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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention primarily concerns restorative dentistry, especially methods
and devices useful for making out-of-the-mouth or extra-oral dental
restorations from dental restorative or composite resin materials. The
invention also concerns a model or die from which a prosthetic dental
restoration can be made and a method of making such a model or die. The
invention further concerns models or dies useful for such purposes as the
restoration of art objects such as statues.
2. Description of Related Art
In spite of remarkable technological advances in prosthetic dental
restorative or composite resin materials, amalgams typically are easier to
install, can be completed in a single visit, and are regarded by many
practitioners as having superior durability. For such reasons, the
amalgams continue to predominate in posterior dental restorations in spite
of their toxicity, aesthetically undesirable color, and the usual need to
remove healthy portions of a tooth in order to interlock the amalgam into
a cavity. Dental restorative or composite resins also can be applied in a
single visit by being tamped into a cavity, shaped or sculptured, then
cured by exposure to light and finished with a bur. The step of shaping or
sculpturing before curing is cumbersome, as is grinding after curing.
Furthermore, shrinkage of the resin during curing produces strain on the
tooth and can result in marginal leakage. Even when shrinkage is minimized
by incremental curing and the dentist has sufficient skill to sculpture
the uncured resin to duplicate the original tooth contour precisely, the
procedure is sufficiently demanding and time consuming that the dentist
may prefer the convenience of an amalgam.
It has been suggested that the effect of resin shrinkage can be minimized
by using a model or die to form an extra-oral prosthetic dental
restoration such as an inlay. Such a model can be formed from dental or
gypsum stone (Plaster of Paris), from thermoplastic resin as in U.S. Pat.
No. 2,136,404 (Wheeler), or from epoxy resin as illustrated in Jensen et
al., "Polymerization Shrinkage and Microleakage," a paper published in
Posterior Composite Resin Dental Restorative Materials edited by Vanherle
et al., pages 243-262 (Peter Szulc Publishing Co., The Netherlands, 1985).
The Jensen article in a table at page 258 lists advantages and
disadvantages of each of in-the-mouth and out-of-the-mouth "`inlay`
posterior composites," the advantages of the latter being:
"Reduced stress on cusps from polymerization shrinkage
Better marginal adaptation at gingivo-proximal (no overhang)
Control of proximal contacts
Better contours and anatomy
Easier to obtain a better surface finish
Possible increased abrasion resistance because resin can be heat cured
under vacuum."
Among the listed disadvantages of the out-of-the-mouth or extra-oral
"`inlay` posterior composite" are that normally more than one dental
appointment is required, thus requiring a temporary restoration, and that
there is increased cost due to laboratory procedures.
While the Jensen article refers to the use of an epoxy die for molding
dental restorations, and the Wheeler patent refers to the use of dental
patterns made from certain thermoplastic resins, such dies or patterns are
more commonly made from dental or gypsum stone. Gypsum stone is generally
regarded as the state of the art molding material against which other
materials are measured. The thermoplastic resins of the Wheeler patent are
said to be grindable but must be melted at fairly high temperatures. This
can cause unacceptable shrinkage of the model and poor restoration fit.
Like epoxy rein, gypsum stone takes a long time to harden, thus requiring
two visits to the dentist and a temporary restoration between visits. The
need for two visits can be exceedingly inconvenient to patients who live
in remote areas, and the need for a dentist to use a dental laboratory can
be troublesome when the closest laboratory is at a distant location.
SUMMARY OF THE INVENTION
The present invention permits an extra-oral prosthetic dental restoration
to be made in a single visit. It can enable attainment of the advantages
quoted above from the Jensen article, while eliminating or minimizing the
above-mentioned disadvantages. The invention can also be used for
nondental restoration work to provide a durable model that can be used
within minutes after it is made. These advantages are achieved by a method
comprising the steps of:
(1) forming a rubbery, heat-resistant impression of an object to be
duplicated (e.g., a tooth, teeth, gingival or gum tissue, or other animate
or inanimate object),
(2) partially filling the impression, e.g., coating or dusting all or a
part of the working (e.g., tooth) surfaces of the impression, with a
liquid or powdered thermosetting resin,
(3) further filling said impression with a molten thermoplastic resin and,
after the thermoplastic resin solidifies and the thermosetting resin is
thermoset,
(4) removing the thermoset resin and solidified thermoplastic resin from
said impression to provide a model of said object.
