The effect of periodontal status and coronal restoration quality on early and late term failures in root canal-treated teeth

Abstract

Background

Understanding the underlying causes of root canal treatment (RCT) failures allows clinicians to more correctly predict treatment outcomes, paving the path for better results.

Aim

This study aimed to evaluate the effect of coronal restoration and periodontal status in teeth with failed RCT.

Materials and methods

The present observational study examined 538 patients to evaluate potential risk factors associated with RCT failure in accordance with the inclusion criteria. Potential failure factors recorded included clinical signs and symptoms, smoking habits, radiological findings, the type and quality of coronal restorations, and periodontal parameters such as the gingival index (GI), periodontal index (PI), periodontal probing depth (PD), and clinical attachment loss (CAL). Univariate and multivariate logistic regressions were used to relate patient and treatment variables with the presence of apical periodontitis (AP) and the timing of RCT. Each association was evaluated by calculating the odds ratio (OR) and its 95% confidence interval (CI). Significance was evaluated at the p < 0.05 level.

Results

The effect of sex, smoking, CAL, age, RCT adequacy, coronal restoration quality, PI, GI, and PD, on RCT time was found to be statistically significant (p < 0.05). According to the multivariate analysis, being male (95% CI 1.061–2.210, OR 1.531, p = 0.023), smoking (95% CI 0.313–0.961, OR 0.548, p = 0.036), CAL (95% CI 1.0.384–0.890, OR 0.585, p = 0.012), and age (95% CI 0.955–0.978, OR 0.966, p = 0.001), were found to increase the risk of being under 5 years by 1.531, 0.548, 0.585, and 0.966 times, respectively. The effects of PI, GI, PD, CAL and age on the presence of AP were found to be statistically significant (p < 0.05). PI (OR: 2.032, 95% CI: 1.391–2.068, p = 0.001) and age (OR: 1.021, 95% CI: 1.004–1.038, p = 0.015) were found to significantly increase the risk of having AP.

Conclusion

The findings of this study highlight the critical role of periodontal health in endodontic outcomes and underscore the potential benefits of educating patients about how smoking adversely affects the prognosis of RCT, which could enhance its long-term success.

Introduction

The primary objective of non-surgical root canal treatment (RCT) is to eradicate infection within the root canal system, create optimal conditions for the regeneration of periapical tissues and guarantee the continued functionality of the tooth within the oral environment [1, 2]. Although previous studies have revealed success rates of RCT up to 86–98% [3], it can often fail when adequate standards of treatment are not met [4]. Endodontic reasons for failure may be loss of working length, insufficient chemomechanical preparation, broken files, ledges, underfilling, overfilling, and perforations. However, some of the cases that were appropriately treated with the highest technical standards may also fail [5]. Despite the development of techniques and materials as well as knowledge and awareness in RCT, the 2% annual loss of root-filled teeth [6] emphasizes the importance of various risk factors contributing to failure [7]. Criteria for evaluating success and failure also varied. Outcomes were evaluated by the functionality of the involved tooth, radiographic changes, and/or the presence of signs and symptoms [8]. The risk of clinical and radiographic failure of root canal-treated teeth depends on several covariates at the tooth, treatment, and patient levels [9]. At the patient level, systemic diseases such as diabetes mellitus have been consistently found to affect endodontic failures [10]. Similarly, smoking and the presence of multiple systemic diseases have also been reported to increase the risk of failure [11, 12]. Treatment-level covariates, such as optimal root canal filling (RCF) and proper and sufficient coronal restoration (CR), have been presented to reduce failure [13]. Tzimpolas et al. [14] noted the most common reasons for RCT failure as prosthodontic (59.4%), periodontal (32%), and endodontic causes (8.6%).

Previous literature has shown that the survival of root canal–treated teeth tends to decrease over time, and that long-term outcomes may be influenced by several factors such as the quality of coronal restoration, patient age, and systemic conditions [15]. Understanding whether the causes of failure differ depending on the time elapsed since the initial treatment may provide valuable clinical insight. Although only a limited number of studies have categorized endodontic failures based on time, a recent study by Jang et al. suggests that early failures are often linked to technical deficiencies such as missed canals or insufficient debridement, while late failures are more commonly associated with long-term issues such as coronal restoration breakdown [16]. The timing of endodontic failure is, therefore, a critical factor that may influence the interpretation of failure causes. Categorizing failures as early (< 5 years) or late (> 5 years) helps distinguish between technical errors and biological or restorative complications that develop over time. This temporal stratification allows for a more nuanced analysis of how variables such as CR quality and periodontal status affect treatment outcomes. Including this timing variable provides a framework for understanding the interaction between treatment-related and biological factors, and may guide clinicians in risk assessment and retreatment planning.

