One of the daily challenges in my practice is assuring that I have created adequate tooth reduction and at the same time not taking away more tooth structure then necessary. Occlusal reduction is a common place that I realize I have under reduced when fabricating the provisional or worse yet when the lab calls or sends the restoration back thin. There are many ways to ensure I have created the right amount of reduction, but before i can do this, I need a goal in mind, so how much space to I need to create.
Having adequate occlusal reduction creates predictability of the final restoration, both esthetically and structurally. I decided to go to the literature and do some research. If we look at success rates of all porcelain restorations in the posterior relative to occlusal reduction, the evidence is clear. Inadequate reduction increases the risk of fracture and failure of the restoration prematurely when placing all porcelain restorations. Due to the inherent physical properties of the porcelain, there are minimal thicknesses for success, and we need to prepare teeth to meet these parameters.
The magic number is 2.0 mm of occlusal reduction, over ALL of the occlusal surfaces. Although there are differences in the fracture resistance of the different types of ceramic materials currently being used for all porcelain posterior restorations, they all do well under these minimal thickness parameters. At first blush I’m sure this number seems high, and may bring up questions about conservative preparation. On the other hand when I teach programs on all porcelain posterior restorations, the most common reason for attendees being there is to learn how to avoid failure and increase their success. Predictably creating a minimum of 2mm of occlusal reduction in all of the load bearing areas is a key to predictable success.
So now that you have the number, create the reduction and verify it before taking final impressions.