Enhancement of internal fit in computer-aided design/manufacturing crowns through optimization of cutting resistance: A quantitative evaluation of feed rate and material properties.

Abstract

Although CAD/CAM technology enables the rapid fabrication of dental crowns, the internal fit of these crowns is generally inferior compared to those produced by conventional fabrication methods. This poorer fit can lead to issues such as plaque accumulation, microleakage, and increased risk of crown failure, as well as other clinical complications including the need for crown repair, veneering ceramic fracture, and secondary restoration replacement. This study quantitatively evaluates how cutting resistance and feed rate influence the internal gap of hybrid resin CAD/CAM crowns. Optimal milling conditions must be identified to ensure both efficiency and accuracy. The cutting resistance of the blanks during milling is a critical factor in determining the milling conditions. This study aimed to investigate the relationships between cutting resistance, material properties, and machining accuracy in dental CAD/CAM crown production at various feed rates. A specialized measurement system with a three-component cutting dynamometer was developed to assess the cutting resistance of dental CAD/CAM blocks during milling, allowing the evaluation of normal, tangential, and axial forces. We set the feed rate to 150, 675, 1000, 1500, and 2000 mm/min. Increasing the feed rate led to a higher cutting resistance and, consequently, larger internal gaps between the crown and abutment tooth. The cutting resistance of the dental CAD/CAM block varied from approximately 4 to 10 N under normal and tangential forces, and from approximately 6 to 25 N under axial forces. A positive correlation was observed between the cutting resistance and the internal gap, suggesting that reducing the cutting resistance can increase the machining accuracy. These findings highlight the importance of optimizing cutting conditions to ensure the production of high-quality dental prostheses with improved fitness.