en Dentistry Dentistry پژوهشي Research <span style="font-size:9pt"><span minion="" pro="" style="font-family:"><b>Introduction</b>: Understanding the reasons for retreatment of dental restorations is crucial to preventing potential tooth failure. Due to limited information on retreatment causes in our region, this study aims to compare the factors leading to the failure of posterior amalgam and composite restorations.</span></span><br> <span style="font-size:9pt"><span minion="" pro="" style="font-family:"><b>Materials & Methods</b><b><span style="font-size:11.0pt"><span new="" roman="" style="font-family:" times="">: </span></span></b>This cross-sectional, observational study was conducted on patients referred to the restorative department of Ilam Faculty of Dentistry and private clinics who required retreatment of existing posterior restorations. Failure criteria for posterior restorations were evaluated based on established reference guidelines.</span></span><br> <span style="font-size:9pt"><span minion="" pro="" style="font-family:"><b>Results</b><b><span style="font-size:11.0pt"><span new="" roman="" style="font-family:" times="">: &nbsp;</span></span></b>The study found that secondary caries had an odds ratio (OR) of 3.08 (95% CI: 2.85 &ndash; 3.29; p < 0.001), indicating a strong association with restoration failure. Additionally, restoration fractures were significantly correlated with retreatment, with an adjusted OR of 2.50 (95% CI: 2.46 &ndash; 2.86; p < 0.001).</span></span><br> <span style="font-size:9pt"><span minion="" pro="" style="font-family:"><b>Conclusion</b><b><span style="font-size:11.0pt"><span new="" roman="" style="font-family:" times="">: &nbsp;</span></span></b>Secondary caries emerged as the most common reason for retreatment, regardless of whether composite or amalgam materials were used. Restoration fractures also significantly contributed to the need for retreatment. Therefore, it is essential to consider factors such as restoration material, classification, and the number of restoration levels to reduce the likelihood of retreatment.</span></span> Dental Restoration Failure, Dental Amalgam, Composite Resins, Retreatment 28 33 http://jbrms.medilam.ac.ir/browse.php?a_code=A-10-233-7&slc_lang=en&sid=1 Fahimeh Feili feili-f@medilam.ac.ir 10031947532846006589 10031947532846006589 No Department of Restorative Dentistry, School of Dentistry, Ilam University of Medical sciences, Ilam, Iran-Oral and Dental Health Research Center, Factulty of Dentistry, Ilam University of Medical sciences, Ilam, Iran Athar Athar Nazifi-a@medilam.ac.ir 10031947532846006590 10031947532846006590 No School of Dentistry, Ilam University of Medical sciences, Ilam, Iran Elham Shafiei eshafiei1524@gmail.com 10031947532846006591 10031947532846006591 Yes Psychosocial Injuries Research Center, Ilam University of medical Sciences, Ilam, Iran Hosein sidkhani seidkhani-h@medilam.ac.ir 10031947532846006592 0000000263151098 No Department of Biostatistics, School of Health, Ilam University of Medical sciences