Comprehensive Notes on Resin Infiltration in Pediatric Dentistry

Resin Infiltration in Pediatric Dentistry: A Review

Introduction

Dental caries is a global issue affecting over 600 million children [1].
Minimal Intervention Dentistry (MID) emphasizes early detection and prevention.
Resin Infiltration (RI) is a key product of acid etching and Bis-GMA resin monomer discoveries.
RI, like Icon® (DMG, Hamburg, Germany), supports MID by infiltrating lesions with low-viscosity resin to arrest caries progression without drilling.
The narrative review explores RI's usage in pediatric dentistry, covering development, applications, shortcomings, and innovations.

Development of Resin Infiltration (RI)

Resin Monomer

The concept of infiltrating early caries dates back to the 1970s [8–13].
1975: Davila et al. [8] proposed adhesives to seal lesions, with acid conditioning enhancing infiltration.
1976: Robinson et al. [9] used resorcinol-formaldehyde resin to occlude pores, reducing demineralization, but discontinued due to cytotoxicity [9, 10].
Ideal resin properties: low viscosity, hydrophilic, anti-bacterial, non-toxic, aesthetic, and able to polymerize [10].
Early studies used unfilled resin (adhesives/sealants) showing caries reduction in vitro [11–13]. Robinson et al. [11] found 60% pore occlusion.
Clinical evaluations by Gomez et al. [14] and Martignon et al. [15] showed limited success due to disintegration or inadequate sealing. Martignon et al. [15] reported a 43.5% lesion progression over 18 months.
Meyer-Lueckel and Paris [17, 18] developed a new material with better sealing and deeper penetration based on the Washburn equation [19].
The Washburn equation describes the enamel structure as a collection of open capillaries that can be penetrated by liquids driven by capillary forces.
High penetration coefficient (PC) materials penetrate deeper and faster [20].
Meyer-Lueckel and Paris [17] suggested materials with PC > 100 cm/s are better infiltrants.
High triethylene glycol dimethacrylate (TEGDMA) concentrations inhibit lesion progression due to superior penetration [17].
TEGDMA, 2-hydroxyethyl methacrylate (HEMA), and ethanol mixtures showed the highest PC due to low contact angle and viscosity.
TEDGMA and ethanol mixtures have high PC value and sufficient hardening [18], suggesting that these two components would be the benchmark for the future development of RI.
Paris et al. [18] found 99% TEGDMA resin had a PC of 204 cm/s, while adding ethanol raised it to 391 cm/s.
Ethanol increases penetration but may cause inhomogeneities and incomplete polymerization.
Low-viscosity RIs (primarily TEGDMA) have high PC and inhibit both artificial [21] and natural enamel caries [22].
High PC allows deep penetration even in low-demineralizing environments [23, 24], with 3 min application sufficient [25].

Etching

Resin infiltration depends on capillary forces influenced by pore volume and radius.
The mineralized surface layer hinders resin penetration into subsurface porosities [26].
Pre-treatment conditioning is needed to eliminate the surface layer [16].
Acid etching with 30–40% phosphoric acid creates microporosities for resin penetration and micromechanical interlocking [27, 28].
Grey and Shellis [29] improved resin penetration using 36% phosphoric acid on artificial caries.
Meyer-Lueckel et al. [30] found different penetration depths between artificial and natural caries due to varying mineral content.
Complete erosion of the surface layer is necessary for resin infiltration.
Meyer-Lueckel, Paris et al. [30, 31] showed 15% hydrochloric acid gel (90–120 s) effectively removes the surface layer.
Mechanical methods (diamond bur, polishing strip) cause smear layer occlusion [32].
Optimum surface erosion is best for low-viscosity resin penetration.

Ethanol

Ethanol reduces resin viscosity and removes residual water in the lesion body.
Meyer-Lueckel et al. [16] found ethanol enhances dental adhesive penetration.
Experimental resins with 10% and 20% ethanol increased PC and penetration depth [17, 18].
Concerns exist regarding inhomogeneities and incomplete polymerization [18].
Paris et al. [33] suggest solvent-free infiltrant is preferable due to ethanol\'s potential impact on polymerization.
Commercial RIs use ethanol pre-treatment after acid etching to remove residual water and protein.

Clinical Usage and Limitations of Current RI

Arresting Non-Cavitated Early Enamel Lesion

RI aims to delay restorative procedures on non-cavitated lesions to avoid the restorative cycle [34].
Fluoride varnish and CPP-ACP, alongside oral hygiene, manage early lesions [35, 36].
Remineralization is time-dependent, reliant on compliance [37], and doesn't fully mask white spots [38, 39].
RI infiltrates lesions, occluding pores, blocking acid diffusion, and improving mechanical strength [17, 24].
Clinical trials show RI reduces caries progression in both primary and permanent teeth [40-46].
Ekstrand et al. [40], Page et al. [41], and Ammari et al. [42] reported reduced caries progression with RI on primary molars, compared to fluoride treatment.
Urquhart et al. [43] reported that RI combined with 5% fluoride varnish was five times more likely to cause caries arrest as compared to no treatment.
Bakshandeh and Ekstrand [47] found reduced lesion progression with RI and fluoride varnish on occlusal surfaces.
Barakat and El-Soud [48] noted better retention with RI than compomer, but higher bacteria due to compomer's fluoride release.
ADA endorses RI for non-cavitated lesions in both occlusal and proximal surfaces, but high cost may limit use [49].
Meta-analyses show significant reduction in caries progression with RI [50, 51].
RI is more effective on permanent teeth due to structural differences in enamel [50]. Primary teeth have less mineralized, more porous, and thinner enamel [50].

