All subjects had reduction malarplasty independently or simultaneously with orthognathic surgery. As described below, the subjects underwent 3D surgical diagnosis for facial deformity/asymmetry and surgical simulation, the surgical guide then being designed using CAD/CAM and 3D printing. The process of the preoperative preparation and postoperative validation is introduced in flow charts (Figs. 1 and 2). This study was approved by the Institutional Review Board.
Surgical simulation and production of a 3D-printed CAD/CAM surgical guide for reduction malarplasty in facial asymmetry
The facial CT data were obtained with less than 1 mm of slice thickness and 512 × 512 of image resolution, then reconstituted into a 3D image set by importing Digital Imaging and Communications in Medicine (DICOM) files into Mimics (version 18.0, Materialise, Leuven, Belgium). A 3D skeletal model was produced to include zygoma and maxillofacial structure.
The maxillary and mandibular dental models were scanned using the optical 3D scanner Rexcan DS2 (Solutionix, Seoul, Korea), their stereolithography (STL) model data being produced for a digital dental model. Using the 3-matic program (version 10.0, Materialise, Leuven, Belgium), the maxillary and mandibular teeth on the 3D skeletal model were replaced by the digital dental model using sequential point- and volume (or surface)-based registration (Fig. 3 A) [8].
The orthognathic surgical simulation was performed to set an appropriate maxillary/mandibular position and facial shape based on clinical evaluation as well as on 3D cephalometry [9], and to construct final occlusion, which was manually set and digitally scanned on the plaster models, and the midsagittal plane, which was constructed with three landmarks and confirmed clinically [10, 11]. The mandible was first set using the predicted postoperative final occlusion and the maxillomandibular complex was positioned for proper skeletal symmetry and balance during the simulation [9]. A horseshoe-shaped orthognathic surgical guide or wafer was designed based on the simulated preoperative mandibular and postoperative maxillary occlusion. An orthognathic maxillary cutting guide was also designed to enable the maxillary cut and interference removal for maxillary repositioning at the maxillary anterior wall near the zygomatic cut (Patent cooperation treaty (PCT) KR2014/010282; Korean patent 10-1478009) [12]. The reference hole for repositioning the maxilla also served to place the cutting guide for reduction malarplasty. During the orthognathic surgery, a 3D-printed surgical wafer was placed after the Le Fort I osteotomy of the maxilla, the 3D-printed cranium-based surgical guides then being connected for more precise movement of the down-fractured maxillary segment to the predicted position (Fig. 3B; PCT KR2014/010284; Korean patent 10-1501447) [13].
The malar shape and its symmetry were evaluated to perform zygomatic simulation surgery with and without orthognathic simulation. The less prominent side of the zygoma was mirrored to the other side along the midsagittal plane and their shapes were compared (Fig. 3 C). The midsagittal plane was manually constructed based on three landmarks, including the orbit and center of the foramen magnum, and clinically adjusted if necessary [11]. The more prominent area of the zygomatic body was positioned for surgical cut, and a one- or two-cutting plane was designed using 3-matic software (Fig. 3D). Care was taken not to expose the maxillary sinus, not to break the continuity of the zygomatic process, and not to damage the zygomaticofacial nerve.
The reduction malarplasty was simulated in a 3D model based on the previously designed cutting plane and the surgical guide outline was designed with cylinders positioned for the reference and stabilizing screws or pins (Fig. 3E). The osteotomy could be simulated using the cutting plane, thus controlling the location and depth of the osteotomy and therefore the amount of bone removal. The surgical guide was designed with careful consideration of device stability and intraoperative visibility. It covered the zygomatic and upper maxillary area and had a cutting slot for accurate cutting location and direction (Fig. 3 F). It also had one or two screw holes for the immobilization of the guide on the zygoma, particularly during the sawing procedure. It had an optional design of shelf-shaped foot-extension, which was to be hooked on the superior border of the zygomatic arch and/or medial side of the zygomaticofrontal process for accurate positioning without a reference point. It can have an additional one foot-extension with a screw hole located at the same reference point produced by the previously described maxillary cutting guide to assist in accurate positioning of this guide in simultaneous orthognathic surgery. The morphological comparison of zygoma after the simulated reduction malarplasty was performed by our new development (Fig. 3G) and the traditional technique (Fig. 3 H).
The zygoma guide without a common reference hole could be positioned based on the superficial contour of the maxillary and zygomatic bones, assisted by the previously described hooking stability of the device’s small extension onto the zygomatic arch or zygomaticofrontal process. It also had a window opening at the lower border to confirm proper positioning by observing intimate contact of the guide with the zygomatic bone. The designed surgical guide was exported in STL data format for 3D printing with a biocompatible material (ProJet 3500 HDMax 3D Printer, 3D Systems, Inc, Rock Hill, SC). The device design was registered to Korean patent (10-1514237).
Reduction malarplasty using simulated surgical guide
The surgical guide for the zygomatic reduction was placed in the zygomatic area after exposing the surgical field by extended intraoral vestibular incision and dissection, stability and conformity to the preplanned location having been confirmed. One or two screws or pins 6–8 mm in length were introduced to fix the guide onto the zygomatic bone surface (Fig. 3I). The osteotomy line was pre-drawn by surgical drill to allow for positioning of a pre-planned osteotomy line while the surgical guide was monitored for inadvertent movement. Controlled reciprocal sawing was performed using the tailored cutting slot of the surgical guide, cut depth being controlled by the length of the saw blade. The surgical guide was removed after the initial sawing, additional reciprocal sawing being performed in the osteotomy line. The zygomatic bone segment was finally cut and retrieved by applying the osteotome, the sharp bony edges around the osteotomy line then being trimmed with burs.
Setting measurement points and measuring errors
Measurements were made at several regional points of the zygoma to evaluate surgical accuracy and morphological differences between the preoperative simulation and postoperative 3D models (Fig. 4 A-E). The simulated and operated 3D craniofacial models of a subject were first superimposed based on the surface registration of the unchanged cranial part (Fig. 4 F).
For comparison of the preoperative (Fig. 4G) and the one-year postoperative (Fig. 4 H) facial appearances, being focused on the contour of the zygoma, of the same subject for simulations and surgery, five reference points were marked on the zygomatic surface of a 3D model: center (C), upper (frontal; A), lower (maxillary; E), anterior (orbital; B), and posterior (zygoma arch; D), based on preoperative planning and immediate postoperative evaluation (Fig. 4I). The shortest 3D distance in absolute value between the two models at a reference point was calculated as the length of a normal line drawn from the reference point of a model to the surface of another model by software function.
One of the authors (KSH) did all the pointing on and measurements of the surgical simulation and postoperative models in order to avoid inter-individual errors. 3D distance between corresponding points from two models was considered surgical error in our reduction malarplasty. We analyzed such errors and verified their significance by Kruskal-Wallis test with a significance level of 0.05, Dunn’s multiple comparisons test, and Bland-Altman analysis using IBM SPSS Statistics 23 (IBM Corp., Armonk, NY, USA).
In order to measure possible errors arising from the measurement process as method error, one author repeatedly (10 times) set a reference point at the zygoma center. In addition, another possible methods error, which could develop during the superimposition process of the cranial structures, was evaluated: two identical cranium models of 5 different subjects were superimposed independently using the same method in this study, and the inter-surface distances were measured and statistically analyzed.