Tensile bond strength of soft and hard reline materials to conventional heat-polymerized, CAD-CAM milled, and 3D-printed denture base resins: an in vitro comparative study

Abstract

Background

Complete dentures require periodic relining to compensate for progressive alveolar ridge resorption and associated loss of denture retention and stability. With the increasing adoption of digitally fabricated dentures using milling and 3D printing technologies, understanding the bond strength between these novel denture base materials and reline materials is essential for clinical success. This study evaluated the tensile bond strength of soft and hard reline materials bonded to denture base resins fabricated by three different manufacturing methods: conventional heat polymerization, milling, and 3D printing.

Methods

Ninety rectangular specimens were fabricated from three denture base materials: conventional heat-polymerized polymethyl methacrylate (PMMA) (Megacryl Hot), CAD-CAM milled PMMA (Voco Ceditec DB), and 3D-printed resin (Arma Resin). Specimens were divided into nine groups (n = 10) based on denture base material and reline material combinations. Three reline materials were tested: laboratory-processed silicone soft liner (Molloplast-B), chairside silicone soft liner (GC Reline II Soft), and chairside hard liner (Ufi Gel Hard C). All specimens were stored in distilled water at 37 °C for 24 h before testing. Tensile bond strength was measured using a universal testing machine at a crosshead speed of 5 mm/min. Failure modes were classified as adhesive, cohesive, or mixed. Tensile bond strength was analyzed with a two-way ANOVA (denture base material × reline material) on log-transformed data after assumption testing (Shapiro–Wilk, Levene), with the Aligned Rank Transform applied as a non-parametric confirmation. Given a significant interaction, simple main effects were compared with Tukey-adjusted tests, and failure-mode distributions with the Fisher–Freeman–Halton exact test (α = 0.05).

Results

A significant denture base × reline material interaction was found (p < 0.001), indicating that the effect of the denture base material on bond strength depended on the reline material used. The highest bond strength was achieved with Ufi Gel Hard C bonded to conventional PMMA (12.56 ± 2.21 MPa), and the lowest with GC Reline II Soft bonded to 3D-printed resin (1.28 ± 0.16 MPa). Differences among denture bases were reline-dependent: with the hard liner, conventional PMMA showed significantly higher bond strength than 3D-printed resin (p = 0.001); with Molloplast-B, all three denture bases were comparable; and with GC Reline II Soft, both PMMA bases outperformed 3D-printed resin (p < 0.001). Adhesive failures predominated in 3D-printed groups (63.3%), whereas cohesive failures were most common in conventional PMMA groups (46.6%); Ufi Gel Hard C showed predominantly adhesive failures (83.3%), while GC Reline II Soft showed predominantly cohesive (56.7%) and mixed (43.3%) failures.

Conclusions

All tested combinations of denture base and reline materials demonstrated clinically acceptable bond strength, exceeding the minimum threshold of 0.44 MPa established for soft denture liners. The hard chairside reline material exhibited superior bonding performance compared with both soft liners across all denture base types. The effect of denture base material was reline-material dependent: conventional PMMA demonstrated significantly higher bond strength than 3D-printed resin when combined with Ufi Gel Hard C, whereas both conventional and milled PMMA outperformed 3D-printed resin when combined with GC Reline II Soft. No significant differences among the denture base materials were observed with Molloplast-B. Although 3D-printed groups exhibited a higher overall incidence of adhesive failures, their bonding performance varied according to the reline material used. Clinicians should therefore consider the compatibility of each denture base–reline material combination, and further optimization of surface-treatment protocols may be beneficial for 3D-printed denture bases.

Background

Complete dentures remain a fundamental treatment modality for edentulous patients worldwide [1]. The success and longevity of complete dentures depend critically on maintaining proper fit and retention, which are progressively compromised by continuous alveolar ridge resorption following tooth extraction [2]. Denture relining is a routine clinical procedure that compensates for these dimensional changes by adding new material to the tissue-bearing surface, thereby restoring proper adaptation and improving patient comfort and function [3].

Traditionally, denture base resins have been fabricated using conventional heat-polymerized polymethyl methacrylate (PMMA), which has been widely used as a standard material for over seven decades owing to its favorable mechanical properties, biocompatibility, and ease of processing [4]. However, the digital revolution in dentistry has introduced alternative manufacturing methods, including computer-aided design and computer-aided manufacturing (CAD-CAM) milling and additive manufacturing (3D printing), which offer potential advantages in terms of precision, efficiency, and reduced processing time [5, 6].

CAD-CAM milled denture bases are fabricated from industrially polymerized PMMA blocks under controlled conditions, resulting in superior mechanical properties and reduced residual monomer content compared to conventional heat-polymerized resins [7, 8]. Three-dimensional printing technologies, particularly digital light processing (DLP) and stereolithography (SLA), enable rapid fabrication of denture bases from photopolymerizable resins with high accuracy and minimal material waste [9, 10]. Several comparative studies have documented the growing body of evidence on the mechanical properties and clinical performance of additively manufactured versus subtractively manufactured denture base resins [11].

Despite the increasing clinical adoption of digitally fabricated dentures, the bonding performance of reline materials to these novel denture base materials remains incompletely characterized. Reline materials are broadly classified into hard and soft types, each serving distinct clinical indications [12]. Hard reline materials, typically based on PMMA chemistry, are used for long-term restoration of denture fit and function [13]. Soft reline materials, predominantly silicone-based elastomers, are indicated for patients with severely resorbed ridges, chronic soreness, or bruxism, providing a cushioning effect and stress distribution [14, 15].

The bond strength between reline materials and denture base resins is a critical determinant of clinical success, as inadequate bonding leads to interface failure, microbial colonization, and compromised denture function [16]. ISO 10139-2:2016 specifies a minimum tensile bond strength of 1.0 MPa for long-term soft denture lining materials [17], whereas a value of 0.44 MPa has been proposed in the literature as the minimum clinically acceptable bond strength [15]. However, the bonding mechanisms and interface characteristics may differ substantially between conventional and digitally fabricated denture base materials due to variations in surface chemistry, polymerization degree, and microstructural properties [18].

