方法：选取40个下颌磨牙近中颊侧弯曲根管，随机分为两组，每组2 0个根管。

Method:40 curved mesial-buccal canals in mandibular molars were selected and divided into two groups randomly. Each group had 20 canals.

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②下颌管中心（M）到与M点在同一水平线的颊侧点（O）的距离；

The distance of Mandible of the Centre (M) and M points to the horizon at the same point of the buccal (O);

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