Note: Descriptions are shown in the official language in which they were submitted.
CA 02246663 1998-09-18
Dental microwave application method, apparatus and polymer based compositions
Objects containing or consisting of polymers are generally used in the dental
arts for the restoration
of lost tissue and the improvement of oral functions. They are required to
have specific physical,
mechanical, chemical and biological properties, for example composite fillings
or dentures should
have adequate strength, durability, processing and dimensional accuracy. They
should be highly
and appropriately polymerized to improve strength and stability, and they
should be biocompatible
and chemically inert. They should be able to be processed rapidly and
conveniently. An example
of a polymer object used in the dental arts is a removable denture. Most
commercial dentures
consist of a methyl methacrylate polymer, a copolymer, a methyl methacrylate
monomer, a
crosslinking agent, an initiator, an accelerator and other additives. The
denture base may be cured
by water-bath, microwave heating or light. Thermal water-bath curing requires
up to 8 hours for
processing flasked denture since plaster molds and polymers are poor thermal
conductors.
Furthermore, due to polymerization shrinkage fit is compromised. Incomplete
polymerization
leaves residual monomer which are toxic and act as an irntant to oral tissues
and compromises the
strength of the denture. Processing by water-bath methods results also in
large numbers of voids
in custom made dental devices such as a denture, which further weaken them.
Improvements in
the properties can be obtained by various processing methods. Examples include
intensive
visible-light exposure, pressurized injection water-bath heat curing and high
intensity microwave
irradiation. Despite improvements, resulting polymerized prosthesis are less
than satisfactory and
varying degrees of micro-shrinkage and porosities are present. Dentures cured
by commercial
microwave oven have improved mechanical properties, and have often better
adaptation than those
cured by the water-bath method. An additional advantage of microwave curing
includes reduced
processing time. Even with these advantages, dentures cured by existing
microwave processing
methods leads to deformations due to the large micro-shrinkage of current
polymers, leading to
fitting inaccuracy and unreliability. Processing porosities in the denture are
not eliminated by
existing microwave high power curing methods.
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CA 02246663 1998-09-18
In the conventional thermal curing method, a temperature differential is
required to force heat by
conduction from the surface to the center, heat penetrates from the outside to
the internal portions
of the material by thermal conduction, overheating and degrading polymers at
the surface. In the
micro-wave curing method the object can be heated uniformly as the
electromagnetic radiation
instantaneously penetrates deeply and heating occurs throughout all three
dimensions of the
irradiated substance. The main advantages provided by microwave energy include
a rapid internal
heating, independent of the heat flow through the surface, resulting in a
reduction of the distortion
of the denture base, compared with water-bath curing and also minimal thermal
lag and thermal
gradients, which result in a more homogeneous cure and better properties.
Additionally, high
temperatures generated in the targeted object by microwave heating with
available ovens and
manufacturers recommendations (3 min., 550 w G.C. Acron) conduct to the
thermal degradation
of many thermosetting materials since very high temperatures are generated
during the hardening
process. Irregularity between available microwave ovens, and insufficient
control of irradiation
power limits the use of microwave for dental device processing.
Another example of polymers used in the dental arts is soft liners. A
permanent soft liner is placed
on the interface between the interior surfaces of the denture and the denture-
bearing mucosa of the
patients. This soft-liner should be permanently resilient, highly stable in
dimension, adhering to
the denture-base polymer, biocompatible, easy to clean and not capable of
sustaining microbial
growth. Several kinds of soft liners including polysiloxane, polyurethanes,
plasticized
polymethacrylates, polyvinyl chlorides and polyphosphazene fluoroelastromers
are currently
employed. Most soft-liners do not fulfill the above requirements due to
inherent disadvantages.
These include the leaching of potentially harmful bonding agents, such as
epoxy and urethane
adhesives, sulfuric, perfluoroacetic acid; poor adhesion to the
polymethylmethacrylate (PMMA)
denture base material; porosity in denture base and the liner resulting from
vaporization of the
solvent; dimensional changes caused by micro-shrinkage, or dehydration and
rehydration steps.
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The improvements of denture soft liners may be based on the use of novel
materials, such as
methacryloxy polydimethylsiloxanes or methacryloxyalkyl-terminated
polydialkylsiloxanes.
A way to improve the fit of existing dentures is to renew its base part. This
procedure which is
usually done in a dental laboratory, is deficient in a manner similar to the
materials that undergo
water-bath hardening. Intra-oral relines can be made using chemically-cured or
light-cured
polymers including polymethylmethacrylates, resulting devices have a
relatively low degree of
cure, are porous since no compression of the dental composition is possible
and often chemical
and physical irntation of the oral tissues are caused.
Polymer matrix-composites are an alternative to mercury-containing dental
amalgam as a
aesthetic restorative biocompatible material. Composites are based on hardened
polymerizable
polyfunctional methacrylates used alone or as mixture with monomethacrylates,
cure initiators
pigments and fillers in a mixture with various comonomers such as
triethyleneglycol
dimethacrylate. The fillers generally consist mainly of pyrogenically produced
microdispersed
silicon dioxide particles.
The half-life of polymer-matrix composites cured by light is on the order of 5-
8 years. The main
deficiencies of composite resins include their surface degradation which leads
to inadequate wear
resistance, polymerization shrinkage and a lack of density. Improving the
degree of
polymerization is generally considered to be one way of improving composite
resin performance
as it would lead to stronger composite fillings and prosthodontic devices
which are less
susceptible to degradation, wear and fracture. It would also lead to improved
biocompatibility,
since there would be reduced amounts of uncured monomer to act as a biohazard.
Shrinkage
produces interfacial gaps, which results in microleakage, the result being
bacterial penetration into
the tooth. This causes adverse reactions of the pulp, sensitivity, possible
pulpal death, and loss of
adhesion.
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The microwave curing of dental compositions under compression while they are
fluid is one way
of reducing problems related to the polymerization shrinkage and porosity.
