Some old crowns were created following an old technology, with a simple metal sheet, soldered in order to create a ring, and fitted as well as possible around the teeth.
The masticatory face is realised at a later stage. Their big inconvenience lies in their bad fitting on the base of the tooth. It creates an important retention of bacteria and food fragments.
There are two kinds of consequences to badly adapted crowns:
1- On the teeth themselves:
The proliferation of bacteria can create cavities on the crowned tooth (a root canal tooth can decay, but we cannot feel any pain) and on the nearby teeth. It is moreover very frequent that these crowned teeth fall out, uncovering a tooth "eaten away" by tooth decay, which can only be repaired with difficulty.
2 - On the gum and the surrounding bone:
The important number of trapped bacteria first generates a gum inflammation, then a "receding of gums", with an osseous loss (decrease of the volume of bone which supports roots).
We now know the major role bacteria play in the periodontal diseases (receding of gums). In the most advanced cases, one of the first actions is to remove these old crowns. Once the gum has healed, a new crown is set.
Setting these old crowns has no longer been taught in dental surgery faculties for about thirty years.