving durable adhesion to veneering composite materials remains a major challenge due to its chemically inert structure. This study presents and validates a standardized, reproducible protocol for evaluating shear bond strength (SBS) between PEEK and an indirect composite resin under different surface treatments, luting cements, and thermocycling conditions. A total of 240 PEEK discs were divided into four groups according to surface treatment: no surface treatment, sandblasting, sulfuric acid etching, and laser irradiation. Surface morphology was analyzed using atomic force microscopy. Each group was then subdivided according to the luting cement used (zinc oxide non-eugenol cement or self-adhesive resin cement; n = 30), and further subdivided into thermocycled and non-thermocycled groups (n = 15). SBS was measured using a universal testing machine, and failure modes were analyzed under a stereomicroscope. Among all experimental groups, specimens luted with self-adhesive resin cement and not subjected to thermocycling demonstrated the highest SBS values (25.134 ± 1.665 MPa), whereas specimens luted with zinc oxide non-eugenol cement and subjected to thermocycling exhibited the lowest SBS values (1.958 ± 0.345 MPa). For all surface treatment protocols, SBS values were significantly higher in specimens luted with self-adhesive resin cement compared with zinc oxide non-eugenol cement (p < 0.001). Thermocycling significantly reduced SBS values across all groups (p < 0.001). This protocol highlights critical procedural steps, including surface treatment selection and cement type, that significantly influence bonding outcomes. The standardized workflow and visual demonstration of key steps provide a reproducible framework for evaluating PEEK–composite bonding.

Introduction

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Polyetheretherketone (PEEK) has emerged as a promising high-performance polymer in prosthodontics due to its favorable mechanical properties, chemical stability, low density, and tooth-colored appearance¹^,²^,³. Compared with conventional materials such as titanium, PEEK offers advantages including ease of chairside modification and reduced esthetic compromise⁴. However, despite these advantages, its clinical application is limited by its chemically inert structure and low surface energy, which result in poor adhesion to resin-based materials⁵^,⁶.

Previous studies have demonstrated that untreated PEEK surfaces exhibit insufficient bonding to composite resins, making surface modification a critical step for achieving clinically acceptable bond strength⁶^,⁷. Various surface treatment strategies have been proposed to overcome this limitation, including sulfuric acid etching⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴, airborne particle abrasion with aluminum oxide⁶^,⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴^,¹⁵^,¹⁶, tribochemical silica coating⁸^,¹⁵^,¹⁶^,¹⁷^,¹⁸^,¹⁹, plasma treatments⁹^,²⁰^,²¹^,²², and laser irradiation⁶^,²³^,²⁴^,²⁵^,²⁶^,²⁷. However, the effectiveness of these approaches varies considerably depending on the protocol parameters and materials used, resulting in inconsistent bonding outcomes across studies¹⁷.

In addition to surface treatment, the type of luting cement plays a crucial role in determining bond strength. Temporary cements, such as zinc oxide non-eugenol formulations, are commonly used for provisional restorations, whereas self-adhesive resin cements provide stronger adhesion through micromechanical and potential chemical interactions²⁴. Furthermore, thermocycling is widely used to simulate intraoral temperature fluctuations and assess the durability of the bonded interface over time⁶^,¹⁷^,²⁵. These variables—surface treatment, cement type, and thermal aging—have been investigated individually; however, their combined effects are often evaluated using heterogeneous methodologies¹¹.

Despite the growing body of literature on PEEK bonding¹¹, there is currently no standardized, reproducible experimental protocol that integrates surface treatment, cementation, and thermocycling into a unified workflow. Differences in specimen preparation, surface modification parameters, cement application procedures, and aging protocols limit the comparability of results and reduce reproducibility across studies. Moreover, the lack of visual demonstration of critical procedural steps further contributes to variability in experimental outcomes. Therefore, the aim of this study is to present and validate a standardized experimental protocol for assessing PEEK–composite bonding under controlled surface treatment, cementation, and thermocycling conditions. The null hypothesis of this study is that surface treatment methods, luting cement types, and thermocycling procedures do not significantly affect the shear bond strength between PEEK and indirect composite materials.

Protocol

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This in vitro. protocol follows institutional guidelines for laboratory-based dental research. Ethical approval was not required because no human participants or animal tissues were involved. All the materials used in this study are described in the Table of Materials.