Microwave curing of
composites while injected into a mold further reduces porosity, and enhances
density, and
consequently improves the biofunctionality and the durability of the dental
restoration. The
physical, mechanical and bio-chemical properties of a composite include
strength, stiffness,
hardness, abrasion resistance, toughness, ideally as similar as possible to
the natural tissues. Most
properties are derived from all three basic components of the composite,
although some are
associated with one of the three constituents. Micro-shrinkage, one of the
main shortcomings of
composites, is primarily due to the resin matrix. The physical and mechanical
properties, such
hardness, stiffness and abrasion resistance and strength, are highly
influenced by resin matrix
when the fillers and coupling agents are fixed. Micro-shrinkage results from
the shorter distance
between atoms in the resin matrix after polymerization. The monomers in the
resin matrix are
located at Van der Waals distance, which becomes a covalent bond distance in
the corresponding
polymer. Commercial resin composites are found to undergo volume shrinkage of
about 1-7%
with most resin composites shrinking 2-3% . Large stresses are built into the
composite by
micro-shrinkage, which results in adhesive failure and cohesive failure of the
composite dental
restoration. The micro-shrinkage also cause volumetric dimensional change
which produces poor
fitting to residual oral tissues. Improvements in resin formulation involve,
for example, the
introduction of spiro orthocarbonates and the stereoisomers. The increase of
degree of conversion
will generally result in the improved mechanical properties. Another problem
caused by the
residual monomers in the composite is the leaching of the unbound materials.
The leaching has
an impact on both the structural stability and biocompatibility. The residual
monomers are eluted
into salivary fluids and brought into contact with mucosal tissues; or be
extracted into dentin and
diffused to pulp. The elution decreases with the higher degree of conversion.
An increase of
degree of conversion will result in improved mechanical properties and
biocompatibility of dental
cured composites.
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The principal method for treating dental caries involves its surgical
excision, using various hand-
held instrumentation. Carious tooth tissue consists of demineralized and
softened enamel or
dentin, and contains micro-organisms. Part of the infected tissue recalcify on
the condition that it
is kept under aseptic conditions. Infected tooth tissue which is not kept
under aseptic conditions
will remain as an active carious lesion, and will continue to cause
progressive and destructive loss
of tooth tissue. The recurrence of caries is primarily caused by
microorganisms, which
recolonizes a sit. In order to slow down infection and promote
recalciflcation, it is necessary to
destroy microorganisms in the carious region.
The constant exposure of dental person and devices to saliva and blood is a
contributor to the
transmission of infection. Available methods of sterilization have drawbacks
in relation to dental
devices including plastic and sharp instruments. Chemical sterilization method
such as Alkaline
glutaraldehyde 2% solution requires 10 hours to kill spore-forming or
tuberculosis
microorganisms, is irntating to human and need to be monitored since its shelf
life is usually two
weeks. Autoclave sterilization method expose dental objects to temperatures
above 100°C for
approximately 1 hour. Rubber and plastic washers and bushings within the
dental handpiece and
sharp edged metallic devices are damaged since heating times are long and
sterilizing warm
vapors are corrosive. When microwaves are used the fact that the many tools
are metallic means
that they will not be heated by microwaves. A second problem is the production
of lighting or
corona discharges between two metallic components, or at the points of
instruments. Such
lighting or arc melt the metal and destroy instruments undergoing
sterilization. Sterilization by
the indirect application of microwaves use microwaves to vaporize a liquid
sterilizing solution and
to expose the instruments to either the vaporized solution alone or to both
microwaves and
vaporized solution, a shielded pressurized atmosphere can be produced by the
vaporized solution.
In another microwave sterilization technique, the instrument is exposed to
microwaves, a placed
in a bag, drawbacks include the need to rotate the objects in three-
dimensional manner and the
CA 02246663 1998-09-18
need to protect the oven from the energy which is not absorbed by the target
and is reflected back
to the microwave source requiring either an absorber of microwaves, such as
water, or an absorber
of microwaves within the oven which also minimize occurrence of arcing. It's
also difficult to
control the required temperature conditions and assure the safety of the
sterilization process. One
way is to surround the tools with a microwave absorbent material which will
prevent the
microwaves from "seeing" the tools but will become hot by itself and transfer
its heat to the tools,
microwave absorbing materials can also be formed into a pocket or box.
Microwave heating of materials stems from dielectric power absorption as
described by the fol-
lowing equation: P = KfE2e' tan 8 . The electromagnetic field energy
dissipated as heat per unit
volume is proportional to the dielectric loss factor (s' tan 8) of a material,
the square of the field
strength (E2) and the frequency ( f ) of the applied fields.
When a curable dielectric resinous material is polymerized, its microwave loss
factor is drastical-
ly reduced.
Microwave power reduction and control is nearly always done by pulsing the
full power on and
off over some duty cycle or time base, wherein a duty cycle or time base is
defined to be the
amount of time from beginning the pulsing of power to the time pulsing is
completed. so, for
example, in an 800 watt oven, it is possible to achieve an average output of
400 watts, or 50%
power, by pulsing the full 800 watts on and off, assuming the pulse width is
equal to half the pulse
period. These time bases are typically long, 20 seconds, full 800 watts is on
for 10 or more
seconds and cause lighting and hot spots problems.
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One apparatus, provided in accordance with this invention, comprise a
microwave applicator
having a three-dimensionally defined irradiation space, including a microwave
applicator having
the format of a box (1), which is open at least on one side and includes a
means of preventing the
escape of microwave through the opening such as a door (2). The door has a
means of being
guided to a precise closing position such as hinges (3), and is able to be
locked. The opening
dimensions are preferably less than those of the walls of the cavity. The door
is made of materials
similar to those used for the cavity, being of good conductivity and
dissipation for the electrical,
thermal and microwave energy, including conductive metals or metal-plated
materials. The
dimensions of the cavity applicator and walls should preferably be set to
minimize
electromagnetic «resonance» or «standing waves» situations which may occur in
some internal
zones of the cavity applicator, thus causing «hot or cold spots». Therefore,
the said dimensions
of the cavity should not be a multiple of the wave length ~,g of the
transmitted microwave energy
or pair fractions of the said wave length such as 1/4, 1/2. For example, for
the frequency of 2.45
Ghz, the wave length is: ~.g=n/f = 4.82 inch.; 9.64 is a multiple of ~,g;
11.24 is a multiple ~,g/3 and
is not "resonant" and is preferred as a cavity wall dimension. A flat flange
(4) made of said
conductive materials is fixed to the opening of the cavity applicator, and
extends outwardly from
the walls, and comes into close contact with a wave trap (5), preferably
mounted on the door, and
which should have a dimension of ~,g/4 of the emitted wave length. Leaky
microwaves will be
90 degrees out of phase when going outward as well as when returning, obliging
the leaky waves
to travel a total of 180 degrees. The returning waves will be in counter phase
with the leaking
waves thus producing an energy cancellation. Each corner of the door is
provided with a curved
band (6) to maintain the said ~,g/4 distance of the emitted wavelength, and
the wave trap's
efficiency. To minimize wave leakage, microwave-absorbing materials may also
be installed in
the wave trap. A means for efficiently locking the closed door is provided
such as a T screwing
handle (7). Safety microswitches (8) are installed in a serial manner to
electrically disconnect the
microwave generator electrical supply when the door is open. A rectangular
wave guide (9) or
a cable, connects operatively the cavity applicator to the microwave source.
