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BACKGROUND OF THE INVENTION
It has previously been known that a post may be placed in an endodontically
treated tooth to retain a superstructure that replaces missing coronal
tooth structure. Thus, the retention of the crown is greatly enhanced.
Posts have been employed in refabrication for several centuries. Over the
years, posts of varying configuration have been cemented with various
cements and with varying degrees of success and acceptance. Presently the
two most popular cements are zinc oxyphosphate cement, which has been used
since 1900, and polycarboxylate cement introduced in 1968.
Placement of an artificial crown requires that the remaining structure of
the tooth be properly prepared by instrumentation after the root canal
space has been cleaned, shaped and filled. Generally, an interior space is
machined into the root canal. Irrigation usually accompanies
instrumentation and serves to cool and lubricate the cutting tool while
flushing the removed material away from the machined site. Once the root
canal is prepared, a post of appropriate size and shape is cemented into
the canal. After the cement has sufficiently hardened, a mass around the
top of the post is built-up of various substances, to form a core.
Generally, preformed crowns are available and have a hollow area to accept
such a build-up mass. These are placed as a temporary cover until a final
crown has been made. This final crown is placed upon the prepared tooth
structure and bonded to the tooth and core using various cements.
In the past few years, there has been considerable discussion about the
inability of chemomechanical procedures to thoroughly remove all the
cellular debris within the root canal space. Silicone models are used to
demonstrate the great irregularity and complexity of the root canal space.
They illustrate that even mechanically well-prepared canals harbored areas
that were never contacted by endodontic instruments. Scanning electron
microscope (SEM) studies of the effects of mechanical preparation revealed
that, regardless of the technique used, often pulp tissue remained and
areas of the canal walls were not instrumented. See Mizrahi et al., "A
scanning electron microscopic study of the efficacy of various endodontic
instruments", J. Endod.1(10): 324-33, (1975); Moodnick et al., "Efficacy
of Biomechanical instrumentation: a scanning electron microscopic study,"
J. Endod. 2(9): 261-66, (1976); Bolanos and Jensen, "Scanning electron
microscope comparisons of the efficacy of various methods of root canal
preparation," J. Endod. 6(11): 815-22, (1980). In addition, other studies
using the SEM found that many of the commonly-used irrigating solutions
were also ineffective in completely removing hard and soft tissue debris,
especially in the apical portion of the canal. See Baker et al., "Scanning
electron microscopic study of the efficacy of various irrigating
solutions," J. Endod. 1(4): 127-35, (1975); McComb and Smith, "A
preliminary scanning electron miscroscopic study of root canals after
endodontic procedures," J. Endod. 1(7): 238-42, (1975); Rubin et al., "The
effect of istrumentation and flushing of freshly extracted teeth in
endodontic therapy: a scanning electron miscroscopic study," J. Endod.
5(11): 328-35, (1979). These results show that the currently-accepted
methods of chemomechanical preparation were inadequate in preparing a
debris-free canal.
Thus, emphasis has been placed on improving the manufacture of endodontic
instruments and developing more effective irrigation techniques and
endodontic materials.
Since instrumentation is not entirely effective in cleaning the entire
canal, the solutions used should help remove pulp tissue remnants,
necrotic debris and bacteria remaining in the prepared root canal space
without irritating the periapical tissue.
Investigators have also described a sludge or smeared layer that exists on
portions of the canal walls. This appears as an amorphous layer on the
canal wall that obstructs the dentinal tubules. It was recently
demonstrated that the smeared layer is primarily calcific in nature and is
created by instrumentation. See Goldman et al., "The efficacy of several
Endodontic irrigation solutions: a scanning electron microscopic study, "
Oral Surg. 52(2): 199-204, (1981).
The smeared layer blocks the dentinal tubules. A recent study has shown
that this calcific layer reduced the permeability of dentin in vitro by
more than forty percent. See Dippel et al., "Influence of the smeared
layer and intermediary base materials on the permeability of dentin," J.
Dent. Res. 60(B):1211, (1981).
Therefore, even well-instrumented canals could contain organic debris such
as pulp tissue as well as inorganic debris such as a smeared layer. Recent
investigations raised the question of removing both layers. See Wayman et
al., supra and Koskinen et al., "Appearance of the chemically treated root
canal walls in the scanning electron microscope," Scand. J. Dent. Res.
