Etching techniques unpacked

November 19, 2020
Laura Dorr
Laura Dorr

Laura Dorr is the executive editor of DPR's Modern Dental Network.

Breaking down indications for etching and how to choose the best approach.

To etch or not to etch? That is the question—and it’s been around for years. Acid etching on enamel was introduced by Dr Michael Buonocore in the 1950s. Buonocore found that bond retention was improved when enamel was etched with 85% phosphoric acid, and the technique has been hailed as the advent of bonding to tooth structure.

While 85% phosphoric acid is seen as overkill today (it’s best to stick to 35% to 40% concentrations), acid etching is still used to increase retention and bond strength of resin-based adhesives.

Over the years, etching has expanded to include not only enamel, but also dentin, which prompted the introduction of alternative etching techniques that didn’t use phosphoric acid.

Etching demineralizes portions of the hydroxyapatite crystals found in enamel and dentin to create a porous surface that allows the formation of resin tags in the dentin’s structure. Essentially, an etched surface has pores that allow the resin to permeate and attach to the tooth.

But etching techniques vary depending on the shape and size of the prep and whether you’re trying to bond to dentin or enamel. Even within those categories, there are 3 approaches to consider: total-etch, selective-etch, and self-etch. But which is the best technique? It depends.

The surface

Etching techniques are dependent on the surface that is being etched, as enamel and dentin respond differently to etching.

  • Enamel etching

Enamel is highly inorganic, while dentin is a more hydrated tissue made up of 20% water and 30% collagen. Composed of approximately 92% inorganic hydroxyapatite, enamel’s structure and crystal arrangement are much more regular than dentin’s structure. This makes enamel etching and bonding significantly easier and more predictable—simply isolate, apply acid etchant for 10 to 15 seconds, rinse, and bond.

  • Dentin etching

Dentin is only approximately 50% inorganic and contains fluid-filled tubules. Since dentin is hydrated, it can make bonding difficult, as bonding is inherently hydrophobic. Additionally, dentin’s composition and permeability vary by region and depth of the tooth, making it less uniform than enamel. Etching on dentin can be tricky, and the results can be more unpredictable than enamel etching. Many practitioners hesitate to etch on dentin at all.

“Less-retentive preps benefit most from etching, although it is much harder to etch dentin, which is normally what there is most of in a compromised crown prep,” says John Flucke, DDS, Dental Product Report®’s technology editor, who practices in Lee’s Summit, Missouri. “The bond strength to enamel is much stronger than to dentin.”

Dentin bonding can be achieved through self-, selective-, or total-etch techniques. While self-etch systems have lower bond strengths, which can be problematic in less-retentive preps, they also generally result in lower rates of postop sensitivity. This can be a serious problem when etching on dentin, particularly when over-etched.

In addition to causing postop sensitivity, over-etching can also be a culprit in bond failure, because if the demineralization penetrates farther into the tubules than the resin can travel, it can result in a gap.

Total-etch

In total-etch procedures, acid etching is performed on the surfaces of both the enamel and dentin layers. This classic technique applies 30% to 40% phosphoric acid to the entire preparation, which removes the smear layer on dentin, opens the underlying dentin tubules, and exposes the collagen fibers, which are infiltrated by a hydrophilic resin monomer to create a bond—all while also creating a suitable etch pattern on enamel. The phosphoric-acid application is followed by a rinse and subsequent application of primer and bonding agent.

Total-etch procedure presents the highest likelihood of postop sensitivity if not executed correctly. Because all of the dentinal canals are opened during total-etch, there is a risk that restorative materials may irritate the pulp if the canals are not closed appropriately by the bonding material. This is a real concern, as technique variability is high in this approach. There is a delicate balance to ensure that the enamel is etched properly without over-etching the dentin. Residual moisture after rinsing dentin and the amount of time it takes to etch the dentin also can be detrimental to the bond.

Selective-etch

In scenarios where the working surface includes both enamel and dentin, selective-etch (or hybrid-etch) technique gives clinicians the ability to apply phosphoric acid only to the enamel area and use a self-etch adhesive on the dentin.

“I personally believe in a selective-etch approach using a universal bonding agent,” says, Rolando Nuñez, DDS, manager of clinical research at BISCO. “Literature has shown that etching enamel with phosphoric acid is of the essence to achieve a predictable and strong bond.”

By providing a deep etch on the enamel, this technique optimizes enamel bond strength, while removing the risk of over-drying or over-etching the dentin, which can lead to postoperative sensitivity. This approach can be particularly advantageous when working with deep areas in the tooth where acid is at increased risk of causing sensitivity.

While selective-etching may take a few more steps than other etching techniques, treating each surface individually can create a stronger bond. However, precision is required: Practitioners must ensure that no etchant reaches the dentin, as it can compromise bond strength.

Self-etch

Self-etching has grown in popularity, as it eliminates some of the risks of total-etch systems (such as over-etching that can lead to postop sensitivity) and performs well for dentin bonding.

By using acidic monomers instead of phosphoric acid to demineralize the tooth structure and remove the smear layer, self-etch bonding systems can reduce the risk of bond degradation caused by matrix metalloproteinase, as well as the aforementioned sensitivity. Self-etch eliminated the need for etching and rinsing required by total-etch, and it also prevents the collapse of the collagen fibers, which can be a problem with the total-etch process due to conditioning and drying.

“Self-etch has proven to work very well on dentin,” Dr Nuñez says. “This approach will yield high bond strength and protect the pulp at the same time. There is no true need to etch dentin with phosphoric acid.”

Self-etch can also be simpler than total- or selective-etch techniques. In addition to not having to worry about over-etching or overdrying dentin, practitioners don’t have to worry about the timing of acid exposure on enamel versus dentin.

“Sometimes I think doctors will first try a self-etching cement because there are fewer steps to its use and controlling the field is easier,” Dr Flucke says. “Proper control of the field is critical to adhesive dentistry, and it’s always much more challenging to try to control the field on a lower second molar with a patient [who] has a very high salivary flow. In cases such as these, it may help to use some adjunct evacuation system such as Isolite (Zyris), Mr. Thirsty (Zirc), or DryShield to help control the situation.”

However, because self-etch systems aren’t as strong as acid etch approaches, bond strengths can often be lower.

A place for all

In the end, there is a time and place for total-, selective- and self-etch approaches when it comes to preparing enamel and dentin for restorations. Clinicians should consider which approach will create the strongest bond with the lowest risk for postop sensitivity—and ultimately, the greatest chance of restoration success.