Abstract:
A dental crown consisting essentially of stabilized zirconia. The dental crown is particularly suited for pedodontic and veterinary applications and most preferably for pedodontic applications. The zirconia crown may be formed by one of several manufacturing processes. The formed crown exterior can be polished and fine polished and can be covered, preferably only partially, with a porcelain glaze to produce an aesthetic outer surface mimicking a natural tooth. Also disclosed is a method for providing and placing a dental crown upon a natural tooth.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/568,195 filed on Aug. 7, 2012, which is a continuation-in-part of application Ser. No. 12/506,193 filed on Jul. 20, 2009, which is a continuation-in-part of application Ser. No. 12/228,783 filed on Aug. 14, 2008, which claims benefit under Title 35, United States Code §119(e) of U.S. Provisional Application No. 60/964,912 filed on Aug. 15, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a novel and useful prefabricated crown for pedodontic and veterinary applications. 
       BACKGROUND OF THE INVENTION 
       [0003]    At present, crowns used to repair adult human teeth are custom fabricated according to a mold of a tooth being repaired which is provided by the dental practitioner. The crowns may be formed of stainless steel or other metal clad with porcelain or a ceramic such as zirconia clad with porcelain. Using the mold as a guide, computer software, through a CAD/CAM technology, directs a milling machine to mill a coping or framework of the crown out of the chosen material, such as metal or ceramic, which is eventually clad with porcelain. Although satisfactory in result, the patient receiving such a crown must make multiple visits to the dental office to achieve this result and the process is time-consuming and expensive. The foregoing procedure is particularly difficult when the tooth being replaced is a child&#39;s (baby) tooth which will eventually be lost and replaced by permanent teeth through the natural maturing process. Veterinary crowns are also produced in the same manner and can be just as difficult to produce. 
         [0004]    Given the difficulties of dealing with a child in a dentist&#39;s chair, dentists have developed prefabricated crowns that do not require molds and multiple visits to the dentist. The use of prefabricated, stainless steel crowns for restoring badly broken-down children&#39;s teeth is now the standard of care in pediatric dentistry. Recently, the American Academy of Pediatric Dentists has encouraged crown restorations due to the poor outcomes resulting from the use of fillings in certain high-risk groups, especially children with the inability to cooperate in the dental chair, thereby necessitating the use of general anesthesia for their dental treatment. Prefabricated crowns are a very efficient and reliable restoration resource and are the restoration of choice in teeth with moderate to severe dental decay. 
         [0005]    Stainless steel prefabricated crowns are the most versatile and widely used crowns in pediatric dentistry. However, due to their unaesthetic appearance, many dentists are turning to other manufacturers of preformed pediatric crowns for a more aesthetic option for their patients. In the late 1980&#39;s, the idea of creating a stainless steel crown and covering the visible surface with a tooth-colored composite material veneer started to gain favor. Cheng Crowns (1987), Kinder Krowns (1989), and NuSmile Crowns (1991) are some of the most notable manufacturers to gain significant market share with this revolutionary new crown for pediatric dentistry. Although these new crowns were much more aesthetic than the all-stainless-steel crowns, they came with some significant drawbacks. 
         [0006]    The main benefit of the pre-veneered crowns was their more aesthetic appearance compared to their stainless steel predecessors. This new product, however, required a new method for preparing the tooth and seating the crowns. Because the plastic material was bonded to the metal substructure, it was recommended that these restorations have a passive fit to the tooth, minimizing the potential to crack the facing. Crimping or altering the metal substructure, which dentists were accustomed to doing before seating a crown, is not recommended in order to avoid weakening the bond between metal and facing. However, due to the increased strength of modern cements, this passive fit method has become accepted and works quite well with most luting agents used on the market today. 
         [0007]    However, because of the interface between the metal and the plastic facing, there is also a tendency for the facing material to crack or chip off the metal substructure. The fragile nature of this interface is a major negative. Dentists often crack a facing when seating the crown or are required to re-treat a returning patient because of an unsightly failed restoration caused by the failure of this weakened interface. 
         [0008]    Since the plastic facing must be applied to the front of the tooth over the metal substructure, the facing has a very bulky and bulbous appearance. This is particularly noticeable when the need arises to crown only one anterior tooth. It is very hard to match the contour of the patient&#39;s natural teeth when using these bulky crowns, necessitating a significant reduction of the tooth structure in order to ensure a proper fit which can lead to unnecessary involvement of the pulp chamber and the need to perform a pulpotomy on the tooth. 
         [0009]      FIG. 11  shows a perspective view of one such bulky crown comprising a stainless steel base  80  and polymeric coating  82 . 
         [0010]    Matching tooth color is another big challenge when using current aesthetic pediatric crowns. The nature of the plastic overlaying the metal substructure gives them their nickname of “Chiclets.” This is a word that is used frequently by both dental professionals and parents when describing the appearance of the current crowns available on the market. 
