Fracture resistance and failure modes in 4- vs. 6-implant supported zirconia full-arch prostheses with and without cantilevers: an in vitro study

PURPOSE. To evaluate the fracture resistance and failure modes of IFCDPs supported by 4 versus 6 implants, with and without cantilever loading. MATERIALS AND METHODS. Forty monolithic zirconia IFCDP specimens (20 with 4 implants, 20 with 6 implants) were fabricated using CAD-CAM technology and bonded to titanium copings on titanium alloy casts. Specimens were divided into four groups (n = 10 each): Group A (4 implants, non-cantilever load at second premolar), Group B (4 implants, cantilever load at second molar), Group C (6 implants, non-cantilever load at second premolar), and Group D (6 implants, cantilever load at second molar). Mechanical testing was performed using a universal testing machine with a three-point bending fixture loading bilaterally. Failure modes (fracture or cement debonding) and maximum failure loads were recorded. Statistical analysis included Fisher's exact test for failure mode distribution and two-way ANOVA for failure load. RESULTS. Non-cantilever loading caused framework fractures at high mean loads (Group A: 25,105 ± 5744 N; Group C: 27,189 ± 5753 N), while cantilever loading led to cement debonding at significantly lower failure loads (Group B: 1,070 ± 404 N; Group D: 828 ± 432 N). Loading site significantly influenced failure load (P < .001), but implant number and its interaction with loading site had no significant effect (P > .05). CONCLUSION. Both All-on-4 and All-on-6 zirconia full-arch prostheses show high static fracture resistance, but cantilever loading greatly reduces strength and induces debonding, underscoring the need to minimize cantilever length or improve bonding strategies.