Impression materials

Therapeutic liners and pulp protection

Specialized liners are used for the therapeutic effect on tooth tissues. There are several variants of their composition and methods of application into the carious cavity.

Non-setting pastes are prepared immediately at the time of the patient's appointment. Their characteristic feature is the absence of a polymerization process (they remain soft). Such compounds are applied to the entire bottom of the prepared cavity, but strictly for a short period, usually not exceeding 1–2 days. After this time, the paste must be removed and replaced with a fresh one.

Combined therapeutic liners have a more complex structure:

• Base component: artificial dentin powder or zinc oxide.
• Active part: anesthetic agents, proteolytic enzymes, as well as broad-spectrum antibiotics.
• Catalyst liquid: a solution of calcium salts (10% calcium chloride is most commonly used).

Such combined agents have a pronounced anti-inflammatory and antibacterial effect, and also stimulate plastic processes in the tissues.

Depending on the clinical picture, the method of protection is chosen:

1. Indirect pulp capping — the liner is applied to the bottom of a deep, but unexposed cavity.
2. Direct pulp capping — used in case of accidental exposure of the pulp chamber. The material is placed directly onto the point of exposure.

The material itself can be applied point-wise (locally to the vulnerable area) or in a continuous layer covering the entire bottom. Both options are relevant for either capping method.

Historical aspects of permanent filling

The main goal of using permanent materials is the reliable and long-term restoration of tooth anatomy. Interestingly, attempts at filling were made even in antiquity.

The first written mentions belong to A. Cornelius Celsus. He advised filling large tooth defects with pieces of lead or lint. However, this was done not for treatment, but before the extraction procedure, so that the fragile crown walls would not crumble under the pressure of surgical instruments.

Later, the prominent physician P. Fauchard (1690–1762) began using lead, tin, and gold directly for tooth restoration. It was Fauchard who gave preference to lead due to its excellent plasticity and ability to tightly fit the edges of the defect. The familiar term "filling" (пломба) originates from the Latin name of this metal.

Classification of dental cements

Modern cements are divided into two large groups depending on the type of acid used. To increase their mechanical strength, silicic acid or glass powder is often introduced into the composition.

1. Mineral cements (based on phosphoric acid):

• Zinc-phosphate
• Silicate
• Silicophosphate

2. Polymeric cements (based on organic polyacrylic acid):

• Polycarboxylate
• Glass-ionomer

The chemical essence of creating any cement boils down to the reaction of the base powder and the acidic liquid. For example:

• If zinc oxide is combined with phosphoric acid, zinc-phosphate cements are obtained.
• Zinc oxide combined with polyacrylic acid yields polycarboxylate cements.
• Aluminosilicate glass with phosphoric acid forms silicate cements.
• The same glass with polyacrylic acid forms glass-ionomer cements.

All mineral compositions are hydraulic binding substrates. Their liquids are solutions of zinc and aluminum phosphate salts in orthophosphoric acid.

Characteristics of zinc-phosphate cements

This group of materials (e.g., Vissin, Adgesor, bactericidal "Doxyvisphate") is available in a "powder-liquid" format.

The powder is produced by sintering oxides at high temperatures followed by ultra-fine grinding into frit. Each component has its own function:

• Zinc oxide (75–90%) guarantees adhesion to tooth tissues and overall plasticity.
• Magnesium oxide (5–13%) enhances mechanical strength.
• Silicon dioxide (up to 5%) gives the finished mass gloss and vitrosity.
• Calcium oxide regulates viscosity and setting time.

The liquid looks like a clear syrup without sediment and odor. It is a 38–44% solution of orthophosphoric acid, with added hydrates of magnesium, aluminum, and zinc oxides for partial neutralization of acidity.

Among the advantages of the material are excellent plasticity, radiopacity, low thermal conductivity and safety for the pulp. Disadvantages include porosity, low resistance to saliva, insufficient aesthetics (mismatch with enamel color), low strength and shrinkage of about 0.5% during setting.

Working rules and mixing technique

The work requires a smooth glass slab and a metal spatula. Standard ratio: 2 grams of powder to 0.35–0.5 ml of liquid (which is approximately 7–10 drops). The entire mixing process must be completed within 90 seconds. The mass begins to set in 2 minutes and hardens completely in 5–9 minutes.

Mixing algorithm: The powder is divided into portions on the glass. First, into 4 equal parts. The last quarter is divided in half (two parts of 1/8). One of the 1/8 parts is halved again (two parts of 1/16). Introduction into the liquid is strictly stepwise:

1. First 1/4 part — triturated for 30 seconds.
2. Second 1/4 part — 15 seconds.
3. Third 1/4 part — 15 seconds.
4. 1/8 portion — 10 seconds.
5. 1/16 portion — 10 seconds.
6. Last 1/16 portion — 10 seconds.

Critical errors:

• It is forbidden to add extra liquid into an already thickened mass — this breaks the crystallization process and makes the filling brittle.
• Components must not be placed on the glass in advance. The powder is hygroscopic and will quickly absorb moisture from the air.
• A new portion of powder is introduced only after perfect trituration of the previous one.

The glass must lie motionless on the table. The spatula is held with the entire hand (index finger on top), its working plane moving strictly parallel to the glass. Start with circular motions, and upon thickening, switch to forceful grinding motions.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Characteristics of silicate and silicophosphate mineral cements
• Specifics of working with polymeric cements (glass ionomer and polycarboxylate)
• The effect of oxide hydrates on the partial neutralization of phosphoric acid in the liquid
• Principles of selecting active components for combined therapeutic bases

Other topics in "Chapter 8. Dental Materials"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.