In the image above, taken several years before the ones below, the yellow arrow points to the area of concern. This x-ray shows a tooth with a large filling that is close to the nerve toward the distal (back of the tooth). In this case, the nerve is clearly visible as an uninterrupted shadow running down the center of both roots and joining in a chamber immediately under the filling. Note that the root is clearly intact as it approaches the furcation.
Upon occasion, when a live nerve becomes irritated (in this case due to the close proximity of the filling), it may become "sick" and forget its usual function of remaining inert and keeping the tooth hydrated. When this happens, it may start to eat away at the very tooth that it is supposed to be protecting. The image above shows a dark (radiolucent) area in the distal (back) root next to the furcation (where the two roots join together). This radiolucency represents a hole in the tooth structure at that point. The nerve simply ate away the tooth from the inside out. This hole is an example of internal resorption.
The reason that this defect is labeled internal/external resorption is that a second phenomenon can cause the same defect. This involves cells in the periodontal ligament which forget their usual function of supporting the root of the tooth. If this happens, these external cells may eat the same hole in the tooth, this time from the outside in. Once the nerve is exposed in the process, as it was in this image, it is impossible to tell from which direction the resorption started.
This x-ray was taken a bit over one year after the first one. Note the radiolucency (darkening) in the mesial aspect of the distal root, emphasized by the yellow arrow. This is the area of resorption. Note also that resorption represents inflammation, and the bone in the furcation is also affected by this. Internal resorption often begins in the pulp chamber, and appears to the dentist like a slightly enlarged pulp. It is easy to miss this type of lesion. The one that appears here was fairly obvious. The patient had presented to the office because of pain in the tooth.
In this case, the internal resorption had destroyed the integrity of the tooth when it perforated through the root into the bone. This meant that the tooth was not savable and had to be extracted. If the process had been noted before the defect had perforated to the outside, a root canal procedure could probably have saved the tooth.
This image shows the tooth after extraction. the area of resorption is obvious. Some of the hypertrophied pulpal tissue still remains clinging to the internal walls of the lesion.