The hedge is over 50 years old. Numerous dead branches can be found within the hedge. Is there an advantage to removing them or should the trees be trimmed only from the outside? From the outside it appears to be in a very healthy condition. I live in the Ottawa-Gatineau area.
It is always a good idea to remove dead material as it is likely to invite pests and diseases. There will be an open spot when you remove dead material but that is preferable to having the tree die altogether.