The truth is we change the occlusion every time we use a handpiece on an occlusal table or incisal edge. Fortunately, the changes we make are within the adaptive range of the majority of our patients and go unnoticed.
There are many occasions in which we set out intentionally to alter a patient's occlusal scheme. We can accomplish this through a variety of means, including appliances, equilibration, restorative dentistry, orthodontics, and orthognathics. These alterations are done with the intent to improve the patient's occlusion.
First and foremost, "improvement" may eliminate symptoms. But beyond that are the signs of occlusal disease. For me, the concept of improvement reminds me of a quote from Dr. Bob Barkley. He told us that our job was "to help our patients get worse at the slowest possible rate." This is true of all we do, and absolutely true with occlusion. I do not believe that altering the way a person's teeth touch will alter what the patient does with his or her teeth.
Yes, it does seem to happen sometimes that patients parafunction less on an appliance or after an equilibration. But those results are not predictable and are often transient since those activities ebb and flow over time. What I can do in the face of the patient's parafunctional activities is alter the tooth contacts to minimize stress to the joints, muscles, and teeth within the patient's adaptive range.
Occlusal adjustments can accomplish only two things.
One of the primary reasons we alter how the teeth touch is to change the amount of force the person can apply. We have good science that supports the fact that posterior tooth contact increases the activation of the elevator muscles, thereby increasing the amount of force or load they can apply. This happens in intercuspal position and excursive movements.
When a patient presents with signs or symptoms that stem from forces or load that is too great, one aim of therapy is to reduce this load. If we look at some general numbers, adding in tooth contacts in the bicuspid region doubles the force over anterior contact. If we add contacts all the way back to the second molars, the force can be as much as five to eight times more than when we have canine contact alone. This force or load is shared by the teeth and joints, and experienced by the musculature.
The percentage of the load distributed between the dentition and the joints is also altered relative to where the teeth contact anteriorly or posteriorly. But, generally speaking, lowering the amount of force is beneficial therapeutically.
The second thing we do when we alter a patient's occlusal scheme is design which teeth touch and the shape and size of those contacts. In this way, we can work to distribute the load across multiple teeth and broad surfaces, or focus it on a few teeth.
One of the primary concepts in adjusting edge-to-edge position is to create big, broad, flat contacts to distribute the load and protect the teeth or porcelain. The concept of canine guidance is to eliminate posterior tooth contact in excursions, thereby minimizing the force. This force is placed all on a small number of teeth. I am sure you can see in your mind's eye a few patients for whom this would not be a desirable situation.
Group function, on the other hand, distributes the force over a larger number of teeth (canines and bicuspids). But the addition of the posterior teeth increases the total amount of force. There is no perfect solution, just the one that is most appropriate for each patient.
Technically, group function is simply when simultaneous contact occurs on multiple teeth on the maxilla and mandible during an excursive movement. The challenge for the practitioner is specifically which teeth are touching. Group function in the current vernacular is anything more than canine contact in excursive movements.
There is a significant difference between group function that partners the canines and bicuspids, and one that includes molars. The further posterior the tooth contact, the greater the muscle engagement and, therefore, the greater the force generated when group function occurs. Conversely, the greater number of teeth engaged, the more the forces are distributed over the multiple surfaces.
This is the dilemma of group function vs. canine rise. Can we reduce the total force significantly by placing the guidance on the canines only, or are we better off distributing the force over multiple teeth?
Limited group function is a phrase I use to describe group function that includes the canines and one or both bicuspids. Is the total force higher than if we used only the canines? Yes. On the other hand, the force is less than could be generated if the contacts went further posterior to include the molars. Are we creating a greater surface area than if we only used the canines for distribution of force? Yes. But we have a smaller surface area for force distribution than if we included the molars. In this way, it is a middle position between the other two guidance schemes.
So when you alter an occlusal contact, you are either attempting to decrease muscle engagement and the force applied, or you are trying to alter how that force is distributed.
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