U.S. patent application number 09/062086 was filed with the patent office on 2001-05-24 for method for manufacturing a dental prosthesis.
Invention is credited to RHEINBERGER, VOLKER, ZANGHELLINI, GERHARD.
Application Number | 20010001510 09/062086 |
Document ID | / |
Family ID | 21933540 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001510 |
Kind Code |
A1 |
RHEINBERGER, VOLKER ; et
al. |
May 24, 2001 |
METHOD FOR MANUFACTURING A DENTAL PROSTHESIS
Abstract
The invention relates to a method for manufacturing a fiber
reinforced composite comprising the steps of (i) preparing a mould;
(ii) filling the cavity of the mould with a fiber-reinforced
polymerizable material comprising an organic matrix and a fiber
component embedded within the matrix; (iii) applying pressure to
the fiber-reinforced polymerizable material; and (iv) curing the
fiber-reinforced polymerizable material. The method is
characterized in that the mould is designed in a way which allows
excess organic matrix material to escape form the cavity during
pressing.
Inventors: |
RHEINBERGER, VOLKER; (VADUZ,
LI) ; ZANGHELLINI, GERHARD; (SCHAAN, LI) |
Correspondence
Address: |
MICHAEL L. GOLDMAN
NIXON HARGRAVE DEVANS & DOYLE
CLINTON SQUARE
P O BOX 31051
ROCHESTER
NY
14603
US
|
Family ID: |
21933540 |
Appl. No.: |
09/062086 |
Filed: |
April 17, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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60044649 |
Apr 18, 1997 |
|
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Current U.S.
Class: |
264/17 ; 249/54;
264/101; 264/16; 264/19; 264/222; 264/257; 264/313; 264/496;
425/389; 425/405.1; 433/215 |
Current CPC
Class: |
A61C 19/003 20130101;
A61K 6/887 20200101; A61C 13/20 20130101; A61C 13/0003
20130101 |
Class at
Publication: |
264/17 ; 264/16;
264/19; 264/101; 264/222; 264/257; 264/313; 264/496; 249/54;
425/389; 425/405.1; 433/215 |
International
Class: |
A61C 013/003; A61C
013/087; A61C 013/20; A61C 013/34; A61C 013/00 |
Claims
1. A method for manufacturing a fiber reinforced composite
comprising the steps of (i) preparing a mould; (ii) filling the
cavity of the mould with a fiber-reinforced polymerizable material
comprising an organic matrix and a fiber component embedded within
the matrix; (iii) applying pressure to the fiber-reinforced
polymerizable material; and (iv) curing the fiber-reinforced
polymerizable material characterized in that the mould is designed
in a way which allows excess organic matrix material to escape form
the cavity during pressing.
2. A method according to claim 1, characterized in that the mould
is provided with one or more grooves connecting the inside of the
cavity with the outside of the mould.
3. A method according to claim 2, characterized in that the grooves
are 0.05 to 1.0 mm wide.
4. A method according to anyone of claims 1 to 3, characterized in
that the mould is provided with one or more drainages.
5. A method according to anyone of claims 1 to 4, characterized in
that the cavity is provided with a void space able to take up
excess matrix material.
6. A method according to claim 5, characterized in that the cavity
is bevelled.
7. A method according to anyone of claims 1 to 6, characterized in
that the fiber-reinforced polymerizable material comprises glass,
ceramic and/or silica fibers.
8. A method according to anyone of claims 1 to 7, characterized in
that the fiber-reinforced polymerizable material comprises 45.0 to
65.0% by weight of the fiber component.
9. A method according to anyone of claims 1 to 8, characterized in
that the fiber-reinforced polymerizable material comprises 31.1 to
48.9% by weight of the organic matrix material.
10. A method according to anyone of claims 1 to 9, characterized in
that the fiber-reinforced polymerizable material comprises a
methacrylate resin, dimethacrylate resin, dimethacrylate-based
aromatic resin, epoxy-based aromatic resin, polymethacrylate resin
and/or urethane methacrylate resin.
