This is a case where, based on the X ray image of a “J” type lesion in combination with a deep narrow periodontal probing, one could easily think of a cracked tooth. However, when observing under high magnification, no crack could be seen from within the root canal mesial wall. This J shaped radiolucency was in fact a narrow desmodontal sinus tract originating from an endodontic infection. Saving that tooth implied a retreatment, a ledge bypass and a few calcium hydroxide dressings replacements. As shown on those control post operative X ray images, a slow but complete periradicular tissues regeneration occured. In this case study, even a CBCT 3D imaging would have shown a deep narrow bony defect that could have misled the practionner to conclude the presence of a cracked tooth. Direct observation under a dental operative microscope showed us otherwise, proving once more how micro dentistry is elevating endodontic care to a higher level.