Crown (dentistry)

The natural tooth crown (Latin corona dentis ) is the part of a tooth that protrudes from the gum and covered with enamel, the hardest substance found in the human body. Derived from this is the term used in common parlance "crown" for an artificial tooth crown of a noble metal alloy or ceramic. The clinical crown of the visible part of a tooth is called.

The term crown or dental crown is because the crown of a human molar tooth, similar to the approximately circular in plan view, arranged cusps, a royal crown.

• 3.1 strip crown
• 3.2 Ring - Lid - crown
• 3.3 Punched crown

The natural tooth crown

The root is the part of the tooth which is anchored in the bone and wears the crown. The transition from the crown to the tooth root is called cervical.

If in pathological changes (eg in inflammation of the gums ( periodontitis), but also in a non-inflammatory atrophy ( periodontitis ) of the gingiva) which goes back to the tooth as a whole holding bone, the part of the tooth from the extended bone protrudes. Thus, the dental crown is relatively longer ( long teeth ) appear.

The ratio of tooth length which is in the bone, the length of the tooth protruding from the bone, called the crown-root ratio. Normally this length from crown to root ratio is about 1:2. If this ratio is 1:1 or even lower, the indication for tooth removal (extraction ) is often given because the root does not find sufficient support in the bone and then often already exposed the bifurcation or trifurcation in the posterior region. The tooth moves, it wobbles and is only partly functional. It is often the case that such a seemingly prolonged tooth with respect to the dental hard tissues (enamel, dentin and cementum ) is fully intact, no caries and must still be removed because of its loosening. Such a degree of damaged tooth should not be crowned because the cost is usually out of proportion to the rest of stay.

The artificial crown

For an artificial crown the natural tooth crown is prepared, that is, it is up to approximately 1 mm by grinding around cylindrical or slightly tapered at the gumline. At the neck of the tooth usually a groove- shaped step is applied. Must be ground down so much even with the occlusal surface or incisal edge, leave enough space to antagonist is. This so- prepared tooth stump is molded with a special impression material ( silicone or hydrocolloid ). Using this impression, the dental technician can then create a working model on which he can then make the crown.

Artificial crowns can be incorporated on stumps, posts and cores, but also on dental implants.

If there is enough remaining tooth structure is present, the tooth stump can be completed by means of a construction fill. In other cases, a pin structure to be anchored in the root canal. In deeply decayed teeth, surgical crown lengthening can be performed.

Prepared tooth stumps of teeth 11, 14 and 16 with fillet preparation below the gum line

Tooth stump in the working model

With partially veneered ceramic PFM crown on the working model

Fully veneered splinted crowns on teeth 24, 25 and 26 on the working model

Full cast crown, Molar, gold alloy

Cast crown from the inside, Molar, gold alloy

Full cast crown on the Sägestumpfmodell ( plaster model )

Tooth crown (probably punched) - no longer corresponds to today's German standard

Defects because approximal crown perforated with completely inadequate Kauflächenmodellation

Materials

Metal -based crowns

The metal-based crown can be made ​​from a high grade gold alloy, a reduced gold alloy, titanium or a base-metal alloy ( NEM). At the cheaper reduced gold alloys can occur under certain circumstances to allergies or discoloration of the oral mucosa by the admixture of less noble metals such as palladium. Therefore, one used today in addition to the expensive " high gold " alloys and non- precious alloys, precious metal alloys or titanium biocompatible as a cheap alternative.

The metal-based crowns have the unfaced full cast crowns, partially veneered crowns and fully veneered crowns are made ​​. The veneering material can be either a veneer resin on composite basis (ie, a mixture of a resin matrix and ceramic fillers ) or ceramic be ( veneering metal-ceramic: VMK ). The latter require because of the high baking- ( 800-900 ° C) special ceramic alloys that form an adhesion-promoting layer of oxide during the firing process. The porcelain veneer is more complex, but has told the plastic veneer the aesthetic advantage of better color stability and increased abrasion resistance. However, this can also be a disadvantage because their extreme hardness prevents a uniform abrasion ( grinding natural ) and the antagonist ( opposing teeth) " bitten " disproportionately. This can result in TMJ symptoms.

Galvano crowns on gold base

An intermediate position take a crowns with electroplated divorced backbones. These are electrochemically prepared at room temperature of pure ( 999 ) Gold and veneered tooth color. They combine excellent biocompatibility, fit and high aesthetics, but are more prone to processing errors.

All-ceramic crowns

All-ceramic crowns have taken the metal skeleton usually a core which is then overburned. The core may be on a CNC drilling and milling machine to mill made ​​of zirconia, and then are veneered with ceramic. There is also the possibility of pressing at high temperature from the liquid ceramic crown exclusively of ceramics ( ceramic press ). In this case, a further blending is not absolutely necessary, but provides advantages for the production of translucency and opalescence. Thus, a natural color effect can be simulated. All-ceramic crowns cause by a lower thermal conductivity less thermal stimuli.

Historical crown types

Strip crown

The strip crown was a previously used type of dental crown. To the prepared tooth stump in this case a metal band was placed, the crown itself consisted mostly of a gold alloy.

