It can be an incredibly frustrating clinical situation, when you have been meticulous about preparing a posterior tooth, (most commonly a molar) for a crown. Using your burs you created depth cuts to ensure adequate occlusal clearance. After the impression you leave your assistant to fabricate the temporary only to have them come get you. Why? Because the temp is thin or perforated on the occlusal. When you go back to check, and have the patient bite, sure enough the opposing tooth is touching your prep.
The most common reason that this happens is because we just prepared away the patient’s first point of contact in centric relation. The lateral pterygoid muscle in coordination with the elevator muscles has a learned pattern of firing that bring the teeth into maximum intercuspal position. This “learned” position is programmed by the patient’s first point of contact when the condyles are seated. For some patients when we remove this contact, and therefore the message that was programming the muscles to locate MIP, they release quickly. When the muscles release and the condyles seat, the occlusion is now totally different than MIP was moments before.
One solution that I considered briefly was to no longer work on molars! Removing this frustration is about understanding which patients are at risk. Identifying Risk begins with the exam, whether we are discussing caries or occlusion. There are several key factors that alert me to this potential issue. I start by identifying the patient’s first point of contact and clarifying if it is on the tooth we are about to prepare. If I am going to prepare FPC away, then I look at the magnitude and direction of the patient’s slide, or the difference between this position and MIP. If the difference is small (1-2mm), then even if their condyle does seat the occlusal difference will not cause an issue for clearance. So large slides (3mm or greater) could cost approximately 1mm of clearance on the prepared tooth. Other factors include whether they have a history of occlusal changes or more than one MIP they can find.
Understanding the risk, still leaves us with the question of how to proceed. That is a longer conversation for another post. However, if we proceed as we would before, at least knowing the risk we can explain this to the patient ahead of time, and help them understand how we would manage it if it happens.
sharon goodwin says
very important point! If you chose not to equilibrate the patient and manintain the existing occlusal relation do you have any tips for allowing the new crown to be made with the occlusal surface the same as before the prep was cut?
Lee Ann Brady says
Unfortunately we do not have a way to replicate the exiting occlusal contours. Making sure you take an accurate bite record at the VDO the restoration needs to be fabricated at, mounting accurately, and equilibrating int he models to get contact is the best way.
Annie Pham says
Thank you for the detailed explanation. Do you have another post for solution? How to “regain” the occlusal clearance if it’s lost. Or how to proceed if the FPC was identified and explained to patient?
Lee Ann Brady says
The next step is to obtain mounted models and look at re-establishing the occlusion with the condyles seated by doing a trial equilibration. The clearance will be gained by more tooth removal if necessary after equilibration.