We have two basic positions for our jaws that we can reproduce over and over again -
- Centric relation - the position where the jaw joints are seated as far back and up as possible.
- Centric occlusion - the position where we have our teeth together.
Ideally, the two positions are the same. So, in the perfect situation, the teeth knit perfectly together when the jaw joints are perfectly seated. Then the chewing muscles (muscles of mastication) are as short and therefore as relaxed as possible.
So what stops them being the same position?
The teeth - if you bring your teeth together and a tooth gets in the way, the only way to get the rest of your teeth together is to push your jaw forwards a bit and maybe slightly to the side. So, you are pushed out of centric relation and you end up with your teeth together but in a different position - centric occlusion. The jaw is pushed out of position and the muscles are stretched.
But the tooth that pushed everything out of line is still there. Every time your jaw tries to find the most relaxed position, centric relation, it bashes into the offending tooth - unless something is done to remove what dentists call that "interference".
The vast majority of people have interferences in the bite - and don't have any problems that they're aware of. So what makes some people suffer when others don't?
Imagine trying to see over a wall that you're not tall enough to peek over naturally. If a pile of bricks had been left for you, you could pile them up one at a time. When the pile was tall enough you could balance on top of them and see the whole landscape over the wall. Remove any single brick and the whole landscape disappears from view. Problems with the bite have many factors (the bricks). If we don't have enough factors piled up we don't get any symptoms. Add just one more factor and the whole syndrome can show up - we call this Temporomandibular Joint Dysfunction Syndrome (TMJ dysfunction). Bearing in mind that most people have bites that aren't perfect, we're interested in getting you below that threshold, so that your symptoms disappear again. Achieving a perfectly accurate bite - what dentists call "balanced occlusion" is extremely difficult (and I would argue, to some extent unnecessary). We can, however, get very close.
So, what symptoms can I get?
To name but a few . . . .
Toothache - the tooth is the first thing you bite on. It's like having a stone in your shoe and standing on it all day. If you tolerated it for a long time, your knee, hip and back would hurt after a while because you'd walk badly. This ties in with . . . .
Headaches and stiff neck - because the muscles are stretched
Clicking joints - because the joint is pulled forward on the disc
Sensitive teeth - because the teeth are attempting to bend when being bitten on. This causes microscopic fracture of the crystals the tooth is made of (abfraction) and can cause vee shaped notches in the tooth at gum level.
Breaking of teeth - when the tooth just can't flex enough to survive. The teeth that break are usually heavily restored, but I've seen perfectly sound teeth split straight up the middle.
How do we balance the bite?
First we have to work out which tooth (or teeth) is interfering in the bite. Sometimes it's very simple. A recently done filling may have a shiny spot where the opposing tooth bites on it. Trim a bit off the filling and the problem goes away because you've been put below the threshold where symptoms kick in.
Sometimes it's more complex. A careful study of the teeth under a good light will often reveal shiny facets on the teeth where they grind against each other. Occasionally, one of them is obviously the problem, for instance when wisdom teeth come through and wedge against other teeth. Trimming some off may solve the problem, but it may return if the teeth move, so in this case extracting the wisdom tooth may be the solution.
If the interference isn't obvious then we set about balancing the bite in much more detail. Measurements are taken of the distances and orientation of the jaws using something called a "facebow". The aim is to reproduce centric relation. These measurements are then translated onto, what is essentially an artificial face, called an articulator - in our case we use the Denar semi-adjustable articulator system.
We're one of the rare UK general practices to own a couple of Denar articulators, as we're into "occlusion". This particular example is being set up to deliberately open the bite to restore the height of the lower third of the face.
By having models of the teeth mounted on an articulator, we have the advantage of being able to see the teeth from all angles. We can also easily reproduce over and over again that holy grail of occlusion, centric relation. And we can do it without fighting against the muscles that have been trained into the wrong position over many years. By putting microscopically thin foil between the teeth on the model we can see where the teeth bite compared to where they ought to bite - centric occlusion. If we then trim small amounts of plaster from the teeth on the model, we can see what effect we have on the bite. Can we get to centric relation easily? Or will we have to do something more complicated? Or destructive!
So, by performing the adjustments on a model we can do a dry run, without destroying the natural teeth. Once it's gone, it's gone! Assuming the interferences are small, we trim small amounts off the natural teeth corresponding to the spots on the model that were trimmed. The results are often instant and remarkable.
What do we aim for?
The occlusion fanatics in dentistry aim for three point contact on all the teeth. Out here in reality, we accept that the time and effort put into that is, to some extent unnecessary. We're back to that threshold thing again.
- To the side (lateral excursion)
- Forwards (protrusion)
We aim for even contact on the teeth on the teeth when touched together in centric. We then aim for one of two types of contact when the teeth slide out to the side
Canine guidance - your canines are your "fangs" (usually the third tooth from the midline). Canines are the teeth that dogs bite you with! They have the largest roots of all the teeth and so can take the most abuse. Sliding out to the side ("lateral excursion") and riding up on the canine in "canine guidance" separates the back teeth and protects them from sideways forces.
Group function - if we can't achieve canine guidance, we'll try to share out the load as much as possible. So we'd set up for all the teeth to touch evenly as they slide out to the side in "lateral excursion". This mutual support is called "group function".
When posturing forward ("protrusive" we would again aim for as many even contacts as we can achieve so that everything slides smoothly without interference.
Something to remember about occlusion is that it is a fluid system.Things change - teeth wear and so do restorations. And restorations are made of all sorts of materials with different rates of wear. So a balanced occlusion today may not be a balanced occlusion for ever. You may have a filling done and the bite may be changed very subtly. And teeth move. Think about how teeth are moved with an orthodontic appliance (a brace). The forces used are deliberately tiny but move the teeth through bone. Habits can change occlusion - some of them are the most innocuous day to day things in our lives. A classic is hairdressers opening hair grips with the teeth. Or chewing a pen or pencil. Or persistent nail biting. Horribly deranged occlusions can come from the most surprising habits.
If we can everything right, when the teeth are together, the jaw joints should be settled deeply into their sockets and movement from there should be a nice even glide to both sides or forwards - "balanced occlusion".
© Hesslewood Lodge Dental Practice, 16.11.2015