Figure 1.

D1 = The distance from the superior border of the mandibular canal (MC) to the superior border of the alveolar process (upper distance).
D2 = The distance from the inferior border of the MC to the inferior mandibular border (lower distance).
D3 = The distance from the buccal border of the MC to the outer buccal margin of the mandible (buccal distance).
D4 = The distance from the lingual border of the MC and the lingual margin of the mandible (lingual distance).