88(5):397, (1980). However, those attempts were unsuccessful.
Similarly, it has been found that the smeared layer is present in a cavity
and prevents an optimum bonding of the filling placed in the cavity. This
permits leakage and subsequent entry of food particles and bacteria, which
cause further deterioration of the tooth structure.
SUMMARY OF THE INVENTION
It has now been found that instrumented canals can be cleaned more
effectively by using one solution to remove organic debris and another to
remove inorganic debris. When the smeared layer is removed and the
dentinal tubules are exposed, a cementing medium which has great
compressive strength can flow into the open tubules, around the post and
can provide a greatly enhanced tensile strength and is therefore a
significant improvement over previous methods.
The present invention presents a novel solution to the aforesaid problems
by providing a method of placing a crown replacement on a tooth, placing a
filling in a cavity cementing of crowns and caps on natural teeth, and
filling a root canal with unfilled resin and gutta percha, which method
provides enhanced foundation strength and security. The machined or
prepared area is washed first with a chelating agent and then flushed with
an organic tissue solvent to remove debris and expose the dentinal
tubules. Then, in a suitable situation, the post and cap are locked in
place with an appropriate cementing medium. The present invention
overcomes the disadvantages of the previous methods by providing a method
of dentinal preparation and placement that significantly improves
retention of coronal structures, as well as promoting retention in other
similar situations.
The invention herein comprises methods and products for attaching a crown
replacement on an endodontically treated tooth. The superior surface of
the tooth is shaped to receive the base of the crown replacement and an
interior space is machined into the root canal to accept placement of a
post. The instrumented area is then flushed with a chelating agent
followed by an organic solvent. A cementing medium having great
compressive strength is placed around a post fitted into the cavity. After
the cement has set, a quantity of filler material is placed around the
remaining exposed part of the post. This post forms the support for the
crown replacement. If desired, a preformed crown replacement may be placed
over the cemented structure until a permanent crown can be made.
DESCRIPTION OF THE DRAWINGS
This invention can be more clearly understood by referring to the
accompanying drawings where in:
FIG. 1 is a sectional view of a damaged pre-instrumented tooth;
FIG. 2 is a sectional view of an instrumented tooth and prepared crown
replacement; and
FIG. 3 is a sectional view of an instrumented tooth having a crown
replacement.
FIG. 4 is a front elevation of a coronal post that can be used in
endodontic restoration.
FIG. 5 is a vertical sectional view of a tooth having a cavity, which tooth
has been restored by use of the principles of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
In one embodiment of the present invention a tooth lacking a substantial
coronal portion is prepared for crown replacement by cleaning the root
canal, forming a abore for placement of a post and forming a counterbore
to aid in post retention. During or following instrumentation a chelating
agent and then an organic tissue solvent are used as a flush to prepare
the instrumented surfaces for crown replacement. Following the surface
cleaning, the post is inserted and cemented in place. A core is formed
around the post with a build-up substance and the crown is fitted in the
proper position. Thus, the surface preparation by the flushing reagents
aids the final placement of the crown.
In FIG. 1 of the drawing, a tooth composed of a crown area 10 and root area
11 is partially illustrated. The tooth, damaged and missing the coronal
portion 12, comprises dentin which surrounds the pulp cavity 1 and its
extension, the root canal 14. In the living tooth, the pulp cavity 13 and
root canal 14 are filled with fine connective tissue which contains
fibroblasts, histiocytes, odontoblasts, blood vessels and nerves. The
oldest or primary dentin 15 lies at the periphery of the tooth and the
secondary dentin 16 lies along the pulp cavity 14, where it is formed
throughout life by odontoblasts. Dentin of the crown area is covered by a
thick layer of enamel 17.
A damaged tooth that has been instrumented and fitted with a preformed
coronal replacement 20 is illustrated in FIG. 2. The instrumented tooth
resting in gum or gingival tissue 35 comprises an instrumented coronal
surface 21, a counterbore 22, and a bore 23. The coronal replacement 20
comprises the replacement shell 24, a post acceptance space 25 and an
instrumented replacement surface 26 that substantially contacts the
instrumented coronal surface 21.
A tooth having a crown replacement installed is illustrated in FIG. 3 and
comprises a bore 23, a counterbore 22, a threaded post 30, the apex of the
thread 31 of the post, the threaded nut or fastener 32, cement or a luting
agent 33, and a formed cored 34.
