Long-term analysis of primary, non-surgical root canal treatments – A retrospective study

Introduction
Studies feature the evaluation of factors with the treatment aim of healing and retention in a symptom- free environment. Evaluation of success/failure is partly based on X-rays only, and partly on X-rays plus clinical situation. According to Schmalz1, X-rays are not an absolutely reliable parameter. Furthermore, studies[2,3] show that the bias of the reviewer influences the interpretation of the X-ray. Finally, various improbabilities affect evaluation.

Ng et al.4 evaluated 76 studies from 1922–2002 in a systematic review. Various studies showed an average success of 74.7 % based on strict criteria, and a success rate of 85.2 % based on loose criteria. When using strict criteria, the success rate increased with duration of follow-up period—from 67.3 % after 24 months to 85.4 % after 48 months. In a follow-up publication based on 63 studies[5], the authors analysed parameters regarding their influence on the success of the root canal treatment (RCT). Using strict criteria, they determined four main factors: apical periodontitis (AP), quality of root canal filling (RF) homogeneous versus nonhomogeneous, length of RF, and quality of restoration. They ascribed a success of 82.5 % to vital teeth, and a success of 73.1 % to non-vital teeth. The lowest success rates could be observed in mandible molars, patient age > 50. In a systematic review of 14 survival studies Ng et al.[6] showed that survival rates concerning extractions showed better results than success rates. The study of Lee et al.[7] features a clear comparison of the evaluated comparison parameters success/survival.

Materials and methods
The author, who started as a dental practitioner in 1969, used the 9,988 non-surgical endodontic treatment cases registered in his patient files from 1985 to 1999. Of these, X-rays were no longer available in 518 cases. Five teeth were extracted immediately upon noticing of via falsa. Eight hundred and twenty-one cases had not returned to the practice after RCT, however they were included in the dropout rate without further analysis. Thus, 8,644 cases of vital and non-vital teeth at 3,951 patients remained for analysis. The non-vital cases had been applied to a former study8 with shorter follow-up-time, exclusion of molars and X-ray-rating of three reviewers. The observation period ended in September 2006. A recall of the patients did not take place. Considering extractions and remaining cases after 5, 10, 15 and 20 years, drop-out rates amounted to 40.6, 58.5, 72.2 and 81.7 %.

RCTs were carried out according to the N2-method of Sargenti.[9] This method includes the use of the paraformaldehyde-containing zinc oxide root canal cement N2 and canal preparation without canal rinsing. Canal preparation was solely done manually by reamer: using step-back technique in vital teeth, crown-down technique in non-vital teeth. The creamy N2 composition was applied to the root canal by lentulo instruments. A gutta percha point was added in wide canals only. The RF-sitting were always finished by a definite filling as well as a provisional crown, if needed. The adequate length of the RF was considered to end in the area of the radiological apex up to –2 mm, an RF level of < –2 was determined as underfilling, an RF level of > 0 as overfilling. RF-quality was judged according to its homogeneity. X-rays were taken and diagnostics were performed by the author himself. Follow-up X-rays were only evaluated one year or more after RCT. According to the European Society of Endodontology (ESE) quality guidelines[10], the ‘follow-up’ diagnoses were classified into three criteria:

• Diagnosis 1 (success): Regular periodontal gap in X-ray, no symptoms, no fistula, no swelling;
• Diagnosis 2 (uncertain result): AP after < 4 years, incomplete healing, not to be evaluated, scar tissue, remaining shadow around overfilled root-canal;
• Diagnosis 3 (failure): AP still existing > 4 years, newly developed AP, clinical symptoms.

In multi-canal teeth, the root canal with the poorest prognosis was selected as being relevant for the analysis. The present study contains an analysis of success/failure and survival of root-filled teeth with reference to re-intervention: extraction, retreatment (RTR), root-end-resection (RER), trephination (TR), hemisection (HEM). Failures were divided into:

• Clinical failure: no follow-up X-ray accompanying acute exacerbation, pain, swelling, fistula;
• Failure with follow-up X-ray.

The statistic evaluation was executed with program R: R Core Team.[11] The survival rates were estimated by nonparametric estimation according to Kaplan & Meier.[12] Group comparison was done by Log Rank test. A p-value of < 0.05 was judged as being significant statistically. Survival publications depending on 14 influence factors (Table 1) in reference to failure and extraction were determined with the help of the multiple Cox regression (Table 2). This include the findings which were relevant in vital and non-vital cases.

