Too large fillings
If an amalgam or composite filling is (too) large, the tooth material may be too weak to withstand large chewing forces. The result can be fracture of the lumps, so that you can no longer chew properly and dentin may be exposed. Exposing dentine can cause pain with heat, cold or sweets. Caries (secondary caries) may also develop along the fracture edge of the filling. If little tooth tissue remains, the hold for composite can be improved by the use of so-called umbrella pins. Umbrella pins are small screws that are screwed into the dentine. With this, good results can be achieved and the restorations are strengthened by it. The pins are mainly used with vital elements.
If this is not possible or there is no certainty that the chewing force will be sufficiently absorbed, it is better to provide the tooth with a crown, in or onlay (see image below).
When a tooth becomes avital (lifeless), the element becomes brittle and pieces of the tooth crown can break off. In addition to caries, the causes of becoming an elemental avital are a blow, a deep preparation or a deep restoration. We generally speak of a trauma.
The element does not have to be lost as a result. Depending on the size of the fractured crown and / or root part, it is possible to repair the element with composite. If large pieces are removed from the tooth, it is better to have a crown made.
Restoration of aesthetics
When restoring aesthetics we think of large and discolored composite fillings in the front, discolored avital elements and developmental disorders that can lead to deformation or discoloration of teeth. Sometimes a small tooth position deviation can also be corrected. In aesthetic dentistry there has been a major shift towards the treatment with composite. The porcelain veneer (facing) has also made its appearance.
Recovery from occlusion and articulation
If the anatomical shape of a tooth is largely lost due to fracture of the tooth crown or extensive restoration, or if an element is lost, this leads to a loss of chewing capacity. The opposite elements (antagonists) can grow and in the case of loss of a dental element the neighboring elements can move. This problem can be prevented with a bridge (see image below).
There are also disadvantages associated with a crown:
- Damage to the pulp. A relatively large amount of tooth tissue is drilled away during preparation. The dentine channels are cut off, which can cause pulp irritation. When cementing a baked restoration, chemical irritation is created by the cement and later thermal irritation can occur.
- The crown cannot be aesthetically pleasing.
- The costs of a crown are high.
- Preparation under the gums (subgingival) causes damage to the periodontium and after placement there will often be a permanent slight irritation of the gingiva.
The manufacture of crowns and bridges is a costly and labor-intensive matter. To prevent disappointments about the premature failure of cast restorations, good oral hygiene is a prerequisite. Caries along the crown edges (outline) and periodontal problems after the crown can be prevented by this. A crown is made to obviate the effects of earlier caries and therefore major restorations, and not to prevent caries!
Crowns and bridges can be made from the following materials.
Metal crowns are no longer made because they are not aesthetically pleasing. However, crowns with a metal chewing surface are made in patients who grind because otherwise their own teeth will be affected. Porcelain is in fact harder than tooth enamel and due to grinding the enamel of the antagonists wear out too quickly. Eligible metals are: gold, noble replacement metals such as palladium gold, semi-precious metal alloys and base metal alloys (with a high tin content). Silver is still used if one wants to make a temporary crown, which is later replaced by one of another material.
Gold is a high-quality, noble, strong, but expensive material. Good alternatives are the precious, semi-precious and sometimes also the base metals. The latter is not entirely clear whether they are tissue-friendly and patients can be sensitive to the nickel that can occur in these metals (see image below).
The alternative materials are cheaper and can be baked just as well or sometimes better with porcelain.
A full porcelain crown is only applied to the front teeth. We then speak of a jacket crown. Advantages are the beautiful color and natural transparency. The disadvantage of a jacket crown is the chance of it breaking.
Fried porcelain on metal substructure
Fried porcelain on a metal substructure is a combination of metal and porcelain. The crown consists of a metal sub-structure on which porcelain is layered on the visible surfaces, previously indicated by the dentist. The baked porcelain crown is often referred to as the VMK crown. This is a brand name, which is actually not correct.
The baked porcelain crown is a strong crown that can be manufactured in the entire mouth with a small chance of breakage. In the (pre) molar region, the outline of this crown is executed entirely as a thin flared metal edge. For the periodontium, it is more favorable if the edge of a restoration ends at or above the gingiva level. In the upper front, this is ugly, because there is then an edge along the gums. The outline was therefore often prepared under the gums. With the porcelain firing technique, this crown can be executed in a shoulder (buccal). The visible porcelain then ends with the outline.
