Smear layer removal and bacteria eradication from tooth root canals by Erbium lasers irradiation

A bibliographic review was performed on PubMed (via the National Library of Medicine), considering such a database includes the major journals in the fields of dentistry and biomaterials. The following search terms were applied: “ultrasonic” AND “Er:YAG” OR “Er,Cr:YSGG” AND “laser” AND “bacteria” OR “smear layer” OR “faecalis” OR “disinfection” AND “root canal” OR “endodontic”. Also, a hand-search was performed on the reference lists of all primary sources and eligible studies of this integrative review for additional relevant publications. The inclusion criteria encompassed articles published in the English language from January 2011 until March 30th, 2023, reporting the effects of ultrasonic activation and Erbium lasers irradiation on the disinfection of tooth root canal dentin surfaces. The eligibility inclusion criteria used for article searches also involved in vitro cell culture assays, randomized controlled trials, animal assays, and prospective cohort studies. The exclusion criteria were the following: papers without an abstract, scarce data (i.e., laser parameters), case reports with a short follow-up period, short number of specimens, and articles assessing only other alternative disinfection methods. Studies based on publication date were not restricted during the search process. The present method was performed in accordance with the search strategy applied in previous studies on integrative or systematic reviews [17, 18, 40‐44].

The selection of studies was conducted in three steps. At first, studies were primarily scanned for relevance by title, and the abstracts of those that were not excluded at this step were assessed. Two of the authors (JCMS and AB) independently analyzed the titles and abstracts of the retrieved potentially relevant articles meeting the inclusion criteria. The total number of articles was compiled for each combination of key terms, and therefore, the duplicates were removed using Mendeley reference manager (Elsevier). The second step comprised the evaluation of the abstracts and non-excluded articles according to the eligibility criteria on the abstract review. Selected articles were individually read and analyzed concerning the purpose of this study. At last, the eligible articles received a study nomenclature label, combining the first author’s name and the year of publication. The following variables were collected for this review: authors’ names, journal, publication year, purpose, study design, bacteria growth conditions and analyses, laser irradiation parameters, ultrasonic parameters, and disinfection solutions. The PICO question was adjusted to the issue where “P” was related to the specimens and “I” referred to the methods of analyses while “C” was related to comparison of findings and “O” to the main outcomes. The data of the reports were harvested directly into a specific data collection form to avoid multiple data recordings regarding multiple reports within the same study (e.g., reports with different set-ups). This evaluation was individually carried out by two researchers, followed by a joint discussion to select the relevant studies.

In previous studies, Er:YAG laser systems with a wavelength of 2940 nm (Fig. 2) were set on a minimum power capacity ranging from 20 to 60 mJ energy and 10 to 50 Hz frequency, while the pulse rate was around 50 μs [4, 20, 22, 24, 26, 33, 45‐57]. Er,Cr:YSSG lasers possess a wavelength of 2780 nm and have been set at low power, 10 to 35 Hz of frequency, 55 to 62.5 mJ of energy, and a pulse rate of 60 or 140 μs [10, 11, 60‐63]. Er,Cr:YSGG laser activation of 2.5% NaOCl or 2% CHX solutions showed to be as effective as ultrasonic-assisted irrigation in percentage reduction of Enterococcus faecalis with the same disinfection solutions [10, 11]. The use of a low concentration of NaOCl (0.5%) in conjunction with laser activation revealed a similar bactericidal effect to that of solely 5% NaOCl solution. That allows the use of a less toxic concentration of NaOCl under Er,Cr:YSGG activation [60]. On the contrary, ultrasonic-assisted irrigation was not enough to enhance the antimicrobial efficacy within 0.5% NaOCl solutions [60, 61]. On multi-species biofilm, laser activation within 4% NaOCl and 15% EDTAC solutions showed similar results when compared to ultrasonic activation, either with 0.5 or 0.75 W of power settings [62]. Er,Cr:YSGG laser activation exhibited high effectiveness in bacteria eradication, considering the irradiance mode over a total activation period of 20 up to 90 s [10, 11, 60‐62].

