CA2212210A1 - Tooth restoration or prosthesis part made from ceramic material and a method of manufacturing the same - Google Patents

Abstract

To prevent the formation of fissures in ceramic restoration components (34) and the widening of any existing fissures, it is proposed that surface areas of the tooth restoration component subjected to particular tensile stress should beprovided with a protective layer (52) which is intimately bonded to the ceramic material and preferably applies a tensile stress to it. The protective layer (52) is fully prepared before incorporation of the tooth restoration component and its free surface is complementary to the preparation surface (12) of a tooth stump (10).

Description

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.

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 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 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 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 - 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 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 (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;

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;

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;

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 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.

CA 022l22l0 l997-08-04 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.

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.

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 CA 022l22l0 l997-08-04 - 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--- 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~.

CA 022l22l0 l997-08-04 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.

- 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 - 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.

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 - 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 ~ 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 ~ 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 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 - 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 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.

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 .

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.

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.

- 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 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 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 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 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 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 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 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 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).

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 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 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 .

Claims (40)

claims

1. Tooth restoration part of ceramic material (58) with at least one external surface and with at least one securement surface (36) that can be joined to a prepared surface (12) of a natural or implanted tooth part (10), characterised in that the surfaces and/or surface edge regions subjected to tensile and/or compressive stresses are at least partially joined to a protective layer (52) counteracting fissure formation and/or propagation of cracks in the ceramic material (58).

2. Tooth restoration part according to claim 1, characterised in that the protective layer (52) is elastically or plastically deformable.

3. Tooth restoration part according to claim 1 or 2, characterised in that the protective layer (52) is stable to hydrolysis.

4. Tooth restoration part according to one of claims 1 to 3, characterised in that it constitutes an individual dental crown (34) that exists alone or in combination with the crown, or exists as a double crown or bridge anchor, and its internal surface (36) is at least partially provided with a protective layer (52).

5. Tooth restoration part according to one of claims 1 to 4, characterised in that the protective layer (52) and the ceramic material (58) interlock with one another on their contact surface (36, 56) via complementary surface roughnesses.

6. Tooth restoration part according to one of claims 1 to 5, characterised in that the protective layer (52) comprises a metallic or ceramic foam material.

7. Tooth restoration part according to claim 6, characterised in that the protective layer (52) comprises an aluminium oxide-foam material.

8. Tooth restoration part according to claim 7, characterised in that the aluminium oxide-foam material is a compactly sintered aluminium oxide material.

9. Tooth restoration part according to one of claims 5 to 8, characterised in that the number of pores (63, 64) per centimetre is between 12 and 48, preferably between 20 and 35, and more preferably between 25 and 32.

10. Tooth restoration part according to one of claims 5 to 9, characterised in that the size of the pores (63, 64) increases or decreases in the direction perpendicular to the internal surface (36) of the tooth restoration part (34).

11. Tooth restoration part according to one of claims 5 to 10, characterised in that the ceramic material (58) forming the tooth restoration part (34) fills at least some of the pores (63, 64) of the foam material.

12. Tooth restoration part according to one of claims 5 to 11, characterised in that the protective layer material fills at least some of the pores (63, 64) of the foam material.

13. Tooth restoration part according to one of claims 1 to 10, characterised in that the protective layer (52) consists at least partially of at least one plastic and/or a natural and/or synthetic resin.

14. Tooth restoration part according to claim 13, characterised in that the plastic is a thermoplastic or thermosetting material.

15. Tooth restoration part according to claim 14, characterised in that the plastic is chosen from the groups comprising polysulphones, acrylates, or from organosilicon polymers.

16. Tooth restoration part according to one of claims 13 to 15, characterised in that the plastic is a mixture of different plastics.

17. Tooth restoration part according to one of claims 1 to 16, characterised in that the protective layer (52) consists at least partially of a plastic and/or resin reinforced by particles and/or fibres.

18. Tooth restoration part according to one of claims 1 to 17, characterised in that the thickness of the protective layer (52) is between 0.1 mm and 1.0 mm, preferably between 0.1 mm and 0.6mm, and more preferably between 0.3 mm and 0.6 mm.

19. Process for manufacturing a ceramic tooth restoration part according to one of claims 1 to 18, characterised by the following process steps:

a) Production of a positive model of the prepared surface;

b) Application of a position-retaining layer on those sections of the positive model that correspond to surfaces and/or surface edge zones of the tooth restoration part subjected to tensile or shear stresses;
c) Shaping the ceramic material over the positive model provided with the position-retaining layer and baking the ceramic material, optionally in several superimposed partial layers;
d) Removal of the position-retaining layer;
e) Mounting the baked ceramic moulded part on the positive model;
f) Introduction of a plastically deformable and hardening or hardenable protective layer material into the mould space formed between the baked ceramic mould part and the positive model;
g) Hardening or the protective layer material.

20. Process according to claim 19, characterised in that the position-retaining layer comprises a material stable under baking conditions, especially a platinum alloy.

21. Process according to claim 20, characterised in that the position-retaining layer comprises a metallic or ceramic foam material, and is partially or completely removed after baking the ceramic moulded part and before applying the protective layer.

22. Process according to claim 21, characterised in that the position-retaining layer comprises a metallic or ceramic foam material and remains on the ceramic moulded part.