When deciding RCT repetition, it is important to identify the main cause of failure and to eliminate this etiological factor for treatment planning. Although various studies [17, 18] on this subject are available in literature, the majority of this research primarily focuses on CR. However, there is very limited scientific evidence on how periodontal status affects the success of RCT. Due to the close relationship between endodontic and periodontal tissues, diseases of these 2 tissues affect each other [19] Therefore, it is important to determine to what extent the periodontal status of the patients affects the RCT. The present study aimed to investigate how CR and periodontal status influence the failure of root canal-treated teeth (RCTT) requiring retreatment, in both the short and long term. The null hypothesis was, “The periodontal condition of patients and the quality of CRs are influential in the short- and long-term success of RCT.”

Materials & methods

This study was conducted in accordance with the ethical principles stated in the Declaration of Helsinki and was approved by the ethical board of Marmara University, Faculty of Dentistry (no: 09.2022.429).

Study design and patient selection

Patients who applied to Marmara University Faculty of Dentistry, Endodontics Clinic between January 2022 and January 2024 and had teeth that failed for various reasons and were given an indication for RCT repetition were included for this study (Fig. 1). As a result of the power analysis, the sample number was determined as n = 264 at 80% power (power: 0.80) and 5% (α: 0.05) error level.

Fig. 1.

Flowchart illustrating data collection process

Inclusion criteria

• Adults over 18 years of age, without any systemic diseases (such as diabetes, osteoporosis) that could disrupt the healing mechanism.

• Patients who underwent RCT at least 1 year ago.

• Patients with an indication for retreatment in a single tooth.

• Mature teeth that have not undergone any endodontic surgery.

• Teeth for which RCT has failed for any reason (symptomatic tooth, tooth with apical periodontitis, etc.) and retreatment indication has been given.

• Cases in which the CR was performed within 3 months after the RCT.

Exclusion criteria

• Teeth that have previously undergone surgical or nonsurgical retreatment.

• Teeth with endodontic periodontal lesions.

• Presence of a history of acute or chronic trauma and teeth with root fracture.

• Incomplete and missing record.

• Teeth with old CR have fallen out.

Patients who satisfied the inclusion criteria were categorized into two groups based on the time elapsed since the initial root canal treatment: less than 5 years (< 5 years) and more than 5 years (> 5 years).

Clinical and radiographic evaluations

Of the 2100 patients, the radiographic examination was performed by 2 experienced endodontists, while the clinical examination was performed by 1 experienced endodontist (1 of the radiographic examiners) and 1 experienced periodontologist. Panoramic and periapical radiographs were taken by a dental radiologist using a PlanmecaPromax 2D panoramic X-ray unit (Planmeca, Helsinki, Finland) and a phosphor plate digital radiography system (Dürr Dental, Bietigheim-Bissingen, Germany), using a parallel technique for standardization.

Prior to the evaluation of radiographic images, each observer participated in calibration training, which included a series of 30 images unrelated to the study sample for interobserver calibration. Cohen’s kappa value was used for interobserver consensus, and this value corresponded to a very good agreement of 0.88. Any differences in radiographic evaluation were resolved by discussion between the 2 observers until consensus was reached. Information regarding the reason for the visit, demographic characteristics, smoking, tooth number, periapical and coronal status, intraoperative findings, and the time elapsed since the first treatment was recorded (Table 1).

Table 1.

Demographic characteristics

		n	%
Sex	Male	227	42.2
	Female	311	57.8
Education level	Primary School	86	16
	Middle School	47	8.7
	High School	215	40
	Licence	183	34
	Postgraduate	7	1.3
Type of tooth	Canine	56	10.4
	Incisor	97	18
	Molar	217	40.3
	Premolar	168	31.2
Smoking	Yes	79	14.7
	No	459	85.3

RCT assessment

RCT failure (RCTF) was defined as the presence of teeth with AP requiring retreatment and/or symptoms such as spontaneous pain, tenderness to percussion, pain triggered by biting, sinus tract formation, or swelling, etc. RCF quality was classified according to the European Society of Endodontology guidelines and criteria below [20, 21]:

1. Adequate filling quality: The RCF material ended 0–2 mm away from the radiographic apex, and root canals were homogenously filled with no gaps.

