Optimization of Bond Strength Between Heat-Polymerized PMMA and Contemporary CAD/CAM Framework Materials: A Comparative In Vitro Study

Abstract

This study aimed to comparatively evaluate the effects of various surface treatment protocols on the shear bond strength (SBS) between heat-polymerized polymethyl methacrylate (PMMA) and different CAD/CAM framework materials, including cobalt-chromium (Co-Cr) alloys, ceramic particle-reinforced polyetheretherketone (PEEK), and glass fiber-reinforced composite resin (FRC). A total of 135 disc-shaped specimens were prepared from Co-Cr, PEEK, and FRC materials. Surface treatments specific to each material, including airborne-particle abrasion, sulfuric acid etching, laser irradiation, plasma activation, and primer application, were applied. PMMA cylinders were polymerized onto the treated surfaces, and all specimens were subjected to 30,000 thermal cycles. SBS values were measured using a universal testing machine, and the failure modes were classified. The normality of data distribution was assessed using the Kolmogorov-Smirnov test, and the homogeneity of variances was evaluated using Levene's test. Group comparisons were performed using the Kruskal-Wallis test, and Dunn's post hoc test with Bonferroni correction was applied in cases where significant differences were detected (α = 0.05). The highest SBS values (~27-28 MPa) were obtained in the Co-Cr group and in the PEEK groups treated with sulfuric acid and primer. In contrast, the PEEK group with additional laser treatment exhibited a lower SBS value. The untreated PEEK group showed significantly lower SBS (~3.9 MPa) compared to all other groups. The Trinia groups demonstrated intermediate SBS values (16.5-17.4 MPa), which exceeded the clinically acceptable threshold of 10 MPa. SEM observations revealed material- and protocol-specific surface responses; plasma-treated specimens maintained topographic integrity, whereas laser-induced surfaces showed localized degradation, particularly following dual-step protocols. Fracture mode analysis indicated that higher SBS values were associated with cohesive or mixed failures. SEM observations suggested that plasma treatment preserved surface morphology more effectively than laser treatment. This study highlights the importance of selecting material-specific surface treatments to optimize bonding between CAD/CAM frameworks and PMMA. Sulfuric acid and primer provided strong adhesion for PEEK, while the addition of laser or plasma offered no further benefit, making such steps potentially unnecessary. Trinia frameworks also showed acceptable performance with conventional treatments. These findings reinforce that simplified conditioning protocols may be clinically sufficient, and indicate that FRC materials like Trinia should be more fully considered for their broader clinical potential in modern CAD/CAM-based prosthetic planning.

Keywords: CAD/CAM frameworks; polymer composites; surface properties.

Figure 1

Minimum, median, and maximum shear bond strength (SBS) values (MPa) of all experimental groups following surface treatment and thermal cycling.

Figure 2

Representative stereomicroscopic images of fracture types observed after shear bond strength testing. (A) Adhesive failure at interface between PEEK substrate and polymer. (B) Cohesive failure within polymer material. (C) Mixed cohesive failure involving both polymer and PEEK substrate. Images were captured under 20× magnification using a stereomicroscope.

Figure 3

Relationship between adhesive failure ratios and mean shear bond strength (SBS) values for each experimental group.

Figure 4

SEM image of untreated BioHPP surface at ×20,000 magnification. A smooth and morphologically intact surface is observed, with no signs of microstructural damage or mechanical roughening.

Figure 5

SEM image of sulfuric acid-treated BioHPP surface at ×20,000 magnification. (a) Submicron-scale pits resulting from amorphous layer degradation; (b) acid-induced porosities indicating enhanced surface activation; (c) irregular erosion patterns and matrix softening suggest partial loss of microscale uniformity. White arrows mark key surface features. Scale bar: 1 µm.

Figure 6

SEM image of BioHPP surface treated with sulfuric acid, primer, and laser at ×10,000 magnification. (a) Local melting of surface due to thermal effects; (b) Flow marks indicating material displacement; partially collapsed pits and surface irregularities are also visible. White arrows mark key features. Scale bar: 1 µm.

Figure 7

SEM image of same group at ×20,000 magnification. (a) Fused pits with indistinct boundaries; (b) matrix deformation visible as blurred and irregular regions; (c) contraction rings indicating localized shrinkage. Black arrows indicate characteristic morphological features. Scale bar: 1 µm.

Figure 8

SEM image of BioHPP surface treated with sulfuric acid, primer, and plasma at ×10,000 magnification. A nano-textured surface with uniform shallow dimpled features is observed, without signs of thermal deformation.

Figure 9

SEM image of same group at ×20,000 magnification. Plasma-induced ravine-like nano-topography is more evident, with preserved matrix continuity and no fiber or resin damage. A representative ravine-like structure is indicated with a black arrow.

Figure 10

SEM image of Trinia surface after CoJet treatment at ×20,000 magnification. (a) Shallow micro-pits formed by silica particle impact; (b) smoothened matrix regions with no visible fiber exposure or structural damage. White arrows mark the key features. Scale bar: 1 µm.

Figure 11

SEM image of Trinia surface after airborne-particle abrasion with aluminum oxide at ×20,000 magnification. Disrupted resin matrix and irregular surface roughness are evident, indicating the mechanical impact of abrasive particles. Scale bar: 200 nm.

Figure 12

SEM image of Trinia surface after airborne-particle abrasion and laser treatment at ×10,000 magnification. (a) Partial matrix melting with evidence of flow deformation; (b) disrupted fiber–matrix interface with loss of continuity. White arrows indicate representative morphological features. Scale bar: 1 µm.

Figure 13

SEM image of same group at ×20,000 magnification. (a) Laser-induced amorphous matrix zones characterized by smooth, glassy surface texture; (b) micro-shrinkage features appearing as localized depressions or contraction rings. White arrows indicate key morphological features. Scale bar: 1 µm.

Figure 14

SEM image of Trinia surface after airborne-particle abrasion and plasma treatment at ×10,000 magnification. Surface shows preserved micro-roughness with localized smoothing and refined texture.

Figure 15

SEM image of same group at ×20,000 magnification. (a) Nano-scale topographic patterns formed by plasma–material interaction; (b) shallow fissure-like features indicating localized degradation. White arrows indicate representative morphological features. Scale bar: 1 µm.