Surface roughness analysis of prepolymerized CAD/CAM dental acrylic resins following combined surface treatments

Polymethylmethacrylate (PMMA) acrylic resins have been extensively used in the fabrication of denture bases for almost 8 decades because of their good mechanical, physical, and aesthetic properties [1,2,3]. However, they possess certain drawbacks, such as dimensional instability, residual monomers, fracture susceptibility, and increased risk of denture-related infections [4, 5]. Furthermore, the chances of surface and subsurface irregularities during processing affect the mechanical characteristics of the fabricated denture and the aesthetic and hygienic outcomes [1].

Light-cured (LC) resin materials have made substantial clinical advancements, reducing dependence on conventional resins. These resins are activated by light in the 460–470 nm range and are frequently based on higher-molecular-weight monomers [1]. Recently, technological advances have led to the incorporation of computer-aided design/computer-aided manufacturing (CAD/CAM) systems to design and fabricate complete dentures by either rapid prototyping or milling by computer numerical control (CNC). These techniques have allowed clinicians to fabricate dentures in only two appointments and made the data retrievable [6]. Since they are prepolymerized, CAD/CAM acrylic resin blocks offer several advantages over traditional processing techniques, including elimination of polymerization shrinkage and porosity, reduced residual monomer, increased denture retention, and overall patient satisfaction [7, 8].

Denture wearers are usually advised to maintain denture hygiene by either mechanical brushing and/or chemical cleansers to prevent biofilm accumulation and the development of oral infections. Although brushing is considered the most efficacious method, the best results could be achieved by combining it with chemical cleansers, especially for subjects with impaired manual dexterity [9]. However, it has been proved that soak-type cleansers alone may not be effective for removing heavy plaque, as demonstrated in an earlier study in which only about 34% of plaque was removed by soaking in perborate-containing tablets [10]. There has been serious concern regarding the effect of brushing and soaking on the surface roughness of the base materials of dentures; several investigators have reported that a rough acrylic surface promotes bacterial accumulation and plaque formation [9,10,11]. The use of a standard toothbrush produced no significant wear of heat-cured (HC), chemically cured, or LC PMMA. Still, significant wear was observed in all types of acrylic resin materials when brushing was combined with abrasive dentifrice [11].

The most commonly used commercial agent for denture immersion is alkaline peroxide (available as a tablet), containing solutions that subsequently release oxygen, leading to debris removal via mechanical means, which has proven effective [9]. It has been reported that the use of alkaline peroxide-containing solutions possibly alters the surface roughness of HC and chemically cured acrylic resin [12, 13]. Another well-known and effective solution is 0.2% chlorhexidine, which has been successfully used as a mouthwash to treat candida-associated denture stomatitis. In comparison, 2% suspension is used as an overnight denture disinfectant.

Chlorhexidine gluconate (CHG) has a bimodal action on Candida species since it is considered fungicidal even at very low concentrations, in addition to its capability to significantly suppress candida adhesion to both inorganic and organic substrates [14]. Both concentrations, i.e., 0.2% and 2%, effectively remove plaque biofilm when immersion is practiced in conjunction with brushing [15]. When 0.12% cleanser was used alone for 8 h, complete disappearance of Candida albicans on the acrylic resin was noted [16]. Immersion in chlorhexidine has been reported to have less effect on the surface roughness of the denture resins [13]. Sodium hypochlorite (NaOCl), when used as a chemical solution at 1%, has been effective against different microbial strains, e.g. Staphylococcus aureus, Pseudomonas aeruginosa, C. albicans, Streptococcus mutans, and Enterococcus faecalis [17]. It has been reported that immersion in NaOCl is less likely to affect the surface of HC or chemically cured acrylic resin. However, the immersion period plays a significant role [13].

The surface roughness of polished smooth acrylic resin may vary between 0.03 μm and 0.75 μm; however, significant bacterial colonization occurs if the surface roughness exceeds 0.2 μm [18, 19]. A recent study by Al-Dwairi et al. [20] reported that prepolymerized CAD/CAM acrylics have superior baseline surface roughness (average arithmetic roughness [Ra] ≤ 0.2 μm) values than HC PMMA. Although numerous studies have shown that prepolymerized CAD/CAM acrylics have better surface properties [7, 8, 20], the surface characteristics following different surface treatments over a prolonged period of use lack scientific evidence.

Therefore, this study aimed to evaluate in vitro the surface roughness of HC, LC, and prepolymerized CAD/CAM resin base materials following mechanical and chemical surface treatments. The null hypothesis was that there is no difference in surface roughness between the tested materials following combined surface treatments.

The results of ANOVA (Table 1) revealed significant interactions between resin type and disinfectants, indicating that the effects of these two factors were interdependent (F=6.60; p<0.001). In the model, both resin type (F=52.70; p<0.001) and disinfectants (F=7.90; p<0.001) are statistically significant.

