Several anatomical studies shows the prevalence of the MB2 canal in the mesiobuccal root of the maxillary first (and even the second) molar to be as high as 90%. This does not mean that all 90% are clinically treatable but if you have difficulty finding and treating anything less than 75% of your maxillary molars’ MB2 canal it means that you require higher magnification, illumination, and/or better practice in looking in the right place for finding this elusive canal.
The MB2 canal is normally about 2mm lingual, and somewhat mesial to the MB1 canal. This canal is often covered by a dentinal shelf that replicates the external anatomy of the tooth, that is the Cemento-Enamel Junction (CEJ). This shelf should be removed with the aid of ultrasonics or a size 2 slow speed round bur in order to get direct line access to the MB2.
The case shown here is a periapical lesion around MB root of tooth #3 that was referred for apicoectomy. An apico should not be performed in these cases as it does not address the remaining bacteria along the whole length of the missed canal. A conventional endo retreatment with microsurgical exploration to find the MB root should be performed to address the missed MB2 root first with the possibility that an apico may still be necessary (only 10% chance). However, if apico is performed after this retreatment the apico success would be exponentially higher than if it were done without the retreatment first (about 40% vs. 90% success for the apico according to the literature).