The wave guide
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CA 02246663 1998-09-18
includes a means of being tuned (10), and in one preferred embodiment,
comprise a directional
coupler (11). An aperture (12) is made both in the wave guide and in a wall of
the cavity, such
that they are juxtaposed. This creates a passage for electromagnetic waves to
enter the cavity. The
aperture preferably has a length corresponding to ~,g/2 of the employed wave
length and a width
equivalent to the wave guide width. A deflecting plate (13) is fixed at one
end of the wave guide
at an angle of about 45 degrees, and causes the incident microwave beam to
deviate into the cavity.
The means of tuning the wave guide and system is advantageously provided on
the wave guide.
For example, three holes can be drill into one wall of the wave guide, and
three tuning screws are
placed into the threaded holes, across the said wave guide wall, the space
between the holes is
preferably at a distance equivalent to ~,g/4. This provides an efficient means
to control and reduce
the standing waves in the wave guide and the microwaves which are returning to
the microwave
source ( 14).
In one embodiment a microwave the probing means consisting of the directional
coupler is
mounted close to the output of the microwave source on the waveguide. This
coupler sense the
transmitted and reflected microwave magnitude and permit the monitoring and
control of
irradiation parameters. The directional coupler includes high frequency
detecting diodes that are
mounted on a printed circuit which is mounted on the wave guide. The output of
detecting diode
is connected to an electronic display to permit the irradiation monitoring and
control of the
transmitted and reflected microwave levels through the process in real time.
Preferably, the
microwave probing means is connected to a central process microcontroller to
follow a preset or
real time self adjusted thermal processing program including irradiation modes
and intensities.
In one embodiment, the control of the microwave generation is accomplished at
the source by
changing adequately the base voltage at the transistor, such as disclosed in
diagram l, for a precise
control of the wave generator output power. For a microwave generator of 2.45
Ghz, such a
magnetron, usually about -3500 DC volts are require to functions. A high
voltage transformer ( 15)
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raises the electrical voltage to about 1750 vac; then, a doubling circuit (
16) composed of a high
voltage condenser and a high voltage rectifying diode brings the voltage to
about -3500 vdc volts.
A secondary low voltage coil of 3 vac supplies the heating filament of the
microwave generator.
The base of the transistor (17) is connected to a micro controller (18). This
transistor can be used
as a variable resistor, to permit monitoring and automated management of the
different irradiation
and timing functions during the process. This providing permits the control of
the microwave out
put power in two ways. First by changing the duty cycles at the transformer
which is insured by
applying pulses to the base connection of the transistor. The second way of
controlling of power
is to reduce conveniently the applied voltage of the primary circuit of the
transformer by changing
adequately the base voltage of the transistor. This embodiment permits a soft
management of
microwave power by avoiding overheating of the microwave source, of sensitive
small or high
absorbency materials and corona discharge when metallic objects are used.
The generated microwave energy travels through the waveguide, is introduced
and radiate into the
defined exposure space from the wave guide aperture. To further reduce the
standing wave
patterns presence in the cavity application system, one or more microwave
stirrer (19) is made
with microwave deflecting blades and installed on an axle through a bushing on
one or more of
the cavity inside walls. The stirrer rotates by means such as a belt, pulleys
and electrical motor
(20). The overall surface of the stirrer can be about 3/4 of dimension of
cavity's wall. Each blade
have a different configuration and passes close to the aperture, causing the
microwave beam to be
oriented and delivered to different areas of the cavity. The materials used
for the fabrication of
the stirrer should have good electrical conductivity. The stirrer shaft is
preferably made of a non
conductive material to minimize microwave conduction and leakage through the
bushing. To
improve the homogeneity of the established electromagnetic fields in the
cavity microwave
applicator, flat or curved reflectors made of conductive, specialty materials
or active
electromagnetic components may be fitted in appropriate locations such as at
the lower corners
useful to enhance energy distribution uniformity. The apparatus is provided
with a stand (21)
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made of microwave transparent material to support suited dental compositions
or objects that are
to be microwave irradiated.
In one embodiment, in order to produce a dental polymer based object with high
flexural strength
and high modulus of elasticity and very low levels of post cure leachables,
being preformed or not,
is irradiated and internally heated while compressed by a fluid such as air or
nitrogen, resting on
perforated tray (22) in a flask (23) made of heat and pressure resistant
microwave partially
transparent materials which may be filled and reinforced such as polyester,
polyethylene,
polypropylene and polysulfone, and having at least two body members and a
means of clamping
such as screws and preferably as the disclosed bracket (24) and a pressure
limiting valve (25).
When used in conjunction with the provided cavity applicator, the flask is
introduced in the cavity
and is connected to a mechanical gas coupling means (26) being positioned on a
wall and or the
bottom of the cavity applicator. This permits the introduction or removal of
gas as needed before,
while and after the irradiation of the processed target. A gas such as air or
nitrogen is introduced
through one of the flask pneumatic connections such as the ring opening (27)
provided with each
body member of the flask and allows easy and fast processing and making of
objects having highly
desirable properties. Preferably, a means of rotary mechanical gas coupling
which employs an
electrical motor (28), permit more uniform microwave exposure of the substance
or object by
entertaining the flask and targeted object in a rotary movement in the cavity
while under pressure
a constant. Microwave absorbing substances such as water can be introduced
into the flask recess
to increase heat or steam generation.
