The primary objective of root canal therapy is to eliminate pulpal and periapical pathology while preserving the patient’s natural tooth.1 Successful endodontic treatment depends on effectively removing all necrotic or infected pulp tissue, thoroughly disinfecting the root canal system, and establishing a tight seal that prevents any microbial penetration..2 Nonsurgical root canal therapy has a high success rate when these goals are met, with reported recovery results ranging from 76% to 86%. Therefore, clinical and radiological follow-ups are crucial to evaluate treatment outcomes and preserve the long-term function of the treated tooth.3 Despite advances in techniques, materials, and clinical protocols, failures still occur.4 The recurrence of clinical symptoms like pain, swelling, or sinus tract formation, as well as persistent or recently established appearance of periapical radiolucency on radiographic examination, are commonly referred to as endodontic failure.5 Previous studies indicates that multiple factors contribute to such failures.6,7 These include inadequate cleaning and shaping, residual necrotic tissues, persistent intraradicular infection, missed canals, iatrogenic errors (such as perforations, ledges, and separated instruments), and improper obturation, including under- or overfilling. The quality of the coronal seal is equally crucial; defective restorations and microleakage have been shown to significantly influence periapical healing and overall treatment prognosis.8 A large percentage of root-filled teeth have radiographic evidence of apical periodontitis, as reported by numerous cross-sectional epidemiological studies, underscoring the urgent need for endodontic retreatment.9 One of the main causes of retreatment cases is frequently found to be missed or untreated canals, which act as a continuous source of infection. Similar to this, inadequate irrigation can leave behind debris and bacterial biofilms, which can jeopardize the effectiveness of treatment. This is especially true if sodium hypochlorite is not delivered to the working length. Even when ideal protocols are followed, anatomical variations or pre-existing conditions such as vertical root fractures or severe periodontal involvement may predispose a tooth to failure.10,11 Epidemiological studies consistently report a considerable percentage of root-filled teeth exhibiting radiographic signs of apical periodontitis, underscoring the need for retreatment in many cases.9 The decision to retreat is often based on clinical symptoms, radiographic findings, and identification of technical shortcomings in the original treatment. Understanding the factors contributing to failure is therefore critical for improving the quality of endodontic care.12 The main objective of this cross-sectional study is to assess the endodontic and restorative factors associated to the failure of teeth that have received root canal therapy. The study aims to encourage better clinical practices and emphasize the significance of cautious endodontic and restorative operations by identifying the main causes of failure.

Study Setting and Design This cross-sectional study was conducted in the Department of Conservative Dentistry and Endodontics department Government. Dental College, Kottayam which is a tertiary dental care hospital in central Kerala India from 2024 June to 2025 June. The study aimed to assess the endodontic and restorative factors associated with the failure of root canal–treated teeth. Ethical Considerations The Institutional Ethics Committee of. Government. Dental College, Kottayam granted the study ethical clearance (Approval No-IEC/M30/2025/617/DCK). The World Medical Association's (2013) Declaration of Helsinki's ethical guidelines were followed in this investigation. The goals and methods of the study were explained to all eligible patients. Prior to data collection, each subject provided written informed consent, and voluntary participation and confidentiality were guaranteed. The study reporting followed the STROBE guidelines for observational studies. Study Population and Eligibility Criteria Outpatients with concerns about permanent teeth that had previously received root canal therapy who came to the Department of Conservative Dentistry and Endodontics comprised