Effect of Angulated Screw Channel (ASC) design on the fracture resistance of implant-supported monolithic zirconia and lithium disilicate crowns: an in vitro study

Abstract

Ti-base abutments with angulated screw channel (ASC) designs have been developed to address the esthetic challenges associated with labial screw access in anterior implant-supported single crowns, as well as the ergonomic limitations caused by restricted mouth opening in posterior crowns. Despite their increasing clinical use, there is limited research evaluating the impact of this design on the fracture resistance of various monolithic restorative materials across different anatomical regions, particularly after aging. This study aims to investigate the effect of angled screw channel design on the fracture resistance of implant-supported single crown restorations made of different materials in different anatomical regions after long-term thermal aging. In the study, a total of 80 implant-supported single crown restorations were prepared and then divided into subgroups ( n = 10) considering anatomical region (anterior/posterior), restoration material (monolithic zirconia/lithium disilicate), and screw channel design (straight 0 ° /angled 20 ° ). Monolithic zirconia and lithium disilicate crowns were fabricated in the form of maxillary central incisors for anterior, and mandibular first molars for posterior specimens. After cementation on Ti-base abutments, the specimens were aged with 40,000 thermal cycles (5 ° C/55 ° C), and the fracture resistance of the crowns was subsequently determined using a universal testing machine under compressive loading. Fracture patterns were examined under a stereomicroscope. Three-way analysis of variance followed by a Bonferroni post hoc test was used in data analysis (α = 0.05). According to the results, anatomical region, restorative material, screw channel design, and the interactions between these variables had a significant effect on the fracture resistance of implant-supported single crown restorations ( p < 0.001). The use of ASC design reduced the fracture resistance of the restoration material in all groups, regardless of anatomical region and material type ( p < 0.001). In the anterior region, using the ASC design resulted in a statistically significant strength loss, which was similar across all materials. In the posterior region, although zirconia material showed significantly higher fracture strength than lithium disilicate, the strength loss was more pronounced. The fracture loads observed in all specimens remained within reported physiological bite force ranges under the present experimental conditions. In conclusion, although the ASC design offers clinical advantages for implant-supported restorations, its effect on reducing the mechanical strength of the restorative material should be taken into account in clinical practice. While the fracture loads observed in this study remained within physiological limits, further investigations under long-term and in vivo conditions are warranted.