Onlay is never a problem. Infact if your dentist is not comprehending a crown lengthening procedure in case of minimal occlusal space, onlay is the right option. A crown done on it will lead to frequent dislodgement of it from the tooth. Onlay is fine with a sturdy core buildup. You dentist has advised you well..

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Hello An onlay filling is better when there is not sufficient tooth structure to hold a full crown. Cost wise also it might be a little cheaper than full crowns. Morover your dentist knows better about the cost etc in their place. So thats not the problem here.

After Root Canal Treatment (RCT) is over your tooth gets brittle. So there is chances of crown fracture while chewing with that tooth. You should go for Artificial Crown over RCT treated teeth. As your doctor is telling that you have less tooth structure remaining after Root canal treatment, so you should have a METAL POST over that RCT treated tooth followed by Artificial Crown over it.

Namaskaram, Crown after a root canal treatment is mandatory as the tooth becomes brittle as it looses its natural resilience. crown preparation requires decent amount of tooth structure . But some times we may not have enough structure to cap the root canal treated tooth in such cases we go for other options like on-lays , post and core etc . your dentist must have weighed all the alternate options and then suggested you the best treatment. kindly get your treatment done as soon as possible before the tooth becomes more brittle and breaks down. Team Shambhala.