Mixed-reality simulation for orthognathic surgery
© Fushima and Kobayashi. 2016
Received: 14 January 2016
Accepted: 26 February 2016
Published: 9 March 2016
Mandibular motion tracking system (ManMoS) has been developed for orthognathic surgery. This article aimed to introduce the ManMoS and to examine the accuracy of this system.
Skeletal and dental models are reconstructed in a virtual space from the DICOM data of three-dimensional computed tomography (3D-CT) recording and the STL data of 3D scanning, respectively. The ManMoS uniquely integrates the virtual dento-skeletal model with the real motion of the dental cast mounted on the simulator, using the reference splint. Positional change of the dental cast is tracked by using the 3D motion tracking equipment and reflects on the jaw position of the virtual model in real time, generating the mixed-reality surgical simulation. ManMoS was applied for two clinical cases having a facial asymmetry. In order to assess the accuracy of the ManMoS, the positional change of the lower dental arch was compared between the virtual and real models.
With the measurement data of the real lower dental cast as a reference, measurement error for the whole simulation system was less than 0.32 mm. In ManMoS, the skeletal and dental asymmetries were adequately diagnosed in three dimensions. Jaw repositioning was simulated with priority given to the skeletal correction rather than the occlusal correction. In two cases, facial asymmetry was successfully improved while a normal occlusal relationship was reconstructed. Positional change measured in the virtual model did not differ significantly from that in the real model.
It was suggested that the accuracy of the ManMoS was good enough for a clinical use. This surgical simulation system appears to meet clinical demands well and is an important facilitator of communication between orthodontists and surgeons.
KeywordsOrthognathic surgery Mixed-reality simulation Computed tomography Motion tracking Facial asymmetry Dental compensation
The purpose of orthognathic surgery is not only to improve jaw morphology but also to correct the inter-occlusal relationship. In patients with jaw deformities, therefore, three-dimensional diagnosis and simulation of skeletal and occlusal problems seem to be essentially important [1–4]. We have developed a new orthognathic surgical simulation system, named mandibular motion tracking system (ManMoS) [5, 6]. ManMoS uniquely integrates the real motion of the dental cast model with the virtual motion of the reconstructed craniofacial model, generating the so-called mixed-reality simulation. The skeletal change of the jaw osteotomy is simulated on the PC monitor while the occlusal change is confirmed by checking the cast model on the simulator. The simulation process is easily repeated, and the operator can make several attempts to determine the final jaw position.
The importance of differential diagnosis of the dental compensation for jaw deformity in three dimensions
Mixed-reality surgical simulation
Jaw repositioning with the priority given to the skeletal correction rather than the occlusal correction
Rigid internal fixation
This article aimed to outline the ManMoS in two orthognathic surgery cases having a facial asymmetry and to examine the accuracy of the ManMoS.
The ManMoS uniquely integrates the virtual dento-skeletal model with the real motion of the dental cast mounted on the simulator, generating the mixed-reality surgical simulation. The method to provide the surgical simulation system is described below. This research has been reviewed from an ethical respective by the ethics committee of the Kanagawa Dental University and approaved by the institute director (No.337).
Motion capture of real dental cast model
Mixed-reality surgical simulation
At the beginning of the simulation, the upper and the lower dental casts are mounted on the simulator while the reference splint is intermaxillary placed between the casts. The stylus receiver is used to digitize three spheres on the reference splint (Fig. 5a), which are also detected in the virtual space of dento-skeletal model (Fig. 3). A bowl-shaped cover is worn on the tip of the stylus receiver, so that the tip of the stylus touch to the sphere’s surface and the long axis of the stylus orients to the center of the sphere (Fig. 5b). Since the diameter of the sphere is 6.0 mm, the center of the sphere locates at 3.0 mm ahead of the tip of the stylus along its long axis. As referred to center coordinates of these spheres, ManMoS integrates the motion data of the real cast model with the virtual dento-skeletal model; consequently, the mixed-reality simulation for an orthognathic surgery is set up (Fig. 1).
ManMoS was applied for two orthognathic surgery cases treated by the sagittal splitting ramus osteotomy (SSRO).
In order to assess the accuracy of ManMoS, positional change of the lower dental arch during the simulation was compared between the virtual and the real model.
At the initial mandibular posture, three landmark points of the central incisor and the bilateral first molars were measured on the lower dental arch of the virtual model. Corresponding points on the real dental cast were also measured directly by using the stylus receiver of the Fastrak. Then, the lower dental cast was arbitrarily moved to the simulator, and three landmark points at the simulated posture were measured both on the real cast and on the virtual lower dental arch in ManMoS. Linear moving distance of each landmark points from the initial to the simulated mandibular posture was calculated and compared between the virtual and the real measurements.
This process was repeated 25 times. Wilcoxon duplicate determination was performed, and the errors of measurement were established.
Linear moving distances of measurement points (mm)
Accuracy of ManMoS simulation
n = 25
Difference between ManMoS and FASTRAK (absolute value)
Facial and oral findings
Coordinate system for 3D diagnosis
For the dental problems assessed in the cranial coordinates (Fig. 9), the lower denture midline is deviated to the left and mesial occlusal relationship on the right side is found along with the mandibular displacement.
Asymmetries in the lower dental arch are assessed in the local MLB coordinates (Fig. 10). For the surgical simulation in ManMoS, in order to correct the facial asymmetry, the MLB (tooth-bearing area of the mandible) should be repositioned symmetrically against the cranial global coordinates. The lower dental arch should be symmetrically aligned referring to the MLB coordinates. As shown in Fig. 10b, the lower denture midline is displaced 3.0 mm toward the right. As referred to symmetrical arch, the left lateral teeth are positioned more mesially and lingually than those in the opposite side.