Table 2 presents the mean baseline surface roughness values of the tested acrylic resin materials. Among the tested materials, CAD/CAM resins demonstrated significantly lower roughness values (0.03±0.02), followed by the LC (0.17±0.03) and HC (0.24±0.06) materials. Although the resin materials underwent identical and standard finishing and polishing techniques, a significant difference was observed in the baseline roughness values (p<0.001).

Figure 2 presents the mean surface roughness of the acrylic materials following various surface treatments. The highest and lowest Ra values were observed in the HC-CHG (4.68±1.18 μm) and CAD/CAM-NaOCl (0.41±0.39 μm) materials. In the HC resins, the disks immersed in Corega and NaOCl did not show any significant differences in roughness values; however, both these disks were significantly different compared to the disks immersed in CHG (p<0.001). On the contrary, CAD/CAM and LC resins immersed in three different disinfectants showed significant differences in roughness values for the immersion mediums tested (p<0.001).

Fig. 2

Among the materials, the HC resins demonstrated the overall highest mean roughness values (Ra = 3.27 μm), followed by LC (Ra = 1.58 μm) and CAD/CAM (Ra = 0.47 μm) resins. For the disinfectants used, the overall highest mean roughness value was observed in the disks immersed in CHG (Ra = 2.32 μm), followed by those in Corega (Ra = 1.71 μm) and NaOCl (Ra = 1.28 μm).

The pairwise comparison (Table 3) of the differences in final roughness values for the resin materials was statistically significant between each pair of resin materials tested (p<0.001). On the contrary, the mean difference between the disinfectants was statistically significant only for NaOCl and CHG (p<0.001). No significant difference was observed between the [Corega and CHG] (p=0.066) and [Corega and NaOCl] (p=0.328) groups.

The profilometer images of representative resin disks for each material and disinfectant are presented in Figure 3. The CAD/CAM materials showed slight surface profile alterations from the baseline to the final measurements for all the disinfectants tested. The maximum changes in specimen surface were found in the HC specimens, with wearing of the surface seen more evidently in disks immersed in CHG. The LC resins demonstrated moderate surface changes following immersion in the disinfectants tested, except for the disks immersed in NaOCl, which showed slight alterations.

Fig. 3

In the present study, we evaluated and compared the surface roughness values of prepolymerized CAD/CAM-PMMA acrylic resins with the values of conventional HC-PMMA- and LC-UDMA-based acrylic resin materials. For the same purpose, the acrylic resins were subjected to power brushing, immersion in disinfectant solutions, and aging by thermal cycling. The null hypothesis was rejected based on the study's outcome because there were differences in the surface roughness values among the HC, LC, and CAD/CAM acrylic denture base materials following different surface treatments.

In this study, surface roughness was measured using an optical noncontact profilometer, which is based on the principle of two-beam optical interferometry without any instrument part being physically in contact with the surface being analyzed. Furthermore, a profilometer is a commonly used tool for determining roughness as it provides a quantitative profile of the specimen surface. The average arithmetic height (Ra) was used to express the roughness measurement because of its simplicity in defining and calculating variations in height [27, 28].

The surface treatment protocol chosen for this study was based on commonly applied and effective denture hygiene methods with the aim of simulating the in vivo clinical conditions as meticulously as possible. Mechanical brushing is considered the most standard and common denture hygiene practice, and hence, it was applied to all the groups. Three types of chemical disinfectants were tested because of the usual practice of overnight immersion of dentures in either water or disinfectant solutions. The reported effect of the chemicals on the wear of acrylic resin surface has varied between studies [13]. Furthermore, aging by thermal cycling was applied in hot and cold water baths to clinically simulate the in vivo thermal changes of the resin material following exposure to repeated cyclic stresses [29].

The surface roughness of denture base materials is affected by the material's inherent characteristics, polishing methods, and the operator's manual skills [20]. Thus, the resin disks were prepared and finished by one operator following a standardized polishing protocol in the present study. However, the CAD/CAM resins demonstrated significantly lower roughness values before and after surface treatment, followed by LC and HC resins in that order.

A roughness value of 0.2 μm is considered the clinical threshold that prevents plaque retention and microorganism adherence to dental restorative surfaces [18, 30]. At the baseline, the roughness values were within the clinical threshold values for all the materials. However, following the surface treatments, all three tested materials demonstrated significantly increased roughness values compared to baseline measurements. These values exceeded the clinical threshold values. However, the values were within the range (Ra = 0.7–3.4 μm) reported by Zissis et al. [31] for denture materials, except for the HC-CHG specimens (Ra = 4.6 μm).

CAD/CAM materials are reported to have exceptional baseline surface quality compared to conventional HC-PMMA [7, 8, 20, 32, 33]. However, previous studies did not compare CAD/CAM resins with LC resins and did not follow surface treatment with disinfectants. Accordingly, a robust methodological approach was designed to compare the CAD/CAM resins with conventional resins following surface treatment. The study outcome demonstrates the dominance of CAD/CAM acrylic resin (Ra = 0.47 μm) in terms of surface roughness compared to both LC (Ra = 1.58 μm) and HC (Ra = 0.47 μm) resins.

