The aim of this study is to compare the clinical success of three different types of prefabricated crowns commonly used in primary molar teeth: nanohybrid composite crowns, zirconia crowns, and stainless steel crowns. Primary molar teeth play a critical role in supporting growth and development by facilitating mastication and nutrition and by maintaining space for the eruption of permanent successor teeth. Current guidelines in pediatric dentistry recommend stainless steel crowns for the restoration of primary molars with extensive carious lesions. However, increasing esthetic demands have led to the development and growing use of alternative crown systems, such as zirconia and nanohybrid composite crowns.

This study will be conducted on 50 children aged 4 to 9 years who present to the Department of Pediatric Dentistry at Izmir Katip Celebi University Faculty of Dentistry, have no systemic diseases, and have no known allergies to local anesthetic agents. Children presenting with carious lesions affecting both mandibular second primary molars will be included. Clinically, teeth with International Caries Detection and Assessment System (ICDAS II) scores of 4, 5, and 6 will be eligible. Radiographically, lesions classified as D1-RA3, D2-RB4, and D3-RC5 will be included.

The study will be designed as a randomized, controlled, parallel-group clinical trial. Participants will be randomly allocated into two groups: in one group, prefabricated zirconia crowns and stainless steel crowns will be placed, while in the other group, nanohybrid composite crowns and stainless steel crowns will be applied.

The primary outcome of the study is to evaluate and compare the clinical success of the three crown types at 3, 6, 9, and 12 months following treatment based on functional, esthetic, and biological criteria. Functional outcomes will include crown retention and material loss, marginal adaptation, antagonist tooth wear, and occlusal height of the crown. Esthetic outcomes will be assessed in terms of surface roughness, color match, and anatomical form. Biological outcomes will include periodontal health. As a secondary outcome, parental satisfaction will be evaluated to assess perceptions of treatment success and factors influencing crown preference.

The results of this study are expected to provide valuable clinical evidence to guide the selection of the most appropriate prefabricated crown type for primary molars in pediatric dentistry.

Inclusion Criteria:

• Children aged 4 to 9 years with no systemic diseases (ASA I).
• Cooperative behavior assessed as Frankl Score 3 (Positive) or 4 (Definitely Positive).
• Written informed consent obtained from the parent/legal guardian and assent from the child.
• Willingness to attend all scheduled follow-up visits.
• Presence of bilateral mandibular second primary molars (teeth 75 and 85) with multisurface carious lesions.
• Presence of an opposing antagonist tooth in occlusion.
• Carious lesions classified as ICDAS II Score 4, 5, or 6.
• Radiographic caries depth extending to D1-RA3, D2-RB4, or D3-RC5.
• Root resorption not exceeding one-third of the root length.
• No radiographic evidence of periapical or furcation radiolucency.
• Normal periodontal ligament space and intact lamina dura.
• No evidence of internal or external root resorption.

Exclusion Criteria:

• Children classified as ASA II or higher or having systemic diseases.
• Frankl Behavior Rating Scale Score 1 or 2.
• Unwillingness to attend follow-up visits or provide informed consent.
• Presence of temporomandibular joint disorders.
• Presence of parafunctional habits such as bruxism.
• Absence of one or both mandibular second primary molars.
• Absence of an opposing antagonist tooth.
• Carious lesions classified as ICDAS II Score 1, 2, or 3.
• Radiographic findings limited to enamel caries or no evidence of caries (E0-R0, E1-RA1, E2-RA2).

Individual participant data will not be shared due to patient confidentiality concerns. The study involves pediatric participants, and all data will be analyzed and reported in aggregate form to ensure privacy and compliance with ethical standards.

This study is conducted as an open-label trial. Due to the visible differences between the crown types, participants, care providers, investigators, and outcome assessors are not blinded.