Metal ceramic crowns placed on teeth

Metal ceramic crowns are strong and durable structures which can be placed on teeth for defect restoration. This prosthetic method makes it possible to restore teeth in the lower and upper jaw and achieve good aesthetic and functional parameters. The substructure is made of a metal alloy and covered with an upper ceramic layer. The prosthesis is strong and very rarely causes allergic reactions.

Types

Metal ceramic crowns which are placed on teeth can be made of different alloys:

• Gold-platinum: the frame does not oxidize and there is no dark border appearing on it. The material does not cause allergy. The drawback is its high price. 
• Cobalt-chromium: the material has a good biocompatibility parameter. Metal ceramic crowns made of this material are noted for their high strength, but there is a risk of material corrosion leading to an allergic response of the organism. 
• Chromium-nickel: the alloy is easy to process. Its low price is an important advantage, but the risk of allergy is high. 

In case of a crown with a milled substructure, the cost of the service will increase. This product is not moulded but milled on a CNC milling machine. The advantage of it is that the product will last for many years and have a perfect fit with the tissues.

The prostheses can be partially or completely lined with ceramics.

Advantages and disadvantages

A metal ceramic prosthesis made of different alloys and placed on an implant or an original tooth can last for up to 12 years without any deterioration of its characteristics. Patient complaints about discomfort or allergic reactions are very rare. In case of small chips, restoration is performed in the oral cavity without removing the structure.

The price including placement and work is lower than that of similar prostheses made of other materials.

Benefits:

• Aesthetics: it is possible to manufacture a prosthesis which will be identical to a natural tooth in terms of shape, structure and shade.
• Functionality: a moulded substructure is securely fixed.
• Strength: metal is resistant to deformation and it is difficult to break.
• Maintaining colour: the cost of a metal ceramic crown is justified as the structure is resistant to coloration, there are no stains and the shade does not dim with time.
• Fit with the gum: the prosthesis fits excellently, so pathogenic bacteria do not affect tissues.

Patients get used quickly to the structure, its anatomical shape and small weight.

Drawbacks:

• A large amount of filing down before placement.
• Chips: there is a risk of the lining getting damaged.
• It is necessary to remove the pulp inside the tooth, making the tissues more fragile.
• A dark border may appear after a long time of wearing the crown. 
• Allergic reactions.

A metal ceramic crown made with a gold substructure can cost more.

Indications for placement

Strong structures are used to eliminate the following defects:

• Teeth have lost their integrity.
• Pathological dental abrasion.
• The enamel has changed its shade due to dental diseases. 
• Missing posterior teeth.
• Incisors are positioned abnormally, missing in the arch or rotated.
• Sustained injuries.
• Aesthetic restoration if ceramic crowns, veneers or inlays cannot be placed. 

A prosthesis can be placed if the patient has developed an allergy to components of existing fixed structures.

Contraindications

Alloy-based structures are not placed in the following cases:

• Severe periodontitis with significant mobility.
• Chronic conditions in an exacerbation stage.
• Malocclusion.
• Bruxism.
• Allergy to alloy components or anaesthesia.
• Bone tissue resorption.
• Live teeth with a wide pulp chamber.

Many of the contraindications can be eliminated by proper treatment. If the dentist has found them to be absolute, it is possible to place biocompatible zirconium dioxide crowns.

Fabrication stages

The fabrication of metal ceramic crowns includes processes which are aimed at creating a structure with an ideal shape and high strength parameters. Axioma Dental Clinic has its own dental laboratory whose specialists manufacture custom made prostheses. The metal ceramic crown manufacturing and placement process includes 5 stages:

Tooth filing

The quality of the performed procedure predetermines how tightly the crown will be fixed on the support structure - either an implant or an original tooth. Filing is done with a special diamond drill while the patient is under anaesthesia:

• Determining the chamfer finish line: placing a thread between the gum and the tooth to determine the operating area and avoid damaging the gum during the procedure.
• Taking an impression: fixing the thickness of the chamfer protecting the tissue.
• A circular chamfer around the prosthesis: the size does not exceed 0.8-1.2 mm and hard tissue integrity is not compromised.

If there is a strong pain after filing, the doctor will establish the cause, e.g. pulpitis, periodontitis.

Impression and cast model

The purpose of the stage is to obtain a precise replica of the shape and a perfect fit of the prosthesis with the gum. An identical pattern is obtained by taking a double impression using plastic silicon mass:

• First impression - thick paste;
• Second impression - less thick mass.

The finished crown will replicate the tooth being restored to the highest possible degree. An identical impression is also obtained by creating temporary plastic analogs subject to quick hardening.

Casting a substructure

When the cast model is ready, a support structure is modeled. A gate system is fabricated, and its precision will affect the quality of substructure casting. The goal is to obtain a product which does not have any pores on the surface.

Important details:

• For a thin-walled section, a more heated alloy is used for casting.
• For thick-walled sections, a liquid metal depot is additionally provided to prevent shrinkage and porosity.

All cast areas are placed in equal conditions. After cooling, the substructure is extracted from the cast mold.

Substructure ceramic coating

The cast base is polished by means of diamond grinders and put into a sandblasting machine. Small abrasive particles clean the surface and make it slightly rough so that it could bond well with ceramic lining.

The base is boiled in distilled water, degreased and baked to obtain an oxide film on the surface. Thermal treatment takes place in a special furnace with the temperature reaching 980 °С.

Ceramic layering:

• Mixing powder with water;
• Obtaining a uniform paste mixture;
• Layering on the substructure using a brush and a spatula;
• Removing excess moisture;
• Preliminary heating of the structure;
• Vacuum firing.

After the structure is cooled, a second final primer is applied, which will hide shrinkage and cracks in the material. After that, dentine mass is applied, which is carefully selected in accordance with the natural colour of the patient’s dental arch.

Placement

Before placing the structure, it is tested to check how tightly it fits with the gum tissue and how well it conforms to the anatomical shape and the colour shade. The crown is degreased, disinfected and fixed on an implant or a tooth.

Process:

• The crown is filled with liquid cement one third full - excess cement leaks out;
• The edges of the prosthesis are covered with cement;
• The structure is fitted on tightly;
• Hardened cement leftovers are removed.

After placement, the patient needs to avoid eating for a while.

Taking care of metal ceramic prostheses

1. Teeth cleaning at least twice a day.
2. Regular visits to the dentist to have plaque removed - this is important if the patient smokes, drinks a lot of coffee or strong tea, and uses food that changes the colour shade of ceramics.
3. Toothbrush: using a soft bristle brush which does not hurt the gum.
4. Toothpaste: using ordinary paste or that which contains bee glue and medicinal herb extracts for gum care.
5. Dental floss: removing food remnants.
6. Rinsing after each meal: water and antiseptic solutions.
7. Bridgework structure: a thorough care of the canal positioned between the gum and the crowns.

Avoid rapid changes of temperature to prevent cracks. Biting down on hard food may result in chipping or bridge breakage.

Eliminating different defects, flawless restorations, fabrication of structures in a dental lab: you are welcome to a consultation.

Medical expert:

Leonid Yakovlevich Kusevitsky

Orhtopaedic dentist, Dr. habil. in Medicine