Recent comparative studies have begun to investigate the bond strength of reline materials to digitally fabricated denture bases, with variable findings. Guntekin et al. [19] evaluated the shear bond strength of hard and soft reline materials to conventional, milled, and 3D-printed denture bases, reporting superior performance for conventional PMMA. Wemken et al. [20] demonstrated that CAD-CAM milled denture base materials exhibited comparable or superior bond strength to conventional PMMA when bonded with auto-polymerizing reline resins. Koseoglu et al. [21] investigated the effect of surface characteristics on bonding performance of 3D-printed denture bases, finding that the highly cross-linked surface structure of 3D-printed resins reduces chemical bonding potential with reline materials.

Studies specifically comparing soft reline materials across different denture base manufacturing methods have yielded important insights. Koseoglu et al. [22] reported that soft liners bonded to 3D-printed denture bases demonstrated significantly lower tensile bond strength compared to conventional and milled PMMA, with adhesive failure being the predominant failure mode. Alfaraj et al. [23] found that both auto-polymerizing and heat-polymerizing denture reliners achieved adequate bond strength to CAD-CAM denture base materials, though values were generally lower than those observed with conventional PMMA. Li et al. [24] demonstrated that the addition of soft reline materials significantly reduced the flexural strength of all denture base types, with the most pronounced effect observed in 3D-printed resins.

The performance of hard versus soft reline materials has been extensively investigated in the context of conventional denture bases. Lau et al. [25] conducted an in vitro study demonstrating that hard reline materials consistently achieve higher bond strength than soft liners, attributed to chemical compatibility and interpenetrating polymer network formation with PMMA-based denture bases. Botega et al. [26] reported that hard acrylic-based reliners exhibited bond strengths approximately four to five times greater than silicone-based soft liners. Kulak-Ozkan et al. [27] evaluated the effect of thermocycling on tensile bond strength of silicone-based resilient liners, finding significant degradation after aging, particularly for chairside-processed materials. More recently, Albazroun et al. [28] confirmed that hard reline materials maintain superior bonding performance across different aging conditions compared to soft liners.

The mechanical properties of digitally fabricated denture base materials have been characterized in multiple studies. Iwaki et al. [8] demonstrated that CAD-CAM milled PMMA blocks exhibit higher flexural strength and elastic modulus compared to conventional heat-polymerized resins due to higher degree of polymerization and reduced porosity. Vuksic et al. [29] compared mechanical properties of 3D-printed, CAD-CAM milled, and conventional denture base materials, reporting that milled PMMA demonstrated the highest flexural strength, while 3D-printed resins showed greater variability depending on printing orientation and post-processing protocols. Li et al. [30] investigated the effect of adding hard reline materials on flexural strength, finding that relining reduced the mechanical properties of all denture base types, with the magnitude of reduction being material-dependent.

Surface treatment strategies to enhance bonding between reline materials and denture bases have been extensively investigated. Akin et al. [31] evaluated the effect of different surface treatments including mechanical roughening, chemical etching, and laser irradiation on tensile bond strength of silicone-based soft denture liners, demonstrating that combined mechanical and chemical treatments yielded optimal results. The importance of surface conditioning is particularly relevant for digitally fabricated denture bases, which may have different surface characteristics compared to conventional PMMA due to manufacturing-specific factors such as milling tool marks or layer-by-layer polymerization patterns in 3D printing [5].

Given the increasing clinical adoption of digitally fabricated dentures and the critical importance of successful relining procedures for long-term denture maintenance, comprehensive comparative data on the bonding performance of both hard and soft reline materials to conventional, CAD-CAM milled, and 3D-printed denture base resins are essential. While several recent studies have addressed specific aspects of this topic [32–34], systematic comparison evaluating multiple reline material types across all three major denture base manufacturing methods using standardized testing protocols remains limited. Therefore, the aim of this study was to evaluate and compare the tensile bond strength of laboratory-processed soft liner, chairside soft liner, and chairside hard liner to denture base resins fabricated by conventional heat polymerization, CAD-CAM milling, and 3D printing technologies.

The null hypotheses were: (Ho1) there would be no significant difference in tensile bond strength among the three denture base material groups; (Ho2) there would be no significant difference in tensile bond strength among the three reline material groups; and (Ho3) there would be no significant interaction effect between denture base material type and reline material type on tensile bond strength.

Methods

This in vitro study evaluated the tensile bond strength of denture base materials produced using three different manufacturing techniques when bonded with soft and hard reline materials.

Sample size estimation was performed using G*Power software (version 3.1). An a priori power analysis for a fixed-effects two-way ANOVA was conducted assuming an effect size (f) of 0.40, a significance level (α) of 0.05, and a statistical power (1 − β) of 0.80. Based on these parameters, the minimum required total sample size was calculated as 90 specimens, providing an actual statistical power of 0.823. Accordingly, 10 specimens per group across nine groups (n = 90 total) were fabricated, consistent with sample sizes reported in comparable in vitro tensile bond strength studies [22, 26].

In this study, three distinct types of denture base resins were utilized, each produced using one of the following methods: conventional, subtractive (milling), and additive (3D printing). These resins were combined with three different types of liners: a soft laboratory-side liner, a soft chairside liner, and a hard chairside liner.

For the additive manufacturing group, specimens were fabricated using 3D-printed photopolymerizable resin (Arma Resin, Arma Dental, Kocaeli, Turkey), composed of diphenyl phosphine oxide, aliphatic urethane dimethacrylate, acrylic acid ester, bio-based aliphatic polyester urethane acrylate, organo-modified filler, organo-modified silicon oxide, and metal oxide pigments. The subtractive manufacturing group was prepared using a pre-polymerized PMMA block (Ceditec DB, VOCO GmbH, Cuxhaven, Germany), designed for milling processes. The conventional heat-polymerized group was fabricated using Mega CRYL HOT (Megadental GmbH, Büdingen, Germany), consisting of PMMA (80–90%) and methyl methacrylate (MMA) (10–15%), along with minor amounts of benzoyl peroxide, pigments, and plasticizers.

For soft lining, heat-polymerized silicone-based liner (Molloplast-B, Detax GmbH & Co KG, Ettlingen, Germany) was used in combination with a silane-based adhesive (Primo, Detax GmbH & Co KG, Ettlingen, Germany). Additionally, an auto-polymerized silicone-based soft liner (GC Reline II Soft, GC Europe N.V., Leuven, Belgium) was applied to a separate subgroup. For hard lining, an auto-polymerized methacrylate-based chairside material (Ufi Gel Hard C, Voco GmbH, Cuxhaven, Germany), containing bisphenol A-glycidyl methacrylate (BIS-GMA), hydroxyethyl dimethacrylate (HEDMA), and urethane dimethacrylate, was utilized.