Mixed-reality simulation for orthognathic surgery
Following the measurements of the three reference spheres, mixed-reality simulation was started. When the operator handles the real cast model, the virtual model of the mandible or the maxilla follows the corresponding dental cast movement in real time. The skeletal change is simulated on the PC monitor, while the occlusal change is confirmed by checking the cast model on the simulator (Fig. 1).
ManMoS occasionally makes us notice the application of unilateral mandibular osteotomy.
Facial and oral findings
3D diagnosis of skeletal problems
Virtual cephalometric image
Since the mixed-reality surgical simulation system ManMoS comes into being through a series of complex operation, there are several error factors. It was confirmed that the accuracy and precision of the tracking device Fastrak itself appeared to be good enough . In the linear measurements of 100 mm, the average error between the measured data and the actual value ranged from −0.308 to 0.136 mm. In this study, with the measurement data of the real lower dental cast as a reference, measurement error for the whole simulation system was less than 0.32 mm. It was suggested that the accuracy of the whole system was sufficient for clinical demands.
As shown in case 1, chief characteristic of a facial asymmetry is the lateral displacement of the mandibular midline. Diagnosis in MLB reference (Fig. 10) represented that remarkable asymmetry was not found at the tooth-bearing part of the mandible, but found at the ramus parts. Therefore, facial asymmetry seems to be mandibular asymmetry, and mandibular asymmetry seems to be ramus asymmetry. And in most cases having a facial asymmetry, the tooth-bearing part of the mandible considers to be displaced due to the asymmetrical growth of the ramus.
It was reported that characteristic dental asymmetries were found in patients with a facial asymmetry [3,4]. As dental asymmetry shown in case 1, the lateral deviation of the lower denture midline and the right-left difference of Angle’s molar relationship were due to the skeletal asymmetry (Fig. 9). Therefore, they should be corrected by means of a jaw surgery. When the lower dental arch was assessed in the mandibular local reference MLB (Fig. 10), the left lateral teeth was inclined lingually. This finding is considered to be a dental compensation commonly found in the side of chin deviation and should be corrected by means of orthodontic treatment. ManMoS provides the differential diagnosis whether dental asymmetries should be corrected by means of a surgery or an orthodontics clearly.
Accurate positioning of the ramus is essentially important and clearly prevents many of the problems associated with the impingement on the condylar proximal segment .
Rigid internal fixation
In ManMoS, jaw repositioning is simulated based on the skeletal correction. Consequently, little occlusal contact occurs and the occlusion is unstable at the final mandibular posture (Fig.13b). Such unstable occlusal condition is a risk to produce oral discomfort and the relapse of the bony segments. To ensure the skeletal correction by the osteotomy, the rigid internal fixation of the bony segments is essentially proposed in ManMoS and usually performed bicortically with the titanium screws.
Occlusal wafer splint
After deciding the final jaw position in ManMoS, the occlusal relationship is transferred with a bite registration material on the simulator and occlusal wafer splint for inter-maxillary fixation is easily fabricated. This seems to be another advantage to use the real cast models in ManMoS.
ManMoS was introduced as mixed-reality simulation for orthognathic surgery.
The skeletal change of the jaw osteotomy is simulated on the PC monitor while the occlusal change is confirmed by checking the cast model on the simulator.
It was suggested that the accuracy of the whole simulation system was sufficient for clinical demands.
ManMoS is effective for the simulation of an orthognathic surgery, especially in cases having complicated skeletal and dental asymmetries.
ManMoS appears to meet clinical demands well and is an important facilitator of communication between orthodontists and surgeons.
Written informed consent was obtained from the patients for the publication of this paper and any accompanying images.
mandibular motion tracking system
quartic curve to the mandibular lower border
rapid maxillary expansion
system electronics unit
sagittal splitting ramus osteotomy
virtual cephalometric image
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- Proffit WR, Turvey TA (2003) Dentofacial asymmetry. In: Proffit WR (ed) Contemporary treatment of dentofacial deformity, vol 1. Mosby, St. Louis, pp 574–644Google Scholar
- Tsurumi F, Takagi H, Fushima K (2000) A multivariate analysis for classification of craniofacial morphology in facial asymmetry. Bull Kanagawa Dent Col 28:15–27Google Scholar
- Saito N, Kobayashi M, Fushima K (2009) Skeletal and dental asymmetry in orthognathic case in Japan. Bull Kanagawa Dent Col 37:19–30Google Scholar
- Fushima K, Odaira Y, Saito N, Tsurumi F, Sato S (1998) Dental asymmetry in facial asymmetry. Bull Kanagawa Dent Col 26:15–21Google Scholar
- Minaguchi K, Fushima K, Kobayashi M (2007) Measurement error in a newly developed mandibular motion tracking system. Bull Kanagawa Dent Col 35:129–137Google Scholar
- Fushima K, Kobayashi M, Konishi H, Minagichi K, Fukuchi T (2007) Real-time orthognathic surgical simulation using a mandibular motion tracking system. Comput Aided Surg 12(2):91–104View ArticlePubMedGoogle Scholar
- Sinclair PM, Thomas PM, Tucker MR (1992) Common complications in orthogonathic surgery: etiology and management. In: Bell WH (ed) Modern practice in orthognathic and reconstructive surgery, vol 1. WBSaunders, Philadelphia, pp 48–83Google Scholar
- Arnett GW, Tamborello JA, Rathbone JA (1992) Temporomandibular joint ramifications of orthognathic surgery. In: Bell WH (ed) Modern practice in orthognathic and reconstructive surgery, vol 1. WB Saunders, Philadelphia, pp 522–593Google Scholar