Abstract: Objective: To evaluate the application value of progressive method for the detection and negotiation of middle mesial (MM) canals of mandibular molars in young and middle-aged patients. Methods: Forty-five mandibular first molars and twenty-five mandibular second molars from 60 patients of 20 to 45 years old were selected. The clinician detected the middle mesial canal in mandibular molars using progressive method. The number of MM canals was recorded and analyzed statistically by chi-square test. The path way of canals was characterized by periapical angulated projection. Results: The detection with naked eye revealed that the presence of MM was three (4.3%), DOM was seven (10.0%), micro-ultrasound technique was fifteen (21.4%) and cone-beam CT combined with micro-ultrasound technique was 17 (24.3%). The statistical analysis showed that there were significant differences between stage four and the first and second stage (P<0.05). There were no significant differences between stage four and the third stage. Among the 17 MM canals, seven showed ''confluent'' anatomy, two showed ''independent'' anatomy, and eight showed ''fin'' anatomy. Conclusion: The four steps progressive method will enhance the ability of dental clinician to locate MM canals in mandibular molars in young and middle-aged patients.

Key words: Progressive method, CBCT , Mandibular molar , Middle mesial canal, Microscopic ultrasound technology