Not every tooth with root canal filling needs a crown. Front teeth and cheek teeth like pre-molars are generally okay with a good white filling material placed unless there’s been a terrible breakdown of the body of the tooth.
Inlays and onlays are necessary to fix the big chewers or the molar teeth at the back. Root canal procedure by itself doesn’t weaken the tooth that much. It’s estimated to cause about 3% reduction in the strength of the tooth. However, teeth that require root canal filling often have very large cavities in them.
If you then take another 3% as the reduction of strength in the tooth, as a result of loss of tooth structure, then you only have half the strength of the tooth left. The risk then is that by chewing all the time and cleansing our teeth at night, we get a vertical fracture running through the root. If this happens, the tooth is lost.
To prevent that, we usually recommend you have what we call the cusp coverage restoration. It doesn’t have to be a crown; it can be an onlay with some gold pasted on top of the tooth, or it can be a full coverage tooth coloured crown if you want to have aesthetics.
You want something to hold the tooth together like the metal straps of a wooden barrel, so if you chew hard food or sleep after cleansing your teeth, you don’t have this destructive separating force going down through the tooth.