Clinical aspects of tooth preparation in the manufacture of porcelain veneers

Indications for the installation of porcelain crowns

The main indication for the installation of porcelain crowns is the aesthetic restoration of the front teeth:

· wedge-shaped defects and chips that cannot be eliminated using filling material;

Complete destruction of the dental crown;

· enamel hypoplasia;

Abnormalities in tooth color or shape;

· a pronounced change in the shade of tooth enamel that cannot be corrected using professional hygiene methods.

In addition, such designs are suitable for patients who are allergic to metal.

Preparation with a ledge: a necessity or a priority

With this approach, the boundaries between orthopedic dentists, therapists and surgeons are “blurred”. And it is no longer so important what specialty the doctor providing dental care is; what is important is the doctor’s choice in favor of one or another preparation technique.

Let's first figure out what is the meaning of creating a ledge on the stump of a prepared tooth. First of all, we note that the ledge is not a supporting element for the crown, although it takes on some of the load. The need to form a ledge is dictated, first of all, by the aesthetic and functional requirements of modern orthopedic dentistry. The thing is that any artificial crown has some minimum thickness; it consists of the thickness of the frame and the thickness of the facing material. For example, the minimum thickness of a base metal frame should be 0.3-0.4 mm. Let's add here the minimum possible layer of cladding to obtain the most modest aesthetics of 0.7 mm. As a result, we get about 1 mm of crown thickness in the edge area. According to modern requirements of dentistry, we must restore the anatomical shape of the tooth without creating overhanging edges, with a smooth transition of the crown into the contours of the tooth. And this, in turn, is impossible without the formation of a ledge. Automatically, the question of the possibility of a non-invasive preparation technique can be “removed” from the agenda. You can talk about the “ledge symbol” only in the case of making a crown with a complete circular metal garland, which can be polished almost “to nothing”, but it is quite difficult to do this efficiently, and you are guaranteed a clear loss in the aesthetics of the cervical area. So is there a need for barbaric preparation without a ledge, for creating a “ledge symbol” that is barely visible to a technician, if you can simply form a high-quality ledge using modern cutting burs? The technology for creating a full-fledged ledge today is as simple as a multiplication table. And in this publication we will try to present it in the form of a certain sequence of actions by a doctor with appropriate explanations. So, step number 1. Creation of occlusal separation. (Fig. 1). We recommend performing this step with a diamond bur No. 909-xxx

rice. 1. Disconnection for the thickness of an artificial crown using a diamond bur No. 909-xxx

Step #2. Removal of large volumes of tissue from the side walls of the tooth. (Fig. 2). We recommend performing this step with an aggressive grinding bur; a TDA series bur is ideal. This stage is usually accompanied by the formation of a preliminary ledge above the gum level.

rice. 2. Removal of large volumes of tissue from the side walls of the tooth with a TDA bur. At the same time we form a preliminary ledge.

Step #3. Formation of a completed ledge at or below the level of the gums (Fig. 3). This stage must be carried out with a carbide tool. The design feature of this bur is the end end of the working head. In Great White Ultra burs, it is increased in size and has a large radius of curvature, with the constant absence of horizontal notches on the curvature of the bur head. (Fig. 4.) This design of the bur makes it simply an ideal tool for forming the final ledge on the tooth stump and finishing polishing the ledge to a glossy surface.

rice. 3. Formation of a complete ledge at or below the level of the gums.

Rice. 4. Simultaneous formation of the final ledge on the tooth stump and final polishing to a glossy surface.

According to the literature, of the many forms of ledges, the most applicable today are ledges of 135 degrees and 90 degrees (most often, its modernized analogue with a semicircular transition between the “wall and bottom” is used).

Stages of manufacturing and installation of porcelain crowns

First of all, at the appointment, the dentist examines the patient’s oral cavity, identifies possible contraindications, and, if necessary, carries out a complete sanitation. Only after this can you proceed to the next stage - preparing the tooth for a porcelain crown and selecting the optimal shade for the future design using a special template.

Next, the doctor takes a combined impression of the tooth and the entire dentition. And if the quality of the casts is satisfactory, it is sent to a dental laboratory, where a plaster model is made based on them.

Direct production of a porcelain crown can be carried out in two ways. The first involves layer-by-layer application of porcelain substance, and the second uses injection molding technology. It is the second option that is more modern and gives the structure high strength,

Therefore, it is used in most dental laboratories.

After the impression falls into the hands of a specialist, he uses thin platinum foil to create a mold for the future dental crown and fills it with a porcelain composition, and then exposes it to high temperatures several times.

The final stage of making a porcelain crown is the correction of the structure, its staining and glazing.

After the crown is ready, it is first tried on. If the result suits both the doctor and the patient, it is finally fixed on the previously prepared tooth.

Features of turning

When carrying out manipulation, you need to remember some features. The main feature is the creation of a special ledge - circular or vestibular. This ledge is necessary in order to subsequently create the edge of the crown part, which is necessary for veneering the ceramic part of the crown. In addition, thanks to the pre-created ledge, the edge of the installed and fixed crown will not injure or injure part of the soft tissue of the gums with which it will come into contact.

Video - Turning with a ledge

• Metal-ceramic crowns - how to save a tooth

Further manipulation and creation of a special ledge depend on the facts of clinical manifestations, such as:

• the degree of destruction of the desired tooth;
• placement of the tooth cavity;
• height of the created metal-ceramic crown;
• patient's age.

The use of special attachments allows you to control the thickness of the edge of mineralized tissue being ground. Using these attachments, you can create special marking grooves, which will later serve as a guide for the specialist. The bottom of the groove should be at the same level as the edge of the gum, this will mean that the necessary part of the tooth has already been cut down, and further manipulations can begin.

