Guided Endodontics: Static vs. Dynamic Computer-Aided Techniques—A Literature Review

(1) Background: access cavity preparation is the first stage of non-surgical endodontic treatment. The inaccuracy of this step may lead to numerous intraoperative complications, which impair the root canal treatment’s prognosis and therefore the tooth’s survival. Guided endodontics, meaning computer-aided static (SN) and dynamic navigation (DN) techniques, has recently emerged as a new approach for root canal location in complex cases. This review aims to compare SN and DN guided endodontics’ techniques in non-surgical endodontic treatment. (2) Methods: an electronic search was performed on PubMed, Scopus, and Cochrane Library databases until October 2021. Studies were restricted by language (English, Spanish and Portuguese) and year of publication (from 2011 to 2021). (3) Results: a total of 449, 168 and 32 articles were identified in PubMed, Scopus, and Cochrane Library databases, respectively, after the initial search. Of the 649 articles, 134 duplicates were discarded. In this case, 67 articles were selected after title and abstract screening, of which 60 were assessed for eligibility through full-text analysis, with one article being excluded. Four cross-references were added. Thus, 63 studies were included. (4) Conclusions: guided endodontics procedures present minimally invasive and accurate techniques which allow for highly predictable root canal location, greater tooth structure preservation and lower risk of iatrogenic damage, mainly when performed by less experienced operators. Both SN and DN approaches exhibit different advantages and disadvantages that make them useful in distinct clinical scenarios.


Introduction
The first stage of non-surgical endodontic treatment is access cavity preparation. This crucial step must allow for a complete location of the root canal system, as well as its cleaning and shaping, in order to maximize microorganism elimination [1]. Inaccurately performing this stage may lead to numerous intraoperative complications such as missed root canals, root perforations, instrument fracture and/or weakening of the coronal structure, which impair the root canal treatment's prognosis and therefore the tooth's survival [2,3].
Teeth exhibiting pulp canal obliteration (PCO) or calcified canals often require a challenging [4] and time-consuming [5] endodontic treatment. PCO occurs by the deposition of hard tissue within the root canal space, thus making it narrower. It generally has no symptoms and can be detected via tooth discoloration and/or radiographical examination. This alteration can be due to chronic caries progression, previous vital pulp therapy procedures, tooth restoration or even luxation injuries [6,7]. Dental trauma, more commonly described in young patients, can instigate partially or entirely calcified root canals [8,9]. Moreover, cases of PCO due to the lifelong apposition of dentine are becoming more frequent as the number of elderly patients and their need for root canal treatment is increasing worldwide [1]. Endodontic therapy is indicated in 7% to 27% of PCO cases when apical periodontitis or acute symptoms are present [10]. Furthermore, anatomic variations such as dens invaginatus or dens in dente are associated with complex internal anatomy, which also renders the access cavity difficult [11][12][13][14].
It is well-known that access cavities should be kept as conservative as possible, following the trend of a minimally invasive dentistry. The achieved tooth preservation may be an efficient way to decrease the occurrence of post-treatment tooth fractures [15]. Access cavity preparations involving both mesial and distal marginal ridges can reduce cuspal stiffness by up to 63% [16]. Although nomenclature in science is crucial to convey ideas and concepts unambiguously, the standardization of access cavity terminology is still an issue. Generally, whether streaming from social media or purely scientific lines, approaches such as "ninja" and truss cavities that preserve the dentinal bridge by producing two or more occlusal access multi-rooted teeth have flourished. The most widely accepted concepts for access cavities are the traditional, the conservative with parallel or divergent walls, the ultra-conservative, the caries-driven, the restorative-driven and the truss cavity [15]. In all the above-mentioned clinical scenarios, preparing an adequate access cavity and identifying root canal(s) orifice(s) can be demanding and may result in an excessive loss of tooth structure, ultimately leading to increased fracture susceptibility. Therefore, preoperative planning is highly recommended and three-dimensional (3D) imaging may be a helpful tool.
Cone-beam computed tomography (CBCT) is considered a valuable tool for the diagnosis and treatment planning in endodontics, frequently helping clinicians in establishing a proper PCO approach [17][18][19][20]. CBCT presents a noninvasive imaging and measuring technology often used in oral implantology for 3D planning, anatomic structures visualization and bone level quantification, as well as guided implant surgery using printed templates [1]. CBCT should not be routinely used, solely being considered when meticulous clinical and two-dimensional radiographic examinations are inconclusive to clarify complex situations per ALARA principles ("as low as reasonably achievable") [21].
Computer-aided SN and DN techniques are based on CBCT datasets. Static guidance refers to the utilization of fixed surgical guides, which are produced using computer-aided design and computer-assisted manufacturing (CAD/CAM) [25,33]. On the other hand, DN requires an optical triangulation tracking system that uses real-time stereoscopic motiontracking cameras to guide the drilling process according to the planned angle, pathway, and depth of endodontic access cavities [2].
The present review aims to compare SN and DN guided endodontics' techniques in non-surgical endodontic treatment.

