Note: Descriptions are shown in the official language in which they were submitted.
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TOOTH RESTORATION OR PROSTHESIS PART M~DE FROM CERAMIC
MATERIAL AND A METHOD OF MANUFACTURING THE SAME
The present invention relates to a tooth restoration or
prosthesis part of ceramic material, according to the
preamble of claim 1 or claim 37.
For the sake of readability tooth restoration parts will
mainly be discussed in the following description; however,
the descripticns and disclosures also apply as appropriate
to other prosthesis parts made from ceramic material.
The ceramic tooth replacement materials from which such
tooth restoration parts are manufactured are characterised
in principle by high compressive strength, wear resistance
and durability, and good biocompatibility They exhibit
only a slight tendency to plaque formation and
accumulation, and aesthetically pleasing tooth restorations
matching individual colour and transparency requirements
can be manufactured from such materials.
The formation of fissures is often found in practice in
such tooth restoration parts despite their inherent good
mechanical properties, which can lead to the destruction of
the restoration part.
The object of l_he present invention is accordingly to
provide an improved tooth restoration part made from
ceramic material, according to the preamble of claim 1,
that reduces the danger of fissure formation and
destruction of the said part.
This object is achieved according to the invention by a
tooth restoration part having the features in claim 1 and
by a method for manufacturing such a tooth restoration part
having the features disclosed in claims 19, 25, 27, 28 and
29.
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A tooth restoration part according to the invention has the
advan.tage that protection against fissure ~ormation is
achieved independently of how or with which securement
material in particular the incorporation is effected, since
the fissure-preventing protective layer is formed under
strictly predeterminable conditions before the
incorporation. The tooth restoration part thus preserves
its integrity and reliability even over prolonged use.
A tooth restoration part according to the invention can be
manufactured using conventional dental laboratory
technology.
Too~ restoration parts according to the invention are
characterised by a high fitting accuracy and pleasing
appea.rance.
Tooth restoration parts according to the invention can also
be manufactured. having very thin walls, and despite this
fact their reliable functioning and satisfactory colour are
ensured, which means that the amount of tooth enamel that
has t:o be cut away in the preparatory work prior to the
actual incorporation can be kept extremely small.
The protective layer provided according to the invention on
the tooth restoration part prevents fatigue fractures
propagating from the critical points of the said tooth
restoration part. Such fatigue fractures may occur during
constant load under antagonistic tooth contact or by stress
prod~lced by mac;tication or biting, since notch stress
ef~ects can induce micro-cracks and promote a subcritical
growt:h o~ the ]atter, which can also be favoured by
hydrolytic effects, moisture expansion and ion exchange
mechanisms (saliva, dentin fluid). Although the elasticity
limit of the ceramic material is generally not exceeded
under mastication or biting stress thanks to receptor-
controlled protective reflexes, nevertheless the constant
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subcritical tensile or shear stresses cause enlargement of
existing micro-cracks. After a critical micro-crack depth
has been reached, a comparatively small load on the tooth
restoration part is then sufficient to produce a
macroscopic crack or fissure and thus damage or even
destroy the tcoth restoration part.
The production and/or growth of microcracks starting from
the critical points of the tooth restoration part is
counteracted by the protective layer provided according to
the invention. The protective layer in an advantageous
manner exerts a constant resistance to the tensile and/or
shear and/or compressive loads produced under the
mastication stresses.
The protective layer provided according to the invention on
critical points of the tooth restoration part is also
stable to hydrolysis.
Tooth restoration parts protected according to the
invention agai~lst crack ~ormation may be fully ceramic
dental crowns, especially porcelain full crowns in the form
of an individual crown or in a crown composite, bridge
anchor, crown ~ramework, partial crowns or double crowns.
They may however also be inlays, the protective layer being
provided along the central longitudinal fissure and
preferably pro-jecting at least partially into the body of
the inlay adjacent to the surface.
Advantageous developments of the invention are disclosed in
dependent claims.
According to a preferred development of the invention the
protective layer material is plastically or elastically
de~ormable, and becomes hard or hardenable pre~erably after
the shaping ancL forming stage. The protective layer is
applied in a continuous manner over the surface of the
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tooth restorat:ion part to be protected so as not to leave
any gaps and a~dheres at least partially in a frictional
manner to the latter, load-induced stress concentrations
thereby being avoided.
s
According to a further development of the invention the
protective layer may be formed at least partially of a
ceramic material. For this purpose organosilicon polymers
can be applied to the internal surface of the tooth
restoration part and at least partially ceramicised after
hardening. This can be performed for example by pyrolysis
of the polymer phase in a low oxygen content gaseous
atmosphere at temperatures below the plasticisation
temperature of the ceramic material used to produce the
tooth restoration part.
According to yet a further development o~ the invention the
protective layer may be formed at least partially of a
plastic material or synthetic or natural resins selected
from generally suitable non-toxic, plastically deformable
and hardenable or hardening materials. ~xamples of
part:icularly suitable materials are thermoplastic or
therrnosetting compounds, resins or polymers.
Further examples of particular suitable protective layer
materials include polysulphones, acrylates or organo-
silicon polymers, for example polysiloxanes.
Suitable protective layer materials are furthermore
mixtures of two or more different plastics or resins.
The protective layer may in turn be built up from different
partial layers.
According to a further development of the invention, at
least a proportion of the volume of the plastic or resin
subsequently forming the protective layer may be provided
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with an inert or reactive filler, as are for example
conventionally used in dental filling plastics or
photopolymerisable glass ionomer cements or more recent
"compomers". 3y adding these generally inorganic,
non-metallic fillers, whose surfaces are preferably
silanised, the mechanical properties of the resultant
composite can be improved after it has been hardened. The
residual shrinkage during the hardening of the composite
can be influenced by the filler content. In this way the
compressive st:ress, among other things, induced on the
adjoining ceramic boundary surface can be controlled. This
compressive stress in addition has a beneficial effect on
the resistance behaviour of the protective layer with
regard to stress-induced crack-opening effects.
As regards the nature, shape, concentration or size
distribution o:E the fillers that are used, every effort is
made to ensure that, under high mechanical loadability,
espe~_ially high tensile elasticity limit, and as low a
brittleness of the protective layer as possible, the volume
shrinkage occw-ring during the hardening o~ the protective
laye:r material is sufficient to build up compressive
stre3ses in the adjoining ceramic surface. At the same
time the protective layer should have a sufficient rigidity
and compressive strength so as not to be destroyed under
the mastication or biting stress of the reinforced tooth
restoration part.
Preferably protective layer materials are used that also
contain inorganic and/or organic particles, short and/or
long fibres, and/or semi-fabricated products made
therefrom, especially plaited ~ibre products. The
protective layer is reinforced by the inclusion of such
products.
As fillers there may also be used metallic particles and/or
shor~ or long metallic fibres, for example made of
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- titanium, though concessions have to be made with regard to the colour of the tooth restoration part.
It has also been found convenient in addition to add photo-
activatable or thermo-activatable reaction initiators
and,/or cataly~ts to the protective layer material,
especially in the case of acrylate-based plastics or
plastics based on organosilicon polymers.
In order to achieve a specific polymerisation activation,
polymerisatiorl inhibitors and their antagonists in the form
of paste-paste systems that are hardenable at room
temperature arld are thoroughly mixed immediately before use
may also ~è acLdèd to-the material used to produce the
protective layer.
