The Hybrid Layer: The Chemistry Lesson You Likely Forgot from Dental School

The Hybrid Layer. Photo courtesy of kajonsakt/stock.adobe.com.

Ah, dental school. You probably remember the first time you had someone watching your every move as you isolated and prepared a tooth for bonding. You can feel the scrutiny, as well as the encouragement. You might even remember the technical advice. But do you remember what happens between the tooth and the material in the hybrid layer that makes adhesive dentistry possible?

If not, never fear. Today, we are reviewing the Hybrid Layer and why you should remember it and how to optimize it to benefit the outcomes of your composite restorations.

So, What is the Hybrid Layer and Why is it Important to Adhesive Dentistry?

The hybrid layer is essential to adhesive dentistry. It is where the dental adhesive system creates a micromechanical bond with dentinal collagen.¹ The hybrid layer is also defined by what it is not. Per the Journal of Esthetic Dentistry, the hybrid layer is “neither resin nor tooth.” Instead, as the name implies, it is a combination of the 2.² Per Nathaniel Lawson, DMD, PhD, Associate Professor, Director, Division of Biomaterials UAB School of Dentistry the hybrid layer is how we bond to dentin. It consists of demineralized collagen that is intertwined with cured adhesive monomer.

The hybrid layer contributes much to the longevity of your dental composite restorations. The polymerization within the living tissue creates a thin layer, which serves as a foundation that joins the different materials. The interface the hybrid layer provides seals the surface of the restoration against leakage and provides acid resistance.² 

The hybrid layer also provides a stress buffer for dental restorations. The collagen present serves to reinforce long-term microtensile bond strength. While studies show that the bond strength decreases for all adhesives, no matter what type of treatment the dentin has, those with depleted collagen decrease even more. In other words, collagen is essential for stress-buffering in the hybrid layer.³ 

Forming a Proper Hybrid Layer

A crucial factor in dentin bonding is the formation of the hybrid layer. The quality of this formation has direct consequences on the resin-dentin interface. Some studies suggest that thicker more uniform hybrid layers increase bond strength, which the researchers attribute to an etch-and-rinse system.²

Dental professionals have dental adhesive system options for creating the hybrid layer. These systems have a dentin bonding agent that facilitates bonding between the dentin and the resin composite material or cement. These light-cured adhesives provide the link between the hydrophilic resin primer and the hydrophobic resin composite.²

The etchants for these dentin adhesive systems can be separate, aka etch-and-rinse, or built-in, aka self-etch. Both have their uses and benefits for how they form the hybrid layer. For example, Dr Lawson explains that in an etch-and-rinse adhesive, it is formed by applying phosphoric acid, rinsing, and applying an adhesive. 

“The phosphoric acid will remove the smear layer and remove some of the hydroxyapatite mineral from the surface of the dentin. Removing hydroxyapatite exposes strands of collagen. When the adhesive is applied, it becomes intertwined with the demineralized collagen,” Dr Lawson explains.

Self-etch adhesives do not have a separate acid-etch step, as the name implies. They condition and prime both enamel and dentin at the same time. These adhesives infiltrate and partially dissolve the smear layer and hydroxyapatite to create the hybrid layer.¹

There are different advantages and disadvantages to etch-and-rinse and self-etch adhesive system regarding the hybrid layer:¹

Etch-and-Rinse Advantages

• Forms a thicker, more uniform hybrid layer
• Facilitates longer resin tags

Self-Etch Advantages

• Has fewer steps than etch-and-rinse systems
• Decreased technique sensitivity due to inability to over-etch the dentin or over-dry the dentin after rinsing off the etchant
• Some have suggested lower sensitivity in deep restorations due to avoidance of phosphoric acid near pulp

In a self-etch adhesive, Dr Lawson explains, the adhesive contains acidic monomers that can solubilize the smear layer and partially demineralize the dentin. That is why etch-and-rinse adhesives have a thicker hybrid layer than self-etch adhesives. However, Dr Lawson says that does not mean that etch-and-rinse hybrid layers are better.

“Preserving some hydroxyapatite can be a benefit as newer adhesives have monomers which can chemically bond with calcium in the hydroxyapatite,” Dr Lawson says. 

A Common Mistake Dental Professionals Make Regarding the Hybrid Layer—and How to Avoid It

Tooth surface preparation is essential for adhesive dentistry. It is well known that bonding to enamel is easier than bonding to dentin. One reason is because the technique gets more complex in the dentin layer than with enamel. Discipline regarding isolation measures to provide a contaminant-free environment is also essential, as is a to-the-letter adherence to adhesive protocol. If the case does not allow for the achievement of these high standards, dental professionals might be better off switching to a less demanding restorative material choice, such as glass ionomers. 

Another common problem is using materials that are not appropriate for working deep in the dentin layer. Unlike enamel, which has relatively little water in its makeup, dentin has lots of water in its composition. Moreover, when you cut and etch the deeper dentin and get closer to the pulp, it releases water, which is not ideal for bonding. Therefore, choosing a primer that is hydrophilic when you are creating a hybrid layer in deep dentin is essential. 

The most common mistakes with the hybrid layer occur because of technique problems, usually with etch and rinse adhesives. For example, sometimes clinicians can over etch dentin layers. Dr Lawson says if you over etch dentin, you expose more collagen. Adhesive is hydrophobic and so it will only penetrate so deep into the demineralized collagen. Therefore, over-etched dentin will have demineralized dentin which has not been infiltrated with adhesive. Enzymes like MMPs (Matrix metalloproteinases) or water, can degrade these areas, Dr Lawson says. 

However, clinicians can avoid this problem. Per the 2020 research published in the journal Polymers, dentists facing over-etched dentin can use a universal adhesive with an application time of at least 20 seconds and expect sufficient bond strength.⁴ 

Dr Lawson says to preserve the hybrid layer, some clinicians will use a 2% chlorhexidine solution after etching prior to applying adhesive. The chlorhexidine will inactivate MMP enzymes that can degrade collagen bonds. Other clinicians may crosslink collagen prior to applying adhesive to strengthen the hybrid layer.

“This can be accomplished with currently with products which contain glutaraldehyde, however there are new molecules which are being studied for their ability to crosslink collagen,” Dr Lawson explains.