         [0011]    In the course of sizing the crowns on the teeth to ensure proper fit, it is often necessary to sterilize and restock the unused crowns. The use of heat sterilizing techniques weakens the facing and the overall integrity of the crown. Therefore, it is necessary to use a 24-hour cold sterilizing technique on these crowns because of their plastic facing/metal interface. However, the use of the cold sterilizing technique tends to alter the shade of the facing from its original color. Due to this fact, many offices must maintain a separate storage container for crowns that have been sterilized. As the sterilized crowns will often not match those that have never been tried in the mouth, dental offices must stock a larger inventory of crowns which is a major unnecessary disadvantage for the dentist. 
         [0012]    Even with all the potential negatives to these aesthetic crowns, until now they have been the best alternatives for dentists and parents who want a more natural smile for their children. 
         [0013]    Various ceramics have been used in dentistry. Alumina, for example, has been used for implant abutments and crown and bridge frameworks and copings. Alumina has more translucency and better matches the translucency of natural teeth, but it lacks in strength and is more prone to failure. 
         [0014]    Zirconia formulations have been used in adult dentistry for several years as a replacement for metal for the manufacturing of crown coping or frameworks. The copings are typically layered with porcelain to build up the entire structure of the tooth restoration and to develop the aesthetic surface characteristics. Zirconia has also been used for implant abutments and as endosseous implant cylinders. Zirconia is white in color, and extremely strong. Zirconia has been replacing alumina as the framework material of choice due to its strength. The downside of zirconia is that it is very bright white and by itself, does not match the human dentition well. 
         [0015]    Dental crowns and impression systems of various sorts have been proposed in the past. For example, U.S. Pat. No. 6,769,913 describes a device for taking dental impressions. The device includes impression cap having an injection port configured to receive material in an inner cavity. 
         [0016]    U.S. Pat. No. 4,492,579 shows a dental crown substrate formed of a noble-based metal formed over a thin metal foil substrate. A veneering material such as porcelain is then coated over the substrate. 
         [0017]    U.S. Pat. Nos. 4,992,049, 5,314,335, and 5,538,429 teach dental crowns utilizing a base of metallic mesh or stranded material covered by a veneer of porcelain. 
         [0018]    U.S. Pat. Nos. 3,058,216, 3,375,582, 4,392,829, 4,846,718, 5,624,261, 6,106,295 and 7,008,229 describe dental crowns utilizing metal or plastic as a coping which is veneered by porcelain or plastic material. 
         [0019]    U.S. Pat. No. 6,663,390 illustrates a ceramic prosthesis in which a metallic core is provided for attachment to an implant in the patient&#39;s mouth. A ceramic crown of porcelain zirconia, or polymeric material then covers the extending metallic core. 
         [0020]    U.S. Pat. No. 1,609,549 shows a telescopic tooth crown in which the interior of the crown includes a number of indents to aid in the adhesion of the inner and outer shells of the crown. 
         [0021]    U.S. Pat. No. 4,766,704 discloses machining a crown from a ceramic blank. 
         [0022]    United States Published Patent Application No. US2006/0154211 describes a pediatric crown which may be prefabricated in various sizes and shapes for primary dentition. Porcelain has been employed as a outer crown material due to aesthetic appeal. 
         [0023]    U.S. Pat. No. 6,638,069 shows a shaping cap using a matrix material including fillers such as zirconia mixed with silica and titanium dioxide. 
         [0024]    U.S. Pat. No. 5,775,913 shows an acrylic material filled with quartz or silicon dioxide, called Artglass. 
         [0025]    U.S. Pat. No. 6,592,373 shows a crown formed form an injection molded acetal homopolymer. 
         [0026]    U.S. Pat. No. 6,974,323 describes a micromechanically acting retention pattern formed by a laser in a zirconia composition implant abutment and dental restoration. 
         [0027]    WO 2007/046693 describes a method for increasing the bond strength of stabilized zirconia to another substance. 
         [0028]    The disclosures of each of the above references are incorporated by reference herein. 
         [0029]    A pre-fabricated strong crown for pedodontic and veterinary applications and process for making the same would be a notable advance in the dental field. 
       SUMMARY OF THE INVENTION 
       [0030]    In accordance with the present invention, a novel and useful crown for pedodontic and veterinary applications is herein provided, most preferably for pedodontic applications. 
