Methods: In this Cross sectional study, 106 posterior permanent teeth which were needed Class II treatment and approximal surfaces of adjacent teeth were sound and free of caries or fillings were selected. Thirty students were studied in 2 groups. After final preparation, approximal surfaces of adjacent teeth were completely dried by air and evaluated by dental chair’s light. In doubtful cases, surfaces were evaluated with ×3 magnifier. Damages were classified in 2 groups; abrasion and groove. The statistical analysis was performed by Fishers Exact, Pearson Chi-Square and linear regression model.
Results: The rate of adjacent surfaces damages were 57.5%. Thirty-one percent of damages were abrasion and 26.4% were groove. Students of grade 3 on operative course 57.5% & students of grade 4 57.5%, girls 56.6% & boys 58.4% caused damage to adjacent teeth. Students who used matrix band and vedge in proximal area as preventive instrument 53% and other students 57.3% caused damages. The rate of damage in upper jaw was 58.2% and in lower jaw was 56.4 %. Damage in Mesio-occlusal cavities was 53.4%, in Disto-occlusal 58.6% and in Mesio-disto-occlusal 61.1%. Students of grade 3 on operative course used matrix band and vedge in proximal area as preventive instrument more than students of grade 4 (24.2% vs. 9.6%, P<0.05). Also, girls more than boys used preventive instrument (24.5% vs. 3.8%, P<0.05).
Conclusions: As result of this study, we have to more focus on provide appropriate condition during cavity preparation, like high-quality isolation, enough light, easy access and use of preventive tools like matrix band to reduce usage of rotary instruments and quantity of damages.