Tooth Bonding Science -

Acid-etch technique. / How the bond is created. / What is dental composite? / Curing lights.

One of the hallmark characteristics of tooth bonding is the great strength with which it adheres to tooth enamel.

And while you might expect that the formation of this bond is somewhat complicated, its principles are actually quite simple and easy to understand.

Here's how tooth bonding works. It's based on what's referred to as acid-etch technique.

A) The tooth's enamel is treated with an acidic conditioner.


Enamel is the most highly mineralized body tissue. (98% mineral content vs. 70% for bone.)

If an acidic solution is placed on enamel (animation frame #2), it will dissolve away some of this mineral content.

And after this treatment, at a microscopic level its smooth surface will be transformed into one that's jagged and rough (animation frame #3).

Animation showing how after acid etching the tooth's surface is very rough.

Acidic etching gel is used to roughen up the enamel's surface.

Takeaways from this section.

The etching gels used in dentistry usually contain around 35% phosphoric acid and are left on the tooth's surface for about 20 seconds. We outline the steps of the etching process involved here.

A familiar comparison.

In terms of something you are probably familiar with, etched enamel is very similar to frosted (etched) glass.

With it, as you run your hand over its surface you can feel that it has a texture. That's because, at a microscopic level, the glass is quite rough, just like very fine sandpaper.

You're probably also familiar with the fact that etched glass has a dull, frosted appearance. In similar fashion, if you dry etched enamel it too shows this same distinctive frosted look.

B) How a bond to the tooth is formed.

Dental bonding science exploits the microscopic surface roughness of etched tooth enamel (animation frame #1 below).

The dentist coats the tooth's etched surface with a liquid plastic referred to as "bonding agent" (animation frame #2 below).

Since it's a liquid, it's able to seep in between the nooks and crannies of the tooth's etched surface. Then, once it's cured (hardened), it becomes locked (bonded) onto the tooth's surface.

Animation showing how dental bonding locks onto the roughened enamel surface.

1) The bonding agent locks on to the enamel.

2) The dental composite, in turn, is bonded to the bonding-agent layer.

The bond is a simple mechanical one.

So, now you know. The attachment that dental bonding creates with a tooth is a simple mechanical one. It's due to an interlocking of the cured bonding agent within the nooks can crannies of the etched enamel surface.

Takeaways from this section.

Although a simple process, acid-etch technique (etching a tooth's enamel surface and creating a bond with it) is responsible for revolutionizing dentistry.

It's the basis of all modern adhesive dental procedures (bonding, porcelain veneers, dental sealants, placing orthodontic brackets) and remains little changed since its first introduction in 1955.

C) How a bonded restoration is created.

At this point, the tooth's surface has only been covered with a very thin layer of plastic (the bonding agent) (animation frame #2).

So to give the dental restoration its needed bulk and shape, successive layers of a dental material called dental composite (see below) are added to the initial sublayer until the restoration takes its needed form (animation frame #3).

As each layer is placed, it creates a chemical bond with the bonding agent sublayer and/or a previously placed layer of composite.

Notice that we said a chemical bond is created, as opposed to a mechanical one like that which exists between the bonding agent and the etched enamel surface.

Bonding to tooth dentin.

Animation showing the enamel vs. dentin portions of a tooth.

A bond can be created with both tooth enamel and dentin.

Creating a bond with tooth dentin, while similar, is a more complicated affair and one whose understanding is still evolving.

What is dentin?

You might not even be aware that a tooth tissue called dentin exists.

Teeth are not solid enamel but instead it's just an outer covering over the part of a tooth we can see.

If a tooth has broken or decayed, or if its gum line has receded, portions of its dentin become visible.

The science of bonding to dentin is fairly involved and beyond the scope of our discussion here. But the fact that it is possible helps to improve restoration integrity and adhesion.

It's all a one-step process.

As a point of interest, the steps your dentist takes when creating this bond are essentially the same as those used with enamel. And, in fact, both processes are accomplished simultaneously using the same agents and materials.

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What kind of material is dental composite?

At its core, dental composite is just a plastic compound. Because the physical properties of this core plastic leave a lot to be desired, fillers and modifiers are added. They help to enhance characteristics such as strength, wear resistance, consistency and color. After all have been added in, the end product is a white, putty-like material.

(The types of fillers usually used are finely ground particles of quartz, glass, zirconium, silica, barium, and strontium.)

The specific types and amounts of modifiers that are added depend on the type of applications the composite is intended for. For example, if it will be used to create white fillings for back teeth, then fillers that enhance strength and wear resistance will predominate.

In comparison, if the composite will be used in applications with front teeth, then fillers that enhance the composite's color, translucency and polishing characteristics will be emphasized.

Animation of a dental composite curing light.

A dental curing light shines blue light that sets dental composite.

Light curing.

Modern dental composites are formulated with a catalyst that triggers its hardening process. What's key is that this catalyst is only activated when it's exposed to a special color of light.

That means the dentist can work with and shape the composite at their leisure. And then, when they're satisfied with their work, they can shine their curing light on it and it will harden within 20 to 40 seconds.

Curing light units.

Dental "curing lights" are hand-held units that produce visible spectrum output (light) that lies within a specific wavelength range. Usually this is blue-colored light that has a wavelength between 420 and 450 nm.



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