Periapical Lesion Anatomy: A Deep Dive Into the Science Behind This Incredible Immune Response

As dentists, we look at radiographs all day long, searching for caries and periapical lesions in the process. But have you ever stopped and wondered what’s really happening on a histological or immunological level in those lesions—or even what periapical lesion anatomy is?

I raise this question because throughout my career as an endodontist and educator, I’ve heard many dentists refer to the periapical lesion as “the infection.” If this is you, I’m glad you’re here! Because that’s not what lesions are at all, and calling them as such can actually do a lot more harm than you think.

So let’s spend some time exploring the anatomy of periapical lesions in detail. I think once we have, you’ll have a better appreciation for why they can be a point of trouble when talking with patients and how you can better communicate when discussing their diagnosis and treatment.

What Is the Anatomy of a Periapical Lesion?

First, let’s get something straight: Periapical lesions (sometimes referred to as periradicular lesions) are barriers that prevent harmful microorganisms from spreading into the tissues surrounding the root of the tooth. In other words, they’re a defense system that the body uses against the threat of microbial invasion.

So when we see a periapical lesion (or dark shadow) on the x-ray, we’re not actually looking at a bacterial infection itself. Rather, we’re looking at bone loss happening as a response to infection. 

But how does this defense response actually work? To answer this question, we have to dive deep into the science behind periapical lesion anatomy!

A Sophisticated Response to Microbial Invasion

We know that periapical lesions develop as a barrier to microbial invasion thanks to a 1965 study by Kakehashi et al. conducted on laboratory rats. (We truly owe so much to those little guys—they’ve helped us make a ton of really important scientific discoveries!) The study aimed to observe the pathologic changes resulting from untreated experimental pulp exposures in germ-free rats as compared with conventional rats with a normally complex microflora.

To make this happen, the scientists drilled into the rats’ teeth so that the pulp would be exposed and then observed them at varying intervals between 1 and 42 days. What they found was that only the conventional rats developed periapical lesions. Absolutely no periapical lesions developed in the gnotobiotic (i.e., germ-free) rats!

How strong of a response there is to a microbial invasion depends on the type and number of bacteria present. Any single microbe that invades the root canal space has the potential to initiate periapical inflammation—but studies show that when different microbes hang out together, that’s when things can get really dicey. That’s because when you mix various bacterial species together, each with a different virulence and pathogenicity, they can all upregulate and downregulate each other. In other words, bacteria that weren’t necessarily harmful on their own become a threat, and this induces a more severe periapical lesion. 

Here’s how it plays out: In the beginning of a root canal infection, facultative anaerobes (a type of bacteria that can live with or without oxygen) dominate the hood and continue to use the oxygen in the area. But as time progresses and the area isn’t treated, the bacteria go further into the tooth and away from the oxygen, and the flora progresses to more obligate anaerobic species (which Möller et. all have shown are typically responsible for the formation of periapical lesions), making the infection worse over time.

Importantly, though, bacteria aren’t the only enemies the body is worried about when it throws up that barrier. Since many of the bacterial species in a root canal infection are gram-negative species, they also produce harmful endotoxins like lipopolysaccharide (LPS). These endotoxins are primarily released after the cell death of bacteria, so you can think of them like a bacterial byproduct. When our bodies sense LPS, we deploy a host of responses to protect the area—but as a result, we also destroy the tissue in that area, leading to the creation of a periapical lesion.   

The Cells of the Body’s Defense System

So, we know what the body is responding to when it forms a periapical lesion, but how does that defense system come together? To understand that and fill in our picture of periapical lesion anatomy, we first have to talk about the different cells that make up the body’s protective response.

PMNs, or neutrophils, are the white blood cells on the front lines of the body’s immune system. They don’t survive too long—only about 3 days—and their primary goal is to locate and destroy invading bacteria, like the ones in a root canal. However, they can also severely damage the host tissues in the process, making them a major cause in the tissue breakdown that happens during the acute phases of apical periodontitis.

If the PMNs fail to get rid of the bugs in the acute phase, the lesion then turns into a more chronic area of inflammation. At that point, the primary cell becomes what’s called the macrophage. Unlike the PMN, macrophages are long-lived and slow moving cells. Once they arrive, they can hang around for months! They secrete molecular mediators like cytokines, IL-1, TNF-alpha, interferons, growth factors, prostaglandins and leukotrienes, which are all important chemicals in continuing to fight the infection.  