2. Inadequate filling quality: The RCF ended > 2 mm from the radiographic apex or extended beyond the radiographic apex (underfilled and overfilled, respectively) and/or was inhomogeneous with gaps in the filling material or a gap between the root filling and dentin (Fig. 2).

Fig. 2.

Representative radiographs evaluating the quality of coronal restoration and root canal filling. a Inadequate quality of CR and RCF. b Inadequate quality of CR, adequate quality of RCT. c, d Adequate quality of CR, inadequate quality of RCT

Periapical status assessment

The periapical status of RCTT was assessed using the PAI scoring system [22]. The radiographic criteria and classification were made according to Strindberg as “healthy teeth (PAI 1, 2) and “diseased teeth (PAI 3, 4, 5)” [23]. In multi-rooted teeth, the periapical condition of the worst canal was considered to represent the tooth.

Periodontal status assessment

For the periodontal examination and evaluation of the teeth, the plaque index (PI) [24], gingival index (GI) [25], probing pocket depth (PD), and clinical attachment loss (CAL) [26] were recorded at 6 sites per tooth (mesiobuccal, midbuccal, distobuccal, mesiolingual, midlingual, and distolingual) using a manual probe (15 UNC Color-Coded Probe, Hu-Friedy, IL, USA) by a periodontology specialist (OB). Intraexaminer agreement was calculated by intraclass correlation coefficient (ICC) and showed an agreement of 0.92.

CR assessment

Modified and simplified Ryge’s Criteria [27]. were used to evaluate the CR as follows:

• ▪ Adequate: Anatomical restoration with no open margins, no overhangs, and no recurrent decay.

• ▪ Inadequate: Restoration with overhangs, restoration with open margin, restoration with unsatisfactory anatomic form, restoration with recurrent decay, fractured, detached, or lost restoration, tooth cusp or tooth wall fracture. 

Statistical analysis

Statistical analyses were performed using IBM SPSS Statistics 22 (IBM Corp., NY, USA). The normality of the data was assessed with the Kolmogorov-Smirnov test. Descriptive statistical methods were complemented by various tests for group comparisons: the Kruskal-Wallis test was applied to compare parameters among multiple groups, followed by Dunn’s test to identify the source of significant differences. For comparisons between two groups, the Mann-Whitney U test was utilized. Qualitative data were analyzed using the chi-square test, Fisher’s exact chi-square test, Fisher-Freeman-Halton exact chi-square test, and continuity correction (Yates) test. Relationships between parameters were examined using Spearman’s rho correlation analysis, while multivariate analyses were conducted through logistic regression. Statistical significance was set at p < 0.05.

Results

The study was conducted with a total of 538 cases, aged between 18 and 76. The mean age was 38.94 ± 15.36 years, and the median age was 40 years. Demographic information, including average age, education level, sex, smoking, and tooth type, is shown in Table 1. Of the teeth analysed, 40.3% were molars, 31.2% were premolars, 18% were incisors and 10.4% were canines. The distribution of study parameters such as time of RCT, clinical findings, RCT adequacy, radiological findings, and CR quality may be seen in Fig. 3. Among all coronal restorations, composite fillings were the most common (49.6%), followed by single crowns (16.4%), amalgam fillings (11.3%), bridges (6.7%), post-and-core crowns (5.8%), post-and-core bridges (5.2%), and onlays (5.0%) (Fig. 4).

Fig. 3.

Distribution of endodontic, periapical, and coronal restoration parameters in root canal–treated teeth according to follow-up duration (< 5 years and ≥ 5 years)

Fig. 4.

Distribution of different types of coronal restorations (%) in root canal–treated teeth

Timing of root canal treatment (RCT)

The rate of inadequate RCT was significantly higher in cases treated within the last 5 years (84%) compared to those treated 5 or more years ago (75.9%) (p = 0.022) (Table 2). Patients with inadequate RCT were 1.658 times more likely to have undergone treatment within the past 5 years. Additionally, the proportion of cases with inadequate coronal restoration (CR) quality was significantly greater among those treated within the past 5 years (67.9%) than those treated 5 or more years ago (59.7%) (p = 0.048), with a 1.438-fold increased risk.