1. Preparation of PEEK specimens

Fabricate 240 disc-shaped polyetheretherketone (PEEK) specimens with a diameter of 8 mm and a thickness of 3 mm from commercially available PEEK rods using a precision cutting device under standardized laboratory conditions (Figure 1A–C).
Mark one surface of each specimen using a permanent marker to distinguish the non-bonding side. Designate the unmarked surface as the bonding surface for all subsequent procedures.
Polish the bonding surface of each specimen using 600-grit silicon carbide abrasive paper under continuous water cooling.
Clean all specimens ultrasonically in distilled water for 10 min.
Air-dry all specimens thoroughly before surface treatment.

2. Surface treatment of PEEK

CAUTION: Handle sulfuric acid in a chemical fume hood while wearing appropriate personal protective equipment²⁸.

Randomly divide the specimens into four groups according to the surface treatment protocol: untreated control, sandblasting, sulfuric acid etching, and laser irradiation (Figure 2A–D).
For the control group, rinse the specimens with deionized water for 1 min and air-dry for 10 s.
For the sandblasting group, abrade the bonding surface using 110 µm aluminum oxide particles at 2 bar pressure for 10 s, maintaining a distance of 10 mm and an application angle of 45°. Ensure consistent application by maintaining identical distance and angulation for all specimens¹⁵^,¹⁶.
For the acid-etching group, apply 98% sulfuric acid to the bonding surface for 60 s¹³.
For the laser group, irradiate the surface using an erbium-doped yttrium aluminum garnet (Er:YAG) laser with a wavelength of 2,940 nm, pulse energy of 150 mJ, repetition rate of 10 Hz, and average power of 1.5 W in QSP mode²³.
Use a non-contact handpiece with a spot size of 0.9 mm and position the laser beam perpendicular to the specimen surface at a distance of 10 mm. Apply the laser in a scanning motion across the surface and perform a single pass per specimen to ensure uniform energy distribution.
Rinse all specimens with deionized water for 1 min to remove surface residues.
Air-dry all specimens for 10 s.

3. Surface analysis by atomic force microscopy

Randomly select 60 PEEK specimens (n = 15 per surface treatment group) for surface analysis.
Perform atomic force microscopy (AFM) in tapping mode under dry conditions to evaluate surface topography (Figure 3A–D).
Use a silicon (Si) probe tip for all measurements and operate the device according to the manufacturer’s standard calibration procedure.
Position each specimen beneath the cantilever and acquire three-dimensional surface images over a scanning area of 10 µm × 10 µm.
Perform three measurements at different locations on each specimen to minimize local surface variability and calculate mean roughness values.
Record root mean square roughness (Sq) and average roughness (Sa) values for each specimen.

4. Preparation of ındirect composite resin discs

Fabricate plexiglass molds to produce disc-shaped composite specimens with a diameter of 5 mm and a thickness of 5 mm (Figure 4A–B).
Place indirect composite resin material into the mold cavities to obtain disc-shaped specimens with a diameter of 5 mm and a thickness of 5 mm.
Polymerize the composite specimens using a laboratory light-curing unit according to the manufacturer’s instructions, ensuring uniform light exposure from all directions.
Remove the polymerized composite discs from the molds and rinse with deionized water for 1 min.
Air-dry the specimens for 10 s.
Abrade the bonding surface of each composite disc using 110 µm aluminum oxide particles at 0.5 MPa pressure for 15 s, maintaining a distance of 10 mm.
Standardize the application by maintaining consistent distance and angulation across all specimens.
Clean all composite discs ultrasonically in distilled water for 10 min and allow them to air-dry.

5. Luting procedure

Divide each surface treatment group into two subgroups (n = 30) according to the luting cement used: zinc oxide non-eugenol cement and self-adhesive resin cement.
For zinc oxide non-eugenol cement, dispense equal lengths of base and catalyst pastes onto a mixing pad and mix for approximately 20 s until a homogeneous consistency is achieved. Apply the mixed cement to the treated PEEK surface using a spatula, applying gentle pressure to minimize air entrapment.
For self-adhesive resin cement, attach an automix tip to the syringe and dispense a small initial amount to ensure proper mixing. Then, apply the cement directly to the treated PEEK surface, keeping the tip in contact to maintain a continuous flow and reduce air entrapment, and cover the entire bonding area with a uniform layer.
Position the composite disc centrally onto the cement-covered PEEK surface using visual alignment and apply a standardized load to ensure consistent seating and cement thickness, avoiding lateral movement during seating.
Apply a constant vertical load of 500 g for 10 s to standardize cement thickness and ensure uniform adaptation (Figure 5).
Remove excess cement from the margins using a microbrush.
For zinc oxide non-eugenol cement, allow a setting time of 6 min under the applied load.
For self-adhesive resin cement, polymerize using a light-curing device (from all directions for a total of 80 s at a distance of 5 mm.
Maintain consistent positioning and alignment of all specimens throughout the procedure to minimize variability.

