Milling is still the most productive and widely accepted fabrication method in dental technology today. There are two main reasons it remains the backbone of the industry. First, milling machines and software have improved vastly over the past 10 years; and second, the growth of the variety of millable materials available is staggering, zirconia pmma, lithium disilicate... you are facing more than 10types of materials every day. Milling machines now even have a number of automated features, such as automatic tool replacement, based on tool life calculators. As in any industry, despite these advances in technology, there will always be issues to troubleshoot. It is important for laboratories to learn how to properly identify issues with their machinery without relying on automated features.
One of the most commonly asked questions regarding milling is, "Why is my zirconia breaking?". There is, of course, a mountain of variables. For example, anyone or any combination of the following could be to blame: bad design; improper nesting; bad connectors or frame design; zirconia not indicated for the dental CADCAM milling machine /dental milling bur/zirconia milling disc; poor milling machine maintenance; bad milling strategies; green state mishandling; bad infiltration liquids; poor drying techniques; sintering oven programs; improper diamonds and rotaries post-sintering; finishing techniques; and oven cycles. This same kind of problem can occur even if the laboratory is using zirconia and burs that are indicated by the milling machine manufacturer.
Considering all that could contribute to such breakage during production, the problem is extremely difficult to diagnose without seeing the zirconia and the machine in question. Most technicians are experienced and know what they are doing, so user error is less likely to be the problem. To help diagnose the issue, first consider what kind of restoration it is. Almost every time it is an all-on-X or some kind of implant-supported arch. This makes sense because fabricators can get away with less-than-ideal practices when doing single units, but those same methods will cause trouble in larger restorations. Second, examining what the fabrication looks like will indicate if it has been designed correctly and is supported well. Most often, these are not the sources of the breakage problem, but they need to be addressed first to be eliminated. Next, it is important for someone new to look at any infiltration techniques, even if the technician has gone through all these things several times already.
With those most obvious potential causes out of the way, there is one common culprit. The most overlooked problem can begin to be identified by asking the technician, "Is your zirconia shiny?" Despite any disbelief as to the validity of that question, the author has found that "shininess" is a symptom of quite a common problem.
For example, the following two pictures show two shade tabs from a shade guide.
In each picture, the tab on the left is smoother and shinier than the tab on the right. At first glance, the technician may think that it is good to have the surface be that smooth straight out of the mill, but actually, it is not. The surface is smooth because the milling burs have started to dull and lose their edge. If the technician regularly mills singles, they may only notice that the crowns are shiny and maybe have some chipped margins. In fact, in first picture, the crown does in fact have some small chips at the margin.
The problem with running dull burs on a large implant-supported case is the implant interface and access holes. The most stressful part of the process is likely when the dull tool starts to bore a hole into the zirconia. Once any initial depth is achieved, internal cavities are more easily milled out, but that initial bore will cause microfractures and stress breaks. On a full crown, such inconsistencies get milled away, but with an implant interface or access hole, there is not as much excess material.
Further proof of this problem is that most such cases will break at or through the abutments. This is not due to the zirconia being thin; it is more likely because of a microfracture. The forces of heating and cooling exploited the microfracture so that it could grow into something much worse.
As processes become more automated and self-regulating, it is easy for technicians to forego learning how to effectively evaluate or control the quality of their equipment. Most milling machines have tool life counters or estimators, but they should not be considered the final word in tool life. The problem is not that they are wrong; rather, it is almost impossible to consider all the variables. For example, even if a laboratory uses the appropriate tools for the mill, those tools will last differently depending on the varieties of zirconia used, the milling strategies employed, bur manufacturers, and so on. This equation changes constantly, so generally the auto-change feature for bur replacement can become inaccurate as a result. Since the shininess of milled zirconia can indicate a dull bur earlier than the machine's tool life monitor, this "symptom" is potentially a better indicator of the need for bur replacement. Laboratories with technicians who know how to monitor their milling equipment can more safely navigate new materials and technology, rather than rely on stock parameters and stock equipment.
To determine the cause of breaking zirconia, there are a whole host of issues and variables to consider and understand; but dull burs is number one. As technology improves, laboratories need to fight the urge to accept that machine diagnostics are always correct. Knowing how to evaluate the technology used in their laboratories, the technicians will maintain control of both their processes and their final products.