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ISO 29022:2013 — Dentistry — Adhesion — Notched-Edge Shear Bond Strength Test
A comprehensive technical guide to the notched-edge shear bond strength test for dental adhesion according to ISO 29022
Introduction to ISO 29022
ISO 29022:2013 specifies a test method for determining the shear bond strength of dental adhesive systems bonded to human tooth substrate, using a specialized notched-edge shear test apparatus. This standard was developed in response to the need for a more reproducible and clinically relevant bond strength test methodology than existing alternatives (such as the conventional shear bond strength test with a wire loop or the micro-tensile bond strength test). The notched-edge design provides controlled stress distribution at the bonded interface, reducing the variability that has historically plagued dental adhesion testing.
For dental materials researchers, ISO 29022 represents a significant improvement over traditional wire-loop shear testing. The standardized notched-edge geometry creates a more uniform stress distribution at the adhesive interface, reducing data scatter and providing more reliable differentiation between adhesive systems. Studies have shown coefficient of variation improvements from 30-50% (wire-loop) to 15-25% (notched-edge).
The standard is applicable to all types of dental adhesive systems used for bonding restorative materials (composites, compomers, resin-modified glass ionomers) to tooth structure. It covers specimen preparation, storage conditions, testing apparatus specifications, loading parameters, and data reporting requirements.
Specimen Preparation and Substrate Handling
Tooth Selection and Storage
ISO 29022 provides detailed specifications for tooth substrate selection and handling, recognizing that substrate variability is a major source of test result scatter. The standard specifies that extracted human molars be used within a defined timeframe after extraction, stored in an appropriate medium, and examined for defects. The tooth substrate preparation involves embedding in acrylic or epoxy resin, exposing a flat enamel or dentin surface by grinding, and standardizing the surface roughness to a defined grit size.
| Parameter | Specification per ISO 29022 | Rationale |
|---|---|---|
| Tooth type | Human molars (caries-free) | Standardized substrate with clinical relevance |
| Storage medium | 0.5% chloramine solution at 4 °C | Preserves tooth structure without altering bond potential |
| Storage time | Maximum 6 months post-extraction | Prevents degradation of tooth substrate properties |
| Surface preparation | 600-grit SiC paper under water | Standardized smear layer for reproducible bonding |
| Bonding area | Defined by adhesive application diameter (2-3 mm) | Controls the test interface geometry |
| Specimen age at test | 24 h ± 2 h after bonding | Standardized storage for consistent adhesive polymerization |
The most frequently overlooked variable in dental bond strength testing is the storage temperature of specimens after bonding. ISO 29022 specifies 37 °C (body temperature) in distilled water for 24 hours. Temperature deviations of even 5 °C can affect the degree of conversion of adhesive polymers, potentially altering bond strength results by 15-30%.
Notched-Edge Shear Test Apparatus
The defining feature of the ISO 29022 test method is the notched-edge shear test apparatus. Unlike conventional shear tests that apply load through a wire loop or flat blade, the notched-edge design incorporates a precision-machined notch that fits around the adhesive bond, ensuring consistent load application at the adhesive interface. The apparatus is mounted in a universal testing machine, and load is applied at a controlled crosshead speed (typically 0.5 mm/min or 1.0 mm/min) until bond failure occurs.
Engineering Insights for Reliable Bond Strength Measurement
The notched-edge design addresses a critical limitation of earlier shear test methods — the phenomenon of non-uniform stress distribution at the adhesive interface. Finite element analysis studies have demonstrated that the notched-edge geometry concentrates stress at the adhesive interface itself rather than in the substrate or restorative material, enabling measurement of true interfacial bond strength rather than cohesive failure within one of the bonded materials. This is particularly important for modern adhesive systems that may achieve bond strengths approaching the cohesive strength of the tooth substrate itself.