11. A method according to anyone of claims 1 to 10, characterized
in that the fiber-reinforced polymerizable material comprises a
mixture of Bis-GMA, decandiol dimethacrylate, triethylene-glycol
dimethacrylate and urethane dimethacrylate.
12. A method according to claim 11, characterized in that the
mixture comprises 24.5 to 38.6 % by weight Bis-GMA, 0.3 to 0.5% by
weight decandiol dimethacrylate, 6.2 to 9.7% by weight
triethyleneglycol dimethacrylate and 0.1% by weight urethane
dimethacrylate.
13. A method according to claim 12, characterized in that a
fiber-reinforced polymerizable material comprising 24.5% by weight
Bis-GMA, 0.3% by weight decandiol dimethacrylate, 6.2% by weight
triethyleneglycol dimethacrylate, 0.1% by weight urethan
dimethacrylate, 3.5% by weight high dispersed silica, <0.3% by
weight catalysts and stabilizers, <0.1% by weight pigments and
65.0% by weight glass fibers is used.
14. A method according to anyone of claims 1 to 13, characterized
in that an elastic membrane is used for applying pressure.
15. A method according to anyone of claims 1 to 14, characterized
in that a pressure of about 2 bar is applied.
16. A method according to anyone of claims 1 to 15, characterized
in that the polymerizable fiber-reinforced material is hardened by
light curing.
17. A method according to claims 1 to 16, characterized in that the
cavity of the mould is overfilled.
18. A method according to anyone of claims 1 to 17, characterized
in that the mould is a silicone mould.
19. A method according to anyone of claims 1 to 18, characterized
in that the fiber reinforced composite is a dental restoration.
20. A method for manufacturing a fiber reinforced composite
comprising the steps of (1) preparing a cast of the tooth which is
to be restored; (2) applying a covering agent to the model and the
cast to cover the cast and leaving only the tooth to be restored
uncovered; (3) placing a fiber-reinforced polymerizable material
comprising an organic matrix and a fiber component embedded onto
the uncovered tooth; (4) applying pressure to the fiber-reinforced
polymerizable material; (5) curing the fiber-reinforced
polymerizable material characterized in that the covering agent is
applied in a way such that a narrow shoulder is formed, i.e. the
shoulder follows the line of the wall of the tooth to be
restored.
21. Method according to claim 20, characterized in that a silicon
covering agent is used.
Description
[0001] The invention relates to a method for manufacturing a
fiber-reinforced composite especially a dental prosthesis such as a
crown, bridge, implant superstructure, inlay bridge or removable
dentures.
[0002] U.S. Pat. No. 4,894,012 and WO 89/04640 disclose a two-step
procedure for producing fiber-reinforced dental appliances. First,
a fiber-reinforced composite material is produced having the
requisite stiffness and strength characteristics and thereafter a
dental device is formed therefrom. The composite material comprises
essentially a polymeric matrix and a fiber component embedded
within the matrix. The materials employed are preferably fully
polymerized thermoplastic materials. Restorations such as e.g.
bridges are prepared by heating the fiber-reinforced composite
material with a heat gun until soft and then forming the material
using a dental cast. Finally, acrylic teeth are fixed thereto.
[0003] U.S. Pat. No. 5,098,304 discloses a method for reinforcing
composite resin systems for restoring or splinting teeth which
utilizes glass fiber material. Bridges are formed by first
preparing the teeth which are adjacent to the missing tooth by
grinding and then fixing a mesh or rope of fiber glass to the
teeth. Thereafter a replacement tooth is formed on the fiber glass
material.
[0004] U.S. Pat. No. 5,176,951 and WO 91/11153 disclose a method of
reinforcing a resin portion of a dental structure, which comprises
the steps or applying one or more layers of a light weight woven
fabric made up of polyaramide or polyethylene fibers to a resin
portion of a dental structure and covering the woven fabric with
more of the resin. In this method the fiber material and the resin
have to be combined by the user when preparing the dental
restoration. This is inconvenient and bears the risk of forming air
pockets which cause destabilization of the restoration.
