The potential for resin infiltration technique in dental practice

Caries infiltration – principle

The aim of caries infiltration is to occlude the pores within the lesion body within enamel of non-cavitated caries lesions with special low-viscosity light-curing resins, so-called infiltrants. That way, diffusion pathways for cariogenic acids are blocked, resulting in a reduction, or even arrest, of lesion progression (Figure 2). To enable penetration of the infiltrant into the lesion body, first the covering pseudo-intact surface layer, as a result of the remineralization processes, has to be removed.4 This can be achieved by etching the lesion for 2 minutes with 15% hydrochloric acid gel.5 Subsequently, the lesion has to be properly desiccated to allow capillary action to soak the resin into the lesion body. Finally, the infiltrant must be applied for sufficient time to infiltrate the lesion deeply and be light-cured.

The limited access to the proximal surfaces is a challenge for any therapeutic intervention on this caries predilection site. Therefore, in contrast to caries sealing where a resin coating is created on the lesion surface, for caries infiltration all resin excess is removed from the surface before light-curing. In the narrow and hardly accessible proximal area, resin layers on the lesion surface are hard to control clinically and might even promote plaque stagnation and gingival inflammation. Therefore, for caries infiltration the resinous diffusion barrier is solely created within the lesion body, which greatly simplifies clinical application.

Proximal application

Resin infiltration may be indicated for active non-cavitated caries lesions extending radiographically into the inner half of enamel (E2) up to the outer third of dentine (D1). In the commercially available treatment kit for proximal infiltration (Icon®, DMG, Hamburg, Germany), all necessary materials are included, with the exception of rubber dam. To allow application without temporary tooth separation for some days,6 special foil applicators have been designed. These applicators consist of two thin, welded foils. One foil (white side of the applicator; Figure 3d) protects the adjacent tooth from contamination, whereas the other foil (green side) has a perforation and thus allows the discharge of the agents in the area of the lesion. The treatment steps for caries infiltration of proximal lesions are illustrated in Figure 3.

The efficacy of caries infiltration has been investigated in clinical trials with a split-mouth design. In a study on German young adults with medium caries risk, 4% of infiltrated lesions progressed radiographically within the three-year follow-up period, whereas in the control group (risk-related non-invasive intervention only) 46% of lesions progressed.7^,8 In a study on children with high caries risk in Greenland, 23% of infiltrated lesions in primary molars showed radiographic lesion progression, whereas for the control lesions progression of 62% could be observed within one year. In this study, fluoride varnish was applied at baseline and after six months in both groups.9 These results on randomized clinical trials are promising, even if further research is needed to evaluate the efficacy of resin infiltration when performed by dental practitioners in daily practice. In clinical studies, more lesion progressions were observed for caries radiographically extending into the outer third of dentine compared with lesions extending just into enamel. This suggests that (at least some) treatment failures might be caused by inadvertently infiltrating cavitated lesions which are more frequent in deeper caries stages, since these caviations can only partially be filled with the resin.10

As with all other therapeutic interventions that aim to arrest lesion progression, such as fluoridation or sealing, the only way to assess the success of proximal resin infiltration is to monitor the lesion radiographically over time. For proximal caries lesions, high quality bitewing radiographs are the most accurate and reliable method to monitor lesion progression. Intervals between radiographs should be adjusted to the individual caries risk. Special care should be taken to align the sequential radiographs similarly, since varying alignments between two radiographs might result in a misleading interpretation of caries progression or regression, even when the lesion is stable.

Aesthetic effect

A positive side-effect of resin infiltration is that fully infiltrated lesions change their whitish opaque appearance and look similar to sound enamel.11 White spot lesions appear chalky due to the high difference in the refractive indices (RI) of apatite crystals (RI: 1.62) and the medium within the lesion pores, which is either a watery medium (RI: 1.33) or air (RI: 1.0).12 The higher the difference of RI, the more visible light is scattered between the crystals and the more whitish the lesion looks. For this reason, desiccated lesions look even whiter than wet ones. When the pores are infiltrated with resin (RI: 1.52) light scattering is significantly reduced because the difference in RI compared with the pores of the apatite crystals is rather low and thus the lesion appears similar to sound enamel.

