Here is an interesting case that is classified as Dens Invaginatus Type 3. The chief characterstics of this anomaly is the enamel lined tract that connects the crown to the root and invaginates the apical or lateral aspect of the root. 
Commonly the root canal that houses the vital pulp is unaffected. This invagination of the enamel tract however, pushes the root canal laterally. Typically the enamel tract opens coronally , gets contaminated, and results in a infection in the bone where it opens apically. Because the enamel tract opens into the “sac” or “pouch” apically, the infection rapidly forms a lesion.
However the vital pulp in the root canal survives as it is not in communication with either the enamel tract or at the periapex.
Treatment may include:
1. Treating the enamel tract orthograde with long term CaOH apexification till the pouch closes. 2. Treating the enamel tract orthograde as well as sealing the apical pouch with root end surgery.
I prefer to do the second approach as I had much success with that approach. So the enamel tract was opened orthograde, drainage obtained, irrigated orthograde. Then using the
surgical approach, raised a flap , through and through flushing and debridement with chlorhehidine, retrograde filling with flowable resin for the deeper part of the defect and MTA for outer rim.
Flap was sutured back, and then finally the enamel tract coronally was sealed with warm obturation with Gp + selaer and resin to seal access.Pulp in the root canal retained its vitality during this whole procedure. Pictures explain the treatment. You can see the appearance of the root end surgically and the canal openings into the root end pouch.
I have published a similar case in the JOE (journal of endodontics) 4 years ago. These cases are complex and very fulfilling when they are treated successfully.
