The septum of this upper 1st molar (Fig.2 S) is wider than that of 2nd molar.  Osteotomy starts in the septum.  MB: mesiobuccal; DB: distobuccal; P: palatal (sockets).

Septum of Upper 1st Molar

Xin Wei, DDS, PhD, MS 1st edition 12/06/2014, last revision 12/06/2014