Introduction

Root canal treatment involves the three-dimensional cleaning and shaping of the root canal system to be obturated by an inert bio-compatible filling material [1]. Yet, the diversity in root canal anatomies makes it impossible to adequately clean the root canal system, relying solely on mechanical instrumentation [2]. This necessitates the additional usage of chemical root canal irrigants for the removal of inflamed and infected pulpal tissue remnants, planktonic microorganisms, bacterial biofilms, smear layer as well as for the effective extirpation of mechanically produced dentine debris [3]. The efficiency of this chemo-mechanical approach in turn affects the long-term success of root canal treatment outcomes [4].

Although, sodium hypochlorite (NaOCl) is currently one of the most widely used intracanal irrigation solutions, its major drawbacks remains to be its high surface tension, possibly affecting its ability for tubular penetration and microbial cytotoxicity [5]. In this context irrigants’ activation/agitation strategies, using sonic (160 Hz–6 kHz) and ultrasonic (25–30 kHz) devices, were introduced to generate flow and shear stresses within the intracanal irrigant fluids [6] and thereby improve the efficiency of NaOCl in eliminating of intra-canal microorganisms by up to 90% [7]. A high frequency polyamide sonic irrigant activation device (EDDY^™-sonic-system, VDW, Munich, Germany), driven by an air-scaler at about 6000 Hz, was introduced as a conservative and cost-effective method for irrigant activation. Its flexible non-cutting polyamide tip of the size 25.04 is postulated to enhance its handling properties and its potential for debris removal from thin intra-canal anatomical groves [8]. Compared to manual irrigation, high frequency polyamide sonic irrigant activation devices achieve a significantly better smear layer and debris elimination [9] as well as a higher efficacy of calcium hydroxide dressings removal from root canal spaces [10]. In addition, compared to ultrasonic devices, they demonstrate superior efficacy in cleaning anastomoses located in the critical apical parts root canals [11].

One of the most common complications associated with the root canal chemo-mechanical therapeutic approach remains to be the accidental occurrence of emphysema, instrument tip fracture, postoperative pain and most importantly intracanal bleeding, potentially affecting the irrigant’s efficiency [12]. Inadequate control of such intracanal bleeding could further result in higher sealer solubility and microleakage of the subsequent root canal filling material [13]. The aim of the present prospective clinical trial was to investigate for the first time the influence of practitioners’ experience levels and patient related factors (age, gender, tooth type, smoking-status, presence of systemic conditions, baseline pain, baseline swelling, fistula and sensitivity to percussion) on the occurrence of intracanal bleedings (primary outcome), postoperative pain, emphysema and polyamide tip fracture (all secondary outcomes), using a high frequency polyamide sonic irrigant activation device. The current study tested the hypothesis that practitioners’ experience levels and patient related factors have no influence on the occurrence of intracanal bleeding, during the use of the high frequency polyamide sonic irrigant activation device.

Subjects and methods

The study was approved by the local ethics committee (D424/20) and was conducted in compliance with the ethical principles of the Helsinki Declaration for medical research involving human subjects as revised in Fortaleza 2013. All subjects gave their informed consent prior to their inclusion in the study. Based on a pilot study with 20 patients, a bleeding rate of 30% was assumed. To detect a clinically relevant difference in the occurrence of bleeding (yes/no) of 15% between patients, with proficiency level as an independent factor, it was calculated that a two-sided binomial test would have a power of 80% when at least 260 patients were enrolled. Considering possible confounders as well as drop-outs, a total of 334 patients scheduled for root canal treatment were included. Patients received a non-surgical root canal disinfection in the course of a complete root canal treatment, using a high frequency polyamide sonic device (EDDY^™-sonic-system) and were followed up for 1 year. All patients undersigned an informed consent. The outcome assessors had no access to information that could identify individual participants during or after data collection.

Statistical analysis

Continuous variables were presented as means +/- standard deviations and categorial data as their absolute (n) and relative (%) frequencies. For further analysis patients’ age was stratified (18–40 years, 41–60 years, 61-80years, 81–100 years). For analyzing this variable in more detail, the subdivision into four categories with a 20 years range could ensures almost equal groups sizes and fits the data sufficiently. For all evaluated parameters the relation to complications induced by sonic irrigation were checked for statistical significance, using the chi-squared test or Fisher’s exact test when appropriate (Tables 1 and 2). Further in a logistic regression analysis with backward selection and the complication as a dependent variable (likelihood ratio criteria with p = 0.05), all statistically significant and clinically relevant independent parameters were estimated and expressed as odds ratios (OR) with 95% confidence interval (95%-CI). As potential confounders for the outcomes the patient related variables: gender, age, smoking-status, compromised healing, baseline swelling, fistula, baseline pain, percussion sensitivity of teeth and treated tooth group were examined. Bleeding and postoperative pain development were also analyzed for their association to each other. All tests were two-tailed with a significance level of 5%. Statistical analysis was conducted using SPSS version 27.0 (IBM, New York, USA).

