I Have an Old Root Canal I Have Been Having Pain Can One Root Be Removed and Keep Other Root and Save Tooth?

The procedure you're asking about is called a "root amputation" and has been used in dentistry for decades.  If the tooth in question is an upper molar, which usually has three roots, then the tooth that is left over will have had 1/3 of its support removed.  If it is a lower molar, which generally has only two roots, than removing the fractured root would mean you've lost 50% of the support.  A tooth with little bone support will often times fail because because of torquing or overload of the tooth.  Be aware that the root amputation procedure is considered dental surgery.  Dentists in general do not recommend root amputation as a first choice because of the advantages and much more successful procedures using dental implants and fixed bridges.

The short answer is YES, it is possible under certain circumstances to remove only the fractured root from a failing lower molar that has already had Root Canal Therapy, in order to save the tooth. It will have a guarded prognosis, however. A more long-term solution would be to remove the entire tooth and replace with an implant-supported crown. This would depend on the amount and quality of jaw-bone available.

I can tell you one thing after 22 years of practice. If it is a back tooth and you remove one root, the longevity becomes questionable. I would rather have you spend the time and the money having it extracted and place an implant. Much more predictable and much more cost effective in the long run. Good luck