The standard also addresses the critical issue of failure mode classification. After testing, each specimen must be examined under magnification (typically 10-20×) to classify the failure mode as adhesive (at the interface), cohesive in dentin/enamel, cohesive in composite, or mixed. The failure mode distribution provides valuable diagnostic information about the adhesive system performance that complements the numerical bond strength data.
When analyzing ISO 29022 test results, engineers should pay close attention to the failure mode distribution rather than only the mean bond strength. A high mean bond strength with 100% adhesive failure suggests potential for improvement, while a lower mean bond strength with mixed or cohesive failures indicates good interfacial adhesion limited by substrate or material cohesive strength.
Frequently Asked Questions
Q1: How does ISO 29022 compare to micro-tensile bond strength testing?
The micro-tensile test (ISO 29022 does not replace it but offers an alternative with simpler specimen preparation and lower technical sensitivity. The notched-edge shear test generally produces higher bond strength values than micro-tensile due to the compressive stress component, but it is more reproducible and less technique-sensitive. Many researchers recommend using both methods for comprehensive adhesive evaluation.
The micro-tensile test (ISO 29022 does not replace it but offers an alternative with simpler specimen preparation and lower technical sensitivity. The notched-edge shear test generally produces higher bond strength values than micro-tensile due to the compressive stress component, but it is more reproducible and less technique-sensitive. Many researchers recommend using both methods for comprehensive adhesive evaluation.
Q2: Can ISO 29022 be used for testing adhesion to dentin only?
Yes, the standard covers both enamel and dentin substrates. For dentin testing, the tooth substrate is ground to expose mid-coronal dentin, and the surface is standardized with 600-grit SiC paper. Dentin bond strengths are typically 40-60% of enamel bond strengths due to the higher water content and organic component of dentin.
Yes, the standard covers both enamel and dentin substrates. For dentin testing, the tooth substrate is ground to expose mid-coronal dentin, and the surface is standardized with 600-grit SiC paper. Dentin bond strengths are typically 40-60% of enamel bond strengths due to the higher water content and organic component of dentin.
Q3: What is the recommended sample size for ISO 29022 testing?
The standard recommends a minimum of 15 specimens per test group, which provides adequate statistical power for detecting clinically relevant differences between adhesive systems. The coefficient of variation for the notched-edge test is typically 15-25%, requiring fewer specimens than wire-loop shear testing (30-50% variation) to achieve the same statistical discrimination.
The standard recommends a minimum of 15 specimens per test group, which provides adequate statistical power for detecting clinically relevant differences between adhesive systems. The coefficient of variation for the notched-edge test is typically 15-25%, requiring fewer specimens than wire-loop shear testing (30-50% variation) to achieve the same statistical discrimination.
Q4: How should pretest failures be handled in data analysis?
Specimens that fail during storage or handling before mechanical testing (pretest failures) must be recorded and reported separately. They should not be included in the mean bond strength calculation but should be reported as a percentage of total specimens, as they provide important information about the adhesive system reliability under clinically relevant storage conditions.
Specimens that fail during storage or handling before mechanical testing (pretest failures) must be recorded and reported separately. They should not be included in the mean bond strength calculation but should be reported as a percentage of total specimens, as they provide important information about the adhesive system reliability under clinically relevant storage conditions.
The ISO 29022 notched-edge shear test has been adopted by major dental adhesive manufacturers as the preferred method for product development and quality control. Its improved reproducibility compared to wire-loop shear testing has enabled more reliable differentiation between adhesive formulations, accelerating the development of next-generation bonding agents with improved durability and reduced technique sensitivity.
The notched-edge shear test method offers specific advantages for testing adhesive systems on different dental substrate materials, including enamel, dentin, and various ceramic and composite restorative materials. The specimen preparation protocol ensures consistent bonding area dimensions, which is critical for calculating accurate shear bond strength values in megapascals. Researchers using ISO 29022 should standardize the crosshead speed, typically 1.0 mm/min, and report the mode of failure (adhesive, cohesive, or mixed) as part of the test results, as the failure mode provides insights into the quality and reliability of the adhesive bond at the interface.
