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"I have one very similar done a few years ago. Root canal failed."
That's what root canals do, sooner or later. Removing the blood supply from the tooth turns it into a brittle piece of dead bone-like material which eventually fractures. If not in the "bite" it may last quite a while. If stressed with normal chewing and biting, it won't last long. At that point it has to be extracted. What you do next is put in some bone grafting material, let it integrate, then drill a hole in it and install an anchor bolt called a dental implant. They cost about $100. Blueskybio is a good source but there are many others. Just for fun, go look and see what these things actually cost and then think about what that dentist charges you for installation - something which takes all of about 15 minutes. Really . . . no joke.
https://blueskybio.com/store/
At least this (installing an implant) is what one
should do. Many irresponsible and greedy dentists will try and talk you into a bridge or other mouth contraption that will compromise some of your remaining intact teeth. That bridge or other contraption will also eventually fail, just like the root canals. Dentists don't advertise that root canals, bridges & other mouth contraptions are temporary fixes. They will encourage you to think, after spending that much money, that you have invested in something permanent. If you die before they fail then I guess you could call them permanent. If you live a while, they'll fail. They are temporary fixes and do a lot of damage to your other teeth and to your wallet before they do.
Implants, as long as you take care of your gums, are permanent solutions. If a tooth is at the point of needing a root canal, it needs to be extracted and the implant process started.
Cost if done in Mexico is about $800 which includes the crown.
I wrote about this not long after joining POA . . . I'll see if I can provide a link to my posting.
http://www.pilotsofamerica.com/forum/showthread.php?p=1698452#post1698452
I haven't re-read it so, knowing me, it was probably too wordy but the essential information was/is there if you're willing to wade through it.
You mention cost . . . before I found my dental surgeon in Tijuana I was paying $1800 per implant and another $1200 for the crown and miscellaneous procedures related to the process (x-rays, etc.) So $3K per tooth, mas o menos.
Your retired dentist is correct in that the seal between gum and crown (called a "margin") can be broken and bacteria get underneath the crown, however this is just as true with all other crowns. Everything changes with time, including margins. With a root canal you will have dead bone-like material underneath the crown which bacteria will enjoy eating. With an implant, you have titanium underneath the crown. Bacteria don't enjoy titanium nearly as much as dead bone-like material. So they don't thrive. Good mouth hygiene is important in either case, i.e., Listerine and a mouth irrigator (waterpik) plus brushing to stimulate the gums.