INTRODUCTION

About 15% of patients undergoing orthodontic treatment experience complications requiring professional restorations, generating annual costs of over US$500 million.¹ Plaque formation increases during orthodontic treatment with fixed orthodontic appliances as a result of mechanical disruption of the self-cleaning mechanisms of the tongue, lips, and cheeks.² Furthermore, oral hygiene becomes more complicated with appliances in situ, challenging patient compliance, and organic acids produced by cariogenic bacteria may demineralize enamel.^3–5 This results in early caries (so-called “white spot” lesions), an unwanted side effect of fixed orthodontic appliances. White spot lesions occur in up to 97% of patients receiving orthodontic treatment⁶ and can be observed around the bracket and the cervical area between the bracket and gingiva⁷ as early as 1 month after starting treatment.^4,8,9

Even though enamel loss caused by demineralization during orthodontic treatment can in part be prevented by prophylactic measures such as daily oral rinsing with solutions containing sodium fluoride,^10–13 poor patient compliance limits their efficacy.¹¹ Other methods to prevent white spot lesions have also been suggested, including the application of fluoride-releasing varnish, concentrated fluoride gels, and chlorhexidine; however, all of these methods are dependent on good patient compliance.¹³

Therefore, a need for effective measures to protect against the formation of early carious lesions remains throughout the duration of orthodontic therapy with fixed appliances. Ideally, these measures should be independent of the degree of patient compliance.² One solution might be to seal the labial enamel surfaces of the teeth prior to bracket bonding by smooth surface sealants. Several studies^2,3,13–17 have tested the effectiveness of the latter, but results are contradictory, in part due to the many different sealing agents and test methods employed. Thus, evidence on the efficacy of smooth surface sealants is limited, and further studies are warranted. Standardized assessment methods and in vitro models would be helpful in that regard.

ProSeal^® (Reliance Products, Itasca, Ill) is a light-curing smooth surface sealant. According to the manufacturer it is composed of >20% urethane acrylate ester, >20% polyethylene glycol diacrylate, and >5% fluoride-containing glass frit.¹⁸ It is unique in that it contains a fluorescing agent, which allows easy visual monitoring of sealant integrity and coverage using near-ultraviolet (UV) fluorescence-inducing light sources. Recent analyses¹⁹ have shown that resin composites containing fluorescent pigments are best detected at approximately 400 nm.

Although the so-called Fluorescence-aided Identification Technique (FIT)²⁰ has been proposed to identify composite restorations with fluorescent properties, there are no data on the use of FIT for clinical monitoring of fluorescing sealants. FIT could be a powerful, noninvasive, and less time-consuming diagnostic tool for identification, monitoring, and even quantification of seal integrity around orthodontic brackets. Here, the integrity of the smooth surface, fluorescent, resin-based sealant ProSeal was examined around orthodontic brackets in vitro by FIT. The null hypothesis was that using the sealant had no effect on integrity and on preventing unfilled areas.

MATERIALS AND METHODS

Testing Procedure

Specimens were collectively exposed to thermocycling (20,000 cycles, 5 ± 2°C to 55 ± 2°C, dwell time 30 seconds, transfer time 10 seconds). Toothbrushing (Braun/Oral-B Ortho Brush Head, Aschaffenburg, Germany) was simulated by using a recently developed device for abrasion testing, allowing linear and oscillating brushing motions simultaneously.²¹ The vertical load was between 1.5 N and 2.2 N. A standard toothpaste (Aronal^®, GABA-GmbH, Hamburg, Germany) was used to prepare an abrasive slurry according to DIN EN ISO 11609. To simulate extensive toothbrush abrasion, each sample was subjected to a total of 15,000 strokes during toothbrush simulation, divided into four cycles of 3750 strokes each, as conducted in earlier works.^2,3 The slurry and heads of the brushes were changed between each treatment cycle.

