Using Free Navigation Reference Points and Prefabricated Bone Plates for Zygoma Fracture Model Surgeries

Abstract

Surgical navigation systems have been an important tool in maxillofacial surgery, helping surgeons create a presurgical plan, locate lesions, and provide guidance. For secondary facial bone reductions, a good presurgical plan and proper execution are the key to success. Previous studies used predetermined markers and screw holes as navigation references; however, unexpected situations may occur, making the predetermined surgical plan unreliable. Instead of determining positions preoperatively, this study proposes a method that surgeons can use intraoperatively to choose surface markers in a more flexible manner. Eight zygomatic fractures were created in four skull models, and preoperative computed tomography (CT) image data were imported into a self-developed navigation program for presurgical planning. This program also calculates the ideal positions of navigation references points for screw holes. During reduction surgery, markers on fractured bone are selected, registered, and calculated as free navigation reference points (FNRPs). The surface markers and FNRPs are used to monitor the position of the dislocated bone. Titanium bone plates were prefabricated on stereolithography models for osteosynthesis. Two reductions with only FNRPs, as well as six reductions with FNRPs and prefabricated bone plates, were successfully performed. Postoperative CT data were obtained, and surgical errors in the six-reduction group were evaluated. The average deviation from the screw hole drilling positions was 0.92 ± 0.38 mm. The average deviation included displacement and rotation of the zygomas. The mean displacement was 0.83 ± 0.38 mm, and the average rotations around the x, y, and z axes were 0.66 ± 0.59°, 0.77 ± 0.54°, and 0.79 ± 0.42°, respectively. The results show that combining presurgical planning and the developed navigation program to generate FNRPs for assisting in secondary zygoma reduction is an accurate and practical method. Further study is necessary to prove its clinical value.

Keywords: Computer-assisted surgery; Fracture fixation; Image processing; Navigation; Registration; Surgical planning; Virtual reality.

Figures

Fig. 1

Generation of FNRPs. a STL image of skull model was imported into navigation program, and dislocated zygoma (red portion) was reduced into its ideal position (green portion). b Transformation matrix was generated automatically from our program, which recorded the virtual displacement and rotation data from manipulating zygoma (hollow arrow). c We selected three points on dislocated zygoma and registered them. Yellow arrow indicates navigation data transmission. d and e In virtual reality, every point coordinate on dislocated zygoma (red dots in d) can be transformed into its reduced position (green dots in e) through matrix transformation (red arrows). Green dots are FNRPs

Fig. 2

Probing procedure. Surgeon used navigation probe to tap already registered point, and relation between the point and its FNRP was shown on screen. Inset shows cone object indicating relative position of registered point in virtual reality; red point near cone object is FNRP of that point

Fig. 3

Use of FNRPs and pre-bent bone plates for zygoma reduction model surgery. a STL image with bilateral zygoma fractures in reduced position. b SLA model produced from STL image. Bone plates were already fixed on it. Screw holes were determined by surgeon. c Registration of screw holes after removal of bone plates and screws. Yellow arrows indicate navigation data transmission. d Transformation matrix (top) and inverse of transformation matrix (bottom). e Inverse matrix can be used to back-calculate screw hole positions on dislocated zygoma (bottom, reddots on red zygoma) from positions that had just been registered (top, red dots on green zygoma). Red arrows indicate coordinate data transformation. f Pre-bent bone plates from SLA model. g Navigation system was used to mark and create screw holes. Surgeon selected three marks on zygoma and registered them (e, yellow dots on red zygoma), and FNRP positions were calculated (e, yellow dots on green zygoma). h Zygoma was reduced with guidance of FNRPs and bone plates

Fig. 4

Distribution of deviation errors for drilling procedure. All deviations were