To calculate the sample size, an online power analysis calculator was used. The mean and standard deviation values were obtained from a previously published study (8). The statistical power was set at 80%, and the level of significance was set at 0.05. Based on these parameters, the calculated sample size indicated the need for ten specimens per group, resulting in a total of 300 specimens were used to test the SBS. In this study, five distinct denture base materials were evaluated: two CAD/CAM milled resins, AvaDent and IvoCad; two 3D-printed resins, FormLabs and NextDent; and one heat-polymerized resin: Major Base 20 was used in this study with one hard denture liner (Ufi Gel Hard C) and one soft liner (KEYSTONE Versacryl Reline Kit—Self Cure Soft Reline) (Table 1).

Acrylic denture bases were prepared in dimensions of 10 × 10 × 3 mm according to the manufacturer's recommendations. All specimens were standardized in terms of the polishing method and were examined by a single investigator. The prepared specimens were polished using silicon carbide abrasive papers available in various grits (500, 800, 1,200) that were mounted on an automated polishing machine (Metaserve 250 grinder-polisher; Buehler, Lake Bluff, IL, USA).

Each specimen was embedded in self-polymerizing acrylic resin with one surface completely exposed using a custom-made, cylindrical-shaped silicon mold measuring 19 × 10 mm. Following polymerization, the exposed surfaces of the specimens were carefully examined, and surface irregularities were eliminated.

Then, the specimens were subjected to 5,000 thermal cycles before the relining procedure to simulate aged denture bases, and after the relining procedure to evaluate the shear bond strength between the reline material and the denture base, 5 °C–55 °C with a 30 s dwell time within a thermocycling machine (Thermocycler, THE-1100/THE-1200, SD Mechatronik GMBH Miesbacher, Westerham Germany).

The specimens were divided into two major groups based on the type of reline materials (n = 150). Each acrylic resin base was further divided into three groups according to the surface treatment: the first group was without surface treatment (control); the second group was treated with sandblasting with 250 μm aluminum oxide particles (SB) using a sandblasting machine (Wassermann Dental Machine, GmbH, Hamburg, Germany) from 1 cm distance for 10 s with 3.5 bar pressure. A customized metal jig with an inner diameter of 6 mm was used to determine the area of sandblasting application (8). The third group underwent surface roughening using a blue-coded tungsten carbide bur. The bur was applied in one direction under light manual pressure, with approximately 12 unidirectional strokes to standardize the surface treatment. In the case of soft liners, the bonding agent was applied in accordance with the manufacturer's recommendations. Two layers of bonding agent were applied to the denture base, with a drying time of 5 min between each layer.

As described in the previous study (10), At the centre of the prepared denture base specimen, a metal cylinder (4 mm diameter × 6 mm length) was fixed within the holder. A customized mold was prepared, into which each specimen was fixed. The reline materials was then mixed and packed in the space created by the cylinder and covered with a glass slap under pressure (1 kg) for 15 min. After polymerization, the specimens were stored in distilled water (37 °C) for two days. Subsequently, the specimens were subjected to 5,000 thermal cycles prior to shear bond strength testing (Figure 1).

The shear bond strength values of the reline specimens were evaluated using a universal testing machine (Instron, Instron Corp., Norwood, MA, USA). Each specimen was mounted onto a specially designed jig, and a chisel (knife-edge shear type) was used to apply a shear load parallel to the bonded interface at a crosshead speed of 1 mm/min until failure occurred (Figure 2). SBS was calculated using the (MPa) = F/A, where F is the force at failure in newtons (N), and A is the bonding area in square millimetres (mm²).

Normality of the data was checked using the Shapiro–Wilk test and was confirmed to be normally distributed. The statistical analysis was processed using one-way ANOVA and an unpaired two-way ANOVA (GraphPad Prism software). Tukey's post hoc test was used for pairwise comparisons. P-value is set at <0.05 (* if P ≤ 0.05, ** if P ≤ 0.01, *** if P ≤ 0.001, **** if P ≤ 0.0001) is considered statistically significant.

Figure 3 displays representative SEM images illustrating the effect of surface treatments on various denture base resins. The specimens without surface treatment (1st raw) showed a smooth surface with minor irregularities and some striations representing the printing layer and/or polishing grits The specimens’ surfaces underwent a dramatic change in surface topography following SB treatment (2nd row). SB resulted in more irregularities in all treated specimens. While bur roughening (3rd raw) resulted in some roughening, it was localized, uneven, and shallower than the SB effect.

Using sandblasting as a surface treatment with milled denture base resins (AvaDent and IvoCad) resulted in a statistically significant increase in SBS compared to both the control group and bur roughening (p ≤ 0.0001). Additionally, sandblasting applied to the 3D-printed NextDent resin exhibits the highest SBS among all groups in hard liner 72.25 MPa (Table 2; Figure 4) with statistically significant differences compared to both the control and bur roughening groups (p ≤ 0.0001). For FormLabs, sandblasting significantly increased SBS compared to the control group (p ≤ 0.01), but no significant difference was observed when compared to bur roughening; the lowest SBS was recorded in the control group of FormLabs at 11.4 MPa. Similarly, the conventional denture base resin treated with sandblasting demonstrated a significant increase in SBS compared to both the control and bur roughening groups (p ≤ 0.0001).

