Last week at the ADA Annual Session I taught a hands on workshop about posterior tooth preparation. Both in the morning and afternoon sessions I was asked the age-old question, how do you decide when to save a cusp or cover it? The answer is ambiguous, much like how we decide as practitioners when to fill a carious lesion we see on a radiographic versus watch it. When we evaluate radiographic caries there are many lesions that all of us would agree need to be filled. Preparing posterior teeth is much like this, there are many clinical situations where full coverage is the correct choice without question. In this day of being conservative and saving tooth structure, it is the restorations that fall between a direct composite and a crown that cause us the most question.
With the minimum reduction numbers in mind for your occlusal reduction the next step is preparing the tooth. There are two critical steps during preparation that predictably create adequate reduction, knowing the dimension of the burs that you are using and beginning with depth cuts to create a visual reference once the prep is underway. There are many different burs, both diamonds and carbides that will work to create depth cuts with adequate occlusal reduction. Find something that works for you, in your hands, but chose the bur with intention and have a consistent system for how you use it.
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.