Evaluation of Effect of Tooth Brushing on the Surface Roughness of Different Implant Superstructure Materials: An In Vitro Study

Introduction

One of the main goals in dentistry is to restore the optimal form, function, and esthetics of the patient. With the increase in the use of dental implants, there has been a corresponding rise in both mechanical and biological complications.¹

The cleansing effect of regular tooth brushing was attributed in different studies to the mechanical effect of the toothbrush along with the mechanical/chemical traits or criteria of toothpaste.^2,3 Despite the proven benefits of tooth brushing, it is also associated with the negative effects such as dental hypersensitivity, tooth surface erosion and even surface wear of few dental restorations.^4,5 The surface roughness (Ra) of dental materials plays a major role in bacterial plaque accumulation and adhesion.^6,7 Bacterial adhesion increases on rough surfaces due to increased surface area; therefore, materials with low Ra values are essential for reducing bioadhesion.^8-10

A wide range of polymers are commonly used for various applications in clinical dentistry.^11,12 Amongst these, poly methyl methacrylate (PMMA) is the most widely used polymer in dental laboratories for fabricating orthodontic retainers, dentures and for repair; in dental clinics for denture relining and preparation of temporary crowns; and in the industry for manufacturing artificial teeth.^13,14 The CAD/CAM (Computer aided design and Computer aided manufacturing) technologies are employed in the production of various ceramic restorations, such as inlays, onlays, crowns, and fixed partial dentures.¹⁵ More recently, several researchers have explored the use of CAD/CAM technologies for fabricating PMMA dental prostheses, comparing the material properties and various aspects of conventional PMMA with those produced using CAD/ CAM methods.^16,17 A range of filler particles, including ceramics and metals, have been explored to enhance the properties of PMMA materials.² These studies clearly demonstrate that the addition of particles did not cause any biocompatibility issues and enhanced various properties of the PMMA, including the mechanical properties, thermal conductivity, dimensional stability, and antimicrobial activity, while also reducing the water sorption and solubility.^{17,18,19,20,21,22}

Although multiple studies have examined the impact of toothbrush abrasion and aging on the surface texture of denture base acrylic resin, composite resin restorations, feldspathic and silica-based ceramic materials, zirconia, not much is known regarding CAD/CAM PMMA, Polychrome composite, and Direct Metal Laser Sintering glazed Porcelain-Fused-to-Metal (DMLS PFM).²³ Thus, the goal of this in vitro investigation was to assess the effect of tooth brushing on the surface roughness of these materials. The null hypothesis of the current study was that brushing would not alter the Ra of implant superstructure materials.

Materials and Methods

Six cuboid-shaped specimens (10×10×2 mm) each from different brands of prepolymerized CAD/CAM PMMA, CAD/CAM polychrome composite were fabricated using VHF K5 Plus 5 axis milling machine and conventional DMLS glazed porcelain fused metal (N=24) was fabricated and grouped (G1, G2 and G3) according to manufacturer’s recommendations (Table 1, Figure 1). The sample size was determined to provide 80% power to detect significant differences, given an effect size of 0.7 and a significance level of 0.05. The initial Ra was measured before tooth brushing simulation using the Mitutoyo Surftest SJ-310 Ra Analyzer (Figure 2a). For the measurement of abrasive wear, a specially designed toothbrush simulator - SD Mechatronik- Brushing Simulator ZM-3.8 (Figure 2b) was utilized with eight stations that had interchangeable brush heads (Colgate zig zag). A 1.5 N tooth brushing load was established. Every item was positioned and Colgate Strong toothpaste was applied. Ten thousand brushing cycles were applied on the materials, effectively simulating one year of tooth brushing.²³ In the tooth brushing system, abrasion was applied to a maximum of eight specimens at once. The specimen compartments were divided from one another such that each specimen could be cleaned and used with its own liquid (such as toothpaste-water mixture). Since each brush was managed by a corresponding pressure mechanism, the selected motion pattern was consistent throughout the trial specimens. One could easily choose the motion series (tooth cleaning strategy) from a combination of forward, backward, and circular movements.²⁴

Post-simulation Ra measurements were done using the Mitutoyo Surftest SJ-310 Ra Analyzer. For statistical analysis, paired t test was employed for intragroup comparisons at two different time intervals within each group. Intergroup comparisons among the three groups at each time interval were analyzed using the F test one-way ANOVA, then Tukey's post hoc analysis for pairwise comparisons. The Statistical Package for Social Sciences (SPSS) version 21 for Windows (SPSS Inc., Chicago, IL) was used for all the statistical analyses.