6. Thermocycling

Divide each cement subgroup into two additional subgroups (n = 15) according to thermocycling conditions: with thermocycling and without thermocycling.
Subject specimens in the thermocycling group to 5,500 cycles between 5 °C and 55 °C (±2 °C) using a thermocycling device²⁰^,²⁹^,³⁰(Figure 6A–B).
Set the dwell time in each bath to 30 s and the transfer time between baths to 2 s.
Maintain consistent cycling conditions for all specimens throughout the procedure.

7. Shear bond strength testing

Embed each cemented specimen (PEEK–cement–composite assembly) in self-curing acrylic resin using stainless steel molds with a diameter of 15 mm and a height of 15 mm.
Mount each specimen in a universal testing machine using a custom jig to ensure stable positioning.
Align the loading blade parallel to the bonding interface and position it as close as possible to the interface.
Apply shear force at a crosshead speed of 1 mm/min until failure occurs (Figure 7).
Record the maximum load at failure (N) for each specimen.
Calculate shear bond strength (SBS) values in megapascals (MPa) using the formula: SBS (MPa) = Load (N) / bonding area (mm²)¹⁴.

8. Failure mode analysis

Examine debonded specimens under a stereomicroscope at 40x magnification.
Classify failure modes according to previously described criteria as adhesive, mixed, or cohesive failures, as defined in the literature³¹.
1. Identify adhesive failure when debonding occurs at the interface between PEEK and the luting cement or between the cement and the indirect composite, with no cement remnants observed on either substrate.
2. Identify mixed failure when partial cement remnants remain on the PEEK surface while other areas of the substrate are exposed.
3. Identify cohesive failure when fracture occurs within the PEEK substrate or within the indirect composite material.
Record failure mode distributions for each experimental group.

9. Statistical analysis

Perform statistical analysis using SPSS software.
Express surface roughness and shear bond strength data as mean ± standard deviation.
Assess the normality of data distribution using the Shapiro–Wilk test.
Assess homogeneity of variances using Levene’s test. The assumption of homogeneity of variances was satisfied (p > 0.05).
Analyze surface roughness data using the Kruskal–Wallis test to compare differences among surface treatment protocols when data are non-normally distributed.
Perform pairwise comparisons using Dunn’s post hoc test with Bonferroni adjustment.
Analyze shear bond strength data using three-way ANOVA analysis to evaluate the main effects and interactions of surface treatment protocol, luting cement type, and thermocycling.
Perform post hoc pairwise comparisons using estimated marginal means with sequential Bonferroni adjustment.
Set the level of statistical significance at p < 0.05.

Results

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This protocol enables reproducible assessment of the effects of surface treatment, luting cement type, and thermocycling on the bonding performance between polyetheretherketone (PEEK) and an indirect composite resin. AFM revealed distinct surface topographies depending on the applied surface treatment (Table 1). Untreated PEEK specimens exhibited relatively smooth and homogeneous surfaces with minimal irregularities. Sandblasted specimens showed increased surface roughness with irregular peaks and valleys, whereas sulfuric acid–etched specimens exhibited a porous morphology characterized by uniformly distributed pits. Laser-irradiated specimens demonstrated shallow, regular surface grooves. Quantitative roughness parameters (Sa and Sq) confirmed these observations, with the highest roughness values recorded for sandblasted specimens and the lowest values for untreated controls. These findings indicate that increased surface roughness does not necessarily correspond to higher bond strength outcomes across all conditions.

Shear bond strength (SBS) values varied according to cement type and thermocycling conditions (Table 2). Specimens luted with self-adhesive resin cement showed higher SBS values than those luted with zinc oxide non-eugenol cement across all surface treatment groups. The highest SBS values were observed in sandblasted specimens luted with resin cement without thermocycling, whereas the lowest values were observed in specimens luted with temporary cement after thermocycling.

Thermocycling reduced SBS values for both cement types (Table 3). Despite this reduction, specimens luted with resin cement maintained higher SBS values than corresponding temporary cement groups. Differences among surface treatment protocols were observed (Tables 4–6). Sandblasted specimens showed higher SBS values compared with other surface treatment groups when resin cement was used. Sulfuric acid–etched and laser-treated specimens demonstrated intermediate SBS values. In specimens luted with temporary cement, untreated PEEK surfaces showed SBS values comparable to or higher than those of treated surfaces.

