PA in Fig.2 is updated lately.  There is a thin sinus septum apical to the 1st premolar (white arrowhead, as compared to Fig.1).  An implant may increase its primary stability if it contacts the stronger distal septum (S).

Sinus Septi

Xin Wei, DDS, PhD, MS 1st edition 02/15/2015, last revision 02/15/2015