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Table 1. Basic characteristics of practitioners’ and patient’s variables in relation to bleeding induced by sonic irrigation.

https://doi.org/10.1371/journal.pone.0285492.t001

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Table 2. Basic characteristics of patient’s and practitioners’ variables related to postoperative pain development after sonic irrigation.

https://doi.org/10.1371/journal.pone.0285492.t002

Results

A total of 350 patients were consecutively screened for eligibility, whereby 16 patients were excluded according to the defined inclusion/exclusion criteria. Finally, 334 patients (176 male and 158 female subjects, mean age 50.03 ± 18.01 years, median 46 years, 25^th-75^th percentile 36–67 years) met the inclusion criteria and were included in the study (Tables 1–4 and Fig 1). In total 91 students performed 182, 4 general practitioners 15, and 2 endodontists 137 root canal treatments, respectively. In 16.8% of all cases (n = 56) pulpitis was diagnosed, 66.4% (n = 222) of the patients presented with an apical periodontitis (173 of them with and 49 without periapical radiolucency) and 16.8% (n = 56) of cases demonstrated symptomatic insufficient root canal fillings requiring retreatment. No relationship was evident between the initial diagnosis and the occurrence of any of the examined complications during sonic irrigation (p>0.05). The study included 76.9% (n = 257) non-smoker, 6.0% (n = 20) former-smoker, 2.1% (n = 7) occasional-smoker, 14.7% (n = 49) actual-smoker and 0.3% (n = 1) passive smoker patients.

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Fig 1. A flowchart of patients included and excluded from the study.

https://doi.org/10.1371/journal.pone.0285492.g001

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Table 3. Results of logistic regression modelling after backward selection (likelihood ratio criteria) for bleeding induced by sonic irrigation.

https://doi.org/10.1371/journal.pone.0285492.t003

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Table 4. Results of logistic regression modelling after backward selection (likelihood ratio criteria) for postoperative pain development (after to prior treatment sonic irrigation).

https://doi.org/10.1371/journal.pone.0285492.t004

The proportion of patients with systemic conditions possibly affecting their healing ability was 10.8% (n = 36) in total, subdivided into 86.1.% (n = 31) with diabetes mellitus and 13.9% (n = 5) having a history of previous chemotherapy (within the last six months). In total 32.0% (n = 107) anterior teeth, 26.9% (n = 90) premolars and 41.0% (n = 137) molars were endodontically treated. Baseline swelling was observed in 6.6% (n = 22) and a fistula in 6.3% (n = 21) of the 334 cases, with 38.0% (n = 127) of the teeth demonstrating baseline sensitivity to percussion.

Discussion

This prospective clinical study evaluated for the first time the effect of the operators’ proficiency levels and patient related factors on possible complications associated with the usage of a high frequency polyamide sonic intracanal irrigant activation device. It should be noted that to date, no universally accepted standardized scientific categorizing of patients into age groups exists. Thus, the current study plausibly grouped patients into four groups (18–40 years, 40–60 years, 60–80 years and 80–100 years), as a linearity of this continuous variable could not be assumed from clinical as well as statistical aspects [17].