[0005] WO 95/08300 relates to a method for manufacturing a dental
prostheses wherein a preimpregnated fabric part is placed on a
shaping model and formed on the model by compression. Then the
organic matrix of the preimpregnated fabric part is cross-linked to
obtain a rigid support shell and successive layers of organic resin
are applied onto the support shell to form an external finishing
coating. The support shell comprises between 20 to 60% by volume of
fibers and other inorganic charges.
[0006] For producing fiber-reinforced bridges it is known to first
prepare a dental cast which is partially covered with silicon to
form a mould leaving a cavity for the restoration to be made. Then
a preimpregnated fabric part is placed in the cavity, formed
according to the model by compression and hardened. This process
allows for the convenient preparation of metal free dental
protheses. However, the use of preimpregnated fabric parts with a
high fiber content requires high pressure during compressing. In
contrast, use of preimpregnated fabric parts with a low fiber
content result in restorations with a limited stability.
[0007] It is the object of the present invention to provide an
improved method for manufacturing fiber reinforced composites with
high fiber content from fabric parts or fiber material
preimpregnated with an organic matrix which process does not
require high pressure for forming the fiber reinforced
material.
[0008] This problem is solved by a method for manufacturing an
fiber reinforced composite-comprising the steps of
[0009] (i) preparing a mould;
[0010] (ii) filling the cavity of the mould with a fiber-reinforced
polymerizable material comprising an organic matrix and a fiber
component embedded within the matrix;
[0011] (iii) applying pressure to the fiber reinforced
polymerizable material; and
[0012] (iv) curing the fiber-reinforced polymerizable material.
[0013] This method is characterized in that the mould is designed
in a way which allows excess organic material to escape from the
cavity during pressing.
[0014] In a preferred embodiment the mould is provided with one or
more grooves connecting the inside of the cavity with the outside
of the mould. The grooves are cut into the mould from top to the
bottom and allow matrix monomer to flow out of the mould after
pressure has been applied. Thus, the volume fraction of fibers is
increased remarkably and the strength of the composite is
increased.
[0015] A schematic view of a mould provided with a plurality of
grooves is shown in FIG. 1. The grooves are preferably 0.05 to 1.5
mm wide, more preferably 0.05 to 1.0 mm, most preferably 0.2 to 1.0
mm.
[0016] Another way to increase the volume fraction of fibers is to
form one or more drainages. A schematic view of a mould provided
with two drainages is shown in FIG. 2. The drainages preferably
have a inner diameter of from 0.05 to 1.5 mm, more preferably 0.05
to 1.0 mm, most preferably 0.2 to 1.0 mm.
[0017] The grooves and/or drainages should be applied on both sides
of the cavity. The number of grooves and drainages depends on the
size of the cavity. Moulds for the preparation of a dental bridge
are usually provided with 2 to 4 grooves and/or drainages on each
side of the mould, preferably 1 groove or drainage every 5 mm, more
preferably 1 groove or drainage every 3 mm.
[0018] Still a further way to increase the volume fraction of
fibers is to provide the mould with void space able to take up
excess matrix material. A preferred way of providing void space is
to make a bevelled cavity as is schematically shown in FIG. 3.
[0019] To ensure a high fiber content of the fiber-reinforced
composite it is preferred to overfill the cavity of the mould with
fiber-reinforced polymerizable material as is indicated in FIG. 3.
During pressing excess matrix monomer flows out of the cavity via
the grooves or drainages or is collected in the void space whereas
the fiber material remains in the mould. It is preferred to
overfill the cavity of the mould by 1 to 10% by volume, more
preferably 5 to 15%.
