Extraction of second permanent molars is contraindicated in the presence of impacted third molars because the third molars may not erupt properly afterward, leading to potential space loss, malocclusion, or impaction complications. Removing the second molar when a third molar is impacted can result in inadequate eruption guidance and poor alignment of the third molar. Preservation of the second molar is crucial to maintain occlusion and arch stability unless the third molar is fully erupted and can functionally replace the second molar. This surgical consideration ensures optimal dental arch integrity and avoids complex future interventions.

Reference: Contemporary Oral and Maxillofacial Surgery, 7th Edition.