M3 positional traits and eruption space measurements were recorded on CBCT derived panoramic radiographs. According to Pell &Gregory classification (Depth: PG-A, PG-B, PG-C; Ramus Relationship: PG-I, PG-II, PG-III) (Fig. 6) [16 (link), 17 (link)] and the angles of WP-based reference frame to classify M3s (A angle: [A1: < 27°, A2:27 ~ 67°, A3: > 67°]; B angle: [B1: < 14°, B2:14 ~ 24°, B3: > 24°]). WP-based reference system is the main reference system in this study, and OP-based reference system was used as an auxiliary system. M3s were classified according to the A and B angles of the former. We divided the A angle and B angle into the three classifications according to the trisection of a sample size to ensure the comparability between samples, individually. Additionally, all M3s also were grouped according to the patient's age, sex, and Angle malocclusion classification, respectively. Group 1 was for male, Group 2 was for female, Group 3 was for class I malocclusion, Group 4 was for class III malocclusion, Group 5 was for 18–27 years old and Group 6 was for 28–40 years old.

The classification criterion of Pell&Gregory. A Depth of Pell&Gregory Classification. PG-A: The highest part of the M3 was on the same level or higher than the occlusion plane of the second molar. PG-B: The highest part of the M3 is below the occlusal plane of the second molar, but higher than the neck of the second molar. PG-C: The highest position of the M3 is below the neck of the second molar. B Ramus relationship of Pell&Gregory Classification. PG-I: Sufficient space available between the anterior border of the ascending ramus and distal side of second molar to accommodate mesiodistal width of the crown of the M3. PG-II: The space available between the anterior border of the ramus and the distal side of the second molar is less than the mesiodistal width of the crown of the M3. PG-III: All or most of the M3 is embedded in the mandibular ramus

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