Table 2.

Evaluations regarding the timing of endodontic treatment

		< 5 years	≥ 5 years		Univariate Logistic Regression
		n (%)	n (%)	p	OR (%95 CI)
RCF quality n (%)	Adequate	39 (%16)	71 (%24.1)	10.022*	1.658 (1.074–2.560)
	Inadequate	204 (%84)	224 (%75.9)
CR quality n (%)	Adequate	78 (%32.1)	119 (%40.3)	10.048*	1.430 (1.002–2.042)
	inadequate	165 (%67.9)	176 (%59.7)
Sex	Female	127 (%52.3)	184 (%62.4)	10.018*	1.514 (1.073–2.137)
	Male	116 (%47.7)	111 (%37.6)
Smoking	Yes	22 (%9.1)	57 (%19.3)	10.001*	0.416 (0.246–0.703)
PI Ort±SS (medyan)		1.7 ± 0.62 (1.66)	1.91 ± 0.81 (1.9)	20.008*	0.670 (0.528–0.849)
GI Ort±SS (medyan)		1.79 ± 0.51 (1.83)	1.91 ± 0.69 (1.99)	20.031*	0.723 (0.546–0.956)
PD Ort±SS (medyan)		3.19 ± 0.82 (3.09)	3.37 ± 1.06 (3.2)	20.060	0.822 (0.687–0.982)
CAL Ort±SS (medyan)		3.6 ± 1.03 (3.52)	3.89 ± 1.27 (3.8)	20.015*	0.811 (0.701–0.940)
Age		34.42 ± 14.28 (32)	42.66 ± 15.24 (45)	20.001*	0.964 (0.952–0.975)

¹ Chi-square test ²Kruskal Wallis Test *p < 0.05

Male patients were more frequently found in the < 5-year RCT group (47.4%) compared to the ≥ 5-year group (37.6%) (p = 0.018), indicating a 1.514-fold higher likelihood of earlier treatment in men. Interestingly, the smoking rate was significantly lower in patients who had RCT within the past 5 years (9.1%) versus those treated 5 or more years ago (19.3%) (p = 0.001), with smoking associated with a 0.416-fold reduced likelihood of earlier RCT. Moreover, the mean age of patients who underwent RCT within 5 years was significantly lower than those treated 5 or more years ago (p = 0.001), with the risk of delayed RCT increasing by a factor of 1.037 per year of age (Table 2).

In terms of periodontal status, patients in the < 5-year group had significantly lower plaque index (PI) levels (p = 0.008), gingival index (GI) levels (p = 0.031), and clinical attachment loss (CAL) values (p = 0.015) compared to those in the ≥ 5-year group. For patients treated 5 years or more ago, the risk of RCT failure increased by 1.492-fold with higher PI, 1.383-fold with higher GI, and for patients treated less than 5 years ago, CAL was associated with a 1.233-fold increased risk of failure. However, probing depth (PD) did not show a statistically significant difference between groups (p > 0.05) (Table 2).

Backward stepwise logistic regression identified sex, smoking, CAL, and age as significant predictors of undergoing RCT within the past 5 years (Table 3). Being male (OR = 1.531, 95% CI: 1.061–2.210, p = 0.023), a non-smoker (OR = 0.548, 95% CI: 0.313–0.961, p = 0.036), having lower CAL (OR = 0.585, 95% CI: 0.384–0.890, p = 0.012), and younger age (OR = 0.966, 95% CI: 0.955–0.978, p = 0.001) were all significantly associated with earlier RCT.

Table 3.

