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Removable Appliances
Removable orthodontic appliances have two immediately apparent advantages:
(1) they are fabricated in the laboratory rather than directly in the patient's mouth,
reducing the dentist's chair time, and
(2) they can be removed on socially sensitive occasions if wires on the facial part of the
teeth would be visible, or can be made almost invisible if fabricated from clear plastic
materials.
In addition, removables allow some types of growth guidance treatment to be carried out
more readily than is possible with fixed appliances. These advantages for both the
patient and the dentist have ensured a continuing interest in removable appliances for
both children and adults.
There are also two significant disadvantages:
(1) the response to treatment is heavily dependent on patient compliance, since the
appliance can be effective only when the patient chooses to wear it, and
(2) it is difficult to obtain the two-point contacts on teeth necessary to produce complex
tooth movements. Because of these limitations, removable appliances in children are
most useful for the first of two phases of treatment, with fixed appliances used in the
second phase and contemporary comprehensive treatment for adolescents and adults
almost always requires fixed, non-removable appliances.
In the 1925 to 1965 era, American orthodontics was based almost exclusively on the
use of fixed appliances while in Europe continued development of removable appliances
and all treatment was done with removables.
There were three major reasons for this trend:
(1) Angle's dogmatic approach to occlusion, with its emphasis on precise positioning of
each tooth, had less impact in Europe than in the United States;
(2) social welfare systems developed much more rapidly in Europe, which tended to
place the emphasis on limited orthodontic treatment for large numbers of people, and
(3) precious metal for fixed appliances was less available in Europe, as a consequence
of the social systems and because the use of precious metal in dentistry was banned in
Nazi Germany.
A major part of European removable appliance orthodontics of this period was functional
appliances for guidance of growth. A functional appliance by definition is one that
changes the posture of the mandible, holding it open or open and forward. Pressures
created by stretch of the muscles and soft tissues are transmitted to the dental and
skeletal structures, moving teeth and modifying growth. The monobloc developed by
Robin in the early 1900s is generally considered the forerunner of all functional
appliances, but the activator developed in Norway by Andresen in the 1920s was the
first functional appliance to be widely accepted.
Andresen's activator became the basis of the "Norwegian system" of treatment. Both the
appliance system and its theoretic underpinnings were improved and extended
elsewhere in Europe, particularly by the German school led by Haupl, who believed that
the only stable tooth movement was produced by natural forces and that alterations in
function produced by these appliances would give stable corrections of malocclusion.
This philosophic approach was diametrically opposite to that espoused by Angle and his
followers in the United States, who emphasized fixed appliances to precisely position
the teeth. These opposing beliefs contributed to the great differences between
European and American orthodontics at mid-20th century.
In the European approach at that time, removable appliances often were differentiated
into "activators," or functional appliances aimed at modifying growth, and "active plates"
aimed at moving teeth.
The two European orthodontists deserve special mention for their contributions to
removable appliance techniques for moving teeth.
Martin Schwartz in Vienna developed and publicized a variety of "split plate"
appliances, which could produce most types of tooth movements.
Philip Adams in Belfast modified the arrowhead clasp favored by Schwartz into the
Adams crib, which became the basis for English removable appliances and is still the
most effective clasp for orthodontic purposes.
Functional appliances were introduced into American orthodontics in the 1960s, through
the influence of orthodontic faculty members with a background in Europe (of whom Egil
Harvold was prominent), and later from personal contact and fixed appliances spread to
Europe at the same time.
A major boost to functional appliance treatment in the United States came from the
publication of animal experiment results in the 1970s showing that skeletal changes
really could be produced by posturing the mandible to a new position and that true
stimulation of mandibular growth could be achieved. (tension of the m. pterygoideus
lateralis provoke gowth in condylar catilage)
At this point, the dichotomy between European and American orthodontics has largely
disappeared. European-style removable appliances, particularly for growth modification
during first-stage mixed dentition treatment, have become widely used in the United
States, while fixed appliances have largely replaced removables for comprehensive
treatment in Europe.
Modern removable appliance therapy consists largely of the use of
(1) various types of functional appliances for growth guidance in adolescents and, less
frequently, in children;
(2) active plates for tooth movement in preadolescents; and
(3) clear plastic aligners for tooth movement in adults.
Functional Appliances
These appliances have no force-generating springs or screws and depend only on soft
tissue stretch and muscular activity to produce treatment effects.
