Bioactive glass as an abrasive in air abrasion technique: application in dentistry

INTRODUCTION

Air abrasion is a method increasingly used in dentistry. It was developed by Black in the 1940s. Black et al. [1] showed many advantages of this technology, such as quick enamel excision, minimal operator fatigue, and minimal painfulness of the procedure. The invention of the air turbine, which was much more convenient to use, in the 1950s made air abrasion no longer useful. With the development of adhesive dentistry, this method is experiencing a renaissance. Abrasion properties, such as leaving a rough and uneven surface, which was a disadvantage in times of widespread amalgam, nowadays, is a great development of the surface for the adhesion of composite material. Many factors, including the propellant gas pressure or the size and type of abrasive particles, influence the effectiveness of abrasion. A review of literature by Szerszen et al. [2] quotes seven abrasives practiced in this technique. The most often used abrasive is aluminum oxide. It is a hard, angular material, with a high abrasion ability to the surface of hard tooth tissues [3]. In contrast, bioactive glass is much less hard, and is often used as a polishing abrasive. The choice of abrasive is a key to achieve the desired result. Due to its’ properties, bioactive glass deserves special attention.

OBJECTIVES

The aim of the study was to show the possible applications and benefits of using bioactive glass as an abrasive in air abrasion technique.

MATERIAL AND METHODS

For the information on the topic presented, PubMed/Medline, EBSCOhost, and Scopus databases were searched using the following combination of keywords: “(air abrasion or air-abrasion or sandblasting) and (bioactive glass or sylc)”. Results were limited to the period between 2010 and 2021. Based on title and abstract, only original papers in English or Polish, with full-text access were included into this study. Articles that did not concern the use of bioactive glass in air abrasion technique were excluded from this research after full-text evaluation.

RESULTS

In the database of PubMed/Medline, there were 32 articles found, in Scopus 28, and in EBSCOhost 21. After removing duplicates, 41 articles were obtained, of which 37 articles met the inclusion criteria and 7 were excluded. The work uses data from 30 scientific articles, which are summarized in Table 1.

CHARACTERISTICS OF BIOACTIVE GLASS

Bioactive glasses are widely used in dentistry. They are chemically composed mainly of silicon, calcium, sodium, oxygen, and phosphorus. The content of many biogenic elements in their composition indicates the high bio-compatibility of these materials. Due to its’ properties, bioactive glasses are used in dental and maxillofacial surgery, periodontology, pediatric dentistry, conservative dentistry, and endodontics [16, 33]. Many types of bioactive glasses are described in the literature. Most of them are experimental materials based on a commercially available composition. Bioactive glasses are commercially available under the trade names ‘SylcTM’ and ‘proSylcTM’. Both products have the same chemical composition and consist of oxides in the following percentages by weight: SiO2, 45%; CaO, 24.4%; Na2O, 24.6%; P2O5, 6%.
These materials are produced by two methods, alloy hardening and by the sol-gel technique. The traditional alloy hardening technique involves dissolving the components of bioactive glass at high temperature, most often above 1,300oC, and cooling it down quickly to maintain the atomic structure. After cooling, the glass is ground to a suitable grain. Unfortunately, alloy hardening reduces the bioactive properties of the material, and does not produce a porous structure. The sol-gel technique has been developed since the 1970s, and allows producing more porous material that takes the form of fibers, coatings, or scaffolds. Bioactive glass produced in this way has a greater ability to form apatites, but has poorer mechanical properties. Currently, bioactive glass for use in air abrasion is produced using the alloy hardening method [22, 33, 34].
The most important feature of this type of material is bioactivity. Bioactive glasses, in contrast to ordinary glasses, are less resistant to chemical reactions occurring in tissue fluids, and therefore, can be a source of ions. This is due to the specific chemical composition, especially the content of phosphates, which as a form of an orthophosphate not bounding to crystal lattice, facili¬tate the precipitation of ions [5, 27]. In the tissue fluid, bioactive glass exchanges H+ ions into Ca2+ and Na+, leading to alkalization of the environment and transformation of the glass surface into a gel rich in ions. In this gel, crystals of amorphous calcium phosphate precipitate and then, they are incorporated into the structures of hydroxyapatites [10, 33].
Another feature of bioactive glass is its’ antibacterial nature. In a study by Drago et al. [35], it was found that it has excellent antimicrobial properties without inducing resistance. This property can be used even in the treatment of osteomyelitis. This is mainly due to the high pH and osmolarity, which are the result of dissolving glass particles [32, 33, 35]. Experiments conducted by Abushahba et al. [22, 31] showed that this material is highly effective against Fusobacterium nucleatum, Por¬phyromonas gingivalis, and Streptococcus mutans. How¬ever, not only the increase in pH is responsible for the antimicrobial pro¬perties. Due to the addition of zinc oxide or strontium oxide, even bioactive glass that gene¬rates a relatively low tissue pH (i.e., 8 compared to 8.8 in SylcTM), exhibits highly antiseptic properties. The mecha¬nism of the antimicrobial action is not based on a simple alkalinization of the environment, but involves complex interactions between individual ions contained in the material [26, 36]. Moreover, the addition of fluoride to the composition increases its’ antibacterial pro-perties [37].
Bioactive glasses used in the air abrasion method are particles with a diameter of typically 38-90 µm. The hardness of this material is low, only 4.5-5.75 GPa compared with 16-18 GPa for alumina. In SEM images, they have a more rounded shape than aluminum oxide, although they are not without sharp edges. Their cutting efficiency is low. Preparation of a glass plate with bioactive glass is almost 8 times slower than preparation with aluminum oxide, but the biological effect makes this material the material of choice for minimally invasive treatment [3, 13, 18, 25]. To increase the speed of work, Tan et al. [17] prepared a bioactive glass with the following composition: SiO2, 37%; P2O5, 6.1%; SrO, 53.9%; SrF2, 3%. This abrasive cut a larger cavity than aluminum oxide at the same time. Farooq et al. [18] used a combination of aluminum oxide and bioactive glass. The speed of preparation was found to be comparable to that of alumina alone. In Figure 1, the most important applications of this material are summarized.

