Over the years the accuracy we are able to achieve in dentistry has greatly increased. While in most cases the more accurate we can be the better, there are areas where too much accuracy can actually lead to increased error. This may sound like it makes no sense at all; however, I'd like to discuss one area were this is indeed the case.
Today various polyvinyl siloxane materials (PVS) are most often selected by us for recording bite records. The choice for using PVS materials to record bites is completely logical because they are easy to use, dimensionally stable and very accurate. While at first it would seem wonderful that PVS bite material flows into the secondary and tertiary anatomy, this actually makes it too accurate.
To demonstrate this concept take a set of well-groomed models and place them into a PVS bite that has captured the occlusal grooves. When you do this you will notice that when you place light pressure on the models they have some "bounce." This bounce comes from the fact that you captured anatomy with the bite that your models could not replicate. In other words, your bite is too accurate for your models and if you use this bite to mount your models, you will have an inaccurate mounting.
As strange as it might sound, the simple way around this is to take some of the accuracy out of your bite in order to increase the accuracy of your mounting. The best way to do this is to trim the bite to remove the secondary and tertiary anatomy. All you really need is the cusp tips. To do this I use a scalpel blade to cut out all the anatomy in the bite until only the cusp tips remain. Once trimmed properly there should be no bounce when light pressure is placed on the models sitting in the bite.
It is important to know that the more anatomy the teeth have, the more trimming will be needed. In many cases the amount of trimming required can be significant. It's necessary to ensure that a bite registration is both thick and wide enough to maintain its integrity. Typically I find that 2-3mm of clearance between the posterior teeth works well. Also maintaining enough volume of material so that it will flow to the buccal and lingual is vital. This will in effect form ribs that will add stiffness to the bite.
John R. Carson, DDS, PC, Spear Visiting Faculty. [ www.johnrcarsondds.com ]