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| study non-surgical treatment 32 roots 3 2016 after 10 years. The difference with/without AP was significant (p = 0.0028 %). Also RF-degree (Fig. 1), RF-quality and restoration had a significant influence on the failure rates. The highest failure frequency could have been observed after overfilling. Adequately filled teeth had a risk of failure of 63.6 % versus underfilled teeth; overfilled teeth had a 1.8-fold higher risk in comparison to un- derfilledteeth.Extractionratesofadequatelyandun- derfilled teeth featured nearly the same extraction frequency, whereas overfilled teeth showed an ex- traction risk of 18 % less. 25 % of all front teeth and premolarRFsaswellas52.6 %ofallmolarRFsshowed apoorRF-qualitywhichendedinatwofoldfailurerisk versus a good RF quality and thus a 16 % higher risk of extraction (Fig. 2). Root-filledteethprovidedwithonefillingonlyhad a higher tendency to failures and extractions. Cox regression proved a failure rate of 28 % less after crowning with build-up pins versus filling therapy. Without build-up pin insertion, crown provision was thereasonfora57 %lowerfailurerate.Crowningre- duced the extraction risk to 31 % of teeth provided with a filling. Three hundred and eighty-two (4.4 %) of frac- turedcanalinstrumentswereregistered—1.83 %in frontteethandpremolars,10.29 %inmolars.Afail- ure was diagnosed in 72 cases (18.8 %) resulting in a 2.2-fold higher failure risk for teeth with frac- turedcanalinstrument.Duringobservationperiod, 108 (23.8 %) teeth were extracted with a fractured canal instrument. The statistic relevance deter- mined by the log-rank test was confirmed in the Cox regression by finding a 32 % higher extraction risk (Fig. 3). The process-related accident of a via falsa with perforationincreasedfailurerisk(75.2 %failure-rate after 8 years) to the 8.4-fold, extraction risk to the 2.5-fold. Extractions represented the main contingent of re-interventions with another 299 treatments (RER, RTR, TR, HEM). Nearly 30 % of re-interventions were needed due to failure. The relation of re-intervention and failure can be learned from Table 5. Table 6 gives a survey about reasons for extraction and their rela- tion to failures. Figure 4 shows the chance of survival ofcaseswhichwerenotsubjecttoextraction,further re-interventions or failure (clinically, radiologically) in a survival curve according to Kaplan Meier.12 After 20 years, 82.3 % (CI 80.5–84.2) of RCTs were not af- fected by radiological and/or clinical failure with n = 381 remaining under risk. Within the first year after RCT the incidents were: 183 of the 199 (92 %) acute exacerbations, 22 of the 514 (4.3 %) failures with fol- low-upX-ray,159ofthe1,883(8.4 %)extractionsand 140 of the 299 (46.8 %) further re-interventions. Discussion ‘Presence or absence of the tooth is not subject to interpretation as would be subjective measurement of radiographic change, clinical signs and symptoms, patient history, etc’. With this statement, Alley et al.13 pointed out the advantages of survival studies, whereasTorabinejadetal.14 limitedtheseadvantages bycallingsurvivalstudiesasbeing‘lessbiased’aswell aslessinformative.Theyfurthernotedthatendodon- n % n % Extraction 1883 21.78 412 21.87 RER 82 0.95 79 96.34 RTR/RER 47 0.54 43 91.48 RTR/TR 12 0.14 11 91.66 RTR 90 1.04 35 38.88 TR 58 0.67 58 100.00 HEM 10 0.11 7 70.00 2182 25.24 645 29.56 RER = root-end resection RTR = retreatment TR = trephination HEM = hemisection Re-intervention Failure Table 5: Re-intervention of all RCT and failure of re-intervention. 32016 Extraction 188321.7841221.87 RER 820.957996.34 RTR/RER 470.544391.48 RTR/TR 120.141191.66 RTR 901.043538.88 TR 580.6758100.00 HEM 100.11770.00 218225.2464529.56