Microimplants
are temporary anchorage devices (TAD's) that looks like a fancy
albeit miniature version of a screw.
A common example of orthodontic
anchorage (relatively immobile blocks of teeth) is the situation
when the orthodontist wants to pull in your front teeth that
are sticking out. He uses the back molar teeth to help him because
they are large, multirooted, and move very little when they
are used to pull back the front teeth. The molar teeth are well
anchored in your jaw.
Another simple way to think
of anchorage: A fisherman is sitting in a row boat and has just
caught a very 150 pound fish. if he has thrown an anchor overboard,
and is strapped into the boat, he can, in time, reel the fish
(tooth) in toward the boat without being pulled out to sea,
or being pulled out of the boat, or the boat moving from its
original position. Woe to the fisherman if he failed to anchor
and was close to a waterfall.
What if the patient has no back
teeth? Solution...the orthodontist places two microimplants,
one on each side of the jaw toward the back where the molars
used to be and attaches rubber bands to the front teeth he wants
to move back.
The screw is about the about
half the size of the width of your thumbnail, and no wider than
the width of the head of a pin. It looks like this: @--- . It
is generally made of Titanium steel, and is placed painlessly
either with a mini-"screwdriver" or a mini-ratchet
wrench following the painless creation of "starter"
hole in the bone. See Tomas page 1 and Tomas page 2.
The microimplant screw is removed
when orthodontic treatment is completed. It takes less than
a minute to remove it, and causes no pain.
In the last 3 years much attention
has been paid to this subject as witnessed by an ever increasing
number of journal articles and presentations by multiple speakers
who have given a host of presentations on the subject at the
international meeting of the American Association of Orthodontist
from during 2004, 2005 and just recently in May 2006 in Las
Vegas.
Like a forest fire burning out
of control orthodontists are coming to use them more and more
to enable them to move teeth in ways they never could before.
It will not be long before the
use of the microimplant (miniscrew, TAD) is being taught and
used in every post graduate orthodontic school in the country
here and abroad.
They are placed into a variety
of locations in the patient's jaw bones usually between the
roots of the teeth as well as other locations nearby the roots
of the teeth and on the palate.
Their use has
made infinitely easier the movement of teeth in certain cases,
that heretofore was accomplished with only the greatest difficulty.
The placement of the microimplant
is becoming one that more and more frequently is being done
by the orthodontist. However, there are many orthodontists who
prefer to have their colleague, the oral surgeon, do it for
them.
The procedure
takes about 20-30 minutes and is accomplished painlessly since
the patient is provided with local anesthesia.
You might think
that driving a screw into the bone is very painful, however,
bone in fact has no nerves inside it, so once the "operator"
passes the screw through the skin that covers the bone, which
is about as thick as this (H) it is done without any sensation
except that of some pressure.
Anchorage
Control
Orthodontic tooth movement is a complex interaction between
a biological process and a mechanical system. When a force is
applied to a tooth another force is generated in the opposite
direction. If this opposite force is applied to a tooth, or
group of teeth, then undesirable tooth movement may result.
The concept
of controlling this unwanted tooth movement is
known as ‘anchorage control’.
Skeletal
Anchorage
Anchorage control
for certain tooth movements can be extremely difficult, requiring
complex mechanics and bulky extra-oral appliances, occasionally
compromising the treatment plan to reduce its side effects.
Over the last few years there has been an exciting development
in the field of anchorage whereby bone is used as an absolute
anchor point. From this skeletal anchorage, forces may be applied
with minimal unwanted tooth movement.
Skeletal anchorage
can be achieved with bone plates (similar to those used in orthognathic
surgery), osseointegrated dental and orthodontic implants and
with ‘mini-screw’ temporary anchorage devices (TADs). There
are many types and brands of mini-screw on the market but most
share similar properties.
Mini-screw
systems
An ideal mini-screw
system is biocompatible, easy to apply and
remove, can be used as anchorage for various tooth movements
and can be loaded immediately.
Mini-screws
are particularly useful for closing spaces from missing teeth,
distalising or retracting teeth, intruding overerupted teeth,
correcting midline discrepancies, reducing occlusal plane cants
and in most situations where insufficient tooth borne anchorage
is available1-12 (Figs 1&2).
Most mini-screws
are manufactured from Titanium alloy or Stainless Steel. There
are numerous head designs, incorporating
various slots, grooves, tunnels and buttons to aid the attachment
of auxiliary appliances such as ligature wires, elastic thread,
elastomeric chain and Nickel Titanium coils. The mini-screws
come in a variety of lengths, usually between 4mm and 12mm,
as well as varying diameters from 1.2mm to 2.0mm. Some of the
mini-screws have surface treatments such as sandblasting, aimed
at improving contact with the bone, or highly polished collars
and heads to aid gingival health and cleaning .