Evaluation of (multivariate) parameters affecting treatment time by logistic regression analysis

Step 4	OR	95% C.I. for OR		p
RCF quality (inadequate)	1.510	0.938	2.431	0.090
Sex (Male)	1.531	1.061	2.210	0.023*
Smoking (Yes)	0.548	0.313	0.961	0.036*
PD	1.624	0.971	2.717	0.065
CAL	0.585	0.384	0.890	0.012*
Age	0.966	0.955	0.978	0.001*
Constant	3.054			0.021*

Variable(s) entered on step 1: RCF quality, CR quality, Sex, Smoking, PI, GI, PD, CAL, Age

*p <0.05

Relationship between PAI and clinical parameters

No statistically significant association was found between periapical index (PAI) scores and the adequacy of root canal treatment (RCT), quality of coronal restoration (CR), patient sex, or smoking status (p > 0.05, Table 4). However, patients with apical periodontitis (AP) exhibited significantly higher levels of plaque index (PI), gingival index (GI), clinical attachment loss (CAL), and probing depth (PD) compared to those without periapical lesions (p < 0.05). Elevated PI, GI, CAL, and PD values were associated with increased risks of AP by 2.045, 1.898, 1.505, and 1.670 times, respectively. Furthermore, the mean age of patients with AP was significantly higher than those without AP (p = 0.003), with the risk of AP increasing by 1.022 times per additional year of age (Table 4).

Table 4.

Assessments regarding the PAI levels

		PAI ≤ 2	PAI > 2		Univariate Lojistik Regresyon
		n (%)	n (%)	p	OR (%95 CI)
RCF quality n (%)	Adequate	12 (%15.6)	98 (%21.3)	10.322	0.684 (0.355–1.316)
	inadequate	65 (%84.4)	363 (%78.7)
CR quality n (%)	Adequate	25 (%32.5)	172 (%37.3)	20.414	0.808 (0.484–1.349)
	inadequate	52 (%67.5)	289 (%62.7)
Sex	Female	47 (%61)	264 (%57.3)	20.535	1.169 (0.713–1.916)
	Male	30 (%39)	197 (%42.7)
Smoking	Yes	7 (%9.1)	72 (%15.6)	10.185	1.852 (0.818–4.188)
PI Ort±SS (medyan)		1.52 ± 0.65 (1.37)	1.86 ± 0.74 (1.77)	30.001*	2.045 (1.411–2.964)
GI Ort±SS (medyan)		1.65 ± 0.63 (1.6)	1.89 ± 0.61 (1.93)	30.002*	1.898 (1.263–2.853)
PD Ort±SS (medyan)		2.91 ± 0.91 (2.8)	3.35 ± 0.96 (3.2)	30.001*	1.670 (1.270–2.196)
CAL Ort±SS (medyan)		3.30 ± 1.12 (2.98)	3.84 ± 1.17 (3.7)	30.001*	1.505 (1.207–1.878)
Age		34.64 ± 15.25 (31)	39.66 ± 15.28 (41)	30.003*	1.022 (1006 − 1.039)

¹Continuity (yates) correction ² Chi-square test ³Kruskal Wallis Test *p < 0.05

Backward stepwise logistic regression was performed to assess the influence of PI, GI, PD, CAL, and age on PAI scores indicating AP (p = 0.001, Table 5). The analysis identified PI (OR = 2.032, 95% CI: 1.391–2.068) and age (OR = 1.021, 95% CI: 1.004–1.038, p = 0.015) as statistically significant predictors of AP presence.

Table 5.

Evaluation of parameters (multivariate) affecting PAI by logistic regression analysis

Step 4	OR	95% C.I. for OR		p
PI	2.032	1.391	2.968	0.001*
Age	1.021	1.004	1.038	0.015
Constant	0.851			0.714

Variable(s) entered on step 1: PI, GI, PD, CAL, Age

*p <0.05

Discussion

Since the success rate of endodontic retreatment is known to be lower than that of initial RCT [28], it is crucial to eliminate or minimize the risks of failure in initial treatment. In this observational study, the factors mainly responsible for RCTF are discussed in order to increase the success of RCT. The findings from the present study suggest that several factors can influence the success of RCT over both the short and long term. In the short term (less than 5 years), inadequate RCT, being male, and inadequate CR were associated with increased risk of treatment failure. In the long term (5 years and above), smoking, older age, higher PI, higher GI, and greater CAL were linked to higher rates of RCTF. Multivariate analysis revealed that being male was the primary factor influencing short-term failure, while age, smoking, and CAL were the most significant factors affecting long-term outcomes. In addition, it was found that the most influential factors for AP were age and PI. Therefore, the null hypothesis was rejected.

