Bacteria that cause dental caries can apparently be kept away from orthodontic plates using nanoscale materials. A study published in the Dental Materials Journal in 2023 demonstrated that adding 7.5% zinc oxide (ZnO) nanoparticles to heat-cured acrylic resin could reduce Streptococcus mutans adhesion by 12.91% compared with the unmodified control group. This finding is important for millions of orthodontic patients in Indonesia, considering that the prevalence of malocclusion in the country reaches 80% and 21.2% of users of removable orthodontic appliances are at risk of developing dental caries.
The study was conducted by drg. Rahmadani Puspitasari, M.DSc, together with a research team from the Department of Dental Biomaterials, Faculty of Dentistry, Universitas Gadjah Mada, namely Prof. Dr. drg. Dyah Irnawati, M.S., and Prof. Dr. drg. Widjijono, SU. The study was carried out at the Integrated Laboratory of the Faculty of Dentistry, UGM, and received ethical approval from the Prof. Soedomo Research Ethics Committee of the Faculty of Dentistry, UGM, on November 22, 2022.
Heat-cured acrylic resin is one of the most commonly used materials for the base plates of removable orthodontic appliances, ranging from myofunctional appliances to retainers. Unfortunately, this material has an inherent weakness: its surface contains microscopic pores ranging from 10 to 500 micrometers in size that are invisible to the naked eye.
These pores provide a favorable habitat for S. mutans, a Gram-positive bacterium commonly found adhering to removable orthodontic appliances. The adhesion process does not occur spontaneously. First, saliva coats the surface of the plate and forms a mucin layer, a lubricating protein produced by the salivary glands. This mucin layer then acts as a “bridge” that allows S. mutans to attach and form a biofilm. The more mucin that accumulates, the greater the risk of bacterial colonies developing and triggering dental caries.
Mechanical and chemical cleaning of the plate is certainly possible, but its effectiveness depends heavily on patient compliance. Children and adolescents, who represent the largest group of orthodontic appliance users, cannot always be relied upon to clean their appliances optimally and consistently.
Rather than relying on patients to clean their appliances diligently, the researchers proposed making the plate itself actively resistant to bacterial adhesion. This was achieved by incorporating ZnO nanoparticles directly into the acrylic resin matrix before polymerization.
The biggest challenge was that ZnO nanoparticles tend to aggregate and become unevenly distributed throughout the resin. To overcome this problem, the nanoparticle surfaces were first modified using a silane coupling agent called 3-(trimethoxysilyl)propyl methacrylate (TMPSM). This silanization process strengthened the bonding between the nanoparticles and the polymer matrix and produced a more homogeneous distribution.
Four groups of disc-shaped specimens, each 10 mm in diameter and 2 mm thick, were prepared: one control group without ZnO and three modified groups containing 2.5%, 5%, and 7.5% ZnO. After being processed using the conventional denture-base fabrication method, the specimens were immersed in distilled water for 48 hours at 37°C to reduce residual monomer levels before testing.
Scanning Electron Microscope (SEM) analysis showed that the higher the ZnO concentration, the fewer microscopic pores were visible on the resin surface. Energy Dispersive X-ray (EDX) mapping confirmed a relatively uniform distribution of zinc (Zn) on the surfaces of the modified specimens.
Surface roughness testing showed a consistent trend: the higher the ZnO concentration, the smoother the resin surface. The mean surface roughness of the control group was 1.65 μm, whereas the 7.5% ZnO group had a roughness of only 1.08 μm.
In addition to becoming smoother, the resin surface also became more hydrophobic, meaning that it repelled water more effectively. The water contact angle in the control group was 86.09°, whereas the 7.5% ZnO group reached 104.15°. A hydrophobic surface interferes with the electrostatic interactions and hydrogen bonds required for mucin adhesion, thereby directly disrupting the bacterial attachment process.
“The 7.5% concentration of ZnO nanoparticles showed the lowest mucin adhesion, at 18.07 ± 0.80 mg/mL, and S. mutans adhesion of 87.09 ± 0.88% compared with the control group.”
This figure means that the 7.5% ZnO group successfully inhibited S. mutans adhesion by 12.91% compared with unmodified resin. The mechanism involves not only surface smoothing and increased hydrophobicity but also the production of Reactive Oxygen Species (ROS), which induce oxidative stress in bacterial cells, inhibit protein synthesis, and damage S. mutans DNA replication.
One interesting finding emerged in the 5% ZnO group: S. mutans adhesion was actually higher than in the 2.5% group. This was attributed to nanoparticle agglomeration at the 5% concentration, which resulted in a lower distribution of zinc on the outer resin surface (2.84%) than in the 2.5% group (5.47%). This uneven distribution weakened the antibacterial activity.
These findings open the door to the development of orthodontic plates with self-cleaning capabilities—plates that can independently inhibit bacterial growth without relying entirely on patient compliance with cleaning routines.
A ZnO concentration of 7.5% was recommended as the optimal formulation in this study. However, the researchers also noted several limitations that should be addressed in future studies, including the lack of measurements of particle size after silanization and the fact that the precise mechanism of Zn ion release from the specimens into the oral environment remains unknown.
This study demonstrates that innovation in dental materials does not necessarily have to be conceptually complicated. Sometimes, all it takes is a nanoscale particle incorporated into a long-established material to produce an impact far greater than its size.
Author: drg. Achmad Zam Zam Aghasy, M.Kes., Annisa Dwi Noviyanti
Photo: FreePik
Source DOI: https://doi.org/10.4012/dmj.2023-016