Dentin – Bonding Agents

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What are Dentin Bonding Agents?

Dentin bonding agents are essential in creating a strong bond between the composite (tooth-colored restorative materials) and the or the enamel layers of the tooth, allowing the bond to withstand mechanical forces and stress. The success of these adhesives depends on their ability to adhere to the natural tooth on one side and the composite restoration on the other side.

There are two different types of bonding agents that are commonly used:

1. Enamel bonding agents (EBA)
2. Dentin bonding agents (DBA)

Bonding can be achieved with enamel and dentin or both, through micromechanical interlocking combined with chemical bonding. The bonding between the composite resin restoration and a bonding agent is attained by co-polymerization of the adhesive resin with the resin matrix of composite materials.

Dentin Bonding Cells  

Dentin bonding cells, also known as odontoblasts, are specialized cells found in the dentin layer of teeth. These cells are responsible for producing and maintaining the dentin matrix, which is the hard, calcified tissue that forms the bulk of the tooth.

When a tooth is exposed to damage or decay, dentin bonding cells may be activated to repair and protect the tooth. This process involves the formation of a layer of reparative dentin, which is produced by the dentin bonding cells in response to the injury. This reparative dentin helps to seal off the damaged area and prevent further decay or infection.

Dentin bonding cells also play an important role in dental bonding, a common dental procedure used to repair and restore teeth. In dental bonding, a resin material is applied to the tooth and then bonded to the dentin layer using a special adhesive that interacts with the dentin-bonding cells. This bonding process helps create a strong, durable bond between the tooth and the restoration material.

Who Invented Dentin Bonding Agents?

In 1949, Oskar Hagger, a chemist at DeTrey/Amalgamated Dental Company, developed an adhesive called Sevriton Cavity Seal. Hagger advocated that this adhesive was an acidic material capable of interacting with the tooth surface on a molecular level. It was later discovered that interaction of the acidic bond with the tooth surface demineralizes the enamel and dentin layers, thus creating microscopic pores or irregularities. This process, (called Acid etching) increases the surface area and roughens the tooth structure, allowing for better adhesion of the dental restorative material.

The acidity is helpful because it promotes a stronger bond between the adhesive and the tooth structure. By creating microscopic pores in the enamel and dentin layers, the adhesive can penetrate and interlock with the tooth’s surface, creating a micromechanical bond. This bond results in a more durable and long-lasting restoration, reducing the risk of failure or complications related to the dental treatment.

Oskar Hagger is now known as the “Father of Modern Dental Adhesives” due to his groundbreaking invention. Hagger’s concept was soon adopted by other researchers, leading to the development of several generations of dental adhesives. Even today, many of Oskar’s inventions remain relevant, with the core concept stating that dental adhesives work through micromechanical retention resulting from acid etching of dentin and enamel.

Bonding to Dentin

Understanding the complex process of bonding to dentin is crucial for achieving better outcomes. Dentin bond strength varies, and several factors govern it:

• Quality of dentin: This includes the number, diameter, and size of dentinal tubules in deep and superficial dentin.

• Dentin permeability: Dentin permeability is not uniform throughout the tooth. It is higher in coronal dentin than root dentin.

• Structural differences: Dentin tubules are more numerous and wider near the pulp, resulting in more fluid and less intertubular dentin in this area. This makes dentin bonding less effective in deeper dentin than superficial dentin.

• Amount of collagen: As dentin ages, there is an increase in mineralization and in the ratio of peritubular/intertubular dentin, as well as a decrease in the number of dentinal tubules. These factors collectively reduce the adhesion quality of dentin.

• Smear layer: The presence of a smear layer on a tooth can interfere with the bonding mechanism. To remove or modify the smear layer, many acids and/or calcium chelators are used, including:
  • Acids: 
    • The most commonly used acid for conditioning dentin is 37% phosphoric acid. It removes the smear layer and exposes the microporous collagen network for resin monomer penetration. It forms an amorphous layer composed of denatured collagen fibers and a collapsed residual collagen layer.
    • Other acids used for dentin conditioning include nitric acid, maleic acid, citric acid, oxalic acid, and hydrochloric acid.
  • Calcium chelators: 
    • These agents remove and/or modify the smear layer without demineralizing the surface dentin layer.
    • Ethylene diamine tetra acetic acid (EDTA) is a commonly used chelator.