Results

The sample size and design employed in the present study made it possible to identify important group differences. Ra was significantly impacted by the substance and brushing interaction. Table 2 and Figure 3 shows the mean Ra values and standard deviations for the materials employed. All the materials of G1 and G3, except for G2, showed notable variations in their Ra prior to and following brushing. There were no discernible differences between the groups when their baseline Ra data were compared. Whereas the Ra values after brushing showed significant differences among G1- G2 and G2-G3 (P <0.05), while no significant difference was found among G1-G3.

Significant statistical differences are observed when distinct capital characters in the same row and different lowercase letters in the same column differ (P<.05)

Discussion

The partial rejection of the null hypothesis indicated that brushing did not alter the Ra of materials used in implant superstructures. Following brushing simulation, notable variations were seen in the Ra of the CAD/ CAM PMMA and DMLS PFM groups, while the surface became smoother after brushing simulation for Polychrome composite material. Increased dental plaque and biofilm formation on rough surfaces has been documented.²⁵ Dental restorations should therefore have a smoother surface. For prostheses to be clinically acceptable, polishing should result in a final Ra below the threshold of 0.2 μm.²⁵ The materials employed in this investigation had mean Ra values that were higher than the tolerable Ra criterion of 0.2 μm, ranging from 0.415 μm to 0.495 μm before brushing simulation, to 1.044 μm after brushing simulation.

Lessly A Garza et al. measured the shade and Ra of the lithium disilicate-based ceramic group, and reported to be significantly affected by toothbrushing time.²⁶ Judy Chia-Chun Yuan et al. evaluated the effect of toothbrushing on the surface of both lithium disilicate CAD and zirconia, finding that brushing made the surface of both materials rougher.^23,5 In the present study, the results found in Group 1 CAD/CAM PMMA and Group 3 PFM specimens, demonstrated that every item that was brushed with the dentifrice was given a rougher brushing. This could be attributed to its low temperature degradation which occurs in moist environments.²⁷ Another explanation could probably be related to the influence of pH variations and the concentration of fluoride on the zirconia’s susceptibility to degradation.^28,3 Although the neutral pH does not alter the microstructural appearance of zirconia surfaces, both alkaline and acidic pH can corrode the material’s surface.²⁸ Low temperature degradation is responsible for grain push-out, increased surface roughness and wear.²⁹

Sharmila R et al. after three-dimensional brushing, reported that heat cure acrylic material and CAD/CAM acrylic material showed superior abrasion resistance compared to pro temp and tooth color acrylic material. ²⁴ In contrast, the Group 2 - CAD/CAM Polychrome composite specimens did not demonstrate any significant difference after brushing when compared to other specimens used in the similar study. This could be attributed to the loss of the glazed surface over time 30 or the difference in thermal expansion coefficients of the core and the glazing material.

This in vitro study had certain limitations. It could not effectively replicate the dynamic oral environment, which involves saliva, microorganisms, masticatory pressures, and pH fluctuations. The specimen's homogeneous, flat surface could have yielded a different brushing result than the crowns curving contour. Nonetheless, no uniform procedure that aligns with the physiological oral environment has been developed till date. Therefore, for replicating in vivo brushing and its effect on the Ra of implant superstructure materials, more parameter-rich research is required in the future.

Conclusion

Within the limitations of this study, the following conclusions could be derived.

• Ra measured before brushing was in the order of PFM > Polychrome composite > PMMA. 
• Simulated one year of brushing has demonstrated most noticeably higher mean roughness changes in PFM.
• The surface of both PMMA and PFM became significantly rougher after one year of brushing. For Polychrome composite, the surface became smoother after brushing.

Conflict of Interest

Nil