In general we can state that each material has its own specific properties and that this has particular consequences for the preparation technique to be used.
In addition to porcelain crowns, crowns can be made from a ceramic material with zirconium oxide. A hood can be made of zirconium, which is then fired with porcelain. But the crown can also be made entirely of zirconium. Zirconium crowns are very strong. Because there is no metal in these crowns, it is a good alternative for people with a metal allergy.
Procedure for the manufacture of a crown for a vital element
The preparation and restoration include the following steps:
- Vitality testing: if the element is vital, anesthesia can be given;
- If there is not enough retention and resistance for the crown to be made, a superstructure must first be manufactured;
- Determine color;
- Manufacture of the emergency crown;
- Print and record the relationship with the antagonists;
- Adjust the crown;
Preparation and printing (and making the emergency crown) can be done in one session. If the gums bleed quite heavily after preparation, it is better to make the impression in a separate session. After the crown has been made in the dental laboratory, it is fitted in the mouth and cemented. When a bridge is made, a separate session can be spent fitting the bridge into bisquit. The porcelain has not yet been flattened. The dentist can still grind the bridge, after which it is further finished in the dental laboratory.
After cementing, if desired, a general inspection and cement inspection can take place in a separate session.
Preparation for a full crown
When preparing an element for a crown, all surfaces of the element are ground, so that space is created for the crown (comparable to a cap). 1 to 2 mm is removed from all surfaces. Depending on the crown to be produced, there are different preparation forms. Usually the occlusal or incisal plane is lowered. Then the buccal and lingual or palatal planes are ground. Slices are then taken from the mesial and distal surfaces. This is called 'slicing'. Finally, the outline is prepared (chamfer, shoulder, command or shoulder command). Retention grooves to improve the grip of the crown can be applied optionally.
After grinding the stump, it is better to replace existing old filler material. The chance that caries remain behind or that the old filling is released later is thus reduced. The aforementioned umbrella pins can be used as a guide for the new restoration material.
The instruments for the crown preparation consist of: the pulp tester to test the vitality, anesthesia requirements, the basic set and a fast rotating corner piece with cooling. The diamond drills used are:
- Short and long conical drills to remove occlusal, incisal, buccal, lingual, palatal, mesial and distal material;
- A flame-shaped diamond drill that is also used subgingivally and with which commands can be applied or a chamfer drill to fit a hollow shoulder;
- The flat, round diamond wheel that is used with front elements to remove palatal or lingual material (see image below).
Manufacture of the emergency crown
The manufacture of an emergency crown, the temporary provision, is necessary for the following reasons:
- Preventing damage to the preparation;
- Maintaining the position of the elements (the neighboring elements and antagonists must have support);
- Protect the pulp from bacterial, thermal and chemical irritation (pain);
- Aesthetic reasons: one cannot send someone home with the stump of a front tooth.
There are two ways to make a temporary provision, namely with the help of a ready-to-wear crown or with the help of self-polymerizing synthetic resin.
1. Confection emergency crowns are preformed crowns, which can have different sizes and shapes. They are delivered in collection boxes. The ready-made emergency crowns can be made of synthetic resin, aluminum or stainless steel. The synthetic resin emergency crowns serve for the front elements and the premolars, while the metal emergency crowns are used for the molars and possibly premolars.
- For emergency crowns made of aluminum or stainless steel, a crown is selected that fits tightly around the preparation. The preparation is rubbed with Vaseline to protect exposed dentine channels from the preparation. After the emergency crown is trimmed to the correct length with a pair of crown scissors and the edges are bent into shape with a so-called festoon pliers, the emergency crown is filled with self-polymerizing resin. The resin is placed in the emergency crown by the assistant. The dentist presses the filled emergency crown over the preparation and removes it several times before the resin has hardened (in connection with any undercuts). After the emergency crown has been hardened, the excess of synthetic resin is ground away. The occlusion and articulation are checked with an articulation paper and the emergency crown is ground if necessary. The edges are smoothed with a rubber disk. Finally, the emergency crown is placed on the stump with the aid of temporary cement.
The technical handpiece must be connected when manufacturing the emergency crown (see images below).