Thus, a total of nine experimental groups were formed, with n = 10 specimens in each group (total 90). All materials were processed and applied following the respective manufacturers’ instructions to ensure standardization and reproducibility across all experimental groups. The materials used in this study, along with their chemical compositions, manufacturing types, and lot numbers, are presented in Table 1.

Table 1.

Materials used in the study along with their manufacturer, chemical composition, and type

Material	Type	Manufacturer	Chemical Composition
Arma Resin	Additive manufactured (3D-printed) denture base resin	Arma Dental Üretim Sis San. Ve Tic Ltd. Şti, Kocaeli, Turkey	Diphenyl phosphine oxide; Aliphatic urethane dimethacrylate; Acrylic acid ester; Bio-based aliphatic polyester urethane acrylate; Organo-modified filler; Organo-modified silicon oxide; Metal oxide pigments
Ceditec DB	Subtractive manufactured (CAD-CAM milled) PMMA	VOCO GmbH, Cuxhaven, Germany	Polymethyl methacrylate (PMMA); Coloring pigments
Mega CRYL HOT	Conventional heat-polymerized PMMA	Megadental GmbH, Büdingen, Germany	PMMA; MMA; Dibenzoyl peroxide; Pigments and stabilizers; Plasticizers
Molloplast-B	Heat-polymerized silicone-based soft liner	Detax GmbH & Co KG, Ettlingen, Germany	Vinyl dimethylpolysiloxane; Benzoyl peroxide
Primo (Adhesive)	Adhesive for Molloplast-B soft liner	Detax GmbH & Co KG, Ettlingen, Germany	γ-methacryloxypropyl trimethoxy silane
GC Reline II Soft	Auto-polymerized silicone-based chairside soft liner	GC Europe N.V., Leuven, Belgium	Silicon dioxide; Methylhydrogen dimethylpolysiloxane
GC Reline II Soft Primer	Primer for GC Reline II Soft	GC Europe N.V., Leuven, Belgium	Methyl acetate; Ethyl acetate; Silicone-modified acrylic resin; Proprietary additives
Ufi Gel Hard C	Auto-polymerized methacrylate-based chairside hard liner	VOCO GmbH, Cuxhaven, Germany	BIS-GMA; HEDMA; Urethane dimethacrylate; Benzoyl peroxide

The machines used for production, curing, surface treatment, and tensile testing are listed in Table 2. For subtractive manufacturing, the Ivoclar Programill PM7 (Ivoclar Vivadent AG, Schaan, Liechtenstein) was used to mill the resin specimens, while the Asiga MAX X (Asiga Pty Ltd, Alexandria, NSW, Australia) was employed for additive manufacturing. Following production, the Dentmate Prodex BL (Dentmate Technology Co., Ltd, New Taipei, Taiwan) was utilized for post-polymerization curing. Surface preparation was carried out using the Phoenix Beta Twin Wheel (Buehler, Illinois Tool Works Inc, United States). The Uniz Ultrasonic Cleaner (Uniz Technology LLC, San Diego, United States) was used to clean the specimens. Tensile testing was conducted using an Instron 3382 Universal Testing Machine (Instron, ITW Test & Measurement, Massachusetts, United States).

Table 2.

Machines used in the production, curing, surface treatment, and tensile testing of the specimens

Machine	Manufacturer	Purpose
Ivoclar Programill PM7	Ivoclar Vivadent AG, Schaan, Liechtenstein	Subtractive milling of PMMA specimens
Asiga MAX X	Asiga Pty Ltd, Alexandria, NSW, Australia	Additive manufacturing (3D printing) of resin specimens
Phoenix Beta Twin Wheel	Buehler, Illinois Tool Works Inc, United States	Surface preparation — wet sandpapering
Instron 3382	Instron, ITW Test & Measurement, Massachusetts, United States	Universal tensile testing machine
Uniz Ultrasonic Cleaner	Uniz Technology LLC, San Diego, United States	Ultrasonic cleaning of specimens
Dentmate Prodex BL	Dentmate Technology Co., Ltd, New Taipei, Taiwan	Post-polymerization UV curing of 3D-printed specimens
OPMI pico/S100 Microscope	Carl Zeiss Microscopy GmbH, Jena, Germany	Failure mode analysis at ×2.5 magnification

From each of the three denture base materials — Megacryl Hot, Voco Ceditec DB, and Arma Resin — rectangular prism specimens (10 × 10 × 20 mm) were fabricated. To standardize the interfacial bonding area, a 10 × 10 × 43 mm reference specimen was fabricated to simulate the combined length of two denture base specimens with a 3 mm gap in between. A negative mold of this reference block was created to serve as a positioning guide. Using this mold, two denture base specimens measuring 10 × 10 × 20 mm were precisely placed at each end, creating a consistent 10 × 10 × 3 mm rectangular cavity between them for the standardized application of liner materials. The bonding surface area used in all bond strength calculations was therefore 100 mm² (10 × 10 mm).

Additively manufactured specimens (Arma Resin) measuring 10 × 10 × 20 mm were designed using TinkerCad 3D design software and exported in STL format. The design file was sent to the Asiga MAX X printer, equipped with a 385 nm LED wavelength light source. Arma Resin denture base resin was used as the printing material. Each specimen was printed in 300 layers, with a layer thickness of 50 μm; the printing process took approximately 26 min per batch.

After printing, specimens underwent a two-stage washing process: immersion in isopropyl alcohol for 2 min to remove excess uncured resin, followed by air-drying to eliminate residual isopropyl alcohol. Post-polymerization was performed using a Dentmate Prodex BL device for 420 s under controlled UV light, following the manufacturer’s recommendations. A total of 60 Arma Resin specimens were produced.

Subtractively manufactured specimens (Voco Ceditec DB) were designed using Ivoclar Programill 2022 software and saved in STL format. The positioning of specimens on the PMMA block was planned to produce 18 specimens from a single block, minimizing material waste. The pre-polymerized PMMA block was securely placed inside the Ivoclar Programill PM7 milling machine, and specimens were carved using multi-axis milling technology to achieve precise 10 × 10 × 20 mm dimensions.