Aesthetic Improvement of Anterior White-Spot Lesion (WSL)

RI treats enamel WSL on anterior teeth by infiltrating enamel porosities, improving aesthetics [52].
WSLs appear as chalky opacities due to differences in optical density, measured as a refractive index [52].
Porous enamel retains air (refractive index = 1.0) and water (refractive index = 1.33), causing light scattering [52].
Poor aesthetic appearance, low self-esteem, and negative psychosocial impact result from anterior WSLs [53, 54].
Post-orthodontic decalcification increases WSL prevalence threefold [55, 56].
Plaque retention elevates cariogenic bacteria, causing mineral loss and subsurface porosities [57, 58].
WSL initiation occurs as early as four weeks post-orthodontic bracket placement [59].
Fluorosis and molar-incisor hypomineralization (MIH) are common developmental anomalies with WSLs [60-68].
Reversal of decalcification and aesthetic improvement are goals in managing anterior WSL [69].
Fluoride-based treatments are first-line, with microabrasion [71, 72] and bleaching [73, 74] also used [70].
Inconsistent outcomes and side effects limit these techniques [38, 75-77]. Bleaching usage is restricted in children in Europe [76].
RI improves refractive index value of porous enamel (1.52), close to normal enamel (1.62) [78].
High patient satisfaction has been reported following the infiltration of WSL [79, 80].
Infiltration improves surface microhardness [81] and shear bond strength while reducing the surface roughness [82].
Systematic reviews show inconclusive evidence due to limited high-quality trials [51, 83-85]. Positive camouflaging effects are reported in clinical trials [86, 87].
Wang et al. [86] reported RI as favorable over fluoride varnish in orthodontic patients.
Knosel et al. [87] noted significant aesthetic improvement with RI, even after 24 months.
Boroumi et al. [85] showed higher optical improvement with RI compared to fluoride varnish, with immediate masking effects compared to six months for fluoride varnish.
Gu et al. [71] and Shan et al. [72] found RI and microabrasion effective, but RI had longer aesthetic outcomes.
Treatment of mild-to-moderate fluorosis with RI has effective masking ability [74].
Schoppmeier et al. [88] concluded that RI with increased etching and infiltration time gave significantly better aesthetic outcomes in comparison to bleaching.
Gencer and Kirzioglu [89] and Zotti et al. [79] observed immediate disappearance of fluorosis WSL following treatment with RI, with significant aesthetic satisfaction reported.
A recently conducted systematic review also favours RI over microabrasion and bleach- ing in the management of fluorosis [90].
Earlier studies found significant color changes in MIH teeth treated with RI after one week of treatment, however, 40% of the infiltrated teeth remained unchanged [93]. Repeated application of acids to encourage more surface erosion might be able to improve the infiltration efficacy in such lesions [93].
Studies showed RI efficacy in masking opacities and improving aesthetics in mild MIH-affected incisors [95, 96].
EAPD suggests RI as an option for MIH-affected incisors, alone or combined with other treatments [68].

Limited Penetration Depth

RI's penetration is limited to enamel and doesn't perform well in dentinal lesions [45].
Peters et al. [45] reported 100% infiltration of inner enamel lesions (E2), but only 64% of outer dentine lesions (D1).
Dentinal tubules enlarge upon demineralization, preventing resin penetration [97].
Schneider et al. [98] found the resin incapable of completely infiltrating advanced carious lesions [98].
In vitro studies show mean penetration depth of RI below 70% [99–101].
Better penetration in enamel lesions (ICDAS 2–3) than deeper lesions [102].
Liang et al. [103] and Soveral et al. [82] described the limited ability of the resin to infiltrate deep into the dentinal lesion.
Penetration depth is better in primary teeth due to less mineralized and thinner enamel [104, 105].
Clinicians should select non-cavitated lesions suitable for RI based on lesion depth [103]. Case selection is important for MIH-affected incisors [103].

Not Radiopaque

Dental materials should be radiopaque for clear distinction on radiographs [106].
ISO recommends radiopacity equivalent to pure aluminum thickness [107].
RI is radiolucent, causing difficulty in assessing penetration depth and caries progression [108].
Periodic radiographs are needed to monitor success, leading to potential medico-legal issues [108].
Efforts are ongoing to develop radiopaque RIs.

