Abstract
Inaccurate visualization of the inter-occlusal relationship has raised an important challenge to virtual planning for orthognathic surgery based on cone beam computerized tomography (CBCT). The aim of this study was to evaluate an innovative workflow for orthognathic surgery planning and surgical splint fabrication. The clinical protocol consists of a single cone beam computerized tomography (CBCT) scan of the patient, surface scanning of the dental arches with an intraoral digital scanner, and subsequent fusion of the two datasets. The “virtual patient” thus created undergoes virtual surgery, and the resulting file with the intermediate intermaxillary relationship is used to obtain the intermediate splint by CAD/CAM technology (computer-aided design and computer-aided manufacturing). A proof-of-concept study was performed in order to assess the accuracy and reliability of this protocol. The study comprised two parts: an in vitro evaluation on three dentate skull models and a prospective in vivo assessment on six consecutive patients. Vector error calculation between the virtually simulated intermaxillary position and the intraoperative intermediate intermaxillary relationship revealed high accuracy. The greatest average variation corresponded to the y axis. Compared to previously described methods for obtaining an augmented three-dimensional virtual model, this procedure eliminates the need for dental impressions, simplifies the necessary technical steps and computational work, and reduces the patient’s exposure to ionizing radiation.
The basis for three-dimensional (3D) virtual planning in orthognathic surgery is to obtain a virtual anatomic model of the patient that includes the facial soft tissue mask, underlying bone, and teeth. Although the incorporation of cone beam computerized tomography (CBCT) in conjunction with appropriate computer software and hardware has provided an unprecedented tool for the diagnosis and treatment planning of cranio-maxillofacial anomalies, inaccurate visualization of the inter-occlusal relationship has raised an important challenge to accurate virtual planning for orthognathic surgery.
The reason for this inaccurate visualization of teeth is the surface representation mechanism of CBCT data. In fact, this is an inherent problem of computed tomography technology. Furthermore, orthodontic appliances and dental restorations may cause significant scattering during the scanning process. As a result, a single scan of the patient does not provide adequate occlusal and intercuspation data for precise orthognathic surgery planning. Thus, for a long time, conventional plaster models have been the only way to accurately establish the occlusion and fabricate surgical splints.
Several research groups have studied the incorporation of plaster dental models into physical bone models. These methods have achieved simultaneous representation of bony structures and accurate dentition, but unfortunately they have not been suitable for computerized virtual osteotomies. In 2003, Gateno et al. reported the first clinically applicable method to integrate an accurate rendition of teeth into the computerized 3D skull model. Their method consisted of laser scanning the patient’s dental impressions with fiducial markers and then incorporating this data into the skull, thereby creating a composite skull model.
Subsequently, Swennen et al. developed an original technique to augment the 3D virtual model of the patient with accurate dental data based on a triple scan procedure (a first CBCT scan of the patient, a second CBCT scan of the patient with a double impression tray in the mouth, and a third CBCT scan of the impression tray alone). Both methods eliminate the need for plaster models; in addition, the technique of Swennen et al. eliminates the need for markers.
The aim of this study was to evaluate an innovative workflow for orthognathic surgery planning and surgical splint fabrication. This workflow is based on a single CBCT scan of the patient, surface intraoral scanning of the dental arches, and subsequent fusion of the two sets of data. The ‘virtual patient’ thus created undergoes virtual surgery, and the file with the intermediate intermaxillary relationship (either mandibular or maxillary repositioning in a mandible-first or a maxilla-first bimaxillary surgery context) is used to obtain the intermediate splint by CAD/CAM technology (computer-aided design and computer-aided manufacturing). We designed a proof-of-concept study prior to the implementation of this protocol at our centre, in order to assess its accuracy and reliability. The study comprised an in vitro and an in vivo evaluation.
Materials and methods
In order to systematically assess our protocol for 3D planning in orthognathic surgery, two separate evaluations were conducted.
Part I: in vitro study
The first evaluation was performed on three dry dentate skull models. A CBCT scan of each skull was obtained with the IS i-CAT device, version 17-19 (Imaging Sciences International, Hatfield, PA, USA). The radiological parameters used were 120 kV, 5 mA, scan time 7 s. The axial slice distance for each scan was 0.300 mm 3 . A 23-cm field of view (FOV) was used. Primary images were stored as 576 DICOM data files. The resulting raw file from each skull was segmented with SimPlant Pro OMS software (Materialise Dental, Leuven, Belgium) in order to obtain a ‘clean’ 3D representation, which was then stored as an STL file ( Fig. 1 ).
Subsequently, surface scanning of each skull’s dental arches was achieved with the Lava Scan ST scanner (3M ESPE, Ann Arbor, MI, USA), thereby producing another STL file. The two STL files were fused using the SimPlant Pro OMS software with a ‘best fit’ algorithm. The system used surface-based rigid registration using ICP (iterative closest point) in order to minimize rotational and translational differences between the two datasets. A ‘virtual patient’ was thus generated from each skull’s corresponding pair of STL files.
The same software was then used to perform a bilateral sagittal split osteotomy (BSSO) in each virtual patient. A ‘mandible-first’ protocol was used. Three different virtual scenarios for mandibular repositioning were recreated: skull 1, mandibular advancement; skull 2, mandibular setback; skull 3, mandibular cant correction. Three separate STL files were thus obtained. These files allowed for CAD/CAM fabrication of three intermediate splints out of photopolymerizable resin.
Subsequently, the splints were used to position each mandible in the corresponding intermediate position related to the immobilized maxilla ( Fig. 2 ). Each skull was then ‘intraoperatively’ rescanned with the IS i-CAT ( Fig. 3 ). The three new STL archives thus obtained reflected the three alternatives for mandibular repositioning that had been planned preoperatively.