After removing the smear layer, dentin priming is performed. Primers are agents containing monomers with a hydrophilic end that has an affinity for exposed collagen fibrils and a hydrophobic end that has an affinity for adhesive resin. Commonly used primers have HEMA and 4-META monomers, dissolved in organic solvents.

Primers are employed to increase the diffusion of resin into moist and demineralized dentin, ensuring optimal micromechanical bonding. For ideal primer penetration into demineralized dentin, multiple coats should be applied. Additionally, it is preferable to keep the dentin surface moist; otherwise, collagen fibers can collapse in dry conditions, resisting primer and adhesive resin entry.

Dentin Conditioner for Glass Ionomer

A dentin conditioner is a solution used to prepare the dentin surface before applying certain dental materials, such as glass ionomer cement. The purpose of the dentin conditioner is to remove the smear layer and debris from the dentin surface, exposing the underlying tubules and creating a more porous surface to enhance the bond strength of the glass ionomer cement.

To remove or modify the smear layer, various acids and/or calcium chelators are used for dentin conditioning:

• Acids: The most commonly used acid for conditioning dentin is 37% phosphoric acid. It not only removes the smear layer but also exposes the microporous collagen network into which resin monomers penetrate. This process forms an amorphous layer, a combination of denatured collagen fibers and a collapsed residual collagen layer, making monomer penetration difficult. To prevent the collapse of unsupported collagen fibers, it is preferred to maintain conditioned dentin in a moist state. Other acids used for dentin conditioning include nitric acid, maleic acid, citric acid, oxalic acid, and hydrochloric acid.

• Calcium chelators: These are used to remove and/or modify the smear layer without demineralizing the surface dentin layer. A commonly used chelator is ethylene diamine tetra-acetic acid (EDTA).

For glass ionomer cement, the recommended dentin conditioner is typically 20% polyacrylic acid, an acidic solution proven to effectively remove the smear layer and debris from the dentin surface. The polyacrylic acid is usually applied to the dentin surface for a specific period, then rinsed off and dried before applying the glass ionomer cement.

Using a dentin conditioner can help ensure a strong and durable bond between the glass ionomer cement and the dentin, improving the overall longevity and success of the restoration. However, overuse or improper application can lead to negative outcomes such as sensitivity, poor bond strength, or damage to the underlying dentin. Therefore, it is essential to use the dentin conditioner carefully and follow the manufacturer’s instructions for use.

Does Composite Bond Better to Enamel or to Dentin?

Due to the morphological, histological, and compositional differences between dentin and enamel, bonding to dentin has proven to be more difficult and less reliable and predictable than to enamel. There are several reasons for this:

• Enamel consists of 95% inorganic hydroxyapatite by volume, while dentin contains only 50%.
• Dentin has a higher water content than enamel.
• Hydroxyapatite crystals in enamel exhibit a regular pattern, whereas in dentin, they are randomly arranged within an organic matrix.
• The smear layer in dentin can make it more difficult for the adhesive to wet the surface.
• Dentin contains dentinal tubules housing vital pulp processes and odontoblasts, making it a sensitive structure.
• Dentin is a dynamic tissue, changing due to aging, caries, or operative procedures.
• Fluid constantly flows outwards from dentinal tubules, reducing the adhesion of composite resin to the dentin bond.

Dentin bonding relies heavily on the successful creation of a hybrid layer, which is the highly organic interface between the dentin substrate and the adhesive resin. To create this layer, mineral components in the dentin are replaced by resin monomers, forming a polymer-collagen bio-composite. Dentin bonding can be achieved with etch-and-rinse or with self-etch adhesives. Both techniques work on the principle of removing the smear layer and minerals to expose the dentin’s collagen network. Removing the smear layer is particularly important for forming a hybrid layer, as it can fill the dentin tubules and form smear plugs, decreasing dentin permeability by up to 90 percent. 

After the smear layer is removed, the adhesive resin can infiltrate the collagen matrix and form the base of adhesion for the restoration.

Dentin Adhesives and Their Applications

Dentin adhesives have revolutionized modern dentistry, allowing for more conservative restorations that preserve more of the natural tooth structure. Also known as dental bonding agents, dentin adhesives are materials used to bond composite resin, glass ionomer, and other dental materials to the dentin surface of a tooth.

