Note: Descriptions are shown in the official language in which they were submitted.
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TOOTH RESTORATION USING FIBRE-REINFORCED COMPOSITE MATERIAL
FIELD OF THE INVENTION
The present invention relates to a tooth restoration system for restoring a
tooth
which has undergone root canal therapy.
BACKGROUND OF THE INVENTION
A human tooth comprises a crown and roots. The crown is the part of the
tooth visible above the gumline while the roots are under the gums. The roots
anchor the
teeth to either the maxillary or mandibular bones. Within the roots are root
canals which
contain pulp. Pulp is live tissue which consists of nerves and blood vessels.
The pulp may become infected and inflamed as a result of dental caries or
physical trauma to the tooth. Endodontics is a branch of dentistry which
treats teeth with
diseased pulps. Endodontics is commonly referred to as root canal therapy. The
goal of root
canal therapy is to save the tooth by removing the diseased pulp. The root
canal is then
cleaned and filled. A crown or cap is typically placed on the tooth to protect
the tooth and
preserve its functionality.
In the prior art, porcelain fused metal ("PFM") systems are commonly used. In
a PFM system, a post and core is fashioned from one of a variety of suitable
precious and
non-precious metals. The post is made to snugly fit the root canal void and
serves to anchor
the core and crown. The core is built up onto the post and forms the support
for the crown
which replaces the injured tooth. The crown is fashioned from porcelain
(ceramic) material.
PFM systems enjoy excellent strength due to the metal post and excellent wear
resistance due to the porcelain. However, a major drawback to PFM systems is
the aesthetic
CA 02234137 1998-04-03
appearance of the completed restoration. The enamel and dentin of a normal
human tooth
allows light to pass through which determines the natural look of a tooth. The
metal post and
core used in a PFM system reflects light passing through the artificial crown
which results in
the restoration looking very unnatural.
A solution to this drawback of PFM systems is to replace the metal post with a
ceramic material which simulates the translucency of normal teeth. An example
is the
CosmoPostTM system available from Ivoclar Vi.vadent, Ivoclar North America
Inc. The post
in this system is a zirconium oxide based ceramic, resulting in an
aesthetically pleasing
restoration. However, this system suffers from certain drawbacks. The posts
are only
available in preformed posts of a certain size and diameter. As a result, the
dentist must
perform the root canal therapy such that the root canal is the exact fit of
the pre-formed post
and to do so requires the use of proprietary root canal reamers.
Another solution to the aesthetic. drawback of PFM systems is to use a carbon
fibre based material such as the ComposipostTM system available from Biodent
of Quebec,
Canada. This prior art system shares the same drawback as the CosmoPostTM
system in that
the posts are only available in preformed sizes and diameters. Again, special
proprietary drills
must be used in the canal preparation.
Posts are subject to high stresses once the restoration is complete and
therefore
the ceramic material used in the CosmoPostTM system and similar systems must
be very
strong. However, that strength prevents flexibility in fashioning the post.
Other ceramic
materials such as commonly available polymer/ceramic composites do not possess
enough
strength to be suitable material for root canal posts.
Therefore, it would be advantageous to have a tooth restoration system which
combines the aesthetic qualities of an all-ceramic root canal restoration
system with the
strength of a conventional PFM system. It would be further advantageous for
such a system
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to allow the convenient fabrication of a post which molds itself to the root
canal and yet still
possesses high structural strength.
SUMMARY OF THE INVENTION
In general terms, the invention in one aspect comprises a tooth restoration
system for insertion into a tooth upon which root canal therapy has taken
place, said system
comprising a post, a core and a crown wherein:
(a) the post is formed from a fibre-reinforced composite material comprising a
polymeric matrix and a reinforcing fibre component embedded within the
matrix, said post having an upper end and a lower end, said lower end molded
to the root canal;
(b) the core is bonded to the upper end of the post and is formed from a
ceramic
material or a polymer/ceramic composite material; and
(c) the crown is bonded to and partially surrounds the core and is formed from
a
ceramic material or a polymer/ceramic composite material.