After milling, each specimen was inspected for dimensional accuracy and surface quality. Any deviations from the required dimensions were corrected using fine polishing and trimming tools. Specimens were then cleaned using an ultrasonic cleaning device to remove residual milling debris. A total of 60 Ceditec DB specimens were produced.

Conventionally manufactured specimens (Megacryl Hot) a digital design measuring 10 × 10 × 20 mm was created using 3D design software. A prototype base was printed using model resin, embedded in laboratory-type silicone, and custom wax models were prepared from this mold. The wax models were placed into a flask, and Type III dental plaster was poured in two stages to encase the wax models.

Wax elimination was achieved by placing the flask in boiling water. The Megacryl Hot material was prepared by mixing acrylic powder and liquid in a 5:2 powder-to-liquid ratio and poured into the prepared cavities. The mixture was subjected to a pressing force of 100–120 bar to eliminate voids. The flask was then placed in a water bath initially maintained at no more than 30–40 °C, gradually heated to boiling temperature (98–100 °C), and polymerised at this temperature for 25–30 min. After cooling at room temperature for 30 min, specimens were deflasked and trimmed to precise dimensions using tungsten carbide burs. Surfaces were polished to ensure a smooth and uniform bonding surface.

The bonding surfaces of all specimens were treated with 240-grit wet sandpaper to consistently prepare the bonding surface and reduce gross surface irregularities, following the methodology described by Kulak-Ozkan et al. [27] that could impact adhesion of the liner material. The sandpapering process was performed under controlled conditions using the Phoenix Beta Twin Wheel to ensure uniformity across all specimens.

A silicone bonding mold of 10 × 10 × 43 mm was created to position two denture base specimens at each end with a precise 3 mm cavity between them for standardized liner application.

Molloplast-B (heat-polymerized silicone-based soft liner)

Two resin specimens were placed at both ends of the silicone mold, ensuring a 3 mm gap verified with calipers. The space was filled with wax to stabilize positioning, then transferred into a flask filled with gypsum. After wax elimination, Primo adhesive (Detax GmbH & Co KG) was applied uniformly to all acrylic bonding surfaces using a fine brush and allowed to dry for 60 min. Molloplast-B was placed into the 3 mm gap using a clean spatula. The flask assembly was compressed in a hydraulic dental flask press at a gauge pressure of approximately 100 bar for 15 min. The flask was then immersed in a water bath at 100 °C and maintained for 2.5 h to complete polymerization.

GC reline II soft (chairside auto-polymerized silicone-based soft liner)

Resin specimens were positioned in the silicone mold, and GC Reline II Primer adhesive was uniformly applied to the acrylic bonding surfaces and air-dried to allow complete solvent evaporation. The GC Reline II Soft material, supplied in base and catalyst cartridges, was loaded into a dispensing gun for automatic mixing (Fig. 1). The freshly mixed liner material was carefully applied to the gaps between the resin specimens to prevent air pocket formation. The assembly was left to self-polymerize for approximately 30 min under ambient conditions. After polymerization, specimens were polished using GC Reline II Point polishing discs.

Fig. 1.

Application of GC Reline II Soft into the standardized bonding cavity between denture base specimens

Ufi gel hard C (chairside auto-polymerized methacrylate-based hard liner)

Resin specimens were positioned in the bonding model, and Ufi Gel Hard C adhesive was applied uniformly to the bonding surfaces and allowed to air-dry for 30 s. The Ufi Gel Hard C cartridge was loaded into a dispensing gun and the liner material was injected directly into the gaps between the resin specimens. The liner was allowed to set for 5 min and 30 s according to the manufacturer’s guidelines. After polymerization, excess liner material was trimmed using a tungsten carbide bur to achieve a consistent surface finish.

All prepared specimens were kept in a light-proof container throughout the preparation process to protect them from external light exposure. Prior to tensile testing, all specimens were immersed in distilled water for 24 h at 37 °C, stored individually to prevent cross-contamination.

Tensile bond strength testing was performed using an Instron 3382 Universal Testing Machine located in the Hard Tissue Research Laboratory. Specimens were mounted in a vertical orientation between the upper and lower parts of the testing machine (Fig. 2). The crosshead speed was set at 5 mm/min in accordance with ISO 10139-2 testing protocols, and the applied force was recorded continuously [17]. Bond strength was calculated using the formula: S = F/A, where S is bond strength (MPa), F is the maximum force at failure (N), and A is the bonding surface area (mm²). The bonding surface area for all specimens was 100 mm² (10 × 10 mm).

Fig. 2.

Specimen mounted in the universal testing machine for tensile bond strength testing

After bond strength testing, all specimens were examined by a single trained observer using a stereomicroscope (OPMI pico/S100, Carl Zeiss Microscopy GmbH, Jena, Germany) at ×2.5 magnification. The failure modes were categorized into three groups: adhesive failure, defined as complete separation at the interface between the denture base and reline material with no reline material remaining on the denture base surface (Fig. 3A); cohesive failure, defined as fracture within the reline material itself with reline material remaining on both bonding surfaces (Fig. 3B); and mixed failure, defined as a combination of adhesive and cohesive failure patterns with partial reline material remaining on the denture base surface (Fig. 3C).

Fig. 3.

Representative failure modes after tensile bond strength testing: A adhesive failure, (B) cohesive failure, and (C) mixed failure

Data were analyzed using IBM SPSS Statistics v29 and R (ARTool package). Descriptive statistics are reported as mean, standard deviation (SD), minimum, and maximum. Because the experiment followed a 3 × 3 factorial design (denture base material × reline material), tensile bond strength was analyzed with a two-way ANOVA including both main effects and their interaction. Normality of model residuals was assessed with the Shapiro–Wilk test and Q–Q plots, and homogeneity of variance with Levene’s test. As the raw data violated these assumptions, a natural-logarithmic transformation was applied, after which both assumptions were satisfied; the two-way ANOVA was therefore performed on the log-transformed values. To confirm the findings without distributional assumptions, the data were additionally analyzed with the Aligned Rank Transform (ART) procedure for a two-way factorial design. Given a significant interaction, simple main effects were examined with Tukey-adjusted pairwise comparisons (denture base within each reline material and reline material within each denture base). Failure-mode distributions were compared with the Fisher–Freeman–Halton exact test. The Type I error rate was set at 5% (α = 0.05).