There are several types of dentin adhesives available, each with its own unique properties and indications for use. Some of the most commonly used types include:

• Etch-and-Rinse Adhesives: These adhesives require the dentin to be etched with an acidic solution to remove the smear layer and expose the underlying dentin tubules. The adhesive is then applied, followed by the application of the restorative material. Etch-and-rinse adhesives are typically used for direct restorations such as composite resin fillings.
• Total-Etch Adhesives: Similar to etch-and-rinse adhesives, total-etch adhesives require the dentin to be completely dried before the adhesive is applied. They are often used for more complex restorations such as crowns or bridges.
• Self-Etch Adhesives: Designed to simultaneously etch and prime the dentin surface, self-etch adhesives eliminate the need for a separate etching step. They are often used for indirect restorations such as veneers or inlays.
• Universal Adhesives: Versatile in nature, universal adhesives can be used with both etch-and-rinse and self-etch techniques. They are often used for direct restorations and can be applied in a single step, simplifying the bonding process.

Dentin adhesives have a wide range of applications in modern dentistry, including the placement of composite fillings, the bonding of veneers and crowns, and the repair of chipped or broken teeth. With proper technique and selection of the appropriate adhesive for each case, dentin adhesives can provide strong and long-lasting bonds that result in successful restorations and improved patient outcomes.

Dentin Bonding Mechanisms and Dental Restorations

The mechanism of dentin bonding involves an adhesive molecule with a bifunctional structure:

M———————R———————X

Where:

• M is the double bond of methacrylate, which copolymerizes with composite resin.
• R is the spacer that makes molecules large.
• X is a functional group for bonding that bonds to inorganic or organic portions of dentin.

It is important to note that dentin bonding agents typically have both hydrophilic (Water-attracting) and hydrophobic (Water-repelling) ends. The hydrophilic end interacts with the dentinal fluid, wetting the surface, while the hydrophobic end bonds to the composite resin.

Bonding to the inorganic part of dentin involves ionic interactions between the negatively charged group on X (e.g., phosphates, amino acids, amino alcohols, or dicarboxylates) and the positively charged calcium ions. Commonly used bonding systems employ phosphates.

Bonding to the organic part of dentin involves interactions with amino (–NH), hydroxyl (–OH), carboxylate (–COOH), and amide (–CONH) groups present in dentinal collagen. Dentin bonding agents contain isocyanates, aldehydes, carboxylic acid anhydrides, and carboxylic acid chlorides, which extract hydrogen from the aforementioned groups and bond chemically.

Dentin and Dental Materials Research

Dentin is the hard, calcified tissue that forms the bulk of a tooth. It consists of a network of tiny, interconnected tubules extending from the tooth’s outer surface to the inner pulp chamber. Dentin is crucial for the structural integrity of teeth and is responsible for transmitting sensations such as pain, pressure, and temperature to the pulp.

Dental materials research is a field of study focusing on developing and improving materials used in dentistry. The goal is to create biocompatible, durable, and aesthetically pleasing materials, including restorative materials (like dental composites and amalgams), impression materials, prosthetic materials, and dental implant materials.

Current research areas in dentin and dental materials include:

 

• Biomimetic materials: Researchers are exploring nature-inspired materials that mimic the properties of natural teeth, such as materials that stimulate new dentin formation or regenerate pulp tissue.

• Adhesion: Adhesion is a critical factor in dental restoration success. Researchers are working to improve the adhesion of dental materials to dentin and enamel, enhancing restoration longevity.

• Biocompatibility: Dental materials must be biocompatible, meaning they do not cause adverse reactions in the body. Researchers are studying new materials’ biocompatibility and enhancing existing materials’ biocompatibility.

• Nanotechnology: Incorporating nanoscale particles and structures into dental materials can enhance their mechanical, chemical, and biological properties, leading to improved performance and longevity. Nanoparticles are also employed to enhance dental materials’ antibacterial properties, helping prevent tooth decay and infection.

• Digital dentistry: Digital technologies like 3D printing and computer-aided design (CAD) and digital imaging are utilized to create custom-made dental restorations with greater precision and accuracy. Researchers are also investigating the use of digital technologies to improve dental implant placement and diagnose dental conditions.

Dentin and dental materials research has the potential to revolutionize dental care by providing more effective, longer-lasting, and patient-specific solutions, ultimately improving oral health and overall quality of life for patients.