I want to learn to read dental x-rays. And I want "whys" addressed in the instructions, as in "when the cavity within the tooth leaves a wall of tooth under x mm thick, it must be filled or else..." It wouldn't be enough to me that there's a cavity.
Reading dental radiographs is something that dentists get better at over the years. Maybe that's why they call a dental business a "practice". One reason they get better at it is that over the years one learns to relate what one sees on a radiograph as "shades of gray", to what one sees clinically, in living color. Typically, the dark area of decay that is visible on the radiograph is smaller than the actual decay evident when one gets into the tooth clinically. This is because a radiograph presents a two-dimensional image of a three-dimensional reality. Although the decay may have progressed farther towards the blood supply of the tooth than the image shows, the closer to the center of the tooth the decay goes, the more bulk of healthy, non decayed tooth structure there is to obscure it on the film.
As to why there may likely be a wall of solid tooth on the edge, with decay underneath the wall, I'm guessing that what you are referring to is the fact that the outer wall of the tooth, (unless we are talking about decay of the root where there is no enamel wall) is covered in enamel, which is many times more mineralized than the inner core of tooth. Due to the mineralization, the enamel is much more resistant to decay. The decay can make a very small hole on the outside of the tooth, but then, once through that wall, the bacterial process of decay can destroy the inner core (dentin) at a much more rapid rate than it destroyed the outer wall of enamel.
You remember from another thread the pictures posted of the very small holes present on the surface of the tooth, and the x-rays of much more extensive damage under the surface. The actual damage from the decay is even more progressed than what the x-ray shows.
There is a school of thought, with considerable evidence to support it, that you can leave the decay in, and seal the tooth up with restorative material and the decay will stop progressing without access to the bacteria from the mouth. In practicality this can be a problem for two reasons. If the seal breaks down, it may go undetected, and then the decay has a head start towards ruining the tooth, and if the decay is extensive enough, it is a mushy base upon which to build the restoration, and upon the load of chewing, the tooth, and/or the restoration (filling) can all cave in on itself, and you can have a big mess.
The best practice unless the decay is really minimal, is to have it removed, get down to solid, clean tooth structure, and restore with appropriate material.
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