Background of the Invention When replacing a portion of a damaged tooth using a dental crown, for example. a tooth preparation is first of all ground and formed from the damaged tooth. This tooth preparation is in the form of an upstanding tooth stud on and over which a crown is placed. The crown comprises a porcelain cap fused to a metal core. The metal core can then be cemented onto the stud. This existing type of crown has a number of problems.

Currently. the porcelain cap is bonded to the metal cap by the following process. An oxide layer or surface is initially formed on the metal core bv heating the core until the metal core's surface oxidises. The oxide layer which is formed is irregular in size and shape and may include raised particles. Heat fusible porcelain is then coated on the oxide layer. The core and porcelain are heated and the porcelain melts and wets the oxide surface and creates a fused porcelain layer over the crown which resembles and is functionable as. a tooth. However. the bond between the porcelain and the metal core via the metal oxide is weak. and this creates problems particularly if the porcelain is of a substantial thickness. Thus, when making crowns, the porcelain thickness has to be very carefully controlled ensuring that the metal core is coated with a very even thickness of porcelain throughout to prevent uneven shrinkage of the porcelain on cooling. Uneven shrinkage would stress the weak oxide bond and cause breaking of the bonds, or cracking of the porcelain.

Because of this need for careful control of porcelain thickness, crowns formed using this process are less translucent than natural tooth due to the metal core located behind the porcelain. (The metal core reflects incident light and makes the tooth look dark) In order to make the porcelain resemble natural tooth. the porcelain needs to be thicker, particularly on the incisal edges and occlusal surfaces of the tooth where ideally, the thickness of the porcelain should be three or more times that of the porcelain covering the sides of the crown.

A further problem arises because once the crown has been made and coated with porcelain and fused, the core cannot be cut or ground without damaging the porcelain outer layer. Often when a crown which has been manufactured for a patient in a dental laboratory is sent to the patient's dental surgery for a fitting, the crown does not fit the tooth stud in a satisfactory manner and adjustment is necessarv. Instead of shaping the tooth stud to fit the core. it is common for the core to be adjusted by cutting and/or grinding. The heat caused by the grinding and cutting tends to cause the metal core to expand. However, the porcelain bonded to the oxide layer does not expand to the same extent as it has a lesser coefficient of thermal expansion than the metal core. This results in the formation of micro-cracks at the junction of the oxide bond radiating out towards the external surface of the porcelain layer. Although the crown may appear satisfactory following the cutting/grinding the micro-cracks eventually tend propogate and to increase in size and. in time, cause failure of the crown.

The above problem also prevents the satisfactory manufacture of preformed crown. bridges and the like since pre-formed crowns cannot be worked on after they have been made.

One other problem with fitting crowns is the problem of the marginal fit of crowns and bridges where the crown or bridge does not perfectly fit its respective tooth stud.

The problem of marginal fit occurs due to the process of manufacturing crowns and the fact that attempts to perfect the marginal fit are done indirectly. That is to say, a perfectly manufactured crown margin is done not directly onto the patient's tooth, but onto a replica of the patient's tooth. The replica of the patient's tooth, inevitably inclues an inbuilt sum of errors compared to the original tooth. Therefore, all attempts to fabricate a perfect margin for the replica of the patient's tooth. even if they are a perfect margin fit on the replica will include errors since the replica is not a perfect model of the patient's tooth. Of course, in practice, the margin for that replica of the patient's tooth includes an error factor which tends to compound the original error and thus, all prefabricated margins that are made indirectly in the manner described above, result in a margin which is deficient in fit.

It is an object of the present invention to alleviate at least some of the above mentioned disadvantages of the prior art discussed above.

Summary of the Invention In its first broad aspect, the present invention provides a dental prosthesis such as a crown or the like. comprising a metal core which is suitable for fixing over and to a tooth stud or the like, a layer of mesh which is fixed to the metal core and an outer layer of fused material typically porcelain. forming an outer body functional as a tooth. and which is mechanicallv bonded to the mesh.

An advantage of the present invention, is that the porcelain or other material which forms the tooth surface, is not bonded to a layer of oxide but is mechanically bonded to the mesh. This provides a very strong bond between the porcelain and the mesh. rather than the existing weak bond between the porcelain and the oxide layer.

The mesh is typically formed from metal with a fine stainless steel being the preferred material.

An important advantage of the present invention is that it allows the metal core to be made from titanium or a titanium alloy. Titanium is a metal which is very inert and is consequently highly compatible and well accepted by the human body in preference almost all other metals, including gold.

However, porcelain does not bond well with titanium. Existing systems for forming a porcelain tooth body on a titanium core have been unreliable and consequently titanium is not widely used for forming dental crowns and the like.

The improvement in the bonding of the porcelain to the mesh. which can expand and contact relative to the metal core. enables the porcelain outer body to be made of uneven thickness without causing cracking of the porcelain. This allows the formation of crowns having more translucent incisor edges and occlusal surfaces. Without a metal core behind the porcelain at the critical areas, the colour of the porcelain becomes more translucent. This results in crowns which are more aesthetically pleasing in appearance.

A further advantage is the improved bonding of between the porcelain outer body and the mesh, is that the metal core can be cut and ground without the porcelain layer cracking and flaking off the metal core.

