Background: Molar Incisor Hypomineralisation (MIH) is a qualitative defect of enamel that shows asymmetrical involvement of one to four permanent first molars (PFM) with or without incisor teeth involvement. MIH molars are prone to post-eruptive breakdown (PEB) and to the development of atypical caries lesions.

Aim of the study: To evaluate and compare the preventive effect of glass ionomer sealant and laser therapy (LT) against dental caries and PEB on molars affected by molar incisor hypomineralisation (MIH) Materials and Method: This study is a two-arm, parallel group, randomized clinical trial. A sample of children (n=134) with an age range of 6-9 years old, having a first permanent molar erupted with MIH without PEB or carious lesion, will be selected from the outpatient clinic of Pediatric Dentistry and Public Health Department, Faculty of Dentistry, Alexandria University after securing necessary consents. All selected 134 first permanent molars will be randomly and equally allocated into two groups. Group 1(n=67): LT and Group 2 (n=67): Glass ionomer sealant (Fuji triage). Dental caries, PEB, retention of sealant and child-self-reported discomfort will be clinically evaluated after 6, 12 and 18months. Associations between dental caries and PEB with independent variables will be evaluated using logistic regression analysis (p < 0.05).

Inclusion Criteria:

• 6-9 years old children (29)
• One tooth with MIH with no PEB according to evaluation criteria proposed by Ghanim (2017) (APPENDIX I) (30) and ICDAS scores 0, 1 and 2 (APPENDIX II) (31)
• Good overall health (ASA I) (APPENDIX III) (32)

Exclusion Criteria:

• First permanent molars with active carious lesions with ICDAS scores 5 and 6 (Appendix II) or defective restorations (31)
• Sufficient dentin loss that requires restorative therapy
• Any clinical signs of failure (abscess, fistula)