The thermoset surfaces of the model can be machined more readily than a
model made entirely from thermoplastic resin. This is important when the
model is of human teeth, because it often is necessary to grind off
material, e.g., at the gingival margins. A surface of cured thermoset
resin is also useful when the model is to be used to shape a dental or
nondental restoration, because the thermoset resin typically will exhibit
good wear resistance. This is important when the restoration is repeatedly
installed on and removed from the model and especially important when the
restoration comprises a metal such as gold. Because the thermoplastic
portion of the model need not be made excessively heat- or
abrasion-resistant, thermoplastic resins with optimal melting temperatures
and shrinkage can be employed. In addition, the cure rate of the
thermosetting resin is greatly enhanced when the impression is filled with
molten thermoplastic resin in step (3), thus rapidly polymerizing the
thermosetting resin to a tough, abrasion- and heat-resistant state, and
enabling the model to be used within minutes after it is molded.
The model produced by the above 4-step model-making method is itself
believed to be novel and has a variety of dental and nondental uses. For
example, the foregoing steps can be followed by the steps of:
(5) applying restorative resin to a portion of the model where restoration
is required (usually after first applying a release agent to the model),
(6) shaping or sculpturing the applied restorative resin to a desired
contour, and
(7) curing the restorative resin to provide a restoration.
When the restoration is a dental restoration and is cemented into the
patient's mouth, the cement can compensate for polymerization shrinkage of
the material used to make the restoration and thus provide good assurance
against microleakage. The same 7-step method can be used for nondental
restorations, for example, to repair art objects such as marble statues,
especially where the restoration should have the same form and contour as
the object being restored.
A primary advantage of the novel method and model in dental use is that the
model can be made far more quickly than the gypsum or epoxy models that
are currently in use. Thus a model of a tooth or teeth can be prepared,
used to make a prosthetic dental restoration, and the restoration can be
bonded to the tooth or teeth, all in a single visit to the dentist. This
eliminates any need for a temporary restoration. While the
impression-forming step (1) of the novel model-making method requires the
same length of time as do methods used for current extra-oral
restorations, steps (2) and (3) of the method provide a substantial time
saving in that a combination of thermoset and thermoplastic resins can
harden much faster (e.g. within a few minutes) than can gypsum stone or
models made entirely of epoxy resin. An additional advantage is that a
dental auxiliary (rather than a dentist) can carry out steps (2) through
(7). Meanwhile the dentist can work on another patient, returning to the
first patient when it is time to install the restoration. This can reduce
cost, since the training and skill of a dentist customarily commands a
wage far greater than that of a dental auxiliary. While the same cost
reduction is available in current extra-oral restorations, that reduction
may be more than offset by the cost of transfering impressions or models
between the dentist's office and a dental laboratory and scheduling an
extra patient visit.
In a preferred embodiment, the novel model-making method can be modified by
including between steps (3) and (4) an additional step of adhering a
flexible, dimensionally, stable support to the thermoplastic resin. Models
made by this modified method can be flexed by hand (or cut with a knife or
other sharp instrument) to form in the solidified thermoplastic resin
clean cracks at one or more locations (e.g., in the interproximal spaces
flanking a replica tooth). With the flexible support serving as a hinge,
the model then can be opened at those cracks to isolate and expose a
portion of the model (e.g., the mesial and distal surfaces of a replica
tooth). In this fashion the model can be easily manipulated to facilitate
access to a portion of the model, e.g., a single replica tooth.
BRIEF DESCRIPTION OF THE DRAWING
In the drawing:
FIG. 1 is a side elevation of a first model made in accordance with the
invention;
FIG. 2 shows the model of FIG. 1 flexed to expose proximal regions of one
of the replica teeth on which a prosthetic dental restoration can be
created; and
FIG. 3 is a side elevation of a second model of the invention, cut away to
a central section.
