Review of Microleakage Evaluation Tools : Journal of International Oral Health (2024)

INTRODUCTION

The advancement of restorative materials and techniques continues to enhance the clinical success of numerous restorative procedures. Despite these new innovations, microleakage persists as one of the main causes of restoration failure. Microleakage is the movement of bacteria, fluids, molecules, and/or ions between the tooth and restoration margins.[¹] Microleakage is a result of the external environment invasion through the margins of the restoration which also can occur internally.[^2,3] Microleakage can cause a variety of adverse effects, such as secondary caries, higher sensitivity of the restored tooth, and interfacial staining leading to pulp pathology.[^4,5] Microleakage most commonly occurs when the gingival margin of any restoration is placed below the cementoenamel junction because bonding to dentin is less predictable than enamel due to its complex pattern and lower mineral content.[^1,4,6]

Another type of leakage, known as nanoleakage, has also been described. Nanoleakage is defined as when fluids move through the bonding between dentin and restorative resin.[⁷] It is mainly caused by dentin acid etching that can provide a way for oral and dentinal fluids penetration into the hybrid layer.[⁸] The amount of this penetration depends on several factors such as type and hydrophilicity of the bonding agent and the application technique. Nanoleakage within the hybrid layer and adhesive–resin interface is an important indicator of a material's sealing ability and the quality of the hybrid layer, which in turn affects the longevity of the restoration.[^9,10]

Microleakage assays provide useful information on the performance of restorative materials. Different techniques for assessing microleakage have been developed and used. These tests which use dyes, radioactive isotopes, air pressure, bacterial activity, neutron activation analysis, scanning electron microscope, dye penetration, and microcomputed tomography (μCT) all come with both advantages and drawbacks.[¹¹] Some of the older methods are no longer used because they do not represent the true nature of microleakage.[¹²]

It is well known that microleakage affects the health of the dental pulp and can cause pain, sensitivity, and discomfort.[²] However, little is known about the cause, mechanism, and nature of microleakage. Therefore, this review outlines and discusses currently available microleakage assessment tools.

DISCUSSION

Several factors are associated with microleakage, such as polymerization shrinkage, which leads to dimensional changes of the material, thermal contraction, water absorption, mechanical forces, as well as the changes in tooth structure dimensions.[¹³] After polymerization, composite resin shrinks to a considerable amount which creates stresses at the restoration margins and thus gaps and microleakage occur.[¹⁴] Moreover, the adaptation of any bonded restorative depends mainly on the shape of the cavity and number of walls bonded (the C-factor).[¹⁵] The coefficient of thermal expansion is another contributing factor to the occurrence of microleakage. Notably, the coefficient of thermal expansion for enamel and dentin is far less than that of composite resin restorations.[¹³]

Leakage is typically evaluated with in vitro models rather than in vivo methods, which can be qualitative as well as quantitative.[¹⁶] The clinical performance of any new dental restorative material can only be tested first using in vitro models. Theoretically, these findings can be transferred to the clinical environment.[¹⁷]

In the literature, in vitro studies use only 10–12 samples per group, which may affect the power of the study statistically.[¹⁸] The International Organization for Standardization (ISO) has established guidelines for studying the physical properties of dental materials, such as flexural strength, water sorption, and solubility (ISO/TS 11405: 2003, Dental Materials).[¹⁹] For microleakage tests, the guidelines emphasize the need to standardize tooth quality, type of cavity preparation, and method used to evaluate microleakage at the margin.[¹⁸] New evaluation tools, such as μCT, can detect microleakage in vivo.[¹¹]

Leakage tests can be subdivided into old and contemporary methods. Old methods were used to test the presence of gaps and the sealing ability of different restorative materials. These methods included air pressure, fluid filtration, electrochemistry, neutron activation, bacteria, and artificial caries.[¹¹] However, these techniques were found to be nonrepresentative of leakage and thus have been replaced by more contemporary methods.

