Restorative dentistry using plastic filling materials – Restorative dentistry

Two phases are distinguished in caries therapy, namely the preparation or preparation of the cavity and the restoration or sealing of the cavity.

Cavity preparation

The following conditions apply to the preparation of a cavity:

• In principle, all caries in the part to be treated is removed from the dental element. The enamel dentin border must in all cases be caries-free.
• The element and the restoration must be able to withstand the chewing pressure. In other words: the treated element offers sufficient (resistance) to the chewing force.
• The pulp and the periodontium are spared and protected as much as possible. Regarding the last point, the preparation is never without consequences for the pulp. During drilling, the odontoblast runners are cut in the dentine. These are foothills of pulp tissue in the dentine canals. Also, frictional heat is created by preparing. To keep the consequences of drilling as small as possible, good cooling is a first requirement. The fast rotating corner piece and the diamond drill are cooled with a water spray that is built into the corner piece. The dentin wound caused by the caries is actually increased by the preparation, because we want to remove all caries (see image below).

The preparation form that is created in this way is called cavity. This term therefore refers to both the carious defect and the preparation.

Nomenclature

The American dentist Black described around five preparation forms around 1900, the so-called standard preparations. The preparations are classified according to the place where the carious defects have arisen. The classification of Black is still used in dentistry and is as follows:

• Class I preparation: preparation for cavities that arise in the fissures of premolars and molars, or in the foramen caecum of the palatal plane of the upper incisions. This preparation is always enclosed by four walls.
• Class II preparation: preparation for cavities starting in the approximate planes of premolars and molars. The occlusal plane is also involved in the preparation, so that preparation and restoration have three possibilities: MO (mesio-occlusal), DO (disto-occlusal) and MOD. Possible extensions to palatal, lingual or buccal are possible.
• Class III preparation: preparation for cavities starting in the approximate planes of cuspidates and incisives. They are so large that the incisal edge remains completely intact during preparation.
• Class IV preparation: preparation for cavities in the approximal planes of cuspidates and incisives, where the incisal edge is involved in cavity and preparation.
• Class V preparation: preparation for cavities that start in the cervical part of the teeth. They can occur on the buccal, labial, lingual and palatal planes (see images below).

Class I preparation occlusally in a premolar.

(a) Class II preparation, DO in a premolar; (b) Class II preparation, MOD in a molar.

Class III preparation mesially in a cuspidate

Class IV preparation mesial in an incisive

Class V preparation buccally in a premolar.

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Based on the class II cavity, the naming is now further discussed.

Preparations in multiple planes in (pre) molars consist of two parts that are referred to as ‘step’ and ‘box’. The scooter is the occlusal part of a multi-plane preparation. The box is the more or less box-shaped extension, usually on the mesial and / or distal side. The two-surface preparation consists of one step and one box, the three-surface preparation consists of one step and two boxes (see image below).

To clearly describe a cavity, the walls are given a specific name. The cavity walls that run more or less parallel to certain outer surfaces are named after this. For example, a cavity can have a mesial, distal, buccal or palatal or lingual wall. Walls that cover the pulp cavity are distinguished according to:

• The pulpal or axial wall; that is, the planes parallel to the longitudinal axis of the element;
• The bottom of the cavity; that is, the horizontal plane perpendicular to the longitudinal axis of the element (see image below).

The surfaces in a class II preparation: a axial wall; b bottom of the scooter; c buccal wall of the box; d the bottom of the box; e the lingual wall of the box; f lingual wall (step); g buccal wall (step).

Restoration of a cavity

A plastic filling that is applied in the mouth is exposed to all kinds of influences. The restoration materials must therefore meet the following requirements:

• Not harmful to health;
• Easy to process;
• Can withstand moisture during application;
• Insoluble in oral fluid;
• Do not expand during curing;
• Sufficient processing time;
• Easy to polishing;
• Tooth-colored;
• Natural transparency;
• Good bond strength to enamel and dentin;
• Durable;
• Shatterproof;
• Insulating at temperature differences;
• Caries inhibiting.

Unfortunately, the plastic filling material that meets all these requirements has not yet been invented.

The following is a discussion of the most commonly used plastic restoration materials. A few material aspects are first discussed for each material and then the application is discussed.

Amalgam

You get amalgam by mixing mercury with an alloy. An alloy is a mixture of different metals. Dental amalgam contains at least silver, tin and copper. There are different brands and types of amalgam on the market, which differ in composition, structure and therefore in properties.

Mercury is harmful to overall health. Mercury can also leak out of a cured filling over the years and end up in the body through ingestion. This is demonstrable, for example in blood and urine tests. The harmfulness of these minimum amounts of mercury has not yet been demonstrated by scientific research.

Some patients with metal fillings complain about ‘streams’ in the mouth. This is caused by contact between two different metals in the moist mouth environment. This can be a contact between two amalgam fillings or between a golden crown and an amalgam fill. By dissolving metal ions, potential differences arise, which the patient perceives and can experience as a nuisance.