DETAILED DESCRIPTION
Referring to FIG. 1, a rubbery poly(vinyl siloxane) dental impression
material (not shown) has been used to mold a model 10 having replica teeth
11a, 11b, 11c and 11d, all made from a thermoset resin, and gingival
tissue 12, made from a first thermoplastic resin. Bonded to the base f he
replica gingival tissue 12 is a flexible support 16, preferably a second
thermoplastic resin that is tough and flexible. The replica teeth are
tough and have good heat- and abrasion-resistance. Their rate of
polymerization (cure) was accelerated by contact with the first
thermoplastic resin in its molten state.
Illustrating a preferred embodiment of the invention, two cracks 17 have
been initiated in the replica gingival tissue 12 at each side of the
replica stump 15. This is readily accomplished by scoring the first
thermoplastic resin with a razor blade, and then flexing the model to
propagate the cracks.
The model is shown in FIG. 2 in its flexed position, providing ready access
to replica stump 15 (shown in phantom). Restoration 21 (a full crown) is
shown in its installed position on replica stump 15. Interproximal
contacts between restoration 21 and adjacent thermoset replica teeth 11c
and 11d can be checked by returning the model to the original unflexed
position shown in FIG. 1.
The first thermoplastic resin preferably is cleanly breakable to facilitate
isolation of replica tooth stump 15. By "cleanly breakable" is meant that
when a panel of the thermoplastic resin 1.27 cm (1/2 inch) in thickness is
scored and flexed by hand at room temperature, it will break at the score
to form two mating surfaces without visibly apparent elongation.
In FIG. 3, each replica tooth of a model 30 (which was formed from a
rubbery dental impression, not shown) has a core 32 of a first
thermoplastic resin and a thin shell 33 of a thermoset resin that provides
good abrasion- and heat-resistance. The thermoset resin can be formed in
place by inverting the impression and coating the replica tooth surfaces
and the gingival margin portion of the model with uncured thermoset resin
in liquid or powder form. The first thermoplastic resin is then added to
the impression in molten form and permitted to harden. Heat from the
molten first thermoplastic resin accelerates the cure of the thermoset
resin. The first thermoplastic resin provides the bulk of the replica
teeth and replica gingival tissue 34, thus providing a cost savings and
rapid hardening. However, the thermoset shell 33 provides wear resistance
and facilitates removal by grinding of replica gingival tissue, to provide
good access for trial installation of a restoration. Bonded across the
base of the replica gingival tissue 34 opposite to the replica teeth is a
flexible support 36 of tough and flexible second thermoplastic resin that
can act as a hinge at cracks 37 through the replica gingival tissue, thus
permitting the model 30 to function in the same manner as shown in FIGS. 1
and 2. The flexible support 36 has serrations 38 and is pressed as a
preformed solid strip into the molten first thermoplastic resin forming
the replica gingival tissue 34. The serrations and relatively weak bond
between the first and second thermoplastic resins permit a replica
prepared tooth 35 and replica gingival tissue between the cracks 37 to be
removed as a unit 39 from the model 30, the unit being repeatedly
returnable to its exact original position.
The impression material from which a mold of the object to be duplicated is
formed, and in which the model is molded, is a rubbery curable material
having sufficient heat resistance to withstand the heat of the molten
thermoplastic resin. Suitable impression materials include addition cure
or condensation cure silicones, polyethers and polysulfides. The silicones
are preferred, since the polyethers and polysulfides generally require the
use of a release agent to facilitate removal of the hardened model.
Alginates and hydrocolloids are at present unsuitable, since they do not
have sufficient heat resistance.
The thermosetting resin should be a one-part or multi-part thermally
curable resinous material that is sufficiently tough and sufficient
heat-and abrasion-resistant in its cured state to enable the model to be
waxed or subjected to repeated trial fits of a restorative. Best results
have been achieved when the thermosetting resin has been a two-part epoxy
resin, but excellent results have also been achieved when the
thermosetting resin has been a two-part urethane resin. Other useful
thermoset resins are described in "Powder Coatings", Kirk-Othmer
Encyclopedia of Chemical Technology, 3d. Ed., Vol. 19, pp. 1-27 (1982),
and include polyurethanes and polyacrylics. Phenolic resins also can be
used. The thermosetting resin may be applied as a liquid or a powder at
elevated or room temperature. Preferably it forms a thin shell on one or
more working surfaces of the model, and has a thickness of less than about
2 mm. For dental models, the thermosetting resin can form an outer shell
or the whole of the replica teeth, and/or a portion of the replica
gingival tissue if desired.
