Abstract
The authors present a new procedure of computer-assisted genioplasty. They determined the anterior, posterior and inferior limits of the chin in relation to the skull and face with the newly developed and validated three-dimensional cephalometric planar analysis (ACRO 3D). Virtual planning of the osteotomy lines was carried out with Mimics (Materialize) software. The authors built a three-dimensional rapid-prototyping multi-position model of the chin area from a medical low-dose CT scan. The transfer of virtual information to the operating room consisted of two elements. First, the titanium plates on the 3D RP model were pre-bent. Second, a surgical guide for the transfer of the osteotomy lines and the positions of the screws to the operating room was manufactured. The authors present the first case of the use of this model on a patient. The postoperative results are promising, and the technique is fast and easy-to-use. More patients are needed for a definitive clinical validation of this procedure.
Genioplasty plays an important role in harmonising facial proportions and profiles, but planning it is difficult because of the limited means of diagnosis, planning and transferring information to the operating room. Specifically, the theoretical virtual anteroposterior and vertical positions of the chin are reduced to the landmarks menton, B point, and pogonion on two-dimensional cephalograms . The authors propose and describe the combined use of three-dimensional (3D) cephalometry, a 3D rapid-prototyping model, and pre-bent titanium plates as a new means of computer-assisted genioplasty.
Method
A young adult patient presented after orthodontic treatment had been completed elsewhere. Clinical examination showed the patient still had a retrusive profile and refused any orthognathic treatment for the occlusion ( Fig. 1 A ). The authors proposed an advancement genioplasty. Approval was received from the local ethics committee (B40320084307) for the clinical application of 3D cephalometric analysis, and the patient gave informed consent for the study. A low-dose CT scan of the head was performed , from which the authors determined the anterior, posterior, and inferior limits of the chin with the newly developed and validated 3D cephalometric planar analysis (ACRO 3D) ( Fig. 1 B) . The osteotomy lines were planned and visualized with Mimics software (Materialize, Leuven, Belgium). The upper osteotomy line was positioned at a distance of at least 5 mm from both mental foramina. The amount of movement was planned virtually with Mimics software in relation to the reference planes from the ACRO 3D analysis ( Fig. 1 B). The authors then built a 3D rapid-prototyping model (RPM) from the low-dose CT scan with a 3D printer (Z-Corp, Burlington, USA) . The 3D RPM was presented as a multi-position 3D model ( Fig. 2 ) with initial, intermediary, and final positions of the bony slices of the chin. The final position of the 3D RPM was used to pre-bend the titanium plates and to indicate the positions of the holes for screws corresponding to the pre-bent plates. The 3D RPM model was repositioned to its initial position and an acrylic surgical guide was made. The role of the surgical guide was to transfer the position of the holes for the screws (in their final position) and the osteotomy lines. The surgical guide was sterilized at 120 °C for 20 min in an autoclave. The pre-bent titanium plates were sterilized . During surgery and before the osteotomy, the acrylic guide was positioned on the patient’s osseous chin. The holes for the screws were drilled through the surgical guide ( Fig. 3 ). With a sterilized pencil the authors drew the osteotomy lines based on the surgical guide ( Fig. 3 ). Osteotomy cuts were performed following the pencil tracings. After complete separation of the bony fragments, the pre-bent plates were positioned and screwed in to place ( Fig. 4 ). A low-dose CT scan showed a good result for the patient’s profile ( Fig. 1 C).
