3D Printing of Monolithic Zirconia Ceramics for Dental Restorations

About 60% of today's indirect dental restorations, or fixed dental prostheses (FDPs) are made of ZrO2 ceramics. They exhibit excellent mechanical properties (strength & toughness), are chemically stable, are biocompatible and have a natural aesthetic. 3 mol% yttria-stabilized zirconia (3Y-TZP) is the most commonly used type of ZrO2 for dental restorations. 3Y-TZP is a submicrometer grained transformation toughened ceramic with mechanical properties allowing it to be used for both anterior and posterior restorations and FDPs. Currently, ZrO2 dental restorations are mostly produced by subtractive manufacturing (SM), using computer-aided design (CAD) and/or computer-aided manufacturing (CAM). CAD/CAM milling uses a small, precise milling tool to create the restoration from a presintered or sintered ZrO2 block. After sintering to full density, these restorations are coated with a colored porcelain layer and thermally treated, creating a fully natural look. Additive manufacturing (AM) is emerging as a new production technique, in which the component is build up layer by layer using an extensive variety of technologies. Over the past forty years, AM has advanced enormously and it is now possible to create components out of a wide range of materials (polymers, metals, ceramics). Although, 3D printing of ceramics is still in its infancy, it has been predicted that the use of AM for dentistry will be one of the fastest growing industries. AM allows to move from mass production to quick production of customized products. The use of AM for dental restorations will help to tackle the typical issues for SM, like milling tool wear, high loss of material (up to 90% waste), inability to create gradient colored materials and the possibility to create specific complex shapes with specific detailed features that cannot be made by conventional milling. The research in this doctoral thesis focuses on producing monolithic fully-dense 3Y-TZP dental restorations using indirect slurry-based AM techniques, i.e. digital light processing (DLP) and material jetting (MJ). The aim is to determine whether these AM techniques can be used to manufacture dental restorations with the same performance as 3Y-TZP manufactured by the conventional subtractive method. The first printing technology investigated was DLP. The DLP equipment used a digital light processor to cure a full layer of UV-curable ceramic slurry to create the ceramic material in a layerwise addition. The right combination of monomers, diluents, ceramic powder and dispersants were determined to create a home-made slurry that could create crack-free highly dense (99.8% TD) 3Y-TZP ceramics. Amongst the investigated printing parameters, decreasing the layer thickness from 30 to 15 µm was found to largely improve the mechanical performance of the printed and sintered material. It was possible to print complex structures with detailed features and accurate dimensions, but the surface roughness was anisotropic. The bending strength along the most favorable X(0°)-direction was lower than conventionally manufactured 3Y-TZP, but in range with values reported in literature for AM-produced 3Y-TZP. The second investigated printing technology was MJ. This technique is also known as ink jetting, where ink droplets containing ceramic particles and a binder are selectively deposited from nozzles on a building platform. The commercial equipment and ink allowed to print 3Y-TZP ceramics with a layer thickness of 10.5 µm. Nearly fully-dense sintered ceramics were obtained (99.7% TD), with highly accurate dimensions, but the surface roughness and mechanical properties were strongly dependent on the building direction. The most favorable orientation was the in-plane orientation (X/0°-direction), allowing to create ceramics with a biaxial strength comparable to that of 3Y-TZP produced by SM. Finally, maxillary four-unit FDPs were produced using MJ and three different types of vat polymerization (both DLP and stereolithography (SLA) based technologies) and their mechanical performance was compared to CAD/CAM milled FDPs. Although it was possible to create highly-dense accurate FDPs, quality control by means of impulse excitation did indicate a high variety in the performance of AM-produced FDPs, whereas SM-produced FDPs showed more consistency. The additively manufactured 3Y-TZP FDPs had lower fracture loads compared to the SM-produced FDPs. Two varieties of the vat polymerisation manufactured FDPs exhibited a slightly higher fracture load compared to MJ-manufactured FDPs. All AM-produced FDPs did survive the bite forces to be considered for clinical application. In summary, both investigated slurry-based AM techniques allow to create nearly fully-dense 3Y-TZP dental restorations with accurate dimensions. The sintered materials have anisotropic bending strength, with the highest strengths when tested perpendicular to the layerwise buildup direction.

Publication year:2022

Accessibility:Embargoed