What Is a Periapical Lesion? Understanding the Meaning Behind That Shadow on the X-Ray

In my 15+ years as a practicing endodontist, I’ve seen many patients’ eyes widen at the sight of periapical lesions—that big, dark shadow they sometimes see surrounding their tooth on their post-root canal x-ray. Whether they voice it or not, I know exactly what they’re thinking: “Why is there still a shadow there?!? Does that mean the root canal didn’t work and I still have an infection?”

If you’ve ever found yourself in this situation, I have some fantastic news: If you see a shadow on your x-ray after a root canal, that doesn’t mean the root canal was unsuccessful!

When done correctly, root canals are a safe and effective procedure to clear your tooth of bacteria, rid your body of infection, get you out of pain, and save your natural tooth. Yet widespread myths like the focal infection theory, misinformation, and poor communication from well-meaning dentists has made patients like you doubt them, and it’s time we’ve fixed that!

That’s why today, I’d like to take the time to explain exactly what a periapical lesion is, how they relate to infection, the incredible science behind your body’s immune response, and the role of root canals in helping to defend your precious teeth. My hope is that after you’ve read this blog, you’ll not only better understand what’s happening inside your mouth, but also trust in the amazing healing power of root canals!

What is a Periapical Lesion?

First and foremost, let’s clear up some misinformation. That dark shadow around your tooth is NOT the infection. It’s actually bone loss that happens as a response to the infection. That may sound scary, but it’s actually a very normal and natural part of your body’s defense system! 

You see, rather than allow harmful bacteria to infiltrate your body and wreak havoc on your health, your immune system creates a protective barrier that helps keep the bad guys at bay until you can get a root canal to treat your infection.

But wait a second… What does bone loss have to do with creating a protective barrier against bacteria??? To answer that question, we have to take a deeper dive into the science behind what a periapical lesion is. 

Don’t worry—I’ll do my best to make it as easy as possible to understand!

“The Microbes Are Coming, The Microbes Are Coming!”

Periapical lesions—or periapical radiolucencies—develop as a response to what we call microbial invasion—a.k.a. harmful bacteria entering your tooth. We know this thanks to an experiment conducted in 1965 by Dr. Kakehashi and colleagues on laboratory rats. (Lab rats have helped us make loads of really important scientific discoveries—we truly owe those little guys so much!)

The experiment involved two groups: One group of rats lived in a completely germ-free environment, while the other had normal bacteria in and around them. To study what would happen, the researchers drilled into the rats’ teeth to expose the pulp (the soft tissue inside the tooth) and watched what occurred over time, from 1 to 42 days. The result? Only the rats with normal bacteria developed periapical lesions, while the germ-free rats had none. This showed us just how important bacteria are in causing these infections!

Research also tells us that the strength of the body’s response to bacteria inside the tooth depends on the type and amount of bacteria present. Even just one type of bacteria entering the tooth can cause inflammation. However, when different types of bacteria are present together, things can get a lot worse. This is because different bacteria can influence each other—some that aren’t harmful on their own can become dangerous when mixed with others. This combination leads to more severe infections around the tooth’s root.

Whether diverse types or very few, the response to these bacteria and their harmful waste products is the same: The body deploys a host of responses to protect the area around the tooth. But as a result, it also destroys the surrounding tissues and breaks down bone, which is what leads to the creation of what we know as a periapical lesion.

The Main Players in The Body’s Defense System

So, now you know what the body is responding to (i.e., bacteria) when it forms a periapical lesion. But how does the defense system actually work, and who are the main players in bringing this sophisticated response together? Let’s review some of them together:

• Neutrophils: These are white blood cells that serve as the front line of defense in your body’s immune response. They only survive about three days, and their main goal is to locate and destroy invading bacteria. However, in carrying out this important duty, they also cause damage, making them a major cause in the tissue breakdown that happens when a periapical lesion begins to form.
• Macrophages: Macrophages appear should the neutrophils fail to get rid of the bugs, and unlike neutrophils, they live a long time. Once they arrive, they can hang around for months, secreting molecular mediators that help to fight the infection.

