How bond strengths are measured

You’re in the market for a new adhesive and see that the latest option on the market is advertising unprecedented bond strength. Since adhesive strength and reliability can make or break the success of a restoration, you’re obviously intrigued. But what exactly does the company mean when they say the product has the highest bond strength-and how did they measure it?

While the numbers might be alluring, it’s cricital to know where they came from. There are many different bond-strength testing procedures, and each method works differently-and produces different results.

“Bond strength is a term and process that can be measured many different ways,” says Dr. John Flucke, DPR’s technology editor who practices in Lee’s Summit, Mo. “If you are going to rely on bond strength numbers from a manufacturer, it’s important to understand how those numbers were determined.”

There are two types of testing (static and dynamic) used to determine bond strength, but within these categories there are numerous methodologies, each with their own strengths and flaws. When comparing adhesive bond strength, it’s good to know which method was used so you can compare products from different manufacturers using the same scale.

Static tests

Static tests, the more common of the two types of testing, measure bond strength through applying a load to a test specimen while the specimen is stationary. Static tests are broken into two categories: Macro tests (where the bond area is larger than 3 mm²) and micro tests (where the bond area is less than 3 mm², and is usually 1 mm² or less).¹

Macro testing

Known for its simplicity, macro testing is divided into three different methods: shear, tensile and push-out.

Macro-shear bond strength (SBS) testing

Macro-shear bond strength testing is the most commonly used approach for evaluating the bond strength of new adhesives. SBS tests “the maximum stress that a material can withstand before failure in a shear mode of loading.”¹ In shear bond tests, two materials are bonded with an adhesive, and loaded in shear until it fractures.

A quick-and-easy procedure, SBS testing is the simplest and fastest method of testing since no further specimen processing or prep is required once the materials are bonded. However, problems arise in measuring SBS as this method puts a lot of stress on the bonded materials (such as the tooth substrate), which can often fail before the adhesive does. As such, it can put the validity of results into question (did the adhesive fail, or was it the tooth?).²

Macro-tensile bond strength (TBS) test

The TBS test is less commonly used than its SBS counterpart but comes in handy when testing the bond strength of cements to hard materials such as metal alloys and ceramics. In TBS tests, a specimen is loaded into a mechanical testing machine and gripped into place. A load is then applied to either side of the specimen perpendicularly so it doesn’t bend (if the specimen is placed horizontally, bending stress will quickly develop).

A benefit of TBS testing is that the distribution of stress in the test is more uniform than in shear tests. This allows the test to provide a more accurate estimate of the stress level that causes bonds to break.

Push-out (PO) test

Push-out tests are designed to test the fatigue resistance of bonds between adhesives and dentin, particularly in the testing of bonds of posts luted in root canals and root-canal sealers. With this method, researchers take a 1-2mm thick dentin slice and punch a tapered cylindrical hole into it. The inside of the hole is coated in adhesive, and then filled with composite material. As explained by El Mourad in a 2018 study, “The composite cylinder is then pushed through the dentin from the smaller diameter side and the bond strength is calculated by dividing the extrusion force by the lateral area of the tapered cylinder.”¹

These tests provide extremely accurate information because, due to the extraction of the composite, failure is caused parallel to the cement and dentin interface, which mimics clinical conditions more closely than shear tests. However, PO tests aren’t particularly popular, because the specimen preparation and methodology are extremely time consuming, and results are difficult to replicate.

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• Micro testing

Micro testing allows for testing of small tooth areas, though many tests are similar in execution as macro tests. Like macro testing, micro testing is divided into three methods: micro shear, micro tensile and micro push-out.

Micro-Shear Bond Strength (µSBS) Test

First introduced in 2002, µSBS testing requires small samples, which allows for preparation of multiple specimens from the same tooth. This gives tests a more uniform approach and eliminates variables that could affect testing outcome. It is typically used to test glass ionomers, enamel or other properties that might be too delicate for the stressors of micro-tensile bond testing.¹

While similar to SBS testing, µSBS testing was found to be less effective at measuring bond strength than the traditional SBS test.

Micro-tensile bond strength (µTBS) test

µTBS testing is incredibly effective and has a lot of perks that shear tests can’t offer, but it comes at a cost: Because it requires extensive specimen preparation, it’s particularly technique-sensitive and labor-intensive.

In µTBS testing, adhesive resins are bonded to the entire flat occlusal surface of a tooth, and then covered in resin composite. The sample is the cured and stored in water before being vertically sectioned with a slow-speed diamond saw. This provides a cross section of the composite, adhesive, and dentin. This technique provides better stress distribution at the interface, which means failure occurs exclusively at the adhesive bond rather than other surfaces like in shear tests. While it generally provides accurate results, µTBS testing is not reliable when measuring bond strengths less than 5 MPa.

