Braces are
impressive little gadgets. Over time, they move your teeth. But
how do they do it?
Braces have
three basic parts (more are shown in this photo):
á Brackets made of metal or ceramic. A bracket
is attached to each tooth.
á Bonding material (glue) or a metal band. This is what attaches the bracket
to the tooth.
á An Arch Wire, which is a thin metal wire that
runs from bracket to bracket and puts pressure on the teeth.
The teeth move when the arch wire puts pressure on the brackets and
teeth. Sometimes, springs or rubber bands are used to exert more
force in a specific direction. Braces
exert constant pressure, which over time, move teeth into their
proper positions.
Your teeth
are surrounded on top by gum tissue (also called Gingiva). Under
the gum tissue, the Periodontal Membrane (sometimes called the
Periodontal Ligament or PDL) encases the bottom portion of the
tooth. Next to that lies Alveolar Bone.
When braces
put pressure on your teeth, the periodontal membrane stretches
on one side and is compressed on the other. This loosens the tooth.
The bone then grows in to support the tooth in its new position.
Technically, this is called bone remodeling.
Teeth Move
Through Bone Remodeling
Bone remodeling
is a biomechanical process responsible for making bones stronger
in response to sustained load-bearing activity and weaker in the
absence of carrying 
a load. Bones are made of cells called
osteoclasts and osteoblasts.
Bone remodeling
works like this: increase the load on a bone and osteoclasts are
created which break it down in response to the load. Remove the
load and osteoblasts are created which create new bony cells.
Repeat the process through repetitive motion and eventually the
bone density increases.
Your teeth
are socketed in bone (your maxilla for the upper teeth, and your
mandible for your lower teeth). As mentioned, surrounding
each tooth is a Periodontal Ligament (PDL) which attaches it to
the surrounding bone.
The PDL as
a sort of messenger between the teeth and surrounding bony sockets.
Pressure between the PDL and bone causes the bone to create osteoclasts
and breakdown the bone to restore the normal spacing between the
teeth and bone. The corresponding tension on the PDL behind the
movement causes the bone to create osteoblasts, effectively building
new bone to fill in the difference and restore the normal spacing
between teeth and bone. Not a whole lot of force is necessary,
only "some" force which is not normally present.
Enter the
brackets and arch wire - the artificial force needed to create
and sustain the pressure.
Arch wires
are interesting things in that they tend to want to retain their
normal shape. They are also made of materials activated by body
heat to increase stiffness. The wire you have presently is what
is called a twist wire which is like a small cable. It wants to
remain straight. When it is put onto your teeth which as a braces
patient are all over the place, and activated by the heat of your
mouth which is 20-25 degrees above room temperature, its desire
to remain straight provides the forces necessary to get the biomechanical
process of bone remodeling to begin and continue.
The solid
wires which come later are made of a nickel-titanium alloy and
while so flexible that you can tie a knot in it, once activated
by body heat becomes quite stiff. The strategic placement of brackets
on teeth and tying of those brackets to this wire complete the
transmission of forces from the arch wire to the teeth and sustained
result in the awesome process of bone remodeling as your teeth
are moved to new positions in your mouth.
The osteoclast
(breakdown) process takes about 72 hours to get fully going, the
osteoblast (rebuild) process about 90 days. Stabilizing the result
takes about 10 months (which is why it is important to wear
your retainer to avoid a relapse of the original or some intermediate
positions).
Illustrations
on this page from:www.vtbraces.com/general_info, www.alm.gr,
and www.stlaurentdental.com/implants.
