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Fully Guided Endodontic Access of Canines and Posterior Teeth: An In-Vitro Analysis of Accuracy and Feasibility


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 Fully guided endodontic access has been used in few clinical circumstances; mainly single rooted incisors. However, these methods are limited or not applicable for teeth with multiple canals. We developed a method to create a single guide for the independent access of canals on canines and posterior teeth using a complete digital workflow. 42 extracted human teeth (maxillary and mandibular) canines, premolars, and molars representing 76 canals were mounted in blocks, and scanned with intraoral and cone-beam CT scanners. Virtual endodontic files were designed and inserted, and direct canal access guides were 3D printed. High speed rotary instrumentation through the guides allowed canal-specific access. Assessment was done clinically via endodontic file placement, and with follow-up scanning. Direct access was created for all tested canals, yielding complete accuracy. Average drilling access depth was 6.67 ± 1.02mm and average divergence of virtual file vs. physical file position after insertion was 1.98 ± 1.06° on average for all canals. For all measured parameters, the only noted differences were tooth surface to pulp chamber and access depths between maxillary and mandibular molars. For specific circumstances, a guide for endodontic access may simplify the procedure clinically, especially for difficult cases, and allow for more conservation of coronal tissue. Using a digital workflow, it is possible to create accurate direct conservative canal access, even in multirootedteeth. Considerations such as drilling protocol require additional examination prior to clinical adoption, but this technique could be applied to conservative or even traditional endodontic canal access.

Despite the enthusiastic support, the current body of studies testing the structural resilience and instrumentation efficacy of teeth with conservative or contracted endodontic access remains unresolved. Specifically, the limitations of the current techniques and technology result in limited fracture resistance when comparing conservative and traditional endodontic cavity access preparations. The limited measured success of these methods may be improved by even more targeted and conservative canal access instrumentation, while the current pulpal debridement and restorative strategies are currently evolving. This is especially true for teeth with multiple canals. A few case reports demonstrate the possibility of fully guided endodontic access to make the procedure more accurate and less invasive, and to treat cases with unpredictable anatomy. A single in-vitro study demonstrates a high level of accuracy for single drill guided access. However, the present state of the art is mainly limited to anterior teeth with a single canal; or to one large access for teeth with multiple canals. One case report exists demonstrating the guided access of maxillary molars, showing a clinically successful result. The theoretical benefits of guided “canal-specific” endodontic access are perhaps more numerous for posterior teeth although this concept has not been thoroughly examined. Following the trend seen in other dental disciplines of full digital planning and case/anatomy specific guided workflow, we developed a method for the fully-guided endodontic access of canines, premolars, and molars. The purpose of this study was to develop and assess a method to independently access canals, especially for teeth with multiple canals, using commercially available CAD/CAM software and three-dimensional (3D) printing technology.

Tooth Blocks:

42 extracted human teeth (having a total of 76 canals) were acquired in compliance with the Medical University of South Carolina IRB. Teeth were seated at the apical extent in rope wax, then the roots were encased in clear orthodontic acrylic resin to create blocks of three to five teeth. The tooth blocks were scanned with the Kodak Carestream9000 cone-beam computer tomography (CBCT) machine at 80Kv, 10mA, and 76 micron slices. Optical surface scans were captured with the PlanmecaPlanscanintraoral scanner. Due to the wide lateral emergence of the canals, we decided not to place molars in proximal contact in the tooth blocks. This was done to prevent guide tubes from separate teeth from interfering with each other.

Virtual Endodontic File Design and Placement:

Optical “intraoral” scan stereolithographicimages were merged with the CBCT in the PlanmecaRomexis3D software module. Virtual endodontic files were custom created in the implant module of the same software at 0.5mm diameter and lengths ranging from 10-18 mm to allow virtual placement with coronal termination of the file near the natural tooth surface. The files were placed into the coronal 1/3 to 1/2 of the canal or until a curvature was encountered (Figure 1d). The files were placed with no consideration of conventional access, file emergence, or estimation of impinging tooth structure for instrumentation. Very simply, the straight virtual files were placed to allow straight vector access based upon the trajectory of the coronal aspect of each canal. For multirootedteeth, all canals were planned if the canals were radiographicallyevident and independent. However, for maxillary molars, we excluded MB2 canals as we were not confident they could be accurately accessed when we planned the study. Based upon the emergence position of the planned endodontic files, measurements were taken from the surface of the tooth to the pulp chamber and from the surface of the tooth to the entrance of the canal. For each canal, ideal instrumentation depth was determined as a length from the tooth surface to approximately mid-pulp-chamber and rounded to a depth that can be measured clinically (to 0.5mm).

