Molar Incisor Hypomineralization (MIH) is a worldwide widespread qualitative developmental defect of the dental enamel with a multifactorial aetiology defined in 2001 as an "hypomineralization of systemic origin affecting one or more permanent molars, usually first permanent molars (FPMs), with or without the involvement of one or more affected permanent incisors". Clinically MIH lesions appear as demarcated opacities with a creamy-white to yellow-brown colour depending on the severity of the defect that is classified as mild or severe (levels of severity) according to the European Academy of Pediatric Dentistry (EAPD) severity criteria. The distribution of the lesions is asymmetrical and their severity varies from a patient to another and also within the mouth of the same patient. Due to its porous structure with an altered prism organization and an increased content of proteins, the hypomineralized enamel has reduced mechanical properties and a lower refractive index if compared to the sound enamel. MIH is associated to a large number of objective and subjective problems as an altered aesthetics, an increased risk of plaque accumulation, caries, post-eruptive breakdown (PEB), reduced retention rates of adhesive materials, hypersensitivity and difficulty in anesthetizing the affected teeth making its management a challenging condition.

Considering MIH histopathological features, while for incisors lesions are mainly an aesthetic issue, for molars they represent a real functional problem. Being FPMs subjected to higher masticatory loads than incisors and being located backward in the mouth, they have an increased risk of plaque accumulation, caries, PEB and are more difficult to be properly brushed especially if hypersensitive. An early diagnosis based on the EAPD judgement criteria and prevention are fundamental for acting with a minimally invasive therapeutic approach gaining the patient's cooperation and avoiding the most frequent related issues. Among preventive measures, pit-and-fissure sealants are a valuable and effective treatment to prevent occlusal caries in FPMs when they are still intact. However, since their efficacy is closely related to the sealant retention, they have to be monitored over time. When the molar to be sealed is fully erupted and isolation is adequate, resin-based sealants are indicated while if the moisture control is inadequate and/or the tooth is hypersensitive and patient is not sufficiently cooperative, low-viscous glass ionomer cements (GICs) are suggested as a temporary measure until the eruption is completed and both symptoms and cooperation are improved. About the use of resinous sealants in MIH molars, to date, literature data are limited and debated. About the use of glass ionomer (GI) sealants, several studies have been performed on sound molars but only a few on MIH molars. If the GI sealants' self-adhesive features, their high fluoride-release, hydrophilic properties and simple use are well known, their bond strength to the hypomineralized enamel and the survival rates on MIH molars are still controversial. To date, the scientific knowledge regarding the use of different type of sealants in MIH affected molars is insufficient to draw exhaustive conclusions and further studies are needed to deepen the knowledge on this topic. The aim of this study is to assess, by clinical examination, the survival rate of a glass ionomer sealant in MIH affected FPMs at 12 months of follow-up.