In one embodiment, a means for vacuum forming method is characterized by the
use of the ring
opening of the lower body member of the flask and a mechanical gas coupler
which is positioned
at the bottom of the cavity wall, in connection with a vacuum source to allow
the thermal
conditioning of thermoplastic softening compositions as well as the cure of
thermosetting dental
material compositions with highly desirable qualities useful in many dental
applications, such as
CA 02246663 1998-09-18
fabrication of dentures, trays or base plates, by attracting with suction the
polymer-based material
before and/or during the irradiation towards the mold, positioned on a dental
model and a
perforated tray to condition thermoplastic or thermohardening dental
materials.
In one embodiment, the lower half of the flask is connected by providing
coupling means to a
vacuum source. A pasty polymer-based material (29) is set on or in a mold or
pattern and
positioned on a perforated tray in the flask. A flexible membrane (30), made
of a material partially
transparent to microwaves, such as silicone rubber is firmly retained by a
means such as a recess
between the two body. members of the flask, permitting the forming of a dental
material by
applying hydrostatic forces while microwave irradiated. Additional pressure
can can be exerted
on the dental material by the introduction of pressurized gas from the upper
ring opening of the
flask. The embodiment is useful in the fabrication of dental devices such as
tray, base plate, fiber
reinforced composite crown and bridge, and molding of thermoplastic based
objects such as vinyl
esther oral protectors, permitting to reduce substantially the size and the
number of the voids.
In one embodiment, a dental model (31) made of materials such as wax or
elastomer, which can
bear components such as artificial teeth, having the forms of the object to be
produced is vested
in a coating material (32) such as plaster in a flask having at least two
bodies members and a
clamping means. First, the cup shaped recess of the lower member of the flask
is filled with the
coating material, then the model which may include a plaster cast is
positioned in the coating
material to a depth that is about half of its total height or to its largest
contour. Once the coating
material is set, a separating medium such as alginate based isolation solution
for plaster is applied
to it's exposed surface. The two parts of the flask are then joined by a
retaining and alignment
setup such as screws and nuts and preferably clamping bracket means. The
jointly clamped body
members of the flask are then filled with more fluid coating material through
its upper ring
opening. Each ring opening (33) can be secured to the flask by mean of
threading or a shoulder.
Once the added coating or mold making material is set, for dental patterns
made of wax or such,
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the complete flask is heated in the apparatus or in a hot water bath a few
minutes to soften and
melt the wax. Subsequently, the flask is split opened after removal of the
clamping mean, thus
exposing the internal forms of the mold and defining the shape of the object
to be produced as well
as holding in position, objects such as artificial teeth. All the parts are
then washed with hot water.
When using thermosetting material, an isolation medium is applied to all
exposed surfaces of the
mold to prevent the adhesion of the polymer material when in close contact
with the mold at the
processing stage. The fabrication method of the mold resembles to the known
technique of lost
wax. A drying treatment of the plaster molds can be done by its irradiation
and heating in the
cavity applicator or an oven. Once the dental mold is made, it's packed and
may also be painted
or sprayed by a dental material composition, the flask members are then
clamped, introduced and
mechanically connected in the cavity applicator to a fluid under pressure such
as air and get
microwave processed.
In one embodiment, the flask is provided with an opening (34) preferably with
the disclosed
means of quick connection permitting the positioning and removal of the
injection nozzle (35)
while flask body members are joined. The mold space within the flask is
operatively connected
to the flask opening through vested runners made of material such as wax,
preferably set on the
model before the second filling of the flask of the coating material. Physical
changes, including
the progressive mold filling densiflcation and the volumetric shrinkage of
many thermally
conditioned polymer based material, is substantially compensated in this
invention with the
pressurized and when needed introduction of the fluid polymer based dental
materials into the
flask. The material injection means includes the use of a fluid conduct (36)
with a male
mechanical hydraulic coupler which allows the introduction of a fluid into the
pressure injection
capsule (37) through the mechanical coupler into the cavity applicator, which
results in the
compression filling of the materials contained in fluid dental material
reservoir (38) into the mold.
When under a hydraulic pressure, the piston (39) forces the material from its
compartment through
the injector (40) and the opening of the flask and (than) the mold is filled.
The cover (41) of the
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capsule is made to be removable by a means such as a threads and is connected
to one or more
injectors in connection with the flask. The capsule can be advantageously
trained into a rotary
motion transmission by means such as a key path (42), on a rotary platform pin
(43) in the cavity
to enhance irradiation uniformity of the mold and dental composition while
submitted to hydraulic
forces. The piston and the capsule are advantageously equipped with sealing
joints (44). For the
processing of some materials such as thermohardening polymers, the said
capsule, conduct, and
nozzle are preferably shielded by being made of microwave impervious materials
such as steel and
conserve the unprocessed material compositions in its original temperature and
fluidity condition,
under pressure and while being continuously available and able to be
introduced as needed in the
mold to compensate for the volumetric shrinkage and to fill voids and/or
compensate for
progressively occurring deformations of the object in thermal process. This
continual pressurized
injection allows a substantial increase of the dimensional precision of the
produced dental objects.
The presence of porosity is significantly reduced and produced objects are
more suited for dental
uses in terms of biofunctionality, fit and durability when compared to objects
such as prosthetics,
produced by the conventional methods and materials. Preferably, a bleeder (45)
made of
microwave transparent materials is employed in an appropriate housing made on
at least one of
the flask members closing surface, and provides a means of hydraulically
connecting the mold to
the exterior of the flask, useful in reducing the energy and time required to
appropriately fill the
mold and also minimize porosity occurrence. Said bleeder accelerate the
emptying of the existing
air of the mold space when introducing resinous materials into it while
preventing the leakage of
resinous fluid dental materials under pressure by moving outwardly and
blocking the external
orifice of housing.