[0020] For pressure application it is preferred to use an elastic
membrane as disclosed in WO 95/08300. Although pressure may be
applied by hand it is preferred to use an automated process and a
machine as described in WO 95/08300.
[0021] A suitable machine is schematicly shown in FIG. 4. This
machine comprises a sealed enclosure 14, a plate 15 receiving a
shaping model 11 in the enclosure 14, a flexible fluid-proof
membrane 9, notably air-tight, separating the enclosure 14 into two
chambers 14a, 14b, means 16, 25, 26 for creating a lower fluid
pressure in the chamber 14b, and means 17, 18, 19, 32 for
cross-linking the parts 7 placed on the shaping model 11 in the
chamber 14a. According to the invention, the cross-linking means
17, 18, 19 are preferably light-curing means comprising at least
one light source 17 located in the chamber 14b opposite the one 14a
containing the shaping model 11. The flexible separating membrane 9
is then translucent or transparent, i.e. it lets light pass. The
cross-linking means 17, 18, 19 comprise at least one light
conveying duct 18 giving out onto the receiving plate 15 to light
from the inside the shaping model 11 itself made of translucent or
transparent material. In this way, lighting from the inside is
achieved and the efficiency of the cross-linking is improved. In
addition, the cross-linking means 17, 18, 19 can comprise a
peripheral mirror 19 surrounding the shaping model 11 to improve
the light diffusion. Instead of, or in combination with the
cross-linking means 17, 18, 19 the machine of the invention can
comprise chemical and/or cross-linking means 32.
[0022] The enclosure 14 is formed by the lower plate 15 receiving
the shaping model 11, a similar parallel upper plate 20 forming a
cover, and a cylinder 21 placed between these tow plates 15, 20 in
a fluid-proof manner. The cylinder 21 can be transparent in order
to visually monitor the execution of the manufacturing process. The
upper plate 20 supports a plurality of small columns 22 with
compression springs 23 located at regular intervals on its
circumference and designed to press against the peripheral edge of
the membrane 9 to wedge it against a cylindrical protuberance 24 of
the lower plate 15. The small columns 22, springs 23 and
protuberance 24 thus form removable securing means of the membrane
9 separating the two chambers 14a, 14b. The membrane can thus
easily be changed as required each time the machine is
disassembled, i.e. each time manufacture of a prosthesis is
prepared. The lower plate 15 is rigidly associated, in a tight but
disassembled manner, to the cylinder 21 in order to enable changing
of the membrane 9 and/or preparation of the shaping model 11 and of
the parts 7 to be polymerized. The light source 17 can be simply
formed by an electrical light bulb. The pressure difference between
the two chambers 14a, 14b can be achieved by inlet of a compressed
fluid such as air or a liquid into the chamber 14b via the orifice
and/or by suction of a fluid form the chamber 14a containing the
shaping model 11 via a suction orifice 26. For example, the suction
orifice 26 and the inlet orifice 25 can be connected to one another
by means of fluid pump 16. Due to the effect of the pressure
difference-thus achieved, the flexible membrane 9 is pressed
against the shaping model 11 and thus presses the pre-impregnated
fabric part 7 against this shaping model 11. The lighting means 17,
18 are then switched on causing photo-polymerisation of the organic
matrix of the preimpregnated fabric part 7 and formation of the
support shell 2. The membrane typically is formed by an elastic
synthetic material such as a copolymer or rubber.
[0023] The method of the present invention allows the manufacturing
and forming of fiber reinforced composites having a final fiber
content of up to 60% by volume by use of a pressure of not more
than 1.5 to 2.5 bar, preferably about 2 bar.
[0024] The fiber-reinforced composite may be further processed by
application of one or more layers of an organic resin as disclosed
in WO 95/08300, i.e. applying at least one layer of an organic
resin to the composite and cross-linking the same.
[0025] The preparation of the mould is well known in the art (see
for example K. H. Korber, Dentalspiegel Labor 3/96; J. Langner,
Quintessenz Zahntechnik 23, 5, 1997, pages 631-646). Preferably a
silicon material such as condensation or addition silicon is used
for forming the mould.
