Abstract:
A near net tooth shaped ceramic prosthesis is provided in a tooth shape to minimize the amount of cutting and baking required to finish the outer crown portion of the dental prosthesis. A metallic core is provided for attachment to an implant in a patient&#39;s mouth. A ceramic crown is then attached to the core.

Description:
This is a divisional of U.S. application Ser. No. 09/855,136, filed on May 14, 2001, invented by Robert L. Riley, and entitled “Near Net Tooth Shaped Ceramic Crown.” 
    
    
     BACKGROUND 
     This disclosure relates to prefabricated ceramic crowns supported on dental implants. 
     One shortcoming of conventional dental implant restorations is that metal abutments can disrupt the translucence of the porcelain used to fabricate the crown. Dental abutments are typically made from titanium or other biocompatible metals. These metals are most often metallic gray in color and hence can have aesthetic disadvantages in dental restorations. In some instances, the abutment can be visible through the gingival tissue and present a grayish color in the transgingival region of the patient. Visibility of the abutment is greatly undesired, especially in the anterior region of the mouth where aesthetics have a crucial importance. In other instances, the tissue and bone surrounding the coronal end of the implant can recede. A portion of the abutment can be exposed and reveal a grayish color in the mouth of the patient. 
     Conventional dental implant restorations have other shortcomings as well. For example, much time and effort are needed to shape ceramic crowns to have a natural tooth-like configuration. Ceramic crowns currently on the market require the dental laboratory either to add or remove a substantial amount of material to the crowns to bring them to the approximate shape of a tooth. Material is added by baking porcelain to the ceramic surface. Many times, multiple layers of porcelain need to be added to achieve a natural shape and color. Each layer must be baked onto the crown before the next layer can be added. Multiple baking cycles can adversely affect the underlying ceramic substrate. 
     As another disadvantage, it is difficult to remove material from a hardened ceramic crown; and ceramics typically are very difficult to cut due to their hardness. Special water cooled diamond tools need to be used for such cuts. The stress associated with cutting the crown can also create microscopic fractures in the ceramic that weaken the material and make it susceptible to fatigue and ultimately failure. 
     Ceramic crowns are sold with various geometric shapes. One dental company sells crowns with a cone-shaped ceramic cylinder having an internal metal core. The ceramic cylinder tapers outwardly from the coronal end of the implant. The metal core is designed with a hexagon or other anti-rotational shape that is selected to match a mating feature on the implant body. Typically, the ceramic cylinder is large enough so excess material can be cut away to shape the crown. If the crown is not large enough, multiple layers of porcelain are baked to the outer surface to form the desired shape. 
     Other dental companies sell ceramic crowns with a generally cylindrical shape. The cylinder is sized slightly larger than the implant to facilitate cutting the crown to the geometry of a natural tooth. At the apical end of the crown, the ceramic tapers inwardly to the implant diameter. The cylinders include an anti-rotational feature to engage a coronal end of the implant. 
     Still, other companies manufacture ceramic caps that are cement-retained to a metal abutment. The ceramic cap has a cylindrical shape and must be cut to the shape and size of a natural tooth. 
     In light of prior ceramic prosthetic teeth and abutments, a ceramic crown that is initially shaped like a tooth would have many advantages over the prior art. The present invention provides such an advantage and other advantages taught herein. 
     SUMMARY 
     The present invention is directed toward a prosthetic tooth that is manufactured to have a shape and size of a natural human tooth. The prosthetic tooth has an internal metallic core and an external ceramic crown. The core generally has a cylindrical configuration, an internal bore, and one end adapted to connect to a dental implant. The crown is formed from a ceramic that surrounds the outer surface of the core. Most importantly, the crown is manufactured to have an anatomical shape and size of a natural human tooth, such as incisors, molars, premolars, or canines. During a dental restoration, the clinician or laboratory chooses a correctly shaped and sized crown according to the tooth or teeth being restored. 
     A principal advantage of the present invention is the crown has a shape that closely resembles the shape of a natural tooth. This near net tooth shape of the crown will reduce the amount of work, time, and expense required to create a final dental prosthetic restoration. Further, the ceramic used to fabricate this crown is compatible with commercially available porcelains so that the gradients of shade and translucence of the natural tooth can be replicated. Also, the crown may be manufactured to have a size that is slightly smaller than the average natural tooth. This difference in size enables the crown to receive an additional layer of porcelain and then match the exact size of the natural tooth. 
