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

Summary

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

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

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 appearance^1,2,3. 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 materials^5,6.

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 strength^6,7. Various surface treatment strategies have been proposed to overcome this limitation, including sulfuric acid etching^{8,9,10,11,12,13,14}, airborne particle abrasion with aluminum oxide^{6,8,9,10,11,12,13,14,15,16}, tribochemical silica coating^{8,15,16,17,18,19}, plasma treatments^9,20,21,22, and laser irradiation^{6,23,24,25,26,27}. 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 time^6,17,25. 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

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

1. 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).
2. 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.
3. Polish the bonding surface of each specimen using 600-grit silicon carbide abrasive paper under continuous water cooling.
4. Clean all specimens ultrasonically in distilled water for 10 min.
5. 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.

1. 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).
2. For the control group, rinse the specimens with deionized water for 1 min and air-dry for 10 s.
3. 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 specimens^15,16.
4. For the acid-etching group, apply 98% sulfuric acid to the bonding surface for 60 s¹³.
5. 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.
6. 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.
7. Rinse all specimens with deionized water for 1 min to remove surface residues.
8. Air-dry all specimens for 10 s.

3. Surface analysis by atomic force microscopy

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

4. Preparation of ındirect composite resin discs

1. Fabricate plexiglass molds to produce disc-shaped composite specimens with a diameter of 5 mm and a thickness of 5 mm (Figure 4A–B).
2. 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.
3. Polymerize the composite specimens using a laboratory light-curing unit according to the manufacturer’s instructions, ensuring uniform light exposure from all directions.
4. Remove the polymerized composite discs from the molds and rinse with deionized water for 1 min.
5. Air-dry the specimens for 10 s.
6. 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.
7. Standardize the application by maintaining consistent distance and angulation across all specimens.
8. Clean all composite discs ultrasonically in distilled water for 10 min and allow them to air-dry.

5. Luting procedure

1. 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.
2. 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.
3. 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.
4. 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.
5. Apply a constant vertical load of 500 g for 10 s to standardize cement thickness and ensure uniform adaptation (Figure 5).
6. Remove excess cement from the margins using a microbrush.
7. For zinc oxide non-eugenol cement, allow a setting time of 6 min under the applied load.
8. 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.
9. Maintain consistent positioning and alignment of all specimens throughout the procedure to minimize variability.

6. Thermocycling

1. Divide each cement subgroup into two additional subgroups (n = 15) according to thermocycling conditions: with thermocycling and without thermocycling.
2. Subject specimens in the thermocycling group to 5,500 cycles between 5 °C and 55 °C (±2 °C) using a thermocycling device^20,29,30(Figure 6A–B).
3. Set the dwell time in each bath to 30 s and the transfer time between baths to 2 s.
4. Maintain consistent cycling conditions for all specimens throughout the procedure.

7. Shear bond strength testing

1. 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.
2. Mount each specimen in a universal testing machine using a custom jig to ensure stable positioning.
3. Align the loading blade parallel to the bonding interface and position it as close as possible to the interface.
4. Apply shear force at a crosshead speed of 1 mm/min until failure occurs (Figure 7).
5. Record the maximum load at failure (N) for each specimen.
6. Calculate shear bond strength (SBS) values in megapascals (MPa) using the formula: SBS (MPa) = Load (N) / bonding area (mm²)¹⁴.

8. Failure mode analysis

1. Examine debonded specimens under a stereomicroscope at 40x magnification.
2. 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.
3. Record failure mode distributions for each experimental group.

9. Statistical analysis

1. Perform statistical analysis using SPSS software.
2. Express surface roughness and shear bond strength data as mean ± standard deviation.
3. Assess the normality of data distribution using the Shapiro–Wilk test.
4. Assess homogeneity of variances using Levene’s test. The assumption of homogeneity of variances was satisfied (p > 0.05).
5. Analyze surface roughness data using the Kruskal–Wallis test to compare differences among surface treatment protocols when data are non-normally distributed.
6. Perform pairwise comparisons using Dunn’s post hoc test with Bonferroni adjustment.
7. 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.
8. Perform post hoc pairwise comparisons using estimated marginal means with sequential Bonferroni adjustment.
9. Set the level of statistical significance at p < 0.05.

Results

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 valley...

Discussion

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, sandbla...

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