Micro push-out (µPO) test

This modification of the traditional PO test has not been investigated as fully as other approaches. Typically used to measure the bond strength of luted fiber posts, it uses dentin disks that are 1 mm or less in thickness. µPO tests usually result in higher values than normal push-out methods, so its accuracy is still undetermined.

Dynamic tests

Dynamic testing more accurately reflects stress that may occur clinically, since a bond rarely undergoes stationary loading. As a result, these fatigue tests generally claim to better predict the long-term effectiveness of adhesives. They are predominantly applied to dentin bonding, since fatigue is much trickier to quantify in enamel bonds.

While perhaps more clinically indicative, dynamic tests are rare because they are more time consuming and labor-intensive than static-bond strength tests. There’s no standard for adhesive fatigue testing available today, which also makes them difficult to quantify or replicate.³ Since no standard exists, numerous different types of fatigue tests have been reported, including macro- and micro-shear, macro push-out, micro-rotary, micro-tensile and micro 3- and 4-point bend methods, with 3- and 4-point bending tests being the most common.¹

Factors that can affect testing

In addition to the methodologies for assessing bond strength, there are other factors that can affect the results of bond-strength testing.

Tooth composition plays a big part. Oftentimes, cow teeth are used in lieu of human teeth in testing simply because they are easier to acquire. However, bovine dentition has biological differences such as larger dentinal tubules and dentin nearer to the pulp, which can affect the accuracy of the test in regard to human teeth. Even within human teeth however, the type of tooth tested can affect test results. For example, using third molars (which are more permeable than erupted teeth) or teeth with preexisting carious lesions can change the results in bond-strength measurement. The size of the specimen can also change the results, as bond strength decreases with increased surface area.

Another contributing factor is dentin depth, as adhesives display higher bond strengths in superficial dentin, with progressively lower bond strengths as dentin deepens. Additionally, elements including pulpal pressure, composite properties, setup and configuration of mechanical testing, and operator error can all influence results in testing.

Even if these factors could be eliminated, and a standard testing protocol introduced, lab testing has its limits in what it can tell us. Measuring immediate bond strength is relatively simple but determining the long-term bond strength (and potential diminishing effectiveness) is hard to pinpoint. Although aging factors are used in lab testing (such as water storage, mechanical loading, enzyme degradation, or thermocycling), they can’t replicate the real-life stressors to which bonding agents are exposed. As a result, lab results will not always be aligned with clinical behavior.¹

“Most of the latest generation of bonding agents (such as universal adhesives) perform satisfactorily on the bench tests,” says Sridhar Janyavula, BDS, MS, a clinical research dentist at DENTSPLY Caulk. “The key question for clinicians to ask is what data is available to assess a material’s long-term performance beyond the 24-hour lab testing that is presented by most manufacturers. With the limited available data, there are speculations that the ‘too good to be true’ adhesives with bold claims to bond to anything on the face of the planet have demonstrated lower bond strengths over time in aging experiments.”

Evaluation of testing

While bond strength is a critical factor in adhesive success, it’s not the only chaacteristic that should be taken into consideration when evaluating the efficacy of bonding agents. Researchers recommend that bond-strength analysis should be combined with results from microleakage tests, bond durability, and gap evolution tests.¹

But if you do examine bond strength alone when deciding on an adhesive, remember that not all bonding tests are created equal and there isn’t a standardized testing protocol to measure standard bond strength. Ultimately, for bond-strength tests to be accurate and provide concrete information to practitioners, testing strategies need to be regulated and uniform.

“If one manufacturer measures it one way, and another uses a totally different testing system, you are comparing apples to oranges,” Dr. Flucke says. “However, if the testing protocols are identical, now you have data you can use.”

References

1. El Mourad AM. Assessment of Bonding Effectiveness of Adhesive Materials to Tooth Structure using Bond Strength Test Methods: A Review of Literature. Open Dent J. 2018;12:664–678. Published 2018 Sep 28. doi:10.2174/1745017901814010664

2. Heintze, SD., Rousson V., Mahn, E. Bond strength tests of dental adhesive systems and their correlation with clinical results - A meta-analysis. Dental Materials. 2015:31(4):423-434. Published 2015 Feb 21. doi: 10.1016/j.dental.2015.01.011

3. Sirisha K, Rambabu T, Shankar YR, Ravikumar P. Validity of bond strength tests: A critical review: Part I. J Conserv Dent. 2014;17(4):305–311. doi:10.4103/0972-0707.136340