This is a really good question that's difficult to answer without seeing xrays or clinical photos. But it sounds like your periodontist feels that significant bone removal will be needed in order to perform the crown lengthening. This is why he/she is saying that tooth #12 may be jeopardized. With that amount of bone removal, I'd seriously consider extracting #11 and replacing it with an implant. Of course, there are other important factors to consider including the amount of resorption (depth into the tooth), the need for a root canal, the length of the remaining root, the condition of tooth #12, how much of your teeth you show when you smile, the condition of your bite, etc. In general, I do not like the idea of allowing the resorption to continue as it will inevitably lead to one of the following: root canal infection & pain, bone loss in the area of resorption with gum inflammation and bleeding, fracturing of the tooth. I feel that it's better to work on the tooth in a controlled setting rather than when something bad happens.