the study population. Teeth that showed clear clinical and/or radiographic indications of endodontic failure were included in the study. Only teeth that had undergone root canal therapy at least six months earlier were taken into consideration. Participants could only be included if they were at least eighteen years old. Sample Size Determination The minimum required sample size was determined using the formula n = 4pq/d², where p represents the prevalence of endodontic failure reported by a previous study12 (43.9%), q = 100−p (56.1), and d = 10% precision. Based on these values, the calculated sample size was n = 100. Sampling Technique and Recruitment Consecutive patients who met the inclusion criteria during the study period were screened and recruited. Detailed clinical and radiographic examination of each included tooth was performed by two calibrated endodontists to ensure assessment reliability. Examiner Calibration and Reliability Assessment Two calibrated endodontists independently evaluated a subset of radiographs and clinical findings. Agreement between the examiners was quantified using Cohen’s kappa (κ) statistic. The obtained kappa score of 0.80 demonstrated strong agreement beyond chance between the two examiners. Clinical and Radiographic Examination and Categorization Criteria The clinical assessment involved examining the nature of pain, checking for tenderness on percussion and palpation, and identifying any swelling, sinus tract, fistula, coronal leakage, loss of tooth structure, or fractures. The radiographic evaluation focused on assessing the periodontal ligament space, the continuity of the lamina dura, detecting any periapical or lateral radiolucencies or resorptive changes, and determining the adequacy of the root canal filling. Standardized intraoral periapical radiographs were obtained for all cases, and radiographic findings were categorized as acceptable, questionable, or unacceptable based on defined criteria. A tooth was considered radiographically acceptable if it exhibited a normal or slightly widened (<1 mm) periodontal ligament space, an intact lamina dura, absence of resorption, and a dense three-dimensional obturation. Radiographic findings were classified as questionable when the periodontal ligament space showed widening of less than 2 mm, the lamina dura displayed uneven thickening relative to neighboring teeth, signs of ongoing resorption were noted, or gaps were detected in the root canal filling, especially in the apical third. Unacceptable radiographic findings included periodontal ligament widening greater than 2 mm, newly developed periapical or lateral radiolucencies, or any periradicular pathology not previously evident. Assessment of Root Canal Filling Quality The quality of root canal filling was also assessed radiographically and classified as adequate when the filling ended ≤2 mm from the radiographic apex, underfilled when the termination was ≥2 mm short of the apex, and overfilled when the material extended beyond the apex. All clinical and radiographic findings were documented using a structured, pretested questionnaire. The two examiners recorded observations independently and resolved discrepancies through discussion. All patient information was anonymized prior to data entry for analysis. Statistical Analysis The data were compiled in Microsoft Excel and subsequently analyzed using SPSS version 25 (IBM Corp., Armonk, NY, USA). Descriptive statistics were used to summarize all categorical variables, including clinical findings, radiographic assessment categories, and root canal filling status. Frequencies and percentages were calculated to describe the distribution of endodontic and restorative factors among the failed root canal–treated teeth. Association between categorical variables—clinical assessment findings, radiographic assessment categories, and obturation quality—were evaluated using contingency tables. Because a substantial proportion of cells in the cross-tabulations had expected counts less than 5, the assumptions for the Pearson chi-square test were violated. Therefore, the Fisher–Freeman–Halton exact test was applied to determine statistical significance for all associations. Exact p-values were reported, with p < 0.05 considered statistically significant.

