Hi.. Can understand your concern.. As per your complain in case if you have had infection below the crown of the root canal treated tooth firstly I would suggest you to get an x ray done and get checked that the root canal filling in the canals is intact and there is no infection below the root tips of the tooth as if it is there it can get activated any time and cause pain.. In case if there is not much of tooth structure remaining then simply filling with the glass ionomer cement will not retain the crown as it will fracture on applying chewing pressure and crown will come out with the filling.. In case if the crown has to be replaced and there is sound bone support around the tooth the treatment modality which is done is POST and CORE TREATMENT in which a small rod (post) of inert material is placed in the largest root canal and filling is done over it ( core build up) and crown is placed over it. The post placed provide anchorage to the filling and it does not fracture on chewing. In case if the natural crown is totally fractured and only roots are present better go for extraction followed by implant placement.. Hope this information helps.. If you find the answer helpful please write a positive review and click on find this answer helpful as a token of appreciation.. Thanks and regards.. Dr.Honey Nandwani Arora..