In the present study, the categorization of treatment timing (< 5 years vs. >5 years) was established to investigate whether the underlying causes of endodontic failure differ according to the duration elapsed since the initial intervention. This approach is supported by recent findings of Jang et al. [16], which demonstrate that early failures are frequently linked to technical factors whereas late failures are more commonly associated with long-term issues such as coronal restoration breakdown. By applying this time-based grouping, we aimed to distinguish failure patterns and risk profiles across different timeframes, thereby providing insights with potential prognostic and clinical relevance.

Yancheshmeh et al. [29] reported that maxillary molars (44.4%), mandibular molars (20%), and maxillary premolars (15.5%) exhibited the highest rates of RCTF. The study also identified short-filled canals (33.3%) and missed canals (17.7%) as the most common causes of these failures. Consistent with these findings, the current study found that molars (40.3%) and premolars (31.2%) accounted for the majority of failed endodontic cases. Moreover, the most frequently observed issues in our analysis were short fillings (71.5%) and insufficient lateral condensation (48.6%), highlighting similar patterns of failure across different studies. Strengthening practitioners’ abilities and knowledge, particularly in handling the complexities of molar and premolar RCTs, will contribute to better overall treatment success rates.

Olcay et al. [17] reported that the CRs of teeth with RCTF for which extraction was indicated were mostly composite. In accordance with their results, it was observed that 49.6% of the CRs of the teeth analyzed in the present study had been performed with composite and 11.3% with amalgam. At this point, the quality of CRs as well as the accuracy of the indications for CRs are questionable. Due to the loss of a significant amount of tooth structure in most of the RCTT, it may be more appropriate to restore these teeth with indirect restorations [30]. However, in our study, only 39.1% of the teeth were restored with onlays and crowns. Therefore, increased physician knowledge and skills in RCTT restoration may be helpful. Another noteworthy outcome of our research is finding that short-term endodontic success is impacted by low-quality CRs. The result corroborates other studies highlighting that insufficient CRs might jeopardize the seal, permitting microleakage and recontamination of the root canal system [31, 32]. The CR serves as a principal barrier against microbial intrusion; hence, its quality is essential for preserving the sterile environment established after RCT. Endodontic and periodontal systems are closely interconnected, influencing each other’s health and treatment outcomes [19]. However, there are a limited number of studies examining the effect of periodontal status on RCTF. In some of these, periodontal status was categorized as healthy or diseased [33]. However, the current periodontological classification [34] is too comprehensive to categorize cases into only 2 categories. Therefore, instead of categorization, we used numerical periodontal parameters including PI, GI, PD, and CAL. This research has demonstrated that poor periodontal health, including increased plaque, gingival inflammation, and attachment loss, can negatively impact the prognosis of RCT. This is likely due to the close anatomical relationship between the endodontic and periodontal structures, as well as the potential for microorganisms from the periodontal tissues to invade the root canal system [35]. Indeed, supporting this argument, it was observed in this study that with increasing age, all periodontal parameters, as well as the likelihood of teeth having AP, also increased. Older patients may be at higher risk due to the cumulative effects of chronic inflammation and attachment loss over time.

Smoking is another well-established risk factor for RCTF and tooth loss, as it is associated with impaired wound healing and increased susceptibility to periodontal diseases [36, 37]. Khalighinejad et al. [33] reported that current smokers had a higher risk of losing RCT T compared to non-smokers or former smokers. In accordance with the results of these studies, in the present study, smoking was found to increase the risk of RCTF in the late period. It can be concluded that smoking has a negative effect on periodontal health and indirectly affects the success of RCT. Indeed, our results suggest that periodontal parameters, along with smoking, stand out as factors that increase the risk of failure in the late period.