Preferably, the fibre-reinforced composite material is uncured or partially
cured
as it is being formed into the post and is completely cured subsequent to
being molded to the
root canal.
In another aspect of the invention, the invention comprises a method of
preparing a dental implant comprising a post, a core and a crown for restoring
a tooth upon
which root canal therapy has taken place, the method comprising the steps of:
(a) creating a post from a fibre-reinforced composite material comprising a
polymeric matrix and a reinforcing fibre component embedded within the
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matrix, having an upper end and. a lower end, said lower end molded to the
root canal;
(b) building up a core onto the upper end of the post from a ceramic or
polymer/ceramic composite material; and
(c) forming a crown around the core from a ceramic or polymer/ceramic
composite
material and shaping the crown to simulate the lost tooth.
Preferably, the fibre-reinforced composite material is uncured or partially
cured
as it is being formed into the post and is completely cured subsequent to
being molded to the
root canal.
A development in the field of passive or non-force imparting dental appliances
provides the preferred material for the post of the present invention. In US
Patent No.
4,894,012 there is described a fibre-reinforced composite material with the
appropriate
properties for use in this invention.
DETAILED DESCRIPTION OF THE INVENTION
There are several aspects to the successful production of an effective
composite
material used for the post of the present invention. These include:
(1) the effective wetting of the fibres by the matrix and the associated
coupling between the fiber and matrix components;
(2) the provision for an even and uniform distribution of the fibres
throughout the matrix material;
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(3) the proper fibre orientation within the matrix for the specific
characteristics and properties desired;
(4) the elimination of voids or air pockets within the composite material;
and
(5) the proper selection of the appropriate fiber and matrix materials.
Suitable composite materials are commercially available. Jeneric~/Pentron~
Incorporated of Wallingford, CT, USA manufactures and sells FibreKorTM in
convenient
ready-to-use sizes. Similarly, Ivoclar North America manufactures and sells
VectrisTM. Both
FibreKorTM and VectrisTM are preferred composite materials for fabricating the
post of the
present invention. They exhibit a high modulus of elasticity and high
strength, matching that
of non-precious alloys. They also exhibit translucency which closely matches
that of human
dental tissue, which is preferred for the present: invention.
The composite material utilized in accordance with the present invention is
composed of two essential components, a polymeric matrix and fibres embedded
within the
matrix. The fibers preferably take the form of long continuous filaments,
although these
filaments may be as short as 3 to 4 millimeters. Alternatively, shorter fibers
of uniform or
random lengths might also be employed.
Although a variety of fibers may be employed, the most commonly used fibers
are glass, carbon and/or graphite, and polyaramid fibers such as the fibers
sold under the trade
name "Kevlar". Other materials such as polyesters, polyamides and other
natural and
synthetic materials compatible with the particular matrix also may be employed
to provide
selected properties.
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The continuous filaments or fibers used in accordance with the present
invention will vary in fiber diameter or denier as well as in fiber length,
and it is preferred to
utilize a range of fiber diameters. Where synthetic materials are employed,
the diameters may
vary from about 1.5 to 15 denier while for inorganic materials such as glass
the fibers are
usually very fine, with diameters falling in the low micrometer to
submicrometer range. A
typical range for glass fibers is about 0.3 to 25 micrometers with the
preferred range being
about 3 to 12 micrometers. Carbon and graphite fibers are typically near the
low end of the
range for glass and preferably exhibit diameters of about 3 to 12 micrometers.
Those fibers
may have an irregular cross section or may be circular or "dog-bone" in
configuration.
