Mesiobuccal root morphology influences biomechanical stability after endodontic microsurgery in maxillary first molars: a finite element analysis study

Endodontic microsurgery (EMS) is an established treatment for refractory periapical lesions; however, late failures related to dentin fatigue and root fracture remain clinically challenging. The mesiobuccal (MB) root of maxillary first molars exhibits pronounced cross-sectional variability that is readily identifiable on cone-beam computed tomography (CBCT), yet its biomechanical relevance in apical surgery has not been adequately investigated. This in vitro study aimed to investigate the interaction between MB root morphology and EMS extent on postoperative stress distribution and root fracture risk. Validated three-dimensional finite element models of post-root canal-treated maxillary first molars were constructed from CBCT data. Two clinically representative MB root morphologies, circular and oval cross-sections, were simulated. For each morphology, root-end resections of 2 mm, 3 mm, or 4 mm, as well as complete MB root amputation, with mineral trioxide aggregate retrograde filling were modeled. An intact tooth model served as control. Oblique functional loading (200 N) was applied. Relative changes in dentin and periodontal ligament (PDL) stress were assessed, and morphology-dependent fracture susceptibility was quantified using Weibull analysis. Increasing root-end resection length produced progressive elevations in dentin stress and leftward shifts in fracture probability curves, with complete MB root amputation showing the most unfavorable biomechanical response. Peak stresses were consistently localized to the mid-to-upper root region. Root morphology significantly modulated behavior: although oval roots exhibited slightly higher baseline stresses, they demonstrated attenuated stress amplification and smaller increases in fracture probability following resection compared with circular roots. PDL stress increased modestly with resection extent, particularly after root amputation, reflecting load redistribution to the remaining roots. In this finite element analysis study, the biomechanical stability after EMS appears to depend on both the extent of root-end resection and MB root morphology. Circular MB roots showed greater vulnerability to stress increases and fracture probability with longer resection, suggesting benefit from more conservative approaches. Oval roots tolerated structural loss better under the modeled conditions. These biomechanical findings suggest a potential role for preoperative morphology assessment in guiding resection extent to optimize root strength.