Results
The observation period of the 8,644 analysed teeth lasted up to 25 years after the RCT (median 6 years). 8,098 (93.7 %) were attributed to vital extirpations (VitE) and 546 (6.3 %) to conservative initial treatment of non-vital teeth. During the observation pe-riod, an average of 5,295 cases attained a minimum of one follow-up X-ray after 7.14 years: 4,886 (60.3 %) in vital teeth, 409 (74.9 %) in non-vital teeth. The follow-up X-ray diagnoses are shown in Table 3.

X-ray diagnosis < 4 years after RCT (n = 1,482) revealed a radiographical failure of 9.9 %. This radiographical failure rate decreased to 8.3 % when diagnosis was made > 4 years after the RCT.

Out of the entity of 713 failures (8.2 % of all RCTs), 514 (5.9 % of all RCTs) were incorporated with X-ray. In doing so, the failure of 465 cases (90.5 %) was proved radiographically; 49 failures (9.5 %) were not substantiated in the X-ray. Another 199 cases (2.3 % of all RCTs) in form of acute exacerbations had to be judged as clinical failures without follow- up X-ray: 139 (1.72 %) in vital, 60 (10.98 %) in non-vital teeth. Fifty-five failures remained without therapy. In 13 cases, therapy was limited to a noncontact grinding. Four hundred and twelve failures (57.8 %) were extracted, failures increased extraction rate to the 2.8-fold.

Fourteen factors had been evaluated (Table 1) regarding extraction and failure rate. Five of these factors (operator, vitality status prior to RCT, preoperative AP, preoperative symptoms, number of appointments) did not have a significant influence to the extractions statistically, whereas three factors (sex, insurance condition, preoperative symptoms) did not have a significant influence to failure.

Of the RCTs, 58.7 % were carried out by the author himself, and 41.3 % by his assistant doctors. Cox regression showed a higher failure risk for the assistant cases versus the author: however, there was no higher extraction risk.

The clientele consisted of 51.4 % male and 48.6 % female patients. Regarding failures and extractions, a significant statistic difference between the sexes could not have been observed in the multivariate analysis (p = 0.417).

With regards to insurance, 59 % of the patients were insured by the RVO health insurances (legal health insurance for workers), 26.6 % by insurances for employees, 12.6 % had private health insurance, and 1.7 % were insured elsewhere. The failure analysis revealed no difference between the individual insurances (p = 0.629). Cox regression showed a lesser (approx. 24 %) extraction risk in employees and privately insured patients.

The average age of the patients amounted to 36.7 years (6–84). Regarding age, the highest failure rate was observed in patients < 30 years, however, they had the lowest extraction rate. According to Cox regression, the middle age group of 30–50 years had nearly 2-fold and the age group of over 50 years a 3.1-fold higher extraction risk versus the patients under the age of 30.

Analysis revealed significant differences (< 0.0001) regarding failures connected with the kind of tooth: molars 18.2 %, premolars 8.8 %, front teeth 8.1 %. Extraction rate of upper incisors was the lowest, extraction rate of the lower incisors the highest. Details can be seen in Table 4. According to univariate analysis, age and kind of tooth were not relevant regarding failure. Considering all analysed influence factors, premolars had a lower extraction risk of 81.5 % versus front teeth, molars a lower extraction risk of 73.1 %.

Aside from 19 teeth (0.23 %), all vital teeth were treated in one appointment. The multi-appointment of non-vital teeth in 257 cases (47 %) led to more failures (p = 0.0496).

Significant failure rate differences could be observed regarding pulp vitality prior to RCT (p < 0.001). Cox regression showed the significantly lower failure risk for vital teeth, which amounted to 30.9 % of non-vital teeth.

Two hundred and fifty-six (46.9 %) of the non-vital teeth featured an apical lesion when starting RCT. These were diagnosed with a failure rate of 34.9 % 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 underfilled teeth. Extraction rates of adequately and underfilled teeth featured nearly the same extraction frequency, whereas overfilled teeth showed an extraction risk of 18 % less. 25 % of all front teeth and premolar RFs as well as 52.6 % of all molar RFs showed a poor RF-quality which ended in a twofold failure risk versus a good RF quality and thus a 16 % higher risk of extraction (Fig. 2).