For the above reasons, amalgam is rarely used in dentistry. There are many patients who still have old amalgam fillings. During the drilling out of these old fillings, the metals enter the mouth of the patient. It is important to vacuum them well and to regularly clean the mouth. The best way to ensure that the patient does not swallow amalgam grit is to remove the filling under a rubber patch (rubber dam) (see image below).

Composite

The area of application of this tooth-colored plastic filling material has been greatly expanded in recent years. In addition to improving the material itself, there is a rapid development in the bonding systems for composite to enamel and dentin.

Composite consists of a plastic matrix containing a filler of very fine glass or quartz particles. The filler particles are covered with a layer of vinyl silane to create a good connection with the synthetic resin. This makes the material stronger and more durable.

Composite is introduced into the cavity as a plastic material and must then cure. This happens because the monomers of the matrix link together into long chains of polymers. This polymerization can be initiated by the addition of an initiator, which is activated by blue light.

The chemically curing composites are supplied as two-component material in plastic pots. Equal amounts of these must be mixed on a mixing block with a spatula, after which hardening occurs.

The light-curing version is packed in plastic syringes or formulas. After application of the material in the cavity, it is polymerized under the influence of light from a halogen lamp with a glass fiber tip or an LED lamp. The processing of the light-curing composites is much easier and gives less chance of inaccuracies.

In the polymerization of, in particular, the light-curing composite, volume shrinkage of 2-5% occurs. This is unfavorable for a dental filling material because shrinkage stress is created in the element. This can lead to pain complaints and formation of edge cracks and edge leakage of the filling. An attempt is therefore made to manufacture composites with the highest possible filler content. The filler does not shrink, is durable and does not deform.

Current composites can have a filler content of more than 80%; nevertheless some volume shrinkage will occur. Large cavities must therefore be filled and cured in layers.

The size of the filler particles largely determines the properties of the composite. A distinction is made between the following types of composite:

• Conventional or macro-filled composites. The filler consists of relatively large particles of 0.5 to 50 µ. It is a very hard fill material. It has the important disadvantage of breaking out the filler particles from the plastic matrix, which is facilitated by the relatively large distance between the filler particles. Another disadvantage is the poor polishability. The result of this is roughness and rapid discoloration.
• Microfine composites. Very small filler particles of 0.04-0.15 are used for this. This has the advantage that these fillings can be polished very smoothly. A high degree of filling can be achieved by, for example, adding small chunks of prepolymerized material with a high filler content. These particles no longer shrink. In general it can be said that microfine composites show little shrinkage and are reasonably durable. They can be polished to a high gloss. The strength is somewhat less.
• Hybrid composites. Both larger and very small filler particles are found in this. The advantages of this are a high filling degree of more than 85% and a decrease in the distance between the filler particles. A smaller amount of resin is necessary here, whereby the strength of the composite material increases and the chance of discoloration decreases. They are durable and easy to polish. The most modern hybrid composites are the so-called micro and nanohybrid composites. These have smaller filler particles than the first hybrid composites.
• Nanohybrid composite. The filler particles in this composite have a size of 0.02 to 0.04 µ. These very small particles ensure that the composite can be polished to a high gloss.
• Unfilled composites. This composite does not contain any filling particles and is not used as a filling material but as a bonding (see images below).

Schematic representation of different composites: a conventional composite; b microfine composite; c microfine composite with prepolymerized components; d hybrid composite.

In addition to the aforementioned functional and material-technical properties, aesthetic factors are important. In order to be able to make beautiful front fillings, the filling must resemble natural tooth material as much as possible. Composites are therefore available in various colors (yellow, gray and brown tones) from light to dark. Many brands use standard Vita colors with the designations A1, A2, B1, B2 and so on. The colors are also available in various translucences. Glaze shines through more than dentin. By using layered layers of opaque (non-translucent) and translucent composite, a natural color gradient can be applied to the composite restoration (see image below).

Composites must be kept cool and dry and, due to their sensitivity to light, in a dark place.

One of the major advantages of composite as a filling material is the very good adhesion to tooth tissue by means of the acid etching technique. This makes it possible to make restorations without an edge gap. In addition, tissue-saving preparations can be made because extensions for retention of the filling material are not required. The adhesion to dental material proceeds as follows:

• The enamel edges of the preparation are etched with a liquid or gel of 37% phosphoric acid for 15-30 seconds. As a result of the action of the etching acid, the lubricating layer resulting from the preparation is removed and etching pits form in and around the enamel prisms of 10-70 microns depth.
• After rinsing the etching acid, the cavity is blown dry. The glaze now turns dull white.
• A primer is applied to improve dentin adhesion. This impregnating agent is dissolved in alcohol. The solvent evaporates by blowing dry for several seconds with the multi-function syringe.
• The bonding is then applied. The bonding is a thin-liquid and unfilled synthetic resin that can be chemically or slightly curing. The bonding flows into the pits in the glaze and thus forms a very strong connection between the glaze and the composite to be applied. Bonding to the enamel is mechanical due to the strong surface enlargement of the etched enamel. Some bondings also attach chemically to the Ca ions of the enamel. The bonding forms a chemical bond with the plastic matrix of the composite (see image below).

