Prevalence

Because of the significant burden of managing MIH-affected teeth, understanding the evidence base for available treatment choices is critical for the health care team. Thus, this review aimed to present a comprehensive review of MIH, focusing on the significant contribution of orthodontists in developing treatment strategies for severe MIH.

Because the term MIH was first introduced in 2001, a wide variation in MIH prevalence has been reported around the world, ranging from 2.8% in Hong Kong to 40.2% in Brazil. . Differences in the prevalence rates exist among continents, regions, and even within countries. Geographic differences on different continents have been described by subgroup analysis, with America having the highest prevalence of MIH (15.3%), followed by Oceania (14.7%), Africa (14.5%), Europe (14.4%), and Asia (10.7%). ^, Although this could reflect real disparities in MIH prevalence among different nations, many commentators put this down to variations in indexes, diagnostic criteria, and age groups; hence, underestimation or overestimation of MIH prevalence in various locations. ^,

A preview study investigating the prevalence of MIH in orthodontic patients showed that 14% of all patients examined presented with MIH on at least 1 tooth. This result was close to the average global MIH prevalence previously reported in a recent meta-analysis that included 116 studies (13.5%).

Etiology

The exact etiology of MIH remains unclear, leading to considerable debate and research. To date, much of the knowledge has been derived from cross-sectional data, which cannot establish a cause-and-effect relationship. The literature has described a wide variety of risk factors for MIH, which often involve insults at a critical phase of tooth mineralization. ^,

Enamel development of the first molars and incisors starts just before birth and is completed during the first 3 years of life. Therefore, disturbances to amelogenesis during this time may cause hypomineralized defects on these teeth. ^, . Among the various etiologic hypotheses mentioned in the literature, some systemic factors that occur during the prenatal, perinatal, and postnatal periods have been associated with MIH.

Some studies have pointed out that systemic exposures such as fever, infections, stress, and respiratory problems affecting children are associated with MIH. Besides, maternal illness, psychological stress, cesarean delivery, and delivery complications seem to play a role in MIH onset. More recently, a genetic predisposition in conjunction with 1 or several other epigenetic factors has also been proposed.

Clinical considerations

Teeth affected by MIH present with demarcated enamel opacities of different colors ranging from creamy white to yellow/brown and distinct borders to the adjacent enamel. This opacity is characterized by a reduced quality of enamel that is of normal thickness but not fully mineralized. Consequently, it is soft and porous and can undergo posteruptive breakdown (PEB) under masticatory forces soon after their eruption. Enamel lost to occlusal trauma may present with irregular outlines or sharp boundaries. The PEB of enamel encourages caries progression, leading to unprotected dentin exposure and increased caries risk. ^,

The high porosity of MIH enamel favors the penetration of bacteria into the dentinal tubules, leading to hypersensitivity. Patients may experience spontaneous pain or discomfort triggered by thermal or mechanical stimuli, increasing the risk of dental fear and anxiety. Hypersensitive teeth pose a concern not only for patients but also for dentists, especially during procedures that involve temperature changes. Therefore, special attention should be given to the use of cold air and water jets or heat from light-curing units.

A typical feature of this condition is the asymmetric appearance regarding the extension and severity of the lesions for the same patient. This means that not all molars will be affected to the same extent. In general, the defects of the incisors are milder than those of the molars and rarely exhibit PEB because of the lack of occlusal load on the opacity. ^,

Diagnostic criteria have been proposed by several different groups to help the clinician and researcher. The European Academy of Paediatric Dentistry developed diagnostic criteria for MIH that include demarcated opacities, posteruptive enamel breakdown, atypical restorations, and extraction because of MIH as features to observe ( Fig 1 ). ^, These criteria have been used by several authors, thus allowing a comparison as well as providing a better insight into understanding and interpreting the lesions and severity of MIH.

Examples of clinical presentations of molar incisor hypomineralization in children: A, White opacity; B, Brown opacity; C, PEB; D, Atypical restoration.

Bonding

Considering the structural specifications of MIH lesions, the affected enamel is characterized by a reduction in mineral quality and quantity, a reduced hardness and modulus of elasticity, increased porosity, and higher protein content. Enamel crystals appear to be less densely packed in MIH than does sound enamel, with thicker prism sheaths and interprismatic intraprismatic organic content.

All laboratory studies also highlighted the weaker bond strength of resin composite to MIH-affected enamel. Besides, phosphoric acid produces a much less pronounced etching pattern and, therefore, limits the adhesive bond of restorations. ^, This is mainly because of the presence of cracks in the porous enamel and the lack of a “uniform hybrid layer” at the enamel-adhesive interface.

Therefore, orthodontists have an important role in treating patients affected by MIH. The success of the bracket bonding procedure depends on the surface characteristics of the enamel. However, cohesive failure is often noted in brackets bonded to MIH enamel because of decreased bond strengths and subsequent enamel surface fracture when the bracket is removed. ^, Frequent rebonding of attachments slows down the progress of treatment with a fixed appliance. It can also be costly in terms of clinical time, materials, and time lost from education/work for the patient. ^,

Although incisors often exhibit milder lesions, in some cases, areas of hypomineralization in bracket bonding regions can compromise the adhesion of these appliances. Various strategies have been explored to enhance adhesion issues; however, a recent systematic review indicated that deproteinization using sodium hypochlorite applied for 60 seconds after etching, particularly for etch-and-rinse adhesives, appeared to improve bond strength. This approach can also be considered for use in cases involving the bonding of brackets to incisors with severe lesions.

