Many times, teeth can be so broken down due to extreme decay or fracture that there is not enough tooth structure available to hold a filling in place. In most of these cases, a crown (cap) would be indicated. However, certain situations may call for a change in plans. Things like inability to afford a crown, an unstable gum situation, waiting to see if a recent root canal will work, or any situation where the long term progression (outlook) is questionable are all possible reasons to go with just a filling. When there is not enough tooth structure to hold a filling, then pin retention is indicated. Pins are tiny threaded screws that are placed about ½ millimeter into dentin. A pilot hole ever so slightly smaller than the pin is drilled into the tooth, then the pin is screwed into place. An amalgam (metal) or composite (tooth-colored) filling is then placed over the top. The pin is only used for retaining the filling on the tooth. It does not reinforce like a rebar does with concrete. Pins have been known to cause microcracks in the dentin, however, I have not seen a fracture of the underlying tooth structure that was severe enough to lose the tooth that was caused by the pin alone.