[0026] The method of the present invention is especially suitable
for producing fiber-reinforced composites such as dental
prostheses, such as crowns, bridges, inlay bridges, implanted
prostheses, implant superstructures, removable appliances,
removable dentures or structural components of dental restorations
such as a support shell.
[0027] A mould for, e.g. preparing a bridge or a structural
component of a bridge is preferably prepared by
[0028] (a) first preparing a cast of the tooth or teeth which is to
be restored;
[0029] (b) forming a wax model of the dental restoration on the
dental cast;
[0030] (c) applying a covering agent to the model and the cast to
form the mould;
[0031] (d) removing the wax model from the mould to leave a cavity
for the restoration.
[0032] This process is comparable to the "lost-wax-technique" and
well-known to the expert in the field.
[0033] The fiber-reinforced polymerizable material used for
manufacturing the fiber-reinforced composites comprises an organic
matrix and a fiber component embedded within the matrix.
[0034] For manufacturing dental restorations the fiber component is
preferably a uniform mesh, a random mesh, or a rope or threat type
material. The fibers may also take the form of long continuous
filaments or may be woven in a leno weave as disclosed in U.S. Pat.
No. 5,176,951. Most preferably a fiber-meshed fabric is used. The
fibers are preferably made from glass, ceramic, silica or organic
materials such as aramid, polyethylene, carbon and boron. Fibers of
ceramic, silica and especially glass are most preferred.
[0035] The fiber content of the fiber reinforced polymerizable
material is preferably in the range of 7 to 94% by weight, more
preferably 28 to 82% by weight and most preferably 45 to 65% by
weight.
[0036] Preferred organic matrix monomers are methacrylate resins,
especially dimethacrylate resins such as dimethacrylate-based
aromatic resins, epoxy-based aromatic resins, polymethacrylate
resins and mixtures thereof. The matrix can also comprise urethane
methacrylate resins. Among the aromatic dimethacrylate resins,
bisphenol-A-derivatives such as bisphenol-A-glycidyl-dimethacrylate
(bis-GMA), urethane-methacrylate (UDMA), triethylene glycol
dimethacrylate (TEDMA) and mixtures thereof are preferred.
[0037] According to the invention, a bis-GMA-base resin can be used
modified by copolymerization with composites of lower molecular
weight, notably as non-restrictive examples bisphenol
glycidyl-dimethacrylates (BIS-MA), bisphenol ethyl-methacrylates
(BIS-EMA), bisphenol propyl-methacrylates (BIS-PMA), ethylene
glycol-dimethacrylates (EGDMA), diethylene glycol-dimethacrylates
(DEGDMA), triethylene glycol-dimethacrylates (TEGDMA), triethylene
glycol-methacrylates (TEGMA), methyl-methacrylates (MMA), and
polyurethane fluor-methacrylates (PFUMA).
[0038] The fiber-reinforced polymerizable material preferably
comprises 31.1 to 48.9% by weight of the organic matrix material.
Preferably a mixture of Bis-GMA, decandiol dimethacrylate,
triethyleneglycol dimethacrylate and urethane dimethacrylate is
used, more preferably a mixture comprising 24.5 to 38.6% by weight
Bis-GMA, 0.3 to 0.5% by weight decandiol dimethacrylate, 6.2 to
9.7% by weight triethyleneglycol dimethacrylate and 0.1% by weight
urethane dimethacrylate. If not indicated otherwise all percentages
refer to the total weight of the fiber-reinforced polymerizable
material.
[0039] The fiber reinforced polymerizable material may additionally
comprise fillers and additives.
