An investigation into fractured surfaces of enamel of modern human teeth: a combined SEM and computer visualisation study.

It has long been recognised that the enamel microstructure may hold important information with regards to phylogeny and masticatory biomechanics. Further, the biophysical and adaptive processes involved in enamel formation and in the creation of different microstructures are poorly understood. This lack of understanding is in part due to technical difficulties when visualising the 3D structure of enamel. Using modern visualisation techniques, models of various regions of different modern human teeth were created. Underlying these models are consistent mathematical representations of the interplay between cell-to-cell adhesion, integrity of the advancing enamel front and (potentially decreasing) constraints on the prism course from the dentino-enamel junction (DEJ) to the outer surface. Seven modern human teeth (I1, 1 lower C, 1 P4, 1 M2, 2 M2 and 1 M3) were fractured longitudinally and formed the basis for the creation of the models. For validation purposes the teeth were then fractured transversely, thus allowing quantitative comparisons between the prism pathways on the newly fractured transverse plane and the transverse pathways as predicted by the model. It was found that these predictions were fairly accurate provided that (a) the light position with respect to the model corresponds with the beam position with respect to the scanned surface and (b) the path of prisms was carefully reconstructed/extrapolated from SEM in cases where prisms were broken. Given that these predictions were based on the mechanisms governing enamel formation as applied to the model, it is suggested that such theories must be reasonable. In other words, biophysical processes, rather than complicated (genetic) positional information, suffice to create different enamel microstructures. In addition, systematic differences were found in prism deviation from their c-axis in different enamel pieces. Given the nature of these differences it is suggested that enamel formation is not only the result of biophysical processes (proximal causes), but could be due to the structures having been selected for in order to counteract masticatory stress exerted during the lifetime of the species (ultimate causes). As to whether and to what extent this may be the case is not yet clear but it is apparent that computer visualisation does have potential to quantify enamel microstructure and to address such questions. Given its non-destructive nature, computer modelling could have particular relevance for studying fragmented fossilised remains.