Objectives: To perform a comprehensive fractographic analysis of split-tooth fractured endodontically treated posterior teeth (ETT) restored with direct resin composite.

Methods: 9 cases of recovered fractured ETT (2 premolars, 7 molars) that had been restored with micro and nano hybrid resin composites (3 MOD, 4 OM/OD, 2 O), were donated from dental offices with time in service ranging between 3 and 8 years. Fracture surfaces of the recovered parts were analyzed using digital 3D and scanning electron microscopy for identification of failure mode, crack origins, direction of crack propagation based on the recognition of characteristic fracture surface features as well as occlusal surface degradation in areas of contact loading.

Results: Mode I fracture (wedge-opening) occurred in all 9 specimens. Fracture origins were located on the restorations occlusal surface and fracture propagated apically with easily recognizable fractographic features (arrest lines, hackle, twist hackle, wake hackle, compression curl) on the resin composites fracture surfaces. Along with enamel cusp contact wear, signs of resin composite fatigue degradation from cyclic loading were cavitations and microcracks located in areas of contact loading and wear near origins.

Significance: Limited to a small number of cases only, this fractographic research provided evidence that mesio-distal split-tooth fractures resulted from fatigue-driven occlusal surface cracks growing critically under functional loading. The presence of marked occlusal contact wear in the vicinity of crack origins on both, the resin composite and the enamel cusps, are clear occlusal stress indicators emphasizing the need for better patient and tooth risk assessment before decision making on the best treatment option.