Ring - lid - crown

In the previously used ring - lid - crown, a ring for the side walls was first soldered from an appropriately cope cut piece of gold plate. This was followed by the dentist in his mouth a matching occlusal surface, the "cover", modeled in wax, cast by a dental technician and then soldered onto the prepared ring. Disadvantage was a not at all or only weakly pronounced tooth equator. The walls of the ring cover crown were in vertical extent practically parallel. means of special pliers ( "hump pliers ") was a tooth equator weakly indicated. the second major drawback was the bad crown marginal adaptation. , the dental technician passed the crown margin at the gingival margin by eye on the plaster model. then does not even fit on the patient, the dentist fits made. because a visual inspection has not been possible while the crown margin often cut deep into the gum line and destroyed the attachment. compared to today's casting technology of the marginal gap was larger by several orders of magnitude.

Stamped crown

A long time still widespread in Russia Production for crowns was punched Krone (" Crown Scharpey "). From pre-assembled sheet metal sleeves (steel) - a ring with a lid available in about 10 different diameters - created with the help of a very stable mechanical special press first the appropriate diameter. Several successive " pressings " stauchten the sleeve together exactly to the required diameter. The principle is similar to the wire manufacturing, in which a solid with a bit too narrow nozzle thick wire becomes longer and narrower. The prefabricated crown sleeves were measured using a stable punch which reached into the sleeve and attached to a 1 m long lever pressed by a slightly smaller hole. The Press- holes were chosen smaller and smaller until the desired diameter. The adjustment of the length of the crown was - as in the ring - cap crown - the plaster model by eye - crown scissor and hump pliers. With the following method you knew the occlusal surface with a Kauflächenprofil ( cusps and fissures ): On the plaster die a tooth was modeled in wax, it only arrived on the modeling of the occlusal surface. On the sides of the tooth model was allowed to have no distinct tooth equator. From this wax crown a plaster cast was together with the individual plaster stump of the tooth taken, and then with a low-melting metal ( tin alloy, the melting Bunsen burner ) poured out. This gave a completely modeled gear in solid metal ( tin alloy ), who later served as a male (positive, Great Parent form). From the male a massive counter-mold was cast: a thin release film came to die and you made ​​a cast with the low-melting metal alloy. This second metal block later served as a template ( depth, negative parent form). The enveloping matrix could only be resolved by the partially enclosed male by the female was cleaved at three break points in three parts. Now the pretreated sleeve (diameter and crown margin fit made ​​) on to the template. The three-part male was placed over the sleeve - everything is done one a funnel-shaped " mounting ring ". With very sharp hammer blows then the occlusal surface of the sleeve between the die and the punch was formed. The disadvantage of the method is that no such sharp cusps or sharp- lined fissures were modeled. In addition, the sleeves at the crown margin are circular, which is almost never match the actual shape of the tooth stump. Accordingly, it often leads to relatively large crown margin columns. They are amazingly resistant to caries in practice often and are bridged by the cement. Such crowns can also be made of gold tubes. Then, the force required is not so great. The remaining mass of Russian repatriates carried gold crowns indicate this Production: they have only a weak hinted modeling of occlusal surfaces and often a large gap edge. The gold crowns are made of a very thin gold sheet, so on radiographs partially transparent. The crowns were made ​​mostly from their own gold of patients, and not, as in usual cast crowns made ​​of special dental gold alloys.

With the stamped gold crowns can also manufacture bridges - for each bridge pier created a gold crown. The single -span bridges are cast separately in gold. Finally, everything is soldered together. Such solder joints are weak points in this bridge design because they are forces which occur often not withstand discolor quickly and solve occasionally.

The gold crowns were manufactured on a large scale even to jewelry purposes for perfectly healthy front teeth ( upper jaw ). ( " Who can afford gold teeth, is wealthy. " ) The wearing of individual gold crowns to jewelry purposes also in African Americans in the U.S. is currently popular. More modern forms of dental jewelry are Twinkles ( precious stones, gold applications ) and barbecues in the hip- hop culture. Until the advent of tooth-colored fillings for anterior teeth it (often with corner building ) to wear in Germany chic (or at least not offensive ) a visible cast gold filling ( inlay ) for the repair of defects in the anterior region. The first tooth-colored fillings - silicate cements - were mechanically rather unstable and were washing slowly out. Only with the advent of tooth-colored composite fillings visible gold inlays came quite out of fashion.

The Russian gold crowns require only a small effort in the preparation of the tooth stump to the manufacturing dentist. It is only a small part of the enamel ground away, just as much that the tooth equator disappears. Often not even completely, which is why the crown edge then inevitably something has to stick out from the neck of the tooth. Also, between the teeth and on the chewing surface must be ground away very little - just enough so that the thin gold plate fits between. Under the pressure to assimilate and as an expression of their integration efforts try many Russian repatriates for replacement of their gold crowns to tooth colored crowns. Surprisingly, the stumps are among the gold crowns ground away sometimes just so weak that they need not be necessarily crowned.

One advantage of stamped crowns is that the dental technician only a very small technical equipment needed - for free medical care in the Soviet Union a very important criterion. Moreover, ( precious) metal is required as for cast crowns for die-cut crowns considerably less.