Age and sex have often been investigated as possible determinants of RCT success; however, the results remain ambiguous. Several studies [33] have revealed no substantial relationship between these demographic characteristics and endodontic results. A comprehensive evaluation indicated that neither age nor sex significantly influenced the success or failure rates of RCT [33]. However, Srinivasan and Raghu indicated, in a study, that female patients may have elevated post-treatment pain or discomfort, perhaps due to variations in pain perception or hormonal factors [38]. In this research, the chance of failure in RCTT rose with age and in male patients. Male patients may be more prone to endodontic problems, presumably because of variations in oral hygiene practices or health-seeking behaviors. The variances might also be attributable to biological and behavioral variables such as pain threshold and immune system abnormalities. Ouarti et al. [18] reported that gingival inflammation, CR quality, short filling, and homogeneity increased the risk of AP in RCTT. In the same study, the prevalence of AP was 72.1%. In our study, unlike these results, there was no significant correlation between CR quality and AP, while PI and age increased the risk of AP according to multivariate analysis. The discrepancy between the studies may be attributed to the inclusion criteria employed by Ouarti et al. and those utilized in our study. Specifically, while Ouarti et al. included only teeth with AP in RCTT, our study encompassed both teeth with and without AP that were identified as failing due to the presence of symptoms. Our findings indicate that, although the majority of unsuccessful cases were AP cases, the exclusive inclusion of AP cases in a study is a limitation, given the considerable number of symptomatic cases that do not exhibit AP 14.3%.

In most of the studies [17, 34, 39] examining the factors that cause RCTF, the survival of the teeth and the reasons for extraction were examined. In our study, we did not analyze teeth that were extracted due to failure. Instead, we focused on teeth that had failed for various reasons and were recommended for RCT. Therefore, causes such as vertical fracture and perforation were not recorded. Olcay et al. [17] reported that the most common reasons for extraction in teeth with failed RCT were prosthetic reasons (20.3%), endodontic failure (9.9%), and periodontal reasons (4.5%). Tzimpolas et al. [14] reported the most common reasons for RCTT extraction as prosthetic failure (59.4%), periodontal reasons (32%), and endodontic reasons (8.6%). In the current study, based on the findings, it can be stated that periodontal factors are a more dominant factor in RCTF. The reason for this difference may be the variations in the methodologies of the studies and the differing socio-economic characteristics of the populations in which the studies were conducted. Additionally, for economic and personal reasons, clinicians and patients do not want to risk the presence of a potentially problematic tooth under the prosthesis [17]. This may be another reason why previous studies found that the decision to extract teeth was mostly for prosthetic reasons. However, since this study focuses on teeth with retreatment indications rather than extraction indications, the causes of failure are relatively more objective.

One of the limitations of the present study may be that the periapical and periodontal status of the teeth before initial RCT is not known. Although some studies [40, 41] report that teeth with large lesions have lower success rates compared to those with small lesions, there are also studies that find no such difference [28, 42]. A systematic review and meta-analysis [43] have reported that the size of the periapical lesion does not have a significant impact on the prognosis of RCT. Our results should be supported by prospective studies in which the preoperative and postoperative endodontic and periodontal status of the patients will be clearly known and the treatments will be performed under the same conditions. However, this study is valuable as a guide for future studies as it draws attention to the importance of periodontal status for the success of RCT.

Conclusions

The findings of this study revealed that RCTF is associated with inadequate RCT and inadequate CR in the short term, highlighting the importance of meticulous execution of endodontic procedures and high-quality CR to optimize short-term treatment outcomes. In the long term, smoking, high PI, GI, and CAL have been identified as significant factors contributing to RCT failure. Accordingly, it has been concluded that regular monitoring of periodontal health and educating patients about the negative effects of smoking and periodontal health on RCT prognosis are essential for improving long-term RCT success.

Abbreviations

CAL

Clinical attachment loss

CR

Coronal restoration

ET

Endodontic treatment

ETT

Endodontically treated teeth

ETF

Endodontic treatment failure

GI

Gingival index

PAI

Periapical index

PI

Plaque index

PD

Probing depth

RCT

Root canal treatment

RCF

Root canal filling

RCTF

Root canal treatment failure

RCTT

Root canal-treated teeth

Authors’ contributions

P.M.D, S.G.K and D.T. participated in this study’s conceptualization and design and formal analysis and drafted the manuscript. P.M.D., S.G.K and Ö.B.A. participated in the data collection and validation. P.M.D and H.S.Ö. participated in the formal analysis and review and editing. P.M.D, H.S.Ö, Ö.B.A and D.T. participated in the interpretation of the data and critically revised the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

The present study didn’t receive any funding.

Data availability

The data underlying this article will be shared on reasonable request to the corresponding author.

Declarations

Ethics approval and consent to participate

This study was conducted in accordance with the ethical principles stated in the Declaration of Helsinki and was approved by the ethical board of Marmara University, Faculty of Dentistry (no: 09.2022.429). Informed consent to participate was obtained from all of the participants in the study.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.