Surgical simulation and the manufacture of a surgical guide for advancement genioplasty
Using facial CT data, with slices of less than 1 mm, we reconstructed three-dimensional surfaces using the Mimics software (version 14.0, Materialise, Leuven, Belgium), using the CT data imported in DICOM format (Digital Imaging and Communications in Medicine). The patient’s cranio-maxilla region was reconstructed as a three-dimensional image, and then a model of the dentofacial skeleton was generated using the CT DICOM data. We then performed a surgical simulation on the maxilla and the mandible, using three-dimensional cephalometry; at this point, the final surgical treatment plan was established. A genioplasty simulation was then conducted using the 3D model of the mandible, according to the surgical plan. We could now rehearse, in silico, the osteotomy of the mentum, paying close attention to the positions of the mental nerve, the mental formen, and the margin between the osteotomy line and the anterior teeth root apex. The outcome of this simulation allowed us to make any final adjustments to the position of the osteotomy line.
Based on the surgical simulation results, we designed a surgical guide using the Mimics software. First, we drew the initial osteotomy line and moved the copy of initial osteotomy line, superior to its initial position, to accommodate the guide (Fig. 1a); the guide must be approximately 4–6 mm wide to ensure stability. Next, the modeled mandibular mentum was cut at the line indicated by the superiorly displaced osteotomy line. The cut chin segments were then moved anteriorly, according to the amount of movement desired (Fig. 1b). In advancement genioplasty, the distance of this move is set as the anterior-posterior thickness of the surgical guide. Then, the original mandibular data was erased from the cut chin segment using a Boolean function. The dimensions of the surgical device were initially designed after applying the inferior cut to the chin segment, as per the initial osteotomy line (Fig. 1c); the resultant device “segment” was then modeled, having completed the osteotomy virtually (Fig. 1d). Handily, the anteroposterior thickness of the device will ultimately guide anterior movement, and the osteotomy line is set at the inferior border of the device.
A full simulated advancement of genioplasty surgery was then conducted, using the model of the surgical guide (Fig. 1e, f). The 3D surface model of the surgical device was used to create an STL dataset, which was then rendered and manufactured using a 3D printer (Fig. 2). A biocompatible material was used to manufacture the surgical device, as is mandatory for all such devices with an intended medical use. In the present example, the ZPrinter 350 (3D Systems, Inc., Rock Hill, SC, USA) was used, in conjunction with medical materials [8].
Advancement genioplasty using a surgical guide
Having completed the surgery simulations, we progressed to actual surgery. The manufactured device was positioned and fitted to the exposed chin bone during surgery. The “fitting process” is guided by the surgical plan, generated in computer simulations of this phase of the surgery. As mentioned, the anteroposterior width of the surgical guide equaled the desired forward movement of the mandible. Then, mini screws, approximately 2–4 mm longer than the width of the device, were used to fix the device into position; screw positioning depended on the method chosen to stabilize the cut chin bone. For central plate positioning, flanking screws must be used, as in the present example. If the metal plate is to be fixed to the left or right, then only one screw must be used, at the center of the surgical guide. In these cases, the surgeon must pay close attention to ensure that the guide does not slip and rotate, while being attached to the chin bone.
Following confirmation of correct fitting, the osteotomy line was drawn, following the inferior border of the guide to check that the surgical device had not moved during the actual surgery. The mentum of the mandible was then cut using a surgical saw against the inferior surface of the surgical device. When cutting, the depth of the saw was adjusted as per the widths of the surgical device and the bone, as measured during the surgical simulation. In this way, we could cut the chin bone cleanly, without lingual soft tissue damage. The cut chin bone was then displaced to the anterior, as per the anteroposterior width of the surgical device (Fig. 3a). In the present example, the chin bone was cut and moved 6 mm. We then confirmed that the chin bone segment had been moved the correct distance before cutting away the central portion of the device where the metal plate was to be fixed (using a bur or cutter (Fig. 3b)). The chin was then fixed to the plate and its stability confirmed before removing the device (Fig. 3c).
Surgical simulation and manufacturing a surgical guide for reduction genioplasty
The preparation for reduction genioplasty, prior to simulation, was identical to that described for advancement genioplasty. The two osteotomy lines were determined, with line adjustment depending on the desired reduction of the chin region (Fig. 4a). The mandibular mentum was then cut virtually using the two osteotomy lines. The osteotomy was confirmed and compared with the final surgical plan by imaging segmentation (Fig. 4b). The modeled mandibular mentum was then cut at the superior osteotomy line, and the cut segments moved anteriorly, according to the thickness of the device. Again, the surgical guide should have an anterior-posterior length of about 4–6 mm to ensure stability. The original mandibular data set was then deleted, allowing the surgical device to be designed using the initial two osteotomy lines (Fig. 4c). The computer-aided design of the device (Fig. 4d) was then tested in a simulated surgery for reduction of genioplasty (Fig. 4e, f). Following its successful performance in silico, the model of the device was converted to STL format, extracted, and manufactured using a 3D printer [8].
Reduction genioplasty using a surgical guide
For this surgery, the superior edge of the surgical device was set as the upper osteotomy line for the chin bone, and the height of the device equaled the intended reduction of the chin bone. Again, the preparation for the reduction genioplasty operation was identical to that for advancement genioplasty. The mentum of the mandible was cut with a surgical saw, along the inferior border of the surgical device (Fig. 5a). The depth of the saw was adjusted as per the widths of the surgical device and bone, measured during the surgical simulation, so that a clean cut of the chin bone was achieved, without lingual soft tissue damage. The chin bone was then cut along the superior border of the device (Fig. 5b), and both the cut chin bone and device were removed. In the present example, 6 mm of the chin bone segment was cut and removed (Fig. 5c). The superiorly moved chin bone segment was then fixed using a metal plate.