Cavities may not always be detectable to the naked eye, which is why your dentist has to take annual x-rays. The purpose of these dental x-rays are to augment the clinical oral examination by providing additional information which we can't see.
Cavities in the mouth which you can see may be much more obvious than those on the x-rays. Tooth decay looks different on the x-ray when compared to the ones you can see in the mouth. Decay in the mouth has color while decay on x-rays are only in black in white.
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What cavities generally look like on x-rays
Since x-rays are in black and white, tooth decay looks different than what they do in the mouth. On x-rays, all structures are classified as either radiopaque or radiolucent.
Radiopaque - shows up as "white" on the radiograph and represent solid structures.
Radiolucent - shows up as "dark" on the radiograph and represent hollow structures.
Your teeth should be solid which you can verify by tapping on it and chomping down a few times. It feels rock solid doesn't it? Since it is a solid structure it will be radiopaque on the x-ray which means it should show up as white. You can see in the x-ray above.
Cavities on the other hand are not solid structures because the literal definition of them is a hole in your tooth. Therefore, a tooth with decay in it will show up with radiolucent areas or spots within it. The radiolucent spots represent non-solidness or in other words, a cavity.
The x-ray above is of an early cavity on an x-ray. You can see there is a little bit of darkness or radiolucent area inside of the enamel. The enamel, which is the outer layer of the tooth is solid so it should be pure white or radiopaque. Unfortunately on this x-ray, the radiopaque area is becoming less radiopaque. That signifies the beginning of an early cavity.
What cavities look like on the chewing surface of teeth on x-rays
For decay that is located on the chewing surface of teeth, they look similar to other types of cavities. They will appear dark or radiolucent on the x-ray. The only difference is the location of the cavity which will be on the chewing surface or the top part of the tooth.
Therefore, what it looks like on the x-ray is a radiolucent area on the top of the tooth. Here is an example of what it looks like.
In case you were curious, here is what it would look like inside of the mouth. Dark to brown color that may be cavitated.
What tooth decay looks like in between the teeth on x-rays
Once again, cavities all look the same on the x-ray with the only difference being where they are located. Thus, cavities that are in between the teeth will just look radiolucent but they are in between adjacent teeth.
For what it looks like inside of the mouth, here is a picture of after your dentist drills into the tooth. You can see the brown parts inside of the dentin layer which comes after the enamel.
What cavities look like underneath of an existing filling on an x-ray
Just because you had the cavity filled before, it doesn't mean that it is immune to future decay. That is right, you can get a new cavity underneath of a cavity filling. We actually call this condition recurrent decay since it isn't the first time that it happened.
What it looks like on the x-ray is a radiolucent area underneath of the filling material. Sometimes these can be difficult to diagnose because the bonding agent used for tooth fillings can also appear radiolucent.
Basically it will look radiolucent underneath of something that looks radiopaque. On the x-ray above, it is on the tooth to your left where you can see a dark circle under the very white looking structure.
Here is a picture of what a different tooth with decay underneath of an existing filling looks like. The exterior may seem solid but you can see discoloration under the cavity filling, which signifies decay.
What cavities into the nerve look like on x-rays
A very large cavity is one which has reached the nerve. What this looks like on the x-ray is a big radiolucent area that is either into the pulp or close to it. The pulp is where all of the nerves of the tooth are housed and it is located right in the center.
For what it looks like inside of the mouth, it typically has a huge cavitation that is a dark brown to black in color. Food and plaque are constantly getting stuck inside of the whole which only exacerbates the entire situation. These may or may not be extremely painful.
What a cavity underneath of a crown looks like on an x-ray
Tooth decay underneath of a crown is one of the most difficult things to detect on an x-ray. In fact, you're not really able to even see anything half of the time. The reason that it is difficult to detect is because most crown materials show up as radiopaque on the x-ray. What the means is that it will cover up cavities on the x-ray.
Since cavities are radiolucent, you can only see them if there is no radiopaque object covering them. In the x-ray above, even if there was decay, there would be no way for you to see it.
Fortunately, with a recent advance in crown technology, there are crown materials that don't look so radiopaque. With these new crowns you are able to detect cavities on the x-ray. Here is an x-ray of one with decay underneath of it which you can see on the left side.
Here is a picture of what tooth decay looks like in the mouth with the crown on and without it off:
What a cavity on the root looks like on an x-ray
Root decay will look radiolucent and they're all located on the root surfaces. This means that they're typically below the level where the enamel is. Depending on if the cavity is on the cheek surface or in between the teeth, you may or may not necessarily see them.
Here is an x-ray of root decay in between the teeth, which you can see. It begins below the radiopaque spot in between the teeth. What it looks like is a dark spot underneath of the white enamel.
The x-ray above is a perfect example of what root caries (decay) looks like. The root cavity is on the right side of the tooth. You can also see a regular cavity on the left side of the tooth. This gives you a good comparison as to what the different kinds of them look like.
Here is an intraoral photo of what it can look like on the root surface below the margins of a crown. It looks brown to black in color near the gumline underneath of where the crown starts.
Hopefully that gives you an idea of what cavities look like on x-rays. For the most part they all look similar in that they are radiolucent on the radiographs. The only difference among the various types of cavities is where they are located.
Aside from that, all symptoms and signs remain the same except for the location of the decay. Unfortunately, there are instances where you are not able to detect cavities on x-rays such as if you have a very radiopaque crown that covers it. If that is the case it will be left up to your dentist's expert judgement.
Related content: In case you wanted more information beyond what cavities on x-rays look like, here is an article about what they just look like in general.
Last but not least, don't forget that all cavities start off small but they progress through the stages of tooth decay as they get bigger. As they get bigger the treatment for them become more complex and also more expensive.
For that reason alone it is in your best interest to take care of them while they're small. This is why it is important to go to your dentist for your routine check ups so that you can minimize your out of pocket expenses!