The Accuracy of Conformation of a Generic Surface Mesh for the Analysis of Facial Soft Tissue Changes.

PURPOSE: Three dimensional analysis of the face is required for the assessment of complex changes following surgery, pathological conditions and to monitor facial growth. The most suitable method may be "dense surface correspondence". MATERIALS AND METHODS: This method utilizes a generic facial mesh and "conformation process" to establish anatomical correspondences between two facial images. The aim of this study was to validate the use of conformed meshes to measure simulated maxillary and mandibular surgical movements. The "simulation" was performed by deforming the actual soft tissues of the participant during image acquisition. The study was conducted on 20 volunteers and used 77 facial landmarks pre-marked over six anatomical regions; left cheek, right cheek, left upper lip, philtrum, right upper lip and chin region. Each volunteer was imaged at rest and after performing 5 different simulated surgical procedures using 3D stereophotogrammetry. The simulated surgical movement was determined by measuring the Euclidean distances and the mean absolute x, y and z distances of the landmarks making up the six regions following digitization. A generic mesh was then conformed to each of the aligned six facial 3D images. The same six regions were selected on the aligned conformed simulated meshes and the surgical movement determined by determining the Euclidean distances and the mean absolute x, y and z distances of the mesh points making up the six regions were determined. RESULTS: In all cases the mean Euclidian distance between the simulated movement and conformed region was less than 0.7 mm. For the x, y and z directions the majority of differences in the mean absolute distances were less than 1.0mm except in the x-direction for the left and right cheek regions, which was above 2.0 mm. CONCLUSIONS: This concludes that the conformation process has an acceptable level of accuracy and is a valid method of measuring facial change between two images i.e. pre- and post-surgery. The conformation accuracy is higher toward the center of the face than the peripheral regions.

Reducing cone beam CT scan height as a method of radiation reduction for photorealistic three-dimensional orthognathic planning.

OBJECTIVES: To determine the superimposition accuracy of full-face stereophotographic images with 22 cm and 13 cm cone beam computed tomography (CBCT) scans. MATERIAL AND METHODS: 22 cm CBCT scans and corresponding stereophotographic images (3 dMD) for 30 subjects requiring orthognathic surgery were randomly selected. A 13 cm CBCT scan was generated from the 22 cm scan for each subject. All scans and images were converted into STL format. For each subject, the 22 cm and 13 cm CBCT scans were imported into CAD/CAM software and each superimposed with the corresponding 3 dMD image. A one-sample t-test was used to test the null hypothesis that the difference in the 90th percentile of the mean absolute distance between the two 3dMD images when aligned on the 22 cm and the 13 cm CBCT scans was not clinically significant (<0.5 mm). RESULTS: The 90th percentile of the mean absolute distance between the two 3 dMD surfaces using the 22 cm and 13 cm CBCT scans was significantly less than 0.5 mm (p < 0.001; 0.2 ± 0.2 mm; 95% CI, 0.16-0.30 mm). CONCLUSIONS: There is no difference in the accuracy of superimposition of a stereophotogrammetry image with a 22 cm CBCT scan or a 13 cm CBCT scan. It should now be possible to use a 13 cm CBCT scan and a full-face stereophotogrammetry image during 3D orthognathic planning to reduce radiation exposure.

Recording of natural head position using stereophotogrammetry: a new technique and reliability study.

The purpose of this study was to develop a technique to record physical references and orient digital mesh models to a natural head position using stereophotogrammetry (SP). The first step was to record the digital mesh model of a hanging reference board placed at the capturing position of the SP machine. The board was aligned to true vertical using a plumb bob. It also was aligned with a laser plane parallel to a hanging mirror, which was located at the center of the machine. The parameter derived from the digital mesh model of the board was used to adjust the roll, pitch, and yaw of the subsequent captures of patients' facial images. This information was valid until the next machine calibration. The board placement was repeatable, with standard deviations less than 0.1° for pitch and yaw angles and 0.15° for roll angles.