CAD/CAM lithium disilicate crown performance cemented extraorally and delivered as a screw-retained implant restoration

Abstract

Purpose: To determine if a novel technique combining the attributes of a cement-retained implant restoration fabricated extraorally and delivered to the patient as a screw-retained implant restoration has the necessary strength to provide a clinically acceptable and predicable restoration.Materials and Methods: Thirty specimens were fabricated and tested in this novel implant restoration technique, in which stock abutment was scanned using a bench top laboratory scanner and 30 lithium disilicate full contour crowns were designed and milled. In the first experimental group, the occlusal access channel was prepared in a pre-sintered crown using new high-speed diamond burs in a high-speed handpiece with ample irrigation as to keep the specimen cool. The access channel was prepared by the same operator for every specimen and the diameter was recorded. The specimens were allowed to air dry for 48 hours prior to being glazed, fired and finished. In the second experimental group, the screw access channel was prepared after the crown was fired and finished. In the control group, no screw access channel was prepared. Each finished crown intaglio surface was silinated per manufacturer specifications and luted with self-adhesive resin cement to its corresponding stock abutment. The cement was allowed to cure for at least 24 hours before testing. Each specimen was individually mounted in a custom-fabricated testing fixture and tested to failure on a servo-hydraulic testing system for static and dynamic tests. Each specimen was vertically loaded at a dynamic rate of 0.100 mm/min until failure and the highest force reached at the point of failure was recorded. Statistical analysis was performed by consultants from the Biostatistical Design and Analysis Center. Results: A total of thirty CAD/CAM lithium disilicate crowns were fabricated and tested to failure. The first experimental group had a mean failure of 990.64N. The second experimental group had a mean failure of 1167.65, and the control group had a mean failure of 188.68N. A two-sample t-test was used to compare the load among the three groups and because there are 3 comparisons, Bonferroni method is applied to adjust p-values for multiple comparisons. The results show that experimental group #1, experimental group #2 and the control group are statistically significantly different from each other. The diameter of the screw access channel did not make a statistically significant difference, most likely because the difference among the diameter wasn't that great between samples.Conclusions: The null hypothesis stated there will be no difference in the axial force required to fracture a lithium disilicate crown with and without a screw access channel prepared. The results of this study support rejecting the null hypothesis and accepting the alternative hypothesis. The preparation of a screw access channel in a lithium disilicate crown has statistical significance and reduces the axial load capacity from a crown without occlusal access. The diameter of the screw access channel did not make a statistically significant difference, most likely because the difference among the diameter wasn't that great between samples.