Root canal treatment is an invasive procedure. It involves hollowing out the center of the tooth in order to eliminate the infected nerve from the pulp chamber. In most cases, this process compromises the natural strength of the tooth structure, and makes it weak. The cavity is filled after the root canal, but generally it is not adequate to restore the full strength of the tooth structure.
Therefore, a crown or a cap is placed on the tooth in order to add to its strength. The crown is made of a strong material that protects the tooth from developing a crack or a fracture. This is important because any crack or fracture on such a compromised tooth will most likely extend down to the root. If that happens, the entire tooth structure needs to be extracted.
It is a standard of care that any posterior tooth, whether premolar or molar that has undergone a root canal treatment usually will require a crown. This is required because the back teeth are usually exposed to maximum biting, chewing and grinding force in the mouth. Without the support of a crown, it may be difficult for the compromised tooth structure to withstand that pressure for too long.
Crowns are also needed in most cases where a front tooth has undergone a root canal procedure. However, there may be some situations where adequate tooth structure is left after the root canal treatment. In such a case, a filling may suffice, and a crown may not be necessary. But this is an uncommon situation.
Crowns for the front teeth are generally made in porcelain because of its life-like aesthetic properties. But for the back teeth, the patient also has an option of gold based restoration which is more resilient than porcelain in the long run.