MTA (mineral trioxide aggregate), ProRoot, Dentsply, is a versatile material that can be used to repair perforations, apexification, it can be used in pulpotomies, apicoectomy and to apical seal zipped or wide open (blunderbuss) apices.  In this case the foramen of tooth number 21 was zipped and control of the apical seal would have been very difficult with gutta percha.

In this case the gutta percha was overextended in tooth number 25.  The gutta percha was removed completely, and replaced with MTA.  The tooth was built up with a glass fibre post and composite core and then crowned.  Note tooth number 24 apical healing.