To simplify the processing of the unfilled or filled or
fibre-reinforced materials from which the protective layer
is made, it has been found convenient to use such materials
in the form of pastes having a consistency or viscosity
customary in dentistry or dental technology. As an
alternative to pastes, plastically deformable products, for
example sheet-like intermediate products formed from the
protective layer materials, may also be used. These
intermediate products may for example be in the form of a
tablet-shaped, ready-made semi-finished product.
A further advantageous way of producing the protective
layer is to start from partially prepolymerised semi-
finished products (prepregs), which first of all becomesignificantly less viscous on heating, thereby making them
easier to shape and form, and which then rapidly
consolidate after shaping and forming or harden under the
action of light and/or heat.
The protective action guaranteed by the protective layer
provided according to the invention on the tooth
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restoration part is, among other things, a consequence of
the different rnoduli of elasticity of the actual tooth
restoration part and of the protective layer. The
protective layer produces a compressive stress on the
coated ceramic surface, which is independent of the
way in which the tooth restoration part is secured to the
tooth stump. 'rhe stress peaks produced under loading of
tooth restoration parts according to the invention, for
example artificial dental crowns, are localised at the
critical points of the tooth restoration part and are there
comp:Letely or partially inactivated, for example by elastic
and/or plastic deformation of the protective layer or by
chanyes in the polymer structure.
If unfilled and/or filled, especially particle-reinforced
or fibre-reinforced plastics are used as protective layer
material, then the protective layer material has a
significantly higher resistance to load-induced fissure-
opening effects compared to the ceramic material. Should
nevertheless a microcrack form, the protective layer
imparts resistance to load-induced crack propagation. In
particular, the resistance to sub-critical crack
propagation is substantially better compared to pure dental
ceramic materials.
The critical points of the tooth restoration part to be
protected are furthermore protected by the protective layer
against degradative hydrolysis and/or corrosion effects,
which may otherwise be caused by dentin fluid reaching the
surface or by cl leakage of buccal fluid that has diffused
out.
The protective effect of the protective layer provided in
the tooth restoration part according to the invention is
promoted by compressive stresses in the adjacent ceramic
boundary surface or interface resulting from the shrinkage
of the plastic phase during its polymerisation
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(thermosetting) or cooling (thermoplast). The build-up of
such compressive stresses on the internal boundary surface
between the ceramic and protective layer as well as within
the respective adjacent material-edge zones is based on the
at least partial frictional adaptation or adsorption of the
protective layer on the ceramic material. The resultant
compressive stress also counteracts tensile stresses
induced under Inastication stress in the ceramic regions
adjacent to the protective layer. The protective layer
seals any fabrication-induced or processing-induced surface
defects at the critical points of the tooth restoration
part and there~y reduces notch stress effects. The same is
also true of tooth restoration parts that overlap tooth
stumps (crowns and the like), as well as tooth restoration
parts that are installed in intra-crown tooth cavities
(inlays).
The invention will now be described in more detail with the
aid of embodiments and with reference to the accompanying
drawings, in which:
Figs. 1 to 7 show various steps in the manufacture of a
ceramic tooth restoration part, namely:
Fig. 1: shows the production of a negative impression of
a prepared tooth stump;
Fig. 2: is a section through a positive model produced by
the ~egative impression of the too~h stump, with
applied position-retaining layer;
Fig. 3: shows the production of a negative impression
from the positive model carrying the position-
retaining layer;
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Fig. 4: shows the modelling of a ceramic dental crown
over a working model produced from the negative
impression illustrated in Fig. 3;
5 Fig. 5: show,s the formation of a protective layer on the
inne:r surface of the shaped and baked dental
crown over the positive model of the tooth stump;
Fig. 6: is a partial sectional side view of the finished
ceramic dental crown;
Fig. 7: is a section through the dental crown cemented
onto the tooth stump;
Fig. 8: is an enlarged partial section through the region
VIII of the incorporated dental crown according
to Fi.g. 7;
Figs. 9 to 11 are sectional views similar to that in Fig.
8, in which modified ceramic dental crowns are
illuc~trated;
Fig. 12: is a sectional enlargement of the region XII of
the d.ental crown according to Fig. 11;
Figs. 13 to 17 are sectional views similar to that of Fig.
12, in which however modified ceramic dental
crowns are shown;
FigsO 18 and 19 are two partial sections in a modified
process for manufacturing a ceramic dental crown
carrying a protective layer, namely:
Fig. 18: shows the forming and shaping of a model of the
subsequent ceramic tooth restoration part over a
positive model of the tooth stump carrying a
releasable position-retaining layer;
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- 10
Fig. 19: shows the formation of a protective layer on the
dental crown blank shaped and subsequently
manufactured according to Fig. 18;
FigsO 20 to 22 show an alternative way of manufacturing a
working model used in the forming and shaping of
the ceramic material and which is oversized
relative to the tooth stump, namely:
Fig. 20: shows the start of the production of a negative
impression of the positive model made from the
tooth stump;
Fig. 21: shows the situation after=tke-hardening of the
casting composition used to produce the negative
impression;
Fig. 22: is a side view of the finished working model;
Fig. 23: is a block diagram of a device for producing
protective layer parts having the external
contour of a positive model of the tooth stump of
corresponding internal contour;
Fig. 2~: is a diagrammatic illustration of the production
of such a protective layer part that is to become
part of a ceramic dental crown;
Fig. 25: is a diagrammatic illustration of the production
of a protective layer part that is to become part
of a ceramic bridge;
Fig. 26: shows the mounting of the protective layer part,
produced according to Fig. 25, on a working model
of the tooth stumps anchoring the bridge;
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11
Fig. 27: showis the built-up o~ ceramic material over the
toot~ stump working model and protective layer
part;
Fig. 23: is an enlarged partial section through the
section XXVIII of the bridge blank of Fig. 27;
Fig. 29: shows the production of additional protective
laye:rs that are intended for the prepared
sur~aces o~ the tooth stumps anchoring the bridge
and other particularly stressed points of the
bridge;
= Fig. 30: is a first sectional enlargement of Fig. 29,
relating to the region XXX therein;
Fig. 31: is a second sectional enlargement of Fig. 29,
relating to the region XXXI therein;
Fig. 32: is a section through the incorporated ceramic
bridge;
Fig. 33: is a side view of an incorporated inlay provided
with a protective layer preventing fissure
~ormation; and
Fig. 34: is a transverse section through the connection
point of a bridge-anchor implant and of a tooth
restoration part joined thereto.
In Fig. 1 a prepared tooth stump is generally indicated by
the reference numeral 10. The preparation surface 12
term:inates in an arched, circumferentially running shoulder
surface 14 lying in the region of the gum papillae 16, 18.
In order to mahe this full crown a negative impression is
made from the upper section o~ the tooth stump. For this
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12
purpose a casting spoon 20 is used that is filled with a
plastic castin~3 material 22.
The ~egative impression formed by the solidified casting
mate~ial is then filled, for example with plaster of Paris.
The resultant positive model of the tooth stump 10 is
worked up in the conventional way (sawn segment model) and
mount:ed together with a counter-jaw model in an
articulator.