The original functional appliance design was a block of plastic covering the teeth of both
arches and the palate. It was made to fit loosely, advance the mandible several
millimeters for Class II correction, and open the bite 3-4 mm. By drilling the plastic body
is possible to guide erupting teeth into the wanted position. In most cases (ClassII) is
needed to impede eruption of upper posterior teeth while allowing eruption of lower
posterior teeth. Lower incisors are capped to control forward displacement of the lower
arch, to prevent them from erupting and control their tendency to tip facially. Usually, the
lower molars are free to erupt, while the upper molars are prevented from erupting by
the acrylic shelf between the teeth.
Later, a lot of modifications named by their authors were described. There was
a tendency for prolonged using of appliance (24 ours a day) and to incease the
effectiveness of the apliances. To obtain this, there wereo ways:
1. to cut – down the acrylic body
2. to include the active components (screws, springs...)
Bionator
The bionator, originated by Balters and sometimes still bearing his name, is best
described as a cut-down activator with an inter-occlusal shelf and incisor capping if
desired. Palatal coverage is eliminated. As with the activator, lingual flanges stimulate
forward posturing of the mandible and shelves or blocks between the teeth provide
vertical control. Long labial bow stimulates the lip seal and distal parts of is eliminates
the pressure of the cheeks. The palatal arch stimulates funtion of the tongue.
There are 2 types, for Class II Div. 1 cases and for anterior open bite.
The Klammt elastic open activator
I tis the most loosely fitted functional appliance without occlusal bite blocks. The small
plasic parts may or may not reproduce the oral teeth surfaces. The 2 labial bows and
palatal arch play the same role as in bionator. It is indicated for Class II Div. 1 and deep
bite cases with good formed dental arches without crowding.
Bimler appliance
The acrylic body is reduced to a minimum and is replaced by wire parts. It is very elasic
so it was thought it is capable to transmit the functional impules to the dentition. Its
construction is very complicated. Bimler described more types for Class II Div. 1, Class
II Div. 2 and Class III malocclusions.
Frankel function regulator
The Frankel appliance (which Frankel called the function regulator) is the only tissueborne functional appliance. A small pad against the lingual mucosa beneath the lower
incisors stimulates mandibular repositioning. Much of the appliance is located in the
vestibule. It alters both mandibular posture and the contour of facial soft tissue. It serves
as an arch expansion appliance in addition to its effects on jaw growth because the
arches tend to expand when lip and cheek pressure is removed. What is more, traction
produced by vesibular shields provoke periosteum to bone apposition on vestibular
surface of the maxilla. Frankel described 4 types but today the appliance is used mostly
for Class III corection.
Twin block
The twin block appliance can be used as either a removable or fixed (cemented) device,
but usually is removable. Its maxillary and mandibular portions are configured so the
interaction of the two parts controls how much the mandible is postured forward and
how much the jaws are separated vertically
The appliance has the advantage of allowing nearly a full range of mandibular
movement, easy acclimation and reasonable speech, so that it can be worn most of the
time. The greatest disadvantage is that displacement of incisors can occur freely.
There are some modifications of activator and bionator designs that include expansion
screws or springs to move teeth. The springs and screws added to a functional
appliance produce tooth movement that often detracts from correction of the underlying
jaw discrepancy. With any functional appliance, there is a tendency for the lower
incisors to be moved facially and springs and screws magnify this undesirable side
effect. For this reason, active components have little or no place in modern orthodontics,
and now are used much less than previously.
The funcional apliances can be used in combination with extraoral force. ( Lehman
appliance, maxilator). The major advantage is possibility to control rotation of the cclusal
plane which increases skeletal effect and helps to corect skeletal dicrepancy.
Clinical acceptability of functional appliances
As a general rule, simple and sturdy functional appliance designs are more effective
than complex and fragile ones. Children and adolescents are not known for their gentle
treatment of removable appliances; if something can be broken easily, it will be. The
original activators, and the twin block appliance, have a significant advantage from this
perspective. The bionator is less sturdy, and its relative fragility is the greatest weakness
of the Frankel appliance.
Vertical control always is a key element, and one of the advantages of functional
appliances in general is the control of eruption they provide. Blocking the eruption of
some teeth and allowing the eruption of others is the key to correcting deep bite or open
bite problems. The activator and bionator designs are particularly easy to adjust so that
some teeth can erupt while others are blocked.
Finally, acceptability to the patient is critical. One important factor in acceptability is the
extent of interference with speech, which can make full-time wear impossible. The
original activators do interfere with speech and for all practical purposes cannot be worn
to school. Removing the palatal plastic and making the appliance into a less bulky
bionator helps significantly (while also making the appliance more fragile, of course).
Twin block appliances produce even less interference with speech and can be
compatible with full-time wear, and the Frankel appliance (after a period of adaptation)
does not interfere with normal speech, so that children can wear it to school routinely.