The combined effect of denture base material and surface treatment on shear bond strength was analyzed using two-way ANOVA. Analysis revealed a significant interaction between the two factors (p = 0.0080) (Table 3).

In this study, the nature of the fracture was analyzed in terms of adhesive, cohesive, or mixed, as described in previous studies. Regarding the nature of failure, adhesive failure was the dominant type in the case of the control group, which accounted for 100% of failures with Avadent, IvoCad, and FormLabs. The other groups subjected to testing demonstrated cohesive and mixed failure modes subsequent to surface treatment, notably with the SB treatment, which exhibited the highest percentage of the previously mentioned types (Figures 5, 6).

The mean and standard deviation (SD) of the shear bond strength for different denture base resins relined with a soft liner are presented in (Figure 7; Table 4). Bur roughening significantly increased the SBS for AvaDent and IvoCad resins compared to the control group (P < 0.0001). However, no significant difference in SBS was found between the bur-roughened and control groups for NextDent (ND) and FormLabs (FL). Conventional denture base resins treated with bur roughening showed significantly higher SBS than the control group (P ≤ 0.01), with the highest SBS value recorded across all soft liner groups (16.82 MPa).

In contrast, sandblasting significantly reduced the SBS of AvaDent, NextDent, FormLabs, and conventional resins when compared to both the control and bur-roughened groups. However, sandblasting resulted in a significant increase in SBS for IvoCad (P < 0.0001).

The combined effect of denture base material and surface treatment on shear bond strength bonded to soft liner was analyzed using two-way ANOVA. The analysis showed a statistically significant interaction between the material and surface treatment (P < 0.0001), as shown in (Table 5).

Cohesive failure was the predominant failure mode observed with the soft liner after bur roughening, accounting for 100% of failures in the AvaDent group and 80% in the conventional denture base group. Additionally, when sandblasting was used, cohesive and mixed failures were frequently observed in both the IvoCad and conventional groups (Figures 6, 8).

In the present study, the milled denture base resins AvaDent and IvoCad demonstrated comparable responses to surface treatment when bonded with a hard reline material. Both materials exhibited a increase in SBS following sandblasting due to increasing the surface area, whereas bur roughening did not result in a significant difference compared to the control group. Similarly, the 3D-printed resins, NextDent and FormLabs, showed a significant enhancement in SBS after sandblasting, with no notable improvement observed with bur roughening. Additionally, the conventional heat-polymerized resin also exhibited a significant increase in SBS following sandblasting treatment. These results suggest that sandblasting is an effective surface treatment method for enhancing the bond strength of various denture base resins to hard reline materials, while bur roughening does not provide a comparable benefit.

Sandblasting enhances shear bond strength by eliminating surface impurities and contaminants from the denture base (30, 31). This process enhances mechanical retention by increasing both the surface area and surface roughness, thereby facilitating improved bonding (31). The roughened surface created by sandblasting promotes better adhesive penetration and mechanical interlocking, resulting in a stronger and more durable bond between the denture base and the reline material (32, 33).

Numerous studies (34, 35) have demonstrated that airborne-particle abrasion with aluminum oxide (Al₂O₃) significantly enhances the bond strength of polymethyl methacrylate (PMMA) denture base resins. Klaiber et al. (34) reported that treating PMMA surfaces with 250 μm Al₂O₃ particles at 0.3 MPa for 30 s significantly increased the bond strength to autopolymerized resin. Erbulak et al. (36) found that, among various surface treatments, abrasion with 110 μm Al₂O₃ at 2 MPa for 30 s produced the strongest bond with autopolymerized acrylic resin. Similarly, Li et al. (37) demonstrated that abrasion using 125 μm Al₂O₃ at 2 bar resulted in the highest shear bond strength (17.31 ± 5.67 MPa), concluding that mechanical surface treatment is effective. Akın et al. (35) also confirmed that airborne-particle abrasion of PMMA surfaces with Al₂O₃ particles improves bonding performance.

In a comparative study, Karaokutan et al. (17) evaluated subtractive, additive, and conventional heat-polymerized resin relined with autopolymerized acrylic resin. Their findings revealed that surfaces treated with 50 μm Al₂O₃ particles exhibited a higher bond strength than tungsten carbide bur. Similarly, our study found that sandblasting provided higher shear bond strength than bur roughening when used with a hard liner. On the other hand, Baig et al. (38) found a statistically significant difference in the SBS of relined specimens (P < 0.05) when various burs were used for surface preparation. However, no significant difference was observed between the standard bur and the control group. Baig et al. concluded that the type of bur used to prepare the denture base for relining may affect the surface roughness and, consequently, the reline bond strength of urethane-based dimethacrylate denture base resin (39).