In accordance with the present invention, it is preferred that a predominant
number of fibers be aligned axially. The orientation results in some degree
from the
production techniques used to form the resultant product, but also is
specifically designed into
these devices. These techniques include molding, such as compression molding,
but the
preferred technique is a form of extrusion known as pultrusion. In the
pultrusion process, a
sizing or coupling agent is applied to the continuous filaments to improve the
wetting thereof
by the polymeric matrix and enhance matrix fiber bond. The treated fibers are
aligned and
maintained in position as they are pulled through a bath of matrix polymer.
The fibers are
maintained under tension while the matrix material, which is in a near liquid
state, ultimately
engages and effectively wets the fibers and results in more effective coupling
and hence
improved mechanical properties. Physically holding the fibers in position
helps to assure even
and uniform distribution of the fibers in the final composite. As the fibers
and matrix leave
the polymer bath, the composite may pass through a series of rollers or dies
to develop a
uniform exterior or outside dimension and assure that the fibers do not
protrude through the
outside matrix surface.
The continuous fibers are usually disposed in a parallel array relative to
each
other and are aligned along one dimension such as the major dimension of the
device being
produced. The continuous filament composite material is capable of providing a
material
having a modulus of elasticity beyond the range available with polymeric
materials used
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heretofore. For example, a continuous filament material can be formulated to
provide a
composite that exhibits a modulus in the range of 1.01 to 60x106 psi and
greater. With glass
or synthetic materials the modulus may be up t:o about 35x106 psi while with
carbon fibers the
modulus may fall within a range up to 40-50x106 psi.
The polymeric materials employed as the matrix for the reinforcing fibers
preferably are fully polymerized thermoplastic materials although a wide
variety of polymeric
materials may be employed, including partially polymerized thermosetting
materials. The
thermoplastics allow ease of formability and the stiffness, strength,
springback and creep
resistance preferable for this invention. For example, the polymeric material
may include
polyamides such as nylon, polyesters, glycol esters such as polyethylene
terephthalate glycol,
polyolefins such as polypropylene or polyethylene, polyimides, polyarylates,
polyurethanes,
styrene, styrene acrylonitrils, ABS, polysufones, polyacetals, polycarbonates,
polyphenylene
sulfides, or a wide variety of other polymeric compositions including
vinylesters and epoxy
type materials. Among this group, the thermoplastic materials are preferred
since they are not
as brittle, exhibit greater toughness and more readily facilitate the
formation of the post.
The method of the present invention begins after root canal therapy has
commenced and a mold is made of the root canal of the subject tooth and the
neighbouring
teeth in a conventional manner. The practitioner of this method commences by
determining
the depth of the root canal and cutting a bundle of the fibre-reinforced
composite material to a
length which will fill the void and protrude approximately 2 to 3 mm. The
bundle is
preferably created from flat pre-formed strips of the material and is used
such that the
orientation of the fibres runs up and down, substantially parallel to the root
canal. The bundle
should be formed to a diameter which snugly fits within the void to ensure a
proper fit in the
tooth restoration. The bundle should be tamped down slightly within the root
canal so as to
better mold it to the root canal. At all time, the material is not handled by
hand so as to
avoid contamination, in accordance with the manufacturer's instructions.
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It is essential that the bundle be uncured or only partially cured when it is
first
formed. Because the bundle is flexible before curing, the bundle will mold
itself to the shape
of the root canal to ensure a proper fit. Once a proper bundle is created, the
bundle may then
be cured to complete polymerization of the matrix material. It is preferable
to use light-
s curing material although other curing methods are available and may be
suitably adapted to
the present method.
The core and the crown are then built up onto the post using conventional and
well-known techniques and materials. Preferred materials for the core and
crown include
polymer/ceramic composite materials such as SculptureTM available from
Jeneric~/Pentron~
Incorporated or TargisTM available from Ivoclar North America. The completed
post/core/crown combination may then be installed in the root canal using
conventional and
well known techniques and adhesives.
As will be apparent to those skilled in the art, various modifications,
adaptations and variations of the foregoing specific disclosure can be made
without departing
from the teachings of the present invention.
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