With a prior pars plana vitrectomy, the anterior hyaloid face is gone and therefore there is less support for the cataract during phaco surgery. This can result in a trampoline effect where the nucleus can bounce up and down while performing phaco within the capsular bag. This places additional stress on the zonular support and can lead to complications.
My preferred method in these cases is to bring the nucleus partially out of the capsular bag and then chop it at the iris plane. This flip and chop technique is gentler to the zonular structures and avoids the reverse pupillary block that is often seen in these cases.
Click the video below to learn why I like flip and chop in post-PPV eyes: