Abstract:
A blank is provided for producing a dental prosthesis (tooth crown) comprising a mechanically processable material block and a holder connected thereto for clamping in an automatic processing tool. Said block is provided with a subgingival anatomic implant connecting part which is protrusively arranged thereon and in which an implant fixture for fixing it to the implant head is formed. The holder is arranged on the surface of the block arrangement side and the implant fixture to a surface on the implant side, thereby making it possible to work the blank by means of a computer-controlled conventional tool. A threaded channel which is embodied in the centre of the block in a parallel direction with respect to the surface on the fixation side, the angular orientation of the mastication surface of the tooth crown with respect to the occlusion vertical and the subgingival anatomic implant connecting part make it possible to fix the prosthetic element (tooth crown) directly to the implant without an abutment and with correct orientation in the row of teeth.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is in the field of dental technology. In particular it relates to a blank and a system for producing a dental restoration by subtractive machining, a method for producing a blank and a method for producing a dental restoration by subtractive machining. 
       RELATED PRIOR ART 
       [0002]    To produce dental restorations, it is known that blanks in the form of a block can be machined subtractively and the contour of a tooth to be replaced can thereby be imported to them. For anchoring the dental restoration in the jaw, an implant is introduced into the patient&#39;s jawbone. After a healing phase, the shape of the tooth to be replaced and its position with respect to the implant are determined, after which the dental restoration can be produced. After the dental restoration has been produced, it can be attached to the implant and thereby to the patient&#39;s jaw. 
         [0003]    Examples of blanks in the form of blocks that can be machined subtractively to produce dental restorations are known from the patent applications EP 000001506745 A1, WO002005016171 A1 and U.S. Ser. No. 02/011,0065065 A1. 
         [0004]    In general it is desirable for dental restorations to have a high strength and a natural appearance. A high strength reduces the risk of breakage and reduces micro-movements, which can have a negative effect on the long life of the dental restoration. In addition, it is desirable to be able to produce dental restorations quickly and in a cost-efficient process. For example, if the dental restoration were produced during a treatment appointment and inserted into the patient, the time required for the patient would be greatly reduced in comparison with a treatment that is performed over a period of two or more appointments. 
         [0005]    Hard materials such as lithium disilicate have the general disadvantage for use as dental ceramics that it is very difficult to machine them subtractively. Subtractive machining of these materials is time-consuming and does not preserve the material well. Wear on the machining tools is also comparatively high. 
         [0006]    One example of subtractive production of a dental restoration made of lithium disilicate with a high strength is described in European Patent EP 1505041 A1, wherein a blank is first produced in the form of a block made of lithium metasilicates, which has a comparatively low strength and therefore can be machined very well subtractively. After the subtractive machining, the lithium metasilicate can be converted by a thermal process into lithium disilicate, which has a high strength. A dental restoration having a high strength can be produced by subtractive machining in this way. According to the manufacturer&#39;s recommendation, lithium disilicate is processed in the translucency stages LT (low translucency) and HT (high translucency) for in the application of fully anatomical restorations. This has the disadvantage that crowns with thick walls based on implant adhesives, for example, have an unnaturally high translucency. Because of this high translucency, the color of the dental restoration does not resemble that of a natural tooth, but instead these dental restorations made of lithium disilicate may have an unnatural appearance. 
       SUMMARY OF THE INVENTION 
       [0007]    The object of the present invention is to provide a blank, a system and a method which will make it possible to finish dental restorations with a high strength and a natural appearance more rapidly than is possible in the prior art. 
         [0008]    The blank according to the invention comprises a structural part and a block part. The block part is connected to the exterior of the structural part in a force-locking manner. In one embodiment, the block part comprises or is made of lithium metasilicate, which is highly suitable for subtractive machining because of its comparatively low strength. The desired shape of the dental restoration, for example, the shape of a lateral tooth, may be imparted to the block part of the blank by subtractive machining. Subtractive machining can be performed in a known way, for example, using computer-controlled CAD/CAM methods. Because of the force-locking connection, the cohesion between the structural part and the block part is not impaired during subtractive machining. This makes it possible for the block part to be held via the structural part during the subtractive machining, for example. 
         [0009]    The structural part comprises zirconium dioxide (ZrO 2 ) and/or titanium. These materials have a very high flexural strength and a very high modulus of elasticity, which impart a very high rigidity and stability to the dental restoration. Micro-movements of the dental restoration implanted in the patient are therefore reduced, so that the dental restoration based on the blank according to the invention has a very long life and is reliable. Furthermore, damage to the restoration such as separation of veneers is prevented. 
         [0010]    Another advantage resulting from the use of the aforementioned materials is a reduced risk of breakage of the dental restoration. This property is important with dental restorations on implants in particular, because they do not have a natural anchoring in the jawbone and therefore the patient lacks the natural reflex arc for his dental restoration. With natural teeth, this reflex prevents the patient from biting down further when he bites on a hard object, for example, a stone, which therefore protects natural teeth from damage. This natural protective mechanism does not exist for dental restorations on implants, so their strength becomes very important. 
         [0011]    The initial strength of lithium disilicate is in the range of approximately 360-400 MPa and the flexural strength of zirconium dioxide (ZrO 2 ), for example, is more than 900 MPa. Based on these properties, the combination of these materials is excellent for reducing the risk of damage with dental restorations and increasing their lifetime. 
         [0012]    To enable the dental restoration produced from the blank according to the invention to be anchored to the patient&#39;s bone, the structural part has an internal contour that corresponds essentially to an external contour of an implant base. The implant base may comprise, for example, a connecting piece, which may be connected to an implant that has healed in the patient&#39;s jawbone. Alternatively, the implant base may also be a section of an implant, which is connected in one piece to the implant anchored in the patient&#39;s jawbone. Since the internal contour of the structural part and the external contour of the implant base essentially correspond to one another, the dental restoration can be placed upon the implant base so that it fits accurately and can be connected to and is in flush contact with it. 
         [0013]    The blank according to the invention offers the great advantage that it can be worked quite well by subtractive machining in a first process step because of the lithium metasilicate, which is contained in the block part and has a comparatively low strength. In a second process step, the lithium metasilicate can be converted into stronger lithium disilicate by a thermal process. Based on the strength of the lithium disilicate, direct subtractive machining thereof would be far less gentle on the material and would also last longer. Furthermore, the wear on the abrasive elements would be extremely high, so that the machining would be uneconomical on the whole or would even be impossible when using small grinding tools. 
         [0014]    However, lithium disilicate has the disadvantage that it has a very high translucency, so that a dental restoration using only lithium disilicate would, to some extent, have an unnatural appearance. Because of the opaque structural part, this disadvantage is not manifested in a dental restoration made of the blank according to the invention because the dental restoration is translucent only at the surface, namely as far as the more opaque structural part and therefore has an appearance very similar to that of a natural tooth (dentin-enamel structure of a natural tooth). The color of the dental restoration can be adapted to the color of a natural tooth through the color of the structural part and also the color of the block part. The subtractive machining and the conversion of lithium metasilicate to lithium disilicate may take place relatively rapidly, so that the production of the dental restoration from the blank according to the invention and the attachment of the dental restoration to the patient can be done in a single treatment session. 
         [0015]    Instead of being made of lithium metasilicate, the block part may also be made of a glass ceramic and/or a glass ceramic precursor. A glass ceramic is an inorganic nonmetallic material in which one or more crystalline phases are surrounded by a glass phase. The term “glass ceramic precursor” in the present disclosure is understood to refer to any material that can be converted to a glass ceramic by a heat treatment. Such a glass ceramic precursor is often porous, and the thermal treatment of the porous precursor will typically have features of a sintering process, in particular including compaction of the material. Another aspect of the heat treatment is a transition of the glass phase to a fine-grained crystalline structure. Herein, it is important that the glass ceramic precursor is not as strong and therefore can be processed more easily than the finished glass ceramic. It should also be pointed out that the “glass ceramic precursor” could in many cases itself be referred to as a glass ceramic, but with a reduced strength in comparison with the condition after the heat treatment, i.e., in the finished dental restoration. 
         [0016]    Instead of the structural part, which can be attached to an implant base, the blank according to the invention may also comprise a structure-connecting part. The structure-connecting part comprises zirconium dioxide (ZrO 2 ) and has a connecting geometry. With the help of the connecting geometry, the structure-connecting part and thus the dental restoration may be attached to an implant. 
         [0017]    The structure-connecting part corresponds to a one-piece combination of a structural part and an implant base. The structure-connecting part may therefore comprise features, which, in the present description, are described only with respect to the implant base and/or the structural part. 
         [0018]    The aforementioned force-locking connection between the block part and the structural part is preferably stable at temperatures up to at least 800° C., especially preferably up to at least 840° C., and in particular up to at least 850° C. The aforementioned force-locking connection is preferably stable at temperatures up to at least the temperature at which at least partial conversion, preferably a complete conversion, from lithium metasilicate to lithium disilicate begins to be possible. Because of the thermal stability of the compound, the force-locking connection between the structural part and the block part is ensured even at high temperatures, so that the aforementioned compound persists even during or after the conversion of lithium metasilicate to lithium disilicate. However, thermal stability of the compound is not absolutely essential because this compound can be replaced by another compound, as described further below, without any loss of the force-locking effect. 
         [0019]    The block part is preferably sintered to the structural part with the help of a joining material, in particular a silicate ceramic material. In addition to sintering, the joining material may also serve to adjust the color of the dental restoration, in particular if the structural part contains titanium. To do so, the joining material is selected in a suitable color, so that the resulting color of the dental restoration corresponds to the color of a natural tooth. 
         [0020]    The joining material preferably has a transformation temperature at which the block part enters into the aforementioned bond with the structural part in sintering with the help of the joining material. The transformation temperature is preferably &lt;850° C. and especially preferably ≦840° C. and/or preferably ≧350° C. and especially preferably ≧400° C. Because of the aforementioned temperature ranges, the structural part can be joined to the block part by a sintered bond in a force-locking manner without complete conversion of the lithium metasilicate to lithium disilicate, for example, so that the blank remains well suitable for subtractive machining. 
         [0021]    In an alternative embodiment of the blank according to the invention, the block part is glued to the structural part with the help of a hybrid material. The hybrid material comprises an organic adhesive and a sintering material. The organic adhesive permits a force-locking connection of the structural part and the block part at comparatively low temperatures, so that the cohesion of the block part and the structural part during subtractive machining is ensured by the adhesive material. The sintering material is suitable for sintering the block part to the structural part at temperatures of &lt;850° C. The adhesive bond can therefore be converted to a sintered bond by thermal treatment after subtractive machining. 
         [0022]    The structural part preferably has a lateral wall thickness d 1  of ≦2.5 mm and especially preferably of ≦2 mm and/or a lateral wall thickness d 1  of ≧0.3 mm, especially preferably of ≧0.5 mm. The outside diameter of the structural part is preferably ≦7 mm and especially preferably ≦6 mm and/or preferably ≧2 mm and especially preferably ≧3 mm. It has been found that very good strength values are obtained within these ranges and that the tooth color and the surface translucency can be adjusted as desired. 
         [0023]    Since more space is usually available for the dental restoration in the occlusal direction than in the lateral direction, the wall thickness in the occlusal direction is preferably chosen to be somewhat thicker than that in the lateral direction. The occlusal wall thickness d 0  of the structural part is preferably ≦3 mm, especially preferably ≦2.5 mm and in particular ≦2 mm and/or preferably ≧0.3 mm, especially preferably ≧0.5 mm and in particular ≧0.7 mm. The occlusal wall thickness d 0  may also vary with the position of the dental restoration: for front teeth it is preferably ≧0.3 mm and for side teeth it is preferably ≧0.5 mm. 
         [0024]    In an advantageous embodiment of the blank according to the invention, the internal contour of the structural part is not rotationally symmetrical with the occlusal axis A o  of the blank. The dental restoration can therefore be attached to the implant base in only one position and is secured against twisting. 
         [0025]    In another advantageous embodiment, the internal contour of the structural part has at least one indentation. The indentation has the advantage that in adhesive bonding of the dental restoration to the implant base, adhesive material can penetrate into the indentation. Therefore the strength of the adhesive bond is increased and the risk of separation of the dental restoration in the occlusal direction in particular is reduced. 
         [0026]    The indentation may be designed to be partial or it may also be designed to be circular on the internal contour of the structural part. 
         [0027]    In another advantageous embodiment, the blank according to the invention includes a channel to receive a plug screw. If the implant base is an abutment that is not connected in one piece to the implant, then the dental restoration can be screw-connected to the implant with the help of an implant screw. After the implant screw has been tightened with a tool through the channel, the channel can be sealed with the help of the plug screw. Alternatively or additionally the plug screw may also serve to fasten the dental restoration onto the implant base (abutment or part of the implant). 
         [0028]    In addition to a blank according to one of the embodiments described above, the invention comprises a system for producing a dental restoration. The system comprises a blank according to the invention having a channel, a plug screw and an implant base. The implant base has an external contour, which corresponds essentially to the aforementioned internal contour of the structural part of the blank according to the invention, so that the blank can be applied to the implant base with an accurate fit. In addition, the implant base contains a screw channel to receive the plug screw, into which the plug screw can be screwed for closing the channel. 
         [0029]    The plug screw preferably comprises a threaded section and a plug section. The threaded section can be screwed into the screw channel of the implant base and the plug section can close the aforementioned channel in the blank according to the invention after being screwed in. 
         [0030]    In a further embodiment, the plug section does not have any thread and/or the sectional diameter of the plug section is greater than the sectional diameter of the threaded part. 
         [0031]    In one alternative embodiment of the system according to the invention, the plug section has threads and/or the sectional diameter of the plug section corresponds essentially to the sectional diameter of the threaded part. 
         [0032]    In addition, the invention relates to a method for producing a blank according to any one of the embodiments described above. This method comprises sintering of a block part on a structural part, wherein the block part comprises lithium metasilicate or a glass ceramic precursor. The sintering is performed by using a temperature profile in which lithium metasilicate is not converted or at least not converted completely to lithium disilicate and/or in which the glass ceramic precursor has not yet been converted completely to a glass ceramic, or at least has not been converted to a glass ceramic having the final strength desired for the finished dental restoration. It is therefore possible to produce a blank which, because of the lithium metasilicate and/or the glass ceramic precursor, can be worked very well by subtractive machining. After conversion of lithium metasilicate to lithium disilicate, which has a higher strength and is sintered in a force-locking manner to the structural part, which has flexural strength, or after the glass ceramic precursor has been converted to the glass ceramic for the finished dental restoration, the dental restoration has a high strength and stability. Despite the lithium disilicate or the glass ceramic, the dental restoration has a natural appearance, namely being translucent at the surface and more opaque at a depth (imitation of the enamel-dentin structure of a natural tooth), because of the opaque structural part and/or the joining material. 
         [0033]    The sintering is preferably performed with the help of a joining material, in particular a silicate ceramic material, e.g., IPS e.max CAD Crystall./Connect (Ivoclar). 
         [0034]    Sintering is performed at a first temperature, which is preferably ≧300° C., especially preferably ≧400° C. and/or preferably &lt;850° C., especially preferably ≦840° C. The duration of the sintering process is preferably ≧10 min, especially preferably ≧20 min and in particular ≧30 min and/or preferably ≦120 min, especially preferably ≦90 min and in particular ≦40 min. Because of the use of these temperatures and times, the lithium metasilicate is not converted completely to lithium disilicate and the block part is nevertheless bonded to the structure in a force-locking manner. 
         [0035]    In an alternative embodiment of the method according to the invention, the block part is bonded to the structural part with the help of a hybrid material. The hybrid material comprises an organic adhesive material and a sintering material that is suitable for sintering the block part to the structural part at a temperature of &lt;850° C. 
         [0036]    The method according to the invention also preferably comprises a subtractive machining of the block part, which is at least partially made of lithium metasilicate and/or a glass ceramic precursor, and heating the subtractively machined blank. The lithium metasilicate of the machined block part is at least partially converted to lithium disilicate due to this heating, or the glass ceramic precursor is converted to a glass ceramic, whose strength exceeds the strength of the glass ceramic precursor. 
         [0037]    After bonding the block part and the structural part to the hybrid material, the method according to the invention preferably also includes a heating step. During the heating, the block part is sintered to the structural part with the help of the aforementioned sintering material of the hybrid material. This heating step can be carried out together with or separately from the aforementioned heating to at least partially convert lithium metasilicate to lithium disilicate or to at least partially convert the glass ceramic precursor to the finished glass ceramic. In doing so, the adhesive bond is replaced or supplemented by a sintered compound. When the adhesive bond is replaced or supplemented, the aforementioned force-locking effect itself is not even temporarily lost. 
         [0038]    To convert lithium metasilicate at least partially to lithium disilicate, the heating is performed at a second temperature, which is preferably ≧800° C., especially preferably ≧820° C., and in particular ≧830° C. and/or preferably ≦950° C., especially preferably ≦900° C. and in particular ≦850° C. The duration of this heating is preferably ≧10 min, especially preferably ≧20 min and in particular ≧30 min and/or preferably ≦120 min, especially preferably ≦90 min and in particular ≦40 min. These temperatures are high enough to convert lithium metasilicate to lithium disilicate, but are not high enough to cause the lithium disilicate to become plastic, so that the shape would change and/or the subtractively machined block part would melt. If a glass ceramic is used for the block part, the temperatures may be adjusted accordingly to convert the glass ceramic precursor into a glass ceramic having the desired strength. 
         [0039]    For subtractive machining of the block part, the blank is preferably held via the structural part. For example, the blank for subtractive machining may be connected to the machine connecting part by means of the structural part with the help of an adhesive bond and/or a screw connection using the plug screw. Alternatively, the blank may also be connected to the implant base for the subtractive processing and may be clamped in the machine by means of this implant base. Since the implant base is preferably metallic, for example, made of titanium, the connection to the implant base must be released before the lithium metasilicate is converted to lithium disilicate and/or the glass ceramic precursor is converted to the finished glass ceramic. Otherwise the implant base would undergo oxidation when heated. The subsequent removal of the oxide would result in an inaccurate fit when the implant base is attached to the implant. In the alternative use of a ceramic implant base, the heating and thus a non-releasable connection (e.g., a sintered connection) are possible. 
         [0040]    In an advantageous embodiment of the method according to the invention, the subtractive machining of the block part also comprises subtractive machining of the plug screw. The blank according to the invention together with the plug screw, for example, is therefore screwed onto the holding device. Next the block part of the blank can be machined subtractively. In doing so, at the same time the plug part of the plug screw is machined subtractively and adapted to the contour of the dental restoration. Next, the external contour of the plug screw may also be provided with a geometric shape, for example, a slot or a cross, which makes it possible to screw the threaded section into the screw channel of the implant base using a tool. 
         [0041]    In an advantageous embodiment of the method according to the invention, the structural part is connected to the implant base, in particular by adhesive bonding and/or screw connection with the help of the plug screw. The structural part can also be connected to the implant base, in particular to a ceramic implant base, e.g., made of ZrO 2 , by sintering. Depending on whether the connection is by adhesive bonding or sintering, the implant base is an implant adhesive base or an implant sintered base, respectively. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0042]    Additional advantages and features of the invention are derived from the following description, which explains preferred exemplary embodiments in greater detail with reference to the accompanying drawings, in which: 
           [0043]      FIG. 1  shows a first embodiment of the blank according to the invention with a plug screw, 
           [0044]      FIG. 2  shows a second embodiment of the blank according to the invention with a plug screw, 
           [0045]      FIG. 3  shows a third embodiment of the blank according to the invention with a plug screw, and 
           [0046]      FIG. 4  shows an embodiment of the system according to the invention, which is screw-connected to an implant, 
           [0047]      FIG. 5  shows a fourth embodiment of the blank according to the invention with a plug screw, and 
           [0048]      FIG. 6  shows a fifth embodiment of the blank according to the invention with a plug screw. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0049]      FIG. 1  shows a vertical section through a blank  10  according to the invention, comprising a block part  12  and a structural part  14 .  FIG. 1  also shows a plug screw  16 , comprising a threaded section  18  and a plug section  20 . The blank  10  contains a channel  22 , which is plugged by the plug section  20  of the plug screw  16 . The structural part  14  has an internal contour  24 , which comprises two indentations  26 . The plug screw  16  comprises a slot  28  to allow it to be screw-connected using a tool. 
