The use of all-ceramic crowns over zirconia abutments is a well-established esthetic treatment option in implant dentistry; however, the effect of the mechanical processing due to abutment preparation has not been investigated under functional loading. The purpose of the study was to evaluate the influence of the zirconia abutment preparation depth and preparation mode on the fracture strength and fracture mode of lithium disilicate crowns after chewing simulation.Material and methods:
Seventy single implant-supported lithium disilicate glass-ceramic crowns (IPS e.max Press, Ivoclar Vivadent) were adhesively cemented (Multilink Automix, Ivoclar Vivadent) onto zirconia abutments (ZirDesign, Astra Tech) using implants with a diameter of 4.5 mm and a length of 15.0 mm (Osseospeed, Astra Tech). Study design concerned the replacement of a maxillary central incisor (11.0 mm in height and 8.0 mm in width). Subgroups (n = 7) were subjected to dynamic loading (C) up to 1.2 × 106 loading cycles at 135° with 98N in a thermomechanical chewing simulator (Kausimulator, Willytech); followed by quasi-static loading at a cross-head speed of 0.5 mm/min until fracture in a universal testing machine (Z010/TN2S, Zwick). Additional subgroups were also subjected to quasi-static loading (S) at 135°. Lithium disilicate implant crowns were divided into five study groups (n = 14) according to the abutment preparation depth [A (control): 0.5 mm, B: 0.7 mm, C: 0.9 mm, and preparation mode [(No label): milling by the manufacturer, (P): copy-milling by the Celay System (Mikrona)].Results:
All specimens survived dynamic loading and mean fracture strengths (N) after quasi-static loading were as follows: Group SA: 384 ± 84; Group CA: 403 ± 67; Group SB: 294 ± 95; Group CB: 374 ± 75; Group SC: 332 ± 52; Group CC: 373 ± 105; Group SPB: 332 ± 80; Group CPB: 499 ± 91; Group SPC: 380 ± 101; and Group CPC: 358 ± 54.Conclusions:
Statistical analysis using multiple linear regression showed that both the preparation depth and mode had no influence on the fracture strength of the implant crowns (P > 0.05); however, fracture strength increased statistically significantly after 5 years chewing simulation (P = 0.01).