Long-Term Colour and Material Stability

Aesthetic demands require color-stable resin-based materials for anterior teeth [109–111].
RI has poor long-term color stability and is susceptible to staining [109].
Significant color changes occur after exposure to coffee, tea, red wine, and grape juice [110–114].
Staining is more apparent in resin-infiltrated teeth than remineralized teeth [115].
TEDGMA, while having high penetration, shows high water absorption [116].
Low filler content leads to polymerization shrinkage and microcracks [110].
Surface roughness causes bacterial adhesion, degradation, and secondary caries [112, 117, 118].
Water absorption leads to surface staining and loss of gloss [119].
Repolishing improves brightness [112, 120], with Paris et al. [78] suggesting excess resin removal and polishing after infiltration.
Bleaching can improve discoloration [116, 121], but caution is advised in pediatric patients.
Clinical studies haven't reported significant discoloration post-infiltration, but most are short-term [87].
Clinicians should warn parents about potential staining, advising regular polishing and avoidance of staining foods [46, 87, 122].

Innovation to Improve the Current RI

RI with Remineralizing and Antibacterial Properties

Incorporating antibacterial or remineralizing agents prevents bacterial colonization and enhances remineralization [123].
Bioactive glass (BAG), hydroxyapatite particles (HAP), and calcium phosphate (CaP) are successful fillers [123].
Fillers reduce polymerization shrinkage and improve hardness [123].
Filler concentration should not exceed 15% to maintain penetration ability [124].
Nano-sized fillers allow incorporation without affecting penetrative ability [125].
Hashemian et al. [125] found good penetration with an experimental RI consisting of $10%$ polyurethane acrylate oligomer (PUA) and $88%$ TEDGMA, with a value of $133 cm/s$ .
Adding HAP or BAG increased microhardness and modulus of elasticity, also water sorption and solubility of the material. The penetration depth was up to $120 \,\mu m$ .
Prodan et al. [127] observed improved DC and strength with fluoride-releasing glass filler.
The addition of CaP increased the surface hardness and the DC value of the RI, and positive calcium and phosphate ion release were observed under pH-cycling challenge for PLA-SiO2-CaP and RI-30% CaP, respectively [129,130]. The deposition of interprismatic mineral crystal was observed by Dai et al. [129], suggesting the remineralising potential of the material..
Silver nanoparticles (AgNP) in RI enhanced antibacterial effects without affecting penetration [97].
Cuppini et al. [135] used the ionic liquid 1-n-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMI.NTf2) encapsulated with methacrylate-based microcapsules as the RI fillers. The addition of 5% and 10% by weight ratio of the fillers reduced the surface free energy (SFE) and increased the contact angle of the material.
Chlorhexidine diacetate salts [132], iodonium salts [133], quaternary ammonium-based resin monomer [134] and guanidine hydrochloride [140] were also being investigated as potential antibacterial RI fillers
Skuca-Nowak et al. [131] and Fisher et al. [136] used metronidazole, an antibiotic that is highly effective against anaerobic bacteria and protozoa mixed with a methacrylate monomer (PMMAn) as the RI filler.

Self-Etching RI

Self-etching RIs reduce lengthy procedures [141].
Wang et al. [142] tested experimental self-etching RI (TEDGMA and phosphoric acid 2-hydroxyethyl methacrylate ester (PAM)) with $75%$ TEDGMA: $25%$ PAM ratio suggested as optimal for penetration.

Radiopaque RI

Radiopaque RI aids in monitoring lesion success [108].
Moeinian [108] tested strontium hydroxyapatite (SrHA) and strontium bioglass (SrBG) nanoparticles, as well as barium and tin methacrylate monomers in TEDGMA. The mixture of SrHA and SrBG with RI monomer resulted in a below-average radiopacity and increased the viscosity of the RI. The incorporation of barium and tin methacrylate produced a homogenous mixture with TEDGMA with low viscosity and contact angle values, but were not clearly visible in the digital radiograph.
Pedreira et al. [143] and Nowak-Wachol et al. [144] made efforts in formulating radiopaque RI, using an experimental RI, using either barium or zirconium oxide particles which served as a filler and incorporated into the RI monomer. The former authors reported the most promising result for $45%$ zirconium oxide and RI.

Micro-Filled RI

Micro-filled RIs contain fillers to improve mechanical properties and reduce polymerization shrinkage [145, 146].
Meyer-Lueckel and Paris [145, 146] mixed organic and glass fillers with TEDGMA, showing filler size influences penetration.
RI with $42 \,\mu m$ organic filler shows penetration percentage close to unfilled RI on artificial caries model [145].
Organic filler increased micro-filled RI bulk, enabling ICDAS 5 cavity to be filled successfully as assessed through the dual-fluorescence staining and confocal microscopy [146].

Conclusion

RI is a micro-invasive technique for non-cavitated enamel lesions.
Ongoing R&D addresses issues like radiopacity, color stability, and usage in deep lesions.
Further clinical trials are needed to validate laboratory findings. While most of the studies were in vitro based, and some did not evaluate the penetration ability of the material on enamel lesion, the potential of those materials are massive and promising.