In the past, I have spoken about the anatomy of posterior teeth but I want now to talk about mandibular incisors. A patient walks into your office and you determine that he or she needs root canal treatment on a mandibular incisor. You may think “Piece of cake!” after looking at the radiograph, which appears to have a single narrow canal. Yes, you are correct that mandibular incisors usually (97-98%) have a single canal but these single canals tend to be wide labiolingually. Therefore, you need to make an oval (or triangular) access at the lingual shoulder to make sure you get direct access to the canal and instrument the canal well labiolingually.
If there is a second canal, which occurs 2-3% of the time, it is usually right beneath the main canal. Meaning, the lingual canal is usually missed. The shoulder conceals the orifice to the second canal, so you need to open up the access more lingually. This happened in my practice recently when a 49-year old patient presented to me with occasional mild discomfort to #23. The general dentist told me that he did the root canal on this tooth about four years ago (Figure 1) but was concerned about the radiolucency that was still present (Figure 2) and the patient’s symptoms. I decided to take a CBCT scan to determine why this tooth was not healing properly and it was clearly seen (Figure 3 and 4) that there was a missed canal. Therefore, I decided to re-treat (Figure 5) instead of doing an apicoectomy, which would have failed since the second canal was completely missed.
This is to show you that although the post-op x-ray of these mandibular incisors may look great, there can be a missed canal. So make sure you extend the access lingually and check for that second canal.