What's new in molar incisor hypomineralization?

Lesion description

Teeth affected by MIH present with demarcated enamel opacities, ranging in colour from creamy white (Figure 1) to yellow/brown (Figure 2).¹ The cervical third of affected teeth is rarely involved.⁹

Post-eruptive breakdown

Upon loading with masticatory forces, MIH-affected enamel is susceptible to breakdown, resulting in irregularly shaped post-eruptive breakdown (PEB) lesions (Figure 3).¹⁰ These lesions have in the past been incorrectly described as hypoplasia but can be distinguished by several features. Rough margins surrounding these lesions suggest shearing of the enamel, as opposed to the developmental absence of enamel seen in hypoplastic lesions. Furthermore, the presence of opacities in the adjacent enamel of the same tooth and/or opacities in other FPMs or incisors are also key points of difference between the two.⁸ Distinguishing hypoplasia from hypomineralization has important clinical implications because hypoplastic enamel is generally normally mineralized and therefore does not pose the same clinical challenges in relation to adhesion of restorative materials. Furthermore, there is a greater risk of PEB in MIH-affected teeth with yellow or brown opacities as compared with those that are associated with white opacities. This difference is reflective of the greater mineral deficit in the darker lesions.¹⁰

Atypical caries

There is mounting evidence that MIH is a risk factor for dental caries with the association being more apparent in countries with low caries rates (Figure 4).¹¹ MIH-affected teeth may be more susceptible to dental caries due to altered properties of enamel, such as increased porosity and decreased hardness. These are likely to mean that the balance between remineralization and demineralization is more easily shifted to favour demineralization leading to the development of carious lesions as the structure of enamel is less resilient. In addition, the poor mechanical properties of MIH-affected enamel may increase the likelihood of cavitation and therefore rapid progression of carious lesions. The avoidance of effective oral hygiene measures due to tooth sensitivity is likely to facilitate the progression of dental caries further.

Distinguishing a carious lesion associated with HSPM from early childhood caries or, in the case of the permanent dentition, MIH from a carious lesion due to traditional risk factors, is important to determine caries risk and prompt appropriate treatment planning and patient counselling. Patients with atypical caries due to MIH may not have the most common risk factors of frequent dietary sugar consumption or poor oral hygiene. In addition, some restorative options may be less successful in MIH-affected teeth.^12,13

MIH distribution

The number of affected teeth may range from one molar to all four first permanent molars, as well as the permanent incisors. Incisor involvement, however, is less frequent and is thought to occur in more severe cases of MIH.^3,14 There is conflicting evidence about whether maxillary or mandibular teeth are more susceptible to MIH and HSPM.^15,16,17,18

White opacities are the most common clinical presentation of both MIH and HSPM, followed by yellow/brown opacities and then post-eruptive breakdown.^15,16 Most lesions on incisors are white and PEB is rare. Increasing numbers of affected molars and involvement of the incisors are indicative of increasing severity of MIH.¹⁹ Findings from several studies indicate that, with increasing involvement of molar teeth, the chance of post-eruptive breakdown and incisor involvement increases.^7,19,20

Symptoms

Teeth with MIH have been reported to be hypersensitive, particularly to cold stimuli, but even spontaneously in severe cases.¹ In addition, clinicians frequently report that it is difficult to gain adequate anaesthesia when trying to restore some MIH-affected teeth.²¹ A greater density of nerves in the sub-odontoblastic layer coupled with increased permeability of hypomineralized enamel allowing bacterial entry, even when the surface appears macroscopically sound, may account for these symptoms.^22,23

The aesthetic problems resulting from MIH lesions on incisors are another significant challenge reported, with affected children being more likely to suffer from unpleasant comments from peers, impacting on confidence and willingness to smile.^21,24

Severity

Several factors can be used to determine severity of MIH, including lesion colour and size, presence of enamel loss, sensitivity, quality of life and treatment required. Generally, teeth with demarcated opacities with only occasional sensitivity have been considered mild.²⁵ Severe lesions are likely to have enamel breakdown, persistent sensitivity and be associated with significant aesthetic concerns. The Molar Hypomineralization Severity Index (MHSI) is a recently described, systematic method for determining the severity of MIH in a dentition, which also factors in the number of teeth as well as the sites affected.²⁶

Differential diagnoses

Other than dental caries and enamel hypoplasia, fluorosis and amelogenesis imperfecta (AI) are possible differential diagnoses. Specific to MIH are the distinct margins of the demarcated opacities which differ markedly from the diffuse opacities associated with fluorosis, for which a history of excessive fluoride exposure is also required. AI can be distinguished by its distribution, as all teeth (primary and permanent) are generally affected and there is often a familial history. In addition, all teeth in AI are similarly affected and present with a much more consistent clinical appearance, compared to MIH which often affects the different FPMs with a striking inconsistency such that, within the individual, one FPM may appear sound and yet another may have substantial PEB.⁸