The present study aimed to assess the endodontic and restorative factors contributing to the failure of root canal treatment. In the evaluated cases, multiple clinical and radiographic parameters showed notable patterns associated with treatment failure. The clinical findings in the present study demonstrate that pain was the most frequently reported symptom among cases of failed root canal treatment, affecting nearly half of the participants (48%). A combination of pain, swelling, and coronal leakage was also commonly observed (20%), followed by isolated coronal leakage or swelling in a smaller proportion of cases. (Table 1). Table 1. Distribution of Clinical Assessment Findings (N = 100) Clinical Assessment Frequency (n) Percent (%) Pain 48 48.0 Swelling 4 4.0 Coronal leakage 4 4.0 Both pain and coronal leakage 16 16.0 Both swelling and coronal leakage 4 4.0 Pain, swelling, and coronal leakage 20 20.0 Both pain and sinus tract 4 4.0 Radiographic evaluation further supported these clinical observations. Almost half of the treated teeth (48%) were categorized as having questionable radiographic outcomes, while an additional 24% were deemed unacceptable, suggesting significant deficiencies in the quality of treatment or persistence of periapical pathology. Only 28% of cases demonstrated acceptable radiographic status (Table 2). Table 2. Distribution of Radiographic Assessment Findings (N = 100) Radiographic Assessment Frequency (n) Percent (%) Acceptable 28 28.0 Questionable 48 48.0 Unacceptable 24 24.0 Assessment of the root canal fillings revealed that 60% of the cases showed underfilling, the most prevalent technical inadequacy identified. Overfilling was observed in only 4% of cases, while adequate obturation was achieved in just 20% of the treated teeth. These findings strongly implicate poor obturation quality—particularly underfilling—as a major endodontic factor associated with treatment failure (Table 3). Table 3. Distribution of Root Canal Filling Assessment Findings (N = 100) Root Canal Filling Assessment Frequency (n) Percent (%) Adequate 20 20.0 Overfilling 4 4.0 Underfilling 60 60.0 A significant association was observed between clinical assessment findings and radiographic assessment categories, as determined by the Fisher–Freeman–Halton exact test. ( p Value <0.001). Clinical symptoms such as pain, pain with coronal leakage, and pain with swelling and coronal leakage were more frequently observed in teeth with questionable and unacceptable radiographic outcomes. Certain clinical presentations, notably pain with sinus tract, occurred exclusively in the unacceptable category, highlighting their potential predictive value for treatment failure. (Table 4). Table 4. Association between Clinical and Radiographic Assessment Findings Clinical Assessment Acceptable (n=28) Questionable (n=48) Unacceptable (n=24) p Value Pain 8 (28.6%) 24 (50.0%) 16 (66.7%) <0.001 Swelling 4 (14.3%) 0 (0.0%) 0 (0.0%) Coronal leakage 0 (0.0%) 4 (8.3%) 0 (0.0%) Pain + Coronal leakage 8 (28.6%) 8 (16.7%) 0 (0.0%) Swelling + Coronal leakage 0 (0.0%) 4 (8.3%) 0 (0.0%) Pain + Swelling + Coronal leakage 8 (28.6%) 8 (16.7%) 4 (16.7%) Pain + Sinus tract 0 (0.0%) 0 (0.0%) 4 (16.7%) There was a significant association observed between clinical assessment findings and root canal filling status using Fisher–Freeman–Halton exact test. (p Value < 0.001). Teeth presenting with pain were primarily associated with underfilled canals. Combined symptoms such as pain with coronal leakage and pain with swelling and coronal leakage were seen in both adequately filled and underfilled teeth. Findings such as swelling alone or coronal leakage alone were restricted to teeth with adequate fillings. These results suggest that worsening clinical signs are strongly indicative of inadequate obturation and may reflect the quality of root canal filling. (Table 5). Table 5. Association between Clinical Assessment Findings and Root Canal Filling Status Clinical Assessment Adequate (n=36) Overfilling (n=4) Underfilling (n=60) p Value Pain 8 (22.2%) 0 (0.0%) 40 (66.7%) <0.001 Swelling 4 (11.1%) 0 (0.0%) 0 (0.0%) Coronal leakage 4 (11.1%) 0 (0.0%) 0 (0.0%) Pain + Coronal leakage 8 (22.2%) 0 (0.0%) 8 (13.3%) Swelling + Coronal leakage 4 (11.1%) 0 (0.0%) 0 (0.0%) Pain + Swelling + Coronal leakage 8 (22.2%) 0 (0.0%) 12 (20.0%) Pain + Sinus tract 0 (0.0%) 4 (100.0%) 0 (0.0%) The analysis revealed a strong relationship between radiographic assessment categories and root canal filling status, demonstrated by the Fisher–Freeman–Halton exact test (p Value < 0.000). Teeth with acceptable radiographic outcomes were primarily associated with adequate fillings, while those with unacceptable radiographic outcomes showed a higher frequency of overfilling and underfilling. Questionable radiographic outcomes were mostly associated with underfilled canals. These results indicate that the radiographic quality of root canal treatment strongly reflects the adequacy of obturation. (Table 6). Table. 6 Association between Radiographic Assessment and Root Canal Filling Status Radiographic Assessment Adequate (n=36) Overfilling (n=4) Underfilling (n=60) p Value Acceptable 20 (55.6%) 0 (0.0%) 8 (13.3%) <0.001 Questionable 12 (33.3%) 0 (0.0%) 36 (60.0%) Unacceptable 4 (11.1%) 4 (100.0%) 16 (26.7%)