Materials and Methods
An electronic search was carried out using the following databases: PubMed, Scopus and Cochrane Library. The search assessed all literature published until October 2021. In Pubmed database, the search terms were applied as follows: (((endodontics) OR ((endodontics) [MeSH Terms])) AND guided). In both Cochrane Library and Scopus databases, the following search key was used: (guided AND endodontics). Search was restricted by language (English, Spanish and Portuguese) and year of publication (from 2011 to 2021).
The inclusion criteria encompassed all types of study designs addressing endodontic access cavities achieved with computer-aided technology (SN or DN techniques) in nonsurgical endodontic treatment.
The exclusion criteria included the use of computer-aided technology in endodontic microsurgery, implantology, bone regeneration, periodontology, laser therapy and restorative dentistry. Other articles non-related to the clinical application of SN or DN techniques in non-surgical endodontic treatment and studies in which CBCT was used only as a diagnostic method were also excluded.
After duplicates deletion, the titles and abstracts were screened based on the inclusion and exclusion criteria. The selected studies were subsequently subjected to a full-text evaluation for eligibility assessment.

Results
After the initial search, a total of 449, 168 and 32 articles were identified in PubMed, Scopus, and Cochrane Library databases, respectively. Of the 649 articles, 134 duplicates were discarded. In this case, 67 articles were selected after title and abstract screening, of which 60 were assessed for eligibility through full-text analysis, with one article being excluded. Lastly, 4 cross-references were added. Thus, a total of 63 studies were included ( Figure 1). Retrieved data from the analyzed studies are described for original research studies (Table 1) and case reports/series (Table 2).
In this case, 14 original research studies focused on computer-aided static technique and 9 on the dynamic navigation method. Only one study comparatively evaluated both guided endodontics' techniques. Moreover, 24 case reports and 6 case series assessed static technique's clinical and/or radiographic outcomes.    The combined use of CBCT and optical scans for the precise construction of a guide rail led to a drill path with a precision below a risk threshold. The present technique may be a valuable tool for the negotiation of partial or complete pulp canal obliteration. Using the AOG-3DP (3D printer) significantly reduced the access opening time for premolars and molars. However, there is a limitation in that CBCT DICOM images must be converted to stereolithographic. stl files in order to be printed via 3D technology. This requires additional preclinical treatment time for imaging and subsequent printing. It could be considered that this can be a useful method in difficult cases. GEA preserved a greater volume of dental tissue in extracted upper human molars than CEA; however, there was no significant difference between CEA and GEA in the volume of dental tissue removed from mandibular incisors.

Wang et al. (2021) [40]
In vitro To evaluate in a laboratory setting, the impact of three designs of endodontic access cavities on dentine removal and effectiveness of canal instrumentation in extracted maxillary first molars using micro-CT. 30  The volume of dentine removed in the crown, pericervical dentine and coronal third of the canal was significantly lower in CEC and GEC groups when compared to that in the TEC group (p < 0.05). There was no significant difference in noninstrumented canal area, canal transportation and centring ratio amongst the TEC, CEC and GEC groups (p > 0.05).
CEC and GEC preserved more tooth tissue in the crown, pericervical dentine and coronal third of the canal compared with TEC after root canal preparation. The design of the endodontic access cavity did not impact on the effectiveness of canal instrumentation in terms of noninstrumented canal area, canal transportation and centring ratio.       The digital planning and guided access permitted to overcome the case limitations and then re-establish the glide path following the original anatomy of the root canals. The guided endodontic represents a personalized technique that provides security, reduced risks of root perforation, and a significant decrease of the working time to access obliterated root canals even in the mesial root canal of mandibular molars, a region of limited mouth opening.
Kaur et al.
(2021) [56] Case Report To describe the use of guided endodontics in calcified maxillary lateral incisor using small diameter bur.

Calcified maxillary left lateral incisor SN Not mentioned Not mentioned
This method demonstrated an ultraconservative, highly reliable, and successful treatment without the excessive removal of enamel and dentin.  [58] Case Report To discuss the impact of new technologies on the treatment of a complex case of a maxillary central incisor with PCO, lateral perforation and apical periodontitis treated using guided endodontics.

Calcified maxillary left central incisor SN 6 months
The periapical radiograph and CBCT scan showed that the guided endodontics approach was radiographically successful.
CBCT contributed effectively for the diagnosis of a perforation that occurred during failed attempts to access the root canal. Guided endodontics, a quick, safe and comfortable intervention, is also very affordable because it does not require any surgical procedures. Case Report To show a modification of guided endodontics with the purpose of reducing the need for interocclusal space using an intracoronal guide technique whereby it can be applied more often in the posterior region.