Fig. 2 shows the resultant positive model 24. A protective
layer/position-retaining layer 26 is applied to the said
model, the layer 26 following the contour of the
= preparation surface 12 and largely-~aving the same
thickness as the embodiment in question. The position-
retaining layer 26 terminates just above the shoulder
surface 14.
Instead of a uniformly thick position-retaining layer 26, a
position-retaining layer whose thickness varies according
to the stresses to which the various surface regions are
subje(_ted may a:Lso be applied: the position-retaining
layer 26 iS thicker over surface regions subjected to
relatively large stresses than in less stressed surface
regions.
The position-ret:aining layer 26 may be made of wax, a
plastic material or another material that can subsequently
be released from the positive model 24 mechanically by
physic:al action or by chemical action. If desired a copy
or "double" can be made from the positive model 24 and the
positive model or copy can then be provided with a non-
releasable position-retaining layer, the model freed from
the position-retaining layer then being used for
manufacturing steps to be carried out later.
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13
In general the position-retaining layer 26 corresponds as
regards shape, layer thickness, position and size to a
protective layer that is subsequently to be present on the
inner surface of a ceramic single crown. As a rule the
position-retaining layer 26 extends over the whole
preparation su:rface 12, the thickness of the said layer
matching the special individual shape of the crown that is
bein~ made and the stresses to which the crown will
subsequently be subjected For average front or side
dental crowns a thickness of the position-retaining layer
26 of about 0.3 mm is suitable. For side dental crowns
subjected to strong mastication stresses, for example in
the case of patients with bilaterally balanced occlusion,
- or for crowns forming bridge anc-hors-, layer thicknesses in
the region of 0.6 mm have proved suitable.
Preferably the region of the preparation surface 12
immediately adjacent to the shoulder surface 14 is free of
the position-retaining layer 26, as shown in Fig. 2, or a
position-retaining layer of reduced thickness (about 0.1 mm
to 0.3 mm) is provided in this region.
The position-retaining layer 26 can be applied to the
positive model 24 by using wax layers, for example by
dipping methods and/or wax modelling. Alternatively,
suitably adapted thermoforming sheets or releasable coating
agents may also be used. In a further modification the
position-retaining layer 26 may consist of several partial
layers formed from one or more of the aforementioned types
of layers. It is important that the position-retaining
layer 26 is on the one hand sufficiently stable so that an
impression can be made from the positive model carrying the
said position-retaining layer, and that on the other hand
the position-retaining layer 26 can be removed from the
posit:ive model 24 without damaging the latter.
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14
A negative impression is then made from the positive model
carrying the position-retaining layer 26, for which purpose
(cf~ Fig. 3) a casting cup 28 is used that is filled with a
casting material 30, for example an addition-crosslinked
silicone, e.g. polysiloxane.
The negative impression that is obtained is cast with a
moulding composition, which may be plaster of Paris or,
preferably, a refractory material.
A working model 32 obtained in this way is shown in Fig. 4,
which represents the positive model of the positive model
24 carrying the position-retaining layer 26.
The working model 32 is aligned in the same position as the
positive model 24 in the saw segment model, a previously
fabricated adjustment key being used, made for example from
silicone. The adjustment key is used in connection with
surfaces of the positive model 24 that are not provided
with the posit:ion-retaining layer 26.
A ceramic material (for example a hard porcelain for mould
sintering suitable for dental technology processing) that
is subsequently to form the dental crown is now built up
over the working model 32. This build-up procedure is
performed using methods known to dental mechanics, the
crown being shaped and contoured individually having regard
to the adjacent teeth and in particular to the antagonistic
tooth in the opposite jaw.
Normally the dental crown 34 is built up and baked in
several layers, adjusted and matched to the articulation
function and polished. The dental crown 34 is then removed
from the working model 32 and carefully cleaned to remove
residues (e.g. of material ~rom which the working model 32
is madeJ. The position-retaining layer is care~ully
removed ~rom the model stump 24.
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The fitting of the dental crown 34 on the positive model
24, which is only a loose fitting on account of the
position-retaining layer 26, is checked together with the
accuracy of the shape of the functional crown surfaces in
the articulator. If desired the fitting in the edge region
or the colour matching for example may also be checked by
examining the dental crown in situ in the patient.
Corrections and adjustments may be made if necessary.
If inaccuracies affecting the fit are detected when the
crown is mounted on the patient's tooth stump, the crown
can be filled with a conventional impression material and
repositioned on the patient's tooth stump. In this case
the crown (or bridge) serves as an individual im~ression
spoon that improves the casting accuracy with respect to
the original overall impression of the patient~s whole jaw.
A new model st~mp is then made that forms the basis for the
further processing, in particular the subsequent
application of a protective layer, as will be described in
more detail hereinafter.
On account of the overdimensioning of the preparation
surface of the working model 32, the inner surface of the
dental crown 34 identified by the reference numeral 36 in
Fig. 5 together with the outer surface of the positive
model 24 defines a cavity corresponding to the geometry of
the position-retaining layer 26. This cavity is filled
according to the invention with a protective layer material
that counteracts fissure formation in the ceramic material
as well as the propagation of any existing fissures in the
ceramic material.
The protective layer may be applied to the baked ceramic
dental crown 34 in the following way:
A cement/position-retaining layer 40 is applied to the
positive model 24. The thickness of this layer is chosen
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16
- corresponding to the thickness of the joint material
(cement) subsequently used to ~ix the dental crown on the
tooth stump 10~ and in practice is about 10 to 20 ~m.
The sur~aces of the model stump 24 that come into contact
with a subsequent protective layer (see below) are
preferably addi.tionally provided with a release agent layer
41. In special. cases such a release layer may be omitted
if the cement/position-retaining layer itself can act as
release agent layer.
Suita.ble available release agents are silicone or alginate
solutions.
The internal surface 36 of the dental crown 34 to be
provided with the protective layer is first of all
degreased with alcohol. Depending on the nature and
composition of the ceramic material of the dental crown 34,
the internal surface 36 is then cleaned by careful blasting
with fine aluminium oxide particles and if necessary
mechanically roughened and/or etched with hydrofluoric
acid, in order lo improve the bonding with the subsequently
applied protect:ive layer. In addition or instead of this a
surface silicate enrichment may be carried out, for example
by tribochemica:L coating.
If conventional dental porcelain is used to make the dental
crownV for example feldspar ceramic or glass ceramic
compositions, the internal surface of the dental crown is
first of all blasted with aluminium oxide particles
(preferred mean grain size about 25 ~m) and then etched,
for example with bifluoride solution or hydrofluoric acid
solution.
After carefully removing residues of etching agent as well
as dissolved precipitates, for example in a water spray, or
after removing residues of blasting agent after tribo-
-
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- chemical surface coating, for example in an air stream
followed by drying, the internal surface 36 to be coated is
silanised in a conventional manner, for example by
applying an orqanosilane solution, and is then ready for
the application of the protective layer.
The protective layer is applied before the dental crown is
mount:ed in the patient's mouth, in other words
independently of the bonding of the dental crown to the
tooth stump using a joint material such as cement. The
dental crown prepared as described above with a silanised
internal surface is filled with a sufficient volume of
plastic protective layer material. Alternatively, the
protective layer material can be applied to the positive
model 24 in the form of a viscoplastic composition, paste,
mouldable sheet, plasticisable tablet or deformable
prepreg. In the last case different partial layers of the
protective layer can be applied in succession to the
posit:ive model 24.