         [0031]    The present invention provides a dental crown composed of substantially pure zirconia (Zr0 2 ) to create a strong crown. The zirconia is stabilized with stabilizers including, but not limited to, magnesium oxide (MgO), calcium oxide (CaO), yttrium oxide (Y 2 O 3 ), cerium oxide (CeO 2 ), aluminum oxide (Al 2 O 3 ), scandium oxide (Sc 2 O 3 ), ytterbium oxide (Yb 2 O 3 ) and hafnium oxide (HfO 2 ). Preferably, the zirconia crown is formed by designing in CAD (Computer Aided Design) followed by milling of sintered stabilized zirconia. Other methods of manufacturing the zirconia crown include electrophoretic deposition, ceramic injection molding or slip casting. 
         [0032]    Various sizes of pedodontic and veterinary crowns may be pre-fabricated for children and animals. 
         [0033]    In addition, the interior surface of the crown preferably includes at least one retention feature which assists in a lasting bond of the crown to the stub of the natural tooth. 
         [0034]    Zirconia possesses a fairly, although not perfect, natural white color. The zirconia crown prepared as described above may be further amplified aesthetically to closely match dental teeth by creating a pearlescent outer surface. Such a result is achieved by pre-polishing the external surface of the cast zirconia crown with a diamond impregnated silicon wheel. A final polish is then accomplished by a brush wheel with the application of a diamond paste prior to placement in a patient. In addition, color modifiers may be added to the zirconia before sintering to modify the color of the zirconium. 
         [0035]    Further, for better tooth appearance, the zirconia crown may be partially coated on the front portion of the crown with a porcelain glazing material. The glazing material presents a more natural tooth appearance. It may be desirable to color the glazing material for an even more natural tooth appearance. 
         [0036]    After the crown has been created, crowns of various sizes may be placed in a kit for use by the dental practitioner. In this regard, the practitioner follows standard preparation guidelines for primary dentition for placement of a pre-fabricated crown. Thus, a pedodontic crown may be installed or placed on a natural tooth in a single visit simply by preparing the tooth stub, and selecting a proper size of a pre-fabricated zirconia crown by its mesiodistal width. The selected crown is then evaluated for fit and possible tissue impingement. A passive fit is preferred where the selected crown may fit loosely on the tooth stub which may not touch the selected crown. Although uncommon to do, any extension of gingival margin is then trimmed where necessary with a fine diamond at relatively slow speed and with copious water spray. The crown is then filled with glass ionomer or self curing composite resin cement which will fill the large gap between the selected crown and the tooth stub. The crown is then seated in the mouth completely. It should be noted that the at least one retention feature in the zirconia crown greatly aids in the fixing of the same to the stub of the natural tooth. Following such seating, the orientation of the crown is checked. The emplaced crown is as hereinabove described mimics a natural tooth in appearance. 
         [0037]    While the present invention is directed to pedodontic and veterinary crowns, and most preferably pedodontic crowns where the present invention offers the greatest advantages, the teachings of the present invention may have applicability to adult crowns. The present crowns may also be used for adults but in this case, the crowns may have a tight fit where the crown fits snugly on the adult tooth stub. There may be a small cement gap that is filled with cement. 
         [0038]    It should be realized that a new and useful dental crown system has been hereinabove described. 
       OBJECTS OF THE INVENTION 
       [0039]    Accordingly, a primary object of the present invention is to provide a pedodontic and veterinary crown which is simple to use and easy to manufacture. 
         [0040]    Another object of the present invention is to provide a pedodontic and veterinary crown in which the crown is essentially formed of stabilized zirconia. 
         [0041]    Another object of the present invention is to provide a pedantic and veterinary crown that is only partially glazed to mimic the natural tooth appearance of a patient. 
         [0042]    Another object of the present invention is to provide a pedodontic and veterinary crown in which a zirconia crown is employed that possesses great strength and durability. 
         [0043]    Yet another object of the present invention is to provide a zirconia dental crown which is suitable for use with front and rear natural teeth. 
         [0044]    Another object of the present invention is to provide a dental crown composed solely of stabilized zirconia whose outer surface has been altered to closely resemble the appearance of a natural tooth. 
         [0045]    A further objective of the present invention is to provide a pedodontic and veterinary dental crown which overcomes the problems of durability, non-uniformity, and the like of prior pedodontic dental crowns. 
         [0046]    Other further objects of the present invention will become apparent from a careful reading of the included drawing figures, the claims and detailed description of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]      FIG. 1  is a side elevational view of the dental crown of the present invention with a directional arrow indicating its installment on a tooth stub. 
           [0048]      FIG. 2  is a top plan view of the dental crown of the present invention. 
           [0049]      FIG. 3  is a sectional view of another embodiment of the dental crown of the present invention. 
           [0050]      FIG. 4  is a block diagram depicting the general process for creating a zirconium crown. 
           [0051]      FIG. 5  is a top plan view of a kit having pedodontic crowns of various sizes which may be employed through a process selection. 
           [0052]      FIGS. 6 and 7  are sectional views of another embodiment of the dental crown showing retention features. 