Marker burs

Teeth must be prepared for further installation of metal-ceramic crowns. This is done with specialized diamond-coated or carborundum-coated nozzles. These types of nozzles can be in the shape of a needle or a flame.

Strong discrepancies in the surfaces that should be in contact make it impossible to install a metal-ceramic crown. Otherwise, a strong touch worsens the process of fixing the crown, which can further lead to injury to the loose and fibrous building tissue of the tooth.

After removing unnecessary tissue between the teeth, several thin, special attachments, cylindrical or cone-shaped, are used and the entire surface that will be in contact is treated. Next, a special ledge will be formed.

The Zekrya gum retractor is designed to protect the free edge of the gums during the preparation process.

Marking grooves

The depth of the periodontal groove at the midpoint on the vestibular side is 2.0 mm

Measurement in the interproximal region, the probe is immersed 4.5 mm, with a sulcus depth of 2.5 mm

A retraction thread was inserted into the periodontal groove of tooth 1-1 to protect the epithelial attachment

Marking grooves are applied in the direction from the incisal to the gingival edge

A carbide bur is used to smooth the surface of the stump

Sanding hard fabrics

The preparation ends with the formation of a ledge

Benefits of porcelain crowns

· High aesthetics. Porcelain has almost ideal optical characteristics. It very accurately conveys the natural shade and translucency that are characteristic of natural tooth enamel.

· Hypoallergenic design and inert material. Porcelain does not have thermal conductivity, so tooth sensitivity to temperature changes is reduced. Since the crown does not contain a single gram of metal, it can be installed in patients who are allergic to it.

· Long service life. With proper care, the structure will last a very long time, approximately 10-15 years, without losing its performance characteristics.

· Resistant to staining and darkening. Porcelain crowns do not lose their shade and shine, maintaining their original appearance for a long time.

· Minimum adaptation time. The patient quickly gets used to the lightweight and comfortable design.

· Easy to care for. Crowns do not require special care. It is enough to use a high-quality toothbrush and toothpaste, and treat them with care.

Disadvantages of porcelain crowns

· High cost of construction. It is due to the complexity of technical work, which requires very high professionalism from a specialist.

· Relative fragility of the material. Patients have to avoid eating solid foods.

· Restrictions on indications. Porcelain crowns are not placed on all teeth.

· Manufacturing of single crowns only. Porcelain is not suitable for creating and installing bridges.

Types of ledges

Before proceeding with the formation of the necessary ledge, the specialist must decide which one will be suitable in this case. There are several varieties, namely:

• a rounded ledge with a groove is the most commonly used option. Most specialists use this option for creating a ledge before installing a metal-ceramic structure. The width of such a ledge ranges from 0.7 to 1.3 mm, which will subsequently preserve the hard tissues of the tooth - enamel and dentin;
• A ledge made in the form of a knife is a good option when installing solid crowns, as well as teeth that have slopes. The width of such a ledge is narrower than rounded. It ranges from 0.4 to 0.5 mm;
• the shoulder type of ledge is the least effective, but the most aesthetic type. It reaches 2 mm in width.

Types of ledges

Various forms of cervical ledges in the manufacture of artificial crowns

Video - Tooth preparation for a crown

Varieties of porcelain designs

Dental porcelain is an excellent material that provides ample opportunities for aesthetic restoration. Several types of structures are made from it:

· Veneers. These are thin plates designed to hide minor defects in tooth enamel. They cover only the front surface of the tooth. Porcelain, composite or zirconium dioxide can be used in their manufacturing process.

· Tabs. These are microprostheses made in a dental laboratory. They help solve more serious dental problems with teeth and are used, for example, in cases of severe decay of the crown or the presence of a large carious cavity.

· Crowns. The most versatile design that allows you to achieve a “Hollywood” smile. They are installed on the front teeth during aesthetic restoration.

The need to create a ledge

Specialists do not always make a ledge when grinding a tooth before placing metal-ceramic crowns. There are several reasons for this:

1. The turning time, which is done without a special ledge, is reduced several times.
2. When creating the necessary ledge, you need to have a specialized set of materials and tools, as well as experience working with them.
3. In order to prepare a tooth and create the desired ledge, it is necessary to have a specialized thread that is laid in the space between the gums and teeth. This technique is necessary in order to protect the gums when working with special attachments and creating the necessary ledge. To place this thread you need a specialized tool.
4. Availability of expensive material from which the impression will be made.
5. A mass is needed from which the so-called “shoulder” will be formed in the future.

The relationship between the edge of the ceramic crown and the gingival ledge of the abutment tooth in relation to the mucous membrane

It is worth noting that a tooth prepared without a ledge can become infected, and the tooth itself can be destroyed. This can cause inflammation of the gums and many complications in the future.

• Ceramic crowns

Below are popular methods for grinding teeth for metal-ceramic crowns.

It often happens that after installing metal-ceramic crowns, the patient experiences discomfort and pain in the area of ​​the new prosthesis. The reasons for this may be:

• violation of the procedure for grinding mineralized tissue.
• inflammation of the apical part of the tooth and processes of inflammatory etiology in the soft tissues of the tooth.

Grinding teeth for crowns

In all possible pathological conditions, it is necessary to contact a specialist as soon as possible to correct these errors. Otherwise, complications may develop.

Metal ceramics or porcelain: what to choose?

This question arises among patients quite often. After all, each design has its own advantages and disadvantages. Thus, metal ceramics are distinguished by their high strength and affordable price, and porcelain has excellent aesthetic characteristics. Typically, the choice between them depends on which tooth needs to be restored. Crowns made of metal-ceramics are most often installed on chewing teeth, and porcelain crowns are installed on incisors and canines. However, if a porcelain crown has a zirconia framework, it can be used to restore any tooth.