In accordance with the invention, a foundation of superior tensile strength
for a coronal structure is obtained by use of a chelating agent and
organic solvent solution to clean the instrumented areas 22 and 23 prior
to cementing of a post 30. Preferably, the bore 23 and counterbore 22 are
rinsed with a chelating agent prior to a flush with an organic solvent.
The luting agent or cement preparation 33 is preferably placed in the bore
23, and the fitted post 30 is seated and held or supported until the
cement 33 solidifies. A core 34 is formed around the exposed post 30 and
fastener 32 and may be composed of the same or similar cementing medium.
Other materials may be used. Following formation of the core 34, the crown
replacement 20 is positioned in place.
in the preferred embodiment, the instrumented coronal surface 21 is formed
from the damaged tooth. Then, the pulp cavity 13 (and/or the root canal
14, depending on the extent of work) is instrumented to form the bore 23.
The depth and diameter of the bore 23 will be influenced by the size of
the pulp cavity 13 and the size of the tooth. Preferably, the majority of
the root canal filling is removed to expose the dentin and provide a
suitable surface to which the cement may adhere. The final shape of the
bore is usually substantially cylindrical, however, the shape may be other
shapes, such as conical or trapezoidal. Since a certain preferred
dimension cannot always be obtained for the bore, the long-term integrity
of the foundation of the coronal replacement 20 depends upon the adherence
of the cement to the instrumented surface.
Preferably, an irrigant is used during instrumentation to aid in removal of
material from the pulp cavity 15 and to lubricate the cutting tool prior
to the final flush following instrumentation. Commonly used irrigants
include sodium hypochlorite (NaOCl) solution, hydrogen peroxide solution,
TEGO, REDTA, RC-Prep, polyacrylic acid (20% aqueous solution, 5000MW) and
water. TEGO is a one percent solution of dodecyldiaminoethyl glycine and
is available from Goldschmidt Products Corporation, White Plans, N.Y.
REDTA is a commercial preparation comprising disodium
ethylenediaminetetraacetate (EDTA) 17.00 gm, cetyl trimethylammonium
bromide 0.84 gm, 5N sodium hydroxide solution 9.25 ml, distilled water 100
ml, and may be obtained, e.g. from Roth Drug Company, Chicago, Ill.
RC-Prep is also a commercialEDTA preparation in a paste form that is used
in conjunction with sodium hypochlorite and comprises urea peroxide 10%,
EDTA 15%, a water soluble base and may be obtained from Premier Dental
Products Co., Philadelphia, PA. Preferably, NaOCl is administered as the
irrigant during instrumentation in an amount that removes a substantial
portion of the pulp debris and lubricates the cutting tool, more
preferably NaOCl is provided in a solution having a concentration of NaOCl
in the range of 1% to 20%, preferably 2-10%, and most preferably in a
solution having a concentration of 4.0 to 6.0 percent NaOCl was found to
be the most effective solution during instrumentation. All concentrations
are by weight unless otherwise noted.
Following instrumentation, the bore 23 is contacted with a chelating agent
and with a solvent or dispensant for organic material. Preferably, the
bore is contacted with the chelating agent prior to a flush with an
organic solvent. The purpose of the chelating agent is to substantially
remove the instrumented or smeared layer which is primarily calcific in
composition. Suitable chelating agents include but are not limited to EDTA
and citric acid. Preferably, EDTA is administered as the chelating agent
in an amount that substantially removes the smeared layer, more preferably
EDTA is provided in a solution having a concentration in the range of 1
percent to 50 percent, and most preferably in a solution having a
concentration of 16.0 to 18.0 percent EDTA. The chelating agent and other
aqueous materials are preferably buffered to approximate neutrality, e.g.
a pH of 7.5. In addition to the concentration of the chelating agent, a
minimum volume of the solution must be flushed through the instrumented
area to substantially remove the majority of the smeared layer. The volume
of chelating agent such as 17% EDTA which may be administered should be
sufficient to do the job, and may broadly range from about 1 cc to 300 cc.
Preferably, less than 10 cc flush per tooth is used, and often a fluid of
2-3 cc is sufficient.
