The key to success in restoring anterior teeth is the precise location of the incisal edges (Figure 1); where those incisal edges are will determine many things regarding the anterior restoration (including long-term success). It’s important to understand that there is no norm that works for every patient. In fact, the anterior guidance on each case will differ notably.
There are very definitive ways of determining where anterior teeth should go and the inclination they should be in so that there is no guesswork required.
If an anterior guidance is going to be made, it must work for both inside/out and outside/in. When we refer to inside/out, this means starting from centric relation and as the jaw moves forward, the lower incisors slide down the lingual inclines of the upper anterior teeth (figure 2). It is a true guidance that is needed to separate the posterior teeth in all eccentric jaw movements. It helps keep the mouth healthy and stable. This is one of the most important things to know in dentistry.
The reasoning why posterior disclusion occurs on this inside/out pathway
Anterior guidance permits a coordinated neuromuscular function. The moment when the anterior guidance separates the back teeth, there’s an entire change that takes place in the masticatory musculature (figure 3).
The anterior teeth, by guiding the separation of the back teeth, now get 100% of the horizontal forces. The masticatory system was designed to allow the anterior teeth to do their job without overloading them horizontally because the moment the posterior teeth are separated and the anterior teeth are carrying the entire load in protrusive, the elevator muscles shut off and reduce the horizontal forces on the anterior teeth.
In addition, there’s another important function anterior guidance performs; it prevents attritional wear. Simply put, when the back teeth are separated, they can’t rub.
Two important principles
You can’t stress what you can’t load. If the elevator muscles shut off, the only elevator muscle that’s left contracting is the anterior fibers of the temporalis–and that’s just minimal. All of the strong elevator muscles are shut off the moment those posterior teeth are separated; you can’t overload the joints in a perfected occlusion.
You can’t wear what you can’t rub. That’s why a perfected occlusion will never wear out, and why anterior guidance plays a crucial role in eliminating wear and breakdown of posterior teeth.
This concept of not stressing the condyles, or not wearing or overloading the posterior teeth is dependent on anterior guidance. If there’s any separation of the posterior teeth before the anterior teeth take over, there will be opportunity for the back teeth to rub and/or overload and activate muscle. It’s very important that anterior guidance provides a stable stop with centric relation.
There are three requirements for ensuring protection of the back teeth for long-term stability, predictable comfort, best phonetics, and best esthetics.
Centric relation: if the joints aren’t completely seated when the anterior guidance is worked out, then the anterior guidance isn’t going to be correct. All occlusal analysis starts at centric relation. That’s the starting point for everything we do to achieve a perfected occlusion.
Anterior guidance: centric relation addresses the back end of the mandible and anterior guidance addresses the front end of the mandible.
No posterior interferences: whenever we talk about centric relation, we expect that there should be no posterior interference to complete seating of the condyles to get centric relation. Once the condyles are completely seated, we should be able to close all the way to front tooth contact in centric relation without requiring the condyles to displace even the tiniest amount in order to get the front teeth into contact (Figure 4).
These are the three requirements necessary for a stable occlusion which includes not only just centric relation, but any excursion as the jaw moves either forward and back or left and right. There is an exception when the anterior teeth are kept from touching by a habitual tongue thrust but that is a separate discussion.
A lack of anterior guidance inside/out results in posterior interferences
When posterior teeth interfere with the anterior guidance, there won’t be anterior guidance, but rather posterior guidance. Posterior guidance creates uncoordinated and hyperactive masticatory muscles. If EMG studies are performed, you’ll see the electrical activity in those muscles spikes up when a posterior interference is introduced. The electrical activity completely shuts off when you remove the posterior interference and allow the anterior teeth to separate the back teeth. Posterior guidance also causes posterior teeth to be overloaded.
The effects of posterior interferences are referred to as occlusal disease, which has the potential to cause wear, mobility, fractures, soreness, pain, and sensitive teeth; all of which is the result of not understanding anterior guidance and the role the anterior guidance has for separating the back teeth the moment the jaw leaves centric relation.
It is impossible to create a correct anterior guidance until all the posterior interferences are eliminated. For example, if dentistry is performed on the front teeth of the type of mouth found in figure 5 and we don’t eliminate the incline interference, we’re not going to create a proper anterior guidance. A back-tooth interference is capable of creating extreme muscle incoordination, headaches, pain. In addition, extreme wear and overload of the joints as well as the anterior teeth. We want to be able to close in a postural closure without striking any incline.