- Ready-made synthetic resin crowns have the advantage that they are aesthetically acceptable in the front. There are also different shapes and sizes of these. The fitting is just like the metal emergency crown. The difference is that when fitting, not a pair of crown scissors but a handpiece with stones is used. The use of the festoon pliers can also be omitted. Sometimes ready-to-wear crowns are made and then completely filled with a temporary cement instead of self-polymerizing resin and attached to the stump.
2. For emergency crowns of self-polymerizing synthetic resin, an impression is made in advance of the element to be ground, the situation or initial pressure. This print can be from alginate or a putty material. After preparation, the spoon is filled with synthetic resin at the place of the ground element and placed in the mouth. The synthetic resin then fills the space between the stump and the original shape of the element, which is fixed in the print. After hardening, the emergency facility is finished with alpine stones and rubber discs. Fixing is done with a temporary cement (see images below).
Crowns and bridges cannot be made directly in the mouth: they are indirect restorations. Therefore it is necessary to have a copy of the prepared element with the surrounding structures. We call the copy the model. To get a model, an impression of the situation in the mouth must first be made; the impression is poured into hard plaster by the dental technician.
The print material must:
- Easy to process;
- Have a very good detail view;
- Taste or smell not too dirty;
- Non-toxic or cause allergic reactions;
- In its cured state, its shape is well retained (elastic and retains its shape);
- Not to be very water-repellent (hydrophobic).
Elastomers or rubbery impression materials and hydrocolloid impression materials are used in crown and bridge work.
Elastomers are soft, rubbery materials. They consist of elongated chains of large molecules that have a weak connection with each other. This can be established by the fact that, as with natural rubber, it is easy to bend or stretch the material, while it is easy to return to its original shape after lifting this bending or stretching force. The elastic impression materials are generally water-repellent. This is unfavorable for making a print in a moist environment and for pouring it out with an aqueous plaster. A search has therefore been made for elastomers that have a hydrophilic character (being water-attracting). The elastomers have an excellent detail display and are reasonably form-retaining.
Within the elastomers group, a distinction is made between polysulfides, polyethers and silicones. There are similarities between these types of printing materials. For example, most are supplied as two components (base and catalyst), which must be mixed in tubes, cartridges, cartridges or pots. Both components are mixed to a homogeneous mass (color). After this there is a certain processing time before curing starts. For some brands, special mixing guns or electric mixing devices are for sale. The material does not have to be created on a mixing block (see image below).
The different types of elastomers are often available in different strengths: putty (available as very solid, stiff, medium and thin-flowing). Depending on the application, one of these types will be chosen.
There are also mutual differences. For example, the polysulfides and silicones are hydrophobic, while the polyethers are somewhat hydrophilic. There are also differences in ease of mixing and smell and taste. The polysulphides and the polyethers score the worst here. With the various elastic impression materials, an adhesive is supplied by the manufacturer, an adhesive which is applied in the spoon to obtain a better adhesion of the impression material to the spoon.
With hydrocolloid impression materials that we use there are materials based on agar-agar, a substance that comes from seaweed. Agar-agar is a water-attracting (hydrophilic) substance. It can absorb and swell water. This is a characteristic that must be taken into account.
An irreversible print material can no longer return to its original state after mixing. Alginate is such an irreversible hydrocolloid. Alginate is used in crown and bridge work for the counterprint, about which more later. The material is supplied in the form of powder. This powder consists partly of a salt of alginic acid. This salt is soluble in water. When alginate powder is mixed with water, a gel-like mass is formed that hardens after a few minutes.
The disadvantage of alginate is its moisture sensitivity. If the alginate impression comes in contact with water, it will swell. If the print is stored dry, water will evaporate and the print will shrink. Alginate powder is also temperature sensitive. The temperature influences the setting time. It is best to use water at 20 ° C. Alginate powder is supplied in measured quantities in bags or capsules or as loose powder in a pack or can. A liquid size and a powder size are supplied with the alginate. One measure of liquid corresponds to one measure of powder.
If an alginate print needs to be stored for a while, it is best to attach a damp cotton roll in the plastic bag. The cotton roll may not come into contact with the print (see image below).
The impression with an elastomer is discussed in the context of crown and bridge work.
