TBU: # 047 The Natural Tooth And The Engineering of Bonding
Apr 29, 2023
New Issue of The Biomimetic Uprising
Read Time: 2 min
The natural tooth is a complex biological structure composed of different tissues such as enamel, dentin, and pulp. Each of these tissues has unique mechanical properties that determine their stress, strain, and modulus of elasticity.
Enamel is the outermost layer of the tooth and is the hardest tissue in the human body. Its modulus of elasticity is approximately 83 GPa, which means that it is relatively stiff and resists deformation when subjected to stress. Enamel is also brittle, meaning it is prone to fracture when subjected to high stress.
Dentin is the tissue beneath the enamel and is less stiff than enamel, with a modulus of elasticity of approximately 18 GPa. Dentin is also more flexible than enamel and is able to deform more without fracturing. Dentin can also absorb more energy before it fails compared to enamel.
The pulp is the innermost tissue of the tooth and contains nerves and blood vessels. It is the softest tissue in the tooth and has a modulus of elasticity of approximately 0.02 GPa. The pulp is also highly deformable and can absorb large amounts of energy before failure.
The tensile force required to pull apart the DEJ (Dentinoenamel Junction) of a natural tooth depends on several factors, including the specific tooth type, the age and health of the tooth, and the testing method used.
Several studies have attempted to measure the tensile strength of the DEJ using different experimental techniques, such as micropush-out tests, nanoindentation, and tensile tests. The reported values for the tensile strength of the DEJ vary widely depending on the methodology used, with some studies reporting values as low as 10-20 MPa, and others reporting values as high as 200-300 MPa.
It is important to note that the DEJ is a complex structure with a gradual transition in composition and mechanical properties between the enamel and dentin layers. This makes it difficult to isolate and test the DEJ independently, and also means that the mechanical properties of the DEJ can vary along its length and between different regions of the tooth.
The tensile strength of the DEJ is an important parameter for understanding the mechanical behavior of the tooth.
Within Biomimetic Dentistry, the micro tensile force of the DEJ is typically considered to be 51.5 MPa. Its important to consider these properties, forces, and concepts when doing our bonding protocols.
As per last weeks article, we should be visualizing what we are bonding to. In dentistry, we bond unfilled adhesive, filled adhesive, flowable composite, short fiber reinforced composite, dentin composite, highly filled 'enamel type' composite, porcelain, zirconia among numerous versions of all of those. We should be careful what get bonded to certain portions of the tooth and in the right sequence. We need to be mindful of the natural tooth and its architecture of the layers. For example, We shouldn't be bonding highly filled enamel like composite to deep areas of the tooth without building up the proper sequence to support that material. This is also one reason biomimetic dentists shouldn't prefer to use zirconia whenever possible. The material is too hard and can cause underling issues to the tooth, especially near the gum line, or bio-rim.
Bonding these different types of materials can have a dramatic impact on the tooth.
When attempting to bond two dissimilar materials, stress and strain can arise due to differences in physical properties such as thermal expansion coefficient, hardness, and elasticity. The stress and strain can be caused by differences in the way the materials respond to applied loads and changes in temperature.
During the bonding process, the materials may undergo significant deformation due to differences in their physical properties. For example, if a hard and brittle material is bonded to a soft and flexible material, the hard material may cause localized stress and strain in the soft material during bonding. This can lead to cracking or failure of the bond over time.
In addition, differences in the thermal expansion coefficient of the materials can cause additional stress and strain during temperature changes. When the bonded materials are exposed to temperature fluctuations, the different rates of thermal expansion can cause the bond to weaken or even break.
To mitigate the effects of stress and strain when bonding dissimilar materials, it is important to carefully select the adhesive and bonding method used. Adhesives with high flexibility and strong bonding properties can help to minimize the effects of stress and strain during bonding. My favorites are Optibond FL by Kerr and SE Protect by Kuraray. Proper surface preparation, including cleaning and roughening the bonding surfaces, can also help to create a stronger bond and minimize the risk of failure due to stress and strain.
The mechanical properties of the natural tooth can vary depending on factors such as age, location, and health of the tooth. However, the overall structure of the tooth is designed to withstand the stresses and strains of normal oral function, such as biting and chewing. With proper bonding techniques and protocols we are able to restore the tooth back to how the tooth was before the decay and damaged happened.
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