Failure mode analysis demonstrated predominantly adhesive and mixed failures across all experimental groups (Table 7). Cohesive failure within the PEEK or composite material was not observed. Failure mode distributions were presented descriptively, as the absence of cohesive failures and low expected frequencies in several categories did not allow for valid statistical comparison. Adhesive failures were more frequently observed in specimens luted with resin cement, whereas mixed failures were more common in sandblasted specimens luted with temporary cement. Overall, the protocol demonstrated measurable differences in bonding performance across surface treatment methods, cement types, and thermocycling conditions.

Microtome setup for precise sectioning, biopsy tool and samples for histological analysis.
Figure 1: Preparation of polyetheretherketone (PEEK) specimens. (A) Sectioning of commercially available PEEK rods using a precision cutting device to obtain disc-shaped specimens. (B) Prefabricated PEEK blank prior to sectioning. (C) PEEK specimens after cutting to standardized dimensions. Please click here to view a larger version of this figure.

Chemical process including beaker, centrifuge, flask, spectrometer; setup for compound analysis.
Figure 2: Surface treatment protocols applied to polyetheretherketone (PEEK) specimens. (A) Untreated PEEK specimens served as the control group. (B) Airborne particle abrasion of PEEK specimens using aluminum oxide particles, demonstrating the sandblasting procedure and resulting surface appearance. (C) Chemical surface treatment of PEEK specimens by acid etching. (D) Laser surface treatment of PEEK specimens using Er: YAG laser irradiation. Please click here to view a larger version of this figure.

Atomic force microscopy diagrams showing surface topography variations for different samples.
Figure 3: Representative atomic force microscopy (AFM) images of polyetheretherketone (PEEK) surfaces after different surface treatments. (A) Control group without surface treatment. (B) Sandblasted PEEK specimens. (C) Sulfuric acid–etched PEEK specimens. (D) Laser-treated PEEK specimens. Please click here to view a larger version of this figure.

Microfluidic chip design and injection setup for lab-on-a-chip applications in biomedical research.
Figure 4: Fabrication of indirect composite resin discs. (A) Plexiglas mold used to standardize specimen dimensions. (B) Placement of indirect composite resin into the plexiglas mold prior to polymerization. Please click here to view a larger version of this figure.

Static equilibrium concept, 500g weight on surface, physics experiment setup, balance demonstration.
Figure 5: Luting procedure of indirect composite resin discs to polyetheretherketone (PEEK) specimens. Please click here to view a larger version of this figure.

Biodegradation bags setup and industrial reactor for polymer decomposition process study.
Figure 6: Thermocycling procedure applied to cemented specimens. (A) Specimen subgroups were prepared for thermocycling. (B) Water baths in the thermocycler are used for artificial aging. Please click here to view a larger version of this figure.

Force application on PEEK-cement interface; mechanical testing setup, stress analysis diagram.
Figure 7: Shear bond strength testing procedure. Representative image of a specimen during shear bond strength testing, together with a schematic illustration showing the direction of force application during the test. Please click here to view a larger version of this figure.

		Descriptive				Kruskal Wallis		Pairwise Comparisons
		Statistic				Kruskal Wallis		Pairwise Comparisons
Variable	Surface Treatment Method	N	Mean	SD	Mean	K-W H	p	Dunn’s
Variable	Surface Treatment Method	N	Mean	SD	Rank	K-W H	p	Post Hoc
RMS Deviation (Sq) (µm)	Control⁽¹⁾	15	190.44	56.15	21.47	27,263	0.000	1-2^a
	Sandblasting⁽²⁾	15	471.23	139.17	50.60			2-3^a
	Sulfuric Acid⁽³⁾	15	206.11	125.89	23-Mar			2-4^a
	Laser⁽⁴⁾	15	223.13	81.66	26.90
Mean Deviation (Sa) (µm)	Control⁽¹⁾	15	151.92	48.13	21.60	26,930	0.000	1-2^a
	Sandblasting⁽²⁾	15	379.95	116.91	50.40			2-3^a
	Sulfuric Acid⁽³⁾	15	163.52	103.01	22.57			2-4^a
	Laser⁽⁴⁾	15	177.51	59.67	27.43

Table 1: Surface roughness values (µm) for each surface treatment group. Roughness parameters (Sa and Sq) obtained from atomic force microscopy measurements are presented as mean and standard deviation values. Differences among surface treatment groups were analyzed using the Kruskal–Wallis test with Bonferroni-adjusted pairwise comparisons. *Significant at the 0.05 level.