Calcified first maxillary right molar SN 2 years
Clinical: Any objective symptoms of apical inflammation. Radiographic: Sustained lamina dura indicative of healing.
The demand for more interocclusal space was solved by transforming the virtual drill path into a composite-based intracoronal guide.

Lara-Mendes et al. (2018) [65]
Case report (2 treated teeth) To describe a guided endodontic technique that facilitates access to root canals of molars presenting with pulp calcifications.
Calcified maxillary left second and third molars SN 3 months Radiographic: Regression of the periradicular lesions.
The guided endodontic technique in maxillary molars was shown to be a fast, safe, and predictable therapy and can be regarded as an excellent option for the location of calcified root canals, avoiding failures in complex cases.

Clinical:
Reduction of pain symptoms and a negative response to the percussion tests.

Radiographic:
Reduction of the periradicular lesions. Case report (6 treated teeth) To report the outcome of guided endodontic treatment of a case of dentin dysplasia with PCO and apical periodontitis based on the use of a 3D-printed template designed by merging CBCT and surface scan data.

Discussion
An adequate access cavity preparation presents a key step for the success of nonsurgical endodontic treatment, with excessive substance loss affecting the long-term prognosis of the tooth [44].
Numerous alterations-calcified canals and several anatomic variations-render nonsurgical endodontic treatment more complex. In order to overcome these difficulties and optimize the outcome of challenging endodontic cases, 3D printed guides were introduced based on principles similar to those of guided implant surgery. In the endodontic field, 3D templates are used for root canal location during non-surgical endodontic treatment, mainly when there are significant risks of procedural errors, including root perforation, which can severely jeopardize treatment outcomes [69]. Connert et al. [46] compared conventional freehand access cavity preparation with the static guided approach using 3D printed teeth with simulated calcified root canals, ultimately showing that access cavities resulting from guided endodontics application allowed maximum tooth substance conservation. In fact, access cavities obtained with guided endodontics were comparable with the recently reported minimally invasive access (MIA) cavities. Additionally, guided endodontics was associated with a significantly lower procedure time [37,51,52]. In addition, while 91.7% of the root canals were located and negotiated using guided endodontics, only 41.7% of the root canals were accessed and negotiated when applying the conventional access preparation technique. In conclusion, these results confirm that the guided technique outperforms the conventional approach regarding root canal location and intervention time [46]. Furthermore, although operator experience seems to play a key role when accessing severely calcified teeth with conventional freehand access, computer-aided techniques have been reported as operator-independent [1,26,44,48,51]. Therefore, digital techniques would be especially valuable for the management of complex cases by less experienced operators.
The workflow of SN technique demands a cone-beam computed tomographic (CBCT) and intraoral scanning to produce a template used to guide the bur through an ideal path previously defined based on the information provided by both techniques [48]. Firstly, a high-resolution preoperative CBCT scan is taken and stored in Digital Imaging and Communication (DICOM) format [44,56,57,65]. Afterward, a digital intraoral impression of the patient's upper and lower arch is obtained using an intraoral scanner [57,70]. This step can also be indirectly obtained from gypsum cast [56,68]. The resulting file must be saved in surface tessellation language (STL). The two files are then uploaded to a specialized image processing software [58,70]. The following step is to design a 3D template using 3D design software, where the bur and the corresponding sleeve positions are planned [44,45,54,56,59,70]. Finally, the 3D guide needs to be printed or milled, and the metal sleeves are incorporated into the template [44,45,56,60,70]. Before the endodontic treatment, the teeth that will support the guide should be isolated with rubber dam and then the guide's fitting and stability must be checked [59,61]. With the 3D template correctly set, the canal can be prepared up to the established working length by inserting the bur through the metal sleeve [34,59]. Once the root canal system is located, it can be negotiated with endodontic hand files until the working length and copiously irrigated. After this, the endodontic treatment can be performed based on conventional root canal preparation and filling techniques [59,61].
DN uses a mobile unit which includes an overhead light, a stereoscopic motiontracking camera, and a computer with implant planning software. These items are used to guide in real time a calibrated handpiece until reaching the reference point previously determined [49]. To perform access cavities through DN, likewise the SN approach, it is first necessary to take a high-resolution preoperative CBCT scan. In order to digitally plan the entry point of the bur, its pathway, depth and angle, the CBCT scan file is uploaded to the dynamic navigation system (DNS) planning software [51]. Before opening the access cavity, the tooth or teeth should be isolated with rubber dam [50]. An initial calibration process of the handpiece with the jaw according to the producer's indications is necessary. The access cavity can be subsequently performed with simultaneous monitoring by the DNS [51,53]. The operator can visually control the progression of the bur by watching it on the laptop screen. In the real-time image, the depth of the bur is indicated by a green bar on the depth gauge; when within 1 mm of the desired depth, the bar's color changes from green to yellow and thereafter to red, when the planned depth is reached [50]. Similarly, to the static approach, once the root canal system is located, it can be negotiated [61].
