Abstract
The purpose of this study was to develop and validate a new chin template system for a two-piece narrowing genioplasty. Nine patients with wide chin deformities were enrolled. Surgeries were planned with the computer-aided surgical simulation (CASS) planning method. Surgical splints and chin templates were designed in a computer and fabricated using a three-dimensional printing technique. The chin template system included a cutting guide and a repositioning guide for a two-piece narrowing genioplasty. These guides were also designed to avoid the mental foramen area and inferior alveolar nerve loops during the osteotomy, for nerve protection. After surgery, the outcome evaluation was completed by first superimposing the postoperative computed tomography model onto the planned model, and then measuring the differences between the planned and actual outcomes. All surgeries were completed successfully using the chin template system. No inferior alveolar nerve damage was seen in this study. With the use of the chin templates, the largest linear root mean square deviation (RMSD) between the planned and the postoperative chin segments was 0.7 mm and the largest angular RMSD was 4.5°. The results showed that the chin template system provides a reliable method of transfer for two-piece osseous narrowing genioplasty planning.
Osseous genioplasty procedures are used widely to correct chin deformities. These procedures are challenging because the chin deformity may exist in all three dimensions; thus it is critical not only to make a correct diagnosis and surgical plan for the genioplasty, but also to transfer the plan precisely to the patient at the time of surgery. The location of the osteotomy and the movement of the bony segment will directly impact the surgical outcome. In a monoblock genioplasty, the bony segment may be controlled simply using a holding screw and repositioned based on intraoperative measurements. However, in a segmentalized genioplasty (e.g., narrowing genioplasty), it is rather more difficult to precisely perform an osteotomy and reposition the two chin piece segments correctly following the surgical plan.
With the rapid developments made in computer-aided surgical simulation (CASS) technology, computer-aided design/computer-aided manufacturing (CAD/CAM) surgical templates are now used by surgeons as a guide to assist them in performing an accurate osteotomy and in moving the bony segment to the desired position, exactly as planned in the computer. There are a few reports in the literature on the use of genioplasty templates and these have indicated good surgical outcomes. However, the design of these templates is bulky, they are difficult to use intraoperatively, and they can only be applied to a monoblock genioplasty. Therefore, the purpose of this study was to develop and validate a new chin template system for a two-piece osseous narrowing genioplasty. This template system includes two surgical guides: a cutting guide that defines the osteotomy cutting line and the screw holes prior to the osteotomy, and a repositioning guide that precisely repositions the two chin piece segments into the planned position and orientation using the predefined screw holes.
Materials and methods
Nine Chinese female patients (median age 22 years, range 18–27 years) with wide chin deformities were enrolled in this study between April and November 2014. Asian women often complain of a prominent masculine chin and square contour of their face. A narrowing genioplasty is a desirable procedure to produce a more feminine facial contour with a slenderer lower third of the face. Therefore, the indication for a two-piece narrowing genioplasty was determined based on the patient’s complaint and the doctor’s clinical aesthetic assessment.
Inclusion criteria for the study encompassed (1) patients who were scheduled to undergo a two-piece narrowing genioplasty as a part of their treatment, (2) patients who were scheduled to undergo a computed tomography (CT) scan as a part of their diagnosis and treatment, and (3) patients who agreed to participate in the study. Patients with a craniofacial syndrome, those who had undergone a previous osseous genioplasty, those with a previous mandibular trauma, and patients requiring only a monoblock genioplasty were excluded. The study was approved by the ethics committee of the hospital prior to initiation. Informed consent was obtained from each patient before enrollment. The main reason for enrolling only female patients is that it is principally Asian women who complain about a prominent masculine chin and square contour of the face and who therefore desire a narrowing genioplasty to produce a more feminine facial contour.
Surgical planning following the CASS protocol
A CT scan of the patient’s head was acquired preoperatively (GE Healthcare, Fairfield, CT, USA). A wax bite was used to slightly separate the maxilla and mandible. The axial slice was taken at a thickness of 1.25 mm with the patient in a supine position. The CT data were imported into planning software (ProPlan 1.4; Materialise NV, Leuven, Belgium) to generate three-dimensional (3D) maxillary and mandibular models. The digital dental models were generated by scanning a set of stone dental models using a high-resolution laser surface scanner (Smartoptics AS, Bochum, Germany). The digital dental models were then imported and merged into the 3D skull model to replace the less-than-accurate CT teeth. This resulted in a computerized composite skull model with accurate rendition of both the bony structures and the teeth.
The composite skull model was then positioned in a unique reference frame. In this study, nasion was defined as the origin of the reference frame for the composite skull model, with the x -axis running in the mediolateral direction, the y -axis in the anteroposterior direction, and the z -axis in the inferosuperior direction. The axial (XOY) plane was a plane parallel to the Frankfort horizontal (defined by the averaged right and left porion and orbitale), dividing the head into upper and lower parts. The midsagittal (YOZ) plane was a vertical plane dividing the head into right and left halves. Finally, the coronal (XOZ) plane was a vertical plane perpendicular to the other two planes.
After the reference frame of the composite skull model was established, a maxillary Le Fort I osteotomy, bilateral mandibular ramus sagittal split osteotomies (BSSRO), and a narrowing genioplasty were simulated in the computer based on clinical examination, cephalometric analysis, and 3D measurements, following the standard planning routine. In addition, during the planning of the two-piece narrowing genioplasty, the inferior nerve canal positions were marked on the mandibular model in order to protect the inferior alveolar nerves ( Fig. 1 ). Once the surgical plan was finalized, the surgical splints and genioplasty templates were designed in the computer using 3-matic software (Materialise NV, Leuven, Belgium) and fabricated using a 3D printing machine (3D Systems, Rock Hill, SC, USA). The surgical splint was designed following the same routine. This splint was used to position the maxillary Le Fort I and mandibular distal segments intraoperatively. The template for the two-piece narrowing genioplasty was also designed in the computer with this new method. These were used intraoperatively for the two-piece narrowing genioplasty. Details are given below.