           [0053]      FIG. 8A  is a side view of a dental crown according to the present invention while  FIG. 8B  is a side view of a dental crown according to the prior art. 
           [0054]      FIG. 9  is a frontal view of natural teeth demonstrating the interproximal line angle. 
           [0055]      FIGS. 10A ,  10 B and  10 C are plan views showing the crowding that can occur with tooth decay and resulting mesiodistal dimension decrease. 
           [0056]      FIG. 11  is a perspective view of a prior crown made of stainless steel with a polymeric coating. 
           [0057]      FIG. 12  is a side view of a dental crown for an anterior tooth according to the present invention. 
           [0058]      FIG. 13  is a cross-sectional view of the dental crown in  FIG. 12  in the direction of arrows  13 - 13 . 
           [0059]      FIG. 14  is a side view of a dental crown for a posterior tooth according to the present invention. 
           [0060]      FIG. 15  is a cross-sectional view of the dental crown in  FIG. 14  in the direction of arrows  15 - 15 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0061]    Various aspects of the present invention will evolve from the following detailed description of the preferred embodiments thereof which should be referenced to the prior described drawings. 
         [0062]    The present invention is directed to pedodontic and veterinary crowns and to a method of providing pedodontic and veterinary crowns for a natural tooth. Pedodontic and veterinary crowns shall be collectively referred to hereafter in the following description as just crowns. 
         [0063]    While the discussion that follows may apply to both pedodontic and veterinary crowns, it should be understood that the present invention is most preferably applicable to pedodontic crowns for restoring baby teeth in patients that are approximately 2 to 10 years old. Baby teeth are temporary teeth which the pedodontic patient eventually loses and are replaced with permanent teeth. 
         [0064]    The ideal crown should have the following characteristics:
       It would be strong and durable;   It would be made of one material to eliminate the problems associated with the fracturing of the aesthetic facing;   It would be thin, like a stainless steel crown, to insure the best emergence profile and overall natural appearance of the tooth. This thinness would also facilitate more conservative preparation of the stump tooth, and therefore reducing chances for nerve damage and increasing retention of the restoration;   It would meet aesthetic scrutiny by having a natural color and surface shine that would mimic adjacent natural teeth;   These crowns would be able to be sterilized with the auto clave for fast turnaround time without the fear of color alteration or compromise of structural integrity;   The method for use would be the same as that used for crowns currently on the market, avoiding the need for dramatic changes in clinical technique; and   The ideal crown would be available in a variety of sizes and at a cost that was competitive in today&#39;s market.       
 
         [0072]    The crowns according to the present invention meet the above characteristics. 
         [0073]    Referring to the Figures in more detail, and particularly referring to  FIGS. 1 and 2 , the invention as a whole is shown in the drawings by reference character  10 . The crown  10  is intended to fit over the stub  12  of a natural tooth and is placed in that position according to directional arrow  14 . Dental crown  10  includes an inner surface  16 , of  FIG. 2 , and an outer surface  18 . Crown  10  is a monolithic structure essentially composed of stabilized zirconia which has been treated to closely follow the color and appearance of a natural tooth. By monolithic, it is meant that crown  10  is only composed of the stabilized zirconia composition and there are no exterior veneer layers of porcelain or plastic. As the crown  10  is sized to fit directly on the patient&#39;s tooth stub, there are no exterior veneer layers or interior layers of any type (other than cement) or interior appliances such as a coping or implant abutment. Further, the crown  10  is formed to be in a net shape for placement on a tooth stub without modification of the dimensions of the crowns  10 , although, while uncommon, it may be necessary to adjust the gingival margin of the crown  10 . When the crown  10  is placed on the patient&#39;s natural tooth using a passive fit (i.e., a very loose fit) for a pedodontic crown, the patient only sees the surface of the stabilized zirconia crown as layering of a plastic or porcelain veneer as shown in the prior art ( FIG. 11 ) does not occur with the present invention. The passive fit should be distinguished from a tight fit when a dental crown may be placed on an appliance such as an implant abutment which requires a very tight and intimate fit between the implant abutment and the dental crown. In this latter case, the dental crown and implant abutment are made as a matched set with an intimate and tight fit and a small, uniform cement gap. 
         [0074]    The stabilized zirconia may consist of 85 to 99.9 weight percent of zirconia and 0.1 to 15 weight percent of stabilizer which may include, but not be limited to, magnesium oxide (MgO), calcium oxide (CaO), yttrium oxide (Y 2 O 3 ), cerium oxide (CeO 2 ), aluminum oxide (Al 2 O 3 ), scandium oxide (Sc 2 O 3 ), ytterbium oxide (Yb 2 O 3 ) and hafnium oxide (HfO 2 ). The presence of the stabilizer is believed to increase the strength of pure zirconia by presenting formation of a monoclinic crystal structure. Included within the foregoing 0.1 to 15 weight percent may be small elements of other compounds such as iron oxide (Fe2O3) or titanium oxide (TiO2) for coloration purposes. 
