An Introduction to Millable Dental Materials

In the following article, the author will discuss a number of the common materials being milled today in laboratories and milling centers across the United States. Those laboratories already milling in-house may discover they can mill additional materials, while others who are considering the purchase of a mill may discover what type of mill they should be looking to purchase. In general, all milled dental materials require secondary processes before being delivered to the dentist.

The raw material milled in a milling machine is referred to as “stock.” Stocks come in a variety of shapes and sizes. Some stock is in the form of small blocks, as seen in Figure 1; note the mandrel on the bottom for the mill to hold during milling. Others are in a frame that the mill grasps on to (Figure 2). Sometimes the stock itself has a lip or edge to be used by the mill for retention (Figure 3).

Some stock may be small enough that only a single unit—also known as a block—can be milled from it. Other stock comes in 98-mm diameter disks that can vary in height from 8 mm to 25 mm (Figure 4). In the case of milling stock, size certainly matters. While it is possible to mill a 5-mm lower-anterior substructure from a disk that is 20-mm tall, it is costly, wasteful, and time-consuming. A 5-mm zirconia substructure milled out of an 8-mm tall stock can cost half as much and mill 25% to 30% faster than the same unit milled from 20-mm stock. Whenever possible, laboratories and milling centers try to mill shorter, single-unit cases in shorter stocks and save the taller stock for multi-unit bridge cases with odd insertion paths or those long single-unit upper anterior cases on implant abutments.

Milled materials are generally not ready to be placed in the patient’s mouth immediately after coming out of the mill. They all require additional processing. Sintering, shading, and polishing are excellent examples of the majority of post-processing steps. While some materials may be available as pre-shaded stock, often the shading is only a base shade that requires further characterization, or the only stock shade available is white, and more in-depth shading is necessary. As the milling process does not usually leave a particularly smooth finish, polishing is usually necessary before the dentist can place the restoration. This is the case particularly for materials such as acrylics and resins. Zirconia and lithium disilicate are examples of materials that are milled in one state and then need to be fired in a furnace to change their properties. Then they may still require the subsequent layering of porcelain, staining, and/or polishing prior to placement.

CP zirconia is by far the most common material milled by dental laboratories and milling centers today. It is available from a number of suppliers with varying degrees of strength (900 MPa to 1400 MPa) and translucency. It starts with zirconia powder that is pressed together or chemically bonded with other compounds. Once in the desired shape, it is pre-sintered by the manufacturer to put it in a form conducive for milling. In its pre-sintered state, CP zirconia is generally chalk-like and relatively easy to mill. Sintering is required after milling and changes the zirconia from a soft material to an extremely strong material. It also shrinks the zirconia a predictable amount, typically around 25%. Manufacturers of CP zirconia typically provide the shrink factor for each stock so it can be programmed into the mill’s CAM software. Stock is available in blocks, disks, and frames.

CP zirconia was initially used for copings and substructures. Recent improvements in translucency and shading have made it increasingly popular for full-contour restorations. Advanced CAD software tools have created the ability to design full-contour restorations with areas “cut back” for layered porcelain and improved esthetics. New shading techniques include pre-shaded stock or the dipping and painting of milled units with a wider assortment of coloring liquids prior to sintering.

HIP zirconia is composed of mainly the same materials as CP zirconia, but it is pressed under high temperatures, eliminating the need for sintering. HIP zirconia is typically provided in disk form for milling. The lack of post-mill sintering and the elimination of material shrinkage make this material ideal for milling large-span bridge frameworks. HIP zirconia does not have as many shading and translucency options as CP zirconia, so it is often used only as a substructure or framework. The biggest drawback to HIP zirconia is that it is extremely difficult, time-consuming, and expensive to mill. However, the dimensional stability for large bridges can make it worth the wait and expense.

Currently, IPS e.max^® CAD (Ivoclar Vivadent, www.ivoclarvivadent.com) is the only lithium disilicate being actively sold in the United States. It is a glass ceramic that is milled from a small block. This material is milled in a pre-crystalized phase and has a characteristic blue color. Thirteen low-translucency shades are available. Since lithium disilicate is more like a glass than a metal or resin, it makes milling challenging; the material is technically “ground” rather than milled.

IPS e.max has a reputation for excellent esthetics and adequate strength (360 MPa) for anterior and posterior restorations. Cutbacks can be designed in the CAD software, or the restorations can be manually cut back prior to crystallization for subsequent porcelain application and improved esthetics. Availability of this material has been restricted to machines that have been approved by Ivoclar Vivadent to mill this material. A number of other manufacturers have similar materials under development.

