May 7, 2019
by Kenneth S. Serota, DDS, MMSc
The Evolution of Surgical Navigation
Transformative technologies are altering the developmental framework in dentistry. Digital imaging, diagnostics and impressions, the use of computer-aided design/computer-aided manufacturing (CAD/CAM) for prosthesis fabrication and lasers for soft and hard tissue alterations can achieve degrees of bio-minimalism that were impossible a few scant years ago.1-5 Nowhere is this more prevalent than in the foundational pillars of implants and endodontics.
Freehand Navigation (FN) is used in implantology to transfer a fixture position from surgical planning to the surgical site. In endodontics, the magnification and illumination properties of surgical operating microscopes (SOM), have enhanced the accuracy of FN access cavity preparations and micro-surgical osteotomies. This has resulted in a paradigm shift towards conservative, more restricted access cavities6 that facilitate the preservation of coronal and radicular tooth structure by optimizing the long axis entry point, the drill angulation and the glide path to the terminus of the root canal space.
In spite of these advances, there are limitations in endodontic clinical scenarios where canals metamorphose in a coronal-apical direction and surgical access is space restricted. While the clinical experience of the operator brings value, altered vertical and lateral angulation of the long axis orientation of the endodontic access cavity presents iatrogenic risk. In both implant surgery and endodontic micro-surgery, small bone volumes or a misdirected osteotomy can injure the inferior alveolar nerve or perforate the maxillary sinus and other critical anatomical structures.7,8
The advent of cone-beam computed tomography (CBCT–DICOM files) has transformed pre-treatment planning. DICOM files are converted into stereolithic files which are used to create Static Navigation (SN) stents (CAD/CAM fabricated). The stents direct the access cavity preparation and micro-surgical orientation, thus avoiding removal of unnecessary tooth and bone structure (Figs. 1A & 1B). Dynamic Navigation offers new vistas and horizons for computer-guided endodontic protocols. Enhanced accuracy due to real-time feedback diminishes the complex impact of access cavity preparation of calcified canals, retreatments and microsurgical procedures.9-11
Each navigation protocol has disadvantages. With FN used for dento-osseous access and surgery, clinical judgment is the pilot. FN depends upon visualization of the anatomical scenario from information provided by casts and radiographs. Significantly more time is required with an FN technique in contrast to a guided technique. Determining the canal path and position or accessing apices is more difficult.
Stereolithic stents [Static Navigation (SN)] require a medium field of view CBCT scan. Polyvinyl siloxane impressions of the arch to be treated are poured and a digital 3D scan of the stone model merged with the patient’s DICOM files. The use of an intraoral scanner is preferable.
In the case of Dynamic Navigation (DN), virtual planning of the endodontic access preparation or the osteotomy can be affected by the resolution of the CBCT scan.12 Flaws in the process of the fabrication of a stent used to position an external fiducial to register the patient to their image can result in inaccurate image acquisition.
Dynamic Navigation facilitates real-time computer guidance technology using an imported CBCT dataset. This is analogous to the use of GPS (Global Positioning Systems) and satellite navigation. An innovative computer-guided technology, Trace and Place (TaP), has been developed by the Canadian company Claronav which eliminates the need for a fiducial stent thus reducing the workflow time as well as the need for an extra registration scan. This also eliminates the concern of needing the fiducial positioned in exactly the same position during the surgery as it was during the scan to ensure accuracy.
An optical tracking device (Fig. 2) tracks a Jaw-Tracker, the optical tracking tag connected to the patient’s jaw and a Drill-Tag, which is the optical tracking tag connected to an instrument specific to the procedure. The tip is superimposed on the CBCT scan which is mapped to the patient’s jaw.
The heightened level of accuracy of TaP technology enhances restricted access cavity preparation and cortical window osteotomy size (high-speed, Piezotome). Ultrasonic tips used for root-end retro-preparation can also be tracked by DN software.
