Do you always need a crown after a root canal?
I was taught, and many of you have heard, that any tooth that’s had a root canal needs a crown.  Recent research has disproved this, at least in some cases.
If a front tooth is basically structurally sound (i.e., not a mass of old fillings), a crown gives no benefit except when it’s needed for aesthetics.  I bond a filling into the access hole I drilled in the back of the tooth to do the root canal, and the tooth becomes structurally sound again.
When a root canal is done on a back tooth, the access hole is made in the chewing surface of the tooth.  Back teeth are subject to much greater chewing pressures than front teeth, so it’s not enough to just fill up the access hole in a back tooth that has had a root canal.  The chewing surface of the tooth has to be covered completely.  This can be done by an onlay, which covers the chewing surface of the tooth and part of the sides of the tooth, or by a crown, which covers the entire tooth, including all the way down the sides.
Onlays are more conservative because they preserve more of the original tooth structure, and they protect the tooth just as well as a crowns do. For this reason,  I try to do onlays on back teeth wherever possible, and only resort to crowns where there is no other choice.

By the way, the above column reminds me of one of my favorite dental course questions.  “What’s the biggest block to new knowledge?”  Answer:  “Old knowledge.”
There is always new research and new information;  that’s why I take so many courses.

Why don’t dental restorations last a lifetime?
When I was in dental school, one of my professors said, “If you want to make monuments that will last until the end of time, be sure you don’t work in the human mouth.”  Another called the mouth “a hostile environment.”   Both were correct.
The bacteria that cause decay are extremely small;  about one micron in diameter. (one thousandth of an inch is about 30 microns).  Silver-mercury fillings corrode over time;  when a space opens up around an old filling that’s as wide as one of the hairs on your head, 150 bacteria can walk abreast into that space.   Result:  decay around the old filling.
That’s one of the reasons that dental restorative materials—fillings, onlays, and crowns—don’t last a lifetime.
Insurance company statistics show that the average crown done in the U.S. lasts about 5 years.  That’s why many insurance company plans will at least partially cover a new crown if it’s needed five years after the original crown was placed.

Is the situation improving?  In some ways, yes.   Here’s how:

A dentist can do things to increase the longevity of dental restorations. The precision of fit is extremely important.  The bonding agent that holds restorations in place is like a thinned-out version of the plastic resin we use for fillings and tooth-colored onlays.  It can’t possibly be as strong as the material it bonds in place.  The more accurately the restoration fits, the less bonding agent is needed to seal the restoration to the tooth.  Since the bonding agent is the “weak link” in the chain, restorations with very precise fit tend to have greater longevity.

A dentist also needs to use advanced materials and techniques to enhance the longevity of restorations.  For example, until about 12 years ago, every available cementing agent for crowns had one very undesirable property;  it could dissolve in saliva over time.  The newer cements are much more saliva-resistant and  actually chemically bond to the tooth. I’m lucky that I’ve developed relationships with the two top experts on bonding  techniques and materials.  That helps keep me up on the best treatment options for you.

Choice of materials is intimately related to their longevity. Some materials intrinsically tend to last longer than others. Laboratory-fabricated plastics are hardened under heat, pressure, and intense light, often in a vacuum or specially controlled atmosphere. These materials are harder and more wear-resistant than the usual plastics used for smaller fillings, and tend to last longer for large restorations like onlays. Recently developed ceramics are even stronger than advanced plastics. Gold tends to last even longer; in fact, gold tends to last longer than any material ever developed for dental use. I’m one of the few dentists in delaware county who frequently places gold restorations. Who wants gold? People who are phobic, who tend to break restorations, or who want the restoration that will last “as close to forever as possible.” it’s not uncommon for gold to last 10, 20, 30, 40 years or longer.

What can you do to help increase the longevity of restorations?   Remember the line about the mouth being a “hostile environment?”  I’ve seen mediocre dentistry placed by mediocre dentists last years longer than it deserved just because the patient was a fanatic about brushing and flossing.  There is no substitute for this effort.
If great home care can make mediocre dentistry last longer, imagine what it can do for really well-crafted dentistry.
The other thing you can do is to keep to whatever checkup schedule we recommend as best for you (three months, four months, or six months).  That way, if you do develop a problem around any restoration, we have the best chance of catching the problem early and doing more conservative treatment to fix things.

This brings up another closely-related issue.  Do you know someone who will only do “point-and-shoot” dental treatment?  By that I mean that the person goes to a dentist only when he or she perceives a problem, points to the problem, gets it fixed, and disappears until the next perceived problem.  People who do this are doing themselves a big disservice.  The most cost-effective service I provide is a treatment PLAN based on a  comprehensive examination, thorough cleaning, and necessary x-rays.  There is no substitute for a thorough overview of your entire mouth.  I  suggest that, if you know a point-and-shoot dental consumer, you nicely slap that person upside the head (just kidding about the slap part) and try to educate that person on the benefits of comprehensive treatment.

Insurance company statistics aside, what is a reasonable life-expectancy for complex dental restorations?  In my experience, the crowns I place last 10-20 years on average, tooth colored onlays 10-15 years, veneers 10-15 years, and gold onlays 10-50 years.

Restorative dentistry: how can “tooth-conserving dentistry benefit you?
I’ve established a relationship with the country’s two top experts in bonding techniques.  One of them coined the terms “tooth-conserving dentistry” and “tooth bank dentistry (i.e.,  all the tooth structure in your mouth is like a bank, and there’s a penalty for premature withdrawal of that structure).”
Today, the goal of restoring a damaged tooth should be to remove as little tooth structure as possible while conserving the rest. 
Why is this vital?  Virtually all restorative procedures weaken teeth at least a little (the exception is a thin porcelain veneer bonded strictly to tooth enamel).  The more tooth structure  removed, the more the tooth is weakened.  Unbonded fillings (like silver mercury materials) greatly weaken the tooth and increases the risk  that the tooth will eventually fracture.   Bonded restorations restore much of the strength the tooth had before it ever had a filling, because the bond mimics the original structural strength that nature originally gave the tooth.
Tooth-conserving dentistry is above all a philosophy of treatment, I can give you several examples of how I practice this philosophy:
☺  I use a laser early-decay diagnosis device called Diagnodent to find cavities on the chewing surface of back teeth far earlier than other diagnostic methods.   Cavities found this early can often be treated by using abrasive particles in compressed air (air abrasion)—no drill, no shot, no pain.  And, far less tooth structure gets removed while still eliminating all the decay.
☺  Whenever possible, I use porcelain veneers on front teeth instead of crowns.  A thin piece of porcelain is bonded to the tooth’s front surface (the part you can see). Veneers are more conservative than crowns, because I only remove a thin layer of frontal tooth structure.  With a crown, I have to remove a thicker layer of  structure around the entire circumference of the tooth.
☺  On back teeth, I use inlays or onlays wherever possible instead of crowns.   Much less tooth structure is removed, and the restoration can be tooth-colored high-strength plastic or, for ultimate longevity, gold.   We’d be happy to show you a model of an onlay so you can see the difference between it and a crown.
I do still place crowns, but only when they are the most conservative treatment choice.  This could occur when a tooth is very badly decayed or broken, or when I have to replace an existing crown.
In all cases, I always recommend the most conservative treatment that will work.