Biofilm Oral

Abstract

Objective

To determine streptococcal adhesion forces with composite resins with different surface roughness.

Methods

Polishing and grinding were applied to obtain smooth (roughness 20 nm), moderately rough (150 nm) and rough (350 nm) surfaces of two orthodontic, light-cured composites. Adhesion forces betweenStreptococcus sanguinis and Streptococcus mutans and the composite surfaces were measured using atomic force microscopy in absence or presence of a salivary conditioning film. Initial adhesion forces were measured as well adhesion after 120 s of contact, as longer contact times are known to result in stronger adhesion forces (“bond-strengthening”). Surface roughness in absence and presence of salivary conditioning films were compared using ANOVA, while adhesion forces were subjected to a Weibull analysis.

Results

Initial adhesion forces in absence of a salivary conditioning film amounted between −0.7 and −0.9 nN for smooth composite resins and increased between −1.0 and −2.0 nN for the roughest surfaces. Streptococcal adhesion forces after bond-strengthening were significantly stronger than upon initial contact, irrespective of the composite type. Salivary conditioning films significantly decreased the surface roughness of the composites, as well as the streptococcal adhesion forces. Yet, also in the presence of a conditioning film, rougher composite surfaces exerted stronger adhesion forces, irrespective of composite type or bacterial strain.

Conclusion

Streptococcal adhesion forces to orthodontic composite resins increase with increasing roughness of the composite surfaces. Composite surface roughness less affects adhesion forces with S. mutans than with S. sanguinis.

Keywords

• Adhesive;

• Composite resin;

• Surface roughness;

• Bacterial adhesion;

• Bacterial adhesion force;

• Biofilm;

• Weibull analysis

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1. Introduction

White spot lesions around orthodontic brackets form one of the most prevalent side effects of orthodontic treatment with fixed appliances, and affect approximately 50% of all orthodontic patients [1] and [2]. These lesions can remain visible as a permanent enamel scar or “tooth decay” even 5 years after treatment [3], compromising facial esthetics after an often lengthy and costly course of orthodontic treatment. The junction between bracket, adhesive and enamel constitutes a favorable place for oral bacteria to adhere and form a biofilm and especially excessive bonding composite around the bracket has been demonstrated to be prone to biofilm accumulation [4]. Lee et al. found that orthodontic composite had a higher ability to retain oral streptococci than metallic brackets [5]. Oral biofilms are only able to maintain position in the oral cavity if the forces by which they adhere are stronger than the prevailing oral detachment forces or the forces exerted by toothbrushing. We have previously investigated oral bacterial adhesion forces to the different materials constituting the bracket–adhesive–enamel junction using atomic force microscopy (AFM) and observed that, in line with the above, different oral bacterial strains adhered more strongly to orthodontic composite resins than to enamel or metal surfaces involved in the junction. It remained unclear though, whether this was due to the hydrophobicity of the composite surface (water contact angle 71°) or its higher roughness (28.5 nm) as compared with enamel (30°; 7.0 nm) or stainless steel (68°; 2.7 nm).

Initial bacterial adhesion on smooth surfaces is reversible and becomes irreversible within seconds to minutes after initial contact [6]. Measurement of bacterial adhesion forces using AFM has demonstrated that longer contact times between a bacterium and a substratum surface results in stronger adhesion forces (“bond-strengthening”) on surfaces in the presence as well as in the absence of a salivary conditioning film[7]. Although it is known that rougher surfaces promote bacterial adhesion [8], [9], [10] and [11] to an extend that exceeds the influence of hydrophobicity or surface free energy [11] and [12], the influence of surface roughness on bacterial adhesion forces and their strengthening over time, has not yet been studied. We hypothesize that bacterial adhesion forces and their strengthening over time are dependent on the surface roughness of the composite surfaces. In order to test this hypothesis, we investigated the influence of surface roughness of two different orthodontic composite resins on the adhesion forces of two oral streptococcal strains using AFM. Force measurements were carried out in the absence or presence of an adsorbed salivary conditioning film.

2. Materials and methods

2.1. Bacterial strains and growth conditions

Streptococcus sanguinis ATCC10556 and Streptococcus mutans ATCC700610 were precultured in 10 mL Todd Hewitt Broth (THB, Oxoid, Basingstoke, UK) for 24 h and then inoculated into 200 mL of THB for 16 h at 37 °C. Bacteria were harvested by centrifugation (5 min, 5000 × g, 10 °C), washed twice with demineralized water, and resuspended in demineralized water for AFM.

2.2. Composite resins and salivary conditioning film formation

Two light-cured composite resins, Transbond XT™ (3M Unitek, Monrovia, CA, USA) and PADLock®(Reliance Orthodontic Products, Inc., Itasca, IL, USA) were included in this study. Transbond XT™ consists of 10–20% Bis-GMA, 5–10% Bis-EMA, 70–80% fillers (silylated quartz and submicron silica) next to <0.2% diphenyliodonium hexafluorophosphate, while PADLock® contains 8–20% Bis-GMA, 15–60% glass filler and 3–12% amorphous silica, next to 1–3% sodium fluoride, as taken from the respective Materials Safety Data Sheets. Both composite resins are commonly used for bonding of orthodontic brackets to enamel surfaces. Composites were made into 1 cm diameter discs with a thickness of 1 mm by pressing between two glass plates, covered with copier overhead films. Composite discs were cured using a halogen lamp (Optilux 501 Curing Light; Kerr/Demetron, Danbury, CT, USA) for 20 s at a distance of 2 mm from the surface. Light intensity was higher than 400 mW/cm2, as verified by a radiometer after every 3 specimens.

Cured discs were used as prepared and after roughening with 0.05 μm wetted aluminum oxide particles (Buehler, Lake Bluff, USA) or with 800 grit wetted sandpaper. Subsequently, discs were cleaned by sonication for 2 × 5 min at 35 kHz in an ultrasonic bath (Transsonic TP 690-A, 154W; Elma, Singen, Germany), and rinsed with demineralized water. X-ray photoelectron spectroscopy (XPS) confirmed the effective removal of grinding and polishing particles from the composite surfaces. In addition, water contact

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