Results

Descriptive statistics for the tensile bond strength values of the nine experimental groups are presented in Table 3. Each group comprised ten specimens. Mean tensile bond strength ranged from 1.28 ± 0.16 MPa for 3D-printed resin bonded to GC Reline II Soft to 12.56 ± 2.21 MPa for conventional PMMA bonded to Ufi Gel Hard C.

Table 3.

Descriptive statistics of tensile bond strength (MPa) for all experimental groups

Group	Denture Base Material	Reline Material	n	x̄ ± SD	Min	Max
1	Conventional PMMA	Molloplast-B	10	2.07 ± 0.34	1.67	2.78
2	Conventional PMMA	GC Reline II Soft	10	1.76 ± 0.15	1.50	1.95
3	Conventional PMMA	Ufi Gel Hard C	10	12.56 ± 2.21	7.62	14.79
4	Milled PMMA	Molloplast-B	10	1.97 ± 0.18	1.74	2.26
5	Milled PMMA	GC Reline II Soft	10	1.92 ± 0.21	1.48	2.14
6	Milled PMMA	Ufi Gel Hard C	10	10.29 ± 1.27	8.95	12.22
7	3D-printed resin	Molloplast-B	10	2.16 ± 0.34	1.70	2.85
8	3D-printed resin	GC Reline II Soft	10	1.28 ± 0.16	1.07	1.60
9	3D-printed resin	Ufi Gel Hard C	10	9.01 ± 1.99	6.08	11.29

SD Standard deviation, x̄ mean

A two-way ANOVA on log-transformed tensile bond strength revealed significant main effects of denture base material (F2,81 = 15.41, p < 0.001) and reline material (F2,81 = 1374.53, p < 0.001), as well as a significant denture base × reline material interaction (F4,81 = 9.16, p < 0.001). The significant interaction indicates that the influence of the denture base material on bond strength depends on the reline material used, and the null hypothesis of no interaction (H03) was therefore rejected. Because of this interaction, marginal (pooled) base means are not interpretable in isolation, and simple main effects are reported instead (Table 4). The same pattern of significance for all three effects was obtained with the non-parametric Aligned Rank Transform analysis (denture base F = 22.02; reline material F = 192.50; interaction F = 12.18; all p < 0.001), confirming that the findings are not an artefact of the transformation.

Table 4.

Two-way ANOVA (log-transformed) and Aligned Rank Transform results

Source of variation	df	ANOVA F	ANOVA p	ART F	ART p
Denture base material	2, 81	15.41	< 0.001	22.02	< 0.001
Reline material	2, 81	1374.53	< 0.001	192.50	< 0.001
Base × Reline (interaction)	4, 81	9.16	< 0.001	12.18	< 0.001

df degrees of freedom (effect, residual), ART Aligned Rank Transform (non-parametric)

Within each denture base material, the hard chairside liner (Ufi Gel Hard C) produced significantly higher bond strength than either soft liner (all p < 0.001), which was the dominant source of the interaction. Among the hard-lined groups, conventional PMMA showed significantly higher bond strength than 3D-printed resin (p = 0.001), whereas the differences between conventional and milled PMMA (p = 0.086) and between milled PMMA and 3D-printed resin (p = 0.206) were not significant. With Molloplast-B, no significant difference was found among the three denture base materials (all p > 0.36). With GC Reline II Soft, both milled and conventional PMMA gave significantly higher bond strength than 3D-printed resin (p < 0.001), while milled and conventional PMMA did not differ (p = 0.196). The simple main effects of denture base within each reline material are summarized in Table 5, and the base × reline interaction is illustrated in Fig. 4.

Table 5.

Simple main effects of denture base within each reline material (Tukey-adjusted)

Reline material	Comparison	p
Ufi Gel Hard C	Conventional vs. 3D-printed	0.001
Ufi Gel Hard C	Conventional vs. Milled	0.086
Ufi Gel Hard C	Milled vs. 3D-printed	0.206
Molloplast-B	All base comparisons	> 0.36 (ns)
GC Reline II Soft	Conventional vs. 3D-printed	< 0.001
GC Reline II Soft	Milled vs. 3D-printed	< 0.001
GC Reline II Soft	Conventional vs. Milled	0.196

Within every base, the hard liner exceeded both soft liners (all p < 0.001)

ns not significant

Fig. 4.

Denture base × reline material interaction

The distribution of failure modes across the experimental groups is presented in Table 6. The Fisher–Freeman–Halton exact test revealed a statistically significant association between experimental group and failure mode distribution (p < 0.001). Overall, the 3D-printed resin groups exhibited the highest proportion of adhesive failures (63.3%). Adhesive failure occurred in 90.0% of Group 7 specimens and all Group 9 specimens, whereas Group 8 exhibited exclusively mixed failures. In the conventional PMMA groups, cohesive failures were the most frequent overall (46.6%), particularly in Groups 1 and 2 (50.0% and 90.0%, respectively). Group 3 differed from this pattern, showing 70.0% adhesive and 30.0% mixed failures. The milled PMMA groups showed a more heterogeneous distribution: Group 4 exhibited predominantly mixed failures (60.0%), Group 5 predominantly cohesive failures (80.0%), and Group 6 predominantly adhesive failures (80.0%).

Table 6.

Distribution of failure modes across experimental groups

Group	Denture Base Material	Reline Material	n	Adhesive (%)	Cohesive (%)	Mixed (%)
1	Conventional PMMA	Molloplast-B	10	1 (10.0)	5 (50.0)	4 (40.0)
2	Conventional PMMA	GC Reline II Soft	10	0 (0.0)	9 (90.0)	1 (10.0)
3	Conventional PMMA	Ufi Gel Hard C	10	7 (70.0)	0 (0.0)	3 (30.0)
4	Milled PMMA	Molloplast-B	10	1 (10.0)	3 (30.0)	6 (60.0)
5	Milled PMMA	GC Reline II Soft	10	0 (0.0)	8 (80.0)	2 (20.0)
6	Milled PMMA	Ufi Gel Hard C	10	8 (80.0)	0 (0.0)	2 (20.0)
7	3D-printed resin	Molloplast-B	10	9 (90.0)	1 (10.0)	0 (0.0)
8	3D-printed resin	GC Reline II Soft	10	0 (0.0)	0 (0.0)	10 (100.0)
9	3D-printed resin	Ufi Gel Hard C	10	10 (100.0)	0 (0.0)	0 (0.0)

When analyzed according to reline material, the Ufi Gel Hard C groups were characterized mainly by adhesive failures, with no cohesive failures observed. Mixed failures occurred in 30.0% of Group 3 and 20.0% of Group 6 specimens but were absent in Group 9. In contrast, no adhesive failures were observed in any GC Reline II Soft group; cohesive and mixed failures accounted for 56.7% and 43.3% of failures, respectively. The Molloplast-B groups demonstrated a comparatively balanced distribution of adhesive, cohesive, and mixed failures (36.7%, 30.0%, and 33.3%, respectively).

