In vitro evaluation of fracture strength and marginal fit of titanium substructures produced by additive manufacturing

Objective: Despite the many advantages of titanium alloy; casting is difficult and manufacturing by milling method is expensive and time consuming. Easily manufacturing with SLM method has eliminated these disadvantages. Marginal fit of prosthetic restorations is of vital importance for the long-term success of the restoration. Therefore, in our study, it was aimed to compare the marginal fit and fracture strength of three-member titanium substructures produced by additive manufacturing (SLM) with Co-cr and zirconium, which are frequently used substructure materials in prosthetic dental treatments, in order to evaluate their alternative potential to substructure materials used with layered porcelain. Materials and Methods: For our study, metal dies made of stainless steel material with 5 mm height, 6 degree convergence angle, 1 mm chamfer step representing the prepared form of mandibular 5 and 7 teeth were obtained. 20 Co-cr substructures produced by direct metal laser sinter method, 20 zirconium substructures produced by CAD/CAM milling method and 20 titanium (Ti6Al4V) substructures produced by SLM method were created on the relevant design. Silicone replica technique was used to evaluate the marginal fit of the produced substructures. The created silicone replicas were divided into 4 equal parts in buccolingual and mesiodistal directions. With the 4 sections separated, the buccal, lingual, mesial and distal regions of the replica were examined under AFM microscope at 100X magnification. The frameworks were then subjected to a fracture test in such a way that the force applied to them would correspond to the central fossa of the pontic of the three-member framework and would also be perpendicular to the occlusal plane. The maximum fracture force and fracture radiographs of the samples were recorded in Newtons (N). Kruskal-Wallis and Man Whitney U statistical tests were used to evaluate the data. Results: When the marginal gap values were examined, the highest value was seen in zirconium substructures in both premolar and molar teeth, while when the internal gap values were compared, no significant difference was found between the materials. In the occlusal gap values, it was found to be higher than the internal and marginal gap values in all samples, and the highest occlusal gap value was shown by the Co-cr alloy. When the fracture strength values of the materials were compared, a significant difference was found between them, the highest value was shown by the titanium alloy, followed by the Co-cr alloy and zirconium (p<0.05). Conclusion: Titanium alloy produced by SLM method does not show any significant difference in terms of marginal fit when compared to Co-cr, which is the most commonly used metal substructure with layered porcelain today; however, it showed significantly lower values in terms of marginal fit when compared to zirconium material, which is a good alternative for patients with metal allergy. Although these results suggest that the use of titanium produced by additive manufacturing as a substructure material is promising, further studies are needed to support the results of this study. Keywords: Titanium, Additive manufacturing, Co-cr, Zirconium, Substructure, Fracture strength, Marginal fit