     As another advantage, the prosthetic teeth of the present invention may be manufactured and sold as a kit. Each kit would include a plurality of prosthetic teeth having different sizes and shapes emulating different sizes and shapes of natural human teeth. A clinician could chose a prosthetic tooth to best match particular needs of a patient. 
     The present invention could also be manufactured and sold as a dental implant prosthetic system. This system would include both a dental implant and prosthetic tooth of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional side view illustrating an embodiment of a dental prosthetic assembly according to one embodiment of the invention. 
     FIG. 2 is a front view of a natural human tooth. 
     FIG. 3 is an exploded cross-sectional side view illustrating an embodiment of a dental prosthetic assembly. 
     FIG. 4 is a perspective view illustrating an embodiment of a core member. 
     FIG. 5 is a perspective view illustrating another embodiment of a core member. 
     FIG. 6 is a side-view illustrating an embodiment of a near net tooth shaped crown. 
     FIGS. 7 a - 7   e  are views illustrating a plurality of tooth shapes for the near net tooth shaped crown. 
     FIG. 8 is a cross-sectional side view illustrating an embodiment of a threaded attachment of a near net tooth shaped crown and a core member. 
     FIG. 9 is a cross-sectional side view illustrating another embodiment of a dental prosthetic assembly. 
     FIG. 10 is a cross-sectional view of an embodiment of a core with a tapered outside diameter. 
     FIG. 11 is a cross-sectional side view showing an angled crown of the dental prosthetic assembly. 
     FIG. 12 is a cross-sectional side view showing another embodiment of an angled crown of the dental prosthetic assembly. 
     FIG. 13 is a three dimensional view illustrating an embodiment of a dental bridge with multiple near net crowns and cores. 
     FIG. 14 is a flow diagram showing a method for utilizing the dental prosthetic system of the present invention. 
    
    
     DETAILED DESCRIPTION 
     A dental prosthetic assembly is generally designated  10  in FIG.  1  and includes a tooth-like prosthesis having a near net tooth shaped crown  12  and a metallic core  16 . The crown  12  has an internal bore  14  to receive the core and is manufactured to have a size and shape of a natural human tooth. 
     The core  16  is connected to a jawbone anchor or dental implant  18 . This anchor  18  may be any one of various dental implants known to those skilled in the art, such as an externally threaded Spline implant, Spline cylinder implant, or an externally threaded implant or cylinder implant with an internal hexagonal connection; these implants are manufactured by Sulzer Dental Inc. of California. 
     FIG. 2 shows a natural human tooth with a root portion  2  and a natural crown portion  4 . The crown  12  of FIG. 1 emulates the natural crown  4  of FIG.  2  and not the root portion  2  of the natural tooth. 
     Looking to FIGS. 1 and 3, a threaded fastener or screw  20  may be used to connect the core  16  to the anchor  18 . The fastener includes a first end  20   a  having threads  22  and a second end  20   b  having a polygonal socket  24 . A tool (not shown) can be inserted into socket  24  to turn fastener  20  into threaded engagement with a threaded bore  19  in anchor  18 . Core  16  includes a screw bore  26  and a screw seat  28 . 
     Screw bore  26  includes an axis C that extends substantially co-axially with an axis A of anchor  18 . Fastener  20  is inserted through core  16  and threaded into anchor  18 . When fully seated, a shoulder  17  of second end  20   b  of threaded fastener  20  is seated on screw seat  28  within core  16 . Further, an axis B passes through the near net crown  12  from an incisal edge  12   i  to a cervix  12   k . In FIG. 3, all axes (A, B, and C) are longitudinal and co-axial. 
     The core  16  is preferably formed of a material selected for fatigue strength suitability such as a metal, like titanium or titanium alloy. The metal core can be fabricated with various shapes, such as a cylindrical geometry (shown in FIG. 4) or a frusto-conical geometry (shown in FIG.  5 ). Further, the core may be formed from one piece (as shown in FIG. 4, for example) or formed from two or more pieces. FIG. 1 shows a core formed from two pieces: a core body  16   a  and a core cuff  16   b.    
     Preferably, the core anti-rotationally engages the implant. The anti-rotational engagement between the core and implant may occur with numerous techniques known to those skilled in the art. Some examples of these techniques include male and female polygonal projections or locking tapers. FIGS. 1 and 3 show a spline connection between the core and implant. In this connection, a plurality of splines  16   c  on the core engage a plurality of mating splines  18   a  on anchor  18 . 
