Subtractive Computer-Aided Manufacturing in Dental Milling

Computer aided manufacturing is not unique or new to the dental discipline. In fact, CAM has been present in dentistry since the 1970s, with the first operatives to explore its applications for dentistry being Duret and Preston,^2,3 followed by the innovative work of Moermann in the 1980s. Their work led to the development of Sirona Dental’s CEREC^® system.^2,4 Prior to reaching the dental industry, CAM was first developed in 1952, when the first 3-axis mill was built by the United States Air Force, which commissioned the Massachusetts Institute of Technology in 1949 to find better and faster ways of machining complex parts.⁵ Integrated CAM technologies were initially applicable in the automotive and aerospace industries, where complicated, customer-specific products could be manufactured rapidly.^6,7 Over the past 50-plus years, most large manufacturing industries have adopted computer numerically controlled (CNC) machining processes (Figure 1). These processes use power-driven tools to mechanically cut (mill) material with the desired geometry while every step is controlled by a computer.²

In dental applications, milling begins with a block of material (Figure 2) and a milling machine controlled by a computer. The milling machine then executes commands for removing material that is not wanted in the final product. This method is also known as “subtractive manufacturing.”² It has been shown that subtractive manufacturing reduces overall production time and yields complex products that would have otherwise been difficult to create through conventional dental processes. Over the years, CAM processes have achieved a significantly high degree of sophistication and complexity in terms of the products that can be manufactured,² and today a vast variety of tool path schematics and materials can be used.

Participation in dental milling manufacturing may be achieved in multiple manners and provide laboratories with several business modalities. There are many dental enterprise entities—including large dental technology manufacturers—that have established milling centers composed of the leading and the most current sophisticated CNC or milling machineries.⁸ This allows a dental laboratory to enter and participate in digital technology with a significant reduction of initial cost. The dental laboratory can accept or scan a case, design it to their specification and skill set, and simply send it to one of their milling partners to perform the machining (Figure 3). Another option would be to purchase the hardware/machine and perform the milling in-house (Figure 4). Prices of these technologies continue to decrease, so the capital outlay and purchase decision procedures should be handled in the same manner as they would for any equipment purchase. The advantage of purchasing the technology is a reduction in the per-unit cost on materials and a decrease in service time with the elimination of shipping between production phases. This option is particularly viable for dental laboratories that participate in digital dentistry and seek to maintain comprehensive in-house control.

Not all milling machines are alike—there are multiple milling options available for different applications within the dental profession. The simplest of all CNC milling apparatuses is the 3-axis milling machine, in which the machine’s tool schematic simultaneously controls bur movement along the X, Y, and Z planes⁹. This type of milling machine is appropriate for most non-complex dental restorations or applications that do not pose an occult surface. If milled components have a tool schematic that requires dramatic curvatures, the 3-axis CNC apparatus could possibly mill with gouging and/or interference. It may be necessary to sacrifice inert detail to prevent this from occurring, which would then limit the precision of the milled product.¹⁰

A second option is a 4-axis CNC milling machine. This type of milling unit provides an additional element of precision. The block table moves along a fourth plane, allowing the tool path to reach more curvature and achieving enhanced detail in the final mill product. In order to produce more complex restorations that require explicate inert details it is necessary to use enhanced axes CNCs that are more sophisticated in their tool schematics.

Also available are 5-axis milling machines, which are even more versatile because they control their tool paths in five motions continuously and simultaneously. Two concurrent motions within the milling machine occur—X-, Y-, and Z-axis movement and A-, B-axis movement in either the cutter spindle or the block table⁹ (Figure 5). The unique advantage to such a tool path schematic is that continuous adjustment of the bur’s orientation while cutting facilitates a significantly heightened precision in the milled product necessary for complex cases such as implants. It is recommended that dental laboratories use discrete 5-axis CNC milling machines, which are able to operate within both 3-axis and 5-axis schema depending on the needs of the case.

Material choices available for use in dental milling have recently witnessed explosive growth. Materials range from wax, PMMA, zirconia, lithium disilicate, lithium silicate, composite resins, CrCo, and many more. With such a wealth of material offerings, CAM technology is now positioned to deliver nearly all dental restorations previously produced manually. In addition, CAM uses homogenous milled blocks of base material such as metal, resin, or porcelain, which prevents the final result from possessing defects typically found with torch casting, polymerization, and porcelain firing.¹ The materials are composed of the purest raw materials, materials that have not been subjected to elements known to potentially cause defects in the materialization of contamination, porosities, and overall weakening of the dental material (Figure 6).

Rapid advances within the CAM realm will result in more sophisticated, intuitive, and significantly less expensive machinery, allowing more entrants into digital dentistry. Computers and their plug-in counterparts will continue to get faster, smaller, and be able to do more for the restorative dental team, and ultimately the patient. This will increase everyone’s accessibility to these emerging technologies and allow for further enhancements within the realm. It has been said that, “Technologies are not driven by what can be built, but rather by what people want to buy.”⁵ CAD/CAM has provided dental professionals with another tool to achieve a consistent and high-quality dental prosthesis that is reproducible with every patient. These emerging technologies—coupled with the appropriate dental technology knowledge, skills, talent, and expertise—will provide the patient with the most optimal dental restoration.

Daniel Alter, MSc, MDT, CDT is a Professor of Restorative Dentistry at New York City College of Technology, City University of New York.