# Decision Support for Removing Fractured Endodontic Instruments: A Patient-Specific Approach
## 3 Discussion  
This is the first work to report the use of digital technologies as decision support between non-surgical and surgical strategies of removal of a fractured endodontic instrument. In the present case, the digital approach allowed us to visualize and anticipate the patient-specific root and sinus perforation, and to predict the mechanical impact of four removal strategies. Studies reported that clinicians have difficulties orienting themselves in space from CBCT slices during their surgical procedure [30]. Herein, VTP was used to simulate the procedures using a multi-label 3D image and to predict the iatrogenicity of the procedure. An increased risk of perforation and complications were reported for the removal of apically fractured instruments [31]. The 3D image enabled us to precisely evaluate the presence of perforation and the position of the sinus using the clinical view. It should be noted that only two endodontic ultrasonic tips were simulated in the current study, but the current proof of concept opens a new way to plan endodontic treatment and develop supplementary digital models of endodontic files. Furthermore, dynamic navigation systems are increasingly being used in the endodontic field [32; 33]; a potential application of the current work could therefore be to implement in these systems the developed digital tools and evaluate dynamically their actions on the tooth structure as it is proposed in the medical field [34]. It is of note that the use of a printed guide increases the accuracy and reduces the risk of sinus perforation during endodontic microsurgery [35]. However, this was not used in the case presented herein owing to the risk of instrument projection into the sinus. Figure 4: Micro apical surgery of the maxillary premolar. (**a**) Initial radiograph after instrument fracture, (**b**) size of the resected apex and of the removed instrument and (**c**) postoperative radiograph at one year. In the current case, surgical strategies present a more favorable stress distribution than non-surgical ones, which supports herein the apicoectomy. This conclusion was also recommended by a previous narrative review promoting a surgical approach in cases of a separated instrument in the apical part of the root [3]. However, a first non-surgical attempt to remove the instrument was also advised before considering surgery [5], which makes the decision-making highly complex. Facing this lack of consensus, the benefit of this patient-specific FEA is to optimize an individual's therapy and reduce the risk of root fracture. As a perspective, this patient-specific stress analysis could also lead the root-end preparation to be customized and the tip size to be adapted according to the anatomy of each root [36]. Regarding the resection level, a 3 mm apicoectomy presents lower stresses than 6 mm, which is in accordance with previous FEA studies [9; 37]. However, Von Mises stress was herein used as a failure criterion under the assumption that dentin could fail after plastic deformation and distortion [38]. Other criteria such as the maximum of principal stress could have been used to predict fracture and provide different perspectives [38]. Despite the apparent value of the presented strategy, several limitations are to be highlighted. The main one is that the accuracy of CBCT is questionable in the occlusal part due to artifacts, whereas it is known that occlusal morphology influences the stress distribution in FEA [38; 39]. Recent technologies such as micro CBCT [40] and the use of an intraoral scanner avoiding X-ray artifacts [41] could improve future simulations. In the present study, the mesh error was considered acceptable [27; 28]. However, FEA results should also be carefully interpreted due to the technical impossibility to identify patient-specific parameters such as force intensity [42] or for ligament modeling [43]. Consequently, the use of software in patient care is a debated topic due to the numerous variables involved in the procedure, making standardization highly difficult [25; 44].