[0040] Suitable fillers are silica-base particles whose diameter
can vary from 0.1 to 100 .mu.m, for example pyrolytic silica,
and/or glass or ceramic-base particles, notably glass or
borosilicate particles, ceramic glasses, barium-aluminium particles
and/or strontium-aluminium particles. Also, radio-opaque heavy
metals can be incorporated, such as niobium, tin and/or titanium,
or organic or mineral pigments. Preferred fillers are highly
dispersed silica and glass or ceramic fillers with a medium
particle size of .ltoreq.1.5 .mu.m.
[0041] The additional fillers are preferably used in an amount of 1
to 30 wt. %, more preferably 2 to 15 wt. % and most preferably 3.5
to 5.5% by weight. It is especially preferred to use 3.5 to 5.5% by
weight of highly dispersed silica as addition filler. Others
additives such as pigments are typically used in an amount of less
than 0.1% by weight.
[0042] The inorganic particles and fibers are treated before being
incorporated in the organic matrix by means of organo-silano
compounds such as aryloxy-silanes and/or halosilanes such as
(meth)acryloyl-alkoxy-silanes.
[0043] The fiber reinforced polymerizable materials contain at
least one polymerization initiator and optionally an accelerator
and/or stabilizers. The materials can be hardened by heat, light or
microwave curing.
[0044] The known peroxides such as dibenzoylperoxide,
dilauroylperoxide, tert-butylperoctoate or tert-butylperbenzoate
can be used as initiators for hot polymerization.
2,2'-azobisisobutyronitril (AIBN), benzpinacol and
2,2'-dialkylbenzpinacols are also suitable.
[0045] In a preferred embodiment the fiber reinforced polymerizable
material contains a photoinitiator such as diketones, preferably
diacetyl and/or quinones such as camphor quinone and acenaphthene
quinone. The photoinitiators may also be combined with an
accelerator such as an amine.
[0046] The concentration of initiators and accelerators preferably
lies in the range of 0.01 to 3.0 wt. %, particularly preferably in
the range from 0.05 to 1.0 wt. %, relative to the quantity of
monomers used in the dental material.
[0047] The total amount of catalysts and stabilizers is typically
in the range of 0.3 to 0.5% by weight, based on the total
composition.
[0048] For producing the fiber-reinforced composites the
fiber-reinforced polymerizable materials are preferably used in
form of a fabric part preimpregnated with an organic matrix. The
fiber-reinforced polymerizable materials may be applied in
successive layers which can be cured before applying the next
layer. Fiber-reinforced polymerizable materials having different
fiber contents may be combined for producing one fiber-reinforced
composite. For preparing a dental bridge or a structural component
of a bridge it is preferred to combine one or more preimpregnated
fabric parts in form of a flat sheet or a disc and a joining
element such as a bar like element.
[0049] Generally, the fiber reinforced polymerizable material can
be of any shape such as a flat sheet, a disc, a bar, or a wire.
Before placing the fiber-reinforced polymerizable material in the
cavity of the mould the material may be cut according to
necessity.
[0050] Table 1 shows the composition of preferred fiber-reinforced
polymerizable materials according to the present invention,
composition No. 2 being especially preferred.
[0051] The external finishing coating can be formed by a filled
cosmetic resin, notably of the type formed by
bis-phenol-A-derivatives such as bis-GMA and the other resins
mentioned above, charged in such a way that it has a high rigidity,
a great resistance to abrasion and a colour shade close to that of
the natural tooth. Charged cosmetic resins of this kind are known
as such.
1TABLE 1 Compositions of most preferred fiber-reinforced
polymerizable materials Com- Com- Com- position No. 1 position No.
2 position No. 3 Composition (% by weight) (% by weight) (% by
weight) Bis-GMA 38.6 24.5 35.2 Decandiol 0.5 0.3 0.4 dimethacrylate
Triethyleneglycol di- 9.7 6.2 8.8 methacrylate Urethane 0.1 0.1 0.1
dimethacrylate High dispersed 5.5 35 5.0 silica Catalysts and
<0.5 <0.3 <0.4 Stabilizers Pigments <0.1 <0.1
<0.1 Glass fibers 45.0 65.0 50.0
[0052] It has been found that the fiber content of a
fiber-reinforced composite could be increased for instance from
43.3 vol. % to 47.7 vol. % if the material is compressed in a mould
according to the present invention using a pressure of about 2 bar
(Table 2). This is an increase of the fiber content more than 10%.