The first thermoplastic resin is, as noted above, preferably cleanly
breakable. Useful cleanly breakable thermoplastic resins which have good
dimensional stability include aromatic thermoplastic resins such as
copolymers of vinyltoluene and alpha-methylstyrene, polyamides, and
polyesteramides. The ability of a panel of the resin to break cleanly can
be enhanced by adding fillers such as quartz, glass microbubbles, aluminum
powder, carbon black, titanium dioxide, or microcrystalline waxes. Clean
breakability also can be enhanced by the addition of glassy modifiers such
as rosin, rosin esters, aliphatic hydrocarbon resins, aromatic hydrocarbon
resins, polyterpenes and combinations thereof.
The first thermoplastic resin preferably hardens as rapidly as possible,
coincident with maintenance of adequate dimensional stability and other
desired physical properties. Hardening can, if desired, be accelerated by
quenching the model in a suitable cooling medium (e.g., water) while the
first thermoplastic resin hardens.
The flexible support optionally used between steps (3) and (4) of the
method is, as noted above, preferably a second thermoplastic resin that is
tough and flexible. The support also may be a fabric (woven or nonwoven)
which can be impregnated with a resin, a plastic film such as
polypropylene or oriented poly(ethylene terephthalate), an adhesive tape,
leather, or rubber. The support can be filled, e.g., with magnetizable
particles to secure the model releasably to a metal sheet such as a metal
wing of an articulating jig. The preferred tough and flexible second
thermoplastic resin can simply be poured to form a layer over the
first-mentioned thermoplastic resin, or can, as noted above, be applied as
a preformed solid strip while the first-mentioned thermoplastic resin is
molten. The preformed strip may, if desired, be formed with serrations or
knobs.
Useful second thermoplastic resins are sufficiently tough and flexible to
permit repeated (e.g., half a dozen times or more) flexing of the model
between the positions shown in FIGS. 1 and 2 without causing apparent
distortion of the model. Such resins include ethylene/vinyl acetate
copolymers, styrene-butadiene block copolymers, butyl elastomers and
polyamides, any of which may be compounded with resins, plasticizers,
extenders and fillers to provide desired physical properties such as
flexibility, adhesion, and dimensional stability.
Dental models of the invention may be used to make any prosthetic dental
restoration including inlays, onlays, veneers, crowns, and bridges. While
each of these can be made entirely from dental restorative or composite
resin, other useful restorative materials such as metals (e.g., gold),
ceramics (e.g., porcelain), and metal-ceramic combinations can be formed
on the novel models disclosed above. Pins or other repositionable locating
means (optionally coated with a suitable release agent) can be installed
in the model if desired, to facilitate removal and replacement of
individual model teeth.
In the following examples of thermoplastic and thermosetting resins and
models of the invention, all parts are by weight.
THERMOSETTING RESIN A
This two-part epoxy resin is a liquid having a gel time of 4 minutes at
21.degree. C. It contains the following ingredients:
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Parts
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Part A:
Poly(glycidyl ether) of Bisphenol A
having an epoxide equivalent weight of
about 200 ("Epon" 828, Shell)
100
Silicon dioxide, mean particle size 4.3
micrometers ("Imsil" A-25, Illinois Mineral)
20
Titanium dioxide, Sp. gravity 3.8-4.3
("Ti-Pure" R-960, E. I. duPont de Nemours)
5
Fluorocarbon surfactant ("Fluorad" FC-430,
3M) 0.5
Part B:
Polymercaptan ("Capcure" 3-800,
Diamond Shamrock) 90
Dimethylaminomethyl phenol ("DMP-30",
Rohm & Haas) 10
"Imsil" A-25 20
"Ti-Pure" R-960 5
"Fluorad" FC-430 0.5
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A slug of Thermosetting Resin A was cast into a bar
12.73.times.1.31.times.1.27 cm (5.012.times.0.516.times.0.500 inches).