In one embodiment, low microwave absorbing materials such as thermoplastic
resins are
indirectly heated with the use of a compression-injection capsule coated or
layered with
microwave absorbing substances such as metal oxides including zinc oxide,
carbon black and
ceramics.
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In one embodiment, an economic manual fluid resin pressurization and injection
device (46) is
provided to remove the need of being connected to an external pressurized
fluid source. A
mechanical force accumulator such as a spring (47) is compressed by turning
the internally
threaded cylinder (48) while holding the device handel (49). A force boosting
piston (50) is
especially useful for molding and filling of composite curable dental
materials. The injection
nozzle and the piston acts as previously described. This embodiment can be
used with the
disclosed cavity applicator and flask or with the mobile microwave applicator.
In one embodiment, a shielded temperature probe such as a thermocouple made
with a
temperature dependent resistor, a fluoro-optic or an infrared temperature
magnitude detecting
means is used with optionally a pivoting electrical connector to permit the
sensing of the thermal
conditions of the microwave irradiated target. This embodiment permits a
precise setting of the
pace of thermal conditioning as well as the indication of the reach of a
specific temperature
magnitude, useful for the thermal processing of delicate materials such
thermoplastics or low
temperature boiling monomers as well as to increase safety in the
sterilization functions and is
preferably used in connection with the central micro controller.
In one embodiment of the present invention, to permit a safe and quick
sterilization of dental
objects without fear corrosion or arc occurrence, a cylindrical column (51),
made of microwave
transparent materials is closed at one end and externally threaded at the
neck, made sufficiently
thick glass or polymer to resist heat and pressure, in used in conjunction
with the provided flask
and cavity applicator. The cylindrical column permits heating of a liquid and
hot steam generation
and optionally the production of a microwave shielding atmosphere is screwed
into the lower flask
half member through it's ring opening with its sealing joints (52). A liquid
such as distilled water
is introduced to fill the column, up to a pre-determined level. A specially
shielded flask
operatively connected or not with the steam generation column is introduced in
the said cavity
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through the door or only its column introduced from the provided top circular
opening (53) into
the cavity applicator which is provided with a disk form closing door. To
sterilize, the steam
having reached the evaporation temperature under microwave irradiation fills
the flask with the
vapor rise up. The upper flask half is preferably made of a heat conduction
and exchanging
material (54) such as stainless steel and comprise a heat sink to cool by
conduction the internally
contacting warm vapor. The condensed and liquefied sterilizing solution return
by gravity to the
base of the column where it's repeatedly heated and evaporated, providing a
constant steam flow
and contact with treated dental objects contained in the flask. To detect the
temperature magnitude
with high accuracy, the temperature probe for microwave environment can be
placed within the
flask. The flask can be sealed immediately following removal of the probe
after sterilization with
the use of an annular elastic sealing coupler positioned on one of the flask
inlets such as the
injection opening or the pressure limiting valve manifold. The means of
microwave and
temperature magnitude detection permits a precise control and delivery of
microwave to a dental
target, useful in avoiding arcing occurrence by generating adequate microwave
power levels
andlor creating a shielding vapor pressure atmosphere inside the flask. The
temperature and
microwave sensing and control is preferably done in an automated manner with
the
programmable micro controller. Once the predetermined temperature is reached,
a signal is sent
to the microcontroller which then reduce the power of emission so as to
maintain a sufficient
amount of time to sterilize (6 min). Equilibrium temperature is reached
quickly since there is no
great swings in the temperature and optimal control of the microwave delivery
is achieved.
In one embodiment, the temperature is safely and economically controlled for
sterilization
function through a gas pressure sensor which is connected to the flask for
example through the
pressure limiter manifold (55) or vent to control the sterilization
temperature inside the flask,
specially when used with the shielded flask, positioned externally with only
its steam column
introduced in the cavity applicator. This pressure sensor is operatively
connected to a
microcontroller to maintain the right warm steam pressure temperature
magnitude and permit
CA 02246663 1998-09-18
monitoring. The temperature sensor for microwave environment can also alone or
jointly with the
pressure sensor be used with the disclosed device. Any increase of temperature
of a gas having a
given volume conduct to an increase of it's pressure. By limiting and/or
controlling the pressure
of the gas, an effective control of flask internal temperature is achieved.
The micro controller
control the flask internal temperature via the microwave generator, using the
provided microwave
power control.
We have conducted complex dielectric permittivity, temperature and
distribution pattern studies
of microwave heated teeth and Simulations of specific absorption rate
distribution. The complex
permittivity was measured on different types of dental tissues, using
extracted teeth, including
enamel, dentin and caries. Reflective coefFlcients has been obtained using a
network analyzer.
The characteristics of enamel caries and dentin are different. Some caries are
soft. The higher the
moisture of caries, the higher is its permittivity. The dielectric loss factor
of caries is fairly higher
than that of normal healthy parts. When the tooth is exposed to microwaves,
caries are
preferentially heated. Temperature rise can kill the microorganisms in caries.
Control and or
extinction of microorganism slows or stops the progress of caries, permitting
previously carious
tissue to recalcify by biological latent support of the pulp. Temperature
distribution measurement
with microwaves heating reveals that the temperature of caries is higher than
that of normal tooth
tissue. These properties are used with the provisions of this invention for
the diagnostic and
treatment of teeth having caries and subsequent internal heat conditioning and
or curing of
provided dental restorative materials. When dielectric loss factor is higher,
the absorption of
microwave is better and local temperature is higher. Microwave energy heats by
radiation and is
able to penetrate through various substances including desiccated tissue and
thus can create an
addressed effect.
In one embodiment, a method of caries control in a non invasive atraumatic
way, without surgical
burs entry and with a reduced risk and necessity of exposing the dental pulp
organ comprise, a
16
CA 02246663 1998-09-18
hand held microwave applicator with an electronic circuit, (diagram 2)
designed to suit a small
microwave generators such as impatt diodes with an output power of about 5
watts which requires
usually an electrical voltage of about 60 DC. The electrical current is
applied through a high
impedance line (56) in order to limit the perturbation of electromagnetic
signals. The power
supply module is provided with a current and voltage limiting mean to permit
the polarization of
the impatt diode in the specific limits with a resonant circuit (57), such as
a 50 ohms line, having
a length preferably equal to the half of the length of the selected frequency.
