Step-Stress Fatigue, Fracture Toughness and Load to Failure of an Additively Manufactured Ceramic Filled Hybrid Crown Material

Abstract

Background: The prevailing trend in dental practice is toward fully digital workflows with additive and subtractive manufacturing. Computer Aided Design Computer Aided Manufacturing (CADCAM) and subtractive manufacturing have led to a proliferation of established restorative materials. In September 2021, the FDA approved Varseosmile Crown Plus, by Bego, for utilization as a permanent crown. This novel composite is the first additive manufactured material to receive approval as a ceramic hybrid for permanent single crowns, inlays, onlays and veneers.[1] The manufacturer categorizes Varseosmile Crown Plus as a ceramic filled hybrid composed of esterification products of 4.4'-isopropylidiphenol, ethoxylated and 2-methylprop-2enoic acid, silanized dental glass.[2] The total content of inorganic fillers (particle size 0.7 μm) is 30 – 50 % by mass.[2] Information published on the mechanical properties of Varseosmile Crown Plus is limited due to its novelty. Hypotheses: There will be no difference in step-stress fatigue on disc specimens, no difference in fracture toughness on bar shaped specimens and no difference in load to failure on tooth shaped specimens between Varseosmile Crown Plus and Vita Enamic. Methods: Two materials were assessed in three tests. Six bars of both materials were utilized to assess fracture toughness following the ISO6872:2015[3] standard. The fracture toughness was assessed utilizing the single edge v-notch beam (SEVNB) in a three-point bending test. Ten samples of each material were used to evaluate the fatigue resistance utilizing the stepwise stress method on disc samples according to ISO6872:2015[3] with a piston-on-three-ball configuration at room temperature under dry conditions. The final material property assessment was load-to-fracture in crown shaped samples adhesively bonded to dentin analogs. A mandibular right first molar (#30) was prepared on a typodont. The preparation was scanned and digitized to make twenty two dentin analogs (NEMA grade G10). A restoration was designed, and eleven crowns were printed or milled from each material. Restorations were bonded to the G10 analog with self-adhesive resin cement. Each crown was axially loaded utilizing a universal testing machine until failure. Results were recorded and compared between groups using two-sample t-tests and Weibull survival analysis. Results: Step-stress fatigue testing revealed Vita Enamic had an average maximum tensile stress of 143.3  12.7 MPa and Varseosmile Crown Plus demonstrated an average maximum tensile stress of 107.5  21.8 MPa. The differences in fatigue failure load (p<0.001), number of loading cycles survived (p<0.001), number of steps survived (p=0.006) and average maximum tensile stress (p<0.001) were all statistically significant between the two materials tested. The fracture toughness testing resulted in an average fracture toughness of 1.343 ± 0.067 MPam for Enamic and 0.878 ± 0.095 MPam for Varseosmile Crown Plus. The difference in fracture toughness between the tested materials was statistically significant (p<.001). The 95% confidence interval for the average KIC value was 1.2735 to 1.4132 MPam for the control and 0.7787 to 0.9771 MPam for the experimental material. The average flexural strength for Varseosmile Crown Plus was 73.619 ± 21.76 MPa. The load to failure testing on crown shaped specimens produced an average failure load of 2342.46 ± 225.46 for Varseosmile Crown Plus and 1871.09 ± 455.07 N for Vita Enamic. Conclusion: A clinical use of Varseosmile Crown Plus should expect a reduction in service life relative to an equivalent restoration of Vita Enamic based on the significantly reduced mechanical properties of fatigue strength and fracture toughness. The load to failure on a crown shaped specimen suggests high initial resistance to compressive force vectors. An established protocol for best practice methods of mixing the composite resin before printing needs to be identified. Once established, the mechanical properties should be re-evaluated, and clinical studies conducted to evaluate for real-world clinical outcomes.