The surface-sealed areas were measured after one, two, three, and four alternating thermocycling and brushing cycles, simulating a period of 6, 12, 18, and 24 months, respectively.²² Digital images of the samples were taken under standardized conditions after each thermocycling and brushing cycle by stereomicroscope (Leica Wild M420, Heerburg, Switzerland) equipped with a digital single-lens reflex camera (Canon 450D, Tokyo, Japan) using a yellow-tinted filter (520 nm) connected in live-view mode to a computer. For FIT, every specimen was placed under the microscope and illuminated with a prototype (modified 20161420-1 unit, KARL STORZ, Tuttlingen, Germany) diagnostic fluorescence-inducing light source (405 ± 7 nm) for visual detection of the sealed area transmitted via fiber-optic cable through a reflecting lens producing a sharply outlined spotlight large enough to illuminate the entire specimen. Each specimen was examined by 15× magnification to determine whether the sealant layer remained bonded to the surface. Afterward, images were captured under identical light conditions in a darkened room illuminated by artificial light. ImageJ (version 1.46r, NIH, Bethesda, Md, USA) was used to measure sealant loss (unsealed area) in the images. Unsealed areas were determined by fluorescence loss, generating a contrast between the sealed (more fluorescent) and unsealed (less fluorescent) areas. The specimens were blinded prior to analysis and, in case unsealed areas were observed, sealant loss was measured in square millimeters and its proportion determined by calculating the sum of the unsealed areas (Figure 1).

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RESULTS

The examination performed confirmed that the samples had an intact and completely sealed area at baseline (100%) after applying ProSeal. The sealed areas were variably affected over the simulated 2-year period. A representative sample is shown in Figure 2. The seal integrity decreased proportionally but variably on all teeth (Table 1). Sealant loss started at the periphery of the sealed area, forming “islands” of remaining sealant, which decreased in size after every thermocycling-brushing cycle. In general, the cervically sealed area of the bracket persisted the longest (Figure 2). Over time, the ProSeal coverage was lost to varying degrees and at different time points throughout the test period. For instance, after the first thermocycling-brushing cycle (6 months) about 3% of the sealant was missing in sample 9 and 30% in sample 3.

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Table 1 Time-Dependent Reduction of the Sealed Areas in the Analyzed Study Samples After Exposure to Each One, Two, Three, and Four Cycles of Alternating Thermocycling and Toothbrushing

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At the end of the simulated period of 2 years, the best-case scenario was that 79% of the tooth surface was still sealed (sample 4); the worst-case scenario was 0% (sample 11). At the end of testing, the mean remaining layer of ProSeal was 54%, and 4/17 samples showed only minimal sealant coverage (at most 22%) and, therefore, the ProSeal was offering little to no protection. The sealed area of the control samples remained intact during the entire testing period. There were no visible changes in fluorescence intensity of the sealed areas even after 2 years of storage in 0.9% sterile saline.

DISCUSSION

This study presented for the first time a practical method to check and monitor the protective layer of smooth surface sealants by means of fluorescence-inducing light throughout orthodontic therapy. Using digital imaging, sealant coverage could be quantified. So far, no quantitative method has been found in the literature to calculate sealant loss on smooth surfaces. The present proposal can also serve as an easy way to calculate sealant loss, even in the in vivo situation. The proposed formula (Figure 1) is a modification of the procedure for the measurement of air bubbles trapped in sealed areas, proposed in a recent work²³ evaluating pit-and-fissure sealants. This kind of analysis is a more objective scoring method, rendering mandatory separate analyses with interexaminer reliability evaluations unnecessary.^23,24 This practice-oriented in vitro and in vivo applicable model refrained from measuring the sealant layer thickness, which would require tooth sectioning, as reported previously,^2,3,25,26 and is, therefore, not applicable to in vivo examinations. Furthermore, layer thickness is not in itself sufficient to assess the sealed area.²⁵ Abrasion may be affected at different places as a result of the different thicknesses of the applied sealant caused by the tooth surfaces being uneven by nature.²⁵

