Fracture Load of Screw-Retained Implant Crowns of Different Contours.

Objectives: To evaluate the effects that variations in cantilever length have on fracture strength and mode of failure of screw retained implant crowns.
Methods: A scan body was mounted in a typodont with an implant replica located in the center of the space of tooth#30. The scan body was scanned using a chairside scanner. Computer software was used to digitally design three crowns for a mandibular first molar with different cantilever lengths. For Group 1 the implant was located in the center of the crown. For Groups 2 and 3, the implant position was repositioned 1.5mm and 3mm distally from the center, respectively. Then, 30 lithium-disilicate crowns were milled, 10 for each group. After firing, crowns were cemented to Vario-base with resin cement. Then, they were subjected to thermocycling for 10000-cycles. Force of fracture and mode of failure were recorded.
Results: After thermocycling, one sample from Group 3 was eliminated due to crack development. Mean and (SD) fracture strength were as follows: Group 1, 1407(454) N, Group 2, 1722(311) N, and Group 3, 1708(293) N. The difference between groups was not significant (p=0.113; one-way ANOVA). Fisher’s exact test comparing the groups’ mode of failure was significant (p<0.001). In post-hoc tests, the difference between Groups 1 and 3 was significant (p<0.001), as was the difference between Groups 2 and 3 (p<0.001), but there was no significant difference between Groups 1 and 2 (p=0.474). For Group 1, 100% of the specimens exhibited only ceramic fracture; for Group 2, 20% of the specimens exhibited abutment fracture and abutment deformation, and 80% of the specimens had a ceramic fracture; for Group 3, 67% exhibited abutment fracture and 33% exhibited ceramic fracture.
Conclusions: Crowns that have large cantilevering length should be handled with caution because of the increased chance for unfavorable fracture.