         [0050]    The block part  12  comprises lithium metasilicate and is connected in a force-locking manner to the exterior side of the structural part  14 . Instead of the lithium metasilicate, the block part  12  may also comprise and/or consist of a glass ceramic precursor, which can be converted by a heat treatment into a glass ceramic having a strength suitable for use in a dental restoration. This variant is not described explicitly below, but it is to be understood that all the advantages and features described below shall also be disclosed in conjunction with this variant. The internal contour  24  of the structural part  14  corresponds essentially to an external contour of an implant base (not shown). A dental restoration produced from the blank  10  may therefore be placed on the external contour of the implant base (not shown) and attached to the patient&#39;s jaw by means of the implant base.  FIG. 1  shows that the internal contour  24  of the structural part  14  is not rotationally symmetrical with the occlusal axis A o  of the blank  10 . The dental restoration produced from the blank  10  can therefore be attached to the implant base (not shown) in only one position and cannot be twisted with respect to this position. The dental restoration produced from the blank  10  can be attached to the implant base (not shown) by means of an adhesive bond, for example, and/or by means of a screw connection with the help of the plug screw  16  over the internal contour  24  of the structural part  14 . Furthermore, the dental restoration can also be sintered to the implant base. The sintered connection preferably uses a ceramic implant base to prevent oxidation. In adhesive bonding, adhesive material can penetrate into the indentations  26 . This therefore increases the stability of the adhesive bond and in particular reduces the risk that the dental restoration will be shifted along the occlusal axis A o  relative to the implant base or released from it. 
         [0051]    With the plug screw  16  shown in  FIG. 1 , the sectional diameter of the plug section  20  is larger than the sectional diameter of the threaded section  18 . Since the diameter of the channel  22  is smaller at the lower end than the sectional diameter of the plug section  20  of the plug screw  16 , the blank  10  and/or the dental restoration produced from it can be screw-connected to the implant base (not shown) with the help of the plug screw  16 . When the plug screw  16  is tightened, the dental restoration is pulled toward the implant base in the direction of the occlusal axis A o . The plug screw  16  may thus also have a fastening function in addition to the plug function in combination with the blank  10  according to the invention. 
         [0052]      FIG. 2  shows another embodiment of the blank  10 ′ according to the invention, which can be used with the plug screw  16 ′ shown herein. In the plug screw  16 ′, the sectional diameter of the plug section  20 ′ and the sectional diameter of the threaded section  18 ′ are essentially the same, so that the plug screw  16 ′ in combination with the blank  10 ′ has merely a plug function. As shown in  FIG. 2 , the structural part  14 ′ has an outside diameter d G , an occlusal wall thickness d o  and a lateral wall thickness d 1 . The embodiment of the blank  10 ′ shown in  FIG. 2  has no indentations on the internal contour  24 ′ of the structural part  14 ′. 
         [0053]      FIG. 3  shows another embodiment of the blank  10 ″ according to the invention, in which the channel  22 ″ has threads (in contrast with the blanks  10  and  10 ′ from  FIG. 1  and  FIG. 2 ). The dental restoration produced from the blank  10 ″ can be used with a plug screw  16 ″ with which the sectional diameter of the threaded section  18 ″ and the sectional diameter of the plug section  20 ″ are essentially the same, and in which both the threaded section  18 ″ and the plug section  20 ″ have threads. The indentations  26 ″ on the internal contour  24 ″ of the structural part  14 ″ and the threads on the plug section  20 ″ prevent the dental restoration, which is screwed and/or glued onto an implant base (not shown), from being removable from the implant base in the channel  22 ″ by pulling in the direction of the occlusal axis A o . 
         [0054]    As shown in  FIG. 6 , the structural part  14 ″″ may also extend completely through the block part  12 ″″—instead of extending only partially up to the channel  22 ″″. In  FIG. 6 , the channel  22 ″″ is bordered in the lateral direction by the structural part  14 ″″—and not by the block part  12 ″″. The internal contour  24 ″″, which corresponds essentially to the external contour of an implant base (not shown), does not comprise the contour of the structural part  14 ″″, which defines the channel  22 ″″. 
         [0055]      FIG. 4  shows a system  30  according to the invention, comprising a blank  10 , a plug screw  16  and an implant base  32 . The system  30  is screw-connected to an implant  36  with the help of an implant screw  34 . The implant base  32  comprises a screw channel  38  and a connecting geometry  40 . The connecting geometry  40  of the implant base  32  can be inserted into the implant  36  with an accurate fit and preferably has an anti-rotational geometry such as a hexagon, an octagon, etc., so that the system  30  cannot be rotated against the implant  36  in a screw connection to the implant  36 . 
         [0056]    It is described below on the basis of  FIG. 4  as an example how, with the help of the present invention, a missing tooth can be replaced by a dental restoration having the tooth contour  42  in a patient. 