Thermocycling and brushing exposure was set at 2 years, following cycle recommendation protocols from the literature,^2,3,22,25 considering that this is the average duration of multibracket treatment. Toothbrushing to simulate mechanical wear in vivo is common in studies assessing the wear of dental materials.^2,3 However, most studies on smooth surface sealants in orthodontic treatment did not bond brackets prior to applying mechanical stress, and so they fail to reflect the clinical reality. It might very well be that bonded brackets have a significant influence on where the sealant is abrading or debonding first. This study showed that the sealant first debonded from the outer areas to leave “islands” of sealed areas, but they continued to decrease over time. To overcome the abrasion problem, the relation of filler amount and filler particle size in the organic matrix of the sealant should be questioned. Some chipping was also observed, leading to the assumption that the material might also lack adhesion. Therefore, using a bonding agent prior to sealant application might be feasible, as has already been proposed for conventional pit-and-fissure sealants.²³

Interestingly, the sealed area cervical to the brackets remained intact the longest but is the most common site for white spot lesions. This may be related to the brushing pattern resulting from the presence of the bracket, leading to inconsistent abrasion of the sealed area. Clinical studies have shown that sealants are less effective when placed gingival to brackets. This may be related to an inadequate sealing procedure, due to the difficulty in maintaining good isolation of the cervical zone of the tooth in the clinical situation.

Several studies examined ProSeal's potential for inhibiting white spot lesions,^{2,3,13–17,26} some showing a positive effect^2,3,15–17 and others showing little to no effect.^13,14,26 In the present study, the unexpected fast but variable reduction in sealant coverage already after the first thermocycling-brushing cycle (6 months) was consistent with previous results¹³ showing that ProSeal only remains intact for less than 30 days. Leizer et al.¹⁴ reported that the additional time and expense of using ProSeal to prevent decalcification does not appear to be justified, as a result of high decalcification rates.

Currently, consistent sealing of dental surfaces during therapy with fixed brackets is known to be only partially possible. This might be because the sealant layer is either not intact from the very beginning or leaks after a short period of time. However, this has yet to be proven directly, since conventional smooth surface sealants are basically invisible. At present, only sealants containing fluorescence pigments like ProSeal allow easy inspection of the sealed area by means of FIT,²⁰ which uses commercially available near-UV fluorescence-inducing light sources for in vivo application. In this way, the loss of fluorescing sealants can be assessed. This method has the particular advantage of identifying a sealant or persistent remainder of it, for instance in monitoring, retreatment, or experimental analyses. The use of FIT does not necessarily require complex and expensive devices, since many simple and practical light sources exist on the dental market for efficient detection of sealants (or even flowable and nonflowable composites¹⁹) that have fluorescence properties. The presented study method describes a new, noninvasive method to assess the sealed area proportion of smooth surface sealants having fluorescence properties quantitatively. Using this method, researchers could easily assess the durability of these sealants at any time during orthodontic treatment.

In summary, the results suggest that the analyzed sealant may not remain intact around orthodontic brackets during the whole duration of orthodontic therapy, and, therefore, a lack of protection against caries might be expected clinically. However, FIT is an easy and effective way, in everyday practice or research activities, to monitor the integrity of sealants that have fluorescent properties. Using FIT, the quality of the sealed area can be verified at any time during therapy. In the case of existing imperfections, reparative, preventive, and/or remineralizing measures can be introduced to minimize progression or even to prevent white spot lesions. Invasive or esthetic damage as well as the need for future restorative interventions can be avoided. Clinical studies are necessary to test the proposed method in the routine diagnostic setting and to establish whether the development or progression of caries during therapy can be avoided.

CONCLUSIONS

• When tested in vitro, ProSeal failed to maintain integrity of the sealed area after exposure to the presented study conditions. Weak bonding and abrasion resistance may be potential concerns of the product properties. Although the results cannot be automatically transferred to the clinical setting, premature sealant loss may represent a significant problem, compromising ProSeal's intended prophylactic function.

• The proposed FIT method represents a new, noninvasive diagnostic procedure for the quantification and monitoring of the integrity of sealed areas around orthodontic brackets when using smooth surface sealants. FIT may be useful for the development of more consistent and verifiable materials for clinical use and research activities.