Contemporary methods

Radioisotope method

A broad range of radioactive isotopes have been used in microleakage studies, including the markers ⁴⁵Ca, ¹³¹I, ³⁵S, ²²Na, ³²P, ⁸⁶Rb, and ¹⁴C. Generally, autoradiography is used to detect the leakage of isotopes at the restoration margins interface of a sectioned specimen.[^20,21,22,23]

Isotopes can penetrate gaps equal to or larger than 40 nm, which is higher than the minimum detectable range of bacteria-based studies. In addition, isotopes appear to be better at demonstrating microleakage than dye penetration tests.[²⁴] On the other hand, this technique requires the use of radiation, and the obtained autoradiograph does not represent microleakage as a three-dimensional (3D) image.

Acetate peel technique

Füsun et al. described the acetate peel technique as a rapid method for preparing many sequential replicas from an etched tooth surface to study dental hard tissue. Acetate films can be used to imprint the etched tooth surface at a micron scale resolution, which gives an overall view of the tooth structure and microleakage at the interface.[²⁵]

Mohapatra and Sivakumar reported that the acetate peel technique is a simple, inexpensive, and fast method for measuring microleakage. Moreover, peels are stable and can be preserved for further evaluation. However, the peel technique is delicate, which may produce artifacts that can be misinterpreted.[¹⁷]

Dye penetration

The staining of microleakage and nanoleakage using colored agents is the most commonly used technique. Dye penetration method involves the use of contrasting dyes as an immersion solution to stain the areas of microleakage, and then the tooth–restoration interface is examined for evidence of staining. Notably, the most commonly used solutions are 0.5% basic fuchsin, 2% methylene blue, and 50% silver nitrate.[²⁶]

The dye penetration assay has many advantages over other techniques. First, no reactive chemicals are used along with no radiation.[²⁷] Second, different dye solutions are available; therefore, the technique is highly feasible and easily reproducible.[²⁸]

The current studies have failed to clearly establish which dyes are suitable for use with microleakage test as some of the dyes can react with dentin such as basic fuchsin.[²⁹] Another important issue with dye penetration methods is the particle size of the used dye which could affect the reliability of the test.[²⁶]

Wu and Cobb[³⁰] used silver nitrate because the strong optical contrast of silver particles is easily detected using microscopy. Silver nitrate staining is the most commonly used material for nanoleakage evaluation as it easily penetrates the interface zone due to its extremely small diameter (0.059 nm). Following its penetration, silver nitrate molecules can become immobile, which prevents further penetration during specimen preparation. Silver nitrate was used to verify the discrepancy between the depth of the demineralized zone and monomer diffusion, which is caused by the presence of water around collagen fibrils (nanoleakage).[³¹]

Some authors reported possible problems arising from the use of 50% silver nitrate solutions or higher and recommended the need to seal the specimens to ensure no other sources of silver nitrate penetration. Moreover, a 24-h immersion in silver nitrate decreased the pH to approximately 3.8.[³¹] In a study by Costa et al., a 24-h immersion in an aqueous solution of 5% silver nitrate was sufficient to detect a loss of the marginal seal in composite restorations.[³²]

Assessing a single section of the tooth is not representative because dye penetration varies from one area to another. Thus, multiple-surface scoring methods are regarded as superior to single-surface scoring methods because the results are more representative of the leakage pattern.[³³] Raskin et al. recommended the utilization of three sections for each restoration to accurately evaluate leakage.[³⁴] Another technique introduced by Gale and Darvell[³⁵] involves grinding of the specimen into sequential slices and then reconstruction of the images by the use of computer software. The authors stated that “3D techniques reveal markedly greater microleakage than 2D assessments.”[³⁵] Moreover, Majety and Pujar[³⁶] recommended completely removing restorations to assess the total amount of microleakage because leakage can vary at different aspects of the cavity. The dye penetration method requires an adequate evaluation tool to determine the true extent of microleakage.[¹¹] Light microscopy, scanning electron microscopy, and transmission electron microscopy are widely used. However, recent methods to better evaluate leakage include 3D methods, such as μCT 1072, confocal laser scanning microscopy (CLSM), and optical coherence tomography (OCT).

