Dental implant surgery is increasingly preferred for the functional and aesthetic restoration of missing teeth or dented jaws. Accurate placement of dental implants along the preoperatively planned path ensures a promising result [1]. Therefore, the surgical navigation system (SNS) is used to guide the surgeon to place the implants according to the planned paths [2]and precise SNS-guided placement of dental implants can be achieved to maximize safety and minimize implant risk compared to freehand implant surgery [3].
However, most of the widely used SNSs have a disadvantage that the preoperative planned implant paths, real-time position and orientation of surgical instruments are displayed on an independent screen that is separated from the view of the surgical field. [4,5]. Thus, surgeons must frequently change the horizon between the surgical field and the computer screen during surgery. [6,7]which hinders the ease and continuity of the operation [8]. Mixed reality (MR) technology now offers a solution. With the introduction of MR-based optical transparent head-mounted displays (OST-HMD), the virtual model, images or text information can be displayed directly to the patient, providing surgeons with no need to take their eyes off the surgical stage to get adequate visual input and a more intuitive and convenient real-time navigation function [9]. In addition, MRI systems help the surgeon filter out unnecessary visual information through contextual awareness, allowing the surgeon to focus entirely on the operation itself [10].
The MRI-based dental implant placement navigation system begins with the creation of the three-dimensional (3D) virtual image, which is created by reconstructing the three-dimensional anatomical structures from the preoperative scanned CT or cone beam CT (CBCT) information. pictures. The generated virtual content is inserted into the real scenes through MR devices such as stereo cameras, integrated videography (IV) images, OST-HMD or even smartphones, and the MR-based navigation systems using HoloLens (Microsoft Corp., Redmond, WA) have become the mainstream of dental implant surgery using OST-HMD in recent years. This alignment between virtual 3D images of the patient’s anatomy and the actual surgical view can be performed using surgical templates [11,12]index based image recording [13,14]or cloud-based image recording point [[15], [16], [17]]. To avoid the problem of markers obscuring the surgical area and the surgeon’s view, marker-free image registration is proposed that extracts the contours of the region of interest from the stereo images to stereo-map the points on the extracted contours and define the 3D contours of targeting surgical area [[18], [19], [20], [21]]. These studies allowed for the accurate display of virtual preoperatively planned pathways in the patient’s actual surgical space [22].
In order to achieve more accurate real-time intraoperative tracking and placement of the patient’s surgical position and surgical instruments after image registration, many studies have integrated SNS with MR to achieve real-time tracking through visual navigation [13,14,[23], [24], [25]]or electromagnetic navigation [26] devices and perform the alignment of virtual and real images under OST-HMD through registration between navigation devices and MR devices.
The clinical application of MRI-based dental implant surgery has yet to be investigated. However, several fantasy or simulation experiments have already shown that the implant placement accuracy of MRI-based dental implant placement navigation systems is similar to that of static navigation or dynamic navigation systems and much better than free-hand surgery [[27], [28], [29], [30]]. The HoloLens device, as an OST-HMD, has been proven to have better security and reliability [31]. However, under surgical scenarios that require high precision and high real-time, the accuracy and real-time of HoloLens are still questionable and need to be further optimized [9,32]. Therefore, automatic and accurate image registration, real-time tracking algorithms, and simplified operating procedures are essential [31,32].
Based on the above, a magnetic resonance imaging-based dental implant placement navigation system (MR-DINS) has been developed to allow the HoloLens-wearing surgeon to see the preoperative planned dental implant path directly at the surgical site under the vision of the OST-HMD . In order to achieve this goal, algorithms based on coordinate transformations are proposed for HoloLens-tracker registration, phantom-image registration, and surgical drill calibration. Finally, phantom experiments are performed to verify the systematic error and accuracy of implant placement.
The main contributions of this study can be summarized as follows:
- (1)
We proposed HoloLens-tracker registration and phantom-image registration algorithms to align the virtual and real scene, using an NDI tracker and an MR HoloLens device to locate and display the preoperative planned implant path in the real surgical area.
- (2)
With a new improvised cubic block, the correspondence between the coordinate systems of the NDI tracker and the MR HoloLens device can be obtained in real time, improving the efficiency and intuitive virtual-real image alignment.
- (3)
We developed an integrated MRI-based surgical navigation system for dental implant placement surgery without hand-eye coordination problem. A phantom experiment was performed to demonstrate the feasibility of the proposed system.