Root-filled teeth provided with one filling only had 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 the reason for a 57 % lower failure rate. Crowning reduced the extraction risk to 31 % of teeth provided with a filling.

Three hundred and eighty-two (4.4 %) of fractured canal instruments were registered—1.83 % in front teeth and premolars, 10.29 % in molars. A failure was diagnosed in 72 cases (18.8 %) resulting in a 2.2-fold higher failure risk for teeth with fractured canal instrument. During observation period, 108 (23.8 %) teeth were extracted with a fractured canal instrument. The statistic relevance determined by the log-rank test was confirmed in theCox regression by finding a 32 % higher extraction risk (Fig. 3).

The process-related accident of a via falsa with perforation increased failure risk (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 relation to failures. Figure 4 shows the chance of survival of cases which were not subject to extraction, 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 affected 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 follow- up X-ray, 159 of the 1,883 (8.4 %) extractions and 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, whereas Torabinejad et al.14 limited these advantages by calling survival studies as being ‘less biased’ as well as less informative. They further noted that endodontic studies come up with another variability making comparability more complicated.

The cases of the present study were treated in the authors office and evaluated by the author himself. Whereas extraction and thus the relevant survival represent a hard, non-discussible fact, success/failure evaluations are subject to bias. According to literature6, 7 the survival rate regarding tooth extractions is higher than the failure rate. Analysis of the practice data showed the opposite: after 20 years, 51.5 % of the teeth were still in situ and 82 % were still saved from failure.

Comparison with some studies can partly not be made, as survival data refer to the initial case numbers and are not in reference to the remaining cases getting less over the years. So the survival dates referring to extractions gained from the insurance registers have to be seen critically. Lazarski et al.[15] indicated that 94.4 % remained in the mouth functionally after an average of 3.5 years. Salehrabi and Rotstein16 calculated a remaining in situ of 97.1 % after 8 years and Chen et al.17 a remaining in situ of 93.3 % after 5 years. Drop-outs are not mentioned in these studies. The fact is that only data known to the insurance are evaluated in those studies, which is rather unlikely in non-treated pathology, non-treated radiographical failure or goodwill treatments. In 914 cases, Stoll et al.[18] noticed 105 (11.5 %) losses (extractions, RER, RTR) after 106 months, which can be interpreted as a cumulative survival rate of 74 %. De Chevigny et al.[19] report about 70–73 % of drop-outs. They judged the remaining cases as being in function (95 %) and as healed radiographically (86 %).

The present study includes a drop-out rate of 40.6 % after 5 years, of 58.5 % after 10 years. As a recall never took place it can be supposed that the drop-out rates would have been more favorable in case of recall management.

The long observation period of up to 25 years may have supported the low failure rate. The later lower failure rate can be led back to the fact that the radiographical failure diagnosis averaged from 9.9 % to 8.3 % after the fourth year. It has to be mentioned that 9.5 % of the failures accompanied by X-ray could not have been verified radiographically. This may be due to the fact that the apical osteitis has not yet rounded the corticalis with sufficient mineral loss what is, according to Bender20, the pre condition for a radiographical AP presentation. Also anatomic features may superimpose an AP. The declining development of apical lesions over time conforms to literature data indicating a better RCT success result[4,21] although Eckerbom et al.[22] noted an increase of apical lesions from 17.3 % to 21.4 % within 20 years. The own, relatively high success rate might be based on a bias with a too positive evaluation of the cases or an over-representation of the VitEs. It might also be due to the used RCT method as observed in epidemiological studies23 in countries with extensive application of formaldehyde-containing materials despite poor technical RF-quality.

Socially advantaged patients were represented with approx. 40 % in the examined patient population. They carried a 24 % lower extraction risk than social weak patients. A different failure risk did not exist between the individual social statuses of the patients. According to Hujoel et al.[24], decision for extraction is determined by sociodemographic factors. A low income status and a low level of education favor the decision for extraction. The authors Jafarian and Etebarian[25] concluded from an analysis of 2,620 extracted teeth that the level of education is of significant importance for tooth preservation. In a company health insurance study[26] representing the working class, 8.3 % of the endodontic cases were extracted within 2 years. After 5 years, 28 % of reinterventions in the form of extractions and RTRs became necessary. In my own practice, the cumulative survival quote of all reinterventions increased to 16 % after 5 years.

