Since #17 and 18 are close to each other (Fig.1), a small elevator is not initially inserted between them without incision.  The elevator slides distal  because of larger distal defect when force is applied (Fig.2 (occlusal view): red outline).  Incision has to be made for better vision. 

Distal Defect     Last     Next Xin Wei, DDS, PhD, MS 1st edition 03/19/2021, last revision 03/20/2021