In office milling cerec

cerec flat top onlay preps

The ugly side of in office milling of dental crowns, which are mostly cerec.

In office milling of dental crowns is gaining in popularity. Unfortunately many dentists that are doing these in office crowns either do not have the best technology or they are just plain old not very good at it.

In office dental crowns such as cerec are often very easy to spot.

What are some of the signs of an in office crown? Firstly most have terrible occlusal anatomy and look more like a white blob than a tooth.  Secondly the fit is often horrible and the phrase “resting in a sea of cement” is quite accurate. The margins are often awful as well. Finally the material is weak. In office milling is weaker than press e.max and much weaker than alternative materials available. Therefore we constantly see fractures and breakages. Frankly, the vast majority of the mills for in office use are just not good enough to use the best materials. The mills typically rotate on fewer axis as well, making the ability to design a nice crown more difficult.

Why are the crowns often so bad?

The dentists are very literally learning how to do them properly on the patients and the learning curve is steep and long. Some never seem to grasp it. In a smaller practice the cost of the mill and software and other equipment causes dentists to continually use the technology even when they know their results are well below what their lab can do. I’m not going to spend a ton of time bashing them. There are some that are good at it. The vast majority I see are barely acceptable to outright terrible.

The doctors that use the technology like to claim that it’s the user not the technology. This only proves the point the technology is insufficient if many of the users have issues. If we see an awful crown in our practice it is likely to be an in office made crown despite the fact they represent a very small single digit percentage of the crowns we see. We have multiple patients that break every single one of these in office milling disasters. The worst dentist using the worst lab couldn’t get such bad outcomes. It’s simply impossible to be that bad. We will use this page to start posting some of the disasters we see. To date they are from all different practices from all over. It’s not an isolated issue.

Techniques that in office milling dentists tend to use.

Many are taught that bonding is all they need to make the crown work. They throw out all mechanical forms of retention and rely entirely on the chemical and micro-mechanical retention. This often fails quickly. Two major examples are the flat top prep and the endocrown.

The flat top prep.

This is just stupid. The dentists simply removes the top half of the tooth and hopes the bond will hold a crown to that surface.

Flat top prep design for in office milling
Flat top prep design results in poor adhesive and this one came off. It was only a few years old while the bridge above it is decades old and going strong.
In office milling flat top broken crown
This is the crown above when it came off. You see several of the other issues we will discuss here as well. The crown broke. The occlusal anatomy sucks.
Flat top design for in office milling
This is even less than a flat top design. Not sure why this wasn’t just a composite filling. Well anyway, like many others we see, it fell off after just a few years.


This design has merit but the in office mills struggle to get this accurate enough to work well. Remember milling requires overmilling and the larger the bur the more overmilling occurs. Thus internal fit is not as great. Margins can still be just as good but not the internal fit. An endocrown relies on a lot of good internal fit. Not going to work very well in anyone’s hands. The dentists that can get it to work are highly skilled but would still have better results with a lab crown.

In office milling endocrown off the tooth
Just another in office milled endocrown that came off after a few years.
Bad cerec endocrown that also has a terrible marginal fit.
Like many in office crowns this one has terrible margin fit, cement all over the place, and the design is awful. It came off after about 2 years. We will try to nurse it along to the 5 year point when insurance will let us replace it.



Results of in office milling

These issues have more to do with the technology itself and the clinical abilities of the dentist.

Poor fit

This is often one of the biggest issues. Margins are often very poor as you can see on many cases here.


Poor margin fit on in office milling crowns
There are 4 in office milled crowns here with 8 IP margins on the xrays. 2 of those margins look acceptable to us. The rest are varying degrees of unacceptable. The worst margin has likely lead to recurrent decay.

Occlusal anatomy from in office milling

Tooth occlusal anatomy is not something we feel is terribly critical. However, our lab does do a nice job of making the top of a crown look like a tooth. We do not spend a ton of time making our composites have perfect anatomy either. However, lack of anatomy of a crown is a tell that the mill is not capable of making precise cuts. It’s more of a grinder than a milling machine.

Poor occlusal anatomy of in office milling crowns
Poor anatomy can be seen on any of the images on this page. Like many of the images on this page these in lays also have terrible margins everywhere.



Breakage of the in office milling crown or inlay or onlay

These in office crowns are a weaker material. There are stronger materials out there and as technology advances there will be stronger and stronger materials. The dental lab will always have a better mill and thus stronger material though. We see these in office crowns break A LOT.

In office milling crown breakage.
We see this a lot of the in office milling crowns. This patient was definitely working with someone that REALLY didn’t know what they were doing. Or are they? Check below! All 5 broke within 2 years. All of them have no anatomy, which is a problem with the mills ability.


In office milled crowns breaking.
So interestingly enough these are the x-rays for the above case. The margins all look really good! We would say they look excellent on here. That is not the easy part of in office milling SO we are back to the fact the materiel and process is NOT up to par. This dentist did good work and the result is still a disaster.

Excess cement left everywhere

This can happen on any crown. However, we see it far more on in office inlays and onlays almost exclusively. This is likely due to the fact they have poor fit and sit in a sea of cement or resin. This of course leads to decay and problems on other teeth.

In office milling crown in a sea of cement
In office milling crown in a sea of resin cement that is causing issues now with the tooth behind it.