The length of the line
may be calculated with the following equation: L= 3x108 / 2Fr E~~1/2. pne end
of the "resonator"
is connected to the impatt diode (58) and the other end of it is coupled (59)
to a transmission line
including an isolator (60) to provide isolation of the microwave source from
the rest of the circuit
in order to avoid frequency variations, caused by a mismatch of the output
(61). A coupler (62)
having a coupling of about -15 dB permit a sampling of the signal emitted by
the microwave
generator in order to measure the forwarded and reflected power. The couplers
should be perfectly
matched at both extremities to permit precise measurements. Matching circuits
(63) at the input
and the output as well as load resistors permit achievement of an adaptation
at each ends, equal or
better than -lSdB. Detecting diodes (64) rectify the radio frequencies signal
in order to convert
the power to a do voltage which can advantageously be subsequently transmitted
to a micro
controller or a "ADC" which converts this voltage to a numerical signal for an
appropriate
processing of the acquired informations and the precise and monitoring control
of microwaves
energy delivery to the dental target. The said control is a means of
controlling the power level,
exposure cycles, processing modes, as well as the selection of the frequencies
of microwave
generation. The control of the microwave generation is preferably made by a
selector (65),
located on the device, allowing to set different power levels and modes.
Between the tip antenna
and the microwave source or amplifier, a shielded cable (66) or wave guide, as
short as possible
is used to operatively transmit the microwave power to the head antenna.
This provided method and device of controlling and treating dental caries in a
non-invasive atrau-
17
CA 02246663 1998-09-18
matic therapeutical approach does not require surgical burs entry and reduces
the risk and the
necessity of exposing the dental pulp.
An insulated connection permits the interchange of different provided head
antennas to match
different applications and enhance energy transmission and deposition on the
dental target. A
means of electrical supply (67), such as a shielded cable, connects the mobile
applicator to the
power supply. The hand held applicator is preferably equipped with a water
cooling system (68)
and a digital display (69).
One head antenna (70) provided for therapeutic purposes to target teeth and
treat and heat or detect
dental caries is made of a highly conductive metal such as copper, platinum or
gold, plated or not,
having the format of a rectangular or a loop shaped band of which one end is
connected to the
internal metallic lead of the device cable and the other connected to the
external metallic lead of
it.
One provided head antenna has the form of an I. This applicator is a short
monopole, made for
example by stripping the outer jacket and the outer conductor of a coaxial
shielded cable, the inner
conductor and dielectric (Teflon) constitute the applicator. A loaded I-
applicator (71) having an
increased energy forwarding capacity is made by soldering a platinum ring over
the outer
conductor of the coaxial cable and soldering a platinum rod to the tip of the
inner lead of the cable.
Another provided head antenna (72) is made of a microstrip being made of
miscible polymeric or
other conductive materials, for example, a square metal skin is positioned on
a non conductive
material with a ground plane on its back, particularly useful for the heating
and/or joining of dental
object.
In one embodiment, an electrically shielded temperature probe is embedded in
the head of the
18
CA 02246663 1998-09-18
hand held applicator antenna to provide a means of monitoring the temperature
of the heated target
for judging the efficiency of tissue heating and to avoid sudden temperature
rises.
The provided head antenna designs help in achieving good impedance matching
and effective
delivery of microwave for internal heat conditioning of dental targets. A
means of safely
containing any escaping microwave energy close to the irradiation space can be
used such as the
disclosed head antenna choke (73), made of microwave absorbing foamy or
rubbery materials.
In one embodiment, head antennas are equipped with a miniaturized version of
the membrane
dental fluid material adapter in order to assist intra-oral tooth restoration.
In general, various polymer based material compositions are useful for the
construction of dental
devices. These compositions may be used in the filling of teeth and the
construction of appliances
used for replacing teeth and other oral structures. One utility of these
compositions is in the
construction and repair of removable dental devices such as dentures and
dental anchored
restorations such as crowns, bridges, inlays, veneers. Also utility is found
in the making of mouth
guards, oral border molding, impression trays, base plates, orthodontic dental
appliances. Various
thermoplastic containing dental compounds are also advantageously thermally
conditioned and
softened while formed with the provided method and apparatus.
One preferred composition for dental aesthetic composite suited to be formed
and hardened in
accordance with the providing of this invention consists of a polymerizable
mixture including one
or a selection from the large family of polyfunctional methacrylate esters,
and oligomers including
the compound prepared from one molecule of bisphenol A and two molecules of
glycidyl
methacrylate called 2,2bis[4(2-hydroxy-3 methacryloyloxy-propyloxy)-phenyl]
propane, known
as Bis-GMA for its lower degree of shrinkage and/or 2,2-bis[4-
methacryloxyethoxy)Phenyl]
propane for its good water resistance properties. Other monomers, such as
triethyleneglycol
dimethacrylate for viscosity reduction, urethane dimethacrylates, spiro
orthocarbontes are
advantageously employed in admixture with silanized inorganic fillers and
organic fillers,
19
CA 02246663 1998-09-18
coupling agents, microwave sensitive cure initiation system including organic
peroxides and
amines and color pigments are advantageously added. The weight of the fillers
as an overall
weight of the composite is preferably in the range of 30 to 90 % and include
silanized silicon
dioxide particles.
In one embodiment, compositions specially suitable for making dental removable
appliances such
as dentures is provided which comprise a liquid and a powdery component.
The liquid component in accordance with the invention contains preferably from
40% to 90% of
mono-, di-, tri-, or multifunctional acrylic monomer, a cross-linking agent, a
plasticizer, a
stabilizer. an accelerator and color pigments.
The mono-, di, tri, or multifunctional acrylic monomer in accordance with the
invention are within
the scope of the formula: ~ R2
[CH2=C]
~ C=O
Rl
where Rl in accordance with the invention is hydrogen, alkyl, substituted
alkyl group, cyclic
hydrocarbon, benzyl, ether, hydroxyalkyl and R2 is hydrogen, halogen, alkyl,
substituted alkyl or
cyclic hydrocarbon group. Monomers within the scope of the following formula
are also
particularly suitable to the invention: O R1
(R)m-IO-C-C=CHJ
n
wherein R is an acrylic-free organic moiety, Rl is hydrogen, hologen, halogen,
alkyl, substituted
alkyl or cyano radical and n is an integer from 1 to 20 and m is an integer
from 1 to 1000.
