The evaluation of the influence of different finishing line depth on strength of CAD/CAM restorations

The aim of this study is to evaluate the effects of different shoulder finishing line depth (0.5 mm, 1.0 mm and 1.5 mm) on fracture strength of coping design as one type of CAD/CAM restorations. Using Lithium disilicate (IPS e-max CAD; Ivoclar Viva dent) and Feldspathic block (CEREC blocks; Sirona) materials for milling of three type of design according to the finishing line depth (FLD), which was done with Cerec inLab software program (Version, 4.2.0.57192) with 120 μ cement film thicknesses. A total of 72 specimens of coping design were prepared as monolithic crown on 72 novel nickel chrome dies. Each with a diameter of 8 mm and height of 6 mm as they were used to support the restoration. 12 specimens of coping design for each material and finishing line depth where prepared, to be in three groups according to FLD. Panavia V5 resin cement system where used as a luting agent for fixation of each coping restorations. For the measurement of fracture resistance value, each sample where subjected to fracture test with a universal test machine.It was found that Lithium disilicate crowns with 1.5 mm thickness of marginal design showed the maximum mean value of fracture strength of about 2721 ± 967 N among all the groups, whereas, the minimum mean valueof fracture strength was recorded with Feldspathic one with 1296.7 ± 642 N at the same depth of shoulder finishing line (1.5 mm). Kruskal –Wallis and Mann-Whitney testsas well as Annova test were performed to compare all groups with different P values. Concluded that the effect of different depth of finishing line on the fracture strength of CAD/CAM restoration material was evedent with variable results wich was related to different factors. As it is clear from the results of the present investigation. Lithium disilicate crown material seems to be increased their strength to fracture as the finishing line depth where increased. While in case of Feldspathic restorations, the strength where increased from 0.5 mm to 1.0 mm depth of finishing line restorations, but the strength were decreased, when the marginal depth increased to be 1.5 mm.