Comparison of mandibular premolars and canines with respect to their resistance to vertical root fracture

Objective. To compare the force required to vertically fracture uninstrumented and instrumented mandibular premolars and canines. Methods. Two groups of single round-canal mandibular premolars and two groups of single oval-canal mandibular premolars were selected after radiographs indicated an internal long:short diameter of 1 or >2 at a level 5 mm from the apex. In addition, two groups of maxillary and mandibular canines were selected. The root canals of three of these groups (one from each category) were conventionally enlarged using same-sized instruments, while the other three groups were uninstrumented. Each root was mounted in an acrylic resin cylinder, and the force required to cause vertical root fracture (VRF) was measured using an Instron tester. Results. No significant difference in resistance to VRF was found between the round- and oval-canal premolars with (p=0.9) or without (p=0.6) instrumentation. The average force required to fracture the uninstrumented premolars and canines was 698 and 566 N, respectively. The average force required to fracture the instrumented premolars and canines was 490 and 554 N, respectively. The force required to fracture the instrumented premolars and canines was 30% (p<0.05) and 2% (p=0.9) lower, respectively, than that required to fracture their uninstrumented counterparts. Conclusions. The instrumented mandibular premolars have a higher risk to fracture than the uninstrumented mandibular premolars.