Endo therapy is performed by creating a very small tunnel in the top of the tooth, using a series of very narrow nickel-titanium files to travel down the pulp space or root canals to remove the affected pulp tissue and then finally shaping, disinfecting and then filling and sealing the pulp space. Once it is filled, the tooth can be restored. Teeth that have undergone endo therapy almost always have to be restored with a crown. This is because once endo therapy has been done, the tooth is no longer receiving any internal circulation and will desiccate or dry out. This is similar to the difference between walking up to a tree and trying to bend one of its branches versus picking one up off the ground that has been there few days and bending it. Obviously, the one off the ground will snap and break much easier while the one on the tree will bend and flex. The tooth is similar. Endo treated teeth have a very high incidence of fracture when they are not protected by a crown.