Several studies confirm the accuracy of both static [1,5,26,44] and dynamic [10,16,47,49,50] approaches. According to a previous systematic review and meta-analysis, there are no statistically significant differences between both computer-aided approaches in terms of root canal location rate, with SN and DN exhibiting success rates of 98.5% and 94.5%, respectively [2]. Moreover, suitability of the static technique for predictable and accurate fiber post removal has also been disclosed [27].
Both static and dynamic systems present correspondent advantages and disadvantages. A major advantage of digital planning is that it is possible to preoperatively visualize the root canal location and plot the navigation in detail without having to mentally transfer the planning to the clinical situation. This will allow dentists to achieve predictable results without extensive endodontic skills being required [61]. One crucial drawback to consider is that the PCO extension might affect the accuracy of both computer-aided techniques [2]. Regardless of the technique, the quality of the CBCT presents a crucial aspect for a correct preoperative planning. Spatial resolution of the CBCT does not always allow for the visualization of the root canal and the presence of highly radiopaque materials can result in radiographic artifacts which jeopardize digital planning [52,53].
The SN technique performed with surgical templates avoids the need for drilling guidance during treatment [53]. It was also mentioned that SN reduced excessive loss of tooth structure and chair-side operating times. Furthermore, using a single bur or two burs ensured the accuracy of the drilling procedure [69]. Despite the advantages of this method, the approach exhibits several disadvantages, one of them being that multiple teeth must be isolated during the procedure because the guide must fit directly on the teeth to ensure its stability [61,62]. Moreover, as the guide restricts visual access to the endodontic cavity, its removal is required whenever the operator aims to confirm the path during the treatment [61,67]. Specifically in posterior teeth, static guidance requires the fabrication of several templates to allow straight access to individual root canals. In addition, it is noteworthy that the accuracy of the technique greatly relies on the surgical template's design and manufacturing process [53]. Regarding clinical application limitations, the static-guided approach requires a straight path to the target, therefore being difficult to use this technique in cases of small or limited mouth openings and/or in posterior teeth with reduced interocclusal space. However, an intracoronal guide has been proposed to overcome this difficulty. In addition, larger diameter slow-speed drills can generate cracks on the tooth surface and produce excessive heat that may harm the periodontal ligament. Finally, the lack of 3D real-time visualization prevents intraoperative changes in the predefined drill trajectory [25,50,52,61,63,64,66].
One of the main benefits of DN is that it enables a direct view of the operatory field and allows clinicians to readjust the direction of the endodontic access cavity bur in realtime [53]. In addition, there is no need for an intraoral scan, the overheating risk deriving from the lack of barrier between the water source and the bur is reduced, and it is especially useful in cases of limited mouth opening or posterior region treatments since a template is not necessary [51]. Additionally, endodontic urgencies can be treated using DN, whereas the static approach requires an additional step of template design and printing [47,51]. Despite the accuracy of DN, disadvantages and limitations were also outlined. The primary disadvantages are the difficulty of keeping the system display in sight during the procedure and the long learning curve of the technique, with proper operator training prior to treatment being required [2,63]. Nevertheless, it has been suggested that augmented reality technology can be employed to transmit the virtual image displayed by the DNS while maintaining the therapeutic area's visibility [2]. Moreover, high initial equipment investment is involved [47].
Overall, guided endodontics allows for obtaining MIA and predictably locating root canals in complex cases while minimizing the potential risk of iatrogenic damage and enabling greater tooth structure preservation [25,39,46], thus presenting a more accurate and safe treatment option over conventional freehand techniques [53]. Since the SN technique was introduced earlier, it is better supported by scientific literature than the dynamic method [2]. The latter, however, shows higher potential for future developments and improvements. Nevertheless, further studies with larger sample size and standardized protocols are currently necessary to consolidate the precision of both guided endodontics' methods [38]. Moreover, research focusing on different clinical scenarios would be of uttermost importance. Although numerous case reports and few case series are currently available, randomized clinical trials would provide valuable data on the clinical outcome of guided endodontics application.

Conclusions
The clinical management of complex endodontic cases, such as severely calcified canals, can be challenging for clinicians. Guided endodontics procedures present minimally invasive and accurate techniques which allow for highly predictable root canal location, greater tooth structure preservation and lower risk of iatrogenic damage, mainly when performed by less experienced operators. Both SN and DN approaches exhibit different advantages and disadvantages that make them useful in distinct clinical scenarios. Therefore, it is important to rigorously evaluate each clinical case for subsequent adequate guided endodontics technique selection.