How Do Braces Work?

by Dr. Joseph Thompson

How an arch wire moves teeth

Many people wonder how braces work?  How do they move teeth with braces?  Here is some information that will help you understand the mechanics of braces.

Bracket Design

On every tooth that an orthodontist wants to move, they place a bracket.  A bracket is a rectangular metal object that is attached to the tooth that has a slot where the wire goes through.  Orthodontists use many different types of brackets.  Most brackets are made of stainless steel and some are made of ceramic, a glass material, for aesthetics.An Orthodontic Bracket

The bracket is attached to each tooth’s enamel with a very special adhesive that will even leach flouride to help prevent cavities from forming.

The slot of the bracket is very important.  It has been determined, through studies, how each tooth should be tipped to obtain a properly aligned occlusion.  That tip and torque is placed into the slot.  So as the wire fills the slot, the bracket moves the tooth to a predetermined position.

A specific bracket is made for each tooth and it has its own prescription of tip and torque.  When the wire is placed into the slot of the bracket, the wire starts to move the tooth to that predetermined position.  If the tooth doesn’t move completely to the correct position, the orthodontist will place bends into the wire to move the tooth.

Some bracket manufactures make a big deal about their brackets being the best for tooth movement.  Tooth movement is roughly the same no matter what type of bracket that is being used.

Root Torque

Arch Wire

The wire that an orthodontist uses is called an arch wire.  The arch wire is held in place with an elastic tie that can come in different colors.  Many patients are excited about what colors they are going to pick.

A tooth moves best when light forces are used.  It is the exact opposite of what you would think.  An orthodontist wants to use a wire that deforms easily and goes back to its original shape.

Arch wires come in different sizes and materials.  In the very beginning of treatment, the orthodontist will want to rotate the teeth and start to align the tops of the teeth (occlusal surfaces) into one plane.  We use light wires that deflect easily but do not obtain permanent deformation.  As the wire, that is deflected, goes back to its original shape, it moves the tooth into a better position.  Some of these light wires are are made of nickel titanium.

You will notice that orthodontists change wires often.  We slowly move from small wires that are round in cross section to heavy thick wires that are retangular.  We are slowly filling up the slot of the bracket with each change of the wire until the bracket and wire moves the tooth into its predetermined position.  We can’t use a heavy wire in the beginning because it won’t fit into the slot and the patient would experience a lot of discomfort.

Once we have an better alignment of the teeth, the orthodontist will change the wire to a material that can be bent, like stainless steel.  The orthodontist will start to bend the wire to fine tune the occlusion.

Tooth movement from pressure with braces

Bone Response

Alveolar bone is very specific in allowing tooth movement to occur.  When pressure is placed on the teeth, the pressure is applied to gum tissue around the root called the periodontial membrane or periodontal ligament.  During that pressure, certain cells, osteoblasts and osteoclasts, move into place so the tooth will move.

Osteoblasts create new bone on the negative pressure side of the tooth and osteoclasts remove bone on the positive pressure side.  This bone resorption and deposition occurs at a certain rate and decreases if too much pressure is placed on the tooth.  The pressure also has to be constant to create tooth movement.

So we have learned that the brackets have a predetermined prescription of tip and torque.  As the wire is placed into the bracket slot, pressure is created around the roots.  The gum tissue that is around the roots (periodontal ligament) signals certain cells to move to the sides of the bone surrounding the roots to start resorption and deposition of bone.  That bone remodelling allows for tooth movement.

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