To obtain maximum effect after instrumentation, the chelating agent should
be followed by an organic solvent or dispensant. The chelating solutions
alone effectively remove the smeared layer but leave varying amounts of
superficial debris, such as tissue and cellular components and possibly
bacteria. Suitable organic tissue solvents include but are not limited to,
sodium hypochlorite and ampholytic soap, such as TEGO or other surface
active agents, emulsifiers, etc. By "organic tissue solvent" is meant a
material which dissolves, or dispenses or otherwise chemically removes the
organic debris left after instrumentation. Suitable naterials are known
and described, e.g. in McCutchen's Publications (1981), the disclosure of
which is incorporated herein by reference. An ampholytic soap forms both
cations and anions and thus may combine the bactericidal activity of
cations with the surface tension reduction and solvent actions of anions.
Preferably, NaOCl is administered as the organic solvent in an amount that
substantially removes the superficial debris. For example, NaOCl may be
provided in solution having a concentration in the range of 1% to 20%,
preferably about 2.8%, and most preferably in a solution having a
concentration of about 4-6%, but most preferably about 5.25% NaOCl,
buffered to pH 7.5. In addition to the concentration of the organic
solvent, a minimum volume of the solution must be flushed through the
instrumented area to substantially remove the majority of the superficial
debris. If an organic solvent such as NaOCl is administered, preferably, a
volume of NaOCl solution is provided in the range of about 1 cc to 300
cc, more preferably 5 cc to 50 cc, and most prefereably about 8.0 to 12.0
cc flush.
Following instrumentation of the bore 23, a counterbore 22 may be
instrumented into the substance of the tooth. The purpose of the
counerbore 22 would be to provide additional area for adhesion of the
cement or a mechanical lock, if shaped appropriately, once the cement
hardens. Preferably, the counterbore 22 is formed at the time of the
instrumentation of the bore 23 therefore permitting administration of the
preferred reagents as irrigant and final flushing agents in the manner
described above.
Subsequently, a post 30 is fitted into the bore 23 to provide a mechanical
anchor for the coronal replacement 20. The post is preferably available
commercially as an endodontic item, more preferable it is manufactured by
any conventional process used for making such posts and most preferably it
is made of a lightweight material having sufficient compressive strength
to withstand the forces generated during mastication and an appropriate
coefficient of expansion substantially similar to the companion materials
employed for coronal replacement. Posts are generally cylindrical along
their long axis and therefore substantially parallel sided, however
tapered posts have also been employed in coronal replacement. Post
retention is also influenced by shape and surface configuration, e.g.,
hexagonal or octogonal sides, length, diameter, and presence or absence of
serrations, such as threads. The diameter and length of the fitted post
are influenced by the size of the bore 23 and the size of the coronal
replacement 20. A threaded post may be screwed directly into the bore 23,
however, the danger of cracking the tooth is a prominent disadvantage.
However, serrated posts, used in conjunction with cement, are more
retentive than smooth posts, and the surface configuration is more
important than length. Various surface configurations, e.g., threads,
serrations, convex protrusions or concave indentations, scales, etc.,
provide an increased surface area on the exterior of the post. Once the
cement has hardened around the post, these surface configurations, such as
the apex of a thread 31, provide a significant barrier, e.g., a mechanical
lock, to removal of the post from the hardened cement.
Preferably, a post with significant retentive properties such as a
substantial gripping surface for the luting agent is employed in this
invention to provide an anchor for the coronal replacement, more
preferably the post would have significant surface configuration such as
serrations, as illustrated in FIG. 3. After the post 30 is attached to the
tooth by hardened cement, a fastener 32 is screwed on or otherwise
attached to the protruding post to aid in anchoring the core 34.
In an alternative embodiment, a substantially cylindrical double-tapered
post 40 illustrated in FIG. 4 may be employed as an anchor for a coronal
replacement. The one-piece double-tapered post 40 comprises a bottom
surface 41, from which the lower tapered portion 42 extends toward the
middle section 43. The middle section 43 is necessarily of a lesser
diameter than the bottom surface 41 and divides the lower tapered portion
42 from the upper tapered portion 44 and top surface 45. The purpose of
the lower tapered portion 42 is to provide a mechanical means of retaining
the post in the hardened cement, similar to the function of the threads of
the post 30 of the preferred embodiment. The upper tapered portion 44
provides an anchor for the core 34 similar to the function of the threaded
fastener 32. The one-piece double-tapered post 40 would preferably be
manufactured by any conventional method of a lightweight material having
the compression and strength characteristics mentioned above.