The increase of fiber content resulted in an increase of flexible
strength and modulus of elasticity of about 15%.
[0053] The fiber content of the fiber-reinforced composites
comprising inorganic fibers is determined via loss of ignition
(LOI). The organic matrix material of the fiber-reinforced
composite is burned at 850.degree. C. for 1.5 hours and the
inorganic reminder (ash or loss of ignition, LOI) determined
gravimetrically. The relation between LOI and the volume fraction
of fibers for Compositions No. 1, No. 2 and No. 3 is shown in FIG.
5. As can be seen LOI and volume fraction of fibers are linearly
correlated. By linar regression analysis the following equation can
be derived from FIG. 5:
vol. %=1.064.times.LOI-23.4
[0054] The fiber content of fiber-reinforced composites comprising
organic fibers can be determined by scanning electron
mircroscopy.
[0055] It was further found that the modulus of elasticity and the
flexural strength are linearly correlated to the volume fraction of
fibers in percent. Thus, fiber-reinforced composites having the
desired physical properties can be produced by adjusting the volume
fraction of fibers to a suitable value. FIGS. 6 and 7 show the
relationship between flexural strength and modulus of elasticity,
respectively, and the volume fraction of fibers for the preferred
material No. 1, and FIGS. 8 and 9 for the preferred material No.
2.
[0056] The fiber-reinforced composite obtained after the first
curing step may be further improved by implying additional layers
of fiber-reinforced polymerizable material. For this purpose the
mould is preferably cut back to lay bare the dental cast and to
form a die. Further layers of fiber-reinforced polymerizable
material are placed on the die as shown schematically in FIG. 10.
FIG. 10 shows a die placed in a machine as shown in FIG. 4. During
pressure application the membrane presses the fiber-reinforced
polymerizable material on the die. Hardening is achieved by
photopolymerisation.
[0057] It was found that the fiber content of the additional layers
is influenced by the form of the die. To increase the strength of
the fiber-reinforced composite it is preferable to make a narrow
shoulder as is schematicly shown in FIG. 11, i.e. the shoulder
follows the line of the wall of the tooth to be restored and steps
are to be avoided. The term "narrow shoulder" refers to shoulders
with steps preferably having an edge length of 0 to 1.0 mm, more
preferably 0 to 0.5 mm.
[0058] For manufacturing e.g. a crown it is usually not necessary
to prepare a mould. In this case the fiber-reinforced composite is
prepared by
[0059] (1) first preparing a cast of the tooth which is to be
restored;
[0060] (2) applying a covering agent to the model and the cast to
cover the cast and leaving only the tooth to be restored
uncovered;
[0061] (3) placing a fiber-reinforced polymerizable material
comprising an organic matrix and a fiber component embedded onto
the uncovered tooth;
[0062] (4) applying pressure to the fiber-reinforced polymerizable
material;
[0063] (5) curing the fiber-reinforced polymerizable material.
[0064] The shoulder of the die is preferably formed as discussed
above.
[0065] FIG. 12 shows a picture of a electron microphotograph of
fiber-reinforced composite prepared by use of a narrow shoulder and
FIG. 13 of a fiber-reinforced composite prepared by use of a wide
shoulder. The volume fraction of fibers was measured at three
different sections of the fiber-reinforced composite by determining
the LOI of different sections of the composite. In the composite
produced by use of a narrow shoulder the fiber content ranges from
30 to 41% whereas in case of the wide shoulder a fiber content from
23 to 41% was found. The strength of the fiber-reinforced
composites can be determined by use of the graphs of FIGS. 6 to
9.
