Superimposition of 3D cone-beam CT models of orthognathic surgery patients

Abstract

Objectives: To evaluate the registration of 3D models from cone-beam CT (CBCT) images taken before and after orthognathic surgery for the assessment of mandibular anatomy and position.

Methods: CBCT scans were taken before and after orthognathic surgery for ten patients with various malocclusions undergoing maxillary surgery only. 3D models were constructed from the CBCT images utilizing semi-automatic segmentation and manual editing. The cranial base was used to register 3D models of pre- and post-surgery scans (1 week). After registration, a novel tool allowed the visual and quantitative assessment of post-operative changes via 2D overlays of superimposed models and 3D coloured displacement maps.

Results: 3D changes in mandibular rami position after surgical procedures were clearly illustrated by the 3D colour-coded maps. The average displacement of all surfaces was 0.77 mm (SD=0.17 mm), at the posterior border 0.78 mm (SD=0.25 mm), and at the condyle 0.70 mm (SD=0.07 mm). These displacements were close to the image spatial resolution of 0.60 mm. The average interobserver differences were negligible. The range of the interobserver errors for the average of all mandibular rami surface distances was 0.02 mm (SD=0.01 mm).

Conclusion: Our results suggest this method provides a valid and reproducible assessment of craniofacial structures for patients undergoing orthognathic surgery. This technique may be used to identify different patterns of ramus and condylar remodelling following orthognathic surgery.

Figure 1

Visualization tool used (Insight SNAP) for visualization of 185 axial, 228 lateral (sagittal), and 203 anteroposterior (coronal) cross-sections for each CT image acquisition. A frontal view of the 3D surface models displays the segmentation of all slices stacked together without any smoothing

Figure 2

Demonstration of superimposition of pre- and post-surgery models of a case treated with maxillary advancement and mandibular setback. This case is shown for illustration only and was not included in this study sample. The surface of the cranial base was used for registration performed with MIRIT. Note that the cranial base colour map is green (0 mm surface distance), showing adequate match of the before and after models for the cranial base Structures

Figure 3

Lateral views of 3D models of a patient treated with maxillary advancement and mandibular setback. The model labelled in red was constructed from a CBCT image acquired 1–2 weeks before surgery. The model labelled green was constructed from a 1 week post-surgery CBCT scan. The other anatomical structures are masked for better visualization of changes in the mandibular ramus and condyle. In the semi-transparent superimposition of mandibular models, red shows the pre-surgery model, blue shows the area where the pre- and post-surgery models overlap, and green shows the post-surgery

Figure 4

Lateral view of regions of interest in the mandibular rami, specifically the condyles (red) and the posterior border (green)

Figure 5

Lateral view of the surface distances between pre- and post-surgery mandibular models of the same patient constructed by three different observers to assess interobserver variability. Note the similarity of the colour maps

Figure 6

Ten subjects were treated with maxillary advancement only. Note that only one subject had a maximum surface distance change 2 mm (red along the posterior border of the rami). For all other patients, mandibular surface displacement was minimal