Document Type : Review Article
Department of Pedodontics and Preventive Dentistry, D Y Patil University – School of Dentistry, Nerul, Navi Mumbai, Maharashtra, India.
The term Molar-Incisor Hypominerlization (MIH) is defined as, ‘Hypomineralization of systemic origin affecting one to four first permanent molars frequently associated with affected incisors.’ [] It was initially observed in maxillary central incisors and identified as non-endemic mottling of enamel by Jackson (1961). [] Although the termed varied from, idiopathic enamel hypomineralization (Koch et al 1987), cheese molar (Amerongen and Kreulen 1995) and Molar-Incisor Hypomineralization (Weerheijm et al 2001); the clinical appearance of these teeth did not differ. [, , ] In 2003, European Academy of Pediatric Dentistry, accepted the term Molar-Incisor Hypominerlization (MIH).
The prevalence of MIH may be underestimated due to lack of standardised tools to record MIH. Schwendicke et al., (2018) conducted a meta-analysis and reported that, the mean global prevalence of MIH is 13.1% and a global incidence of 17.5 million people in 2016. [] Research on MIH has shown that up to 71.6% of affected children may have incisors involved along with first permanent molars. []
The aetiology of MIH is multi-factorial.[,,] It develops due to interference with the calcification or maturation phase of amelogenesis resulting in qualitative defects of enamel (hypomineralization). [] This interference may occur due to complications during delivery, preterm birth, low birth weight, the use of antibiotics, corticosterioids and β2 agonists [] during the first year of life, upper respiratory infections, diseases such as asthma, pneumonia [] etc.
Histologically, the MIH opacity is more porous than sound enamel because of its lower mineral density (18% lesser compared to normal tooth structure) due to which the tooth is more prone to breakdown. [] The resulting enamel is not completely mineralised resulting in hypersensitivity.
Clinically MIH is classified according to the level of severity as mild, moderate and severe. [] Mild MIH presents with demarcated opacities in a non-stress bearing areas of first permanent molar, without caries, loss of enamel, fracture or dentinal hypersensitivity. Moderate MIH has atypical restorations, demarcated opacities on occlusal or incisal third of teeth with post-eruptive enamel breakdown limited to one or two surfaces without cuspal involvement. In Severe MIH, post eruptive enamel breakdown with crown destruction, dental sensitivity and aesthetic concerns are present.
Young permanent teeth are those recently erupted teeth in mixed dentition phase (7-12 years) in which normal physiological apical root closure has not occurred. [] These teeth have a large pulp chamber and with incompletely mineralised enamel. Therefore, commonly used aesthetic treatment modalities such as veneers or bleaching are contraindicated since it leads to an inflammatory response. []
The management of such young permanent anterior teeth is a challenge to the Pediatric dentist due to limited evidence-based options available. The most commonly encountered problems in MIH affected anterior teeth are hypersensitivity, discoloration and break down. Young patients frequently comment on aesthetic discomfort if anterior teeth are involved leading to psychosocial issues. [] The following review aims to provide an overview of the same.
Review of Existing Literature
Mild MIH presents with demarcated opacities without dentinal hypersensitivity and generally requires no treatment. (Figure 1).
Figure 1: Management of different types of MIH
Moderate MIH presents with demarcated opacities with a possibility of post-enamel breakdown. Silva et al (2011) noted that the risk of breakdown is higher in MIH affected teeth with darker opacities than lighter ones. [] The darker the opacity, it would contribute to lower mineral density and hardness value resulting in lower mechanical resistance, increased porosity and post-eruptive breakdown. This could help clinicians determine a risk-based treatment for children with MIH. The clinical management for these opacities includes resin-infiltration (Kim et al 2011) [] and micro-abrasion (Wallace and Deery 2015). []
Resin Infiltration was initially described by Davila et al in 1975. [] The concept was first developed in Germany, at the Charité University Hospital in Berlin, from in vitro studies on the penetration of resin into caries and marketed under the brand name of Icon® (DMG America Company, Englewood, NJ 2009). [] Resin infiltration masks the enamel opacity which is based on changes in light scattering within the lesions. [] The refractive index (RI) of the porous enamel, filled with water is RI=1.33, resin infiltrate shows a RI of 1.52 which is closer to the enamel hydroxyapatite that is 1.62, decreasing the appearance of opacity. The mechanism of resin infiltration is to perfuse the porous enamel with resin by capillary action with a low-viscosity resin into the inter-crystalline spaces of hypocalcified enamel. [] In this procedure, 15% hydrochloric acid is applied for 120 seconds, rinsed and dried followed by application of 95% ethanol. Resin (Icon) is applied and light curing is carried out for 3-minutes. If reapplication is required, it can be carried out followed by light curing for one-minute. If the opacity is still visible, the depth of the lesion is deeper than 40um and micro-abrasion should be employed. []
Micro-abrasion involves acid-etching with 10% hydrochloric acid in combination with an abrasive medium which is silicon carbide, available as a gel and applied using a rubber-cup on low-rotation handpiece introduced by Croll in 1986. [, , ] This technique removes the surface porosities by the combination of erosive and abrasive effect. [] Hoeppner et al., (2010) reported micro-abrasion with 35% phosphoric acid and found it to be more resistant to demineralization for up-to 4 months. [] This effect is limited to a lesion depth of no more than 100 μm deep (0.1mm) [] Each application removes 10 μm of enamel surface (maximum 10 applications) to remove the stain; however, if the stain persists after 10-applications of micro-abrasion, the lesion must be considered too deep for this technique to be employed. []
The etch-bleach-seal technique is another method to manage yellow to brownish MIH opacities which allows tissue preservation. [] It was introduced by Wright (2002).[] The tooth is cleaned with pumice, etched with 37% phosphoric acid for 60 seconds, followed by application of 5% sodium hypochlorite for 5 to 10 minutes. The bleaching process (5% sodium hypochlorite) can be repeated if little or no change is seen during the first application. The tooth is re-etched and covered with clear fissure sealant or composite bonding agent. []
In moderate and severe MIH, hypersensitivity can be present. These teeth can be sensitive to a current of air, cold and warmth (even with enamel that has not disintegrated) and mechanical stimuli like tooth-brushing. The reported prevalence of hypersensitivity in MIH affected teeth is 34.7% which was statistically significant than in non-affected molars.[] Tooth sensitivity can be reduced with the use of professional varnish application, tooth mousse containing CPP-ACP and fluoridated dentifrice.
