Challenges of photogrammetric intra-oral tooth measurement

Recording the surface shape of a living tooth would seem to be a straightforward photogrammetric task, but the researchers' experiences in developing a procedure for routine intra-oral tooth measurement shows that, in practice, photogrammetric measurement of the living tooth in the mouth has difficult challenges which are predominantly photographic. Although the problem of access can be overcome by using specialist intra-oral dental cameras, arranging multiple images is problematic. A second challenge arises because of the optical characteristics of dental enamel: The tooth surface is featureless and is unsuitable for photogrammetric mapping without some augmentation, and the enamel is also highly reflective. Some imaging tests have been carried out on teeth in the mouth in the search for an automated measurement technique, but, to avoid patient discomfort, further investigations have been carried out on an extracted tooth, whose characteristics are similar to those of the live tooth. In an example cited here, a pair of images of an extracted frontal incisor was collected with a single camera, a camera base of about 6 mm and a base-To-height ratio of 1:2. To make the enamel surface both opaque and textured, it was painted with a weak water colour solution. Features detected with an interest operator were matched by area-based matching, and coverage was of acceptable accuracy and density except in areas of illumination glare. The measurement should be repeatable with living teeth by duplicating the conditions of the photography, but procedures with living patients are noticeably more awkward than working with inert objects.