         [0075]    Inner surface  16 , takes the form of a hollowed out area which is intended to fit the stub  12  of the natural tooth. In addition, the interior surface  16  of crown  10  includes at least one retention feature which assists in holding the crown  10  to the tooth  12 . Such a retention feature is important for the following reason. Once sintered, zirconia has a smooth surface finish. This smooth surface finish is satisfactory for the outer aspect of the restoration which will be polished, however, the inside of the restoration is better if it is not smooth. 
         [0076]    Metal and ceramic restorations are typically microblasted with abrasive particles and air, and ceramic restorations are then additionally hydrofluoric acid etched to further roughen the internal aspect. Roughening the surface allows better mechanical retention of the cement layer. However, sintered zirconia, unlike traditional dental ceramics and porcelains, is too hard to be air abraded and does not etch with hydrofluoric acid. Furthermore, if the zirconia is drilled on by the dentist to add grooves, the heat generated can weaken and fracture the zirconia molecular structure. All manufacturers of zirconia restorations do not recommend drilling on the zirconia for that reason, unless it can be put back into the sintering furnace to heal any fractures. This is impossible in a clinical situation since sintering takes many hours and requires a special furnace of over 1000 degrees centigrade. 
         [0077]    The present inventors have proposed a preferred novel step over milling process which creates macroscopic apertures on the inside of the restoration to aid in retention by locking in the cement layer. The step over milling process can create spirals, grooves, random cuts, indentations or other designs to help improve retention of the cement layer to the restoration. Shown in  FIG. 6  is a cross section of a crown in which grooves  40  are cut into the interior surface  16  while  FIG. 7  is a cross section of a crown in which ledges or short segments  42  are cut into the interior surface  16 . The grooves  40  and short segments  42  should have a width  92  of about 0.05-0.7 mm and a depth of about 0.05-0.4 mm. In addition, the groves  40  and short segments  42  should be spaced apart  94  from each other by about 0.05-0.7 mm. Alternative retention features may include a plurality of holes or pits  20  in the interior surface  16  of the crown  10  which create interstices to accept cementitious material when the crown  10  is eventually placed over tooth stub  12  and allowed to set. Such retention features help to retain the crown  10  on the tooth  12 . 
         [0078]    Since the crowns according to the present invention are not made by a layering process wherein plastic or porcelain veneer is layered over a metal crown, the crowns according to the present invention can be kept thin at the incisal edge. Referring to  FIGS. 8A and 8B , a conventional crown is on the right ( FIG. 8B ) and the inventive crown is on the left ( FIG. 8A ). It can be seen that the incisal edge  88  which is determined by measuring the last 1.5 mm of the biting edge buccolingually (lip side to tongue side of the tooth) across the crown as indicated by  86  is about 2.5-3.5 mm for the prior art crown and is about 0.4-2.0 mm for the crown of the present invention. This indicates that the prior art crown has a more rounded and undefined incisal edge  88  which reduces the cutting efficiency of the tooth. Also important to note is that the crowns of the present invention have a bevel  90  on the lingual side of the tooth which give the crowns of the present invention more of a cutting edge like a natural tooth. The angle β of the bevel should be about 28 to 70 degrees measured from the vertical as shown in  FIG. 8A . 
         [0079]    In an exemplary embodiment, the dental crowns may have an aesthetic glaze that covers only part of the dental crowns. Referring now to  FIG. 12 , there is shown a first dental crown  94  for an anterior tooth that has a front surface  96  (buccal surface) that faces outwardly to the front of the patient&#39;s mouth and a rear surface  98  (lingual surface) that faces toward the patient&#39;s tongue. The front surface  96  may contain a glaze  106  that extends from the biting surface  100  to the margin  102  and wraps about half-way around the crown  94  stopping at  104 . 
         [0080]    A cross section of dental crown  94  in the direction of arrows  13 - 13  is shown in  FIG. 13 . The glaze  106  wraps approximately half-way around the circumference, stopping at  104 . Glazing the rear surface  98  is undesirable as it can cause abrasion of the opposing anterior tooth. 
         [0081]    The dental crown  94  may be polished to have a pearlescent polished surface except where the glaze  106  is present. That is, the pearlescent polished surface may be the rear surface  98  (extending around and stopping at  104 ) while the glaze  106  will be on the front surface  96 . The glaze  106  presents a surface appearance that greatly mimics the natural tooth appearance of a patient. Since it is the front surface  96 —that is, the glaze surface —that the patient and others may see while looking at the patient, the appearance of the dental crown  94  with the glaze  106  presents a more natural, pleasing appearance to the patient and others. 
