**1. Introduction**

A molar-incisor malformation (MIM) is a recently reported dental anomaly of the permanent first molars, deciduous molars, and permanent maxillary central incisors [1]. MIM anomalies of the permanent first molars and deciduous molars may be characterized by normal crowns with a constricted cervical region and thin, narrow, and short roots, while the affected maxillary central incisors may exhibit a hypoplastic enamel notch near the cervical third of the clinical crown [2]. Although the etiology of MIMs remains uncertain, it is thought to be attributable to systemic disease associated with the neural system during infancy [3–6].

MIM was named by Lee et al. [1] in 2014. In addition, there have been papers published this dental anomaly as 'Root malformation associated with a cervical mineralized diaphragm' or 'Molar root-incisor malformation'. According to Vargo et al. [3] in 2020, a total 87 cases have been published in articles so far, and all except one case have been affected by the permanent first molars. The average diagnostic age was nine years.

**Citation:** Lee, H.; Kim, H.J.; Lee, K.; Kim, M.S.; Nam, O.H.; Choi, S. Complications of Teeth Affected by Molar-Incisor Malformation and Pathogenesis According to Microbiome Analysis. *Appl. Sci.* **2021**, *11*, 4. https://dx. doi.org/10.3390/app11010004

Received: 13 October 2020 Accepted: 18 December 2020 Published: 22 December 2020

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MIM has been mistaken for molar-incisor hypomineralization (MIH) until recently. MIM has similarities with and differences from MIH. The similarities include the involved teeth; both MIM and MIH appear most frequently in the maxillary central incisors and the permanent first molars [1,2]. There are differences, however, in the affected tissues. In MIH, the enamel is affected, whereas in MIM, dentin and cementum are affected. In incisors affected by MIM, the cervical enamel notch may appear. The exact cause of both is unknown, and treatment is difficult [3]. Since it has been the biggest problem in children's oral health recently, it is important to identify the correct treatment.

MIM teeth are associated with complications such as dentoalveolar infections, early exfoliation, space loss, spontaneous pain, impaction, and poor incisor esthetics [3]. Dentoalveolar infection is the most common complication of MIM. In severe cases, inflammation surrounds the entire roots, resulting in an abscess, fistula, and vertical mobility. A MIM with complications is usually extracted due to the poor prognosis reported in previous case studies [1,7,8]. The early loss of the first permanent molar can negatively affect occlusal stability and craniofacial development [9,10].

Various factors can affect the development of a dentoalveolar infection, and the oral microbiome is one of them. The oral microbiome is described as a group of resident oral microorganisms in the host. Oral diseases, such as dental caries and periodontal disease are caused by dysbiosis of the host and oral microbiome [11]. Next-generation sequencing (NGS) is an effective method used to conduct research on the microbiome [12]. NGS is a molecular technique that reads more sequences faster and more economically and is widely used to identify microorganisms.

In this study, we analyzed the oral microbiomes around the MIM teeth to provide evidence for the pathogenesis of the complications because the oral microbiome is associated with oral health and oral disease. The aim of this study was to report the clinical process of MIM teeth and investigate the pathogenesis by microbiome analysis.
