Onlay

As I have mentioned previously, existing restorations in our teeth do not last forever. Due to a constant wear and tear, older fillings tend to break down over the years. It can be somewhat confusing to the average person as to what is an appropriate replacement for an old and defective filling. There are many choices but as a general rule the proper choice for a replacement is directly proportional to how much tooth structure is remaining after the excavation. So the size of the original restoration is definitely a factor. Another factor is the nature and severity of the defect, meaning how much recurrent decay is present at the time of diagnosis. It is also very important to be conservative and not remove any more healthy tooth structure than is necessary.
In my experience most existing defective fillings are best restored with an Onlay. An Onlay is a type of dental restoration that is the best of both worlds not only in the sense that it provides proper coverage but also it is not overkill in preparation. What happens in most situations is that after a defective filling is cleaned out, the walls that contained the old filling will be too thinned out to support another direct filling especially at the very top of the tooth. Those thin walls are then reduced by a couple of millimeters and the Onlay is fabricated in the Laboratory from an impression of the teeth. The analogy that I like to use is a mushroon shape in cross section where the "cap" covers most of the biting surface and the "stalk" sits in the tooth where the filling was. This design is ideal because we are essentially only removing what is defective. It is also relatively more conservative in preparation than some other choices such as a crown for example. The idea is that the more of your own tooth we can hold on to now, the better off you are in the long run and more options are available in the future.
Onlays are made of a type of porcelain and are bonded in with a resin cement. This is a very strong combination and teeth that are restored in this way will typically not require further treatment for many years.