     The outer surface of the core may have various textures, coatings, and configurations. FIG. 4, for example, shows core  16  with a textured coating  16   e  on the outer surface. FIG. 5 shows core  16  having a plurality of grooves  16   d . The various textures and coatings can enhance the strength of connection between the core and crown. 
     While ceramics can be strong, they are often brittle. The addition of a metallic core adds strength to the overall assembly. This added strength is especially important at the implant interface where forces are transferred from the restoration to the anchoring implant. 
     Crown  12 , FIG. 6, is formed of an aesthetic suitable material, such as a ceramic material, an unfired ceramic material, a polymer material, or a combination of ceramic and polymer materials. Preferably, the crown is made from a ceramic, such as aluminum oxide, zirconium oxide, or a composite thereof. These materials can be made to have mechanical strength sufficient to support occlusal forces and are relatively inert when exposed to body fluid and tissues. These materials also allow for the addition of porcelain to their surface to provide shading to the unique color of the adjacent natural dentition. A clinician, laboratory, or the like may add a layer of porcelain to the outer surface of the crown to match the aesthetics of adjacent natural teeth. The crown can also be manufactured and sold with a thin layer of porcelain  12   b  already applied to its surface. This latter application facilitates minor modifications to the final prosthetic restoration. 
     In one embodiment, the crown may be manufactured to have a size that is slightly smaller than the average natural tooth. For example, the crown can be manufactured to have an outside surface or outside diameter that is 0.5 mm to 1.5 mm smaller than the natural tooth to be replaced. This difference in size enables the crown to receive an additional layer of porcelain and then match the exact size of the natural tooth. 
     One important advantage of the present invention is that the crown is manufactured to have shapes approximately equal to the natural shapes of human dentition. The crowns, manufactured in these shapes are thus prefabricated and sold to clinicians, laboratories, and the like in the shape of human teeth. Since ceramic materials are typically difficult to shape using machining techniques, the present invention significantly reduces or completely eliminates the amount of machining required to create the shape and size of the final prosthetic restoration. 
     Crown  12  may be provided in a kit to have a plurality of different sizes and shapes that mimic the sizes and shapes of natural human teeth. These shapes, for example, could include tooth shapes such as an incisor  12   c , FIGS. 7 a ,  7   b , a canine  12   d , FIG. 7 c , a premolar  12   e , FIG. 7 d  and a molar  12   f , FIG. 7 e.    
     Crown  12  may be attached to core  16  by various means known to those skilled in the art. In FIG. 1, the bore  14  in the ceramic crown  12  is made slightly larger that the outside diameter of the core  16 . This difference in size creates a cement gap  17 . The cement gap is a space for dental cement that holds the crown to the core. In FIG. 7, an alternative connecting method is shown, a threaded fastener  30 , such as a set screw, is used to attach crown  12  to core  16 . 
     FIGS. 9 and 10 show another embodiment of the present invention and in particular illustrate an alternative way to attach crown  12  to core  16 . A layer of material  29  is provided between the crown and the core. This material is suitable for bonding the two components when the components are heated. This layer of material may be a heat activated adhesive or may be formed from precious metals, such as gold, silver, platinum, palladium, or alloys formed from these metals. 
     In the preferred embodiment, the core is fabricated from gold (or a gold alloy) and then gold (or a gold alloy) is used to bond the core and the crown. Gold is advantageous since it is both strong and biocompatible. Further, dental gold alloys are capable of withstanding higher temperatures than titanium. 
     Preferably, the gold is applied to the inner bore in the crown. The gold core and crown are then connected together, and heat is applied to bond them permanently together. The bonding may occur after an outer layer of porcelain is applied to the crown and subsequently heated or baked to bond the porcelain to the ceramic crown. This latter step often occurs since dental laboratories bake shades of porcelain onto the ceramic crown to match color of natural teeth. The heat during this operation melts or activates the layer of material  29 . After the prosthesis is heated, the porcelain baked, and the crown and core bonded, the prosthesis is ready to be implanted into the jawbone of the patient. As shown in FIGS. 9 and 10, a hole  21  may be left in the crown to provide access to the screw  20 . 