BIOACTIVE GLASS IN THE TREATMENT OF CARIES

The method of air abrasion is more and more often used in cariology. According to the assumptions of minimally invasive dentistry, the first therapeutic intervention at the stage of a white carious spot is re-mineralization. To make it as effective as possible, it is recommended to ensure good access of ions to the inside of the cavity. Re-mineralization can be carried out using traditional methods, such as the use of fluorine compounds, but as reported by Milly et al. [16], air abrasion using bioactive glass is also an effective method of treating pre-cavity caries [38]. In their experiment using optical coherence tomography, air abrasion with the use of bioactive glass reduced substrate dispersion by 3 times compared with etching and applying a paste with bioactive glass content [16]. The combination of two re-mineralization methods yields even better results. The use of NovaMin® (bioactive calcium-fluorosilicate glass) prior to application of polyacrylic acid and bioactive glass air abrasion resulted in re-mineralization of as much as 91.6% of the original mineral content within a month [27]. To improve the re-mineralization properties, Taha et al. [24] created a bioactive glass containing calcium fluoride. Its’ use provided lower surface roughness and light backscattering values compared with SylcTM bioactive glass.
In case of cavity caries, non-contact preparation of hard tissues of teeth due to the avoidance of vibrations is more pleasant for patient than the classic, invasive preparation with the use of an air turbine [39]. The most common abrasive for this purpose is alumina. Its’ great hardness and sharper edges determine great clinical effectiveness. However, the biggest drawback is the lack of any biolo¬gical activity [3, 6, 13]. An alternative to alumina is bioactive glass. Although its’ cutting speed is poor, this material through the exchange of ions supports re-mineralization of tissues de-mineralized by caries. Combined with antimicrobial properties, bioactive glass is a material that fits perfectly into the concept of minimally invasive carious cavities debridement [6, 17]. Due to its’ low cutting speed, it is more conservative and does not lead to over-preparation, removing almost only caries tissue [6, 7].
In the air abrasion technique, particles of the mate¬rial hit the surface of a soft carious lesion and get bogged down in it, losing their kinetic energy. The size of particles is directly proportional to the kinetic energy imparted by the air stream. Larger particles cut tissues less efficiently, while they sink deeper into the carious lesion [40]. Due to their re-mineralization properties, particles embedded in de-mineralized areas can efficiently deliver ions. In addition, bioactive glass does not reduce the adhesive forces generated by the bonding systems. In case of de-mineralized enamel, the adhesive force generated after preparation of the cavity with bioactive glass is higher than in processing with aluminum oxide [11, 12]. When using bioactive glass to modify dentin, no greater bond strength is achieved compared with conventional adhesive protocol. However, high pH following air abrasion may interfere with some self-etching bonds [9, 23, 32].
Another advantage of bioactive glass is the possibility of modifying the smear layer before the application of glass ionomer cement. The traditional conditioning method with 10% polyacrylic acid is less effective compared with applying air abrasion. Reports by Sauro et al. show that the use of bioactive glass air abrasion after 24 hours does not yield significant differences compared with conventional conditioning. However, after 6 months in artificial saliva solution, when the material was subjected to loads, the adhesive force was almost twice as high after abrasion. Bioactive glass can also be used in combination with polyacrylic acid. The use of such a technique for the preparation of cavities enhances the bonding durability of glass ionomer cement. Due to its’ hydrophilicity, a high concentration of polyacrylic acid necessitates the use of bonding systems containing high vapor pressure solvents, as in the case of adhesive restoration [8, 23].
Bioactive glasses are also used in the primary prevention of caries; they can be used to clean the fissures before sealing. A study by Bagheri et al. [21] showed that the use of bioactive glass abrasion reduces micro-leakage compared with no modification or application of an adhesive system. Air abrasion by removing the super¬ficial layer of aprismatic enamel, improves the pene¬tration of acids and enables the creation of favorable etching patterns. Interestingly, the use of alumina in fissure cleaning provides a better sealant retention than the use of bioactive glass. This is due to the properties of both materials. As a harder material, aluminum oxide effectively removes hard tissues, quickly causing their roughness [3, 20].