Linear shrinkage of the bar in the long dimension was 0.13%.
THERMOPLASTIC RESIN B
Using a hot plate (Corning PC-35) and a band heater (Tempco Electric Heater
Corp.), 50 parts of modified hydrocarbon-based resin, acid No. 90-100
("Pexalyn" A500, Hercules) were melted at a temperature of about
171.degree. C. (340.degree. F.). While stirring with an air mixer, 150
parts of a copolymer of vinyltoluene and alpha-methylstyrene ("Piccotex"
100, Hercules) were added to the melt. When a homogeneous mixture had been
obtained, 225 parts of "Imsil" A-25 were added incrementally using
high-shear mixing, followed by incremental additions of 30 parts "Ti-Pure"
R-960. The temperature was raised to 232.degree. C. (450.degree. F.) for
about 12 minutes with continued mixing followed by removal of the resin
and casting into a slug mold suitable for use in a hot melt gun. The slugs
were identified as "Thermoplastic Resin B".
Linear shrinkage of a bar of Thermoplastic Resin B (molded in the mold used
for Thermosetting Resin A) was 0.52%. The bar was cleanly breakable when
flexed by hand.
THERMOPLASTIC RESIN C
Using the same procedure used for Thermoplastic Resin B, the following
ingredients were mixed with heating and stirring:
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Ingredient Parts
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"Piccotex" 100 150
Microcrystalline wax, m.p. 84-87.degree. C.
("Bowax" 993, Boler Chemical)
34
"Imsil" A-25 200
"Ti-Pure" R-960 10
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Linear shrinkage of a bar of the resulting Thermoplastic Resin C was 0.043
cm (0.017 in.) or 0.34%.
THERMOPLASTIC RESIN D
Using the same procedure, the following ingredients were mixed with heating
and stirring:
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Ingredient Parts
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"Piccotex" 100 370
Polyethylene glycol dibenzoate
(Benzoflex" 2-45, Velsicol)
30
"Imsil" A-25 250
"Ti-Pure" R-960 200
Carbon Black ("Sterling" R-V7688, Cabot)
3
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Linear shrinkage of a bar of the resulting Thermoplastic Resin D was 0.005
cm (0.002 in.) or 0.04%.
THERMOPLASTIC RESIN E
Using the same procedure the following ingredients were mixed with heating
and stirring:
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Ingredient Parts
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"Piccotex" 100 240
Ethylene/vinyl acetate copolymer, 18% vinyl
acetate ("Elvax" 410, E. I. du Pont
de Nemours) 88
Modified rosin, acid No. 94 ("Regalite" 355,
Hercules) 160
"Imsil" A-25 246.4
"Ti-Pure" R-960 40
Red iron oxide 1.6
Hollow glass microbubbles, avg. density
0.23 g/cm.sup.3 (3M) 24
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Linear shrinkage of a bar of the resulting Thermoplastic Resin E was 0.056
cm (0.022 in.) or 0.44%.
EXAMPLE 1
A dental model was made as illustrated in FIG. 3. The teeth were replicated
substantially entirely from thermosetting resin and the gingival tissue
was replicated substantially entirely from two thermoplastic resins. The
model was formed using a rubbery dental poly(vinyl siloxane) impression
material ("Express" Type 1, 3M) molded upon a "Typodont" model ("R862",
Columbia Dentoform) of two molar and two bicuspid teeth. One of the molar
teeth had been prepared to receive a standard MOD restoration. Using a
double-barrelled syringe equipped with a static mixer ("EPX", 3M),
Thermosetting Resin A was injected into the impression to approximately
the gingival margins. Over this, Thermoplastic Resin E was injected from a
hot melt gun ("Polygun" TC, 3M) at a melting chamber temperature of
approximately 199.degree. C. (390.degree. F.). While Thermoplastic Resin E
was still molten, a serrated strip of tough and flexible thermoplastic
resin was pressed into the molten resin. The strip was about 100 mm in
width, 5 mm in average thickness, and 40 mm in length and was made by
molding a hot melt adhesive having a Brookfield viscosity of 5,000 cps at
191.degree. C. (375.degree. F.), a tensile strength of 2.76 MPa (400 psi)
and an elongation of 750% ("Jet-Melt" 3792 hot melt adhesive, 3M) in a
mold with a serrated face. After 10 minutes, Thermoplastic Resin E had
hardened, Thermosetting Resin A had been cured by heat from the
thermoplastic resin, and the resulting model could be removed from the
dental impression. An excellent replica was obtained. Its replica tooth
areas were readily cut away with a bur. When a heated waxing spatula at a
temperature of about 260.degree. C. (500.degree. F.) was laid upon the
replica teeth for several seconds, no damage to the teeth was noticed.