     Gold soldering or a brazing process can be used to join the core to the crown. A dental laboratory, for example, can add the soldering or brazing gold, or the gold can be supplied as a preform coating installed during the manufacturing stage. The preform coating can also be added using an electroplating process that metallizes the surface of the internal bore and bonds the crown and core. 
     In another embodiment of the invention, the crown may be angled to provide proper alignment or angular correction of the prosthesis in the jawbone of the patient. FIGS. 11 and 12 show two different embodiments for angling the crown  12 . In FIG. 11, a central axis C extends downwardly through the core  16  and anchor  18 . An incisal axis B extends through the crown  12  and at an angle to axis C. As shown, crown  12  has two portions: a top coronal portion  50  and a bottom apical portion  52 . The coronal portion  50  is canted with respect to central axis C to provide the noted angulation. 
     Looking now to FIG. 12, the core  16  has two portions: a top or upper portion  54  and a bottom or base portion  56 . The upper portion  54  is angled or tilted relative to the base portion  56 . Further, axis C and axis B show the relative angulation of the core. 
     If it is desired to restore multiple adjacent teeth, an interconnecting bar member  40 , FIG. 13, may be used. For example, a pair of spaced apart anchors  18 ,  18 ′ may be implanted in the jawbone. Each anchor  18 ,  18 ′ includes a respective core  16 ,  16 ′ attached thereto as discussed above. The bar  40  includes opposite terminal ends  42 ,  43  that are each attached to one of the anchors  18 ,  18 ′, respectively. Bar  40  carries another, or mid-position core  16 ″ which is positioned between the cores  16 ,  16 ′ attached to the anchors  18 ,  18 ′. Thus, when the bar  40  is attached to the anchors  18 ,  18 ′, the mid-position core  16 ″ is aligned with the cores  16 ,  16 ′ attached to the anchors  18 ,  18 ′. Crowns  12 ,  12 ′,  12 ″ can then be attached, via their core receiving bores, to a respective one of the cores  16 ,  16 ′,  16 ″ so that a multiple unit bridge is formed of near net tooth shaped crowns. The bar  40  can be formed using a variety of methods including a metal casting, or a ceramic pontic. Several methods including the application of an acrylic or porcelain to form a “ridge lap” can be used to hide the bar and create an aesthetic result. 
     FIG. 14 illustrates a method for utilizing a dental prosthesis of the present invention. The present invention will work with single and multiple restorations and extraction and edentulous sites. For illustration purposes, the method for a single-tooth restoration is discussed. 
     As shown in block  70 , in the first step, the implanting doctor (i.e., dental implantologist) examines the patient and determines the tooth or teeth that need to be replaced. Per block  72 , the next step is to create a surgical template. The template helps to determine the form and position of the tooth replacement. Based on the information from the template, the implanting doctor can then select the proper near net shaped ceramic crown, as shown in block  74 . These near net crowns are manufactured to have a shape of a human natural tooth and then offered for sale to the clinician, laboratory, implanting doctor, or the like. 
     Next, per block  76 , the doctor implants the anchor into the jawbone of the patient using the surgical template as a guide. The present invention will support various implants known to those skilled in the art. Next, per block  78 , the doctor records the position of the dental anchor and surrounding dentition. An impression can be taken or the surgical template can be used to record these positions. Once the information is recorded, it is transferred to a dental laboratory. 
     As shown in block  80 , the dental laboratory uses the recorded information to create a stone model of the anchor and surrounding dentition. At this point, as shown in block  82 , minor corrections (such as a reduction) may be made to the shape of the near net crown. These corrections may be made, for example, to account for differences in position and form between the near net crown and an ideal prosthetic restoration. In the next step, block  84 , the dental laboratory adds porcelain to the surface of the near net crown. The addition of the porcelain helps to achieve the exact dimensions required and helps to achieve the correct color and shading to match adjacent dentition. 
     In block  86 , the laboratory creates a restorative template on the stone model to record the position and orientation of the finished ceramic crown in relation to natural dentition or other anatomical features. In block  88 , the laboratory transfers the finished crowns, cores, and templates to the restorative doctor. 
     In block  90 , the restorative doctor assembles the cores to the coronal end of the implants. A retaining screw may be used to connect a core to an implant. Lastly, in block  92 , a finished ceramic crown is cemented on the end of the core. The restorative template is used to place the crown in the correct position and orientation. 
     Although illustrative embodiments have been shown and described, a wide range of modification, change, and substitution is contemplated in the foregoing disclosure without departing from the scope of the invention.