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Gao Y, Shen Y et al. Aust Endod J 2015 10.1111/aej.12103

This was a 'virtual' experiment and of elegant design. The investigators used micro-computed tomography to measure the thickness of remaining dentine, including that area adjacent to the 'danger zone', following the removal of a 'virtual' fractured file in each of 37 extracted and then scanned maxillary first molar teeth. The 'danger zone' is the distal concavity in the root between the first and second mesiobuccal canals. The method that was used to remove the 'virtual' fragment comprised creating a staging platform with a Gates Glidden bur and then using ultrasonic tips to trephine around and expose the coronal 1.5 mm of the fragment. If a size 25 file was fractured at a depth of 5 mm from the canal orifice, the thickness of dentine at the 'danger zone' was reduced from some 0.7 mm before recovery to 0.4 mm (368 ± 161 μm). It has been estimated by others, that when the thickness of dentine in less than 300 μm, condensation forces generated by obturation may exceed the structural integrity of the root dentine.