Luting or cementing agents are utilized to affix the post 30 to the dentin
15 and may be used to form the core 34 as well. The strength of the
foundation is also influenced by the type of cement employed to anchor the
post. Commonly available cements for luting preparations include, but are
not limited to, zinc oxyphosphate, polycarboxylate, cyanoacrylate and
various resins. Preferably, the luting agents employed have substantial
adhesive properties and are easily prepared and employed in the method of
the invention, more preferably the luting agents have a high compressive
strength in the range of 12,000 lb/in.sup.2 to 30,000 lb/in.sup.2 and tend
to resist the tensile and shear forces placed on the material by
mastication. Most preferably the luting mixtures of cements have a
compressive strength of at least 20,000 lbs/in.sup.2. A suitable luting
agent is an unfilled resin, Bis-gamma methacrylate (Bis-GMA), combined
with the catalyst therefore, which has a compressive strength approaching
20,000-30,000 lbs /in.sup.2 and which has a watery consistency, and flows
easily with a low surface tension. Thus, these properties enhance its
usefulness since it is easily mixed and applied. Preferably, Bis-GMA is
applied in solution in a concentration that would provide a compressive
strength of at least 12,000 lbs/in2, more preferably at least 20,000
lbs/in.sup.2 and most preferably approximately 30,000 lbs/in.sup.2.
Once the post 30 has been fitted to the tooth, a quantity of cement 33
sufficient to contact the dentin 15 and the fitted post 30 is deposited in
the bore 23. The length of the post is adjusted during fitting so that a
portion of the post protrudes from the bore. Once the post is anchored in
the hardened cement, the threaded fastener 32 is rotatably attached to the
protruding part of the post 30. The threaded fastener 32 is preferably
made of a lightweight material having a compressive strength similar to
that of the post 30. The purpose of the threaded fastener 32 is to provide
a prominent surface around which a core 34 may be formed. This prominent
surface and the counterbore may be used together to provide a mechanical
means of retaining the core 34 once it has hardened.
The core 34 may be composed of a composite, cement or luting agent similar
to the core cement, or a different substance, such as amalgam. Preferably,
the core 34 should be composed of a material having sufficient bonding and
compressive strength to withstand the pressures encountered. In addition,
the core substance must have substantial mechanical and/or chemical
affinity for the coronal replacement 20.
The coronal replacement 20 is preferably performed by any manufacturing
process used in molding crowns, and comprises a replacement shell 24 and a
post acceptance space 25. During the fitting procedure the coronal
replacement 20 is substantially shaped to create an instrumented
replcement suface 26 which conforms to the contours of the instrumented
coronal surface 21. Once the coronal replacement 20 has been fitted to the
tooth, a core 34 is formed around the threaded fastener 32 and post 30
providing an amount of core substance to substantially fill the
counterbore 22 and post acceptance space 25 of the coronal replacement 20.
Although not wishing to be bound by theory as to why the structure prepared
by this invention reduces the possibility of loosening, it is believed
that when the organic debris and the smeared layer are removed from the
machined bore 23 and counterbore 22, the dentinal tubules are exposed. The
most effective final flush after instrumentation was found to be 10 cc of
17 percent EDTA followed by 10 cc of 5.25 percent NaOCl. The best result
is obtained when a 5.25 percent solution of NaOCl is employed during
instrumentation to lubricate and remove the majority of the pulp debris.
Thus, a cementing medium which has great compressive strength, such as
bis-GMA resin, can flow into the open tubules and into the serrations of
the post and provide greatly enhanced tensile strength.
The following examples are set forth to further illustrate the present
invention.
PREPARATION AND ANALYSIS OF MATERIALS
One hundred and twenty freshly extracted singly-rooted teeth were selected.
The crowns were removed at the cervical line and a post preparation either
4 mm or 7 mm was made with the appropriately sized burr to receive a 0.050
parapost (Whaledent Corp.). The teeth were then divided into two groups of
60 teeth each. Each group was further subdivided into two groups, each of
which had 30 teeth with 4 mm preparations and 30 teeth with 7 mm
preparations.
In Group A, all the post preparations were flushed with a syringe and 23
gauge needle using 2 cc of 5.25% NaOCl and the canals were dried with
paper points and air. In Group B, all the post preparations were flushed
with a syringe and 23 gauge needle using 1 cc of 17% EDTA solution
followed by 1 cc of 5.25% NaOCl after which the canals were dried wity
paper points and air. Both groups were then divided into three subgroups
(Table I).