Surface treatments	TempBond NE	TempBond NE	RelyX U200	RelyX U200
	+	+	+	+
	thermocycling	no thermocycling	thermocycling	no thermocycling
Control	4.014 (±0.558)	6.772 (±0.946)	6.871 (±0.719)	13.346 (±1.948)
Sandblasting	1.958 (±0.345)	3.120 (±0.743)	17.488 (±2.073)	25.134 (±1.665)
Acid etching	2.562 (±0.315)	4.790 (±0.277)	9.405 (±0.837)	14.699 (±1.506)
Laser irradiation	1.968 (±0.544)	3.190 (±3.190)	11.727 (±1.724)	16.854 (±1.517)

Table 2: Means and standard deviations of SBS values (MPa) of specimens with different surface treatments and luting cements. Mean shear bond strength (SBS) values (MPa) and standard deviations for all experimental groups according to surface treatment method, luting cement type, and thermocycling condition. Each group represents a combination of these variables.

Source	Type III Sum of Squares	df	Mean Square	F	Sig.	Partial Eta Squared
Corrected Model	1,07,08,655	15	7,13,910	5,10,632	.000*	.972
Intercept	1,94,13,135	1	1,94,13,135	1,38,85,457	.000*	.984
Roughening Method	7,03,557	3	2,34,519	1,67,742	.000*	.692
Cement	71,20,405	1	71,20,405	50,92,947	.000*	.958
Thermal Cycle	9,54,869	1	9,54,869	6,82,980	.000*	.753
*Surface Treatment Method Cement**	16,08,654	3	5,36,218	3,83,536	.000*	.837
*Surface Treatment Method Thermal Cycle**	19,233	3	6,411	4,585	.004*	.058
*Cement Thermal Cycle**	2,76,479	1	2,76,479	1,97,754	.000*	.469
*Surface Treatment Method Cement * Thermal Cycle**	25,460	3	8,487	6,070	.001*	.075
Error	3,13,172	224	1,398
Total	3,04,34,962	240
Corrected Total	1,10,21,827	239

Table 3: Statistical analyses of between-subjects effects. Results of three-way ANOVA evaluating the effects of surface treatment method, luting cement type, and thermocycling on shear bond strength. Main effects and interaction effects are presented with corresponding F-values, degrees of freedom, significance levels, and effect sizes. *Significant at the 0.05 level.

(I) Surface Treatment Method	(J) Surface Treatment Method	Mean Difference	Std. Error	Sig.^b	95% Confidence Interval for Difference ^b
(I) Surface Treatment Method	(J) Surface Treatment Method	(I-J)	Std. Error	Sig.^b	Lower Bound	Upper Bound
Control	Sandblasting	-4.174*	.216	.000	-4,749	-3,600
	Sulfuric Acid	-.113	.216	1,000	-.688	.461
	Laser	-.684*	.216	.010	-1,259	-.110
Sandblasting	Control	4.174*	.216	.000	3,600	4,749
	Sulfuric Acid	4.061*	.216	.000	3,486	4,636
	Laser	3.490*	.216	.000	2,915	4,065
Sulfuric Acid etching	Control	.113	.216	1,000	-.461	.688
	Sandblasting	-4.061*	.216	.000	-4,636	-3,486
	Laser	-.571	.216	.052	-1,146	.004
Laser irradiation	Control	.684*	.216	.010	.110	1,259
	Sandblasting	-3.490*	.216	.000	-4,065	-2,915
	Sulfuric Acid	.571	.216	.052	-.004	1,146

Table 4: Comparisons of shear bond strength (MPa) between surface treatment methods. Pairwise comparisons of shear bond strength (MPa) between surface treatment methods based on estimated marginal means. Bonferroni adjustment was applied for multiple comparisons. *Significant differences at the 0.05 level.