[0066] In the following the present invention will be further
illustrated by use examples.
EXAMPLE 1
[0067] A silicon mould with a cavity of 3.times.3.times.36 mm was
made. The cavity was filed with material No. 2 (see Table 1 above)
and covered with an elastic membrane. The membrane was pressed onto
the mould with a pressure of approximately 2 bar in a machine as
shown in FIG. 4 (VECTRIS.RTM. VS1, Ivoclar). 2 minutes after
pressure application the light source was switched on and the
material cured within 7 minutes. In a first test series the cavity
of the mould was underfilled, in a second series over-filled. This
procedure was repeated with moulds provided with a 3 grooves having
a width of 1 mm or 2 drainages having a inner diameter of 1 mm on
each side. In a further test bevelled and non-bevelled moulds were
used. The flexural strength and the modulus of elasticity of the
bodies prepared was tested according to ISO 10477. The results are
shown in Table 2.
[0068] Table 2 shows that moulds with grooves and drainage tubes
generally result in a higher fiber content. The highest strength
and fiber volume was achieved with an overfilled mould with
grooves. Bevelled moulds also resulted in an increase of fiber
content and strength.
2TABLE 2 Fiber content and mechanical properties of fiber
reinforced composites mould mould with without mould with drainage
drainage grooves tubes underfilled ash 61.2% 61.8% 61.5% not Vol.
fraction in % 41.7% v 42.3% v 42.2% v bevelled flexural strength
1105 MPa 1129 MPa 1125 MPa modulus of elast. 37140 MPa 37936 MPa
37804 MPa overfilled ash 62.7% 66.8% 65.1% not Vol. fraction in %
43.3% v 47.7% v 45.9% v bevelled flexural strength 1170 MPa 1347
MPa 1275 MPa modulus of elast. 39263 MPa 45101 MPa 42713 MPa
underfilled ash 61.6% 63.5% 63.8% bevelled Vol. fraction in % 42.1%
v 44.1% v 44.5% v flexural strength 1121 MPa 1202 MPa 1218 MPa
modulus of elast. 37671 MPa 40325 MPa 40855 MPa overfilled ash
60.6% 64.9% 62.8% bevelled Vol. fraction in % 40.8% v 45.4% v 43.2%
v flexural strength 1068 MPa 1254 MPa 1165 MPa modulus of elast.
35946 MPa 42048 MPa 39130 MPa
EXAMPLE 2
[0069] A dental cast was made from a tooth prepared for receiving a
crown. The cast was covered with condensation silicon mass
(Optosil.RTM., Bayer) in a way that only the tooth stump to be
restored remained uncovered. In the first test a narrow silicon
shoulder was prepared and in the second test a wide shoulder. A
disc shaped preimpregnated fabric part (Table 1, No. 1) was placed
on the stump and shaped on the model by compression with a flexible
membrane as shown in FIG. 10. The preimpregnated fabric part was
light cured as described in Example 1. Then the fiber content at
three different sections of the fiber-reinforced composite was
determined by cutting the composite into pieces and measuring the
LOI (% ash). The strength at the three sections was estimated using
the graphs of FIGS. 7 and 8. The results are shown in Table 3 and
FIGS. 12 and 13.
3TABLE 3 Fiber content and mechanical properties in different
sections of fiber reinforced coposites Narrow wide silicone
shoulder silicone shoulder occlusal % ash (LOI) 60.9% 60.6% section
Vol. fraction in % 41.4% v 41.1% v flexural strength 892 MPa 883
MPa modulus of elasticity 29094 MPa 28762 MPa middle ash 56.5% 44%
section Vol. fraction in % 36.7% v 23.4% v flexural strength 757
MPa 376 MPa modulus of elasticity 23887 MPa 9153 MPa gingival ash
50.5% 43.5% section Vol. fraction in % 30.3% v 22.9% v flexural
strength 574 MPa 361 MPa modulus of elasticity 16797 MPa 8599
MPa
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