Group A - all teeth flushed with 2 cc of 5.25% NaOCl and dried with an air
blast and paper points.
Group A 1--Zinc oxyphosphate cement powder and liquid were dispensed onto
the clean, dry surface of a glass slab and carefully mixed according to
manufacturer's directions to a creamy consistency suitable for
cementation. A lentulo spiral was used to place the cement in each post
preparation. The post was then coated with the cement and seated to place
and held until initial setting occurred.
Group A 2--Polycarboxylate cement powder and liquid (Durelon-Premier) were
dispensed onto a clean dry slab and carefully mixed according to
manufacturer's directions to a creamy consistency. A lentulo spiral in a
suitable handpiece was used to place the cement in the post preparation.
The post was coated with the cement and seated to place and held until
initial setting occurred.
Group A 3--A 30% mixture of Bis-GMA resin with the catalyst, TEGDMA, was
dispensed onto a paper mixing pad, the catalyst added and mixed. A lentulo
spiral by hand was used to place the resin in post preparation until it
was completely coated. Resin was used to coat the post which was seated
and held until initial set.
Group B - all teeth were flushed with 1 cc of 17% EDTA followed by 1 cc of
5.25% NaOCl and then dried with an air blast followed by paper points.
Group B 1--Posts cemented with zinc oxyphosphate cement and same procedure
as Group A 1.
Group B 2--Posts cemented with polycarboxylate cement and same procedure as
Group A 2.
Group B 3--Posts cemented with unfilled Bis-GMA resin and same procedure as
Group A3.
After cementation, all teeth were stored in 100% humidity at room
temperature for at least one week before testing.
The teeth were then tested for tensile strength on an Instron testing
machine at a cross head speed of 0.1 in/min.. The tensile strength of the
post was tested to failure.
The results were statistically analyzed using a 3-dimensional factorial
analysis of variance. Results were obtained for 114 teeth. The tensile
strengths are summarized in Table I.
Without wishing to be bound by theory, it is believed that the improvement
obtained by use of the particular solvent combination, removes both the
organic and inorganic debris, and thus eradicates the smeared layer which
would otherwise overlay the tubules. When this layer has been removed, a
light, low-viscosity, low-surface tension cement (such as unfilled gamma
methacrylate) can penetrate these pores, thus to establish a mechanical
seal in which the cured resin protrudes into the pores and sets up a
mechanical lock inside the tubules. The strength of this bond is believed
to be such that bond failure generally occurs within the adhesive itself,
rather than to the interface.
TABLE I
______________________________________
RELATIVE TENSILE STRENGTH OF
POSTS USING DIFFERENT BINDING
AGENTS AND DIFFERENT RESINS
EDTA
& Recent
Group I - 4mm. Posts
NaOCl NaOCl Improvement
______________________________________
Zinc Phosphate
37.59 52.73 clcl
Polycarboxylate
29.03 40.05 38%
Resin 51.55 97.40 89%
Group II - 7 mm. Posts
Zinc phosphate
51.31 63.40 24%
Polycarboxylate
41.55 46.37 12%
Resin 55.00 124.00 126%
______________________________________
The very clear cut superiority of the posts cemented with bis-GMA resin
following an EDTA-NaOCl flush was indisputable. As shown in Title I, it
was significantly better in every case and under every circumstance. The
strongest post in the zinc phosphate group was the 7 mm one following the
EDTA-NaOCl rinse. The 4 mm post cemented with bis-GMA after an EDTA-NaOCl
rinse had an 89% improvement in tensile strength, as compared with the
same post and resin, but utilizing only an NaOCl resin. The 7 mm post
cemented with bis-GMA after an EDTA-NaOCl rinse had more than twice the
tensile strength. The strongest post in the polycarboxylate group was the
7 mm one cemented after an EDTA-NaOCl flush and here the 4 mm bis-GMA post
was twice as strong and the 7 mm bis-GMA post was three times as strong.
These results indicate that when the smeared layer is removed and the
dentinal tubules are exposed, a cementing medium which has great
compressive strength such as the bis-GMA resin which is not soluble in the
oral fluids, can flow into the open tubules on the one side and into the
serrations of the post on the other side can provide a greatly enhanced
tensile strength.