In a recent previous study, an infection model with Enterococcus faecalis was established in sixty-six extracted maxillary first molar root canals after instrumentation up to ProTaper Universal F2^TM. Specimens were divided regarding the following disinfection methods: conventionally invasive access group (CIA), computer-guided minimally invasive access group (MIA), conventional irrigation (CI), passive ultrasonic agitation (PUI), and Er:YAG laser-activated irrigation (LA). Microbial samples were collected from tooth root canals by the paper tip method and cultured, and the colony-forming unit (CFU) values of each sample were calculated. Then, the root canals were enlarged to the size of F3, and dentin debris was collected from the F3 file. After dilution and culturing, the CFU values were calculated for each group. The disinfection effect of Er:YAG laser or ultrasonic-assisted computer-guided minimally invasive access was similar to conventionally invasive access, while Er:YAG laser showed higher bacteria eradication than ultrasonic activation [58].

Laser-assisted irrigation requires the positioning of the fiber tip at 5 mm from the working length and vertical movements while withdrawing the tip surrounded by disinfection solutions [24, 46, 51, 57]. When inserted into the canal, the tip must be centered, avoiding contact with the canal surfaces [46, 51]. Different studies have confirmed that the laser tip does not need to be positioned at the apex because the cavitation bubbles also allow cleaning in the apical region [45]. Thus, the spatial requirements (high taper and conicity) are less important for the action of the pulsed Er:YAG laser when compared to the PUI [57]. On the Er,Cr:YSGG laser, the endodontic tip was placed 2–5 mm into the canal or above the orifice of the pulp chamber with short up and down movements [11, 60, 62, 63]. As for the Er:YAG laser, extreme caution should be taken when activating irrigating solutions close to the apical constriction and preferably positioning the laser fiber at the entrance of the root canal to reduce the risk of extrusion [61]. Therefore, Erbium lasers are also appropriate in curved root canals, unlike PUI approaches. That could also be an advantage of Erbium-based LAI over solely ultrasonic activation, as the tip position is less affected by the complexity of the tooth root canals. Moreover, positioning the laser tip at the canal entrance may decrease the heating of the tooth structure or surrounding tissues such as the periodontal ligament and alveolar bone [61]. However, a significant bacterial reduction was reported on positioning the tip 6 mm from the working length instead at the canal entrance, as well as increasing the pulse energy from 20 up to 40 mJ. The effect of higher pulse energy can probably be explained by higher peak powers which yield larger primary cavitation bubbles at the fiber tip and hence larger liquid displacement and violent shock waves [33]. However, radial shock waves gather at the bottom part of the fiber tip in a conical-shape tip or PIPS^TM due to inner reflections that result in a higher power density in comparison with a flat tip of similar diameter [15].