Cement	(I) Surface Treatment Methods	(J) Surface Treatment Methods	Mean Difference	Std. Error	Sig.^b	95% Confidence Interval for Difference ^b
Cement	(I) Surface Treatment Methods	(J) Surface Treatment Methods	(I-J)	Std. Error	Sig.^b	Lower Bound	Upper Bound
RelyX U200	Control	Sandblasting	-11.202*	.305	.000	-12,015	-10,390
		Sulfuric Acid	-1.944*	.305	.000	-2,756	-1,131
		Laser	-4.182*	.305	.000	-4,995	-3,370
	Sandblasting	Control	11.202*	.305	.000	10,390	12,015
		Sulfuric Acid	9.259*	.305	.000	8,446	10,072
		Laser	7.020*	.305	.000	6,207	7,833
	Sulfuric Acid	Control	1.944*	.305	.000	1,131	2,756
		Sandblasting	-9.259*	.305	.000	-10,072	-8,446
		Laser	-2.239*	.305	.000	-3,051	-1,426
	Laser	Control	4.182*	.305	.000	3,370	4,995
		Sandblasting	-7.020*	.305	.000	-7,833	-6,207
		Sulfuric Acid	2.239*	.305	.000	1,426	3,051
Temp Bond NE	Control	Sandblasting	2.854*	.305	.000	2,041	3,667
		Sulfuric Acid	1.717*	.305	.000	.904	2,530
		Laser	2.814*	.305	.000	2,001	3,626
	Sandblasting	Control	-2.854*	.305	.000	-3,667	-2,041
		Sulfuric Acid	-1.137*	.305	.001	-1,950	-.324
		Laser	-.040	.305	1,000	-.853	.773
	Sulfuric Acid	Control	-1.717*	.305	.000	-2,530	-.904
		Sandblasting	1.137*	.305	.001	.324	1,950
		Laser	1.097*	.305	.002	.284	1,909
	Laser	Control	-2.814*	.305	.000	-3,626	-2,001
		Sandblasting	.040	.305	1,000	-.773	.853
		Sulfuric Acid	-1.097*	.305	.002	-1,909	-.284

Table 5: Comparisons of shear bond strength (MPa) between surface treatment methods for each cement. Pairwise comparisons of shear bond strength (MPa) between surface treatment methods for each luting cement type based on estimated marginal means. Bonferroni-adjusted multiple comparisons were performed. *Significant differences at the 0.05 level.

Thermal Cycle	(I) Surface Treatment Methods	(J) Surface Treatment Methods	Mean Difference (I-J)	Std. Error	Sig.^b	95% Confidence Interval for Difference ^b
Thermal Cycle	(I) Surface Treatment Methods	(J) Surface Treatment Methods	Mean Difference (I-J)	Std. Error	Sig.^b	Lower Bound	Upper Bound
No	Control	Sandblasting	-4.068*	.305	.000	-4,880	-3,255
		Sulfuric Acid	.315	.305	1,000	-.498	1,127
		Laser	.037	.305	1,000	-.776	.850
	Sandblasting	Control	4.068*	.305	.000	3,255	4,880
		Sulfuric Acid	4.382*	.305	.000	3,570	5,195
		Laser	4.105*	.305	.000	3,292	4,917
	Sulfuric Acid	Control	-.315	.305	1,000	-1,127	.498
		Sandblasting	-4.382*	.305	.000	-5,195	-3,570
		Laser	-.278	.305	1,000	-1,090	.535
	Laser	Control	-.037	.305	1,000	-.850	.776
		Sandblasting	-4.105*	.305	.000	-4,917	-3,292
		Sulfuric Acid	.278	.305	1,000	-.535	1,090
Yes	Control	Sandblasting	-4.281*	.305	.000	-5,094	-3,468
		Sulfuric Acid	-.541	.305	.466	-1,354	.271
		Laser	-1.406*	.305	.000	-2,218	-.593
	Sandblasting	Control	4.281*	.305	.000	3,468	5,094
		Sulfuric Acid	3.740*	.305	.000	2,927	4,552
		Laser	2.875*	.305	.000	2,063	3,688
	Sulfuric Acid	Control	.541	.305	.466	-.271	1,354
		Sandblasting	-3.740*	.305	.000	-4,552	-2,927
		Laser	-.865*	.305	.030	-1,677	-.052
	Laser	Control	1.406*	.305	.000	.593	2,218
		Sandblasting	-2.875*	.305	.000	-3,688	-2,063
		Sulfuric Acid	.865*	.305	.030	.052	1,677

Table 6: Comparisons of shear bond strength (MPa) between thermal cycle groups for each cement. Pairwise comparisons of shear bond strength (MPa) between thermocycling conditions for each luting cement type based on estimated marginal means. Bonferroni adjustment was used for multiple comparisons. *Significant differences at the 0.05 level.

