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Endodontics Supporting Your Practice

What are the outcomes of the Toronto Study: Phase 3-4: Orthograde Retreatment?


Dentures.This summary is based on the article published in the Journal of Endodontics: Treatment Outcome in Endodontics: The Toronto Study—Phases 3 and 4: Orthograde Retreatment

Cristian de Chevigny, DMD, MSc, Thuan T. Dao, DMD, MSc, PhD, Bettina R. Basrani, DDS, PhD, Vincent Marquis, DMD, MSc, Mahsa Farzaneh, DDS, MSc, Sarah Abitbol, DDS, MSc, and Shimon Friedman, DMD

This study is a reference to the Oasis Case Conference with Dr. Mary Dabuleanu 


  • The Toronto Study Project was established in 1993, with the intention to augment the evidence supporting endodontic treatment by prospectively investigating the 4- to 6-year outcome of treatment provided by endodontic residents.
  • The modular design included recall of treated subjects in 2-year phases and pooling of successive samples to improve the power of statistical analysis and the resulting ability to identify significant predictors of outcome. For example, in the successive reports on initial treatment in Phases 1 (1993–1995), 2 (1996–1997), and 3 (1998–1999) 1-3, the number of significant outcome predictors identified by multivariate analysis increased with each added phase.
  • In the previous study on orthograde retreatment in Phases 1 and 2 4, three significant outcome predictors were identified:
    • Preoperative perforation,
    • Adequate quality of the previous root filling, and
    • Lack of a definitive restoration; all had a negative influence on healing.

Purpose of the Study

  • The pattern demonstrated in the initial treatment reports suggested that addition of the next phases of the Toronto Study might also identify additional outcome predictors for orthograde retreatment.
  • The purpose of this study was 2-fold:
    • To systematically assess the 4- to 6-year outcome of orthograde retreatment in Phases 3 and 4 of the Toronto Study
    • To examine outcome predictors for orthograde retreatment in the pooled samples of Phases 1–4. 

Main Findings

  • The significant outcome predictors identified by the multivariate analysis included preoperative root filling quality and perforation, both previously identified in Phases 1–2 4 and preoperative AP.
  • Previous root filling quality, a combination of length and density, was the most important outcome predictor. The associated difference in healed rate was entirely attributed to teeth with AP, where a 36% difference was recorded.
  • The intracanal microbial flora sustaining post-treatment AP in teeth with inadequate root filling might resemble that of primary AP and be susceptible to orthograde treatment5. In contrast, the flora in well-filled canals might be more resistant to orthograde treatment 5.
  • In teeth with apparently adequate root fillings, disease might occasionally be sustained by extraradicular biofilms6, 7, apical cysts8, foreign-body reactions 9, or undiagnosed root cracks. All of these would be refractory to orthograde treatment.
  • The healed rate in teeth with a preoperative perforation was 31% lower than in teeth without perforation.
  • When teeth were retreated where the previous root fillings appeared too short, 74% of the canals were renegotiated to an acceptable length, same as in Phases 1–2 4 and in another previous study 10. The results suggested that a short root filling should not be considered a technical contraindication to orthograde retreatment. On the contrary, the outcome was better in these teeth than in those in which the previous root fillings appeared adequate.

In summary:

  • 4–6 years after orthograde retreatment, 82% of teeth healed, whereas 94% remained asymptomatic and functional.
  • Addition of Phases 3– 4 of the Toronto Study identified an additional significant outcome predictor to the 2 identified in Phases 1–2, whereas another previously identified predictor was no longer significant.
  • The quality of the previous root filling, presence of a perforation, and AP were identified as the outcome predictors.
  • The suggested predictive role of the number of treatment sessions in teeth with preoperative AP requires confirmation from randomized controlled trials.


  1. Friedman S, Abitbol S, Lawrence HP. Treatment outcome in endodontics: the Toronto Study—Phase 1: initial treatment. J Endod 2003;29:787–93.
  2. Farzaneh M, Abitbol S, Lawrence HP, Friedman S. Treatment outcome in endodontics: the Toronto Study—Phase II: initial treatment. J Endod 2004;30:302–9.
  3. Marquis VL, Dao T, Farzaneh M, Abitbol S, Friedman S. Treatment outcome in endodontics: the Toronto Study—Phase III: initial treatment. J Endod 2006;32:299 –306.
  4. Farzaneh M, Abitbol S, Friedman S. Treatment outcome in endodontics: the Toronto study—Phases I and II: orthograde retreatment. J Endod 2004;30:627–33.
  5. Sundqvist G, Figdor D. Life as an endodontic pathogen: ecological differences between the untreated and root-filled root canals. Endodontic Topics 2003;6:3–28.
  6. Tronstad L, Sunde PT. The evolving new understanding of endodontic infections. Endodontic Topics 2003;6:55–77.
  7. Siqueira JF Jr. Periapical actinomycosis and infection with Propionibacterium propionicum. Endodontic Topics 2003;6:78 –95.
  8. Nair PNR. Non-microbial etiology: periapical cyst sustain post-treatment apical periodontitis. Endodontic Topics 2003;6:96 –113.
  9. Nair PNR. Non-microbial etiology: foreign body reaction maintaining post-treatment apial periodontitis. Endodontic Topics 2003;6:114 –34.
  10. Bergenholtz G, Lekholm U, Milthon R, Heden G, Ödesjö B, Engström B. Retreatment of endodontic fillings. Scand J Dent Res 1979;87:217–24.



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