trends _ re-treatment I Fig. 3 Fig. 4 this reason treatment will have a much better chance of long-term success. The difference between the two sets of outcomes is in large measure related to the different levels of preoperative risk assessment. 2) Preoperative diagnosis and assessment of risk factors One aspect of the determination of restorability is whether the tooth is vertically fractured and/or whether treatment will make vertical fracture likely. In addition, if the tooth has not had an overt iatrogenic event, the clinician should determine whether the contemplated treatment will lead to one. Near strip perforations through overzealous shaping can lead to overt strip perforations, should the removal of existing obturation material not be performed passively and with the correct methods (heat removal first, mechanical second, solvents and patency files third). The placement of highly tapered rotary nickel titanium (RNT) files into large canals at high speed is predictive of midroot strip perforation. Minimising this risk is addressed in detail below. 3) Access If at all possible, the crown should be removed. Leaving crowns in place and creating access risks leaving portals for coronal microleakage, unset restoratives, caries and fractures. It also minimises access for evacuation of the obturation material and removal of objects of all types that may be lodged in the canal system (such as posts and RNT file fragments). A compromised access will limit both the tactile and visual control of the clinician and as a result some teeth that could otherwise be re-treated successfully are compromised. It is noteworthy that the vast majority of failed root canals show evidence of overt coro- Fig. 5 Fig. 6 roots 4 _ 2009 I 25