Sandblasted specimens predominantly exhibited cohesive and mixed failure modes, in contrast to the adhesive failures observed in untreated controls, indicating superior bonding performance. SEM analysis confirmed increased surface irregularities following sandblasting, which probably enhanced mechanical interlocking. As a result, sandblasting significantly improved the shear bond strength of all tested denture base resins relined with a hard liner.

Based on the findings, sandblasting is recommended as an effective surface treatment for CAD/CAM milled 3D-printed, and conventional denture base resins when used with a hard liner. In contrast, bur roughening showed no significant effect on bond strength across any of the denture base types tested in this study when used in combination with a hard liner.

In the present study, the milled denture base resins AvaDent and IvoCad, as well as the conventional heat-polymerized resin, exhibited higher bond strength with the soft liner when treated with bur roughening. However, NextDent and FormLabs, both 3D-printed resins, showed no improvement with bur roughening and demonstrated a negative effect when treated with sandblasting. These findings suggest that bur roughening is an effective surface treatment for improving the bond strength of various denture base resins, conventional and milled, to soft liners, whereas sandblasting does not offer a similar advantage.

The superior shear bond strength observed with PMMA may be attributed to the chemical compatibility between acrylic-based soft liners and conventional denture base materials, largely due to the similarity in their chemical structures (39–41). In contrast, 3D-printed denture bases are composed of light-polymerizable resins that contain additional additives to facilitate photopolymerization. These materials require curing in specialized light-curing units, a process that can alter the surface energy of the printed denture base (41, 42). This explains why 3D-printed denture bases tend to exhibit lower SBS compared to PMMA.

Silicone-based denture liners demonstrate superior mechanical strength and durability compared to resin-based liners; however, they suffer from poor chemical bonding capabilities, with adhesion failures often linked to the bonding agent used (43, 44). Studies have shown that adhesive failure rates between silicone liners and PMMA denture bases can increase significantly, from 13.8% to 60%—after 30 days of water storage indicating a progressive decline in bond strength over time (45). In our study, the aging process was simulated over a six-month period of use, which correlated with a reduction in bond strength. Efforts to improve adhesion using silica air abrasion and salinization were unsuccessful, as the surface roughness generated by 30 μm particles did not permit sufficient liner penetration into the acrylic base. However, mechanical surface roughening by bur may provide better penetration, potentially explaining the relatively high mechanical strength observed despite the material's weak chemical bonding.

Minami et al. (46) reported that roughening the denture base resin surface using air-particle abrasion did not effectively enhance the failure load or improve the long-term bond durability of a soft denture liner to the denture base. Atsü et al. (47) found that sandblasting surface treatments applied to the denture base resin did not improve the tensile bond strength of a silicone-based soft liner to heat-cured acrylic resin. In a systematic review, Özdemir et al. (48) reported that although sandblasting was frequently used as a surface treatment for resilient lining materials, it was ineffective in enhancing bond strength. Due to the limited existing research on the use of bur roughening in combination with soft liners, direct comparisons have been challenging. This study contributes to addressing the existing gap in the literature by demonstrating that bur roughening is recommended with milled and conventional denture bases bonded to soft liners.

Bur roughening surface treatment resulted in an improvement in SBS for milled and conventional denture base resins relined with a soft liner. This enhancement in bond performance was further validated by SEM analysis of the failure modes, which predominantly exhibited cohesive and mixed failures in Bur roughening specimens.

Based on the findings, bur roughening is recommended as an effective surface treatment for both CAD-CAM milled and conventional denture base resins when used with a soft liner. In contrast, neither sandblasting nor bur roughening is recommended for 3D-printed denture base resins in combination with a soft liner.

For clinical implications, it is recommended that sandblasting be an effective surface treatment for enhancing the bond strength of hard liners to milled, 3D-printed, and conventional denture base resins. In contrast, bur roughening is recommended for improving the bond strength of soft liners with milled and conventional resins. Although ISO 10139-2:2016 (49) specifies that soft long-term denture liners must demonstrate a minimum bond strength of 1.0 MPa in at least 8 out of 10 tested specimens, all groups in this study, whether control, sandblasted, or bur-roughened, exhibited higher bond strength values. However, neither sandblasting nor bur roughening is advisable for 3D-printed denture base resins when used with soft liners.

Limitations of this in vitro study include the absence of a complete intraoral environment, such as oral microbiota, salivary pH changes, and occlusal forces. Furthermore, only a 0-degree printing orientation was tested, despite known variations in material properties associated with different printing orientations, and the specimens did not replicate actual denture configurations. Further research is needed, as this study utilized only one type of each liner and examined only two surface treatment methods. Additional investigations employing various types of carbide burs and different sizes of aluminium oxide particles for sandblasting are recommended. In addition to the recommendation of further investigations in condition-simulating oral conditions and in vivo studies.