Caries into the pulp. Clearly, endo is indicated, but how do you build up you temporary prior to extirpation and canal preparation?
One method is to pop a little cavit in the chamber then restore as normal and reaccess through the restoration. This works fine when you feel confident you can easily reproduce the access cavity with little risk of deviation.
To be honest, endo access for calcified or thin teeth makes my cortisol spike. So when there is clear caries to the pulp chamber and I’ve got access, I prefer not to completely close it, only to need to reaccess again.
To keep patency, try placing a large paper point in the Canal. Then your free to get a well sealed temporary in, easily maintaining your conservative access. Once the temp is done, ensure your isolation is still ideal and get on with the endo without wasting time finding that access again!