If you’re confused about what centric relation is, I don’t blame you. There are many definitions (26, according to Wikipedia) that don’t always agree about what this concept is and its significance for dentistry.
First of all, if you have any interest in the TMJ and how it is related to occlusion, pick up a copy of this book:
“Functional Occlusion: From TMJ to Smile Design” by Pete Dawson changed my life. Really. I’ve read A LOT of books on occlusion and this is the one that made it all clear. It’ll take a couple of reads to really sink in, but it’s totally worth the effort.
My concept of centric relation is largely based on Dawson’s work. Here’s his definition:
I love that definition. I’m going to get that tattooed on my chest. No, wait… bad idea.
So what’s so special about CR that it gives prosthodontists goosebumps? A couple of things: (1) it’s a jaw position that should be comfortable for the muscles/nerves/etc. of the TMJ, and (2) it’s a jaw position that can be consistently found regardless of the teeth.
That’s why it’s so useful. You can find it on anyone, anytime and it should be the most comfortable position for them.
Now that that’s clear, let’s look at the three most common misconceptions about CR.
(1) If you can make maximum intercuspation coincident with centric relation, you’ll resolve TMD
TMJ symptoms, or TMD, has several possible etiologies. If the patient’s pain is due to a history of trauma, an auto-immune disease, or severe parafunctional habit, for example, then occlusal adjustments probably won’t help at all. My first goal when treating a TMD patient is to determine if there is a good chance that the occlusion is a cause.
But even if occlusion is a factor and you successfully equilibrate the bite through selective grinding, orthodontics, etc, there is a chance that the symptoms will not fully resolve. Chronic stress to the muscles, nerves, ligaments, and blood supply to the TMJ area may cause permanent damage that will never fully heal. That’s why I never promise to cure all symptoms of TMJ; just to hopefully make them better and prevent them from getting worse.
(2) All occlusal records should be taken in centric relation
So should you use a CR bite record for a single crown? How about for your amalgam and composite restorations?
The only time I record a bite in CR is when I am completely redesigning a full arch (or arches). Complete dentures, full crown and bridge rehabilitations, full implant reconstructions, and full mouth occlusal equilibrations. I will also sometimes use CR for less than full mouth reconstructions if I am replacing/changing a significant number of teeth that I know the bite will be altered.
When I have a chance to design a bite from scratch, I’ll make maximum intercuspation coincident with centric relation.
If a patient has not been equilibrated so that maximum intercuspation is coincident with centric relation, then that patient will continue to bite into maximum intercuspation during function. So if you’re doing a crown, bridge, filling, removable partial denture that only replaces a few teeth, etc, etc, then you gain nothing by recording a centric relation bite.
(3) Centric relation is the most posterior position of the condyle-disk assemblies in the eminentiae
Notice in the above Dawson definition that he says “most superior” position, not “most posterior.”
Older definitions of CR thought of the most posterior (most retruded) condyle position as the goal. One of the possible causes of TMJ pain is a stretched or torn posterior ligament. So it follows that a retruded condyle position would help relieve the stress on that ligament.
In many cases, the most posterior position will be clinically identical to the most superior position, so the debate is largely academic. But it is important to recognize that the rationale for using CR comes from it being the most superior position of the condyle-disk assemblies.
In this position, the condyle can be loaded through the disk and supported by the eminentiae on the other side, free of any muscle activity.
For more information about the TMJ, centric relation and occlusal adjustment, check out my e-book, “The TMJ and Occlusal Adjustment”