Background: Mandibular 3rd molars (M3) are frequently get impacted and resulted in various sequelae on mandibular 2nd molars (M2), including; caries, periodontal bone loss (PBL) and external root resorption (ERR). The aim of this study was to assess the incidence of different patterns of impacted M3 and their association with the severity of various complications affecting the adjacent M2 using 3 dimensional CBCT. Methods: A total of 246 impacted M3 were retrospectively selected and categorized based on angulation, occlusal plane level with M2, and their relation to anterior border of ascending ramus by 2 radiologists using CBCT images. M2 were evaluated for the presence of distal caries, distal PBL and ERR. Chi-square test and one-way ANOVA test were used for data analysis. Results: The highest occurrence of different impaction patterns was mesioangular 48%, level A 45.9%, class I 59.3%. PBL was the most common complication detected in adjacent M2 at 65%, followed by ERR 38.2%, and caries 30.9%. There was a significant association between caries severity and impaction angulation (p=0.002). PBL and ERR severities and all impaction patterns were significantly associated (p <0.001). Conclusion: There was an association between different patterns of impacted M3 and the severity of various complications in the adjacent M2. Clinical relevance: Observing and assessing the level, depth, and position of lower impacted M3 is crucial in anticipating potential future complications that might affect the adjacent M2