Three-dimensional methods

The 3D analysis was first used by Gale et al.[³⁷] The technique involves the production of sequential slices of the samples using a water-cooled wire saw in 200-μm thick and separated by 280 μm. Specialized computer software is then used to reconstruct the images and create 3D models. Then, the surface area of dye leakage as well as volume of leakage is calculated manually. 3D analysis was found to give more accurate information than 2D analysis as the samples are examined thoroughly.[³⁸] However, the method is highly subjective and destructive. The samples are destroyed during the sequential grinding and slicing procedure. Moreover, this slicing can affect the restoration tested and thus alter the results.[³⁹]

Microcomputed tomography

μCT is a modified version of medical computed tomography that creates a 3D visualization of dental structures without destruction.[⁴⁰] μCT starts from a set of 2D images taken along the rotational axis, which is then transferred to a computer program that produces a 3D image of the sample. These 3D reconstructions have a resolution of a few microns. One of the important features of μCT is that the 3D reconstruction can be sectioned in any direction to gain accurate information of the sample's internal geometric properties and structural parameters.[⁴¹] Moreover, μCT results in a precise 3D reconstruction of the sample.[⁴²] Burghardt et al.[⁴³] and Ibrahim et al.[⁴⁴] recommended μCT as the gold standard for assessing bone morphology and microarchitectures.

De Santis et al. was the first to use μCT in a microleakage evaluation study.[⁴⁵] This technique was also been used to evaluate mechanically stressed dentin and adhesive–composite interfaces as well as detect marginal leakage at the restoration–tooth interface in vivo. One of the main advantages of μCT is that the 2D section information is maintained; thus, all margins are available for visual inspection.[^46,47] In addition, μCT was suggested by Özkan et al.[⁴⁸] as an alternative method to histological examination for in vitro studies. Eden et al.[⁴⁹] evaluated the reliability of a marginal leakage assessment of self-etch adhesive Class II resin composite restorations in primary molars prepared in vivo using μCT. These authors concluded that μCT could be used to develop a standardized method for measuring marginal leakage from in vivo samples. In combination with a 4-h immersion in 50% silver nitrate, the marginal leakage along the restoration–tooth interface was accurately and reliably measured.[⁴⁹] However, μCT alone is not a substitute for the histological examination of leakage in in vitro samples.[⁵⁰]

Confocal laser scanning microscopy

CLSM is used to detect subsurface structures up to 100 μm in size without sample destruction.[⁵¹] Tangsgoolwatana et al.[⁵²] compared the degree and pattern of microleakage in bonded amalgam restorations treated with fluorescent dyes and a⁴⁵ Ca radioisotope using CLSM and autoradiography, respectively. The authors reported a high correlation between the results of the fluorescent and radioisotope studies, indicating that these two microleakage methods can be directly compared.[⁵²] Another study by Grobler et al.[⁵³] stated that CLSM could detect resin tag formation, penetration of the bonding agents deep into the tubules, and hybrid layer formation for any bonding material.

Optical coherence tomography

This evaluation tool was first introduced in 1998 by Colston et al.[^54,55] to evaluate dental hard and soft tissues in pigs. In 2008, Drexler and Fujimoto[⁵⁶] used OCT to visualize dental tissue in vivo. Since then, OCT use in dentistry has become increasingly more common.[⁵⁷]

OCT produces 3D images for the qualitative and quantitative evaluation of dental hard and soft tissues in vivo.[⁵⁸] This method enables clinicians to accurately and rapidly detect the early stages of dental caries, periodontal problems, and oral cancers.[^59,60] Researchers have used OCT to clinically evaluate dental restoration margins intraorally.[⁶¹] Moreover, OCT can be used to quantify bubbles and voids within composites and most dental restorations by detecting the reflection of infrared light waves from internal microstructures, similar to the ultrasonic pulse echo method.[⁶²] It has been also reported that OCT can detect enamel cracks at the margins of composite restorations noninvasively.[⁶³]

Bakhsh et al.[⁶⁴] evaluated the tooth–restoration interface using OCT and CLSM and reported that OCT can be used to quantify interfacial gaps at a micron scale. Another study by Nazari et al.[⁶⁵] used OCT to detect voids and gap formation with flowable resin composite. The authors concluded that OCT is a unique method to noninvasively and precisely assess restoration materials. The same conclusion was reported by Shimada et al.[⁵⁹] in 2015.

CONCLUSION

The microleakage assessment tools available to researchers each have their own advantages and disadvantages. Notably, the microleakage testing of different restorative materials should use similar methodologies to reduce variability in the results. Finally, OCT is a new tool with a great potential to test restorative material behavior in vivo.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

REFERENCES