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The aim of this study was to evaluate the reproducibility of the head position for a three-dimensional soft tissue laser scan (lasergraph) using thin-plate splines, for orthognathic surgery planning and follow-up. 60 laser scans of five subjects (12 scans per subject) were obtained at specified intervals. The head was positioned in the lateral view using a spirit level, an engineering device for setting horizontal surfaces, to adjust the Frankfort horizontal plane parallel to the ground. The projection of a narrow beam of longitudinal laser light was used to adjust the axial plane for the frontal view. These scanned images (lasergraphs) were digitised and the co-ordinates of the landmarks recorded. The digitised laser scans were compared using thin-plate splines analysis. The mean difference between the scans due to variations in head position was 0.0135 +/- 0.0109 g x cm2/ sec2 in the lateral view and 0.0090 +/- 0.0054 g x cm2/sec2 in the frontal view. This represents an overall distortion error of less than 2% when following up the surgical change of a typical bimaxillary osteotomy case with 6 mm maxillary advancement and 3 mm mandibular set-back. It is concluded that facial laser scans (lasergraphs) with the Frankfort horizontal plane set using a head rest and spirit level, and the axial plane set using projection of a vertical laser light on the facial midline, are highly reproducible.

The use of a facial midline indicator (FMI) to transfer the midline and long axis of the face from the patient to the articulator for orthognathic planning is described.