A significant
difference between mini-screw systems can be
their mode of placement. Mini-screws are generally either selftapping
mini-screws or self-drilling/self-tapping mini-screws.
Self-tapping
mini-screws require initial placement of a pilot
hole in the bone, normally utilising a pilot drill in a handpiece.
The mini-screw is then inserted by a hand screwdriver into the
hole. Although this was the method for the first mini-screw
systems, it has largely been superseded by the introduction
of
the self-drilling/self-tapping mini-screw.
The newer Self-drilling/Self-tapping mini-screws are able to
penetrate cortical bone by hand screwing, without the need for
a pilot hole. Depending on the point design, the cutting point
of the mini-screw either removes the bone in its path or compresses
it to the sides. The benefits of these mini-screws lie in the
simplified placement procedure, reduced equipment inventory,
and decreased insertion time.
There are occasions
however where even these self-drilling/self tapping miniscrews
need to have a pilot hole placed first, for example when extremely
thick and/or hard cortical bone is encountered.
Site
Selection
Bone
Density and Thickness
Bone density
and thickness are the key indicators for the success of the
mini-screw. Essentially, the mini-screw is held in place by
its engagement with the bone, so the denser and thicker the
bone, the more contact there is between the miniscrew and the
bone, resulting in greater stability. Some studies have suggested
that the self-drilling/self-tapping mini-screws generate a closer
interaction with the bone than the selftapping mini-screws,
hence their higher success rate.
When inserting
the mini-screw the aim is to engage as much of
the cortical bone as possible. This could mean placing the miniscrews
at an angle to maximise contact area.
Bone density
and thickness vary considerably from patient to patient and
even from site to site within the same patient, requiring careful
assessment when considering mini-screw positioning. Generally
the site must be an area where there are no significant anatomical
structures that can be damaged, such as tooth roots, nerves
and blood vessels.
Maxilla
Buccal maxillary
bone is generally thin and of variable quality, whilst palatal
bone is more favourable in both thickness and density. The anterior
nasal spine and zygomatic buttress are also solid insertion
points. The main nerves and vessels to avoid involve the greater
palatine foramen and the incisive papilla. In younger patients
it is also necessary to avoid the mid-palatal suture as the
bone may not be fully developed and there is a risk of damaging
the nasal septum.
Mandible
Buccal mandibular
bone is usually of sufficient quality for miniscrew
placement, although close proximity of the roots means care
should be taken. The ramus and retromolar regions are excellent
regions for mini-screw placement since there are no teeth to
consider.
Soft
Tissue
Failure of mini-screws
has been associated with the type of
soft tissue through which they have been inserted. Mini-screws
inserted through attached mucosa have a higher success
rate than those placed through movable and thin unattached
mucosa.
Attached
Mucosa
Attached Mucosa
is the best area for insertion as the soft tissue is tightly
bound to the underlying bone and does not twist up along the
mini-screw as it is being inserted. Different areas of the mouth
have varying thicknesses of attached mucosa, for example, the
palatal mucosa. This must be taken into account when selecting
the correct length of mini-screw.
Unattached
Mucosa
Although it
would be ideal to always place the mini-screws through attached
mucosa, this is not always possible so careful preparation of
the site may be required. This may involve raising
a flap or using a tissue punch, as well as adding an auxiliary
wire to the mini-screw that can penetrate through the healed
mucosa. Irritation of the unattached mucosa is common and must
be considered in the site selection, particularly if there are
frenum interferences.
Mechanics
The position
of the mini-screw must, of course, offer some mechanical advantage,
taking into account the desired tooth movements and the range
of action required. Immediate loading of the mini-screw has
been shown to be more successful.
Insertion
Procedure
Patient to rinse with a chlorhexidine
mouthwash for
30 seconds.
Apply topical anaesthetic.
Apply local anaesthetic.
Create a positioning jig.
Take periapical radiograph to determine interradicular
space.
Mark insertion point on mucosa with a sharp probe.
Determine mini-screw length and diameter.
Use tissue punch on unattached mucosa if required.
Place pilot drill hole if required.
Insert mini-screw.
Take periapical radiograph to determine correct
position.
Load immediately
Removal
Procedure
It is important
to remember that mini-screws are not osseointegrated and are
simply removed by un-screwing them. Local anaesthetic is not
usually required for removal. Some mini-screws are lost spontaneously
and patients must be warned that a certain percentage fail immediately
or part way through the procedure.
When mini-screws
were fi rst developed the failure rate was as high as 60%, it
is now down to only 10 to 20%.
Conclusion
Even though
this technique is relatively early in development, rapid
advances in its scope and application are occurring. Mini-screws
have turned previously diffi cult or almost impossible cases
into routine and predictable ones. As mini-screws improve and
the technique is refined, they will become a standard feature
of the orthodontist’s armamentarium.