         [0082]    Referring now to  FIG. 14 , there is shown a second dental crown  108  for a posterior tooth that has a front (buccal) surface  110  and a rear (lingual) surface  112 . The front surface  110  may contain a glaze  114  that extends from a line  116  just below the biting surface  120  to the margin  118  and wraps around the rear surface  112  of the crown  108 . In an embodiment that is not as preferred, the glaze  114  may only wrap half-way around the dental crown. The glaze  114  preferably should not be on the biting surface  120  since it is abrasive and can cause unusual wear on the opposing posterior tooth. 
         [0083]    A cross section of dental crown  108  in the direction of arrows  15 - 15  is shown in  FIG. 15 . The glaze  114  wraps around the entire circumference of the dental crown  108 . 
         [0084]    Again, the dental crown  108  may be polished to have a pearlescent polished surface except where the glaze  114  is present. The pearlescent polished surface ay also include the biting surface  120 . 
         [0085]    The purpose of the glaze  106 ,  114  is for aesthetics only to present a more pleasing appearance for the patient. 
         [0086]    In the prior art embodiments where there is a porcelain or plastic veneer over a ceramic or metal core, the veneer may have a thickness of around 0.5 to 1.5 millimeters and as explained previously, results in a restoration that is not aesthetically pleasing and may result in chipping or spalling of the porcelain or plastic veneer. 
         [0087]    The glaze  106 ,  114  is much thinner, on the order of 0.005 to 0.1 millimeters. More preferably, the thickness range of the glaze  106 ,  114  should be at a minimum 0.02 millimeters to provide sufficient coverage and wear resistance and at a maximum 0.06 millimeters which should provide sufficient wear resistance and aesthetics without being too thick. With the glaze  106 ,  114  being so thin, the shape of the crowns  94 ,  108  are not affected by the glaze  106 ,  114 . Also, the glaze  106 ,  114  will not chip or spall off the crowns  94 ,  108 . 
         [0088]    The dental porcelains that may be used for veneers or even for the entire crown are usually composed of kaolin clay (may also be in the form of kaolinite (Al 2 Si 2 O 5 (OH) 4 ), a feldspar and some additional proprietary ingredients formulated by the suppliers of the dental porcelains. Feldspars are aluminosilicates, frequently alkali aluminosilicates, and may have the compositions KAlSi 3 O 8 , NaAlSi 3 O 8  or CaAl 2 Si 2 O 8 . These dental porcelains containing feldspar may also be called feldspathic porcelains. Porcelains other than feldspathic porcelains known to those skilled in the art may be used in the exemplary embodiments. In particular, dental porcelains other than feldspathic porcelains that bond to stabilized zirconia may be used in the exemplary embodiments although feldspathic porcelains right now are the preferred porcelains for use in the exemplary embodiments. 
         [0089]    The glaze  106 ,  114  may be made by diluting a dental porcelain in water or organic solvent and then applying it as a very thin layer. Glazes may come premixed, in a paste or in powder form. If in powder form, the glaze powder and the solvent provided by the manufacturer may be mixed together until the desired consistency is obtained. The precise composition of the glaze may not be known as it is often a proprietary formulation by the glaze manufacturer. 
         [0090]    It is preferred that feldspathic porcelains be used in the exemplary embodiments. Again, the glaze  106 ,  114  is several orders of magnitude thinner than a porcelain veneer and is not subject to the chipping and spalling problems of porcelain veneers. 
         [0091]    The process for applying the glaze  106 ,  114  may be as follows. The dental crowns  94 ,  108  are microabraded using 50 micron alumina particles where the glaze is to be formed. The microabrasion roughens the surface of the stabilized zirconia crowns  94 ,  108 . Microabrasion is preferred to allow the diluted porcelain solution to wet the surface of the zirconia. Without the microabrasion, the diluted porcelain solution may simply bead up on the surface of the stabilized zirconia. The diluted porcelain solution may then be applied, such as by painting with a fine brush, to the desired portion of the dental crowns  94 ,  108 , preferably that portion that has been microabraded. In the case of the dental crown  94  for an anterior tooth, it may be applied only to the front  96  of the dental crown  94 . In the case of the dental crown  108  for a posterior tooth, it may be applied only to a portion of the front  110  and rear  112  of the dental crown  108 , preferably that portion that has been microabraded. Thereafter, the dental crowns  94 ,  108  may be fired at a temperature of about 850° C. for about 25 minutes to cause the diluted porcelain solution to form the glaze  106 ,  114 . 
         [0092]    The stabilized zirconia material of the dental crowns  94 ,  108  is chemically inert so that the glaze  106 ,  114  does not chemically bond with the stabilized zirconia. However, quite surprisingly and unexpectedly, the glaze  106 ,  114  forms a strong mechanical bond with the stabilized zirconia. While not wishing to be held to a particular theory, it is believed that the glaze  106 ,  114  shrinks during firing, mechanically locking the glaze  106 ,  114  onto the underlying stabilized zirconia of the dental crowns  94 ,  108 . 
