We know what acids do to enamel, but what are they doing to the enamel replacements? Five recent studies may reveal the answer.
We know what acids can do to natural teeth, but do we know what happens when CAD/CAM resin materials make contact with acids? Some recent studies aimed to answer these questions definitively.
Acid is no friend to teeth; exposure to acids results in tooth erosion, no matter the source. Because enamel is one of the toughest materials on earth, this doesn’t bode well for restorative materials. However, although manufacturers test ceramics extensively for their level of acid resistance, German investigators indicated that CAD/CAM resins are not—or at least not to standardized evaluation methods. Therefore, in 2022, they undertook an in-vitro study investigating the long-term acid resistance of BRILLIANT Crios, CERASMART, Grandio blocs, Lava Ultimate, and SHOFU Block HC.1
The team exposed the materials to tonic water, acetic acid, hydrochloric acid, and demineralized water to determine the damage these could cause the resin materials. They assessed surface roughness with confocal laser scanning microscopy and changes in surface hardness with Vickers hardness test. They also employed scanning electron microscopy, along with energy-dispersive x-ray spectroscopy and x-ray microcomputed tomography.1
The research revealed minimal changes in both surface roughness and surface hardness when exposed to one of the acidic media, although CERASMART had the most significant deterioration in surface properties. The team saw leaching of barium, aluminum, and titanium from filler on the rough surface (in a 2-µm zone) but not on the polished one. The team concluded that within the confines of the study, polished CAD/CAM resin composites resisted damage from acid exposure and were safe to recommend to patients who might have erosive oral conditions.1
Acid Increased Surface Roughness But Not Flexural Strength
Other research from earlier this year also explored these effects. Investigators in Egypt published a study in January 2023 that explored the effects of acid on the flexural strength and fatigue resistance of CAD/CAM materials.2
The investigators experimented using high-translucency zirconia (Ceramill Zolid HT+), lithium disilicate (IPS e.max CAD), hybrid ceramic (Vita Enamic), and nanohybrid resin composite (Grandio blocs). The team immersed the products in gastric hydrochloric acid, white wine, Coca-Cola, orange juice, and artificial saliva for 24 hours. They then used a scanning electron microscope and stylus profilometry to assess surface topography and roughness. The team tested flexural strength using a universal testing machine and cyclic fatigue loadings for each of the 2 halves of the samples.2
The zirconia and lithium disilicate had the highest initial flexural strengths, whereas the resin composite and hybrid ceramics had the lowest. The cyclic fatigue reduced the initial flexural strengths for all the materials except for the resin composite and hybrid ceramics. Meanwhile, the most consistent materials (shown by high Weibull moduli values) were zirconia and lithium disilicate, and the least were resin composite and hybrid ceramics.
The investigators determined that the erosive liquids induced significant changes in surface roughness for most of the CAD/CAM materials tested except for zirconia and lithium disilicate. However, those changes did not compromise the flexural strength of the material. Notably, resin composite and hybrid ceramics had greater resistance to cyclic fatigue.2
Resin Materials Are Susceptible to Erosion
This isn’t the first study on the subject. Another in-vitro study out of Italy in 2021 investigated the effects of distilled water, soda, and energy drinks on the volumetric wear and surface roughness of CAD/CAM monolithic materials.
In this study, the research team prepared 48 rectangular materials using nanohybrid composite, resin, lithium disilicate, and high-translucency zirconia. After immersing in distilled water for 2 days, the team used a stainless steel ball as an antagonist in a chewing machine and exposed the specimens to Coca-Cola, Red Bull, and distilled water. The research team then analyzed the wear and surface roughness using a cutting-edge 3D profilometer and scanning electron microscope.3
The findings revealed a significant disparity in the volumetric wear experienced by resin-based materials compared with ceramics (P = .00001). In particular, distilled water induced less volumetric wear than the other solutions tested (P = .0014). High-translucency zirconia exhibited superior surface roughness characteristics in comparison with all other materials examined.
The investigators concluded that these results underscore the critical consideration of monolithic CAD/CAM material selection when restoring worn dentition stemming from erosive processes. In particular, resin-based materials appear to be more susceptible to the erosive effects of acidic environments when subjected to a 2-body wear test.3
Ceramics Experience Changes in Surface Roughness and Microhardness
Another 2021 study out of Saudi Arabia assessed ceramic materials in an acidic environment. The research team wanted to study what happened when lithium disilicate, monolithic zirconia, and feldspathic ceramics were exposed to acidic conditions regarding flexural strength, elastic modulus, microhardness, and surface roughness.4
The team prepared 180 rectangular specimens (30 specimens of each of the ceramic materials). The investigators took initial surface roughness measurements on 90 specimens (30 per material) using a state-of-the-art optical noncontact profilometer, then immersed 30 specimens of each material in 1 of the following solutions: citric acid, an acidic beverage, or artificial saliva (their control). After immersion, the team assessed postimmersion surface roughness, flexural strength, and elastic modulus using the same optical noncontact profilometer and a 3-point bending test. Finally, the research team immersed the remaining 30 specimens of each material, then performed a microhardness test using a Vickers diamond microhardness tester. They used a scanning electron microscope to determine surface characteristic changes.4
The research team found that there was no significant impact on the flexural strength and elastic modulus of lithium disilicate or zirconia. However, microhardness and surface roughness were significantly altered in all groups.4
Zirconia-Reinforced Lithium Silicate Glass Ceramics Experience Increased Surface Roughness
Prior to these studies, a 2019 study in Iran assessed how 3 common beverages–soda, orange juice, and black tea–affected the surface roughness of zirconia-reinforced lithium silicate glass ceramics. The research team had 104 rectangular samples that were 2-mm thick and cut from VITA SUPRINITY blocks. They were then split into 2 groups; group 1 had a 2-step polishing treatment, whereas group 2 was heated and glazed per the manufacturer’s recommendations.5
The investigators measured the baseline surface roughness of the material using a profilometer; divided the samples into 4 groups that were immersed in artificial saliva, soda, orange juice, and black tea; and then took the surface roughness measurements again.5
The research team’s conclusions found that the highest and the lowest average surface roughness values were in the orange juice and saliva groups, regardless of the finish on the material. There was also an increase in surface roughness following immersion in orange juice and soda, again with no significant difference between finishes. However, the investigators noted that the surface roughness was slightly higher (although not statistically significant) in the polished surfaces compared with the glazed ones. So when it comes to surface roughness, orange juice and soda increase it.5
There you have it—5 studies and their conclusions about what acidic environments can do to your CAD/CAM restorations. One thing all these studies have in common is that they sought to provide more information for your materials selection process. This information will help you select what’s best for the case in front of you so that when your patients have a soda, they still have something to smile about.