Osteoclasts: Normally, the body creates a lot of osteoblasts to build and strengthen bone. But when there’s an infection created by harmful bacteria, the body sends signals for the osteoclasts to take over, and their job is to break down bone and other hard tissues of the tooth as a protective measure. This bone resorption is what causes that big, dark shadow you see on the x-ray!

So when we think about what a periapical lesion is, it’s not an infection wreaking havoc on your body. Instead, it’s the result of your body putting up one hell of a fight against the bacteria and that it could use some extra support! In other words, as scary as it may look on an x-ray, that lesion is a good thing, because it’s a signal that your body is doing exactly what it’s supposed to in order to protect itself from harm. Freaking incredible, right?!?

The Defense System In Action

To give you an even better appreciation of the body’s sophisticated response to bacteria, let’s go just a bit deeper to look at how all these cells we just talked about work together. I know this is a lot of science, but I’m confident you can handle it, and I think it will help you trust your body more than ever before!

Think of your body’s cells as an army. Just as in a real war, your body doesn’t employ all of its defenses at the same time and in the same way! That would be a huge waste of resources. Instead, each type of cell serves as a separate line of defense, with the goal being to prevent the bacteria from spreading further and infiltrating the body.
We call these different lines of defense “Zones of Fish,” because they were first discussed in a 1939 research article by a man named E.W. Fish, and they look a little something like this:

The zone closest to the root of the tooth is the zone of infection, which consists mainly of the bacteria and the neutrophils—those white blood cells that act as a sort of border patrol. In fact, researchers have found that, thanks to these guys, bacteria are very rarely able to leave the canal! Next, there’s the zone of contamination, which contains toxins and dead bone cells. Then, you have the zone of irritation, which is where the macrophages and osteoclasts hang out. Finally, there’s the zone of stimulation, which contains fibroblasts and the osteoblasts we mentioned earlier.

By working together, the cells in each of these zones prevent the bacteria from escaping from the canal and wreaking havoc on your body. Isn’t it amazing how powerful your body is? I hope you’re really starting to understand what a periapical lesion is and how this defense system works to protect you!

How Root Canals Help Treat Infection

Before we get into the role root canals play in this process, let’s recap!

The dark shadow you may see on your x-ray is not the infection itself. Instead, it’s part of an extremely sophisticated defense system that’s meant to protect your body from harmful bacteria. It’s a process that I call bioflow!

When the body detects bacteria, it sends different cells to the tip of the tooth to fight off the infection. As part of this process, a chain of biological reactions occurs, causing the surrounding tissues to break down and the bone around the tooth pull away, creating the periapical lesion or dark shadow you see on x-rays and 3-D imaging, like these here:

And that, my friends, is what a periapical lesion is!

Importantly, once it has formed, the bacteria will go in and out of balance as your body fights the infection. Sometimes the bugs will take over the party, causing you to feel more pain and discomfort. But as soon as they get out of hand, your body’s immune cells will see the need to take action, causing those symptoms to go away and making you think the problem is gone. This cycle will continue until your infection is treated.

That’s why when you experience symptoms of an infection, such as pain, swelling, and sensitivity, it’s important to see your dental provider right away, even if they suddenly disappear. As incredible as your body’s own defense system is, it can’t hold those bad bacteria off forever! You need a root canal to be able to get rid of the infection, feel permanent relief from pain, and restore the natural balance of your mouth’s microbiome.

When root canal therapy is done correctly (something I’ve dedicated my career to helping dentists do!), they’re successful 97% of the time. And even when they fail—such as when the infection is left for too long without treatment and a cyst is formed—we have another procedure called an apicoectomy that can still eradicate the bacteria and clean the bone, save your tooth, and keep your body healthy and happy!

Feel Empowered in Your Dental Decisions

Root canals are a safe and powerful tool for relieving your pain, eliminating infection, and restoring your body’s natural balance, but there’s a lot of misinformation out there that makes patients like you doubt their efficacy and even lose trust in their dentist. 

That’s why I created Tooth Wisdom: The Empowered Patient’s Guide to Oral Health. It’s a self-paced online course designed to empower you through fun and accessible endodontic education, rebuild your trust in your own body and in your dental provider, and fully understand your treatment options so you can make the decisions that are right for you.

– Sonia