In addition to PMNs and macrophages, the body also sends what are called osteoclasts—cells whose job is to break down bone and other hard tissues of the tooth. In a normal physiologic world, bone-building cells (osteoblasts) are the ones in control. But when there’s an infection, osteoclasts get signals from other immune cells (like T and B lymphocytes, plasma cells, epithelial cells, etc.), leading to the resorption of bone, cementum, and dentin. As you can see, there are a lot of parts to the anatomy of a periapical lesion!

The 4 “Zones of Fish” and the Defense System in Action

Let’s take a look at a graphic to help you visualize how these different cells work together to protect the body: 

This image demonstrates what we call the “Zones of Fish,” which come from a 1939 study by E.W. Fish that was the first to recognize 4 zones of response to pathogens in the root canal system. 

Closest to the foramen, you have the zone of infection, which consists mainly of the bacteria and the PMNs—cells that you can think of as a sort of border patrol. Next, you have the zone of contamination, which contains toxins and dead bone cells. Then there is the zone of irritation, which contains the macrophages and osteoclasts. Finally, you have the zone of stimulation, which contains fibroblasts and osteoblasts that secrete collagen in order to limit the infection from spreading. 

In essence, the cells in each of these zones act as separate lines of defense to prevent the bacteria from escaping from the canal and wreaking havoc on your body. Isn’t it amazing how powerful the body naturally is? I hope you’re gaining a better appreciation for how fascinating periapical lesion anatomy is!

Now, you may be wondering: What happens when the bacteria get past the body’s defenses and actually do jump into the periapical area? Because, let me remind you, typically the bacteria DON’T do this! 

Well, it is totally possible to have an extraradicular infection, which is why root canal therapy can’t be 100% successful. As amazing as the body’s defense system is, every now and then it fails—or maybe the infection hangs around too long without treatment and a cyst is formed (another way our body naturally protects us). 

Fortunately, that’s why apicoectomies were created. Good root canals (emphasis on “good!”) work about 97% of the time—but for those 3% of root canals that fail, endodontists can perform an apicoectomy to completely eradicate the bacteria and clean the bone, save the tooth, and keep the body healthy!

Putting It All Together

When we put all of these parts of periapical lesion anatomy together, it becomes clear why it’s inaccurate to say that a lesion is “the infection.” In reality, the formation of the lesion is part of an extremely sophisticated defense system that’s meant to protect the body from harmful bacteria—a.k.a bioflow, baby!

When the presence of bacteria is detected, the body mounts an immune response, sending more and more immune cells to the apex of the tooth to try to fight the microbial invaders. Then, through a series of biological actions, the host responds to those bacterial invaders, and the bone starts to resorb and pull away from the tooth, creating the lesion we see on x-rays like the ones below. This is your body giving us the sign that help is needed!

(And for the record, can we just say that the cardinal rules of root canal therapy have SO been violated here! This would NOT be in the category of “good” root canals that are 97% successful.)

Once the periapical lesion has formed, it will go in and out of balance. Sometimes the bugs will take over the party, which will probably be accompanied by some pain for the patient. And when those bad boys get out of hand, your body’s immune cells will then see the need to take over the party, causing those symptoms to go away. This is why patients don’t have constant pain. It becomes intermittent as the immune system takes over, backing away as it feels necessary.

Make sense? The lesion will then continue to be dynamic like that until the root canal infection is treated… and then we can create a whole other blog all about wound healing to talk about it!

When Communicating With Patients, Clarity is Key

The main takeaway here is that we need to be careful how we describe those “dark shadows” on the x-rays to our patients. As you now understand after learning more about periapical lesion anatomy in this blog, the periapical lesion is not the infection itself, but rather our body’s response to that infection.

While it may seem harmless to call the lesion “the infection” as a sort of verbal shorthand when talking to patients about their diagnosis and treatment plan, it actually causes a lot of confusion and can diminish their trust in both you and the efficacy of root canals. 

Why? Because when they see that lesion staring back at them from the radiograph AFTER the root canal treatment is over, they think the infection is still there and that the root canal was unsuccessful. On the contrary, we do root canals to remove the bacteria so the body can respond differently and regenerate that bone!

So although I understand the motivation behind keeping things short and simple, we need to make sure that simplicity isn’t taking the place of clarity. Of course, keep things as easy to understand as possible, so as not to cause patients unnecessary stress and anxiety—but make sure that in doing so, you’re also providing them with accurate information that empowers them to make informed decisions about their dental health.

I know communicating with patients isn’t always easy. That’s why I created Tooth Wisdom: The Patient’s Guide To Endodontic Empowerment. It’s a self-paced online course that helps patients understand the role of endodontics in caring for their teeth and feel confident in making decisions that are right for them.

You can share this valuable resource with your patients by encouraging them to join the waitlist.

– Sonia