In a related aspect, the invention provides a method of making a dental prothesis such as a crown or the like comprising the steps of: a. forming a metal core :

b. bonding a mesh to the metal core ; c. coating the metal core with fusible porcelain or the like: and d. heating the core and the porcelain to fuse the porcelain or the like to the mesh to form an outer body functionable as a tooth or part thereof.

Typically, the mesh is bonded to the core by welding.

The fact that the metal core of the crown can now be ground and cut, enables the mass production of porcelain crowns. This has the advantage of reducing the cost of crowns and improving their quality. It can also reduce the number of visits necessary by a patient to obtain a crown and also reduces the requirement for dentists and dental laboratory to retain expensive equipment for manufacturing custom built crowns. The invention also allows bridges to be made from pre-formed components.

In a preferred embodiment, crowns are formed which include projecting wings. Those wings can be inserted into channels in a pontic bridge tooth and retained there using filler material or the like. The wings can be adjusted to suit the patient's mouth without damaging the porcelain coating on the crown.

The present invention can also be applied to preformed crowns and pontic bridges.

In a vet further aspect of the present invention. there is provided a method of improving the margin of fit between a crown and the tooth margins of a stud by interposing a compressible layer of gold between the crown and the stud around the margins (lower side walls) of the stud and burnishing the gold against the margin between the crown and tooth stud.

Typically, the compressible gold layer is formed from a very thin sheet of gold. of less than 10 microns thick which is crinkled.

Brief Description of the Drawings Specific embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a schematic section through a crown; Figure 2 is a schematic side view of a pontic bridge : Figure 3 is a schematic plan view of the pontic bridge on lines III-III shown in Figure 2 :

Figure 4 is a schematic section through a crown illustrating a gold margin : Figure 5 is a cross-section on lines IV-IV of Figure 4: and Figure 5a is an enlarged view of a crinkled gold foil.

Detailed Description of the Preferred Embodiment Referring to the drawings. Figure 1 shows a abutment crown for an incisor tooth. As is well known in the art. a projection or tooth stud is formed from the remains of the patient's damaged incisor tooth which is to be crowned. A metal core 12 is constructed to fit over the tooth stud. The metal core can be formed from any suitable inert metal. However. the preferred material is titanium or a titanium alloy as it is inert and has a high degree of acceptability by the human body. Titanium or stainless steel mesh 14 is then placed over the core 12 and welded to the core. A layer of fusible porcelain is then coated over the core in varying thicknesses. In particular there is a greater thickness of porcelain at the top 18 of the tooth where the incisor's cutting edge is to be defined. The porcelain is then heated to fuse it to the mesh and to form an outer body which simulates and functions like a tooth.

As discussed in the introductory portion of the specification, because the porcelain is bonded to the mesh, a very strong mechanical bond is provided between the porcelain and the mesh and this allows the crown to be formed with an outer bodv 16 of porcelain which is of uneven thickness. As can be seen. there is no metal core behind the porcelain in the upper part 18 of the crown at the incisor edge. thus the porcelain becomes highlv translucent and closely resembles a natural tooth in appearance.

Figure 2 shows a pontic bridge assembly. The bridge assembly has been formed from three pre-formed components-abutment crowns such as 20 and 22 and pontic bridge tooth 24. The pontic bridge tooth 24 replaces a missing tooth and is supported between the two adjacent crowns 20 and 22.

The crowns 20 and 22 define wing posts which project from the sides of the crowns and are welded to the core. The pontic bridge tooth 24 has a channel 28 which extends through the centre of the tooth. As is best seen in Figure 3. the sides of the channel converge outwards so that the channel is widest at the centre of the pontic bridge tooth. The wing posts which also flare outwardlv as is best seen in Figure 3. are then inserted into the channel and a composite filling used to stabilise and lock the wing posts in the channel.

Figures 4 and 5 illustrate a yet further embodiment of the invention which addresses the problems of marginal adaptation when applying the crown 50. which may be a crown of the type described above, to a tooth stud 52. The tooth stud is a smooth upstanding block formed on the remains of the patient's tooth which has a flat top and side walls. The lower visible parts of the side walls between the gum line 53 up to about mid-way towards the top. define the tooth margin which extends around the tooth stud 54.

With reference to Figure 5, the present invention involves the use of gold foil which is crinkled so that it is compressible in the margin. An enlarged view of the foil is being shown in Figure 5a. The foil is placed around the tooth margin. fixed to the tooth stud using a laver of glue or adhesive 58. Adhesive (not show) is also applied to the lower part of the interior of the crown 50 which contacts the outside of the layer of gold foil 56 and the crown is applied over the tooth stud compressing the gold margin into a thin layer and creating a very tight fitting contact between the margin of the crown and tooth stud. Since pure gold is very soft, it compresses to form a perfect fit between the crown and the margin of the tooth stud and the lower part of the gold foil 59 which projects beyond the base of the crown 50 can be burnished using an ultrasonic scaler 60 or the like which will burnish and remove excess gold to provide a good finish.

The yellow gold will also have the effect of blocking any dark shadow effect coming from the dark metal core of the tooth stud.

Instead of crinkled gold foil. a layer of anv compressible gold such as spongiform gold could be used.

It will be appreciated by persons skilled in the art that numerus variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.