Intracanal bleeding, which uncontrolled could pose one of the major reasons for endodontic failure and apical microleakage [18], was defined as the study’s primary outcome. Endodontic inflammatory periapical lesions are usually accompanied by a local activation of immune cells and osteoclasts resulting in an increased blood flow [19], which in turn could result in a heightened probability for the occurrence of iatrogenic intracanal bleeding and postoperative pain. Generally, intracanal bleeding during or after sonic activation can be caused by different factors. On the one hand, the sonic irrigant activation could improve the canal cleanliness and vital pulp tissue remnants removal, possibly stimulating intracanal bleeding. Alternatively, a too intense irrigant activation could irritate periapical tissue with resultant intracanal bleeding. This unwanted side effect could delay the healing process and lead to postoperative complications. The present study recorded only intracanal bleeding events that occurred during the final sonic irrigant activation stage, following a complete mechanical instrumentation, pulpal tissue removal and canal preparation, prior to root canal filling or medication placement. Thus, it is plausible to concluded that the recorded intracanal bleeding events could be primarily ascribed to an unwanted irritation of the periapical region. It was further previously reported that sonic activation could cause a significantly higher irrigant extrusion [20] compared to manual, ultrasonic or laser irrigation activation systems, with an additional possibility of inducing intracanal bleeding or emphysema. Yet in the current study, only a proportion of the sonic root canal irrigations (26.6%; n = 89) were associated with bleeding, with no relation to the operators’ proficiency levels. In addition to excluding patients with bleeding tendencies, a possible explanatory approach could rely on the assumption that the enrolled patients, even the ones presented with baseline swellings, had non-exuberant chronic inflammatory periapical reactions with lower blood flow and a subsequently reduced risk of bleeding [19]. Yet, age was significantly associated to intracanal bleeding, demonstrating a lower bleeding tendency in patients from the oldest age group (81–100 years, OR = 0.111, 95% CI = 0.02–0.50, p = 0.007), which could be reasonably explained through the aging processes of the dentin-pulpal complex [21]. A lifelong formation of secondary/tertiary dentin as well as the apical and coronal dentin sclerosis, beginning in the third decade of life, leading to a closure of secondary canals and isthmuses, and a narrowing of the dentin tubules and the pulpal chamber with advancing age [22], progressively separates the root canal vascular system from the periapical tissues, possibly culminating in the presently observed reduced intracanal bleeding.

In accordance with earlier studies investigating different root canal therapeutic approaches, gender [23] and the presence of fistula [24] were not related with any of the complications under investigation. A recent meta-analysis [14] reported a notable association between the number of cigarettes consumed as well as the intensity and duration of tobacco smoking, and a higher prevalence of apical periodontitis, with possibly higher tendency for intracanal bleeding. However, as none of the included cross-sectional studies controlled for confounding factors, it was difficult to establish a cause-and-effect relationship. In contrast, results of the current investigation did not establish a correlation between smoking status and the occurrence of intracanal bleeding, using the high-frequency polyamide sonic activation device. Further studies are warranted to examine whether this absence of bleeding could be generally ascribed to the patient’s smoking status or if the sonic activation device presents a safer alternative in smoking patients requiring root canal treatment, reducing their tendency for intracanal bleeding. Although, the presence of systemic conditions such as diabetes, chronic stress or hormonal disorders were held responsible for limited periapical healing rates and successful endodontic outcomes [15], in the current study the presence of these systemic conditions was not related to any of the complications under investigation. No emphysema or fractured polyamide tips occurred in any of the treated patients, making the device a safe sonic irrigation activation system.

Pain is an important indicator during endodontic diagnosis and therapy. Earlier results showed that postoperative pain values were significantly lower in treatment groups, using sonic irrigant activation compared to no sonic activation [25]. In the current investigation pain was documented, using the established numerical rating scale. Proficiency levels demonstrated a significant association with postoperative pain development, with the highest pain induced by practitioners of lower proficiency levels. These findings are similar to previous studies on different root canal treatment modalities, which hypothesized that less experienced operators (e.g., undergraduate students) could have a higher chance for over-instrumentation or periapical extrusion of debris, medicaments, sealer, gutta-percha or irrigants [26], with a resultant higher post-operative pain [27,28]. Furthermore, similar to earlier studies [23,29] postoperative pain levels were clearly associated with baseline pain levels, a finding that can be explained by the assumption that the periapical tissue and the root canal system hitherto irritated by the presence of infection/inflammation become secondarily stressed by endodontic instrumentation [30]. Patients who already have higher baseline pain, with elevated inflammatory cytokine levels and pain mediators at the infected/inflamed area, are likely to experience more pain during and after root canal treatment [31].

Yet, the present study’s results should be carefully interpreted in light of its limitations. Firstly, uneven number existed for practitioners from each proficiency level, according to a normal German university hospital setting. Secondly, patients were assigned to the practitioners, according to their treatment capacity. Thirdly, blinding of operators and patients was not applicable to practical as well as ethical reasons due to educational as well as consenting reasons for the practitioners as well as the patients respectively. Finally, although patients reported their pain perception on a well-established numerical rating scale, these results remain to be subjective and could be influenced by multiple factors including a gender-dependent perception of pain [32], with a limited reliability.

Conclusion

Within the current study’s limitations, the use of the examined high frequency polyamide sonic irrigant activation device appears to be safe irrespective of the operators’ proficiency levels, with regards to intracanal bleedings, emphysema or high frequency polyamide sonic activation instrument fractures. Yet, sonic irrigation in the hands of undergraduate students tended to induce higher postoperative pain, especially in younger patients with higher baseline pain scores. Further research is needed to compare sonic irrigant activation to other activation techniques in the hands of operators with different experience levels.

Acknowledgments

We would like to thank all endodontists, dentists and students, who treated the patients in this study.