These monomers may be used alone or in admixture.
The microwave sensitive initiators in accordance with the invention includes
benzoyl and
peroxide, dilauroyl peroxide up to 2,5%.
The polymerization accelerator in accordance with the invention is a
quaternary ammonium
CA 02246663 1998-09-18
chloride, which is easily soluble in the methacrylate monomers and reacts with
barbituric acid
derivatives. A preferred compounds are the quaternary ammonium with an alkyl
of 1 to 20
carbons, such as, dodecyltrimethylammonium. These quaternary ammonium
chlorides may be
added in alone or in admixture from 0,09 to 1,5 %.
The crosslinking agent in accordance with the provided microwave hardening
material
compositions is a polyfunctional monomer wherein at least two carbon-carbon
double bonds, such
as 1,3-butanediol dimethacrylate, 1,4-butanediol dimethacrylate, 1,4-
butanediol divinyl ether,
di(ethylene glycol) dimethacrylate, di(ethylene glycol) divinyl ether,
pentaerythritol diacrylate
monostearate, ethylene glycol dimethacrylate, trimetylolpropane
trimethacrylate, pentaerythritol
triacrylate, pentaerythritol tetraacrylate, trimetylolpropane triacrylate. The
crosslinking agents
may be used alone or in admixture.
Polymerization promoters for the monomers of the provided curable material
system including
acrylates consists of a barbituric acid derivative which, when irradiated with
microwaves, rapidly
react with the quaternary ammonium chloride to produce radicals, which
promotes a rapid and
uniform polymerization in the composition and a higher degree of conversion.
The barbituric acid
derivative in accordance with the invention include 1,3,5-trimethylbarbituric
acid, 1,3-dimethyl-
5-isobutylbarbituric acid, 1,3-dimethyl-5-phenylbarbituric acid, 5-n-
butylbarbituric acid, 5-
ethylbarbituric acid, 1-cyclohexyl-5-ethylbarbituric acid and 1-benzyl-5-
phenylbarbituric acid.
These acid derivatives may be used alone or in admixture in very small
amounts.
The polymerization stabilizers comprise hydroquinone, hydroquinone monomethyl
ether or 4-
ethoxyphenol which are usually added to the liquid component of dental
compositions (up to 4%).
The plasticizer in accordance with the invention is generally a low molecular
weight ester, such
as dibutyl phthalate or phosphates .
The composition for a one component microwavable curable material system in
accordance with
this invention is approximately the same as the one for the two component
materials with some
variations mainly in the initiation system. Preferred initiators for a one
component dental
composition for denture or such need to be thermally stable at room or higher
temperatures such
21
CA 02246663 1998-09-18
as 50°C and initiate polymerization at higher temperatures such as
benzopinacole, tert-
butyleperbenzoate, and 2,2'dichlorobenzopinacol.
The powder component in accordance with the invention includes from 20% to 80%
of mono-di-
tri, or multifunctional acrylic or acrylate ester polymer. The powder may
advantageously include
from 5% to 40% of a copolymer. The powder component in accordance with the
invention may
advantageously include from 0,1 % to 3 % of an initiator for radical
polymerization such as
organic peroxides. The powder component in accordance with the invention can
include up to 1%
of a barbituric acid derivative to promote chemical reaction.
The mono-, di, tri, or multifunctional acrylic polymer used in denture base in
accordance with the
/ R2
invention are:
[CH2_C] n
~ C=O
O
\R1
where the RI in accordance with the invention is hydrogen, alkyl, substituted
alkyl group, cyclic
hydrocarbon, benzyl, ether, hydroxyalkyl, R2 is hydrogen, halogen, alkyl,
substituted alkyl group
and n is an integer at least equal to 2.
The copolymer in accordance with this invention are mainly composed of methyl
methacrylate
polymer or a mixture of methyl methacrylate polymer and an methacrylate
polymer other than
methyl methacrylate polymer.
Inorganic and organic fillers may be added into the compositions of one or two
components
denture base. Useful inorganic fillers include glass, metal ceramics, silicon
dioxide in powdery or
fiber format, which are preferably silanized with coupling agent, such as 3-
methacryloxloxypropyltrimethoxy. Organic fillers include splinter or bead
polymers of high
molecular weight , or fibers such as aramide fibers, polyacrylate fibers,
polyamide fibers and
polyacrylonitrile fibers. Organic fillers may be used alone or mixed with
inorganic fillers.
Microwave curing resilient compounds for making devices such as denture liners
is molded and
22
CA 02246663 1998-09-18
cured with the provided novel method and apparatus including
organopolysiloxanes and
phosphonitrilic fluoroelastomers [poly(fluroalkoxy)phosphazene] with a
crosslinking agent, a
filler and an initiator. Silicones are containing a repeating silicone-oxygen
backbone with organic
side groups attached via carbon silicone bonds. One composition for soft
denture liners in
accordance with this invention contain silicones within the scope of the
structural formula:
CRnSiO(4-n/2J
m
Wherein n=1-3 and m>l. R groups are usually methyl, longer alkyl, fluoroalkyl,
phenyl, vinyl,
alkoxy or alkylamino. One preferred silicone compound is polydimethylsiloxane
(PDMS) of the
following structure: i H3 i H3 i H3
~I i-O-I i -O-I i_O~
CH3 CH3 CH3
Methacryloxy-terminated polydimethylsiloxanes are particularly useful since
they bond well to
PMMA made dentures due to the chemical similarity.
The crosslinking agents for soft liners are normal multifunctional monomers
wherein at least two
carbon-carbon double bonds. Preferred crosslinking monomers are acryloxy or
methacryloxyalkyl-terminated siloxane monomers, such as 1,3-bis[(p-
acryloxymethy) phenethyl]
tetramethyldisiloxane, 1,3-bis(3-methacryloxypropyl) tetramethyldisiloxane,
due to chemical
similarity.
