Fig.3 (preop PA): The distance between the gingival margin and the upper border (red line) of the Inferior Alveolar Canal (IAC) appears to be 20 mm. 

Fig.5 (preop CBCT (sagittal section) taken 2 years earlier): The lower (not upper) border of  IAC is 20 mm from the gingival margin.  A severe miscalculation occurs using PA image!  It may lead to a surgical complication.

Overcautious or Not

Xin Wei, DDS, PhD, MS 1st edition 12/22/2014, last revision 12/23/2014