en Oral & maxillofacial surgery Oral & maxillofacial surgery Original article Original article <strong>Background</strong><strong> and aim</strong>: Efficient bonding techniques should be employed for strengthening tooth structure. Whether to preserve intact dental tissue or to sacrifice some undermined parts can still be challenging. We aimed to evaluate the fracture strength of cusp coverage with composite versus unsupported enamel reinforced with composite in posterior restorations. <span dir="RTL"></span><br> <strong>Materials </strong><strong>and m</strong><strong>ethods: </strong>In this in-vitro study, over-impressions were made from 36 sound human maxillary premolars using bleaching shields. The teeth were randomly divided into three groups (n=12): group 1) intact teeth, group 2) teeth with MOD cavities without cusp coverage, and group 3) teeth with MOD cavities and 1.5 mm of buccal and palatal cusp coverage. Wide MOD cavities were prepared such that only 1 mm of intact enamel was left unsupported at margins. The cavities were restored using light-cure glass ionomer and P60 composite using the over-impressions to achieve the normal tooth anatomy. The teeth were stored in water at 37&deg;C for a week, and their fracture resistance was assessed using a universal testing machine. The load at fracture was recorded in Newton (N). Data were analyzed using one-way analysis of variance (ANOVA) and Tukey&rsquo;s test with a 95% confidence interval (CI). <span dir="RTL"></span><br> <strong>Results: </strong>The mean&plusmn;standard deviation (SD) of fracture load was 1834.62&plusmn;104.04 N in group 1, 750.34&plusmn;147.46 N in group 2, and 1211.30&plusmn;210.85 N in group 3. One-way ANOVA revealed significant differences between the groups (P=0.001). Likewise, Tukey&rsquo;s test showed that the difference between the groups was statistically significant (P<0.05).<span dir="RTL"></span><br> <strong>Conclusion: </strong>Using composite in restoring unsupported enamel must be combined with cusp coverage to increase fracture strength. کامپازیت رزین .استحکام شکست . انله Composite Resins, Fracture Strengths, Onlays 26 32 http://jrdms.dentaliau.ac.ir/browse.php?a_code=A-10-617-2&slc_lang=en&sid=1 SH Jalalian shahariar.jalalian@yahoo.com 100319475328460010608 100319475328460010608 Yes Assistant professor, Restorative Dept, faculty of Dentistry,Islamic Azad University of Medical Sciences, Tehran, Iran. M Emami Arjomand mansoureh.emami@gmail.com 100319475328460010609 100319475328460010609 No Restorative Dept,faculty of Dentistry A Mahavi u_worth.living@yahoo.com 100319475328460010610 100319475328460010610 No Dentist