Posterior Crown Preparation – Step by Step

While it may seem silly to spend time reviewing the protocol for one of the most common procedures in dentistry, you would be shocked at how much this little review can help you. The goal for any procedure in practice should be to achieve the ideal result with extreme efficiency. What most of us don’t realize is how much a standard protocol will help in this goal.

At a recent visit to Glidewell Dental Lab in Newport Beach, CA, I was privelaged to walk through the lab and get a better understanding of their inner workings. First, I must admit that Glidewell is committed to excellence and consistency. They literally have every piece of technology and if they don’t have it – they invent it! Well in walking through the lab and talking with the team leaders I was quickly made aware of the single biggest problem for the laboratory – what dentists send them.

First issue is the impression itself. You would be amazed at what I saw. Impressions with no visible margins, impressions for final restorations in dried alginate, impressions that weren’t full polymerized, and the list goes on and on. The other issue revolved around the preparation itself. The biggest problem was under-reduction. As dentists we always jokingly say that ceramists ‘always want more room’, but it goes beyond that. It goes to the point that we typically don’t give the laboratory enough reduction for the minimum requirements for the material. This leads to restorations that are weak due to inadequate thickness and unaesthetic and/or bulky because lack of room to build in beauty.

In an effort to ensure proper reduction for the chosen material, I found that I dramatically increased my efficiency at the same time. By following a connect the dots approach and using burs that are designed to achieve adequate reduction, I was able to make preparations ‘assembly line’ like in procedure.

Let’s take a look at a preparation for a typical all-ceramic posterior crown – whether zirconia, procera, or empress. The manufacturer recommends 2.0mm of occlusal reduction and a minimum of 1.0mm of axial reduction.

The first key is to use a occlusal depth reduction bur to ensure occlusal reduction. Here a 2.0mm depth bur was used.

The interproximal should be reduced to open contacts.

Now the occlusal depth cuts are connected to achieve full occusal reduction. Notice how interproximal has been reduced.

Using the same 856-016 diamond facial and lingual depth cuts are placed to half the depth of the bur. This will give about 1.0mm axial reduction.

The depth cuts are connected and the margin is refined with a fine diamond. Finally retraction cord is placed to allow for an excellent impression.

While it may seem elementary to spend time discussing such a simple procedure or reviewing areas to make it better. It is natural to feel that we are fast enough and good enough already, but this technique is simple and only requires 3 burs.

1. Meisenger 2.0mm Occlusal Router
2. 856-016 coarse diamond
3. 856-018 fine diamond

Having a particular setup and sequence allows your auxillary to be prepared for you and have all the tools out in advance. Aditionally it will reduce your inventory and allow you to cut down on time spent using multiple different burs. Finally, by using a fine diamond to smooth your preparation will give extra detail and send a message to the laboratory that you require excellence. And most importantly, the use of burs that are designed to give appropriate reduction for materials will ensure a restoration that meets guidelines for maximum strength and aesthetics.