One-piece zirconia endocrowns: An in vitro study comparing the bacterial adhesion, fracture resistance, and failure type

Abstract Introduction: Zirconia endocrowns (ECs) combine high fracture resistance with low bacterial adhesion potential, yet the optimal axial height for clinical success remains unclear. This in vitro study assessed and measured the effect of different axial height-length designs on the bacterial colony-forming unit (CFU), fracture resistance, and fracture patterns in one-piece molars’ ECs. Materials and Methods: Forty fresh extracted maxillary and mandibular molar teeth were categorized according to the remaining axial height (extending from 2 to 4 mm on all surfaces), with an additional 2 mm intrapulpal extension. Specimens underwent thermocycling for 10,000 cycles during aging, and then the bacterial CFU of Streptococcus mutans bacteria was counted. A universal testing device was utilized for testing the fracture resistance and failure types, which were classified under magnification. Data were analyzed using the SPSS to assess significance between and within groups, where a P ≤ 0.05 indicated statistical significance. Results: The count of adhered S. mutans bacteria was high in all groups, 498 ± 5.027 and 516 ± 7.967 (CFU/mm²), without significant differences, P > 0.05. Fracture force analysis in Newton indicated the highest values for 2 mm mandibular one-piece ECs for molars (2658.15 ± 118.41), with significant differences compared to maxillary one-piece ECs molars at 2 mm ( P < 0.05) but with no significant differences at 4 mm ( P = 0.72). Failure patterns showed that Type V fractures (irreparable) were most common in mandibular one-piece ECs, whereas Type III failures (reparable) were least frequent. Conclusion: One-piece ECs with a 2 mm axial height in both arches showed the highest bacterial CFU and fracture resistance. Irreparable fractures were more common in mandibular one-piece ECs, whereas most of the maxillary molars show an irreparable fracture type.