to evaluate and compare the clinical performance and periodontal responses of four commercially available one-step universal adhesives with different formulations in Class II composite restorations over a three-year period.

The experimental design description adhered to the guidelines provided by the Consolidated Standards of Reporting Trials (CONSORT) statement. The present study is planned as a double-blinded randomized clinical trial, ensuring that both the patients and the examiner are unaware of the treatment allocation. The trial will follow a split-mouth design. A total of forty-eight adult patients, who are seeking dental treatment, will be enrolled in the study. The participants will be recruited from the Outpatient clinic at Faculty of Dentistry, Mansoura University.

No active advertisement will be used for participant recruitment, and instead, a convenience sample will be formed. Prior to participating in the study, each patient will be required to provide informed consent by signing a consent form. The protocol of the study was approved by Mansoura University's ethics committee before initiation

Inclusion Criteria:

• Patients must present with at least four posterior primary compound proximal caries involving two surfaces, specifically occluso-mesial or occluso-distal cavities, with a corresponding International Caries Detection and Assessment System (ICDAS) score of 4 or 5.
• The buccolingual width of the lesion should be approximately less than two-thirds of the intercuspal distance.
• Teeth to be restored must be in normal occlusion with the natural antagonist teeth.
• Vital teeth with no signs of pulpal inflammation or pathological lesions.
• Normal periodontal status and good oral hygiene.
• Good likelihood of recall availability.

Exclusion Criteria:

• Patients with proximal cavities that extend beyond two surfaces or have a lesion buccolingual width greater than two-thirds of the intercuspal distance, or requiring cusp coverage.
• Partly erupted or endodontic treated teeth.
• Poor oral hygiene or severe periodontitis.
• Absence of neighboring or antagonist teeth.
• Patients with known allergy to any component of the study materials.
• Patients with unstable medical conditions.
• Patients with known unavailability to attend recall visits.
• Pregnant or lactating females.
• Patients with heavy para-functional habits, fractured, or visibly cracked teeth.