         [0093]    The crowns according to the present invention have been designed so that they have a normal emergence from the root structure. Referring to  FIG. 9 , the inventive crowns have an interproximal line angle, a, of 5 to 12 degrees to allow for a healthy interproximal papilla. The prior art crowns do not allow for such an angle, thereby causing problems for the patient later on. 
         [0094]    The crowns according to the present invention have a facial profile similar to that of normal teeth. Referring back to  FIG. 8 , it can be seen that both teeth have a convex facial profile  60  but the inventive crown is much less convex and more in line with a normal curvature of a natural tooth. 
         [0095]    The crowns according to the present invention use zirconia that has been specially formulated so that it is strong, aesthetic and, most importantly, thin. These properties enable the best of both worlds—room on the inside for passive fit, and contours on the outside of the crown that blend in with the natural tooth shape. Since the inventive crown is thinner than the current aesthetic crowns, the restoring dentist will not have to prepare the tooth as much, allowing for more retention and decreased chance of damage to the nerve. Referring back to  FIG. 6 , the approximate thickness of the crown axially as indicated  64  is about 0.1 to 0.4 mm while the approximate thickness of the crown from the axial-occlusal line angle to the occlusal surface as indicated at  66  is about 0.4 to 0.8 mm. These should be compared to the prior art aesthetic crown whose thickness of the crown axially on the buccal side is about 0.5-2.0 mm ( 64  in  FIG. 6  for the inventive crown) and of the crown from the axial-occlusal line angle to the occlusal surface ( 66  in  FIG. 6  for the inventive crown) on the buccal side is about 0.8-2.5 mm. The thick crown wall of the prior art aesthetic crown creates a dilemma for the dentist. The tooth has to be aggressively prepared so that the thick walled prefabricated pediatric crown will fit passively on the tooth stump. In the situation for baby teeth, the tooth is small already, and grinding them more to allow for the thickness of the crown increases the chance that the crown will fall off after it is cemented, or cause permanent damage to the nerve of the tooth. 
         [0096]    The crowns according to the present invention have a thin margin, again indicated by  64  in  FIG. 6 , of about 0.1-0.4 mm around the circumference of the opening of the crown  10 . Referring to  FIG. 8A , the margin  64  is about 0.1-0.4 mm on both the buccal side  64 A and the lingual side  64 B. The advantage of a thin margin is that the margin can fit under the gum  68  as shown in  FIG. 8 . With the prior art crown, shown in the right half of  FIG. 8 , the margin may not fit under the gum  68 , since the prior art crown has a margin thickness of about 0.5-1.5 mm on the buccal side  64 A even though the margin  64 B on the lingual side may be 0.2-0.4 mm, thereby causing the margin  64 A to impinge on the gum tissue  68 , potentially creating inflammation and swelling. 
         [0097]    Referring back to  FIG. 6 , the crowns according to the present invention have a thickness of the occluasal surface  84  of about 0.5-0.9 mm while the prior art crowns have a thickness of the occlusal surface of about 0.8-2.5 mm. 
         [0098]    A further advantage of the present invention is that crowns can be prefabricated with a normal mesiodistal dimension as well as a reduced mesiodistal dimension as might be needed because of space loss due to decay. When teeth decay and loose tooth structure, the space that is created from the loss of tooth structure allows the adjacent teeth to drift and fill in the space. Referring to  FIG. 10A , three normal child&#39;s teeth are shown. The mesiodistal dimension is indicated by  70  while the buccolingual dimension is indicated by  72 . In  FIG. 10B , the middle tooth  74  has some tooth decay  76 . Over time, the adjacent teeth can crowd tooth  74  so that the mesiodistal dimension  70  could be reduced as shown in  FIG. 10C  while the buccolingual dimension stays the same. Current preformed pediatric crowns do not fit properly in this situation as they are too wide mesiodistally for the appropriate buccolingual dimension. The dentist has to squeeze the preformed crown mesiodistally to make it fit and it ends up bulging out buccolingually. The inventive crown is offered in alternative sizes wherein the standard size has been modified to be narrower mesiodistally while keeping the normal size buccolingually. This allows a better fit in space loss secondary to caries and tooth migration situations. 