Buccal white spot lesions are frequently observed after treatment with fixed orthodontic appliances. After de-bonding of the brackets the lesions can be arrested, usually with non-invasive interventions like biofilm control and local fluoridation. However, because remineralization is usually limited to superficial layers, only rather shallow lesions disappear. Most lesions remain as enamel scars even when they are arrested and aesthetic improvement can often only be achieved using quite invasive interventions, like enamel micro-abrasion or composite restorations. Resin infiltration can be used to camouflage active non-cavitated buccal lesions that are perceived as aesthetical impairment. The treatment steps are similar to the proximal application (Figure 4).

In a South Korean study, 61% of post-orthodontic caries lesions could be masked completely using caries infiltration and a partial masking effect could be observed in another 33% of lesions.13

The masking effect can be insufficient, if only an incomplete removal of the pseudo-intact surface layer is accomplished. In this case, only incomplete infiltration of the underlying lesion body occurs. Because rather inactive lesions exhibit quite thick surface layers, it is often necessary to repeat the etching step until the surface layer is completely removed. During each application, for 2 minutes, an approximately 30 µm thick enamel layer is eroded by the HCl-gel. To check if the surface layer is completely removed, water or ethanol should be dropped on the desiccated lesions. When the penetration of these liquids into the desiccated lesion results in a quick change (within the first 3–5 seconds) in lesion appearance (rapid change of refractive index within the pores), this effect can also be expected during the application of the infiltrant and the procedure can be continued. If this effect fails to appear, the etching should be repeated until the masking effect can be observed. Of course, the amount of enamel erosion should be balanced with the aesthetic impairment caused by the lesions and alternative therapeutic options such as remineralization and restorations should be considered.

For the masking of developmental defects like molar-incisor-hypomineralization, hypomineralization of a permanent tooth after apical trauma of the deciduous tooth or fluorosis, resin infiltration cannot be recommended at present, because these lesions cannot be that easily infiltrated.13 Moreover, in contrast to the resin infiltration of caries, the infiltration of developmental defects has not been thoroughly scientifically investigated.

Caution is also advised with cavitated lesions. Nonetheless, infiltration of enamel parts of caries lesions can be easily combined with composite restorations. Since the infiltrant has similar chemical properties to bonding agents, composite resins can be placed on to and adjacent to infiltrated lesions. Cavities extending into dentine, however, have to be restored conventionally with dentine adhesives, because the infiltrant is far too hydrophobic to be used for this purpose.

Summary

Caries infiltration follows the principle of caries sealing. However, the resinous diffusion barrier is created inside the lesion but not on the lesion surface, whereby clinical application is simplified. Clinical studies have shown that caries infiltration in combination with non-invasive measures is more efficacious to arrest proximal lesions extending radiographically up to the outer third of dentine compared with non-invasive measures alone. Thus, particularly for proximal caries lesions radiographically extending around the enamel-dentine junction, caries infiltration is efficacious to slow down or even arrest caries progression in order to postpone or even avoid the first restorative intervention. A positive side-effect of caries infiltration is that infiltrated lesions lose their whitish appearance. Buccal white spot lesions can be arrested with non-invasive interventions but often remain as whitish or brownish enamel scars which are perceived as being unaesthetic. Here caries infiltration can be used to camouflage these defects, combining hard tissue preservation on the one hand and a good predictability of the treatment outcome on the other hand.

Disclosure statement

The two authors receive a research grant and royalties from DMG, Hamburg. This article is partly based on a previous publication: Paris, Dörfer, Meyer-Lückel: Kariesinfiltration. Zahnärztliche Mitteilungen 2011; 101: 38–41.