What is shear and tensile bond strength of dental resin composite?

May 29, 2022

A dental restoration is used to restore the strength and functional component of the tooth following the removal of the carious portions. Any dental restoration must be able to resist masticatory forces and also be esthetic at the same time. The resin composite is one such material.

Introduced in 1960s, the composite resin has seen eight generations of the materialistic changes. A composite resin usually has a main resin component, filler particles and a coupling agent that binds the resin component to the filler particles. The resin component forms the primary polymer network of the material. But it is the filler particles that ensure that the material has higher wear resistance and better strength.

Advancements in composite resin have been because of the manner in which the size of the filler particles have changed over the years. Starting from minifillers to microfillers to nanofillers, the composite resins have evolved in their strengths and wear resistance. Some of the composite resins today have hybrid fillers, that is a combination of nano, micro or mini fillers. Fillers particles can be of glass, zirconia, silicon or a combination of these materials.

Shear and tensile bond strengths refer to the resistance of the composite resin to masticatory forces when it is bonded to the tooth. Tensile strength refers to the maximum stress a material can withstand before it dislodge. Shear strength refers to the resistance of a material before it dislodge against a shearing mode of loading.

Tensile strength of a material is calculated by forming a specimen of a material in a cylindrical mould. It is then subjected to a diametral tensile test. In this test, the specimen then is pulled via a testing machine till a threshold is reached. The values are then calculated by multiplying the load applied into two and dividing it by the diameter and thickness of the specimen.

Shear strength of a material is calculated to check the shear strength between two surfaces. In this case, the strength between the material and tooth surface is calculated. The force applied during the punch out test is divided by the diameter and thickness of the specimen.

The measure of a material's shear or tensile strength might not be entirely suggestive of its durability. It has been debated that the success of a material's bond to the tooth lies at the interface between the tooth and the material, and not the inherent properties of the material itself. A lot of factors dictate the shear or tensile strength of the resin.

• Etching

Etching refers to a process where an acidic agent is used to create tags within the tooth's surface. The composite resin then binds to the tooth chemo-mechanically by interlocking with the resin tags. The time till which the acid is made to stay on the tooth's surfaces before washing and drying the tooth also affects the formation of tags. Moreover, the concentration of the acid used also plays a role in the formation of pores.

• Priming/Bonding

The type of bonding agent used also influences the adhesion of the material to the tooth. This bonding agent is used to facilitate the bond between the material and the dentin of the tooth.

• Polymerization shrinkage

Resin materials once set, tend to shrink. This can cause a gap between the tooth and the resin material, leading to compromised bonding. The gap can be a source of microleakage that can lead to failure of the material.

• Stress Distribution

The way the composite material is placed on the tooth is also important. If the filling has high points, then the masticatory forces may lead to improper stress distribution and cause cohesive failure of the material.

The tensile strength of composite based on the type of filler content is as follows:

Hybrid: 30 - 55 MPa
Nanohybrids: 81 MPa
Microfilled: 25 - 40 MPa

The shear bond strength of resin composite can vary between 10 MPa to 21 MPa.

Tensile and shear bond strength of a material are an important feature for ensuring their durability. They represent the material's mechanical properties and vary based on their composition. However, these factors are as important as the clinical technique for ensuring better results.

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