The normal initiators in the soft denture liners in accordance with the
invention are general
peroxides, such as benzoyl peroxide, lauroyl peroxide, which are usually added
to the powdery
component of resilient compositions in small amounts.
The phosphonitrilic fluoroelastomers (poly(fluoroalkoxy)phosphazenes) in
accordance with this
invention are polymerized by monomers within the following formula:
~OCHZ(CF)2nX
N=P
'OCH2(CF)2 n X
wherein X is H or F, and n is usually from 1 to 11, 30 to 60 °lo of
total ingredients for a firmer liner
23
CA 02246663 1998-09-18
and up to 90 % for softer one.
The crosslinking agent suitable for the fluoroelastomers are monomers with at
least two functional
groups, such as tetraethylene glycol dimethacrylate, ethylene glycol
dimethacrylate, 1,6-
hexamethylene glycol dimethacrylate, trimethylopropane trimethacrylate,
pentaerythritol
triacrylate, pentaerythritol triallyl ether, pentaerythritol tetraacrylate.
The flllers,which are preferably mainly hydrophobic improve hardness and the
ability to grind and
polish the cured resilient materials and the bond durability between the liner
and base. Particles
of fillers may be beads or fibers, pigments and other additives can be added
to the soft material
system (fillers 7 % for soft, 30 % for firm liners).
Thermoplastic compounds such as poly functional methacrylate, polycarbonate,
polysulfone,
fluoropolymers, elastomers, polyurethanes, impression compound, wax,
polycapratone and
mixture of thermoset and thermoplastics are advantageously heat processed with
the provided
method and permit dental rehabilitation.
Microwave absorbing substances can advantageously be incorporated into
disclosed thermoplastic
and thermohardening material compositions, to decrease internal heat
generation of compositions
which does not have sufficient dielectrical loss when microwaved nor does they
have sufficient
heatability for a desired speed of heating. These microwave «absorbers» are
also useful when the
employed polymeric material has only a low microwave absorption behavior at
low temperatures
such as many thermoplastic polymers including polycarbonate and also for
substantially
increasing the speed and the adressability such as in welding and joining
functions. These
absorbers may be powdery, hollowed, coated and comprise ferromagnetics,
metallic oxides or
speciality ceramics.
All ratio for materials are expressed in weight.
24
CA 02246663 1998-09-18
Examples
Cavity applicator dimensions
- Cavity: 32 cm x 32 cm x 28 cm made of steel
Wave guide: 3.8 cm x 7.6 cm x 45.7 cm such as WR 284 made of copper
- Steerer: 20 cm made of steel
Flask (made of polypropylen)
- diameter interior: 8 cm - bleeder 2 mm diameter 3.5 mm long
diameter exterior: 13 cm - membrane thichness: 3 mm
- recess depth: 1.5 cm - rind: 3.5 cm
Injection capsule dimensions
(made of stainless steel /
wall thickness 6 mm)
Diameter Stroke Piston height
- Dentures: 10 cm 5 cm 2 cm
- Manual: 5 cm 6 cm 2 cm
- Composite boosting piston: 2.5 1.5 cm
3.5 cm cm
Process programmable micro controlle
Micro-controller Pic of Microchip inc. or Parallax
Microwave frequency
- Magnetron frequency: 2,45 GHz Output power 600 W
- Impatt diode frepuency: 24 GHz Output power SW
- Vacuum source such as an 600W cleaning aspirator for dental
Vacuum forming of resinous or microwave softened dental materials
- The steam generation column is made of polycarbonate with it walls having a
tickness of 1 cm,
6 cm inside diameter and a height of 12 cm
Pressure limiting valve
Aperture: 4 mm2
- Weight: 80 g
- Pressure: 24 PSI
Sterilization method ~ Rapid heat & steam generation ~ Warm steam maintenance
Water 200 cc sterilization equiped flask
with only its column introduced in the 450 W, 2 min, ~ Max 25 PSI, temperature
100° C
cavity. Pressure limiting valve set a 25 water is brought to the boiling 125
W, 5.5 min
Experiment of decay control in the cavity microwave applicator
Pre station Microwave Incubation Results
irradiation
Section of decayousSurface desinection,1.5 Wlcm2 Culture of Delivered
freshly energy irradiated rradiation
extracted human15 seconds density of decayous teethdestroy completely
teeth deeping irradiation
prepared, 2 in cloramine(200 W in section in carious zone
mm3 T the cavity medium at
solution application)37C 24 h. microorganisms
60 sec.
CA 02246663 1998-09-18
Examples of microwave procressingSteps of the
procedure
in order
of polymer based material Compression, Microwave Bench cooling
forming irradiation
Aesthetic com site
A 100 de 3M inc, color ivory,
0.15% of 1 min 3 min 450 3 min
benzoide proxide for initiation. W
lcc Example 1
* Example 2 2 min 5 min 250 3 min
W
* Mechanical test (3 pointsSize: 25 x Load at max Displacement
bend, failure) of 2 x 1.75 at
specimens of example 2 mm max
25 PSI membrane
Testing specifications, compression 45 N 0.42 mm
crosshead speed , flask
2 mmm/min Instron device & luster mold
Mold i 'actionMicrowave Bench cootin
fillin irradiation
GC Acron resine for dentures
, 40 cc 100 si 3 min 7 min,. 225 6 min
Flask with bleeder plaster W +
mold 1 min 400
Large capsule Example 1 W
Exam le 2 100 si 3 min 4 min 450 6 min
W
Soft materials 12 min ,225W
Mollo last B Re nesi & co 80 si 5 min + 6 min
GER 40 cc 1.5 min 400
W
G-C ACRON, denture repair material ~ 25 psi, air pressure - 80 g pressure
limiting valve weight on a regular
powder & fluid I dental index made of plaster
Microwave softening of thermoplasticMicrowave irradiationAdaptation
dental material ~ ~ time
Border moldin coum ound in a 4 min 200 W 2 min
cc satin ue
Dental custom tray, polycapratone2 min, 300 1.5 min
sheet thermosoftening W
Thickness: 3 mm
26