This could be attributed to the exclusive processing technique of CAD/CAM-PMMA billets, which involves high temperature and pressure values during polymerization. Furthermore, CAD/CAM resins have less residual monomer and lower porosity, contributing to exceptional stability and lower microbial adhesion to the surface [34, 35].

This study was consistent with the findings of Shinawi [36], who demonstrated comparable CAD/CAM resin surfaces initially, as well as clinically acceptable surface characteristics following 3 years of brushing simulation. However, no comparable groups were used in that study. Al-Dwairi et al. [20] compared two brands of prepolymerized CAD/CAM resins with conventional HC-PMMA resins. The authors found encouraging surface characteristics compared to the HC groups. Differences were also found between the two brands of CAD/CAM groups, indicating that different brands of CAD/CAM-PMMA resins may present internal differences in surface properties.

LC resins produced substantially less roughness than HC-PMMA, similar to the results reported by Haselden et al. [11]. The better roughness values of the LC resins relative to HC-PMMA may be attributed to the former's structure, which contains a dimethacrylate resin with a large urethane backbone. Furthermore, an even distribution of the activating agent through the chain is likely to ensure fewer variations during polymerization [11].

Effervescent compounds such as sodium perborate or sodium bicarbonate are used in denture disinfectants. When the sodium perborate in these effervescent tablets dissolves in water, it decomposes to form an alkaline peroxide solution, releasing oxygen and loosening the debris mechanically. Furthermore, these effervescent compounds reduce the malodor by neutralizing the bacterial by-products [37]. Chemical denture disinfectants have different probabilities of producing a change in the surface roughness, and they also produce a varying effect on different materials [16, 38].

The variation in the roughness of the denture resin materials could be attributed to the molecular mechanisms that have been proposed for the alteration of the denture base surfaces following immersion in disinfecting agents. Firstly, the leaching of initiators, plasticizers (e.g. di-n-butyl phthalate), and free monomer present in the acrylic resins leads to an interaction of these components with the disinfectants, resulting in alteration of the physical properties of polymers and leading to the dissolution of particles. However, this interaction might depend on the ionic concentration of the disinfectant [39]. The second possible mechanism is that the acrylic PMMA resin, a known polar material, can absorb water and disinfectant solutions. Water molecules interfere with the attachment of polymer chains, thereby changing the physical properties of the polymer. Absorption of water initially causes softening of the polymeric resin, which affects the frictional forces between the polymer chains. These repeated absorption–desorption cycles may eventually cause irreversible damage to the material due to hydrolytic degradation of the polymer, thereby causing the formation of microcracks [40, 41]. However, these two mechanisms are highly dependent on the immersion time and the concentration of the disinfectant solutions, which might explain the varying surface roughness results in the literature [20].

The results of this study show that CHG (Ra = 2.32 μm) produced the most significant change in Ra values, followed by the corega (Ra = 1.71 μm) and NaOCl (Ra = 1.28 μm) groups, respectively. This outcome contrasts with the systematic review by Schwindling et al. [13]; however, the previous review did not include studies on CAD/CAMPMMA materials.

Regardless of the effectiveness of this solution in contributing to denture cleansing, few considerations could restrict their routine use. The use of chlorhexidine has been reported to affect patient compliance by contributing to brown stains on the acrylic resin [16]. Corega denture cleanser contains oxygen-releasing agents along with enzymes, supporting the hypothesis that oxidation in combination with a strongly alkaline solution can be harmful [42]. Although NaOCl is an effective and commonly recommended disinfecting agent, its use is restricted because it causes the whitening of acrylic resins [17, 43].

This study had a few limitations. Firstly, the study could not simulate the exact in vivo conditions despite the strong methodology applied. In the oral cavity, all three surface treatments (brushing + immersion + thermal cycling) occur simultaneously, whereas, in this study, it was applied independently. Thus, different roughness characteristics of the resin surface could be expected. Furthermore, the inconsistencies in brushing, immersion, and diet between individuals may limit the generalization of the study outcomes. Finally, this study's results could not be compared with previous literature due to the lack of studies on prepolymerized CAD/CAM blocks exposed to disinfectant solutions. Future studies should be directed toward relating the roughness values with plaque accumulation and microbial activity.

Within the limitations of the methodology and testing conditions, prepolymerized CAD/CAM acrylic resin demonstrated superior surface quality following different surface treatments. The roughness of the HC and LC resins exceeded the clinical threshold and the reported roughness values for acrylic resins following surface treatments. Among the disinfectants tested, NaOCl produced overall low roughness values compared to other disinfectants. CAD/CAM acrylic resins could be considered promising materials for the fabrication of dentures considering the quality of surface roughness presented. This could be beneficial in the prevention of plaque accumulation and colonization of microorganisms.