Clinical Evaluation of BioFlx Crowns and Preformed Zirconia Crowns on Primary Second Molars

The goal of this clinical trial is to clinically evaluate the performance of BioFlx in comparison to preformed zirconia crowns in the restoration of primary second molars in pediatric patients. The main questions it aims to answer are: Is there a difference between BioFlx crowns (Group A) and one zirconia (Group B) in terms of crown retention (primary outcome)? Is there a difference between BioFlx crowns (Group A) and one zirconia (Group B) in modified gingival index, plaque index, occlusion, surface roughness, stain resistance, wear of opposing tooth, color match, anatomic form, marginal integrity and discoloration, proximal contact, and recurrent caries at the crown margins? Researchers will compare the clinical performance of Bioflx crowns and zirconia crowns to determine if they are similar. Participants: * Participants will be asked to attend two appointments. During the first visit, a BioFlx crown will be placed on the primary lower second molar, and during the second visit, a zirconia crown will be placed on the contralateral primary lower second molar. * Participants will be recalled and evaluated after 3, 6, and 12 months.