Indication of functional appliances
1. in cases where the main problem is the altered function ( habits like sucking, lip
biting, tongue thrusting, mouth breading, insertion of the lower lip under the
incisors....)
2. in cases where the dentoalveolar compensaton was lost (contact between he
incisors is he impuls for A-P compensation, occlusal contact of the distal teeth for
vertical compensation)
3. in adolescents (in maximum growth spurt) to stimulate the mandibular growth
Removable Appliances for Tooth Movement
Tooth movement with removable appliances in children almost always falls into one of
two major categories:
(1) arch expansion, in which groups of teeth are moved to expand the arch perimeter;
(2) repositioning of individual teeth within the arch.
Active plate cosist of:
1. baseplate
2. retentive components (clasps, labial bow)
3. active components (springs, screws)
1.The framework of an active plate is a baseplate that serves as a base in which screws
or springs are embedded and to which clasps are attached. It followes the lingual
surface of the teeth and alveolar process and extend as far as the first molars. The
upper plate is reduced and in the midline ends at first premolars region.
2.Retention of an active appliance is critical to its success. The best springs are
ineffective if the appliance becomes displaced.
By far the most useful and versatile clasp for contemporary removable appliances is the
Adams crib. This clasp is made of 0.7 mm wire.The retentive points of the clasp must fit
well into the undercuts for good retention. When this clasp is used for children, it
may be necessary for the points to slip slightly into the gingival crevice. This step is
accomplished by trimming away stone interproximally on the laboratory cast, so that the
clasp can fit far enough to engage undercuts below the height of contour.
When a new removable appliance is received from the laboratory, or when a patient
returns for adjustments, it is often necessary for the dentist to tighten the clasps by
simply bending the clasp slightly gingivally from its point of attachment. It is also
possible to bend the retentive points inward to obtain better contact in the undercut
areas. As a general principle, the more active a removable appliance is to be and the
greater the force applied during its use, the more clasping is required to hold it in place.
There are another clasps used: Schwartzs, drop shaped, triangle shaped,...
Labial bow may be only retentive component or may be activated for incisor movement.
There are many variations in its shape depending on the desired teeth movement. It is
usually made of 0,8 mm steel wire.
3.The active element is a jackscrew placed so that it holds the parts of the plate
together. Opening the screw with a key then separates the sections of the plate.
Position of the screw depend on desired teeth movement. It can move the single tooth
or a teeth group.
The screw offers the advantage that the amount of movement can be controlled, and the
baseplate remains rigid despite being cut into parts. The disadvantage is that the force
system is very different from the ideal one for moving teeth. Rather than providing a light
but continuous force, activation of the screw produces a heavy force that decays rapidly,
and rapid reactivation of the appliance has the potential of damaging the teeth.
If the force levels become too high, the appliance is likely to be displaced. This is the
most common problem with expansion plates: activating the screw too rapidly results in
the appliance being progressively displaced away from the teeth rather than the arch
being expanded as desired.
In contrast to the heavy, rapidly decaying forces produced by a screw, nearly optimum
light continuous forces can be produced by springs in a removable appliance. Like the
edges of an active plate, however, these springs contact the tooth surface at only one
point, and it is difficult to use them for anything but tipping tooth movements.
The guideline for tooth movement with a spring from a removable appliance therefore is
that this is acceptable for a few millimeters of tipping movement. Root control is needed
for more than 3 to 4 mm of crown movement.
In designing springs for tooth movement, two important principles must be kept in mind:
(1) the design must ensure adequate springiness and range while retaining acceptable
strength. This usually means using recurved or looped wires for additional length,and
(2) the spring must be guided so that its action is exerted only in the appropriate
direction.
Because smaller wires are not strong enough, it is unwise to fabricate springs for
removable appliances from steel wire smaller than 0.5 mm. Larger diameter wires
usually are preferred. In general, it is better to use a larger
wire for its (considerably) greater strength, and then gain springiness and range by
increasing the length of the spring, than to use a smaller wire initially.
.
As treatment proceeds, three adjustments are necessary when an active removable
appliance is being used appropriately: tightening of clasps when they become loose,
activation of the spring or springs, and removal of material from the baseplate.
Activation of the springs must be done carefully, and not more than approximately 1 mm
at a time. The more the spring is activated, the more difficult it becomes to keep it in the
proper position. Too much activation usually displaces either the spring or the whole
appliance. Baseplate material must be removed from the path of a tooth that is to be
moved. Failure to relieve the baseplate near a spring is a common error.