An integrated system for simulation and image-guidance for orthognathic surgery

Abstract

Accurate orthognathic surgery planning and planning transfer in the operation theatre are important factors for a successful surgery outcome with appropriate esthetical and functional improvements. However, conventional surgery has inherent limitations because the surgery planning is performed based on 2-dimensional cephalograms and the surgery operation depends on the experience and skill of surgeons. Moreover, 3-dimensional natural head position (NHP) information was not contained in not only the cephalogram but also CT data. To overcome these limitations, a new integrated system covering from virtual surgery planning to surgery guide system for orthognathic surgery has been developed. A new method for 3-dimensional simulation of NHP was also developed using POSIT algorithm. An accuracy evaluation was performed for the NHP simulation and the surgery guide system. In the virtual surgery planning system, realistic virtual osteotomy was performed on a 3-dimensional surface model reconstructed from the CT data using a haptic device. After designation of the landmarks on the surface model for a bone relocation procedure, bone segment was relocated by a combination of rotational and translational movements for each axis. The simulation of bone segment relocation was done according to the conventional surgery planning method. Movements of the designated landmarks were calculated for each relocation step and the final positions of the landmarks were compared with the conventional surgery plan to verify the simulation result. Relocated final position of the bone segment was recorded for the surgery guide system. For the NHP simulation, a photograph of subject was taken after attaching ceramic sphere markers on the subjects maxillofacial area. CT data of the subject was also acquired while the ceramic markers were attached on the same positions. The POSIT algorithm was performed to acquire the relationship between the photograph coordinate system and the CT coordinate system. The 2-dimensional and 3-dimensional marker positions were acquired from the photograph and the CT data, respectively, and these positions were used for the POSIT algorithm calculation. The transformation matrix between the two coordinate systems was acquired as a result of the transformation calculation. Acquired transformation was applied to the CT data of the subject and the CT data was reoriented to the NHP. A Phantom was designed for an accuracy evaluation of the NHP simulation. Discrepancy between the simulated orientation and a virtual NHP of the phantom was determined as an accuracy of the simulation. The result of the accuracy evaluation of the NHP simulation showed clinically acceptable errors (degree) with 0.05±0.19 in roll, -0.56±0.37 in pitch and -0.01±0.29 in yaw. Intra-observer and inter-observer variation was evaluated for the NHP simulation and showed good agreement between measurements (p-value = 0.05). The surgery guide system tracked the maxilla bone segment intra-operatively using a 3-dimensional optical tracking system. Specially designed registration body was fabricated to improve the efficiency of the surgery. The point-to-point registration method was implemented to acquire the relationship between the optical tracking system coordinate system and the CT coordinate system. With the help of newly developed registration body, the registration procedure could be performed preoperatively. After the registration was completed, real-time maxilla bone segment position was tracked by the system. The result of the surgery planning which was previously acquired from the virtual surgery planning system was imported into the surgery guide system. During the tracking procedure, discrepancy between the planned position and the real-time intraoperative position of the maxilla bone segment was shown in 3-dimensional virtual environment. The discrepancy information of the points of interests on the bone segment was also quantitatively calculated. An accuracy of the surgery guide system was evaluated and compared with the conventional surgery guide method. The mean error was 0.47 ± 0.22 mm for the developed method and 1.05 ± 0.49 mm for the conventional method. The error of the developed method was significantly lower than the conventional method for all direction of movements. The accuracy of the surgery guide system was also evaluated in more clinical condition using the result of the model surgery. The CT data of the patients dental casts was acquired with the registration assembly. Registration between the CT coordinate system and the articulator coordinate system was performed, and the surface model of the dental cast was reoriented to the articulator coordinate system. The maxilla bone segment was relocated according to the model surgery result and tracked by the surgery guide system. Discrepancy of the landmark positions between the virtual surgery planned position and the actual position according to the model surgery was calculated and used for the evaluation of the system accuracy. Total of 7 cases of experiment was done and 5 measurements were implemented on 8 landmarks for each case. Total average of the absolute error ranged from 0.06 to 0.87 mm and the RMS error was 0.54±0.21 mm. The evaluation results showed clinically acceptable accuracy. In this study, integrated system for orthognathic surgery was developed. The system contained the virtual surgery planning system, the NHP simulation and the surgery guide system. The developed system helped the surgeons to predict the result of orthognathic surgery preoperatively and verify the exact intraoperative transfer of the surgery planning. In conclusion, this system was compatible with the clinical environment and increased the efficiency and the accuracy of the orthognathic surgery from the surgery planning to the surgery operation. The developed system is expected to be adopted in routine clinical situation and could be useful for the overall improvements of facial deformities after surgery.