Changes in face topography from supine-to-upright position-And soft tissue correction values for craniofacial identification

Soft tissues of the human face hang from the skull under the downward vector of gravity. Subsequently, the fall of the tissues is not likely the same between supine, prone or upright positions with ramifications for soft tissue measurements such as average soft tissue thicknesses used in craniofacial identification. Here we use high-resolution Dimensional Imaging® DI3D stereo-photographs (Glasgow, Scotland) to map the shape change between upright and supine position in the same 62 participants and encode the surface shell differences as greyscale pixel intensity values. Statistical tests were conducted using MANOVA at 31 capulometric landmarks, with posture as the independent factor in a repeated measures design, and sex, somatotype and age (two groups of <50 and>50 years) as independent factors in a between subjects design. Results indicate that facial morphology changed in characteristic fashion between the positions: when supine, the soft tissue extruded inferior and lateral to the eyes (Δmin = +1.2 mm; Δmax = +3.0 mm, p < 0.05) and retracted lateral to the mouth and around the nasolabial fold (Δmin = −1.0 mm; Δmax = −2.4 mm, p < 0.05). These patterns were more marked in older subjects (posture = p < 0.01, η2 = 0.55; and age = p < 0.01; η2 = 0.29). By calculating mean heat maps for the faces, this study clearly demonstrates that posture influences the cheeks/eyes as well as the nasolabial fold, thereby holding broader ramifications for face morphology than previously reported. Since many prior facial soft tissue thickness studies report data for supine subjects, correction factors are provided for converting supine facial soft tissue thickness data to upright estimates. Out-of-sample performance tests of posture-corrected supine means derived from two CT samples (using upright B-mode ultrasound data from living subjects as ground truths) confirmed the utility of the correction factors for landmarks that fall in zones affected most by the posture change (lower standard errors after correction). The standard error improvements were −0.9, −0.6, −0.5, and −1.4 mm respectively for the mio-mio′, go-go′, zy-zy′ and mr-mr′ landmarks (reductions indicated by the negative sign).