This study assessed the 4- to 6-yr outcome of initial endodontic treatment during Phase 1 of the "Toronto Study" project. A total of 450 teeth were treated, alternating the "Schilder" technique and step-back with lateral condensation, and examined clinically and radiographically by an independent examiner. Apical periodontitis was recorded as absent (periapical index < 3) or present (periapical index > or = 3) and outcome dichotomized as "healed" (no apical periodontitis, no signs or symptoms) or "disease." Univariate, bivariate, and multivariate analyses were performed (p = 0.05) on 120 examined teeth. The "healed" rate (81% overall) was significantly higher for teeth treated without apical periodontitis (92%) than with apical periodontitis (74%). Several other factors were associated with healing rate differential of 10% or higher, but no statistical significance. This study confirmed apical periodontitis as the main prognostic factor in initial endodontic treatment. Continuation of the project will allow assessment of other prognostic factors with better power.

The prevalence of AP in root-filled and untreated teeth was comparable to that reported in previous methodologically compatible studies. The quality of both the root filling and the restoration were found to impact on the periapical health of root-filled teeth, with the impact of the restoration being most critical when the quality of the root filling was adequate.

Conservative endodontic cavity (CEC) may improve fracture resistance of teeth but compromise instrumentation of canals. Extracted human intact maxillary incisors, mandibular premolars and molars were imaged with micro-CT and assigned to CEC or traditional endodontic cavity (TEC) groups (n=10/group/type). Canals were prepared and post-treatment micro-CT images obtained. These teeth along with the negative controls were then loaded to fracture. Mean proportion of untouched canal-wall was significantly higher only in distal canals of molars with CEC compared to TEC. Mean dentin volume removed was significantly smaller for CEC in all tooth types. Mean load-at-fracture for CEC was significantly higher in premolars and molars without differing significantly from the negative controls. While CEC was associated with compromised canal instrumentation only in the distal canals of molars, it conserved coronal dentin in all three tooth types and increased resistance to fracture in the mandibular molars and premolars.iii ACKNOWLEDGEMENTS

The 4- to 6-year outcome of orthograde retreatment was assessed for Phases I and II of the Toronto Study. In total, 523 teeth in 444 patients were retreated. With 395 teeth lost to follow-up and 25 extracted, 103 teeth (34% recall) were examined by two independent, blinded, calibrated examiners for outcome: "healed" (absence of apical periodontitis, signs, or symptoms) or "diseased" (presence of apical periodontitis, signs, or symptoms). The "healed" rate (81%) differed significantly for preoperative apical periodontitis (absent, 97%; present, 78%) and perforation (absent, 89%; present, 42%). Logistic regression revealed an increased risk of disease for preoperative perforation and adequate root filling quality, and postoperative lack of definitive restoration (odds ratios = 26.5, 6.6, and 14.0, respectively). Without perforation, inadequate intraoperative root filling length was also identified (odds ratio = 6.8). This study suggested that apical periodontitis, although a strong predictor, was secondary to preoperative perforation and root filling quality, and to postoperative restoration, in predicting the outcome of retreatment.

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