The model could be used to create dental restorations such as inlays by
injecting dental restorative or composite resin into the cavity of the
replica prepared molar. To facilitate doing so, the replica gingival
tissue formed by Thermoplastic Resin E was scored at the mesial and distal
sides of the replica prepared molar. Then upon flexing the model by hand,
the replica gingival tissue broke cleanly at each score, permitting the
model to be hingedly opened in the manner shown in FIG. 2 of the drawing.
By grasping the replica prepared molar between the fingers, it and the
replica gingival tissue between the cracks were separated from the
serrated surface and lifted out to form a unit similar to unit 39 in FIG.
3, but having a replica molar formed entirely of thermosetting resin.
EXAMPLE 2
A dental model was made as described in Example 1 except that instead of
injecting a liquid thermosetting resin, a thermosetting epoxy resin powder
("Scotchkote" 203, 3M) was sprinkled into the inverted dental impression
from a plastic squeeze bottle. To facilitate application of the powder,
the impression had been preheated in an oven at 65.degree. C. The dental
impression was turned over, allowing excess powder to fall out, and
leaving a uniform layer of powder covering the tooth surfaces and adjacent
portions of the gingival surfaces.
The impression was once again inverted. Onto the layer of powder,
Thermoplastic Resin E was injected from a hot melt gun as in Example 1 to
fill both the tooth and gingival portions of the impression. A serrated
strip of tough and flexible thermoplastic resin as used in Example 1 was
pressed into the molten Thermoplastic Resin E. The model was allowed to
cool, hardening within 10 minutes, at which time the model could be
removed from the dental impression and put to immediate use to create a
dental restoration. The thermosetting epoxy resin powder had been fused
and cured by the heat of the molten Thermoplastic Resin E to provide a
thermoset shell having a uniform thickness of approximately 1 mm. Surfaces
of the model that had been covered by the thermoset shell were readily cut
away with a bur. When a heated waxing spatula touched the shell for
several seconds, no damage to the teeth was noticed.
EXAMPLE 3
A dental model was made using a rubbery dental impression as in Example 1.
Then using a double-barrelled syringe as in Example 1, a two-part
thermosetting urethane resin composition ("Dyna-Cast", Kindt-Collins) was
injected into the impression to approximately the gingival margins.
Immediately thereafter, a tough and flexible thermoplastic resin having a
Brookfield viscosity of 14,500 cps at 191.degree. C. (375.degree. F.), a
tensile strength of 3.3 MPa (475 psi), and an elongation of 600%
("Jet-Melt" 3758 adhesive, 3M) was injected from a hot melt gun into the
impression to fill the gingival portion of the impression. After cooling
for about 10 minutes, the resulting model was removed from the impression
and was ready for immediate use in making dental restorations. The model
was an excellent replica, its thermosetting resin having been cured by
heat from the thermoplastic resin. The replica teeth of thermoset urethane
resin could be readily cut away with a bur. When a heated waxing spatula
touched the replica teeth for several seconds, no damage to the teeth was
noticed.
The replica gingival tissue of the model was cut on either side of the
replica prepared molar to about half of the gingival tissue thickness of
12 mm. The uncut replica gingival tissue then served as a hinge to provide
good access to the proximal surfaces of the replica tooth and to return
the row of replica teeth approximately to the original configuration. Even
though the opposing surfaces at the cuts did not precisely mate with one
other, the resulting inaccuracy was deemed to be of only minor
significance in the formation of typical restorations.
Various modifications and alterations of this invention will be apparent to
those skilled in the art withut departing from the scope and spirit of
this invention and the latter should not be restricted to that set forth
herein for illustrative purposes.
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