         [0057]    First, instead of the missing tooth, the implant  36  is implanted in the patient&#39;s jawbone. After a healing phase which is necessary in order for the implant  36  to be able to heal in place in the patient&#39;s jawbone, the treatment is continued. With the help of the present invention, during a single treatment session, the dental restoration can be produced from the blank  10  according to the invention and can be attached to the implant  36  after the implant  36  has healed in place. 
         [0058]    In this treatment session, the tooth contour  42  of the dental restoration to be produced is first determined by a known method (for example, scanning, CAD construction). Next the tooth contour  42  is transferred to the block part  12  of the blank  10  by subtractive machining, for example, with the help of a computer-controlled CAD/CAM method. To do so, the blank  10  may be accommodated in the machine by means of a machine connecting part (not shown). The machine connecting part has a section (not shown) with an external contour (not shown), which corresponds essentially to the internal contour  24  of the structural part  14 . For subtractive machining, the blank  10  may be screw-connected to the machine connecting part with the help of the plug screw  16 , for example. Another possibility consists of releasable adhesive bonding of the structural part  14  of the blank  10  to the machine connecting part. In both cases, it is of great practical importance that the structural part  14  of the blank is already connected to the block part  12  in a force-locking manner, even before the subtractive machining. The machine connecting part preferably comprises a screw channel (not shown), into which the plug screw  16  can be screwed. In the screwed-in state, the channel  22  of the blank  10  is plugged or closed by the plug section  20  of the plug screw  16 . 
         [0059]    After being chucked in the machine tool, the block part  12  is machined subtractively, wherein the plug section  20  of the plug screw  16  is preferably also machined subtractively, so that the desired tooth contour  42  is also created in the position of the channel  22 . After subtractive machining of the plug section  20 , a geometry is preferably created in the plug section  20 , for example, a slot, a cross slot, a hexagon socket head (Allen wrench head), Torx, etc. Therefore, the plug screw  16  can also be unscrewed from the screw channel by means of a tool and/or subsequently screwed into the screw channel  38  in the implant base  32 . 
         [0060]    Since the block part  12  of the blank  10  comprises lithium metasilicate, subtractive machining can be performed in a very time-efficient manner that saves on material. The wear on the machining tools is much lower than with harder materials, so that cost savings are possible. The waiting time for the patient is shortened because of the rapid subtractive machining, and this facilitates the treatment in one appointment. 
         [0061]    The force-locking connection between the block part  12  and the structural part  14  makes it possible to accommodate the blank  10  for subtractive machining by means of the structural part  14  in the machining tool. This has the advantage that the block part  12  can be machined completely and there are no regions for example, because of fastening in the machine tool that are excluded from the subtractive machining and have to be remachined later. The dentist and/or dental technician also typically could not produce the force-locking connection himself in his practice because this would include, for example, presintering, which must be carried out by skilled workers using special equipment. If the structural part were not connected to the block part  12  in a force-locking manner even before the subtractive machining, this connection would have to be performed at another location after the machining of the block part  12 , so the dental restoration could not be produced and inserted within one treatment session. The cost and effort of such a sintered joint after subtractive machining are very high, and they require technical dental knowledge and skills and are therefore unsuitable for “chairside use” (=use in a dental practice). 
         [0062]    After subtractive machining, the machined blank  10  is exposed to the process conditions already described in order to at least partially convert the lithium metasilicate of the machine block part  12  into lithium disilicate. The aforementioned machine connecting part can be released from the structural part  14  before or after this conversion step. 
         [0063]    The blank  10  may optionally be cast in plastic (e.g., polyurethane) for subtractive machining, in which case, after the subtractive machining, the restoration is held by retaining webs on the remainder of the blank  10  that has been machined out and is thus held in the plastic embedding. 
         [0064]    Alternatively, the blank  10  for the subtractive machining can also be held in the machining tool via the implant base  32 . Since the implant base  32  preferably comprises a metal, it must be released from the structural part  14  of the machined blank  10  before the of transformation process, because the metal of the implant base  32  would otherwise already be oxidized during the transformation process. The removal of the oxide would lead to an inaccurate fit in a subsequent connection to the implant  36 , which would be undesirable. However, the implant base  32  need not necessarily be metallic but instead may also be ceramic. In this case, the implant base  32  can be exposed to the process conditions of the transformation process without oxidizing. The connection between the implant base  32  and the structural part  14  after the transformation thus can remain in existence. 