According to Lee et al.[7], the median survival limit regarding extraction had been reached after 21 years, thus approximately corresponding to my own. The median failure limit of Lee et al. was after 119 months (about 10 years). It has to be added that the authors scheduled their study 2 weeks after RCT only, so very early extractions and failures might have been skipped from the study.

Torabinejad et al.[14] pointed out that in their study they categorically assigned pain after RCT to the failures. The same principle is valid for the present study. All acute exacerbations (clinical failures) were treated. 10.5 % of the failures accompanied by X-ray did not undergo therapy. The postoperative pain sensations amounted to 1.7 % after vital, after completion of a non-vital treatment to 11 %. A little more than a third could be contributed to overfilling: 30.9 % after overfilling of vital and 41.7 % after overfilling of non-vital teeth. The Gesi et al.27 study on vitally extracted teeth mentions a pain rate of 13.3 % within the first week after VitE. 30.7 % of the pain arose in overfilled teeth. In the own study, 69 overfillings (34.2 %) were responsible for 199 postoperative pain sensations.

Neither the sex, nor the preoperative pathology or the number of appointments were decisive for extractions or failure. However, the failure risk of vital teeth only amounted to 30.9 % of non-vital teeth. The multivariate analysis emphasised the results of the univariate analysis regarding the extraction risk subject to age and kind of tooth. The two older age classes attained higher extraction rates compared to the age class of <30. With increasing age, tooth loss rises even in non-root canal treated teeth by carious destruction and periodontal diseases. According to
Eriksen et al.[23], epidemiologic studies reflect the endodontic performance of general dentists with success rates of 60–75 %. In his own practice, the author achieved a failure rate of 10.3 % in vital and 28.7 % in non-vital teeth.

It is also to be expected that different levels of experience are responsible for different success rates. The cases treated by assistant doctors had a statistically significantly higher failure rate compared to the practice senior while the extraction rates of assistants and practice senior were on the same level. Cox regression proved that RF-degree and RF-quality do hardly play a role for the extraction risk the more however for the failure risk. An RF with ‘voids’ and incomplete canal wall adhesion allows a bacterial augmentation/ invasion, an overfilling at least a periapical irritation caused by canal content flowing over the apex and root canal filling material.

According to Ng et al.[6], the most significant criteria for survival were: crown, two proximal contacts, no use as prosthetic post and non-molar. In their studies, crowned teeth with or without build-up pin showed the lowest extraction risk versus all other researched variables, crowned teeth without build up pin simultaneously featured the lowest failure risk. Crowns offer a high fracture protection and are an expensive investment, where the patient does not like to separate from. It has to be considered though that crowning of teeth is preceded by an estimation of the survival prognosis[6] and the better case material will be considered first when it comes to crowning.

Regarding the incidents of perforation and fractured canal instrument, the author found evidence from Ng et al.[28] in the survival literature, where 76 (4.7 %) of 1,617 cases were subject to perforation and 105 cases (12.2 %) out of 858 secondary root canal treatments subject to fracture of a canal instrument. The Cox regression proves a 3.7-fold extraction risk for perforations, for the cases with fractured canal instrument a 3.1-fold extraction risk. Marquis et al.[29] added 11 fractured canal instruments and 18 perforations in 369 endodontic cases to the intraoperative complications which affect the success result. Their own study proved a 2.5-fold extraction and a 8.4-fold failure risk for via falsa perforations. In case of fractured canal instruments the risk of extraction increased 1.3 times and the risk of failure to 2.2 times.

Conclusions
More than 8,000 endodontic primary cases had been observed for up to 25 years. Quality and length of RF, the type of restoration, the fracture of root canal instruments and the incident of perforation were responsible for the failure and extraction risk. The position of the tooth, age and social status had an influence on extraction frequency, the pulp state and the operator on the failure frequency. A symptom-free tooth remaining in situ of 73.2 % after 10 years and 51.5 % after 20 years speaks for a successful endodontic treatment therapy under practice conditions.

Acknowledgment: The author would like to thank Dr rer. nat. Monika Kriner for performing the statistics.

Editorial note: A list of references is available from the publisher.
This article was published in roots - international magazine of endodontology No. 3, 2016.