In fact, ultrasonic-assisted irrigation increases the flow of liquid and improves both the solvent and antibacterial capabilities and the removal effect of organic and inorganic debris from the tooth root canal surfaces [66]. The physical principle of passive ultrasonic irrigation (PUI) involves the formation of bubbles due to the pressure in the liquid under ultrasonic vibration. That becomes unstable, and then the bubbles collapse, causing an implosion comparable to a vacuum decompression. The collapse of the bubbles releases the impact energy responsible for the detergent effect. In the selected studies, various ultrasonic devices were used, such as the Newtron P5 XS^TM, EMS piezoelectric ultrasonic unit, SybronEndo^TM, EndoUltra^TM, VDW.ULTRA^TM, or PerioScan^TM. Such ultrasonic devices are commonly used in clinical activity and remain an affordable alternative to lasers. In determining the optimal ultrasonic irrigation effects, high frequencies ranging from 28 up to 40 kHz were assessed in the selected studies, of which 30 kHz was the most common assessed frequency. Most of the studies applied the device continuously for 60 s, although no consensus has been established. One previous study pointed out that PUI showed the lowest biofilm removal over the first 20 s [56]. In most studies, the ultrasonic power was not specified and could differ when compared to other devices, leading to some limitations in the correlation of parameters. The success of the PUI approach varies concerning the complexity of the tooth root canal anatomy. The tip of the ultrasonic file must be cautiously positioned to achieve adequate biofilm removal in the presence of an isthmus [56]. A high taper might allow a better disinfection solution flow into the canals and ensure a good oscillation of the ultrasonic tip. Thus, contact between the root canal surfaces and the ultrasonic endodontic file must be avoided since that decreases the amplitude and the irrigant streaming speed [61]. In the selected studies, the tip positioning of the ultrasonic device was usually located at 1 mm up to 4 mm from the working length. The absence of tip positioning in the apical third of the root can result in low bacteria eradication [60]. Those findings may emphasize the need for tip positioning at the apex to increase the bactericidal effect in the apical third. Therefore, the positioning of the tip can become limited in curved root canals, as tips must be placed at 1 to 2 mm from the working length [61]. In the selected studies, a decreasing pattern of disinfection solution penetration depth was noticed regarding the canal thirds. Passive ultrasonic irrigation was more efficient in the coronal third than in the middle and apical thirds and revealed a low penetration depth in the apical third. Those findings were reported with 5–5.25% NaOCl and 17% EDTA solutions [24, 48, 64, 65].

On photon-initiated photoacoustic streaming (PIPS^TM), low pulse Er:YAG energies around 30 mJ have been used within a very short pulse length (50 μs), and a 20 Hz frequency, resulting in high peak powers and hence more efficient cavitation generation [20, 22, 24, 26, 33, 46‐50, 52‐54, 56, 57]. On the PIPS^TM technique, the conical tip is usually placed at the entrance of the pulp chamber or a few millimeters into the canal and held stationary over laser activation [20, 22, 24, 26, 33, 46‐50, 52‐54, 56, 57]. Even though Er:YAG activation with the PIPS^TM technique has shown potential outcomes, no significant differences between PIPS^TM and PUI taking into account bacteria eradication were found in three of the selected studies [26, 33, 49]. On the contrary, PIPS^TM showed a higher percentage of bacteria eradication when compared to PUI in several other previous studies [47, 52, 53, 56]. The antibacterial action of Er:YAG laser activation combined with 2.5–6% NaOCl solutions was highly effective [26, 33, 47, 49, 52, 53]. PIPS^TM was even improved when a chelating agent was added [52]. Er:YAG laser removed a higher amount of apical biofilm and infected dentin tubules when compared to solely ultrasonic-assisted irrigation [55]. The bacteria eradication is illustrated in Fig. 2. Differences in disinfection efficiency between the activation types can be explained by the movement of NaOCl out of the canal with every pulse, which led the PIPS^TM tip into a fluid-free canal after 4 s [46]. Those results indicate a stronger effect of LAI that is attributed to the extremely turbulent action of the disinfection solution induced by pulsed Erbium lasers [33]. In addition, the findings are in agreement with the fact that Erbium laser activation is known to enhance the release of Cl and O ions due to the heating effect [67]. Additionally, when the PIPS^TM tips were positioned in the canal, some melting effect was noted when the tip was kept too long against the surfaces, but that did not negatively influence the scores. No damage to the models occurred when the laser tips were positioned at the entrance level; thus, care has to be taken when positioning the tip close to the apex for a long time [50]. Furthermore, a shock wave-enhanced emission photoacoustic streaming named SWEEPS^TM consists of emitting pulse pairs into a liquid, leading to a higher amplification of the pressure waves when compared to the PIPS^TM single irrigation [36]. That increases the disinfection efficiency of the photoacoustic streaming. Thus, SWEEPS^TM consists of positioning a laser fiber tip in the access cavity filled with a disinfection solution that is quite similar to PIPS^TM, although SWEEPS^TM sends pulse pairs into the liquid [23, 34, 48, 68]. In a previous study, SWEEPS^TM was associated with different concentrations of NaOCl solutions (0.5%, 1.0%, 2.0%, and 5.25%) for the disinfection of bovine root canals infected with E. faecalis. The colony-forming unit (CFU) for viable bacteria decreased after the tooth root canal disinfection procedure on SWEEPS^TM in association with NaOCl low-concentration [36].