Remineralization and fissure sealants

Minimally invasive dentistry has an important role to play in the management of MIH, particularly whilst teeth are erupting, where the goals of treatment are to provide relief from sensitivity, facilitate oral hygiene and increase the resistance of the enamel to breakdown.²⁵ Placement of intermediate fissure sealants using glass-ionomer cement is an effective interim dressing during, or immediately after, eruption when moisture control may be challenging.⁴⁸ In addition, the use of remineralizing agents such as fluoride varnish and CPP-ACP has been recommended to improve mineral content and reduce sensitivity.⁴⁹ Evidence for this approach has been emerging, with recent clinical and in vitro studies showing an improvement in mineral content and reduction in porosity.^50,51 The possibility of ‘maturing’ hypomineralized enamel with such agents has exciting potential for a ‘medical management’ whereby MIH-affected teeth are spared the restorative cycle of increasingly destructive treatment. Further, in cases where restorative treatment is needed, such as in teeth with PEB, it is much more likely to succeed if the enamel is of better quality. However, the success of remineralization is dependent on application as early as possible, as the effectiveness of remineralizing agents may be impaired by the highly mineralized surface layer which forms post-eruptively.⁵¹

Fully erupted FPM with mild MIH defects would benefit from resin-based fissure sealants. Unfortunately, the retention of such sealants is often dependent upon the degree of hypomineralization.^52,53 The bond strength of resin composite fissure sealants to hypomineralized FPM may be improved by pre-treating with 5% sodium hypochlorite for one minute after etching, as well as use of a bonding agent, although evidence from clinical studies of MIH-affected teeth is still lacking.^54,55

Posterior teeth

Extraction

Ultimately, timely extractions are often the preferred mode of treatment of severely affected and symptomatic FPM. Facilitating the eruption of a healthy, unaffected second permanent molar into the space of the FPM eliminates the burden of ongoing restorative treatment.¹³ Studies suggest that, if planned appropriately, a favourable occlusion may be achieved without fixed orthodontic appliance therapy.^13,62 Nevertheless, a full discussion of possible orthodontic sequelae and treatment needed is of utmost importance.⁶³

A range of considerations, including the severity of MIH, patients' aesthetic expectations and suitability for orthodontic treatment, other orthodontic concerns including crowding and facial profile, missing or supernumerary teeth and the presence of third molars all influence the final decision on whether to restore and retain or extract MIH-affected molars. It is critical that clinicians recommending extraction of FPM in the mixed dentition also warn patients and families about the risk that other, as yet, unerupted teeth (such as the second permanent molar) may also be affected by MIH-like lesions that may compromise the long-term viability of those teeth. A recent unpublished study has revealed that 13% of children in an orthodontic cohort had MIH–like lesions in both the FPMs and the second permanent molars.⁴ In addition, management of children with MIH should be based on a careful assessment of the risks and benefits of retaining MIH-affected teeth and the implications for the future.

Extraction of the FPMs between 8 and 10 years of age, when crown formation of the second permanent molar is complete and the bifurcation has commenced mineralization, may result in the eruption of the adjacent tooth into a favourable position.⁶² Extraction of the mandibular FPM before 8 years increases the risk of distal drift of the unerupted second permanent premolar, whereas extraction after 10 years is less likely to result in mesial migration of the second permanent molar (Figure 5).⁶⁴ Consideration should be given to the need for a compensating extraction of the opposing, maxillary FPM when a mandibular FPM is indicated for extraction in order to prevent super-eruption of the maxillary FPM, which could interfere with the mesial migration of the mandibular second permanent molar into a favourable position. The timing of maxillary extractions appears to be less critical, although tilting and rotation of the second permanent molar around the palatal root can occur if the extraction of the FPM occurs after eruption of the second permanent molar.⁶⁵

In patients with a Class II incisor malocclusion, the extraction of an asymptomatic but poor prognosis FPM may be delayed until after the second permanent molar has erupted. The space resulting from the extraction of the FPM can then be used to correct the Class II malocclusion.⁶⁵

Anterior teeth

Simple non-invasive treatments can improve the aesthetics of MIH-affected incisors and lead to significant improvements in affected children's satisfaction.²⁴ One such technique is the etch-bleach-seal technique, which involves repeated cycles of etching with 37% phosphoric acid and application of 5% sodium hypochlorite until the discoloration improves⁶⁶ (Figures 6 a, b). The lesion is sealed with a clear resin composite, or a resin infiltrant.⁶⁷ In some cases, bleaching may be so effective that further invasive treatment is not required. Microabrasion has also been recommended but may be less likely to succeed as the opacities tend to extend through the full thickness of enamel.⁶⁸ Even if the opacity cannot be eliminated completely through bleaching, the use of direct and indirect composite veneers may provide an effective medium term means of improving aesthetics with minimal biologic cost.⁶⁹ More aggressive options, such as ceramic veneers, should be delayed due to the risk of complications such as short clinical crown height, risk of irritation of the pulp of immature teeth and instability of gingival margins as the teeth erupt.²⁵

Advanced behaviour guidance

The high rates of dental anxiety amongst children with MIH has been attributed, in part, to difficulties in achieving adequate local anaesthesia during the treatment of MIH-affected teeth.⁴⁵ Consideration of patient comfort and perception during treatment is important because past negative experiences are a major risk factor for future dental anxiety and onset can occur in childhood.^12,70,71 In addition, lack of patient co-operation is likely to impair the ability of a clinician to provide good quality treatment, increasing the prospect of treatment failure.

Due to the difficulty obtaining effective anaesthesia, high treatment burden of MIH and young age of patients, use of appropriate behaviour management techniques is a key component of an effective management strategy. The use of nitrous oxide sedation and general anaesthesia may be indicated where restorative treatment or extractions are to be undertaken. However, in addition to the benefits associated with moisture control, the routine use of rubber dam when restoring MIH-affected teeth can also improve patient comfort by protecting other teeth from water spray.