Summary:

• Purpose: investigate the anatomic features of the pulp floor and root canal system in 3 rooted mandibular  1st molars.
• n= 122 mandibular 1st molars Inclusion criteria: native Chinese, intact roots.
• n= 25 (2 rooted), 20 (3 rooted) mandibular 1st molars.

Materials/Methods:

• To calculate the frequency, each extracted tooth was stored in 10% formalin and its age, sex, side, and root number of the specimen  were recorded.
• To study the morphology of the pulp floor. Teeth were cleaned with 5% NaOCl and ultrasonic scaler.  Then scanned at fixed resolution using micro CT. Opacity, view angle and magnification were adjusted to study the root canal system.
• Interorifice distance measured between all orifices.
• Angle formed by DBO-DLO  and MLO-DLO was measured (fig1).
• Vertucci classification was used.

Most highlighted Results:

• frequency of three-rooted teeth was 31.97%.
• 3 rooted teeth had mostly 4 canals, 2 rooted teeth had mostly 3 canals. The third root usually curved severely in proximal view.
• Grooves were often present between DBO and DLO in the three-rooted first molars.
• significantly longer interorifice distance in distal orifices than mesial orifices 2.93 as shown in Table.3.
• Mean angle formed by DBO-DLO  and MLO-DLO  was 75⁰±10 ⁰.
• In both 3 and 2 rooted 1st molars: mesial roots had type II (2-1), while distal canals had type I (1-1)
• DL root rarely has  lateral canals, most of the lateral were found in mesial root of 2 rooted mandibular molar.

Clinical significance:

Understanding the anatomy of 3 rooted mandibular 1st molar would aid in both access and instrumentation.

**(DBO:distobuccalorifice, DLO:distolingualorifice, MLO:mesiolongual orifice, MR: mesial root)

Figure.1