Tooth mousse containing casein phosphopeptide-amorphous calcium phosphate (CPP-ACP) was introduced by Maki Oshiro et al in 2007. [] In MIH-affected teeth, topical application can deliver amorphous calcium phosphate (ACP) which maintains a supersaturated environment resulting in inhibition of sensitivity. [] The products commercially available are Tooth mousse or MI paste and MI Paste Plus which contains fluoride (GC Corporation, Tokyo, Japan.)  If patient presents with hypersensitivity, it is recommended that a peanut-size amount of the tooth mousse is smeared on the surface of teeth using a cotton swab or gloved finger, twice a day, after brushing. The patient is advised not to eat or rinse for 30 min. Apart from tooth mousse, CPP-ACP is also available as sugar-free chewing gum (Recaldent, Melbourne, Australia) and lozenges (Adams/Cadbury Schweppes, Morris Plains, NJ). CPP-ACP products are contraindicated in children who are allergic to milk as it contains casein. 
Toothpastes such as Pro-Argin was introduced as SensiStat in the late 1900s and since 2009 has been known as Colgate Sensitive Pro-Relief Densensitizing paste. It helps to reduce hypersensitivity. The main component of this paste is 8% arginine and calcium carbonate. [] It has been seen that hypersensitivity reduces with the use of pro-argin as it physically plugs and seals the exposed dentin tubules. [] While, Toothpastes such as Enamelon containing stannous fluoride (970 ppm F) and amorphous calcium phosphate, elmix gel (2500 ppm amine fluoride and NaF 1000ppm) induce remineralization and decrease sensitivity by occluding dentinal tubules. Further research needs to be conducted to evaluate the same. []
In severe MIH, in addition to the above-mentioned conditions, widespread post-eruptive breakdown is noted leading to the loss of enamel in association with the affected dentine. The clinical management is to provide a composite restoration which is aesthetically pleasing. Micromechanical adhesion decreases in MIH affected teeth and pre-treatment is recommended to increase the bond strength of the resin restoration. [] Lagarde et al., 2020 conducted a systematic review and recommended that self-etch adhesives should be used when bonding with MIH affected teeth, as the absence of rinsing could possibly eliminate the interference of residual water on the bond, promoting adhesion. []
Research conducted on the above-mentioned clinical techniques are discussed.
