The Effect of Isthmus on Vertical Root Fracture in Endodontically Treated Teeth

• The effect of an isthmus connecting root canals on vertical root fracture (VRF) is studied experimentally and analytically.
• Cross sections of roots with VRF were examined for interior damage.
• An analytic model to assess the apical force applied to gutta-percha that is needed to cause VRF was developed.
• The presence of isthmus drastically enhanced the likelihood of VRF; apical loads as little as 10 N may cause VRF in 2-canal roots.

IntroductionVertical root fracture (VRF) from apical condensation of gutta-percha is a common failure mode in endodontically treated teeth. Virtually all previous studies of VRF are limited to 1-canal roots. In this study, we consider experimentally and analytically VRF in roots with 2 canals.MethodsThe interior root morphology in mandibular molar teeth extracted from patients due to VRF or other reason was examined from a series of polished horizontal cross sections. A 2-dimensional fracture mechanics analysis was used to determine crack growth from the canal surface to the outer root surface and evaluate the apical load needed to cause VRF, Fmax.ResultsFrom a mechanistic viewpoint, the isthmus connecting root canals can be regarded as a natural weak plane or crack. The results expose the prime role of isthmus in reducing Fmax, from ≈ 50 N with no isthmus present to ≈ 10 N.ConclusionsTwo-canal mesial roots are much more prone to VRF than 1-canal distal roots. We suggest that VRF may occur during clinical condensation of gutta-percha in mesial roots of mandibular molars as well as other roots with canals connected by isthmus.