By WcLy of modification part of the protective layer
material may also be applied to the model stump and a
further part of the protective layer material may be
applied to the internal surface of the dental crown.
The clental cro~n 34 is then completely slipped over and
mounted on the positive model 24. In the case of
protective layer materials having a high viscosity,
especially thixotropic protective layer materials, this
procedure is a~ditionally assisted by oscillating,
preferably ultrasound-activated tools. For this purpose a
compression instrument 42 connected to the power take-off
part 44 of an ultrasound generator 46 grips the upper
surface of the dental crown 34, as illustrated in Fig. 5.
The ultrasound generator is driven by a power supply device
48 and has a handle 50 suitable for exerting axial pressure
(cf. arrow in Fig. 5~.
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18
Alternatively, in order to slip on and mount the dental
crown 34 the v:Lscosity o~ the protective layer material can
also temporarily be reduced by the application of heat, the
dental crown 3'L or t~e positive model 24 preferably being
heated to the appropriate temperature beforehand. If
necessary the dental crown 34 can also be slipped over the
positive model 24 in stages, the dental crown being removed
between each of the stages so that the protective layer
material can be directly reheated again. The last-
mentioned procedure is used in particular in the processingof thermoplastic protective layer materials.
In general the amount of protective layer material used is
suf~iciently large that excess material is squeezed out
when the dental crown 34 is completely pressed down onto
the positive model 24. A~ter carefully removing all the
expressed material, the fitting of the dental crown, the
contact points with the adjacent teeth as well as the
functional occlusion contacts and articulation contacts can
be checked.
When using acrylate-containing protective layers it can be
ensured by applying glycerol gel, preferably to the edge
regions of the not yet hardened protective layer
corresponding to the crown edge region, that no oxygen-
induced surface polymerisation inhibition layers are
formed. Reproducible mechanical properties of the
protective layer up to its edge regions are obtained in
this way.
The polymerisation o~ the protective layer material is
effected for example by irradiation with light of suitable
wavelength and/or the action of heat, or also auto-
catalytically. In the case of thermoplastic protective
layer materials the protective layer is compacted and
hardened by coo:Ling.
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- By using the aforedescribed technique thinly exposed edges
o~ dental crowns or other tooth restoration parts can also
be reproducibl~ reconstructed, which is not possible using
conventional dental ceramic techniques since with thinly
exposed edges t:he latter can break on account of the
brittle fracture behaviour of the ceramic material.
When the dentaY crown 34 is mounted on the positive model
24 an adjustment aid, often termed a key, is preferably
used in order to ensure that the axial alignment and the
angular position of ~he dental crown relative to ~he tooth
stump axis are correct. Such a key may be unnecessary
where the shou]der surface 14 or another surface section
not covered by the position-retaining layer 26 is already
properly formed and shaped. Normally such a key is made of
plaster of Paris or hard silicone and has surface
impressions of the crown and model stump. In order to
mount the crown in position this key engages for example
with the labia] surface of the positive model and the crown
is then rotated, tilted and displaced on the positive model
(with expulsion of the already applied protective layer
material) unti] its surfaces engage exactly in the
complementary surface impressions of the key.
Instead of a key, small positioning notches and
corresponding positioning ribs on the positive model and
the dental cro~m can also be used, the positioning means on
the dental crown being removed in a last stage of the crown
manufacture.
After the protective layer has completely hardened the edge
region of the dental crown is finished off and polished.
After removing the finished dental crown 34 from the
positive model 24 and cleaning the dental crown to remove
residues of re]ease agent, the dental crown is now ready to
be mounted in position by the dentist. This state is
illustrated in Fig. 6. It can be seen that the dental
CA 02212210 1997-08-04
- crown according to the invention differs from conventiona
ceramic dental crowns by the protective layer 52 provided
on the internal surface 36, the free surface of the said
layer being exactly complementary to the positive model 24
5 and thus to the tooth stump 10.
The dental crown 34 is secured on the tooth stump 10
preferably by conventional cementing using a zinc
oxide/phosphate cement or a glass ionomer cement or a
compomer or a similar cement. The dental crown can also be
secured in position with a polymer-containing mounting
composite, the time-consuming etching and silanisation of
the i,nternal surface of the crown that is otherwise
necec;sary with ceramic tooth restoration parts being able
to be omitted in this case. Conventional cementing is
however more a~vantageous since the crown can be
incorporated considerably more quickly and simply, without
leaving any residues of joint material.
Fig. 7 is a section through the incorporated dental crown
34 and the tooth stump 10 together with the cement layer 54
lying between the dental crown and the preparation surface
12.
Fig. 8 shows an enlarged section from Fig. 7. It can be
seen that the external surface 56 of the protective layer
52 and the internal surface of the ceramic body 58 of the
dental crown 34 have mutually complementary and irregular
roughnesses so that the protective layer material and the
ceramic material 58 locally firmly interlock with one
another. This interlocking is advantageous having regard
to the tangential compressive stress exerted by the
protective layer on the ceramic body. This interlocking
engagement of the protective layer material and ceramic
material is also advantageous as regards increasing the
contact surface between these two materials.
CA 02212210 1997-08-04
In the dental crown 34 whose manufacture has been described
above with re~erence to Figs. 1 to 8, the protective layer
52 extends to t:he vicinity of the shoulder surface 14, but
not right up to the latter.
According to the variant illustrated in Fig. 9, the
protective layer 52 of the dental crown 34 runs up to the
shoulder surface 14.
In the embodiment according to Fig. 10, the lower end of
the protective layer 52 runs parallel to the shoulder
surface 14 and as far as the outside of the dental crown
34.
In the embodiment according to Fig. 11 the protective layer
52 comprises a ceramic foam material that is applied,
before the manufacture of the tooth restoration part,
instead of or in addition to a position-retaining layer on
the model stump or on a further model stump preferably made
of a refractoryr material and prepared after casting this
first: model stump. The refractory material may be an
inoryanic material, for example a ceramic material,
especially an aluminium oxide ceramic, zirconium oxide
ceramic, silicc,n carbide ceramic, etc. The ceramic foam
material is preferably in the form of a prefabricated part
that is matched to the model stump by for example manual
grincling or by pressing the model stump into the foam
material under controlled fracture of the trabecle The
degree of expansion and thickness are controlled manually,
for example with handcarving tools or with rotating
diamond-tipped tools. Alternatively mechanical machining
may be used, as described hereinbelow.
The restoration work is then carried out under an at least
partial infiltration of the ceramic composition forming the
crown, into the foam structure. The foam structure remains
as a constituent part of the definitive restoration and is
CA 02212210 1997-08-04
- preferably add:itionally infiltrated with protective layer
material on the side ~acing the stump. If foams or metal
foams produced by pyrolysis are used, the foam can also be
produced to start with (before baking the crown) or
s subsequently (in situ in the finished crown).
Such a protect:ive layer can also be produced by introducing
in the aforedescribed manner an at least partially organo-
silicon material into the mould cavity defined by the
dental crown 3~1 and the positive model 24, the said organo-
silicon material then forming an at least partially ceramic
foam material after pyrolysis. Suitable organosilicon
materials are :Eor example polysilazanes, polysiloxanes, or
composites pr-epared therefrom that are ceramicised by
pyrolysis.