Estimates place the global population over 65 at 615m.13 Years of dentate and periodontal disease can impact on the pulp, the periapex and peri-radicular tissues. With longevity will come increasing numbers of complex endodontic procedures as age and treatment induce sclerotic changes in the pulp canal space. As such, the use of DN will prove to be of significance in endodontic therapy.
Prior to the Appointment
The first stage of TaP [Trace and Place] workflow is the importation of the patient’s CBCT [DICOM] dataset into the Dynamic Navigation planning software to reveal the dentition. The screen shows the stream video, panoramic view, target view, depth indicator, bucco-lingual and mesio-distal section views (Fig. 3). The access point of entry, the axis orientation/angulation and the depth of the access cavity are planned. For micro-surgical procedures, the Piezotome pathway is based on the dimensions of the osseous pathology surrounding the root apex (Figs. 4A-4C).
The planning stage can be done at any time prior to the procedure provided the CBCT scan is consistent with the current dentate condition. As a preliminary step prior to the Trace Registration itself, three to six trace starting points (landmarks) are chosen and marked on visible and accessible teeth. When the mouse pointer is positioned over the 3D model, a 2D cross-sectional view presents. The red cross-hair sticks to landmark, its centre on the surface (Fig. 5). The software advises the clinician if it suspects the landmark is in a position it deems may be difficult to delineate.
The Jaw-Tracker (mandible or maxilla) or Head-Tracker (maxilla) is securely fastened to the jaw to be treated (Fig. 6). It should be noted that Jaw-Tracker can be positioned at a distance from the rubber dam unlike a Jaw-Tracker attached to a fiducial stent which is more positionally restricted.
When the three landmarks have been determined, the optical tracking sensor tracks the Tracer Tool as it is brushed around the landmarks on the facial, lingual, occlusal surfaces in a manner similar applying etching or bonding solutions. The software shows the number of points contacted as a percentage (Fig. 7).
Calibration of the Drill
The Drill-Tag is attached to the hand-piece and the drill axis and drill tip are calibrated. The optical tracking sensor continuously tracks the Drill-Tag and the software shows the location and position of the drill or Piezotome. The software will issue a warning if the Drill-Tag or the Jaw-Tracker is out of view of the camera (Figs. 8A & 8B).
The navigation screen is active when the system identifies the calibrated instrument as it approaches the patient’s jaw. The target view measures the distance between the instrument’s tip and central axis of the designated access penetration point, the glide path or the osteotomy. The central axis length of the planned procedure is represented by the center of the static white target, the tip of the drill is indicated by the moving black cross following the drill tip movement. The real-time direction of the drill is represented as a cone in the head of the handpiece (Figs. 9A & 9B).
During the drilling, the moving cross and cone are tracked. The cone will turn green when the instrument tip is within 0.5 mm and has a less than 3° angulation to the planned glide path or osteotomy. When the drill tip reaches a distance of 1 mm from the apical or horizontal extent of the planned depth landmark, the depth indicator turns yellow.
Innovation in dentistry occurs when there is a willingness to explore and improve both diagnosis and treatment. The challenge is to cohesively marry the equipment and materials to new software applications. Real-time feedback using digital imagery, tracking systems, targeting and registration to navigate hard tissue is an integral part of surgical procedures in medicine. Safer and less invasive protocols are the direction of dentistry’s future. Dynamic Navigation is proving to be the pilot. Improvement in stereoscopic cameras, the resolution of computer screens, optical markers and the reference array to the patient and the instrumentation will herald an unprecedented level of accuracy in all dental procedures.
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About The Author
Kenneth S. Serota, DDS, MMSc, graduated from the University of Toronto Faculty of Dentistry in 1973 and received his Certificate in Endodontics and Master of Medical Sciences degree from the Harvard-Forsyth Dental Center in Boston, Massachusetts in 1981. Active in online education since 1998, he is the founder of the Endodontic forum ROOTS and the interdisciplinary Facebook forum NEXUS. Dr. Serota is a clinical instructor in the University of Toronto postdoctoral endodontics department. He is the social media and marketing director for Navident Dynamic Navigation.