Two-piece narrowing genioplasty template system
To design the two-piece narrowing genioplasty templates, the 3D models of the distal mandible and the chin segment in both their initial position and final position are used. This new genioplasty template system includes two surgical guides: a cutting guide and a repositioning guide. The cutting guide is used to assist the surgeon in performing the osteotomy and for pre-drilling the screw holes ( Fig. 1 ). During the design of the cutting guide, all 3D models of the bony segments are located in their original positions. The upper portion of the cutting guide is designed like a dental splint, serving as a locking mechanism to firmly attach the whole cutting guide onto the mandibular teeth. The lower portion of the guide is designed to indicate the cutting lines for the two-piece genioplasty and the trajectory of the cutting plane (indicated by the thickness of the cutting guide). It is important to note that the cutting guide should not be extended to the mental foramen area. In addition, six screw-hole drilling guides, two on the distal mandible and two on each chin segment, are designed on both sides of the osteotomy cutting lines ( Fig. 1 ). These screw holes on each bony segment serve as bony reference landmarks for automatic repositioning of the chin segments in the next step. Finally, a solid vertical bar is designed to rigidly connect the upper and lower portions of the cutting guide.
The repositioning guide is used to automatically reposition the two chin segments into their final positions ( Fig. 2 ). During the design process, both the original and the planned positions of the two chin segments are used. The upper portion of the repositioning guide is designed to firmly attach the whole guide to the distal mandible using the two screw holes created previously by the cutting guide. The lower portion of the repositioning guide includes four repositioning screw holes, two for each chin segment, in their final planned positions. In order to achieve this, the chin segments are first positioned in their original locations. The screw holes created by the cutting guide are linked digitally to their corresponding chin segments. Each chin segment is then moved into its planned final position, bringing the screw holes along with it as an integrated unit. At the time of the surgery, the new location of the screw holes on the repositioning guide will automatically bring the chin segment into its final planned position as the screws are placed into the appropriate screw holes and tightened ( Fig. 2 ).
Surgery
All surgeries were performed by a single attending surgeon who is experienced in orthognathic surgery. During surgery, the CAD/CAM occlusal surgical splints were used to place the maxilla and mandible into the final planned positions as routine.
All patients underwent a two-piece narrowing genioplasty using the new template system. The anterior surface of the chin was exposed as for a routine genioplasty procedure via intraoral approach. The first guide, the cutting guide, was positioned on the mandibular dentition exactly as planned in the computer. The lower portion of the guide was then attached firmly to the chin with six screws using the drilling holes designed on the cutting guide ( Fig. 3 A) . Once the two-piece genioplasty osteotomy cutting lines were marked with a surgical saw as planned, the cutting guide and the screws were removed and the osteotomy continued ( Fig. 3 B).
After the osteotomy was completed, the second guide, the repositioning guide, was installed firmly onto the distal mandible by aligning the two screw holes on the guide to the two corresponding screw holes on the distal mandible ( Fig. 4 A) . Afterwards, each of the osteotomized chin segments was moved and rotated until the two corresponding screw holes on the chin segment and the guide were aligned. As the screws were placed and tightened, the chin segment was automatically moved into its final planned position and secured temporarily. Each chin segment was then stabilized by rigid fixation ( Fig. 4 B), following which the guide and associated screws were removed ( Fig. 4 C) and the surgical wound closed as usual.
Outcome evaluation
A CT scan was acquired 3 days postoperatively. The postoperative CT scans represented the actual surgical outcomes. The outcome evaluation was started once all postoperative CT scans had been obtained. The accuracy of this new two-piece narrowing genioplasty template system was assessed by comparing the actual postoperative outcomes to the planned outcomes.
The postoperative 3D maxillary and mandibular models were generated using the same planning software (ProPlan). The postoperative 3D models were then imported into the same design software (3-matic). The outcome evaluation was completed by first digitizing a group of anatomical landmarks on the planned and the postoperative models. The differences in position and orientation between these landmarks were calculated. Details of the evaluation procedure are described below.
The premise was adopted that three points are sufficient to define the position and orientation of an object in 3D space. Three corner points were digitized on each chin segment while in its planned final position ( Fig. 5 ). The landmarks utilized were (1) the superomedial corner point on the buccal surface of the chin segment (point 1); (2) the inferomedial corner point (point 2), similar to menton; and (3) the posterolateral corner point on the buccal surface (point 3).
A ‘reversed’ routine developed by Xia et al. and Hsu et al. was used to ensure the correspondence of landmarks between the planned and the postoperative chin segments. First, the three landmarks were ‘glued’ onto the planned chin segment. Second, using the surface-best-fit method, each chin segment was registered to the corresponding postoperative chin segment. The three landmarks were also brought along with the chin segment accordingly. Third, the three landmarks on the planned chin segment were duplicated. The duplicated landmarks became the landmarks for the postoperative chin segment. Finally, the planned chin segment together with its landmarks was moved back to the originally planned position. Then the postoperative chin segment was registered to the planned chin segment using the surface-best-fit method. The coordinates of all of the landmarks in the planned and postoperative positions were recorded.
The linear differences were calculated using the centroids of the planned and postoperative chin segments. The centroid coordinates ( x c , x c , x c ) of each chin segment were computed using the following equations:
x c = x 1 + x 2 + x 3 3 ; y c = y 1 + y 2 + y 3 3 ; z c = z 1 + z 2 + z 3 3 ,
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