Unveiling the secrets of monomer molecules in dental composites

To achieve a successful resin restoration, you need the right resin composition—one with monomer molecules that polymerize well and form minimal stress. This article will reveal what monomers are worth knowing about and how your resin choice impacts your finished resin restoration.

How do you choose the resin product you use in your clinic? The factors can be diverse, including the tooth and location being treated, the procedure requirements, and the desired physical, mechanical, and aesthetic properties. The vast array of dental resins available on the market, each with its own unique properties, advantages, and disadvantages, can make it difficult to choose. In reality, dental composite resins are complex systems, and the final performance of the product depends largely on each component, how they are combined, and how they interact. Understanding your dental resin product goes beyond simply knowing the resin brand; it’s also important to understand its composition.

The monomer molecules in dental resins are often overlooked, yet they are present in every completed resin filling and serve as a crucial building block. Much like choosing the right bricks for building a wall, understanding the fundamentals of resin monomers and how they are incorporated into your resin can significantly impact the outcome of your resin filling.

What is the function of monomer molecules?

While not all resin systems on the market are identical, they generally consist of the following key components: a resin matrix, fillers, coupling agents, initiators, stabilizers, and dyes, which are mixed in varying proportions and configurations to achieve specific performance effects. But what role do monomers play in this process? On their own, monomers are like individual bricks. Once combined, they form the resin matrix, a common component of resin products.

Monomers are specialized small molecules that, under conditions of light, heat, or pressure, can crosslink and bond with each other to form more complex molecules known as polymers. Polymers are found in nearly every material we encounter in our daily lives, as well as in many dental products. In dental resin products, polymers form a crosslinked, three-dimensional network structure known as the resin matrix. Fillers are then added to reinforce this crosslinked structure. The resin matrix not only provides primary support for the resin filling but also plays a crucial role in the resin’s physical, mechanical, and optical properties.

Select the chemical composition of the matrix

There are many different monomers available on the market. The most commonly used monomers by dental product manufacturers are bisphenol A glycidyl dimethacrylate (BisGMA) and its ethoxylate (BisEMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA). These acrylic monomers are often used in varying proportions or combinations to complement each other’s strengths and weaknesses.

BisGMA

BisGMA was first used in dental composite resins in 1962 and remains the most commonly used monomer in dentistry. Its high molecular weight and small number of double bonds result in low polymerization shrinkage, high reactivity, low toxicity, and rapid polymerization cure, allowing it to form a strong cross-linked polymer structure. 2-6,8 However, its high viscosity not only affects the resin’s handling properties but also reduces filler-matrix compatibility and monomer conversion, which negatively impacts the mechanical properties of the resin filling, including wear resistance and volume shrinkage. Therefore, BisGMA is often combined with other low-viscosity monomers, such as TEGDMA, to improve the resin’s handling and physical properties.

TEGDMA

TEGDMA is a highly tough, low-molecular-weight, low-viscosity monomer molecule. It is often used as a diluent for other high-viscosity monomers to improve resin handling properties, increase filler content, and enhance copolymerization. Unfortunately, TEGDMA also has its drawbacks: it results in higher water absorption, lower mechanical properties, and poor color stability.

BisEMA

It is essentially a slightly modified form of BisGMA, boasting high molecular weight and hardness, but low viscosity. This results in higher monomer conversion, lower polymerization shrinkage, lower water absorption, and improved mechanical properties. Therefore, it is often used in composite resin matrix systems as a replacement for TEGDMA.

UDMA

UDMA was developed to overcome the limitations of BisGMA. It is a high-molecular-weight, low-viscosity monomer that combines high strength and toughness. Clinically, UDMA resins demonstrate not only higher flexural strength, elastic modulus, and hardness, but also higher monomer conversion rates compared to BisGMA-based resins.

However, it’s not perfect. UDMA has a higher viscosity than TEGDMA and BisEMA, which may limit handling and filler compatibility. Furthermore, it has a lower refractive index than both BisGMA and BisEMA, which may reduce its optical compatibility with common radiopaque filler systems.

However, not every resin on the market has the same formulation or uses the same monomers. Therefore, it can be difficult to accurately determine the exact composition of the resin you’re using and whether you’ll truly reap all the benefits promised by the manufacturer. Think of it like building a wall with stones of varying materials and sizes. Without the right cement to strengthen and bind the stones, a homogeneous and strong wall structure is impossible.

AFM

Unlike the monomer molecules of traditional dental resins, AFM possesses a third reactive site that breaks and recombines during polymerization, thereby relieving the internal stress of the polymer crosslink. The broken monomer molecules then repolymerize under low stress, relieving internal stress without compromising the mechanical properties of the polymer.

AUDMA

AUDMA has a higher molecular weight than traditional dimethacrylates, which has the advantage of limiting the number of monomer molecules that can be accommodated within a space. This reduced number of monomer molecules means less volume shrinkage caused by polymerization and also controls the rate of hardness development in the final polymer. Both of these factors help reduce shrinkage stress.

Importance

Choosing the right dental resin can be daunting, especially with the plethora of products available, each with varying compositions, properties, and benefits. However, it’s also important to understand the resin’s composition, including its polymer and monomer components.

Traditional resin systems, due to their long history of clinical use, are often preferred. However, these systems have their own limitations, and resins have evolved significantly since their introduction in the 1950s. Exploring new resin systems can offer new advantages to your practice, overcoming many of the limitations of traditional resin systems and making your treatment easier and more controlled.