The coronal portion of the developing upper third molar may be impacted by the developing roots of the second molar. At the relevant age – roughly from 10 to 13 years – both organs are largely soft tissues when and where they collide. It is obviously not possible to obtain tissue from both teeth whilst they are developing, but a frequent after-effect of such collisions is extraction of the third molar. Prior to the Human Tissue Act 2004, we were free to collect these from collaborating general dental practitioners and such were fixed and stored in 70% ethanol. From an archive of >1,000, we selected an initial 5 with obvious indentations of the mesial side of the crown. These were scanned using high contrast resolution X-ray MicroTomography on the MUCAT-2 Time Delay Integration scanner developed at QMUL (90kV, 180µA, ~18 hours scan duration, 15 micron voxel size: XMT). 3D visualisations were made using Drishti (Australian National University). Teeth were then cleaned of non-calcified soft tissue surface layers by treatment with either a bacterial protease enzyme detergent or hypochlorite bleach, washed, dried and examined unmounted and uncoated in a Zeiss EVO-MA10 SEM operated at 50Pa chamber pressure, 20kV, using backscattered electrons (BSE-SEM). Enamel was missing from mid-lateral and cervical regions of crowns with severely indented mesial surfaces, and, where this occurred, underlying dentine was also thin. Patches of osteoclastic resorption of completed, post-mature enamel surfaces were also frequent in these regions. Both enamel-free and resorbed enamel surfaces were covered with thin layers of Sharpey-fibre rich cementum. Deep fissures, local focal areas of hypoplasia and surface-overlapping projections of enamel added after normal enamel completion were all frequent. The mesio-buccal root of the teeth studied to date was split into buccal and palatal divisions: this, of course, must have happened at a much later stage of development. We show here how the aftermaths of collisions between developing tooth germs can be reconstrued from combined XMT and BSE-SEM studies long after the events.
Ethics statement: the teeth that we studied came from an anonymised archival collection of third molars extracted before 2000.