Considering that bonded molar tubes show a higher failure rate and more decalcification than do molar bands, it seems reasonable to choose orthodontic bands in patients with severely MIH-affected molars. It is important to emphasize that orthodontic bands can create additional areas for dental plaque retention, making proper oral hygiene more challenging. Therefore, patients using orthodontic bands should be especially vigilant about their oral hygiene. Molar banding is a well-established procedure in orthodontics as it yields proper retention and resistance to orthodontic forces. In addition, the conservative approach foresaw the use of orthodontic bands cemented with glass ionomer cement, increasing the molars’ resistance to masticatory forces and reducing the risk of experiencing PEB.

Severely affected teeth with MIH and extensive restorations can greatly benefit from the use of orthodontic bands, which serve as temporary retainers, maintaining these restorations for a period sufficient to postpone definitive rehabilitation after orthodontic treatment. ^{, ,}

Beyond the consideration of adhesive challenges, cases involving functional appliances using molar clasps (eg, Twin-block) for retention may require modification of the appliance design or consideration of alternative treatment approaches. Careful planning is essential when indicating treatment, and this may include evaluating the condition of the affected molars.

Management

The management of MIH is considered challenging for the patients, the caregivers, and the dentist. Various therapeutic approaches are available to manage teeth affected by MIH. These include preventive measures, desensitizing agents, remineralizing products, fissure sealants, both direct and indirect restorations, and crowns. The selection of an appropriate treatment modality depends on factors like the severity of the condition, the patient’s age, socioeconomic status, and treatment expectations. Restorative procedures are commonly chosen when teeth exhibit structural loss. Clinicians can consider several materials and techniques, such as resin composite and glass ionomer cement, to address MIH-affected teeth. In addition, management of incisors with microabrasion, resin-infiltration, external bleaching, and a combination of approaches was reported, providing clinicians with a range of options to tailor their treatment strategies to individual patient needs.

However, it is crucial to recognize the critical role that adhesive procedures play in these cases. MIH-affected molars that have been previously restored often require frequent retreatment cycles. Studies have demonstrated that children with MIH undergo dental treatments for their FPMs approximately 10 times more frequently than do children without MIH. ^{, ,} Therefore, more invasive procedures, such as extractions, are adopted for teeth with major structural impairment, aiming for orthodontic treatment after dental surgery.

Orthodontic approach

In managing patients with severely affected molars because of MIH, a critical treatment decision involves choosing between restoration and extraction. A comprehensive clinical, radiologic, and orthodontic examination is essential to determine the most suitable approach. Although some orthodontists may prefer to preserve the permanent first molars because of their occlusal significance, timely extractions often become the preferred treatment option for these teeth. In patients with severely affected teeth, scheduled extractions might be an option, as the spiral of escalating reinterventions and the associated burden of dental procedures could be avoided. ^,

Therefore, the decision to extract any of the FPM may be thoroughly evaluated, and early discussions with an orthodontist are essential to anticipate favorable outcomes. Extracting affected molars at 8-10 years old has been demonstrated to result in optimal spontaneous space closure. At this stage, the crown formation of the second molar is usually complete, facilitating the mesial migration of the second molar closer to the second premolar. However, spontaneous space closure has been observed in up to 82% and 63% of maxillary and mandibular second molars, respectively. In patients in which spontaneous alignment does not occur, orthodontic therapy might be necessary. After 10 years old, if extraction is considered, there is a higher likelihood of experiencing failure in spontaneous space closure, poor angulation, and an unsatisfactory contact point relationship with the second premolar. ^,

As such, orthodontists may face crucial decisions when considering whether to extract an opposing or contralateral healthy first molar to balance or compensate for the missing severely MIH-affected tooth. When a maxillary first molar is extracted, compensatory extraction of the healthy mandibular first molar is generally unnecessary because of the rapid mesial migration of the maxillary second molar. However, for mandibular first molar extraction, orthodontists should consider a compensatory extraction of the healthy maxillary first molar, as overeruption is more likely to occur. It is essential to note that there are differing opinions on the practice of balancing extractions. ^, Although some authors recommend the extraction of the contralateral molar as a preventive measure to avoid centerline shift, other studies have reported a lack of evidence to support this claim.

The planned extraction approach depends on multiple factors, including the amount and location of crowding, type of malocclusion, and patient and parent motivation. Similarly, the health of the permanent second molar plays a crucial role when extraction of the permanent first molar is proposed. This is a challenging and debatable clinical choice because it is made before the eruption of permanent second molars, and there is uncertainty about whether these molars will also be affected by MIH. However, it has been suggested that mild lesions in the second molars tend to be more frequent when the first molar is severely affected.

In addition, orthodontists should confirm the presence of the third permanent molar once studies have suggested a possible association between MIH and dental agenesis. However, the development of the third molars occurs later than the optimal age for planning about extraction. The first appearance of a third molar bud is approximately at 9 years on average, and the mean age of complete crown formation is around 14 years. The decision-making process regarding the extraction of FPMs is indeed challenging for orthodontists. Waiting for the third molar germ to appear to confirm its presence before extracting the FPMs might result in missing the ideal age for FPM extraction. In contrast, opting for early extraction of the FPM at the ideal age carries the risk of the patient not developing a third molar. This choice should be approached with caution by the orthodontist, as it requires careful consideration of the patient’s dental development and specific needs.

In cases involving severely affected molars in adult patients, another treatment option to consider is dental extraction followed by the direct placement of dental implants. Dental implants have demonstrated excellent long-term results in restoring masticatory function and esthetics. This approach may be particularly suitable for adult patients who have exhausted alternative treatment options and have severe molar involvement.

Conclusions

The orthodontist’s involvement in the management of children with MIH is essential for making treatment decisions and ensuring the best possible outcomes. Through early evaluation, timely extractions, and effective orthodontic interventions, orthodontists can significantly contribute to the oral health and well-being of children affected by MIH.