Surface Treatments	Cement Types	Thermal Cycle	Failure Modes				n
		(+/-)	Adhesive	Cohesive		Mixed
			(PEEK/Cement)
Control	Temp Bond NE	+	13 (86.7%)	-		2 (13.3%)	15
		-	10 (66.6%)	-		5 (33.4%)	15
	Rely X	+	15 (100%)	-		-	15
		-	15 (100%)	-		-	15
Sandblasting	Temp Bond NE	+	-	-		15 (100%)	15
		-	4 (26.7%)	-		11 (73.3%)	15
	Rely X	+	15 (100%)	-		-	15
		-	7 (46.7%)	-		8 (53.3%)	15
Sulfuric Acid Etching	Temp Bond NE	+	3 (20%)	-		12 (80%)	15
		-	2 (13.3%)	-		13 (86.7%)	15
	Rely X	+	9 (60%)	-		6 (40%)	15
		-	11 (73.3%)	-		4 (26.7%)	15
Laser Irradiation	Temp Bond NE	+	12 (80%)	-		3 (20%)	15
		-	7 (46.7%)	-		8 (53.3%)	15
	Rely X	+	14 (93.4%)	-		1 (6.6%)	15
		-	13 (86.7%)	-		2 (13.3%)	15
Total			150		-	90	240
			(62.5%)			(37.5%)

Table 7: Distribution of failure modes (adhesive and mixed) across all experimental groups. Each group represents a combination of surface treatment, luting cement type, and thermocycling condition. Failure mode distributions are presented descriptively.

Discussion

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The present study introduced a standardized in vitro. protocol for evaluating the combined effects of surface treatment protocols, luting cement selection, and thermocycling on the shear bond strength between polyetheretherketone (PEEK) and an indirect composite material. The results demonstrated that these variables significantly influenced bonding outcomes, thereby rejecting the null hypothesis. Surface treatment constitutes a critical component of the protocol. Consistent with previous investigations, sandblasting with aluminum oxide particles produced the highest surface roughness values, confirming its effectiveness in generating micromechanical retention on PEEK surfaces⁶^,⁹^,¹¹^,²⁴.Chemical surface treatments, particularly sulfuric acid etching, have been widely reported to modify PEEK surface chemistry and enhance bonding potential⁸^,⁹^,¹⁰^,¹¹^,¹²^,¹³^,¹⁴,while laser-based surface treatments have been shown to increase surface roughness without the use of aggressive chemicals¹⁷^,²³^,²⁹^,³²^,³³^,³⁴^,³⁵^,³⁶^,³⁷.However, the present protocol demonstrates that increased surface roughness alone does not uniformly correspond to increased bond strength, highlighting the importance of evaluating surface modification strategies in combination with other variables.

The type of luting cement also plays a decisive role in the protocol. In agreement with previous studies, specimens luted with self-adhesive resin cement demonstrated higher shear bond strength values than those luted with temporary cement across surface treatment protocols⁹^,¹⁹^,²⁴^,³⁸^,³⁹^,⁴⁰.This finding indicates that cement type plays a critical role in interfacial bonding performance. While resin cement may provide improved adaptation to the substrate, temporary cements exhibited lower bond strength values, which may be associated with differences in both their chemical composition and physical properties. In particular, the higher viscosity and limited penetration of temporary cements into surface irregularities, along with their lack of resin-based adhesive components, may contribute to reduced interfacial bonding performance⁴¹. Furthermore, the lack of improvement in bonding performance with surface-treated specimens luted with temporary cement suggests that increased surface roughness does not necessarily enhance bonding when the cement cannot adequately infiltrate the modified surface. This may result in insufficient adaptation and potential air entrapment, ultimately reducing effective interfacial contact and bond strength.

Thermocycling constitutes an essential component of the protocol for simulating intraoral aging conditions, with the selected number of cycles corresponding approximately to 4 to 5 years of clinical aging¹⁴. Consistent with earlier reports, thermocycling resulted in reduced shear bond strength values across all groups⁶^,¹⁷^,²⁵^,³⁵^,⁴²^,⁴³.These reductions may be attributed to water sorption, thermal stresses, and interfacial degradation during repeated temperature fluctuations. The extent of this reduction appears to vary depending on the surface treatment and cement type, suggesting differences in interfacial stability under thermal stress conditions. Therefore, the inclusion of thermocycling enhances the ability of the protocol to assess bonding durability under simulated aging conditions.

