The removal of separated instruments that have not penetrated the apex commonly involves a combination of procedures. The use of Masserann trepans coupled with ultrasonic vibration can be used to extract the fragment as described in the silver points removal page.
In the event of failure to remove a separated instrument it may be possible to bypass it, although in our experience it is difficult to bypass a separated instrument which lies in the apical third because the root canal is usually more circular in cross section in this area.
See clinical sequence here.