Fracture Load of Lithium Disilicate Screw-Retained Implant Crowns of Different Contours: A Comparative Stud

Aim.;The aim of this study was to evaluate the effects that variations in cantilever length have on fracture strength and maximum load of screw retained implant ceramic crowns that are constructed using CAD/CAM technology.;Materials and Methods.;A scan body was mounted in a customized typodont with an implant replica located in the center of the space of tooth #30, 3 mm below the buccal cemento-enamel junction of the adjacent molar #31. The scan body was scanned using a chairside scanner. The computer software was used to digitally design three clinically appropriate anatomical crowns for a mandibular first molar with different cantilever lengths. For Group 1 the implant position and the screw openings were located in the center of the crown. For Groups 2 and 3, the implant position and the screw openings were repositioned 1.5 mm and 3 mm distally from the center. After size and contour of the crowns had been inspected and confirmed digitally for each group, 30 lithium disillicate crowns were milled, 10 for each group. After firing, each crown was luted to its respective Vario-base with a universal resin cement under a standard weight of 5 kg. After cementation, all samples were subjected to thermocycling for 10000 cycles Then, all groups were loaded until catastrophic fracture occurred.;Results.;The mean fracture strength for Group 1 (equal ceramic dimension mesial and distal to screw hole) was 1407 N (SD = 454 N), for Group 2 (7 mm mesial "cantilever" extension) 1722 N (SD = 311 N), and for Group 3 (8.5 mm mesial "cantilever" extension) 1708 (SD = 293 N). The difference between groups was not statistically significant (p-value = 0.113; one-way analysis of variance). For the maximum load, Group 1 demonstrated the lowest mean value of 1453 N (SD = 449 N) followed by Group 2 with a mean value of 1733 N (SD = 313 N), while Group 3 showed the highest mean value of 1939 (SD = 116 N). The difference between the groups was statistically significant (p-value = 0.012; one-way analysis of variance). Post-hoc tests showed that there was a statistically significant difference between Group 1 and Group 3 (p-value = .009; Tukey's HSD). However, there was so statistically significant difference between Group 2 and Group 3 (p-value = 0.376; Tukey's HSD) and Group 1 and Group 2 (p-value = 0.158; Tukey's HSD).;Conclusion.;Within the limitations of this study, different mesial extensions of lithium disillicate screw-retained implant crowns did not lead to significant differences in fracture strength values. Lithium disilicate screw-retained implant crowns with equal mesiodistal crown dimensions demonstrated statistically significantly lower maximum load than crowns with a cantilevering extensions of 8.5 mm. Crowns that have a substantial cantilevering contour should be handled with caution because of the increased chance for unfavorable fracture patterns.