Base denture resins

Classification of restorative materials

Although various compositions are used in clinical practice, special attention is paid to materials capable of compensating for the drawbacks of traditional fillings and ensuring reliable functionality.

Composite materials They are characterized by relatively low polymerization shrinkage (1.6–1.8%). They are versatile and used for filling Class I, II, and Class V cavities in posterior teeth. They are used in layered restoration techniques, for core buildup, splinting, and the fabrication of indirect restorations.

Compomers These are hybrid composite-ionomer compositions. Their structure includes acid groups of glass-ionomers and photopolymerizable resins of composites. Curing takes place in two stages:

1. Light exposure triggers composite polymerization.
2. Moisture from the oral cavity activates the glass-ionomer reaction, creating a fine internal structure.

This mechanism ensures fluoride release and chemical adhesion. Water sorption leads to slight expansion of the filling (up to 3%), which counteracts shrinkage, though it requires control to prevent overhangs. Compomers are ideal for primary teeth, Class III and V cavities, and temporary fillings, but they are inferior in strength to conventional composites.

Ormocers Organically modified ceramics. The matrix of this material occupies an intermediate position between an inorganic silicate network and organic polymers, while glass and ceramics serve as the filler. They possess high strength, biocompatibility, and are indicated for Class I–V cavities.

Principles and types of dental adhesion

Since composites cannot chemically bond to tooth tissues on their own, adhesive systems (bonding agents) are used for their fixation. Adhesion can be of several types:

• Mechanical. Typical for cements and amalgam. Retention is achieved by creating retention points, smooth walls, and forming a box-shaped cavity.
• Chemical. Characteristic of polycarboxylate and glass-ionomer cements. The bond is formed at the molecular level (e.g., the formation of chelate compounds with proteins or an ion-exchange layer involving calcium, fluoride, and aluminum ions).
• Micromechanical. Used for composites. Requires preliminary tissue demineralization and the application of a bonding agent to form a hybrid zone.

Composition of a classic adhesive system

To ensure micromechanical bonding, a three-component system is used, and each step of its application is strictly regulated.

1. Conditioner. Most often, this is 37% phosphoric acid. On enamel, it dissolves inorganic components to a depth of 5–50 µm, removing the surface layer (about 10 µm) and eliminating the organic pellicle. Enamel takes on a «honeycomb» appearance. On dentin, the acid opens the dentinal tubules and exposes collagen fibers.
2. Primer. A mixture of hydrophilic monomers in a solvent (alcohol or acetone). The primer penetrates into the moist dentinal tubules and collagen network.
3. Adhesive (bonding agent). A hydrophobic unfilled resin that polymerizes within the created micropores, forming rigid tags.

The dentin problem and the hybrid layer

While bonding to enamel is easily achieved, dentin presents a serious challenge. Dentin tissue always remains moist due to fluid circulation within the dentinal tubules.

If a hydrophobic enamel bond is applied directly to moist dentin, debonding—the detachment of the filling—will occur, leading to postoperative sensitivity and pulp inflammation.

The primer was introduced specifically to solve this problem. Due to its hydrophilicity, it penetrates through moisture. After its polymerization, a hybrid layer is formed—a unique zone where tooth tissues and polymer resins intertwine. This layer not only firmly binds the composite to the tooth but also serves as a hermetic barrier protecting the pulp from bacteria.

A special role in this process is played by the smear layer (smear layer)—a mixture of hydroxyapatite debris, collagen, saliva, and microorganisms left behind after drilling. Some modern adhesive systems do not remove it with acid, but impregnate it with special monomers, turning this debris layer itself into a strong bonding link.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of chemical adhesion of glass-ionomer cements
• Sandwich technique: using base liners under a composite
• Features of bonding systems that preserve the smear layer
• Morphology of etched enamel and micropores

Other topics in "Chapter 8. Dental Materials"

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