         [0099]    With respect to  FIG. 3 , another embodiment  10 A of the present invention is depicted.  10 A includes a crown  22  formed similarly to crown  10  of  FIGS. 1 and 2 , namely being composed substantially of pure zirconia. Crown  22  includes an outer surface  24  and an inner surface  26 . In addition, embodiment  10 A is formed with an inner band or sleeve  28  which is flexible. The edge portion  30  of sleeve  28  may be cut and/or crimped to fit cervical margins during preparation of the natural tooth. Sleeve  28  may be formed of any suitable material such as metal, plastic, and the like. For example, stainless steel may be employed in this regard. Sleeve  28  is bonded to inner surface  26  of crown  22  via an adhesive layer  32 . Adhesive layer  32  may consist of Panavia F or other suitable bonding agents. Embodiment  10 A of the present invention can be employed on the rear teeth of a patient, while the embodiment  10 , depicted in  FIGS. 1 and 2 , can be used on the front or rear teeth of a patient. 
         [0100]    It should be realized that crowns  10  or  10 A of the present invention may be formed by various methods, including CAD/CAM machining, electrophoretic deposition, ceramic injection molding or slip casting. The most preferred method is by CAD/CAM machining. According to this process, as shown in  FIG. 4 , a presintered block of stabilized zirconia is obtained, step  44 , and then precisely machined using CAD/CAM machining to form the crown and any desired retention features, step  46 . The crown is then conventionally sintered to harden the stabilized zirconia, step  48 . The outer surface of the crown is then polished with a polishing material such as a diamond paste to result in a pearlescent appearance, step  50 . 
         [0101]    The process continues by storing the crown in a kit of various sizes, step  52 . Thereafter, the crown may be retrieved from the kit for placement on a tooth, step  54 . The crown is tried on the patient&#39;s tooth for fit and adjusted as necessary. If this crown has an unsatisfactory fit, the process continues by selecting another crown. When the fit of the crown is satisfactory, the crown is cemented into place, step  56   
         [0102]    Turning now to  FIG. 5 , there is represented a kit  36  in which a plurality of zirconia crowns  34 , fabricated according to the present invention, and of various sizes are displayed on tray  38 . The dental practitioner may select a particular crown  10 ,  10 A for a particular use and insert the same over the stub  12 ,  FIG. 1 , of the natural tooth quickly and easily without taking a mold of each and every pre-existing tooth to be crowned and milling a new tooth through a CAD/CAM process, or the like. It should be realized that the system of the present invention saves time and is more economical than the prior methods found in the pedodontic and veterinarian fields. 
         [0103]    While the present invention has particular application to pediatric and veterinary dentistry because of the lack of a suitable dental crown for these applications on the market today, the present invention nevertheless is also suitable for prefabricated crowns for adults. 
         [0104]    The following Examples are intended to illustrate the invention of the present application but are not deemed to limit the scope of the invention in any manner. 
       Example I 
       [0105]    The shape of a natural tooth was designed in CAD. From such design, a hardened zirconia block was milled to produce a crown which mimicked a natural tooth. The zirconia block consisted of pure zirconia stabilized by yttria sold under the designation “ZS-blank” by Kabo Dental Corporation, Lake Zurich, Ill. Following such production of the crown, it was pre-polished by applying a diamond impregnated silicon wheel (CERAGLAZE 1, NTI Axis Dental) to remove all surface irregularities. A final polish was achieved with a Robinson brush/wheel and a 40 millimicron diamond paste. (Frontier Dental Laboratories, El Dorado Hills, Calif.). 
       Example II 
       [0106]    A crown prepared according to Example I was placed in the mouth of a child patient. A dental practitioner prepared the natural tooth using standard guidelines for primary dentition to allow the patient to receive the pre-fabricated crown of Example I. That is to say, a stub similar to stub  12  of  FIG. 1  was created. The prepared zirconia crown was of the proper size having an acceptable mesiodistal width. The fit of the crown was evaluated and possible tissue impingement was determined. Extension of gingival margin was trimmed with a fine diamond wheel on slow speed with a copious water spray. The crown was filled with glass ionomer to cement the same to the natural stub of the tooth. It should be realized that self-cure composite resin cement may have also been used in this regard. The crown was then seated in the mouth completely. Practitioner then checked and adjusted occlusion. Finally, the final polish was applied to the crown using a diamond impregnated silicon wheel. 
       Example III 
       [0107]    To attain a natural appearance of the crown of Examples I and II, a pre-polish was applied to all external surfaces to remove surface irregularities. A diamond impregnated silicon wheel (CERAGLAZE I, NTI Axis Dental) was employed. After such pre-polish, a final polish was accomplished by using a Robinson brush wheel with 40 ml diamond paste (Frontier Dental Laboratories of Eldorado Hills of California). The finished dental crown appeared to very closely resemble, on an aesthetic basis, the natural tooth of a human. 
         [0108]    While in the foregoing, embodiments of the present invention have been set forth in considerable detail for the purposes of making a complete disclosure of the invention, it may be apparent to those of skill in the art that numerous changes may be made in such detail without departing from the spirit and principles of the invention.