effect of diameter and preparation method of occlusal screw –access channel on fracture strength of cement retained implant - supported posterior monolithic zirconia crowns

Abstract

The favorable fracture strength of implant-supported crowns is one of the effective factors in gaining the long-term success of dental implants. The Use of zirconia crowns can be an acceptable treatment in relation to metal-ceramic crowns for gaining the best clinical success in dental implants. Purpose: Considering that there are few studies on the effect of the diameter of the occlusal access channel on the fracture strength of these restorations, and currently there are no accepted guidelines for determining the appropriate diameter of the occlusal access channel, the present research aims to evaluate the effect of diameter and occlusal screw-access channel preparation method on fracture strength of cement-retained implant-supported posterior monolithic zirconia crowns with the aim of their retrievability. Material and methods: A total number of 56 cylindrical implant fixtures with internal hexes with 4,5 mm diameter and 10 mm length and 56 abutments with 4,5 mm diameter were selected. Full-contour posterior Multi-layer monolithic zirconia crowns were designed with and without occlusal screw-access channel. For crowns with an initial occlusal screw-access channel , a computer model was designed with an access channel in the center of the occlusal surface with 2.3 mm and 3.5 mm diameters, 10 pieces each (20 pieces in total) While the computer model for the crowns without access channel as the control group or the crowns in which the occlusal screw access channel is prepared with a Coarse Grit diamond round bur was designed as an intact occlusal surface in the number of 10 and 20 , respectively (30 in total). In addition, 6 crowns with intact occlusal anatomy to perform SEM and evaluation of surface defects caused by preparation with a bur with diameters of 2.3 mm and 3.5 mm were designed, 3 each. The obtained data were collected and analyzed in SPSS 17 software by one-way analysis of variance and then by Games-Howell post hoc test. Results: The Maximum value of fracture strength was related to the control group with a mean of 3436.74 N. after them, the highest value of fracture strength was in the initial access channel group with a mean of 3260.93 N. the minimum value of fracture strength of groups were in crowns with a screw-access channel prepared with a bur with 3.5 mm diameter with a mean of 1936.43 N. the results of this study showed that there was a significant difference between the mean of fracture strength of groups (P<0.01). The highest difference in the mean of samples with occlusal access holes was between A1 and B2 groups. And also, the results of this study showed that the mean fracture strength in the groups with a 2.3 mm diameter was higher than in groups with a 3.5 mm diameter (P<0.05).but, from the fracture strength point of view, the method preparation of access channel and it’s diameter had no synergistic effect on each other (P>0.05). Examining the samples under the electron microscope confirmed the presence of obvious cracks in access channel prepared with bur groups compared to the initial ones . Conclusion: the results of this study showed that in clinical conditions, using an initial access channel with a diameter of 2.3 mm is the best choice to increase the fracture strength of hybrid implant-supported posterior monolithic zirconia crowns, But in a situation where the clinician has to prepare an occlusal access hole with a bur, the clinician should keep in mind that increasing the diameter of the channel will significantly affect the strength of the crown.