23. Process according to claim 21 or 22, characterised in that the position-retaining layer or a relief remaining in the surface of the ceramic moulded part after removal of the said layer is infiltrated at least partially with the protective layer material.

24. Process according to one of claims 21 to 23, characterised in that the position-retaining layer is at least partially infiltrated with the ceramic material when the ceramic material is built up over the position-retaining layer.

25. Process for manufacturing a ceramic tooth restoration part according to one of claims 1 to 18, characterised by the following process steps:

a) Production of a positive model of the prepared surface;
b) Application of a position-retaining layer on those sections of the positive model that correspond to surfaces and/or surface edge regions of the restoration part that are subjected to tensile or shear stresses;
c) Preparation of a doublet or copy of the positive model provided with the position-retaining layer;
d) Shaping the ceramic material over the doublet of the positive model and baking the ceramic material, optionally in several superimposed partial layers;
e) Removal of the position-retaining layer from the positive model;
f) Mounting of the ceramic moulded part on the positive model;
g) Introduction of a plastically deformable and hardening or hardenable protective layer material into the mould space formed between the ceramic moulded part and the positive model;
h) Hardening of the protective layer material.

260 Process according to claim 25, characterised in that the position-retaining layer comprises a meltable or soluble material and the removal of the position-retaining layer is carried out under the action of heat or a solvent.

27. Process for manufacturing a ceramic tooth restoration part according to one of claims 1 to 18, characterised by the following process steps:

a) Production of a positive model of the tooth surface;
b) Production of an oversize copy or doublet of the positive model;
c) Shaping of the ceramic material over the doublet of the positive model;
d) Baking the ceramic material;
e) Mounting the ceramic moulded part on the positive model;
f) Introduction of a plastically deformable and hardening or hardenable protective layer material into the space formed by the ceramic moulded part and the positive model;
g) Hardening of the protective layer material.

28. Process according to claim 26, characterised in that to make the oversize doublet a modelling composition is used to form a negative impression of the positive model, the modelling composition undergoing a contraction in volume during its consolidation, and the oversize doublet being formed by casting from the negative mould.

29. Process for manufacturing a ceramic tooth restoration part according to one of claims 1 to 18, characterised by the following process steps:

a) Production of a positive mould of the tooth surface;
b) Shaping of the ceramic material over the positive model;

c) Baking the ceramic material;
d) Removing ceramic material from those sections of the ceramic moulded part corresponding to surfaces and/or surface edge regions of the ceramic tooth restoration part subjected to tensile or shear stresses;
e) Mounting of the ceramic moulded part on the positive model;
f) Introduction of a plastically deformable and hardening or hardenable protective layer material into the space formed between the ceramic moulded part and the positive model;
g) Hardening of the protective layer material.

30. Process for manufacturing a ceramic tooth restoration part according to one of claims 1 to 18, characterised by the following process steps:

a) Production of a positive model of the prepared surface;
b) Measurement of the prepared surface on the positive impression and storage of the details of the measured shape contour;
c) Shaping and forming a protective layer part or position-retaining part under numerical control, its one boundary surface corresponding to the stored surface contour and its second boundary surface corresponding to a second surface contour spaced therefrom and derived from the first surface contour, for example by parallel displacement;
d) Shaping of the ceramic material over the formed protective layer;
e) Baking the ceramic material.

31. Process for manufacturing a ceramic tooth restoration part according to one of claims 1 to 18, characterised by the following process steps:

a) Production of a positive model of the prepared surface;
b) Measurement and dimensioning of the positive model and storage of details of its surface contour;
c) Shaping of ceramic material over a protective layer material part preferably consisting of metallic or ceramic foam material;

and d) Baking the ceramic material;
e) Fabricating an internal surface in the protective layer material part under numerical control according to the stored surface contour details;

or d') Fabrication of an internal surface in the protective layer material under numerical control according to the stored surface contour details;
e') Baking the ceramic material.

32. Process according to claim 30 or 31, characterised in that the pores of the protective layer material consisting of foam material are at least partially infiltrated with a plastic material.

33. Process according to one of claims 19 to 32, characterised in that the surfaces of the ceramic moulded part to be coated with the protective layer material are at least partially enriched with silicate by tribochemical surface treatment or are etched and/or silanised.

34. Process according to one of claims 19 to 33, characterised in that the positive model is provided with a release agent before the mounting of the ceramic moulded part and the shaping of the protective layer material.

35. Process according to one of claims 19 to 34, characterised in that the shaping of the protective layer material in the space defined by the ceramic moulded part and the positive impression is effected by simultaneous action of heat and/or mechanical vibrations, for example ultrasound and/or pressure.

36. Process according to one of claims 19 to 35, characterised in that the protective layer material consists at least partially of organosilicon polymers and is at least partially ceramicised.

37. Medical prosthesis, characterised in that it comprises a foam material volume (52; 142) that is at least partially impregnated with an infiltration material (60, 61; 58; 166).

38. Prosthesis according to claim 37, characterised in that the foam material volume (52; 142) comprises ceramic or metallic foam material.

39. Prosthesis according to claim 37 or 38, characterised in that the infiltration material is a plastic material (60; 166) or a ceramic material (61; 58).

40. Prosthesis according to one of claims 37 to 39, characterised in that the infiltration material is elastically or plastically deformable.