         [0065]    After the transformation, the dental restoration based on the lithium disilicate and the structural part both have a high stability and strength. The grayish appearance of lithium disilicate typically occurring when the wall thickness is large, which is due to a comparatively high translucency can be corrected by the opaque structural part  14  and/or the joining material, by means of which the block part  12  is connected to the structural part  14 . The dental restoration is therefore translucent only at the surface and therefore resembles the appearance of a natural tooth. If the structural part  14  includes zirconium dioxide, then the desired tooth color can be adjusted very well by adjusting the color of the structural part  14 . The glaze and stain firing can be performed separately or in combination with the crystallization firing (conversion of lithium metasilicate to lithium disilicate). 
         [0066]    After the transformation, the dental restoration may be attached to the implant that has healed in place. To do so, on the one hand, the implant base  32  is screw-connected to the implant  36  with the help of the implant screw  34  and, on the other hand, the structural part  14  is attached to the implant base  32 . 
         [0067]    The structural part  14  and the implant base  32  can be joined with the help of the plug screw  16  for example, after the screw connection of the implant base  32  with the help of the implant screw  34 . Alternatively or additionally, the implant base and the structural part  14  may be adhesively bonded or sintered. If the implant base  32  consists of an abutment, the adhesive bonding is preferably performed before the screw connection with the help of the implant screw  34  because it can then be performed outside of the patient&#39;s oral cavity. To improve the stability of the adhesive bond, the internal contour  24  of the structural part  14  and/or the external contour of the implant base  32  may have indentations  26 . These indentations  26  may be designed to be partially or completely circular. The indentation profile may be designed in various geometric shapes, for example, as a semicircle, a semioval, a rectangle, etc. 
         [0068]    After the machined blank  10  that has been converted to lithium disilicate has been joined to the implant base  32  and the implant base  32  has been screw-connected to the implant  36  with the help of the implant screw  34 , the plug screw  16  can be screwed into the screw channel  38  of the implant base  32 . In the screwed-in state the subtractively machined plug part  20  of the plug screw  16  closes the channel  22 . The external contour of the machined plug section  20  corresponds to the respective section of the desired tooth contour  42 . It is not necessary to fill the channel  22  with a filling material to close it. 
         [0069]    It is pointed out that the implant base need not necessarily represent a part that is separate from the implant—as described with reference to  FIG. 4 . Alternatively, the implant base may also be designed in one piece with the implant, in particular forming an implant section. 
         [0070]    The embodiment of the blank  10 ′″ shown in  FIG. 5  comprises a structure-connecting part  44 , which combines the functions of the implant base and the structural part. As described in the preceding description for the structural part, the structure-connecting part  44  may also be connected to the block part  12 ′″ of the blank  10 ′″ from  FIG. 5 . The structure-connecting part comprises a connecting geometry  40  with which a dental restoration produced from the blank  10 ′″ can be connected to an implant  36  ( FIG. 4 ). 
         [0071]    Although preferred exemplary embodiments are shown and described in detail in the drawings and in the preceding description, these should be regarded merely as examples of the invention and not restrictively. It should be pointed out that only the preferred exemplary embodiments are depicted and described and all the changes and modifications that currently and in the future will lie within the scope of protection of the invention should be protected. The features shown here may be important in any combinations. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           10 ,  10 ′,  10 ″,  10 ′″,  10 ″″ blank 
           12 ,  12 ′,  12 ′″,  12 ″″ block part 
           14 ,  14 ′,  14 ″,  14 ″″ structural part 
           16 ,  16 ′,  16 ″,  16 ′″,  16 ″″ plug screw 
           18 ,  18 ′,  18 ″ threaded section 
           20 ,  20 ′,  20 ″,  20 ′″,  20 ″″ plug section 
           22 ,  22 ″ channel 
           24 ,  24 ′,  24 ″,  24 ″″ internal contour 
           26 ,  26 ″ indentations 
           28  slot 
           30  system 
           32  implant base 
           34  implant screw 
           36  implant 
           38  screw channel 
           40  connecting geometry 
           42  tooth contour 
           44  structure-connecting part