# Accuracy of Guided Endodontics in Posterior Teeth
## 2 Materials and Methods  
Sixty-seven premolars and molars extracted for reasons unrelated to this study were used to construct three maxillary and four mandibular models. Only premolars and molars with complete root formation and without resorptions or restorations were included. Teeth with profound caries extending to the pulp were excluded. The extracted teeth were placed in their anatomically correct arch positions and fixed onto a permanent methyl methacrylate copolymer base. Afterwards, optical surface scans of the models were made using an intraoral scanner (3 Shape TRIOS 3; Institut Straumann AG, Basel, Switzerland) and saved in surface tessellation language (STL) format. Additionally, CBCT scans of each model were made with a voxel size of 0.16 mm, 90 kV, 6 mA and an 8 \(\times\) 8 cm field of view (3D Accuitomo 170; Morita Manufacturing Corp., Kyoto, Japan) and saved in digital imaging and communications in medicine (DICOM) format. The STL and DICOM data sets were then imported into 3D planning software (coDiagnostiX; Dental Wings Inc, Montreal, Canada) and virtually matched based on the available matching tool without further corrections. For each main root canal of the 67 posterior teeth, a guided access cavity was planned in the 3D planning software by virtually positioning a true-to-dimension bur to allow straight-line access to the root canal orifice, followed by the construction of templates (Figure 1). A total of 11 templates were virtually designed with computer-aided design/computer-aided manufacturing (CAD/CAM) and manufactured from polymethyl methacrylate discs (98.5 mm; Yamahachi Dental, Gamagori, Japan) in a 3D milling machine (Motion 2; Amann Girrbach AG, Koblach, Austria) [16]. The fabricated templates were checked for proper fit on the models and adjusted as needed. Afterwards, the tip of the guided endodontics bur was stained with caries marker solution (Karies Marker, V  
Figure 1: Digital workflow: (**A**) virtually planned straight-line access cavity, (**B**) multiple access cavities planned for posterior teeth in a maxillary model, (**C**) virtually designed template, (**D**) 3D rendered view of accuracy evaluation revealing the deviation between planned and performed access cavity preparation. Germany). The template, with the sleeve inserted, was placed on the model in the end position and the stained bur was inserted in the sleeve, leaving a mark indicating the occlusal position of the access cavity. The enamel at the access cavity site was then removed using a diamond bur in a contra-angle handpiece without using the template. With dentin exposed, the endodontic bur (ATEC Endoseal, steco-system-technik GmbH & Co KG, Hamburg, Germany) was set to a speed of 10,000 rpm and guided through the sleeve of the template using slow and short up-and-down movements while regularly clearing debris from the bur and access cavity. Guided access cavity preparation was completed once the bur reached the defined bur stop. Endodontic hand files were then utilized to confirm the root canal location (Figure 2). After access cavity preparation for each tooth, the bur was inspected for deformation or damage, and was replaced after completing all teeth on a model or beforehand if any damage was observed. After all access cavities were prepared, postoperative CBCT scans of each model were created using the same CBCT machine settings as those used preoperatively. Using the planning software, the pre- and postoperative CBCT data were matched in the 3D planning software. The treatment evaluation tool integrated into the software was used to determine the accuracy of the prepared vs. virtually planned access cavities. Because the direction of each access cavity could be derived from the postoperative CBCT data, virtual bur placement was possible for each prepared access cavity. The software then automatically calculated the angular and spatial deviation in different planes at the tip and base of the bur. Descriptive analysis was performed for all access cavities (Table 1).