Do I Really Need the Crown Lengthening?

To begin with, a crown lengthening would be and should be performed by a periodontist. This is not something that an endodontist does. Having or not having a root canal isn't really involved in why a tooth needs crown lengthening. Most likely, the decayed (cavity) area that made a root canal necessary was very large. The dentist who places the crown needs to be able to place a nice margin, beyond the filling material that he or she placed in the tooth after your root canal. This is also called the build-up. If the crown margin is impinging the gum tissue and is too close to the underlying bone, you will have constant inflammation, bleeding gums and possibly pain. This is called a biologic width invasion. This situation could eventually result in future bone loss, as your body tries to "retreat" from this impingement. As another doctor mentioned, a radiograph or X-ray would help us in our answer to you. You should be glad that your dentist is suggesting crown lengthening. That means he or she is knowledgeable. The procedure doesn't take long and your periodontist will help you manage the pain. I hope this helps. 

If you broke a substantial portion of your tooth, then what your dentist is recommending (a crown lengthening) is not uncommonly needed before the tooth will support a new crown.  There are two problems your dentist is thinking about.  One is will there be enough tooth structure for the crown to stay cemented onto the tooth. The shorter or smaller the tooth, the more likely that the tooth will come off.  Longer teeth, and very short teeth that have had crown lengthening generally keep their crowns on long term.  The other concern he may have is how deep the cavity was to the bone under the gums.  If it is too close, violating something dentists call "biologic width", then a crown placed close to the bone will cause irritation and a constant gum imflammation.

Regarding who should do the crown lengthening?  Most dentists can do the procedure and endodontists (root canal specialists) have been trained in this procedure.  However, generally speaking the specialist who does the most of the crown lengthening procedures are periodontists (gum and bone specialists).

You will have some post operative pain after the crown lengthening procedure.

Another option that might have been discussed with you prior to the root canal was just having the tooth extracted and have it replaced with an implant and implant crown.  Sometimes that is a better treatment with longer longevity than the root canal, crown lengthening, post and core and crown.  Financially, they are pretty much equal.  However, since you have already had the root canal I would recommend going ahead with the crown lengthening by the endodontist.  If you want a second opinion, suggest a consultation with a periodontist (gum and bone specialist).