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Introduction

Fixed appliances for the removal of malpositioned teeth are used in orthodontics for both adolescents and adults. Even today, difficult oral hygiene and the associated increased accumulation of plaque and food residues during therapy with multibracket appliances (MBA) represent an additional caries risk1. The development of demineralization, causing white, opaque changes in the enamel are known as white spot lesions (WSL), during treatment with MBA is a frequent and undesirable side effect and can occur after just 4 weeks.

In recent years, increased attention has been paid to sealing of the buccal surfaces and the use of special sealants and fluoride varnishes. These products are expected to provide long-term caries prevention and additional protection against external stresses. The various manufacturers promise protection between 6 and 12 months after a single application. In the current literature different results and recommendations can be found regarding the preventive effect and benefit for the application of such products. In addition, there are various statements regarding their resistance to stress. Five frequently used products were included: the composite based sealants Pro Seal, Light Bond (both Reliance Orthodontic Products, Itasca, Illinois, USA) and Clinpro XT Varnish (3 M Espe AG Dental Products, Seefeld, Germany). Also investigated were the two fluoride varnishes Fluor Protector (Ivoclar Vivadent GmbH, Ellwangen, Germany) and Protecto CaF2 Nano One-Step-Seal (BonaDent GmbH, Frankfurt/Main, Germany). A flowable, light-curing, radiopaque nanohybrid composite was used as the positive control group (Tetric EvoFlow, Ivoclar Vivadent, Ellwangen, Germany).

These five frequently used sealants were investigated in vitro towards their resistance after experiencing mechanical pressure, thermal burden and chemical exposure causing demineralization and consequently WSL.

The following hypotheses will be tested:

1.Null hypothesis: Mechanical, thermal and chemical stresses do not affect the sealants investigated.

2.Alternative hypothesis: Mechanical, thermal and chemical stresses affect the sealants investigated.

Material and method

192 bovine front teeth were used in this in vitro study. The bovine teeth were extracted from slaughter animals (slaughterhouse, Alzey, Germany). The selection criteria for bovine teeth were caries- and defect free, vestibular enamel without discoloration of the tooth surface and sufficient size of the tooth crown4. Storage was in a 0.5% chloramine B solution5, 6. Before and after bracket application, the vestibular smooth surfaces of all bovine teeth were additionally cleaned with an oil- and fluoride-free polishing paste (Zircate Prophy Paste, Dentsply DeTrey GmbH, Konstanz, Germany), rinsed off with water and dried with air5. Metal brackets made of nickel-free stainless steel were used for the study (Mini-Sprint Brackets, Forestadent, Pforzheim, Germany). All brackets used UnitekEtching Gel, Transbond XT Light Cure Adhesive Primer and Transbond XT Light Cure Orthodontic Adhesive (all 3 M Unitek GmbH, Seefeld, Germany). After bracket application, the vestibular smooth surfaces were cleaned again with Zircate Prophy Paste to remove any adhesive residue5. To simulate the ideal clinical situation during mechanical cleaning, a 2 cm long single archwire piece (Forestalloy blue, Forestadent, Pforzheim, Germany) was applied to the bracket with a preformed wire ligature (0.25 mm, Forestadent, Pforzheim, Germany).

A total of five sealants were investigated in this study. In selecting the materials, reference was made to a current survey. In Germany, 985 dentists were asked about the sealants used in their orthodontic practices. The most mentioned five out of the eleven materials were selected. All materials were used strictly according to the manufacturer’s instructions. Tetric EvoFlow served as the positive control group.

Based on a self-developed time module to simulate the average mechanical load, all sealants were subjected to a mechanical load and subsequently tested. An electrical toothbrush, Oral-B Professional Care 1000 (Procter & Gamble GmbH, Schwalbach am Taunus, Germany), was used in this study to simulate the mechanical load. A visual pressure check illuminates when the physiological contact pressure (2 N) is exceeded. Oral-B Precision Clean EB 20 (Procter & Gamble GmbH, Schwalbach am Taunus, Germany) were used as toothbrush heads. The brush head was renewed for each test group (i.e.6 times). During the study, the same toothpaste (Elmex, GABA GmbH, Lörrach, Germany) was always used in order to minimize its influence on the results7. In a preliminary experiment, the average pea-sized amount of toothpaste was measured and calculated using a microbalance (Pioneer analytical balance, OHAUS, Nänikon, Switzerland) (385 mg). The brush head was moistened with distilled water, moistened with 385 mg average toothpaste and passively positioned on the vestibular tooth surface. The mechanical load was applied with constant pressure and reciprocal forward and backward movements of the brush head. The exposure time was checked to the second. The electric toothbrush was always guided by the same examiner in all test series. The visual pressure control was used to ensure that the physiological contact pressure (2 N) was not exceeded. After 30 min of use, the toothbrush was fully recharged to ensure consistent and full performance. After brushing, the teeth were cleaned for 20 s with a mild spray of water and then dried with air8.