Discussion

This in vitro study systematically evaluated the tensile bond strength of soft and hard reline materials bonded to denture base resins fabricated by three different manufacturing methods: conventional heat polymerization, CAD-CAM milling, and 3D printing. The results revealed significant effects of denture base material and reline material, together with a significant denture base material × reline material interaction, indicating that the influence of denture base material on tensile bond strength depended on the reline material used. Therefore, the denture base materials could not be ranked independently of the reline material, highlighting the importance of selecting compatible denture base–reline material combinations in clinical practice.

The most important finding of the present study was the significant interaction between denture base material and reline material. Although significant main effects were identified for both factors, the interaction demonstrated that their effects on tensile bond strength were not independent. Rather, the bonding performance of each denture base material varied according to the reline material with which it was combined. Therefore, the third null hypothesis (H0₃), which assumed no interaction between denture base and reline material, was rejected. This finding also precludes interpretation of the marginal denture base means as a general ranking of conventional, milled, and 3D-printed materials. Instead, the results should be interpreted through the simple main effects within each reline material. Conventional PMMA showed significantly higher bond strength than 3D-printed resin when combined with Ufi Gel Hard C, whereas milled PMMA did not differ significantly from either material. With GC Reline II Soft, both conventional and milled PMMA produced significantly higher bond strength than 3D-printed resin, while no significant differences among denture bases were observed with Molloplast-B. These findings indicate that bonding performance is determined by the compatibility of the specific denture base–reline material combination rather than by the denture base manufacturing method alone.

The finding that all tested combinations exceeded the minimum clinically acceptable threshold of 0.44 MPa is consistent with recent comparative studies. Guntekin et al. [19] reported higher bond strength for conventional PMMA than for digitally fabricated alternatives when bonded with both hard and soft reline materials. In the present study, a comparable pattern was observed for Ufi Gel Hard C, with conventional PMMA showing significantly higher bond strength than 3D-printed resin; however, no significant differences among the three denture base materials were detected with Molloplast-B. Nevertheless, the mean bond strengths of 12.56 MPa for conventional PMMA with Ufi Gel Hard C and 2.07 MPa with Molloplast-B were numerically comparable to their reported values of 11.8 MPa and 3.2 MPa, respectively.

Wemken et al. [20] investigated the bond strength of auto-polymerizing reline resins to CAD-CAM milled denture bases and reported values ranging from 10.5 to 13.2 MPa, comparable to our findings for milled PMMA with hard liner (10.29 MPa). Their observation that milled PMMA achieved bond strengths statistically equivalent to conventional PMMA is corroborated by our results, where no significant difference was detected between these two materials when bonded with the same reline material.

The significantly lower bond strength observed for 3D-printed resin when bonded with GC Reline II Soft is consistent with the findings of Koseoglu et al. [22], who reported tensile bond strength values of 1.5–2.8 MPa for soft liners bonded to 3D-printed resins, compared with 2.8–3.9 MPa for conventional PMMA. The value obtained for the 3D printed resin–GC Reline II Soft combination in the present study (1.28 MPa) was close to this reported range and further indicates the difficulty of achieving optimal bonding with this specific material combination. However, this pattern should not be generalised to all reline materials, as no significant differences among the three denture base materials were observed with Molloplast-B, whereas with Ufi Gel Hard C, a significant difference was detected only between conventional PMMA and 3D-printed resin.

Alfaraj et al. [23] evaluated both auto-polymerizing and heat-polymerizing denture reliners on conventional and CAD-CAM denture base materials, reporting that CAD-CAM materials achieved adequate but slightly lower bond strengths compared to conventional PMMA. Our findings support this observation, though the differences between conventional and milled PMMA in our study were not statistically significant for any soft reline material type.

Within each denture base material, the hard liner (Ufi Gel Hard C) demonstrated significantly higher tensile bond strength than both silicone-based soft liners (all p < 0.001), consistent with the established literature. Lau et al. [25] reported that hard acrylic-based reline materials achieve superior bond strength compared with silicone-based soft liners, which they attributed to greater chemical compatibility and interpenetrating polymer network formation with PMMA-based denture bases. The present findings support this observation at the level of the individual denture base–reline combinations: Ufi Gel Hard C produced mean bond strengths of 12.56 MPa, 10.29 MPa, and 9.01 MPa with conventional PMMA, milled PMMA, and 3D-printed resin, respectively, whereas the corresponding values ranged from 1.28 to 2.16 MPa for the two soft liners.

Botega et al. [26] reported that hard acrylic-based reliners exhibited bond strengths approximately four to five times greater than silicone-based soft liners, which aligns closely with our findings. The superior performance of hard liners can be attributed to chemical bonding mechanisms involving residual monomer diffusion and polymer chain entanglement at the interface, whereas soft silicone liners rely primarily on mechanical interlocking and adhesive primers for bonding.

The two soft liners showed denture base–dependent differences rather than a uniform overall pattern. Molloplast-B yielded numerically higher mean bond strength than GC Reline II Soft when bonded to conventional PMMA (2.07 vs. 1.76 MPa) and 3D-printed resin (2.16 vs. 1.28 MPa), whereas similar values were obtained with milled PMMA (1.97 vs. 1.92 MPa). Because analysis did not establish an overall pooled difference between the two soft liners, these findings cannot be interpreted as demonstrating the general superiority of the laboratory-processed liner over the chairside material. Kulak-Ozkan et al. [27] reported higher bond strength for laboratory-processed silicone liners, potentially owing to more controlled processing conditions and a higher degree of polymerisation; however, the present findings suggest that the relative performance of the two soft liners may depend on the denture base material.