A major contribution of this protocol lies in its standardization of critical experimental parameters, including surface treatment application, cementation procedures, and mechanical testing conditions. Variability in shear bond strength testing has been widely reported in the literature, often due to differences in specimen preparation, cement thickness, alignment, and load application. By controlling parameters such as cement thickness through standardized loading, centralization of the composite disc, and alignment of the loading blade, the present protocol reduces methodological variability and improves reproducibility across experiments. Failure mode analysis further supports the methodological value of the protocol. In agreement with previous studies evaluating pretreated PEEK surfaces, adhesive and mixed failures were predominantly observed across the groups, and no cohesive failures were detected³².These findings highlight that surface modification and cement selection influence not only the magnitude of bond strength but also the failure mechanism. The lack of cohesive failures indicates that the tested protocol primarily challenges the interfacial integrity rather than the bulk properties of the materials. Incorporating failure mode analysis into the protocol provides complementary information that aids the interpretation of shear bond strength data.

Several limitations of the protocol should be acknowledged. As an in vitro. method, it cannot fully replicate the complex mechanical loading, moisture exposure, and biological factors present in the oral environment. In addition, shear bond strength testing represents a simplified loading model and does not account for multidirectional stresses encountered clinically. Further limitations include the use of a single type of indirect composite material, the absence of surface chemistry analysis, and the lack of fatigue or cyclic loading tests, all of which may influence bonding performance under clinical conditions. Nevertheless, the protocol provides a standardized, adaptable framework for systematically comparing bonding strategies between PEEK and indirect composite materials.

Overall, this protocol provides a reproducible and structured framework for evaluating PEEK–composite bonding under controlled conditions. It enables systematic comparison of surface treatments, luting cements, and aging procedures, and may serve as a reference method for future studies aiming to improve consistency and comparability in bonding research involving high-performance polymers.

Within the limitations of this in vitro. protocol, the bonding performance between polyetheretherketone (PEEK) and an indirect composite material is influenced by the combined effects of surface treatment, luting cement type, and thermocycling. Sandblasting significantly enhances shear bond strength when used in conjunction with self-adhesive resin cement, whereas surface treatments do not improve bonding when temporary cement is used. Thermocycling reduces bond strength across all groups, indicating degradation of the bonded interface over time. The standardized protocol presented in this study enables reproducible evaluation of PEEK–composite bonding performance and provides a structured framework for comparing surface treatment, cementation, and aging conditions.

Disclosures

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The authors declare no conflicts of interest.

Materials

List of materials used in this article
Name	Company	Catalog Number	Comments
Polyetheretherketone (PEEK) rods	Mitsubishi Chemical Group (MCAM)	KETRON® CLASSIX LSG PEEK (white)	Used to fabricate disc-shaped specimens
Precision cutting device	Buehler	IsoMet 1000	Used for sectioning PEEK rods into standardized discs
Polishing device	Buehler	Phoenix Beta	Used for standardized surface polishing
Silicon carbide abrasive paper (600-grit)	3M	01993 (Wetordry™ Sandpaper Sheet, 600 grit)	Used under water cooling for surface polishing
Ultrasonic cleaner	Tecno-Gaz	Astra S	Used for cleaning specimens
Aluminum oxide particles (110 µm)	—	—	Used for airborne particle abrasion
Sandblasting unit	Danville Engineering	MicroEtcher ERC	Used for surface roughening
Sulfuric acid (98%)	Sigma-Aldrich (Merck)	258105	CAUTION: Corrosive; handle with appropriate protective equipment
Er:YAG laser system	Hoya ConBio	—	Used for laser surface treatment of PEEK
Atomic force microscope	Quesant Instrument Corporation, USA	—	Used for surface topography analysis
Indirect composite resin	GC Corporation	Gradia Indirect	Used for fabrication of composite discs
Zinc oxide non-eugenol cement	Kerr	TempBond NE	Temporary luting cement
Self-adhesive resin cement	3M ESPE	RelyX U200 Automix	Resin luting cement
Light-curing unit	Kerr	Optilux	Used for polymerization of resin cement
Thermocycler	Salubris Technica	Dentester	Used for artificial aging
Self-curing acrylic resin	Bayer Dental Ltd.	Meliodent	Used for specimen embedding
Universal testing machine	Instron	3345	Used for shear bond strength testing
Stereomicroscope	Nikon	SMZ 800	Used for failure mode analysis

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Standardized Protocol to Evaluate the Effect of Surface Treatments, Luting Cements, And Thermocycling On PEEK–Composite Bond Strength

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Summary

Abstract

Introduction

Protocol

Results

Discussion

Disclosures

Materials

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