Regarding the previous studies, smear layer removal was more effective when 1–2.5% NaOCl and 17% EDTA solutions were combined during ultrasonic activation [45], although smear layer removal was more efficient in the coronal third, followed by the middle third [21, 51]. The effect of ultrasonic irrigation with 17% EDTA on smear layer removal applied to the coronal and apical thirds of the root canal was found to be depth-dependent [4]. The smear layer was removed from all regions of the root canal; however, the dentinal tubules were only open in the coronal and middle thirds [4, 63]. Ultrasonic-assisted irrigation was shown to be more efficient than laser-assisted activation when near-infrared lasers were used [20‐22]. On the debris removal assessment with 2.5% NaOCl, ultrasonic irrigation performed similarly to near-infrared laser-assisted activation, although none of the tested methods was able to completely free the root canal systems of debris [46, 50, 54, 57].

In a previously selected study, the effectiveness of ER:YAG LAI was assessed on eighty-eight human maxillary molars with either a mesiobuccal or distobuccal canal. After instrumentation to a size of 25/0.02 taper, a ledge was produced at 2.2–0.7 mm short of the apex and checked using micro-CT. Specimens were divided into four groups: syringe irrigation (SI), ultrasonic-activated irrigation (UAI), agitation with the XP-endo finisher (XP), and LAI. The apical 1-mm region was examined using SEM to evaluate the remaining debris and smear layer. LAI showed significantly lower smear layer scores than SI and XP (P < 0.05), with an insignificant difference as compared with UAI. Also, LAI revealed significantly higher debris removal scores than SI (P < 0.05) when solely NaOCl solution was used [59]. Another previous study reported a higher smear layer removal when using SWEEPS^TM or PIPS^TM than that recorded for traditional disinfection methods with EDTA and NaOCl or ultrasonic irrigation [69]. Another previous study revealed a significantly higher removal of smear layer and debris by using SWEEPS^TM when compared to the use of traditional syringe irrigation; however, the results were not significantly different when compared to passive ultrasonic Irrigation [70]. Thus, SWEEPS^TM can enhance the effects of disinfection solutions such as NaOCl solutions [36].