Schematic representation of glaze: a un-etched; b after etching; c the bonding has flowed into the etching pits (mechanically); d the composite adheres chemically to the bonding.

Composite can be used as filler material in caries therapy. In principle, all cavities can be restored with composite. The work site must be completely dry to make a composite filling.

The modern composites are all hybrid composites. For occlusally loaded fillings, a composite with relatively large filler particles is usually used in connection with the breaking and compressive strength. For fillings where the aesthetics weigh heavily, composites with relatively small filler particles are used. These have a natural translucency and can be polished to a high gloss.

Composites are also used in adhesive techniques such as adhesives for crowns, bridges, etching bridges, wire splints, mounting pins and orthodontic brackets.

Glass ionomer cement

Glass ionomer cement (GIC) consists of a powder of aluminum silicate glass and a liquid of polyacrylic acid. Glass ionomer cements are capable of releasing fluoride for a long time. By regular administration of fluorine, by means of toothpaste, fluoride application, etc., the cement is able to charge itself with fluoride. The great advantage of glass ionomer cement is the good adhesion to enamel and dentin. Furthermore, the cement is bactericidal and pulp-friendly.

Due to the solubility during the first 24 hours, the surface of a glass ionomer restoration must be protected against moisture with, for example, a lacquer layer. The cement must also not be dried out. If glass ionomer is applied to dentin that has dried out too much, there is a risk of pain. Because GIC needs water to be able to react, this will partly be taken from the environment, the tubules; the occurring pressure differences will irritate the pulp.

Separate types have been marketed for the various applications of this group of cements. Silver powder can thus be added to make the material stronger.

The applications are:

• As a liner, for under composite fillings;
• As a cement floor, temporary filling or building material;
• For the permanent cementing of, for example, metals and porcelain crowns and bridges;
• For cementing orthodontic brackets;
• As a permanent filling material for class V restorations;
• As permanent filling material in milking elements;
• As a sealant.

Compomer

Compomer is a mixture of glass ionomer cement and composite. It would combine the good properties of both materials, such as

• Good adhesion to enamel and dentin (glass ionomer cement);
• Fluoride release (glass ionomer cement);
• Strength and durability (composite).

The material is easy to process, is light curing and is supplied in spray capsules.

The applications are:

• Caries sensitive patients;
• Class V fillings;
• Fillings in milk elements.

Cements

The area of application of dental cements is very extensive. They are used:

• As a permanent filling and construction material;
• As an underlay material to protect the pulp;
• As a temporary filling material;
• As a fastener for, for example, a crown, cast superstructure or root pin (see image below).

After deviating, a liner has been applied at the deepest places. The cavity has been reduced to standard depth by applying a cement floor.

The number of different types of cement, each with their specific characteristics, is decreasing considerably. Composite cement and glass ionomer cement in particular are frequently used. Other species are becoming less and less or not at all.

Composite cements and glass ionomer cements have the same properties as the composites and GIC used as restoration material.

The applications are:

• As building material;
• For permanent cementation of, for example, ceramic crowns, etching bridges, in- and onlays and facings;
• For cementing orthodontic brackets;
• As a sealant.

Ca (OH) 2 preparations are usually supplied in two components that must be mixed together. Light-curing forms also exist. Ca (OH) 2 (calcium hydroxide) induces the odontoblasts to form tertiary dentin.

The applications are:

• As lining cement in deeper parts under restorations;
• For endodontic treatments;
• Direct pulp covering: if a small part of the pulp is left open after complete caries removal (exponation), a thin layer of Ca (OH) 2 can serve as a wound dressing;
• Indirect pulpa shelter: when there is a high risk of a pulpa exponation, it is sometimes decided to leave a thin layer of hard but carious dentine on the bottom of the cavity. This layer is then covered with a Ca (OH) 2 preparation (see image below).

Varnishes

Varnishes are supplied in the form of a liquid. The liquid is a natural or synthetic resin dissolved in an organic solvent (e.g. acetone, ether or chloroform).

The application are:

• On exposed, sensitive root dentine. A varnish seals off the dentine canals and thus protects the pulp from harmful stimuli.
• As a preventive means to prevent caries or to promote remineralization of white spot caries.

Remaining materials

The ideal plastic filling material has not yet been discovered. At the time of writing, there are mainly developments in the field of bioactive filling materials. These materials have a hydrophilic character: they need a little moisture to adhere well to dentin. Because it adheres directly to dental tissue, no bonding system is needed. The material responds to pH changes in the mouth and is able to exchange calcium, phosphate and fluoride with the immediate environment.