When teeth pulp devitalizes due to advanced caries or trauma, and the tooth is immature and the root apex is not fully formed, there are 2 ways to treat this:
Apexification whereafter the canal is cleaned, a bioceremic material like MTA is placed as a solid plug in the apical foramen area. The canal is then sealed and restored.
Regenerative Endodontics is a new treatment procedure where after the canal is cleaned and disinfected, bleeding is introduce into the canal by poking at the apical tissue beyond the apical foramen. The access is then sealed off. With time the blood clot differentiates into dento-pulpal complex and root continues to form and mature.