Albazroun et al. [28] recently confirmed that hard reline materials maintain superior bonding performance across different aging conditions compared to soft liners, with less degradation after thermocycling. While our study evaluated specimens after 24-hour water storage without thermocycling, the substantial difference in initial bond strength between hard and soft liners suggests that this performance gap would likely persist or widen under aging conditions.

The significantly higher bond strength of conventional heat-polymerized PMMA compared with 3D-printed resin in the Ufi Gel Hard C and GC Reline II Soft groups may be attributed to differences in polymerization chemistry and surface characteristics. Conventional PMMA undergoes thermal polymerization and may retain greater amounts of residual monomer at the bonding surface, facilitating monomer diffusion and polymer-chain interpenetration with the reline material [35]. In contrast, 3D-printed resins are photopolymerized layer by layer and subsequently post-cured, resulting in a highly cross-linked polymer network with fewer available reactive sites and a reduced capacity for interfacial monomer diffusion. However, no significant difference between conventional PMMA and 3D-printed resin was observed with Molloplast-B, indicating that the influence of these substrate-related characteristics also depends on the chemistry, processing method, and adhesive system of the reline material used.

CAD-CAM milled PMMA did not exhibit a uniform intermediate bonding pattern, as its numerical position varied according to the reline material used. It showed a mean bond strength between those of conventional PMMA and 3D-printed resin with Ufi Gel Hard C, the highest numerical mean with GC Reline II Soft, and the lowest numerical mean with Molloplast-B. Nevertheless, CAD-CAM milled PMMA did not differ significantly from conventional PMMA with any of the three reline materials. Iwaki et al. [8] characterized the mechanical properties of CAD-CAM milled PMMA blocks, reporting a higher degree of polymerization and reduced porosity than conventional PMMA because of industrial polymerization under controlled pressure and temperature. Although these properties improve the mechanical strength of milled PMMA, they may theoretically reduce its surface reactivity for bonding. In the present study, mechanical surface preparation with 240-grit abrasive paper followed by the application of the corresponding chemical primer resulted in bond strength values statistically comparable to those of conventional PMMA across all reline materials. However, identical abrasive protocols applied to materials with different microstructures do not necessarily produce equivalent surface roughness, and the comparable bond strengths should therefore not be attributed solely to the standardized surface-preparation procedure [36].

Vuksic et al. [29] compared the mechanical properties of 3D-printed, CAD-CAM milled, and conventional denture base materials, reporting that milled PMMA exhibited the highest flexural strength, whereas 3D-printed resins showed greater variability depending on the printing parameters. In the present study, failure modes also varied according to the specific denture base–reline material combination. Adhesive failures were most frequent overall in the 3D-printed groups (63.3%), whereas cohesive failures occurred more frequently in the conventional PMMA (46.7%) and milled PMMA groups (36.7%). However, the failure pattern among the 3D-printed groups was not uniform: adhesive failure predominated with Molloplast-B (90%) and Ufi Gel Hard C (100%), while the GC Reline II Soft group exhibited exclusively mixed failures (100%). Therefore, the interface appeared to represent the weakest component in the 3D-printed resin–Molloplast-B and 3D-printed resin–Ufi Gel Hard C combinations, but this interpretation cannot be generalised to all 3D-printed groups. Moreover, none of the Ufi Gel Hard C groups exhibited cohesive failure; instead, this hard liner showed predominantly adhesive failures overall. These findings indicate that failure behaviour was determined by the interaction between the denture base and reline material rather than by the denture base manufacturing method alone.

Failure mode is not merely a descriptive outcome — it carries important clinical implications. Adhesive failures, as predominantly observed in 3D-printed groups, indicate interface separation and may predispose to complete liner debonding in clinical use. The 100% adhesive failure rate in Group 9 (3D-printed + Ufi Gel Hard C), despite a relatively high mean bond strength, suggests that while initial bond strength may be adequate, the interface quality may be insufficient for long-term clinical durability. Cohesive failures, more common in conventional PMMA groups, indicate that the bond is stronger than the liner material itself, which may be clinically more favorable as it implies predictable fracture behavior within the liner rather than sudden debonding.

The exclusively mixed failure pattern in Group 8 (3D-printed + GC Reline II Soft) is noteworthy, as it suggests a transitional bonding quality that may be sensitive to aging and cyclic loading. Clinicians should consider these failure patterns when selecting reline materials for 3D-printed dentures, as the predominance of adhesive failures may necessitate more frequent reline replacement and closer clinical monitoring.

The predominantly adhesive failure mode observed in 3D-printed groups (63.3% overall, with Groups 7 and 9 showing 90.0% and 100.0% adhesive failures, respectively) suggests that surface treatment protocols optimized for conventional PMMA may be insufficient for 3D-printed resins. Koseoglu et al. [21] investigated the surface characteristics of 3D-printed denture bases and their relationship to bonding performance, finding that the highly cross-linked surface structure limits chemical bonding potential. Their results suggest that more aggressive surface treatments, such as airborne particle abrasion or plasma treatment, may be necessary to improve bonding performance for 3D-printed denture bases. Supporting this, Htat et al. [37] demonstrated that 3D-printed denture base resins require more aggressive surface conditioning than milled resins to achieve comparable bond strengths, reporting TBS values of 1.8 MPa with 240-grit sandpaper alone versus 3.1 MPa after airborne particle abrasion. Similarly, Dederichs et al. [38] and Al Taweel et al. [41] confirmed that surface preparation protocols optimized for conventional PMMA are suboptimal when applied uniformly to 3D-printed substrates.

The chemical composition and surface characteristics of 3D-printed resins differ substantially from conventional PMMA. Most 3D-printed denture base resins are based on methacrylate or acrylic oligomers with photoinitiators and cross-linking agents, resulting in a highly cross-linked polymer network after photopolymerization and post-curing. This dense cross-linked structure may limit monomer diffusion and polymer chain interpenetration at the interface, reducing chemical bonding potential. Additionally, the layer-by-layer fabrication process in 3D printing creates unique surface topography that may influence mechanical interlocking and adhesive penetration.

