This is a situation most dentists face every week. A patient comes in with a broken tooth (either with or without prior root canal treatment)and would like their tooth fixed. This poses a dilemma for dentists. Most are aware that teeth do not have enough exposed tooth available to safely hold a crown are more likely to fail than teeth that have do have sufficient 'ferrule'(sound tooth structure around the base of the tooth that can be used to anchor crowns). In my experience, it is extremely desirable for teeth to have at least 1.5 mm of ferrule in order for me to fabricate a predictably long term restoration.
What should a dentist do if a patient wants his tooth fixed and it doesn't have sufficient ferrule? First he or she must make the patient aware of this issue and its ramifications. The fact is that if a crowned tooth has insufficient ferrule, it is more likely to fail. The remaining tooth structure is much more likely to fracture and can even lead to the loss of the tooth. With this fact in mind what should a dentist do?
Ideally a patient who wants this sort of tooth restored has two good options. The first is the road least travelled. It involves having the tooth orthodontically extruded and after it stabilizes having a crown lengthening procedure. This will allow for an aesthetic result and a more predictable restoration since the resulting restored tooth will have the desired amount of ferrule.
Another option is to for go orthodontic extrusion and just perform a crown lengthening procedure ( removing bone and gum from around the tooth in order to expose more tooth structure). This can work well but can result in removing supporting bone from adjacent teeth and can cause aesthetic problems, since the resulting tooth can look too long and appear to have experienced gingival recession and 'black holes' interproximally(missing papillas).
Often patients offered either of these two options are hesitant to give approval to either and are insistent that the tooth be fixed as is. Although dentists sometimes go along with this request, it can lead to the eventual loss of the the tooth. How long such a restoration will last varies, but it is not a gamble that patients should take, if they want to keep a tooth long term.
Still another option, not yet mentioned, is that some broken teeth should be extracted and have bone grafting and an implant placed, while there still is sufficient bone available at the site. Implant restorations can be extremely predictable when placed in good candidates with the proper amount of bone support.