Most back teeth (premolars, molars) that require root canal treatment have extensive cavities (decay) and/or fillings undermining the structural integrity of the remaining tooth structure. The weakened remaining tooth structure is prone to breaking after root canal treatment is completed since the filling material used to fill up the hole left behind by the decay and root canal treatment is unable to splint the tooth externally.
Cracked teeth that have never had decay or a filling are also at risk of the remaining tooth structure splitting apart under the force of biting.
A crown is the only way to keep the remaining tooth structure together to prevent an unrestorable tooth fracture after root canal treatment, that may require extraction of the tooth.