Finally, the p:rotective layer may also be built up from
metallic powde:rs, for example titanium and gold powders
that are introduced in the form of a paste, as described
above, and whi,-h are then firmly sintered together under
the action of heat or with the formation of porosities, for
example as foam structures, or may be applied by sputtering
or any other suitable known method for applying metal
layers. In the case of high melting point metals, a
metallic or ceramic binder that melts at lower temperatures
may optionally be added.
In the embodiment according to Fig. 12 the protective layer
52 again comprises a ceramic foam material, a partial layer
60 of the open-pore protective layer adjacent to the
internal surface 36 having been infiltrated with plastic
material, while a partial layer 61 of the protective layer
adjacent to the ceramic mass has been infiltrated with
ceramic material.
Such a material is prepared by first of all forming the
protective layer 52 over the positive model, producing it
CA 02212210 1997-08-04
~ from a prefabricated part or at least partially baking,
sintering or pyrolysing the layer so as to obtain an open-
pore ceramic foam material layer, and then building up the
dental crown 3'L over the resultant open-pore protective
layer 52, the initial portions of the ceramic material,
which may have a clay-like consistency, penetrating the
external regions of the protective layer 52. Alternatively
the foam struct:ure can be removed from the model stump
a~ter matching and the crown can be built up on the latter,
for example of a hard ceramic material, a refractory stump
being omitted i.n this case.
Where. for example glass ceramic production methods or crown
materials are used a model o~ the crown, for example of wax
or plastic, is made with the incorporation of the foam,
this model is embedded in a negative mould and glass or
ceram.ic material is pressed or ~orced in, with an at least
partial infiltration of the foam, after thoroughly baking
the model material in a muffle.
After baking the dental crown 34 the region of the open-
pore protective layer 52 facing the internal surface 36 is
then infiltrated with plastic material.
The embodiment according to Fig. 13 resembles that
according to Fig. 12, except that in this case the
protective layer 52 was built up on a working model
produced using a position-retaining layer, as is
illustrated in :Fig. 4. By mounting the resultant dental
crown on the positive model 24 and filling the mould cavity
contained between the dental crown and positive model with
plastic, the st:ructure shown in Fig. 13 is then obtained,
in which the pa:rtial layer 60 of the open-pore protective
layer 52 ad~acent to the internal surface 36 is infiltrated
with plastic material, though in this case a further
partial layer 6:2 consisting only of plastic (optionally
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~ provided with fillers) now lies over the thus infiltrated
open-pore protective layer 52.
In the embodiment according to Fig. 14 the open-pore
protective layer 52, which is again infiltrated from one
side with ceramic material and from the other side with
plastic material, has an anisotropic pore structure.
Larger pores 63 exist in the vicinity of the internal
surface 36, while smaller pores 64 are present in the
regions adjacent to the body of the ceramic material.
Oxide-ceramic foams are produced by compressing foam
material, dipping the latter in a slip composition, and
expanding the foam in the impregnated state. Slip is
thereby sucked into the pores. The foam can then be baked
under special firing conditions (very high temperatures),
with the formation of a compact sintered foam structure.
These foams are nowadays used as filters, for example for
casting aluminium, and are commercially available as
prefabricated components.
Anisotropic oxi,le-ceramic foams are produced from
anisotropic foam composites of complementary pore
structure.
Such foam mater:ials can also be prepared for example by
building up the protective layer 52 from partial layers
obtained by sinlering together particles of larger and
smaller diamete]s. Alternatively partial layers of
different plast:ic materials produced by pyrolysis of
ceramic foams of different pore sizes can be built up on
top o~ one another.
The ernbodiment according to Fig. 15 resembles that
according to Fig. 14, though in this case the large pores
63 ex:ist in the vicinity of the ceramic material body,
while the small pores 64 are adjacent to the internal
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surface 36. Also, in this case too in a partial region of
the internal surface there is only a partial layer 60
consisting of plastic material.
The use of foarn materials in the preparation of the
protective layer 52 has the advantage that part of the
desired elasticity for the protective layer is obtained
from the pore structure of the relevant materials and does
not have to be provided from volumetric properties of the
10 material
This applies in particular when using centrically hollow
particles, especially metallic hollow spheres, to produce
the foams. FUI. ther advantages of the use of foam
structures inc]ude:
- the relatively simple production of a type of
framework or structure on which or in which the
restoratic~n part is built up;
- the preparation of a multi-part gradient structure,
the foam material occupying only a small volume
(crowns have only a limited layer thickness);
25 - the individual polychromatic end result of the
restoration work on account of the composition being
infiltrated, e.g. ceramic or plastic composition (the
colour effect can also arise from the interior,
whereas up to now only a surface layer of the tooth
restoration has been responsible for the colour).
An anisotropic foam material with both large and small
pores is preferable since the most favourable conditions
for the relevant material being infiltrated can be
predetermined in this way. For example, due to the
different pore sizes it can be ensured that the more poorly
infiltrating material fills up the large pores reliably and
CA 02212210 1997-08-04
26
- substantially completely and reliably wets the individual
foam trabeculae all the way round, whereas the smaller
pores, which are to be infiltrated with another material,
initially remain unfilled. These smaller pores can then
subsequently be filled with this other material, whose
viscosity or wetting behaviour with respect to the
trab~eculae is adjusted differently, optionally with the
help of special wetting agents and, likewise, optionally
with additional assistance from pressure, heat or
ultrasound.
A ~urther advantage of the use of foam materials for the
protective layer is that an etching of the internal surface
of tl-Le ceramic crown can be dispensed with.
Part:icularly suitable foam materials are foams formed from
aluminium oxide ceramics. Suitable pore densities are in
the range from 30 pores per inch to 120 pores per inch
(12-~8 pores/cm), preferably 50 pores per inch to 90 pores
per inch (20-3', pores/cm), and particularly preferably in
the range from 65 pores per inch to 80 pores per inch
(25-32 pores/cm).
In the aforedescribed embodiments the protective layer
consisted at least partially also of plastic material.
In the embodiment according to Figs. 16 and 17 the
protective layer consists of foam material that is
infiltrated up to the vicinity of the internal surface 36
with the ceramic material that also forms the ceramic bulk
58. As described above, the foam material may again be a
ceramic foam material or a metallic foam material.
As an alternative to the examples according to Figs. 16 and
17 foam structures may also be used in which the foam
material, for example metallic foam material or organo-
silicon foam material, is converted or baked in part
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pyrolytically at least partially in the manufacture and
baking of the crown or in a subsequent tempering of the
latter, or undergoes a volume shrinkage. In this way a
negat:ive of the relief of the foam structure remains in the
dental crown.