The present study evaluated the endodontic and restorative factors contributing to the failure of root canal treatment by examining clinical symptoms, radiographic findings, and the technical quality of root canal fillings. The study identified clear patterns indicating that treatment failure is multifactorial, with clinical, radiographic, and procedural factors often overlapping. In the present study, pain was the most frequently reported clinical symptom, affecting nearly half of the participants. This observation is consistent with previous literature, which highlights persistent postoperative pain as a common reason for patients seeking endodontic consultation.12 Coronal leakage frequently accompanied symptoms such as pain and swelling, suggesting reinfection resulting from inadequate coronal sealing. This aligns with the findings of Rao et al. who reported that 79.6% of teeth demonstrated radiographically inadequate coronal restorations, while 58.4% showed clinically deficient margins indicating widespread shortcomings in coronal rehabilitation.3 As coronal leakage is a well-established route for microbial ingress into the root canal system, the present study further reinforces its substantial contribution to treatment failure. Radiographically, nearly three-quarters of the examined teeth presented with questionable or unacceptable post-treatment appearances, indicating significant shortcomings in treatment quality or ongoing periapical pathology. This aligns with previous literature emphasizing that radiographic success depends heavily on accurate working length determination, effective apical sealing, and proper canal shaping.13 In the present study, only a small proportion of teeth were classified as radiographically acceptable, highlighting the persistent difficulty in achieving predictable radiographic success in endodontics. Specifically, 72% of cases displayed either questionable (48%) or unacceptable (24%) radiographic findings. These results are in agreement with the study by Nouman Noor et al.14, which also reported a high prevalence of radiographic inadequacies, particularly in multirooted teeth, where untreated or unfilled canals (40.9%) and underextended fillings (29.5%) were identified as major contributors to failure. Although both studies underscore radiographic inadequacy as a key determinant of treatment failure, they differ in their reporting approach: the present study categorizes radiographic quality into defined grading levels, whereas the comparative study identifies specific technical errors such as missed or inadequately filled canals. Nonetheless, the collective evidence reinforces the critical role of radiographic integrity in determining endodontic treatment outcomes. In terms of clinical presentation, Salehrabi et al.15 found pain on percussion to be the most common symptom (66%), followed by combinations of spontaneous pain, swelling, and sinus tract formation. In contrast, the predominant symptom in the present study was spontaneous pain, noted in 48% of cases, with an additional 20% exhibiting combined symptoms such as pain with swelling and coronal leakage. Unlike the findings of Salehrabi et al., where pain on percussion was most frequently observed, our study indicates that spontaneous pain and signs of coronal compromise were more prominent clinical indicators of failure. This may reflect more advanced reinfection or increased bacterial penetration resulting from inadequate coronal sealing. Although infrequent across both studies, the presence of a sinus tract consistently corresponded with severe radiographic pathology and poor obturation quality, underscoring its association with advanced endodontic failure. The findings of the present study are in strong agreement with previous literature identifying the technical quality of root canal fillings as a major determinant of treatment outcome.11 In the present study, obturation quality emerged as a key contributor to treatment failure, with underfilling being the most frequently observed technical error. The underfilling was the predominant inadequacy, observed in 60% of failed cases, reinforcing that insufficient apical extension remains a critical factor in post-treatment disease. Underfilling compromises the apical seal, allowing residual or recolonizing microorganisms to persist within the canal system, thereby sustaining periapical inflammation. Adequate obturation was achieved in only a minority of cases, indicating a potential gap in technical execution during the primary treatment procedures. Although relatively few cases exhibited overfilling, this error may still contribute to persistent symptoms when associated with periapical irritation. The present findings align closely with Iqbal