We used to call these FORLs – Feline Odontoclastic Resorptive Lesions. The dental specialists decided to drop "feline" because they started finding them in dogs, too. Odontoclastic referred to cells that work at reabsorbing the roots of baby teeth so that the permanent teeth can come in. Since we now realize that we don't actually understand the mechanism, that term has been dropped out.
Now it's just "tooth resorption" or "resorptive lesions". It looks like a cavity that people get when they don't care for their teeth, the kind of tooth decay that's called "caries" in human dentistry. It looks like it, but it's not. If you drill them out and fill them, they just keep dissolving away. You can't fix them.
Another difference is that they often occur on the root below the gum-line, so you can't see them by just looking into the cat's mouth, even if the cat is really cooperative. Alas, most cats are not all that cooperative about letting you hold their mouths wide open while you poke around in there. But even if they did, you frequently cannot see them.
We often discover them while cleaning the teeth. They may have been hidden by tartar. Sometimes as you clean below the gum-line, the tooth starts to bleed – NOT normal. And we find them when we take the dental X-rays.
Talk about sensitive, his OWNER is sensitive, sensitive to her cat's needs, that is. I've had patients whose mouths were horrible and the owner was pretty much unaware that anything was bothering the pet. Toby's mouth looked okay except for a little bit of redness around the gum-line where there was just a little bit of tartar visible on the teeth.
Our technician, Anna, cleaned his teeth, and still didn't see much, but when she took the X-rays, it was a different story.
Here is the reason that Toby was uncomfortable. Look at that hole in the tooth. It was pretty much concealed by tartar and the gums. The tooth used to have two roots, and one is pretty much dissolved away. The tooth just to the right (behind it in the mouth) is normal.
He had some discomfort following the extraction, even with pain medicine, but by the next day, he was a happy guy again, eating very well. His owner was kind enough to send me a picture of him at chow-time, and nothing is holding him back now.