If the ceramic restoration that is not provided with a
protective layer has at least partially open porosities on
the side facing the stump or such porosities can be
produced for example by physical treatment, such as by sand
blasting and/or by chemical treatment, for example by
application of acid or basic solutions, and/or
electrochemical treatment, preferably by dissolving out
foam structures introduced into these boundary surfaces,
for example foam structures with metallic, organic or
inorganic constituents, or metallic or ceramic foam
structures, to a depth that corresponds to the minimum of
the otherwise conventional thickness of the protective
layer, the application of a position-retaining layer before
the production of this ceramic restoration can then be
omitted. In these cases the protective layer is formed by
infiltration of these preferably intercommunicating
porosities. The boundary surfaces may be treated, in
particular silanised, before the infiltration to improve
the adhesion of the protective layer. In this case
preferably less highly filled plastics have proved suitable
as composition that can be infiltrated. Due to the
relat:ively large surface area of the contact zone between
the ceramic phase and the layer that has penetrated or
infiltrated into the porous boundary surface, the thickness
of the said layer can be reduced compared to a layer
applied to the site to be protected of the restoration
part, while preserving the same functional security. With
a pore fraction of 50~ referred to the overall volume of
the boundary surface to be infiltrated, an infiltration
depth of 0.1 to 0.2 mm is generally sufficient.
CA 02212210 1997-08-04
28
It is also possible to combine a protective layer
infiltrated into a porous ceramic boundary surface with a
layer applied to this boundary surface. These layers may
be applied in a single combined work stage or in several
work stages, preferably in each case with the complete
repositioning of the restoration part on the model stump.
A further possible way of manufacturing restoration parts
according to the invention is for the position-retaining
layer formed on the model stump or on a duplicate model of
the latter hav:ing the same shape and size to consist of a
material that :remains dimensionally stable and accurate
under the baking temperatures required to produce the
eeramic restoration part. The coefficient of thermal
expansion of such a material should correspond to that of
the chosen ceramic compositions, and the position-retaining
layer should be able to be removed from the model stump
without causing deformation or distortion, and should be
able to withstand the manual forces while the facing
compositions are being sintered on. The ceramic
restoration part can then be formed directly on this
pbsil_ion-retaining layer, which for dental crowns is shaped
somewhat like a small cap. Suitable materials for this
posil_ion-retaining layer are for example sheets of platinum
or platinum alloys. Also suitable are for example
posi~ion-retaining layers produced from organosilicon
mate:rial precursors, which are pyrolytically ceramicised at
the same time as the baking on of the first ceramic layer
or in a preceding edge cycle.
A similar effect can also be obtained by selectively
remo~ing, for example by etching, part of the trabeculae.
The foam is then produced for example by incorporating
polymers, metals or inorganic materials or composite
mate:rials into this negative relief, which has the finished
c rown .
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Foam structures may also be produced by baking a normal
dental ceramic composition on a refractory model stump, the
sturnp materia] releasing gas bubbles, for example nitrogen
bubbles, at e]evated temperature (nitrogen from urea, or
the like). These gas bubbles form open porosities in the
edge region of the crown that can be infiltrated with
plastic, metal or ceramic material and form a composite
material with the aforementioned materials. The foam
structure may, insofar as it can be pyrolysed, also be at
least partially ceramicised.
Another possible way of forming the cavity or space for the
subsequent accommodation of a protective layer is
illustrated in Figs. 18 and 19: a position-retaining layer
26 of wax or plastic is mounted over the positive model 24.
The dental crown 34 is in turn built up over the layer.
After taking of~ the shaped dental crown and removing the
position-retaining layer, this is embedded in a refractory
embedding composition. The wax or plastics material flows
out and bakes ~luring the thermal treatment and the
restoration part is ~inished by impressing or forcing in
glass ceramic or ceramic material and is then optionally
tempered to form or expand crystals contained in the body
of the materia].
In ~he embodiment illustrated in Fig. 19 the positive model
Z4 i'3 already not made of a refractory material, and the
position-retaining layer 26 comprises a refractory
material, for example platinum. The ceramic material of
the cLental crown 34 is built up over this position-
retaining layer 26. The dental crown can be baked without
the positive model 24 only on the position-retaining layer
26. The position-retaining layer 26 is mechanically
removed after the baking. The protective layer 52 is then
applied to the dental crown 34 as described hereinbefore
with reference to Fig. 5.
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Fig. 20 and 2i show another possible way o~ making an
oversized wor~ing model 32.
The casting Cllp 28 contains a casting material 30 whose
volume shrink~, on hardening. Using this material a
negative impression is made from the positive model 24.
Fig. 21 shows the situation a~ter the casting material 30
has harden2d. An intermediate space 66 that predetermines
the geometry of a protective layer to be subsequently
~orrned on the crown can be recognised between the internal
surface of the nega~ive impression and the external surface
of the positive model 24. The casting cup is provided with
peri-orations or coatings so that the material forms a
shrink fit~
The casting cup 28 is shaped and dimensioned so that the
thickness of the intermediate space 66 formed by shrinkage
is smaller in the sections adjacent to the preparation
boundaries than in those regions adjacent to the occlusal
surface of the positive model 24.
If the negative impression shown in Fig. 21 is cast using a
refractory laminating composition, then the working model
32 shown in Fig. 22 is obtained, which compared to the
positive model 24, whose contours are shown by the dotted
lines, is overdimensioned corresponding to the intermediate
space 66. The working model 32 according to Fig. 22 can
then be further used in a similar way to the working model
illustrated in Fig. 4.
Alternatively a dimensionally accurate negative impression
can be made and the working model 32 can be produced using
an expanding modelling composition. Such modelling
compositions are ~or example materials whose volume is
increased by crystallisation during heat treatment.
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- A further possible way of producing a working model, which
however is not shown in the drawings, is to make a negative
impression of the positive model 24, similarly to Fig. 3,
though without: providing a position-retaining layer.
Starting ~rom this negative impression a working model is
then produced using a special modelling composition. This
modelling composition has the property that it can become
solid and ~irm to such an extent as soon as it comes into
contact with the negative impression that it can be handled
and retains its geometry. The material of the working
model can then. be expanded by a subsequent heat treatment,
crystallisatic~n, or by a chemical treatment. This
expansion may for example be a residual part of a foaming
process. The oversize working model is then used in the
same way as described above with reference to Fig. 4ff.
In a further modification of the invention part of the
overdimensioning of the working model can be produced by
making a negative impression of the positive model 24 using
a shrinking modelling composition, as described above, and
filling the resultant oversize negative impression with a
modelling composition that expands according to the shape.
Particularly large oversizes can be achieved in this way.
2S In principle it is also possible to form a model of the
restoration part directly on the tooth stump 10 in the
patient's mouth, at least as regards the internal surface
36. In this case a release agent is then preferably
applied to the tooth stump 10. Material can then be
remo~ed by machining from the prehardened blank of the
restoration part in order to create a space for the
protective layer 52. If desired, instead of removing this
material a position-retaining layer 26 can also be built up
over the tooth stump 10, the ~urther procedure being as
described above. As an alternative or in addition, an
oversize working model can also be produced by removing
material chemically or chemically and mechanically from the
CA 02212210 1997-08-04
negative impression. If such a removal is to be performed
di~ferently in different surface regions of the negative
impression, the latter can be coated with a light-sensitive
protective lacquer that prevents material being removed
chemically or chemically and mechanically from the
corresponding sites remaining after the development.
In such cases in which the protective layer 52 comprises a
foam material, the position-retaining layer 26 can be made
~0 from this material to start with and then combined with the
ceramic material.
Such foam materials can also be obtained in the form of
plastically deformable sheets-, which can then be adapted to
the model stump by bending and pressing.