The printing spoons to be used come in various shapes and sizes. First of all there are the clothing spoons made of plastic or metal that cover the entire jaw or part of the jaw in terms of size. Metal spoons are used most often. They are strong and rigid, so the print does not deform when taken out. They are available in a sealed and perforated version. The perforated spoons give the impression material a good hold (see image below).
A second possibility is the individual spoon. For this purpose, the dentist makes an alginate impression of the jaw to be treated (half). A suitable spoon, an individual spoon, is made on the plaster model thereof. This can be made of perspex or synthetic resin. With a round drill in a handpiece, perforations can be made in the spoon if desired. The individual spoon is used when printing more crowns or bridge (s).
Print with elastic material
If the ready-made spoons are used, a spray print can be made in two ways:
- The single print method (one print is made);
- The double-print method (two prints are made with one and the same spoon).
The two methods are essentially the same, but there is one difference: making the spoon individually suitable. The confection spoon is not adjusted for the single-print method. With the double-print method, a precisely fitting individual spoon is made on the basis of a first print.
If the double-printing method is used, the confection spoon is filled with a putty material or with a sturdy polysulfide material. The spoon is placed in the mouth and removed from the mouth after curing. At the location of the prepared element, material is cut away in the impression to make room for the accurate, second precision impression. This is possible with a sturdy washing knife or a scalpel knife. The printout must be cut away on all sides in such a way that the distance to the prepared element is at least 3 mm, ie halfway to the neighboring elements. There is now a precisely fitting spoon with space for the actual impression material at the place of the preparation.
The further course of the prints is identical for both methods, and is described below.
Draining the work site
Drying the work site with cotton wool rolls and a saliva plunger, in the case of preparations in the lower jaw a tongue shield, is important. As we have seen in the discussion of print material, the elastic print materials are sensitive to moisture. If blood, sulphus fluid and saliva residue reach the material during printing, the chance of success is rather small. A dry work site is therefore a prerequisite. The use of an anesthetic with a vasoconstrictor (a blood vessel constricting agent) can prevent bleeding in the work area.
Widening of the gingival sulcus
An impression with elastic materials is only possible if the edge of the preparation (the outline) is free. For the parts of the preparation where the outline is below the gingiva (subgingival), this means that the gingiva must be pushed aside. We call this sulcus widening. This expansion must be good, but reversible (repairable). Because the process can be painful, anesthesia is usually required. The sulcus widening can take place in a number of ways: with retraction wire, with retraction paste and with an electrosurgery device.
- Retraction wire. A piece of double-wound wire is cut to length, enough to be applied twice around the preparation. For a full crown this will be approximately 4-6 cm. With an ash 6 the thread is pushed distal and just below the preparation edge into the sulcus by the dentist. The thread is then pressed all the way into the sulcus and this process is repeated with the second half of the thread so that the sulcus is properly opened. The emergency crown is placed on the preparation, so that the wires remain clamped in the sulcus. The thread must remain in place for five to ten minutes, depending on the gingiva condition. By inserting the thread, pressure is exerted on the gingiva. This reduces the blood flow. Yet there is a chance that the gingiva will bleed as a result of the insertion. For this reason chemical substances that use astringent are often used. The retraction wire can be impregnated with this or the agent can be used locally as a liquid or paste.
- Retraction paste. The paste contains astringent agents and is applied to the sulcus with a thick, blunt needle. Because the pasta is very stiff, the sulcus is widened by the pressure. A distinction is made with regard to the astringent products: astringents and vasoconstrictives. An adstringens is a substance that causes blood proteins to precipitate. This results in a superficial batter, which stops the bleeding. An example is alum. A vasoconstrictor is a substance that narrows blood vessels. Because the small blood vessels in the sulcus are squeezed shut, the blood flow is reduced and the bleeding stops. Adrenaline has a stimulating effect on the heart muscle in addition to a local blood vessel constriction. The use of adrenaline-containing retraction thread should be used with caution in cardiovascular patients. A piece of retraction wire of 5 cm can contain an amount of adrenaline equal to that in 14 carpets of Xylocaine®.
The threads slide easily out of the sulcus through saliva, while, moreover, the chemicals 'wash away' from the threads. It is therefore important to keep the work site dry (see image below).
