Both root canal treatment and extraction are viable options for a severely decayed tooth.
Factors to consider include:
1) Patient age
Both root canal treatment and tooth replacement (implants, bridges or dentures) have potential complications. Some complications may be easier to manage than others (root canal treatment failure vs implant failure). Some tooth replacement options (bridges) may require irreversible treatment for the adjacent teeth.
Since ALL dental treatments will need replacing at some point in the future, I would consider treatment that preserves existing tooth structures before artificial replacements. For example, if you are in your 30s, your teeth would have to last at least another 50 years based on current estimates. That would mean some dental work would need replacing about 4-5 times.
2) The amount of tooth structure remaining
A severely damaged tooth may be more prone to an irreparable fracture later on despite being crowned. The amount of healthy tooth structure can only be properly assessed after ALL decay and existing filling material has been removed.
3) The presence of endodontic (root canal-related) infection
A chronically infected tooth may not completely heal even after root canal treatment. However, the size of the infection does not necessatily indicate the relative chance for success. This is assessed using xrays of the tooth roots.
4) Biting pattern
If you are missing an opposing molar, then the tooth may be classified as non-functional (not participating in chewing function). The strategic importance of a particular tooth needs to be assessed.
5) Cost
The cost of a root canal treatment and crown may be lower than that for an implant.
