Studies suggest a high correlation between tooth fracture to large fillings and root canal treated tooth. A study by Larson (1981) determined that fillings that take up 1/3rd of the “intercuspal distance” of a tooth decreased the resistance to fracture by more than 50%.
Larson TD, et. al. Effect of prepared cavities on the strength of teeth. Oper Dent. Vol. 6, No. 1. 1981
Geurtsen (2003) found tooth’s risk for fracture increased substantially as the filling approached 50% of its intercuspal distance. Geurtsen’s recommendation was that fillings should not exceed 1/3 to 1/4 this distance.
Geurtsen W, et. al. Diagnosis, therapy, and prevention of the cracked tooth syndrome. Quintessence Int. Vol. 34, No. 6. June 2003.
Depending on how the tooth has fractured, these natural teeth can, occasionally be restored or saved without major consequences. However, prognosis drops significantly when fracture tooth is presented with pain, worse still, a tooth which has already fractured in half can only be removed.
American Association of Endodontists has a great article with animations to better explain the different scenarios and treatment options.