We found the digital workflow outlined above to be very predictable after some trial and error. The access guides had accurately printed guide holes, even in cases when they were overlapping, and we had no issues with guide fit on the tooth blocks. Drilling was done under irrigation although it was blocked by the guide, and we did not monitor heat generation. Even in the absence of metal sleeves, the guides gave the tactile sensation that the bur was directed with minimal deflection. 
  After access drilling, we hand placed endodontic C-files into the holes and were directed passively to the canal entrance for every canal. Therefore we had 100% success and accuracy from the standpoint of direct clinical canal access. In 3 cases the bur removed tooth structure at or around the canal entrance, but we had no perforations, or loss of access to the canal. We did note on multiple occasions that the surgical burs used for access slipped inside the friction grip of the high-speed 
handpiece, changing the depth of the bur. We speculate this may be the cause of the error in the 3 cases. 
  For the teeth randomly selected for this study, we measured surface to chamber, and surface to mid-chamber (ideal drilling depth) lengths. No significant differences were seen between canines, premolars, and molars comparing teeth within each arch. A significant difference was measured when averaging all canals for mandibular versus maxillary molars for distance and access depth. The average distance from the enamel surface to the pulp chamber for all teeth was 5.25 ± 1.22mm and the average drilling access depth was 6.67 ± 1.02mm (table 1). 
  To determine the deviation between the planned straight vector canal access and seated endodontic file position we overlaid scans of files placed in the accessed canals and scans of the files placed in the same canals, to the same depth, but with the clinical crown removed (Fig 1h). This also served as an indirect measurement of access accuracy. Average file angle deviation was 1.98 ± 1.06° for all canals. No significant differences were seen between tooth types in each arch, nor between arches. File deviation ranged from 0.23° to 5.28°.

Endodontic Access Guide Design and Fabrication:

After virtual file positioning, the access guides were created using the Romexisimplant guide design module. Parameters were set: guide thickness 2 mm, guide tube length 7 mm, gap to tube 1.5 mm, and tube internal diameter 1.45 mm. Stereolithographyformat computer files of the designed guides were exported to the FormlabsPreform software and supports were added for printing with careful attention to add the supports only to the external surface of the guide, and with no supports terminating within the guide tubes. The guides were printed with dental model resin with the Formabs2 3D printer. Support removal and processing of the guides was done per Formlab’sinstructions. 
Endodontic Access Instrumentation:

Guides were fully seated on the tooth blocks after 24 hour hydration of the teeth/blocks in 0.9% normal saline. Access hole drilling was completed with #4 surgical length round carbide burs, using a fresh bur for each canal. The burs were placed into friction fit high-speed dental operative electric handpiecesand seated at the appropriate depth as measured with an endodontic ruler based upon the ideal instrumentation depth noted in the file design section of the methods. Drilling was done at 150,000 RPM under irrigation with a single forward motion to depth until the head of the handpiecemet resistance at the guide tube.

Post operative analysis:

After endodontic access, the guides were removed and CBCT images were captured (Figure 1e). Each hole was evaluated for passive canal access using a 0.6 C-file, and additional CBCTs were captured with the files in place at a maximum of one file, per tooth, per image to reduce radiographic artifact. In an effort to determine the accuracy of our anticipated path of access, and to determine the difference between that path, and the natural path of the physical endodontic file placed to length in the canal, we developed a protocol to measure the deviation between estimated file path and true file emergence. To do this the clinical crowns of the teeth were sectioned away with a high-speed handpieceunder irrigation to the level of the CEJ. The files were replaced and once again, CBCT images were taken. The DICOM data from the initial images taken with files in place were merged with the images of the files in place with the crowns missing (Figure 1h). We were therefore able to see the two files superimposed and were able to measure differences in angulation and position. Variation was measured from the first perceivable point of the vertex (point prior to separation), and rays were marked on the same side of the files to yield an angulation. For each canal, the files were observed circumferentially and the direction of greatest variation was recorded between the files as the angle deviation.

Despite the significant trend in digital and 3D applications, the likely reason we have not seen any degree of success in a study like ours prior is the complexity of the guide design. Clinical cases and studies rely on the use of implant planning software, which struggle with implants in overlapping proximity. With new software, we were able to create a work-around protocol. Guided access to the canals is so close together and sometimes merged or overlapping, we relied on the tubes created in the guide material for drilling rather than seated metal sleeves. Immediate improvements may use an alternative protocol including small fixed drills directed by metal sleeves. The drawbacks would be increased cost and the need for independent guides for each canal in cases of teeth with canals emerging in close proximity. We did not plan access for MB2 canals. If they are radiographicallyevident, we are optimistic that we could create accurate guidance. In some cases the access emerged or overlapped at the tooth surface creating a single access hole, but allowed passive file placement. In teeth with canals accessed independently, this new strategy will be useful to preserve tooth structure. This preservation of tooth structure may result in improved structural integrity, but will also complicate pulpal debridement which are both subjects of active investigation. In addition to the theoretical benefits of conservative endodontic access, a guided approach could prove useful for the following: limiting iatrogenic trauma, increasing instrumentation accuracy, reducing structural removal to the minimum, access in canal calcification and through restorations. We speculate that access through radiopaque restorative materials will be successful as the access planning is mostly driven by the coronal third or half of the canals. Since an intraoral scan is merged to define the tooth and tissue surfaces, the problems associated with radiographic scatter artifact on the CBCT are nullified. Our analysis of file deviation with and without the clinical crown in place was an objective measurement of access accuracy equaling results seen in guided implant dentistry. It will be up to the practitioner to decide the balance between minimally invasive conservatism, especially at the cervical area, versus traditional straight line access. We want to emphasize that we are not supporting any philosophy of endodontic access in this report, and understand the complications and limitations of minimally invasive access. Rather, we offer our model as a potential tool to be used for traditional, minimally invasive, or new creative approaches to endodontic access.

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