Suitable foam materiais, especiaiiy aiuminium oxide ceramic
foam materials, are however also commercially available for
protective layers, and are ideally suited to cutting or
machining. In this case a position-retaining layer part or
a protective layer part to be integrated subsequently into
the ceramic material can then be produced in the manner
described in mo:re detall hereinafter with reference to
Fig. 23:
Fig. 23 shows diagrammatically a device for producing an
oversize workinq model starting from a positive model 24
secur,_d to a sp:indle 68 driven by an electric gear motor
70. An angle setting device 72 is provided to determine
the spindle setiing.
The gear motor 70 rests on a carriage 74 that runs on a
guide rail 76 and is driven by a threaded spindle 78. A
gear motor 80 in turn drives the spindle. A further angle
setting device ~32 serves to determine the angular position
of the threaded spindle 78 and thus the axial position of
the carriage 74~ The gear motor 80 is arranged on a plate
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33
84 that is mounted on a stationary frame part 86 of the
device.
A first linear setting device 88 mounted on a frame part 90
of the device frame cooperates with the circumferential
surface of the positive model 24. A further linear setting
device 92 arrar.~ged on a carriage 94 that runs on a guide
rail 96 and is driven by a threaded spindle 98 determines
the geometry of the front face of the positive model 24. A
drive motor 100 operates the threaded spindle. The radial
setting of the linear setting device 92 relative to the
axis of the spindle 68 is measured by an angle setting
device 102 that cooperates with the threaded spindle 98.
The output signals from the various linear and angle
setti.ng devices are coupled to interfaces of a process
control computer 104 that produces a digital image of the
surfa.ce of the positive model 24 from the various position
signa.ls obtained on scanning the surface of the positive
model 24 and stores this image on a bulk storage facility
106, for example a hard disk. The various programs used to
control the process computer are also stored on
the latter, including on the one hand a program that
generates the digital image of the positive model 24, and
on the other hand a program that generates an oversize
contour of the surface of the positive model 24 that is
displaced in a defined manner, and thus produces a digital
image of a position-retaining layer or of the outside of a
protective layer.
The process computer 104 in addition cooperates with a
monitor 108 as well as with a data entry keyboard 110 in
order to receive control commands and to output data and
information on work that has been carried out.
A first spindle drive 112 that operates on a tool 114 that
is particularly suitable for processing and working
CA 022l22l0 l997-08-04
34
external surfaces is controlled by the process computer
104. In addition the process computer 104 controls a
spindle drive 116 that drives a tool 118 particularly
suitable for machining and working internal surfaces. Five
coordinate drives 120, 122, 124, 126 and 128 are controlled
by the process computer 104 so as to move the tool 114 in
the space as necessary to produce the external surface of
the position-retaining layer or protective layer. This
movement control is effected by using the calculated
digital image of these surfaces stored in the bulk storage
106.
In addition the process computer 104 controls five
coordinate drives 130, 132, 134, 136 and 138 that serve to
guide the tool 118 over a surface corresponding to the
digital image of the external contour of the positive model
24, which is likewise stored in the bulk storage 106.
In this way the device illustrated in Fig. 23 can thus in
general produce a dimensionally stable part whose geometry
corresponds to the position-retaining layer 26 or to the
protective layer 52 of the embodiment according to Figs. 1
to 8.
As an alternati~e to the aforedescribed numerically
controlled production of a protective layer part or
position-retaining layer part, these parts can also be
produced in mechanical copying and grinding processes, the
model being gea:red down to the tool workpiece. Some semi-
finished products, for example semi-finished products
consisting o~ ~oam, can also often be directly shaped and
~ormed by hand.
Fig. 24 diagrammatically illustrates the production o~ a
truncated conical protective layer part 142 starting from a
; block-shaped blank 140 of ceramic-foam material, this
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material having smaller pore sizes in the bottom half of
the block than in the upper half of the block.
The protective layer part 142 is then chemically treated or
coated in order to produce a better bonding with the
infiltrating material, different sections o~ the protective
layer part bein~ able to be treated differently. Such a
treatment may a~ain for example comprise a silanisation.
The ceramic dental crown can then be built up over the
protective layer part 142. This can be effected in such a
way t]~at the ceramic material at least partially
in~iltrates the protective layer material. One can then
furth-e~ infiltrate the protective layer part from the
inside with plastic material or metal, or optionally at
least partially pyrolyse organosilicon materials
(infiltrated materials), as already described hereinbe~ore.
The part produced under numerical control by the device
accorcling to Fig. 23 can however also be used as a
position-retaining part. In this case a~ter finishing the
dental crown 34 the foam material is then removed in order
to create free space for the protective layer 52. If a
pyrolysable material is used this can be effected by heat
treatment, while if a thermostable foam material is used
this can be effe~ted by an at least partial removal of the
position-retaining layer part. After the complete
removal an internal surface of the dental crown remains
that has a very irregular and rough shape according to the
pores of the removed foam material, similar to the
situation shown in Fig. 8.
The position-retaining layer can be completely removed for
example by chemical dissolution or by sandblasting the
inner surface 36 of the dental crown 34. As described
above, the inner sur~ace of the dental crown 34 is then
preferably tribochemically surface-coated or etched ancl
CA 02212210 1997-08-04
36
silanised before the protective layer material is
introduced into the mould space formed between the dental
crown and positive model.
5 In those cases in which the part produced with the device
according to Fig. 23 is to act as a position-retaining
layer part that is subsequently completely removed ~rom the
dental crown 34, the measurement and dimensioning of the
positive model ~eed not be carried out with great accuracy.
It is simply sufficient if the external and internal
surfaces of the position-retaining part only roughly follow
the contours of the preparation surface. If need be, it is
sufficient to carry out only a rough analysis of the
positive model t:o speci~y the external and internal
surfaces, which can also be carried out by hand or
optically (measurements made ~rom a video picture),
following which the most suitable match in each case is
found from a predetermined set of standard surfaces.
It is clear that: a non-cutting and machineless treatment
can a]so be considered instead of a machining or cutting-
type treatment: with foam materials a shaping and forming
can also be accomplished by variable local compression.
Material can also be removed by using ultrasound erosion
equipment or spark erosion equipment. Finally, some foam
materials can also be shaped and formed by chemical-
mechanical treatment, for example by local treatment with
acid. All these procedures can be carried out under
numerical control in a similar way to the mechanical
machining processes described above. It is clear too that
the matching and shaping operation can also be carried out
by hand by machining, using a suitable hand-held tool.
In the case where a self-supporting position-retaining part
or protective layer part is to be produced from a very
light or unstable foam material, the latter can temporarily
be infiltrated with a stabilising composition, for example
CA 02212210 1997-08-04
37
a wax or a subsequently removable plastic, for the shaping
and forming operation. In the case where a protective
layer part is involved the stabilising composition is then
removed, before building up the ceramic crown, from the
shaped ~oam part to at least the same extent as the ceramic
composition is to infiltrate the said foam part. The
extent of removal of the stabilising composition can be
predetermined by for example the selected temperature
range. In the case of a position-retaining layer part the
reinforcing composition can remain in the foam material
until the position-retaining layer is removed. If a
silicone is used as reinforcing material, this can in turn
be partially removed by pyrolysis.