The time module used is based on the assumption that the average cleaning time is 2 min9, 10. This corresponds to a cleaning time of 30 s per quadrant. For an average dentition, a full dentition of 28 teeth, i.e. 7 teeth per quadrant, is assumed. Per tooth there are 3 relevant tooth surfaces for the toothbrush: buccal, occlusal and oral. The mesial and distal approximal tooth surfaces should be cleaned with dental floss or similar but are usually not accessible for the toothbrush and can therefore be neglected here. With a cleaning time per quadrant of 30 s, an average cleaning time of 4.29 s per tooth can be assumed. This corresponds to a time of 1.43 s per tooth surface. In summary, it can be assumed that the average cleaning time of a tooth surface per cleaning procedure is approx. 1.5 s. If one considers the vestibular tooth surface treated with a smooth surface sealant, a daily cleaning load of 3 s on average can be assumed for twice daily tooth cleaning. This would correspond to 21 s per week, 84 s per month, 504 s every six months and can be continued as desired. In this study the cleaning exposure after 1 day, 1 week, 6 weeks, 3 months and 6 months was simulated and investigated.

In order to simulate the temperature differences occurring in the oral cavity and the associated stresses, artificial ageing was simulated with a thermal cycler. In this study the thermal cycling load (Circulator DC10, Thermo Haake, Karlsruhe, Germany) between 5 °C and 55 °C at 5000 cycles and an immersion and dripping time of 30 s each was carried out simulating the exposure and ageing of the sealers for half a year11. The thermal baths were filled with distilled water. After reaching the initial temperature, all tooth samples oscillated 5000 times between the cold pool and the heat pool. The immersion time was 30 s each, followed by a 30 s drip and transfer time.

In order to simulate the daily acid attacks and mineralization processes on the sealants in the oral cavity, a pH change exposure was carried out. The solutions selected were the Buskes12, 13solution described many times in the literature. The pH value of the demineralization solution is 5 and that of the remineralization solution is 7. The components of the remineralization solutions are calcium dichloride-2-hydrate (CaCl2-2H2O), potassium dihydrogen phosphate (KH2PO4), HE-PES (1 M), potassium hydroxide (1 M) and aqua destillata. The components of the demineralization solution are calcium dichloride -2-hydrate (CaCl2-2H2O), potassium dihydrogen phosphate (KH2PO4), methylenediphosphoric acid (MHDP), potassium hydroxide (10 M) and aqua destillata. A 7-day pH-cycling was carried out5, 14. All groups were subjected to 22-h remineralization and 2-h demineralization per day (alternating from 11 h-1 h-11 h-1 h), based on pH cycling protocols already used in the literature15, 16. Two large glass bowls (20 × 20 × 8 cm, 1500 ml3, Simax, Bohemia Cristal, Selb, Germany) with lids were chosen as containers in which all samples were stored together. The covers were only removed when the samples were changed into the other tray. The samples were stored at room temperature (20 °C ± 1 °C) at a constant pH value in the glass dishes5, 8, 17. The pH value of the solution was checked daily with a pH meter (3510 pH Meter, Jenway, Bibby Scientific Ltd, Essex, UK). Every second day, the complete solution was renewed, which prevented a possible drop in the pH value. When changing samples from one dish to the other, the samples were carefully cleaned with distilled water and then dried with an air jet to avoid mixing the solutions. After the 7-day pH cycling, the samples were stored in the hydrophorus and evaluated directly under the microscope. For optical analysis in this study the digital microscope VHX-1000 with VHX-1100 camera, the movable tripod S50 with VHZ-100 optics, the measuring software VHX-H3M and the high-resolution 17-inch LCD monitor (Keyence GmbH, Neu-Isenburg, Germany) were used. Two examination fields with 16 individual fields each could be defined for each tooth, once incisal and apical of the bracket base. As a result, a total of 32 fields per tooth and 320 fields per material were defined in a test series. To best address the everyday important clinical relevance and approach to visual assessment of sealants with the naked eye, each individual field was viewed under the digital microscope with a 1000 × magnification, visually evaluated and assigned to an examination variable. The examination variables were 0: material = the examined field is completely covered with sealing material, 1: defective sealant = the examined field shows a complete loss of material or a considerable reduction at one point, where the tooth surface becomes visible, but with a remaining layer of the sealant, 2: Material loss = the examined field shows a complete material loss, the tooth surface is exposed or *: cannot be evaluated = the examined field cannot be represented optically sufficiently or the sealer is not sufficiently applied, then this field fails for the test series.