Resin infiltration is capable of penetrating MIH enamel lesion but, the, the pattern, extent and change in hardness produced are currently unpredictable. It has been reported that the average resin penetration depth in MIH lesion is 0.67 ± 0.39 mm [] as compared to sound enamel where the maximum depth of penetration was 6.06 ± 3.31 μm. []
Kim et al., (2011) compared the effect of resin-infiltration on MIH affected young permanent anterior teeth and teeth with post-orthodontic decalcification. Evaluation was done using standardized photographs at the end of one week. The author concluded that the masking effect was believed to be dependent on the lesion depth of upto 40 μm and there might be little or no masking effect in deeper lesions.  Elbaz et al., (2017) compared the effectiveness of resin infiltration and fluoride varnish in masking white spot lesions. Children between the age of 9- to 14-years with white spot lesions of central incisors were evaluated at baseline and after one month. They reported that resin infiltration showed statistically significant improvement in masking of the lesions. [] Bhandari et al., (2018) evaluated the aesthetic results of resin infiltration on MIH affected teeth. Photographic evaluation of color change was assessed at 1- and 6-month intervals in 7 – 16-year-old children. They reported that resin-infiltration showed immediate results with no statistically significant difference at 6-month evaluation. [] Nogueira et al., (2020) compared the efficacy of fluoride varnish, enamel etching with 37% phosphoric acid followed by fluoride varnish, and resin infiltration in 6 to 12 year old. They concluded that the loss of integrity was minimal and was statistically significant with resin infiltration at 18-months follow-up. []
If the depth of the opacity is deeper than 40 μm, micro-abrasion can be carried out. It produces a shiny, lustrous, glass-like surface of enamel which reflects the light differently. This optical property provides the camouflage for the opacities seen.  Bhandari et al., (2014) compared micro-abrasion and micro-abrasion followed by CPP-ACP in mild MIH affected teeth. Statistically significant results were seen when CPP-ACP was used following micro-abrasion at 6-month follow-up. [] CPP-ACP stabilizes high concentration of calcium and phosphate that bind them to dental plaque and pellicle. Under acidic conditions, they release calcium and phosphate ions, maintaining a supersaturated mineral environment, reducing demineralization and enhancing remineralization. [] Ozukoc et al (2018) reported a case where moderate-MIH (yellowish-brown) discolorations on central incisor in a 12-year-old child was treated with micro-abrasion followed by composite restorations. The micro-abrasion technique complemented with or without composite restorations will allow for significant improvement in aesthetics. []
With deeper opacities ranging from white to yellowish-brown color the etch-bleach-seal has shown promising results. Homme et al., (2017) reported three cases between 7 – 11 years of age with improved results. Etch-bleach-seal may also be used in combination with resin infiltration and micro-abrasion in yellowish opacity to obtain satisfactory results.
Tooth mousse promotes the deposition of high concentration of calcium, phosphate and fluoride ions which occlude the dentinal tubules reducing dentinal hypersensitivity. [] Pasini et al., (2018) compared tooth mousse containing CPP-ACP and fluoridated toothpaste in 7- to 13-year-old children. They reported that thermal and mechanical sensitivity reduced significantly in the group using tooth mousse with CPP-ACP. [] Cardoso et al., (2019) reported a case where MIH lesions were treated with professional application of fluoride varnish containing CPP-ACP complex (MI Varnish, GC Corporation) once a week for three weeks and the patient received fluoridated toothpaste containing CPP-ACP (MI Paste Plus) as an adjunct. Tooth sensitivity test were conducted using thermal stimulus and wong-baker scale which were evaluated at the beginning, during and at the end of treatment. The patient reported no post-operative pain or sensitivity at 6-months follow-up. []
Bekes et al., (2017) evaluated the efficacy of a single in-office treatment with toothpaste (elmex Sensitive Professional) in 6- to 14-year-old children. The applications were performed with rotary cup filled with paste on the affected tooth. This was followed by 8-weeks of brushing twice daily with the same paste. Schiff Cold Air Sensitivity Scale (SCASS) was recorded at baseline and at the end of 8-weeks. The study concluded that the application of the desensitizing paste decreased hypersensitivity significantly immediately and at 8-weeks. [] Restrepo et al., (2016) evaluated the effect of fluoride varnish (5% NaF) weekly application for 4-weeks with usual home-care and the control group with only home-care treatment in 9–12-year-old children. Both the groups were instructed to use fluoridated tooth-paste (Colgate Total; 1450ppm F) twice a day. They concluded that application of varnish did not provide any additional benefit over the use of fluoridated tooth paste. []
In Molar-Incisor Hypomineralization it is seen that, molars with brown enamel defects have 15 to 21-fold higher protein content while yellow and chalky enamel showed about 8-fold higher protein content than sound enamel. This increased protein content in MIH-affected teeth will reduce the micro-mechanical adhesion of the restoration. [] Pre-treatment of enamel with 5% sodium hypochlorite removes the enamel proteins enhancing the ability to etch the surface, thereby improving the likelihood of successful resin restoration. Chay et al (2014) conducted a study to evaluate if adhesion of composite restorations can be improved by pre-treatment and concluded that it increases bond strength. []A systematic review reported a significant increase in bond-strength and survival rate of composite when 5 % NaOCl was applied for 60 seconds after etching as it increased protein degradation. [] In determining the cavity margin placement, for composite restoration, it should be placed when clinically all defective enamel is removed and leaving the cavity margins in sound enamel. [7, 10] Lagarde et al stated in a systematic review, that when it is possible, a total removal of the defective enamel would ensure a better adhesion.  Sonmez et al (2017) evaluated deproteinzation and cavity designs in MIH-affected molars of 8- to 12-year-old children. He concluded that failure of restorations was statistically significant when hypomineralised tissue was left surrounding the cavities and pretreatment significantly increased the bond strength seen at 24-month follow-up. Thereby, total removal of the defective enamel whenever possible would lead to a better adhesion of the resin restoration. []
Depending on the severity of MIH, minimally invasive techniques such as Resin infiltration, micro-abrasion and etch-bleach-seal along with non-invasive long term treatment modalities like tooth mousse gives satisfactory results. Decrease in bond strength of restoration to MIH affected teeth should be kept in mind. Appropriate management strategies as discussed above would help in restoration longevity.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Conflict of Interest
The authors declare no conflict of interest.