et al., who reported that underfilled canals accounted for 33.3% of endodontic failures, identifying it as the most common technical deficiency.16 According to the systematic review by Ng et al., teeth that receive root fillings placed flush with the apex exhibit significantly better survival compared to those with short fillings. This aligns with the current study’s findings, which show that insufficient obturation, especially short fills, is closely linked to persistent symptoms and radiographic evidence of pathology.17 In addition, Segura-Egea et al. reported that 73.9% of teeth with inadequate filling adaptation were associated with apical periodontitis.18 This is comparable to the current findings, in which 72% of teeth exhibited questionable or unacceptable radiographic outcomes, most of which were linked to underfilled or poorly adapted fillings. Collectively, these studies substantiate the conclusion of the present work: underfilling and inadequate adaptation of obturation material remain the most significant technical contributors to root canal treatment failure. Consistent with previous studies indicating that obturation defects such as voids, inadequate density, or insufficient canal preparation are more prevalent in molars and strongly associated with treatment failure, the present study likewise identified underfilling as the most frequent technical deficiency, occurring in 60% of cases. Radiographically questionable or unacceptable outcomes were also common, together accounting for 72% of the examined teeth. These findings mirror widely reported trends in the literature, which underscore that optimal canal debridement, proper shaping, and high-quality obturation remain essential determinants of long-term endodontic success.3,11 A notable strength of the present study is the statistical analysis demonstrating significant associations among clinical symptoms, radiographic assessment categories, and obturation quality using the Fisher–Freeman–Halton exact test. Pain and combined symptoms were strongly associated with questionable and unacceptable radiographic findings, while pain with sinus tract occurred exclusively in teeth with unacceptable radiographic status. These associations were not statistically analyzed in many previous studies, although clinical examples such as pain following overextended fillings, were mentioned descriptively.11,19 Further, the present study established a robust link between clinical symptoms and obturation status: teeth with pain were predominantly underfilled. Such specific symptom–finding mapping provides valuable clinical insight that is not addressed in the earlier studies.19,20 The present study has several limitations that should be acknowledged. First, its cross-sectional design restricts the ability to establish temporal or causal relationships between the evaluated endodontic and restorative factors and the observed treatment failures. Radiographic interpretation was performed using two-dimensional periapical images, which provide limited visualization of complex root canal morphology and periapical structures; the use of CBCT imaging might have enabled more accurate and detailed assessments. Important patient- and tooth-related variables—such as age, tooth type, tooth location, and number of roots—were not available, preventing evaluation of how anatomical complexity may have influenced failure patterns. Similarly, information regarding the operator who performed the original root canal therapy was lacking, precluding assessment of the potential impact of clinician experience or training. Although coronal leakage was documented clinically, the study did not include a comprehensive assessment of restorative quality, including restoration type, marginal integrity, or the time elapsed since placement. In addition, certain clinical and radiographic subcategories had small sample sizes, necessitating the use of Fisher–Freeman–Halton exact tests and limiting the generalizability of the findings. The absence of microbiological analysis also restricts insight into the etiological basis of persistent periapical pathology. Furthermore, the study did not collect information regarding the interval between primary treatment and failure presentation, limiting the ability to determine whether failures occurred early or following long-term function. Since the present study did not document the time interval between treatment and failure, it is not possible to directly compare the findings with previously reported relapse patterns.

The present study demonstrates that persistent pain, compromised coronal integrity, and underfilling of root canals are the dominant features of failed endodontic cases in this cohort. Although the lack of tooth-level and demographic details constrains certain comparative analyses, the observed relationships between clinical symptoms, radiographic status, and obturation adequacy provide actionable insights to improve endodontic outcomes.

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