In vitro study of fracture resistance and color stability of 3D-printed, milled hybrid ceramic, milled PMMA and prefabricated zirconia crowns for restoration of primary incisors

Background: The advancement of digital technology has expanded the options for pediatric dental restorations, offering more esthetic alternatives and optimizing tooth structure preservation. However, the mechanical properties of these emerging materials remain insufficiently investigated. This study evaluates the fracture resistance and color stability of 3D (Three-dimensional) - printed resin, milled hybrid ceramic, milled PMMA (Polymethyl Methacrylate) and prefabricated zirconia crowns in restoring primary incisors. Methods: An intact naturally exfoliated maxillary primary central incisor was collected and prepared for restoration. Digital impressions of the prepared incisor were taken, and resin dies were printed and used as supporting materials. Crowns were digitally fabricated using 3D-printed (RC), milled hybrid ceramic (HC), milled PMMA (PC) with thicknesses of 1 mm and prefabricated zirconia crowns (ZC) with the same thickness (n = 20 crowns/group) were prepared. After cementation using resin-modified glass ionomer cement, it was subjected to 1000 thermal cycles (5–55 ◦C). Fracture resistance was assessed using a universal testing machine with axial loading. Color stability was evaluated with a spectrophotometer after two weeks of immersion in orange juice and a carbonated beverage. Data were analyzed using one-way analysis of variance and post hoc tests (α = 0.05). Results: All crowns exhibited fracture resistance above 400 N. A statistically significant difference was found between RC and ZC (p = 0.0398). The lowest value is in the RC group (433.1 ± 61.9 N), and the highest is in the ZC group (511 ± 65.0 N). No significant differences were observed among the other groups. RC exhibited the highest color change (∆E = 5.4 ± 0.6), while HC, PC and ZC demonstrated better stability. Conclusions: 3D-printed resin, milled hybrid ceramic crowns and milled PMMA crowns demonstrated good fracture resistance and acceptable color stability for primary incisor restorations. These materials have potential clinical applications but require further in vivo studies to assess long-term effectiveness.