Assessment of volumetric changes with a best-fit method in three-dimensional stereophotograms

Different three-dimensional stereophotogrammetry systems and analyzing methods exist that often use landmarks for comparison. Measurement errors in landmark or surface comparison are mostly within 1 mm, which seems clinically acceptable. The aim of this study was to validate a three-dimensional stereophotogrammetric best-fit method of assessing volumetric changes and to compare three devices.

The validation of the best-fit method was at first done on a life-size dummy head. Scans were made in the ideal position, as well as in four additional positions, and a scan was made in which a soft putty specimen was added to the dummy head. The comparison was executed with a best-fit method using triangulation. Student's t tests were used to detect statistically significant differences. Second, comparisons were made among scans of a white man in the ideal position and with volume changes added.

The different positions tested for the dummy head showed no significant volume differences within each system or among systems. The differences found when adding a soft putty specimen fell into the same range as the differences between various positions. The differences within a live situation were 10 times greater compared with the dummy-head situation.

In a dummy-head situation, the different systems gave similar results when tested with a best-fit method. However, in live situations the differences may become 10 times greater, possibly due to different facial expressions. These differences may become clinically relevant and, therefore, further research in volumetric changes is needed.