The smear layer removal by Er:YAG laser activation appeared to be more effective in the coronal, middle, and then apical thirds [20, 45]. However, at subablative setting activation of 5.25% NaOCl with LAI showed poor results at 1, 3, and 5 mm from the apex. Laser activation was equal to or less effective than PUI, depending on the depth [51]. Laser activation with 2.5% NaOCl and 17% EDTA solutions was more effective than with solely NaOCl [45]. On the combination of EDTA and NaOCl solutions in the pulp chamber, laser activation removed more smear layers from both the middle and apical thirds of the tooth root canals than using PUI. On irradiation time, Er,Cr:YSGG laser activation with 17% EDTA revealed higher smear layer removal over 60 s than that for 30 s [63]. That indicates that the activation time of laser irradiation also affects the canal debridement, although other factors also have effects such as the tip, medium (solution or dry conditions), irradiance, and activation mode. Also, Erbium-based LAI for 30 or 60 s performed better than passive ultrasonic activation for 60 s. Laser-assisted irrigation in EDTA solution revealed effective removal of the smear layer and debris from the root canal apex [63]. Nevertheless, different Er,CR:YSGG laser units (i.e., Waterlaser MD^TM and iPlus^TM) have different pulse durations, and therefore, protocols have changed, that cannot be parallel-compared, despite comprising the same components and wavelength. On PIPS^TM within NaOCl and EDTA solutions, activation was slightly more effective than ultrasonic-assisted irrigation on smear layer removal [20, 22]. PIPS^TM provided less debris than solely laser- or ultrasonic-assisted techniques [46]. Er:YAG was more efficient in the middle third, than in the coronal and apical thirds, whereas the lowest smear layer removal was detected after using the PIPS^TM technique at the coronal third [22]. Er:YAG and PIPS^TM laser activation did not exhibit significant differences in smear layer removal in one previous study [20, 50]. The tip positioning did not alter the effectiveness of the smear layer removal by using laser activation within solely 17% EDTA, unlike with the use of ultrasonic activation [4]. On different combinations of disinfection solutions, Er:YAG laser activation performed similarly to ultrasonic activation with slightly less amount of remaining debris [20, 46, 50, 54, 57]. Considering the previous findings, near-infrared laser irradiation (i.e., Nd:YAG or Nd:YAP) was not capable of improving smear layer removal and canal debridement. In association with NaOCl and EDTA during 20 s to 80 s, near-infrared lasers did not succeed in efficiently removing the smear layer or debris in comparison to Er:YAG lasers or ultrasonic activation [20‐22].

Laser-activation irrigation or PIPS^TM within 17% EDTA and 5% NaOCl solutions for 30 or 90 s exhibited a significantly higher penetration area when compared to other groups [24, 48]. Er:YAG laser irradiation combined with 5% NaOCl and 17% EDTA solutions showed penetration depth mean values of 961 μm into canal dentin using the LAI PIPS^TM approach, while a 722 μm penetration depth was recorded for LAI SWEEPS^TM. Passive ultrasonic irrigation reached a mean penetration depth of 823 μm in canal dentin [48]. However, ultrasonic irrigation, LAI, and PIPS^TM revealed decreasing penetration depth coronally to apically [24, 48]. Considering the minimally achievable penetration depths, PIPS^TM achieved the deepest penetration in the middle and apical sections, with significant differences compared to all other groups [48]. The lower penetration depth of the antimicrobial solution can be caused by the following factors: higher smear layer remnants, the melting effect of laser on dentin tissues, and occlusion of dentinal tubules [64].

The efficacy of novel Er,Cr:YSGG laser radial firing tips (RFT) has been reported in the literature [27‐30]. The laser irradiation with the RTP (140 μs pulse at 20 Hz and 37.5 or 62 mJ irradiance) has been compared with solely endodontic irrigation in 3% NaOCl solutions and interim calcium hydroxide paste in necrotic tooth root canal with chronic apical periodontitis. Anterior and premolar teeth were randomly assigned and radiographically assessed for a 6- or 12-month follow-up [27, 29]. Er,Cr:YSGG laser irradiation with RTP provided statistically significant decreases in periapical index scores regarding radiographic evaluation [27]. That approach has also been shown to be effective for the endodontic treatment of a transplanted multi-rooted tooth with apical periodontitis regarding a 3-year follow-up [28]. Thus, radial firing tips provide more homogeneous laser irradiation of root canal walls, as revealed by the removal of bacteria and the smear layer from tooth root canals in an in vitro study [30].

In fact, a variety of ultrasonic- and laser-assisted parameters are assessed in the studies. However, laser-activated irrigation is effective in a short time frame, and laser activation for 20 s is similarly effective as 3 ultrasonic activation sets [50]. Additionally, the selected studies differ considering the biofilm composition, incubation time, and bacteria culture methods. Also, some of the simulated in vitro methods can reveal different outcomes when compared to in vivo studies or clinical situations. Thus, no consensus currently exists on appropriate setups and incubation times in order to develop a representative biofilm model [33].