The effect of nickel-titanium rotary systems on the biomechanical behaviour of mandibular first molars with curved and straight mesial roots: a finite element analysis study

The objective of the study was to discuss the effect of root canal preparation using files with varying tapers and active cutting parts on stress distribution in mandibular first molars with curved and straight mesial roots via 3-dimensional (3D) finite element analysis (FEA). Mesial root canals of six artificial mandibular molar models were prepared up to ProTaper Universal F2 (size 25/0.08 taper) (Dentsply Sirona Endodontics, Ballaigues, Switzerland), ProTaper Universal F3 (size 30/0.09 taper ), FlexMaster (VWD, Munich, Germany) (size 25/0.04 taper), FlexMaster (size 30/ 0.04 taper), Hero Shaper (Micro-Mega, Besançon, France) (size 25/ 0.04 taper) and Hero Shaper (size 30/ 0.04 taper). The 3D models with curved mesial roots were constructed from the micro-computed tomographic images of the six prepared models and also one unprepared artificial model. Subsequently, 3D molar models with straight mesial roots were developed. Occlusal force of 200 N was applied and FEA was performed using the Abaqus software (Abaqus 6.14, ABAQUS Inc., Providence, RI, USA). Maximum Principal Stress (PMax) values and distributions were calculated. In both curved and straight rooted groups, the highest Pmax stresses were recorded for ProTaper F3 model and the lowest Pmax stresses were recorded for intact molar model, followed by Hero Shaper (size 25/ 0.04 taper) model. All of the models in straight rooted group exhibited lower Pmax values than their counterparts with curved mesial roots. Curved mesial root anatomy may increase the risk of fracture in mandibular molars compared to straight mesial root anatomy. Nickel-titanium rotary systems with a short active cutting part and a low degree of taper may be advantageous in terms of reducing fracture risk in mesial roots of mandibular molars.