An important aspect of this technique lies also in the fact that since a 4
mm post cement with the unfilled bis-GMA following an EDTA-NaOCl rinse,
can have one and a half times the tensile strength of the best 7 mm post
cemented with zinc phosphate, shorter posts can and should be used. These
are much easier to prepare and inadvertant perforations of the side of
slender or curved roots would be much less likely to occur. If a longer
post is chosen, then the 7 mm post cemented with unfilled resin after an
EDTA-NaOCl rinse has tensile strength equalling or at least approaching
the tensile strength of known screw-in posts. This allows a simpler
procedure without the danger of cracking the root which is inherent with
the screw-in post.
When the invention is used to provide a foundation for a coronal structure,
the danger of the core loosening or being damaged due to the tremendous
pressure encountered by the dental structure is significantly reduced.
Reduction of loosening problems will be effected by using any preparation
in accord with the invention have a prepared dental area flushed by a
chelating agent followed by an organic solvent and a luting agent of high
compressive strength employed to anchor a post inserted therein for a
coronal structure.
FIG. 5 is a cross-sectional view of a tooth 100 shown as a molar that has
been restored in accordance with the teachings of the present invention.
The tooth has a crown 101, a neck 102, and roots 103. Extending around the
outer periphery is a layer of enamel 111. Immediately within the enamel is
the dentin 112. Centrally of the tooth is the pulp chamber 113. The outer
surface of the roots 103 are provided with cementation and the crown is
shown as provided with cementation and the crown is shown as projecting
above the gum 115.
The tooth is shown as having a cavity 116 which has been provided with a
filling 17 of bis-GMA resin and an amalgam 118.
The method used for restoring the tooth 100 is very similar to the method
used to produce the post and crown restoration of FIG. 3. The method
consists of mechanically preparing the tooth to receive the filling,
including cleaning debris from the cavity 116 and possibly undercutting.
The prepared area is then flushed with a chelating agent selected from the
group consisting of citric acid and ethylenediaminetetraacetic acid and
then flushing the prepared area with a solvent selected from the group
consisting of sodium hypochlorite, surface active agents and emulsifiers.
The solvent dissolves, dispenses or otherwise chemically removes organic
debris. Then a low-viscosity luting agent is provided in the cavity and
later placing the filler 117 is inserted in place, contacting the filler
and the dentin of the tooth with the luting agent. The filler is allowed
to harden and the amalgam is inserted in the remainder of the cavity and
is faired to the tooth surface.
Since it has been difficult in the past to bond to the dentin (as
contrasted with bonding to the enamel), the present system solves a
long-standing problem. In the past, using traditional methods, a bonding
strength of 400 psi was considered optimum; with the present method it has
been possible to obtain bonding strengths 1600-1800 psi. Apparently, the
technique of the invention acts to remove the smear layer from the dentin
and expose the tubules to access by the low-viscosity cement. The smeared
layer is very tenacious, since it consists of a very greasy combination of
inorganic and organic elements. The present inventive method is very mild
and does not affect the remaining calcified tissue. The method is
particularly effective in situations in dental restoration where bonding
must take place almost entirely with dentin and where enamel is not
available to support the restoration materials.
The procedures described above can be effectively used in connection with
the cementation of crowns and caps on natural teeth, particularly in
situations where the cap is in contact with the dentin. In this case, the
outer layer of enamel and some dentin is removed, to provide room for the
crown. After this removal, the dental surface is washed thoroughly in
accordance with the invention as described above; then, the surface is
dried carefully and a thin layer of a bis-GMA low-viscosity unpolymerized
resin (containing its own catalyst) is painted over the entire surface of
the prepared tooth. This is allowed to polymerize and an impression is
made to permit preparation of the crown. Once prepared, the crown is then
cemented to the prepared surface with a bis-GMA cement; this provides a
strong chemical and mechanical bond which prevents marginal micro-leakage
and prevents bacterial penetration. A similar situation exists in filling
an instrumented root canal; the tubules in the dentin are exposed by the
process of the invention and a low-viscosity unfilled resin is used. The
resin enters the tubules and the pre-fitted gutta percha point is
inserted. This acts as a pressure medium to force the resin into the
tubules and other irregularities in the surface of the canal. The resin
then polymerizes under the action of its contained catalyst; thus creating
a filling of any gaps between the gutta percha and the canal surface. This
prevents micro- | | |