Millable blocks of feldspathic ceramic were originally designed for the CEREC^® chairside milling system. CEREC^® Blocs (Sirona Dental, www.sirona.com) and Vitablocs^® (Vident, www.vident.com) offer numerous shades with varying translucencies that are ideal for esthetic restorations. The relatively low strength (140 MPa) typically limits their use to anterior restorations and areas of low occlusal impact. Restorations from the pre-shaded blocks can be simply polished after milling, but they benefit from improved strength and esthetics if they are stained and glazed prior to seating. These blocks are available to laboratories with Sirona inLab^® mills, but are far more commonly used in clinical chairside mills.

Ivoclar Vivadent offers millable blocks of a leucite-reinforced glass ceramic under the name of IPS Empress^® CAD. Available in 16 different pre-shaded blocks, restorations from this material have excellent esthetics but relatively low strength (160 MPa). Milled restorations should be polished but can also be stained and glazed for additional characterizations and strength. These blocks are available to laboratories using the inLab or E4D milling systems (D4D Technologies, www.e4d.com), but again are far more commonly used in clinical chairside mills.

3M produces Paradigm™ MZ100 and Lava™ Ultimate (3M Espe, www.3m.com). Paradigm MZ100 is a classic millable indirect composite available in block form. Lava Ultimate is an advanced nano-ceramic material incorporating nano-sized particles and clusters of silica and zirconia in a resin matrix. It is available in block form as well as a frame. These materials mill quickly and only need polishing after milling. Both materials can be further characterized with light-curable stains and do not require firing in an oven.

Machinable wax is used just like traditional dental wax for casting metals or pressing ceramics. Wax suppliers often have different blends available, which are typically differentiated by color. Some blends tend to be stronger with better handling characteristics but are not easy to modify. Others are more easily adapted but may not be as durable. Blue, brown/red, and green are the most common colors available, but there is no standard as to which color is more durable and which is easiest to modify. Burnout temperature for machinable wax tends to be slightly higher than traditional dental waxes.

Polymethyl methacrylate (PMMA) is also known as “acrylic” or “resin.” It is a biocompatible material with moderate strength and is available in a variety of colors, shapes, and sizes. Clear acrylic is often used to test mill complex cases for fit or design, and tooth-shaded stocks are used for temporary crowns and bridges. Several companies are now offering computer-designed removable prosthetics and milling portions of them from blocks or disks of pink acrylic.

Chrome cobalt is primarily used as a base metal material for copings and frameworks. It is widely accepted and used in Europe, but is not yet as popular in the United States. It is not a highly challenging material to mill, but it can be time-consuming and tool life can be relatively short. Post-processing of chrome cobalt substructures requires de-gassing prior to layering porcelain.

Titanium is a lightweight, strong, and biocompatible metal for substructures and full-contour restorations, as well as implant abutments and bars. It is available in several different grades, based on purity. Grade 5, also known as Ti-6Al-4V, is a titanium alloy and is the most common grade used in dental applications. It is a significantly stronger formulation and less prone to flex. Implant abutments are often milled from block forms, while substructures, full-contour restorations, and bars are milled from disks. Post-processing for substructures involves sandblasting, an oxidation wait period, and application of a bond coat prior to applying specially designed porcelain.

Polyurethane is a rigid polymer. It is a durable yet easy to mill material, making it an excellent choice for dental models. As with most plastic materials, many shades are available and it can be formed into nearly any shape. After milling, some cleaning of model parts may be necessary.

A multitude of millable materials are available in all shapes, sizes, and sometimes colors and shades. Dental laboratories need to determine which materials best suit their individual business models and/or rely on their milling center partners. All of these materials, except for the model material, are subject to FDA approval. Due diligence should be taken to ensure approved materials are used for any restorations being placed in patients’ mouths.

Future materials are likely to involve improvements in strength, esthetics, and durability. Most materials today can be wet-milled, but not all can be dry-milled. It is difficult to say if these materials are capable of being dry-milled or if they will have to be wet-milled. All we know for sure is that the industry continues to evolve at a blistering pace. It should be safe to assume that the new materials on the horizon will most likely blend the best characteristics of the materials currently in use.

About the author Chris Brown, BSEE, is the business manager at Apex Dental Milling in Ann Arbor, Michigan.