- Electrosurgery device. Except for use in surgery, the electrosurgery device can be used to dilate the sulcus. With the help of an electrode, the blood vessels in the sulcus are seared shut and sulcus tissue shrinks, causing the sulcus to open. This sulcus broadening is reversible; that is, there is no lasting change with correct application. A well-arranged, dry work site is obtained, but a disadvantage is that scorching can cause a nasty odor. The risk of bone burning and damage to the element is high if the electrode is not only held against the sulcus tissue.
Spraying the preparation
The print sprayer is laid out and the tip checked. A special print sprayer is used to get an exact print of the subgingival edges. The tip of the syringe must be 1 mm internally (to be checked with the pointed tip of the ash 49). If the opening is too small, the print material comes out of the syringe too slowly and one has to use too much force. After widening the sulcus, the retraction threads or paste can be removed and the impression mass can be applied. If the single printing method is used, a somewhat stiffer polyether or polysulfide printing material is preferred. If the double-printing method is followed, then somewhat less stiff material is used. There must be enough media to fill the syringe and spoon (see figure below).
While the dental assistant fills the syringe and spoon with impression material, the dentist removes the emergency crown. When the assistant has arrived to fill the impression syringe, the dentist removes the threads from the sulcus with tweezers. If the sulcus is still moist after the removal of the wires, it may be necessary to blow dry with the air syringe. The assistant indicates the syringe and the dentist sprays the preparation (s), after which the spoon is filled in with the remaining amount of impression material.
Homogeneous mixing of the impression material is a requirement. Therefore, use of a mixing gun or mixer is preferable to hand mixing.
Curing of the material (polymerization)
To achieve a good cure, the material must remain in the mouth for a sufficient length of time. The dentist or assistant holds the impression tray in place until the impression material has sufficiently cured.
Removing and cleaning the print
the patient's lips can be greased for spraying and inserting the impression, making it easy to remove the impression material. After removal, the sulcus must be carefully inspected, as residual elastomer can cause inflammation.
Blood and saliva are removed from the impression with the spray and the impression of the preparation is blown dry for inspection (see image below).
Register the antagonists and their position in relation to the preparation
The antagonists of the preparation must also be included in the model. For this a print is made of the antagonists. A total or partial alginate imprint is taken from the antagonists. To record the occlusion (bite registration) a soft bite is made of soft red wax (see image below).
After inspection and cleaning of the mouth, the emergency crown can be cemented (again) and the end of the first or second session has been reached. The prints are cast by the assistant or sent to the laboratory where models are made. The first technology phase is pouring out the print. The model on which the casting is made is called the working model. A hard plaster or synthetic resin is used for this. On the working model, the element on which a casting must be made is removable. To this end, the model is sawn through at a few places and the individual parts are provided with pins that can be replaced in one foot (see image below).
The dentist can best indicate on the working model where the outline of the crown will be. On the stump a border of plaster is put away with a straight enamel knife up to the outline. This is called the notching or addition. If the dentist wants to do the carving himself, the model will come back one more time. After notching, the crown is made in the laboratory. The following details can then be stated on the laboratory receipt:
- The name of the dentist;
- The patient's name;
- The type of crown (ceramic, metal or a combination of both materials);
- The color and shape design;
- The date on which the crown must be ready.
Fit the crown
When the crown is ready, the crown will be adjusted in the second or third session. The emergency crown is removed and the preparation stump is cleaned with a probe and water spray. The crown is slid over the preparation stump and checked for fit, occlusion and articulation (using mirror, probe, floss and articulating paper). The color is also considered for a ceramic crown. Sometimes it is necessary to correct the crown or metal part slightly. If the crown meets the set requirements, it can be cemented on the stump.
Cementing the crown
The cementing procedure can proceed as follows.
- The cleaned stump is drained: below with cotton wool and svedopter, above with cotton wool and saliva cleaner.
- The cement is prepared according to the mixing rules supplied by the manufacturer. Composite cement or glass ionomer cement is used as cement. The creation is done by the assistant.
- The crown is filled with cement and handed over to the dentist.
- The dentist places the crown. Bite logs are often used for this. Bite holes cannot be used with a full porcelain crown, because the risk of breakage is high.
- After curing, the dentist removes the excess cement with mirror, probe, scaler and interdental with floss
If the dentist considers this necessary, the patient must return again. When the gingiva is completely healthy again (after the irritation caused by the impression, emergency crown and cementation), a new check is made to ensure that there are no cement residues in the sulcus. This is done with a mirror, a probe and a scaler.