Dental milling, often called Computer-Aided Design/Computer-Aided Manufacturing (CAD/CAM) milling, is a digital method for creating precise dental restorations. This technique uses computerized machinery to carve the restoration’s final shape from a solid block of material. It is a subtractive manufacturing process, meaning the machine removes material from a blank to achieve the required anatomical form. This technology enables the fabrication of accurately fitting crowns, bridges, and other prostheses with enhanced efficiency compared to traditional methods.
The Digital Foundation of Dental Milling
The milling process relies on a digital workflow to create the blueprint for the physical restoration. This begins with capturing the precise structure of the patient’s mouth using a specialized scanner. Dentists typically use an intraoral scanner to create an accurate, three-dimensional digital model of the prepared tooth and surrounding anatomy.
This data is transferred to Computer-Aided Design (CAD) software, where a dental technician or dentist designs the restoration. The software allows for adjustments to ensure the restoration has the correct shape, size, and contact points. Once the design is finalized, the CAD file is sent to Computer-Aided Manufacturing (CAM) software, which translates the design into toolpath instructions for the milling machine.
The Subtractive Manufacturing Process
Dental milling is a subtractive manufacturing method because it physically removes material from a solid block or disc to produce the restoration. This contrasts with additive manufacturing, like 3D printing, which builds an object layer by layer. The milling machine uses high-speed rotary instruments, or burs, that move along multiple axes to carve the anatomical features of the design.
Many machines operate with five-axis movement, allowing the cutting tool to reach the restoration from nearly any angle. This ensures complex undercuts and margins are precisely rendered, producing accurate, well-fitting restorations. The milling process is divided into two main categories: wet milling and dry milling, determined by the material being used.
Wet milling utilizes a stream of liquid coolant, typically water, during cutting to reduce frictional heat and prevent the material from cracking. This method is reserved for hard, brittle materials like glass ceramics, such as lithium disilicate, which are sensitive to temperature changes. The coolant also aids in achieving a smoother surface finish, beneficial for aesthetic restorations.
Dry milling operates without liquid coolant, relying instead on compressed air and a vacuum system to clear debris and dust. This technique is used for softer materials, such as pre-sintered zirconia, polymethyl methacrylate (PMMA), and wax. Dry milling is faster and simpler to maintain, making it the choice for materials that can tolerate the heat generated during cutting.
Applications and Material Selection
The CAD/CAM milling process fabricates a wide array of dental prostheses and appliances. Common restorations include single crowns, bridges, veneers, inlays, onlays, and custom implant components like abutments and surgical guides. The choice of material is tied to its function and the required milling process.
Zirconia is frequently milled due to its high strength and biocompatibility, and it is usually dry-milled in its pre-sintered state. Materials like PMMA, an acrylic resin, are typically dry-milled for temporary restorations. Glass ceramics, such as lithium disilicate, are used for aesthetic restorations. These materials are harder and heat-sensitive, requiring wet milling to protect their structural integrity and achieve a smooth finish.