The above embodiments relate to dental crowns. It should
be understood that larger tooth restoration parts, which in
turn are provided in fissure-threatened regions with a
protective layer, can also be manufactured by the
aforedescribed method. The manufacture of a three-part
bridge will be described hereinbelow by way of example.
In Fig. 25 a blank for a bridge-bridge framework/protective
layer part 142 is identified by the reference numeral 140.
This composite part serves as a filler material, as a base
or model for building up the restoration material by
infiltration and overcoating, and also as a protection
against fissure formation and propagation. The part is
made from an aluminium oxide-ceramic foam material. The
size and geometry of the blank 140 are dimensioned so that
it can equally ~e used for commonly available one-part
bridges.
An oversize working model 144, which is shown in Fig. 26,
is made in a similar way to that described above with
reference to Figs. 1-4, using a positive model of the tooth
gap (alveolar saddle) and the two adjacent anchoring tooth
stumps (bridge ~?illars). The blank 140 is machined and cut
CA 02212210 1997-08-04
38
by hand until the protective layer part 142 shown in Fig.
26 is obtained, which abuts in a positive locking manner
the tooth stum~s 146, 148 of the working model 144 and
whose underside runs at a predetermined distance over the
saddle 150 o~ the missing tooth. The bridge framework thus
receives the shape, optionally reduced in size in surface
sections, of the subsequent bridge intermediate member that
~ogether with the adjacent bridge anchors is individually
formed, baked and worked up as regards shape, colour,
translucency, tooth position and sur~ace by infiltration
and/or overcoating with ceramic materials.
Alternatively, the protective layer part 142 may be matched
and adapted to the working model 144 by operating under
manual guidance of the tool 114.
The ceramic material o~ the one-part bridge 152 is then
built up over the refractory working model 144 and around
the protective layer part 158, as illustrated in Fig. 27.
There then remains between the underside of the ceramic
material and the top side of the working model 144 in the
transition region between the middle bridge segment and
right-hand bridge segment associated with the premolar
bridge pillar, an intermediate space 162 for the top side
of the working model 144, as can be seen in more detail in
Figs. 27 and 28.
After baking the ceramic material the bridge part 152 is
slipped over the positive model 156 of the restoration
region, which differs from the working model 144 on the
tooth stumps 158, 160 by virtue of the position-retaining
layer. Mould spaces are again obtained between the left-
hand bridge segment and the tooth stump 158 and the right-
hand bridge segment and the tooth stump 160, which are
similarly filled with plastic material as described in
detail above with reference to Fig. 5, protective layers
162, 164 thereby being obtained. Part of this plastic
CA 02212210 1997-08-04
39
material penetrates into the intermediate space 154 shown
in more detail in Fig. 28 and thus provides improved
protection against ~issure formation in the transition
region between the middle and right-hand bridge segments by
virtue of an additional protective layer 166, shown here
outlined more heavily. The corresponding section of the
finished bridges 152 is shown on an enlarged scale in Fig.
30.
In the transition region between the left-hand and the
middle bridge segments on the other hand the ceramic
material extends as far as the underside of the bridge.
The-saddle segment of the positive model 156 is not a
casting mould or other type of mould directly involving a
material to be shaped, but is an aid by means of which the
dental technician can shape and form the underside of the
bridge. In this connection the technician maintains the
distances, visible in the drawing, between the underside of
the bridge and the top side of the saddle segment.
Fig. 32 shows the incorporated bridge on the tooth stumps
170, 172. The bridge is secured to the tooth stumps by
jointing material layers 174, 176, for example cement
layers.
In the aforedescribed bridge stress fractures are avoided
in the Iarge ceramic voIume of the middIe bridge segment
(through the protective layer part 142) and in the vicinity
of the securement surfaces on the tooth stumps 170, 172,
through the protective layers 162, 164. A special
reinforcement was provided in the severely stressed
transition region between the middle and right-hand bridge
segment associated with the premolar bridge pillar
(protective layer 166).
CA 02212210 1997-08-04
In the case o~ bilaterally severely stressed bridges such
an additional protective layer, corresponding to the
protective layer 166, can also be provided in the
transition region between the left-hand and middle bridge
segments.
Conversely, in the case of less severely stressed bridges
an additlonal protective layer 166 can be omitted in the
transition region between the middle bridge segment and the
edge bridge segments.
In the aforedescribed ceramic tooth restoration parts the
protective layer was provided either in the interior of the
ceramic volume (middle bridge segment of the bridge
according to Fig. 29) or on cup-shaped internal surfaces of
crown parts or crown-like bridge segments. In the case o~
other tooth restoration parts, where the particularly
fissure-susceptible material regions run differently, the
protective layers are also arranged correspondingly
differently. As an example of such another type of tooth
restoration part, Fig. 33 shows an inlay made from a dental
ceramic (similar procedure to onlay, overlay and partial
crown). This inlay is firmly inserted into a recess
(cavity) 178 extending from the biting surface of a molar,
using a cement layer 180. An approximately wedge-shaped
depression 182 is made in the mod longitudinal fissure,
which i9 filled with a protective layer material. The
protective layer 184 thus freely extends into the biting
surface and is there shaped and formed corresponding to the
desired fissure relief.
Preferably at least part of the mod longitudinal fissure of
the inlay, overlay or partial crown are provided with the
protective layer.
In the case of ,-rowns protective layers may be provided for
example externally (additionally) either in regions of the
CA 022l22l0 l997-08-04
41
fissures (side teeth and front teeth) or externally (for
example in the palatinal region of front teeth).
The aforedescribed examples of tooth restoration parts
provided with protective layers were intended to illustrate
the incorporation on natural tooth stumps. It is obvious
of course that such protective layers are also useful for
tooth restoration parts that are secured on artificial
tooth stumps, for example implanted tooth stumps. Since
implant tooth stumps as a rule have specially made joining
surfaces ~or bonding for example to stump structures or
other superstructures, the position-retaining layer parts
or protective layer parts can be prepared as standard
parts; a special shaping and forming of at least the
internal surface of these parts is then unnecessary, and
the external surface has to be shaped if necessary
according to the individual circumstances. The invention
has been illustrated and described above with reference to
tooth restoration parts. It is clear however that other
medical and dental ceramic implants or prostheses can also
be ma:nufactured according to the aforedescribed principle.
These are characterised by the fact that they are lighter
than conventional implants and may have a more compact
geometry. The :Lnvention can in particular also be used for
skeletal implants.
In the case of implants protective layers may also be
formed as a damping element against the background of an
elastic deformation, for example to inactivate problematic
torsional moments acting in each case on the longitudinal
axis of the imp]ant. For example, an internal hexagon or
external hexagon of the implant may frequently be connected
to a complementary protective layer of a restoration part
so that the combination at the same time performs a damping
function.
CA 022l22l0 l997-08-04
42
Fig. 34 shows diagrammatically a transverse section through
the joining site between a diagrammatically illustrated
implant 186 and a prosthesis part 188 joined thereto.
An internal hexagon 190 is provided at the end o~ the
implant, while the prosthesis part 188 has an external
hexagon 192. ~ protective layer 194 iS arranged over the
external hexagon 192, which layer on account o~ its
elastici~y permits slight rotational movements between the
internal hexagon 190 and the external hexagon 192, such
relat:ive movements being heavily damped by the internal
damping exertecl by the protective layer 194 .
