A root canal removes the nerve from your tooth.

Think about what that means for a second. The nerve is the alarm system. It’s the thing that screamed at you loudly enough to get you into a chair in the first place.

Once it’s gone, that alarm is disconnected permanently.

So when a root canal starts to fail years later, the warning you’d expect often never arrives. The infection returns quietly, dissolves bone around the root tip, and shows up on an X-ray at a routine checkup while the patient insists the tooth feels completely fine.

That’s the part nobody explains. Here’s what to actually watch for.

The 7 Signs

1. Pressure or a dull ache when you bite down. 

Not sharp pain. More like the tooth feels bruised, or slightly taller than its neighbors.

2. Tenderness in the gum above or below the tooth. 

Press a fingertip along the gumline. A failing root often makes that specific spot sore while everything around it feels normal.

3. A small bump on the gum that comes and goes. 

This is a sinus tract, and it’s the infection draining. It may release a bad taste, then vanish for weeks. Disappearing is not healing.

4. Intermittent swelling in the face or jaw. 

Even mild puffiness that shows up and resolves on its own deserves an exam.

5. The tooth darkening. 

Gradual graying or yellowing of a treated tooth, especially a front tooth, can signal what’s happening inside.

6. A crown or filling that feels loose, cracked, or catches floss.

Bacteria leaking back down through a compromised restoration is one of the most common failure routes. The root canal may have been perfect. The seal on top wasn’t.

7. Absolutely nothing. 

The most common sign is no sign. Which is exactly why the imaging at your checkup matters more than how the tooth feels.

Why Root Canals Fail

The single biggest culprit is anatomy nobody found the first time.

Your upper first molars usually have a fourth canal called the MB2. It’s narrow, it hides, and on a flat two-dimensional X-ray it’s frequently invisible. CBCT studies put missed canals in maxillary first molars somewhere in the 40% range, and MB2 accounts for the large majority of them.

The consequences are not subtle. A meta-analysis found post-treatment apical periodontitis in 85.1% of root-filled teeth with a missed canal, versus 56.3% without one, translating to roughly a sevenfold increase in odds. Another study found maxillary first molars with a missed MB2 were 4.38 times more likely to show a lesion at the root tip.

The other common causes: bacteria surviving the original cleaning, root filling that stopped short of the apex, and coronal leakage from a crown or filling that failed over time.

None of this means your first dentist did poor work. It usually means the tooth had a canal that couldn’t be seen with the tools available that day.

Does Retreatment Actually Work?

This is the question patients really want answered, so here are real numbers.

A systematic review of 29 studies found pooled success rates for nonsurgical retreatment of 78% under strict criteria and 86.4% under looser criteria. A separate longitudinal study reported 80.6% and 93% respectively at two to three years.

One finding is worth knowing before anyone pushes you toward surgery. Compared head to head, endodontic surgery looked better at the two-to-four-year mark (77.8% versus 70.9%). By four to six years the relationship flipped, with nonsurgical retreatment at 83.0% versus 71.8% for surgery. Retreatment tends to win the long game.

And the honest boundary: retreatment isn’t right for everything. A vertical root fracture, advanced gum disease around the tooth, or a tooth without enough structure left to restore are all situations where saving it isn’t realistic. We’d rather tell you that upfront than take your money for a procedure with a poor prognosis.

Why We See So Much of This in Fort Bend County

Sugar Land grew fast, and it grew by people moving in.

A large share of the retreatment cases we evaluate at our Williams Trace office involve a root canal done somewhere else entirely. Another city, another state, sometimes another country, often fifteen or twenty years ago. The original films didn’t come along.

That matters, because without a baseline it’s harder to tell whether a shadow at the root tip is old scar tissue or an active problem. It’s one reason we look carefully at treated teeth in new patients from Missouri City, Stafford, Richmond, Meadows Place, Sienna, and Riverstone rather than assuming previous work is fine because it isn’t hurting.

If a Treated Tooth Feels Off

Get it looked at while it’s still small. Lesions at the root tip grow slowly and quietly, and a tooth that’s straightforward to retreat this year can become non-restorable in three.

Schedule a consultation at our Sugar Land office. You can also meet our doctors or read more about root canal retreatment and endodontic treatment.

Frequently Asked Questions (FAQ’s)

Can a root canal fail years later? 

Yes, a root canal can fail months or even decades later, most often from a missed canal or bacteria leaking back in through a failing crown.

Does a failing root canal always hurt? 

No, many failing root canals cause no pain at all because the nerve was removed, so they are often found only on X-rays.

What is the success rate of root canal retreatment?

Nonsurgical root canal retreatment succeeds in roughly 78% to 86% of cases, depending on the criteria used to judge healing.

Is retreatment better than pulling the tooth? 

Retreatment is usually preferred when the tooth is restorable, since keeping a natural tooth preserves bone and avoids the cost of replacement.

What causes most root canal failures?

Missed canals are the leading cause, particularly the MB2 canal in upper first molars, which is associated with a sevenfold increase in root tip infection.