The findings of this study have several important clinical implications for denture relining procedures in the era of digital dentistry. First, all tested combinations of denture base materials and reline materials achieved bond strengths exceeding the minimum clinically acceptable threshold, indicating that relining of digitally fabricated dentures is technically feasible and can be expected to provide adequate short-term performance.

Second, the superior bonding performance of hard reline materials compared to soft liners across all denture base types suggests that hard relining should be the preferred option when long-term dimensional stability and mechanical durability are primary concerns.

Third, the bonding performance of 3D-printed denture bases was reline-material dependent. Significantly lower bond strength than conventional PMMA was observed with GC Reline II Soft and Ufi Gel Hard C, whereas no significant difference was detected with Molloplast-B. Although 3D-printed groups showed a higher overall incidence of adhesive failures, the failure pattern also varied among material combinations. Clinicians should therefore exercise particular caution when relining 3D-printed dentures with GC Reline II Soft or Ufi Gel Hard C. Additional surface-treatment protocols beyond mechanical roughening and primer application may be beneficial for improving the bonding performance of these specific combinations.

Fourth, the comparable bonding performance of CAD-CAM milled PMMA and conventional PMMA is clinically reassuring, indicating that milled dentures can be relined using standard protocols with predictable outcomes.

Fifth, the choice between laboratory-processed and chairside soft liners should consider clinical convenience together with the compatibility of each liner with the selected denture base material. Although Molloplast-B and GC Reline II Soft exhibited different numerical bond-strength patterns across the denture base materials, the updated analysis did not demonstrate an overall, denture base–independent superiority of either soft liner.

Several limitations of this study should be acknowledged. First, the in vitro testing protocol evaluated bond strength after 24-hour water storage at 37 °C, which represents initial bonding performance but does not simulate long-term clinical aging. The choice of a 24-hour storage protocol follows ISO 10139-2 standards for initial bond strength assessment; however, future studies should incorporate thermocycling, mechanical cycling, and extended water storage to evaluate bond durability [17]. Janyaprasert et al. [32] demonstrated that thermocycling significantly reduced bond strength for all denture base and reline material combinations, with the magnitude of reduction varying by material type.

Second, this study evaluated tensile bond strength using a specific testing configuration and loading rate. While tensile testing is widely used and standardized in ISO protocols, clinical loading of denture-reline interfaces involves complex combinations of tensile, shear, and peel stresses. Azpiazu-Flores et al. [33] evaluated adhesive strength using a different testing methodology and reported somewhat different relative rankings of materials, highlighting the importance of testing methodology in bond strength assessment.

Third, only one product from each material category was tested. Given the wide variety of denture base resins and reline materials available commercially, the results may not be generalizable to all products within each category. Future studies should evaluate multiple products within each category to identify material-specific factors that influence bonding performance.

Fourth, surface treatment protocols in this study followed manufacturer recommendations, which were developed primarily for conventional PMMA. Optimization of surface treatment protocols specifically for digitally fabricated denture bases, particularly 3D-printed resins, represents an important area for future research.

The use of identical 240-grit silicon carbide wet-grinding across all three fabrication methods constitutes a methodological limitation that warrants explicit acknowledgment. Although this approach ensures procedural consistency, it does not guarantee equivalent surface roughness (Ra) outcomes across materials with fundamentally different microstructures [36]. Ozden et al. [36] demonstrated that fabrication method significantly affects the resulting Ra after identical polishing sequences applied to conventional, CAD/CAM-milled, and 3D-printed denture base resins. Furthermore, 3D-printed photopolymerized resins possess an oxygen-inhibited surface layer that may not be adequately removed by 240-grit abrasion alone, potentially compromising the availability of reactive surface sites for bonding [37, 38]. Htat et al. [37] and Dederichs et al. [38] both reported that standard sandpaper abrasion protocols yielded significantly lower bond strengths for 3D-printed substrates compared to airborne particle abrasion or plasma treatment protocols. The lower tensile bond strength observed for 3D-printed resin with specific reline materials may therefore partly reflect the interaction between the material and the surface preparation protocol employed, rather than solely the inherent properties of the 3D-printed resin. Future studies should incorporate quantitative surface roughness measurement alongside bond strength testing to decouple the effects of surface topography from material composition [36–41].

Conclusions

Within the limitations of this in vitro study, the following conclusions can be drawn:

1. All tested combinations of denture base materials and reline materials demonstrated tensile bond strength exceeding the minimum clinically acceptable threshold of 0.44 MPa.

2. Hard chairside reline material (Ufi Gel Hard C) exhibited significantly superior bonding performance compared to soft liners across all denture base types.

3. A significant interaction was observed between denture base material and reline material, indicating that the effect of denture base material on tensile bond strength depended on the reline material used.

4. With Ufi Gel Hard C, conventional PMMA demonstrated significantly higher tensile bond strength than 3D-printed resin, whereas milled PMMA did not differ significantly from either material. With Molloplast-B, no significant differences were observed among the denture base materials. With GC Reline II Soft, conventional and milled PMMA demonstrated significantly higher bond strength than 3D-printed resin, while no significant difference was found between the two PMMA materials.

5. The lower bond strength observed for 3D-printed resin in specific reline-material combinations may partly reflect its material-specific response to the surface preparation protocol rather than solely its inherent material properties.

Abbreviations

PMMA

Polymethyl methacrylate

MMA

Methyl methacrylate

CAD-CAM

Computer-aided design and computer-aided manufacturing

DLP

Digital light processing

SLA

Stereolithography

ISO

International Organization for Standardization

SD

Standard deviation

ANOVA

Analysis of variance

BIS-GMA

Bisphenol A-glycidyl methacrylate

HEDMA

Hydroxyethyl dimethacrylate

STL

Standard Tessellation Language

UV

Ultraviolet

MPa

Megapascal

Ra

Surface roughness

Authors’ contributions

B.A.S. conceived and designed the study, performed specimen preparation, conducted the experiments, analyzed and interpreted the data, prepared Figs. 1, 2 and 3, and drafted the manuscript. E.K. supervised the study, contributed to data interpretation, and critically revised the manuscript. Both authors reviewed and approved the final manuscript.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Data availability

The datasets generated and analysed during the current study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable. This study was conducted entirely in vitro using commercially available dental materials and did not involve human participants, human tissue, or animal subjects.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.