When treatment planning a tooth with a large failing restoration, I often find that the tooth structure will not be sufficiently supported by another possibly larger new filling.When this is the case, it's best to restore the tooth with a crown (or an onlay). I often explain this to my patient and add that when it's time to fix a restoration, it's best to fix it right and choose the best option available.
Patients sometimes counter that they cannot afford to spend the money involved with having a crown made ( or would rather not). After hearing this I often point out that we can go ahead a place a large filling but if it may put the tooth at risk of developing a more significant problem. If their tooth later ends up needing a root canal or even needs to be extracted, a significantly greater sum will be required. I add this information because I need to confirm that my patient truly understands the reasons for my recommendation and that I have informed consent to proceed with placement of a large filling. Often, after some back and forth conversation, my patient may change their mind and have a crown made instead of a filling , but if they don't, I can often place the large filling, with a clear conscience!