BIOACTIVE GLASS IN THE TREATMENT OF NON-CARIOUS LESIONS

Non-carious lesions are becoming an increasingly important problem in a dentist’s practice. Increasing life expectancy, stress, acidic diet, and greater care for teeth make attrition, abrasion, and erosion commonly encountered. Classic methods of treatment are based on the modification of the surface of the defect (e.g., with a laser) or the creation of a protective layer on its’ surface (using various fluorine compounds or bioactive glass). Using air abrasion with bioactive glass ProSylcTM, Dinostypulos et al. [28] achieved 2 times slower de-minerali¬zation progress in dentine compared with control sample. In addition, the increase in roughness after acid attack, in case of the surface subjected to air abrasion was 3 times lower than in control not subjected to any protective measures. In case of enamel, bioactive glass air abrasion resulted in 2 times lower micro-hardness loss and significantly lower surface roughness decrease [30].
Due to their hardness (range, 4.5-5.75 GPa) that exceeds the hardness of enamel (3.5 GPa), bioactive glass can remove mineralized tooth tissues. This is a significant disadvantage in the treatment of non-carious defects. Compared to using only tin fluoride, abrasive blasting results in a slightly greater overall loss of hard tissue volume. However, re-mineralization properties of bioactive glass mean that cavities protected in this way show significantly greater hardness and less increase in roughness over time [24, 28]. At the expense of a slightly greater loss of dentine volume, a well mineralized acid-resistant layer can be obtained.

BIOACTIVE GLASS IN THE TREATMENT OF DENTINE HYPERSENSITIVITY

Dentin hypersensitivity is an increasingly common condition. The increasing number of gingival recessions and abfraction losses in the population, predisposes them to hypersensitivity reactions. In its’ treatment, various methods and substances are applied to close the dentinal tubules, including bioactive glass. The conventional method is to apply an agent containing bioactive glass to the tooth area with symptoms of hypersensitivity [37]. However, an innovative method of bioactive glass application in hypersensitivity treatment is to use it as an abrasive in a sandblaster. The mechanism of action is based on the occlusion of dentinal tubules and the reduction of their permeability. Contrary to sandblasting with sodium bicarbonate, which increases hypersensitivity, bioactive glass significantly reduces hypersensitivity and is more pleasant for patient [4, 19]. As shown by SEM electron microscope studies, accelerated abrasive particles occlude the dentin surface, closing 100% of dentinal tubules. Even after etching the surface prepared in this way with citric acid at pH 3.2, 94% of channels remain closed [3, 5].

BIOACTIVE GLASS IN PROSTHODONTICS AND IMPLANTOLOGY

Although air abrasion with aluminum oxide is mostly used in prosthodontics, bioactive glass can also find some purposes. It cannot be used in roughening prosthetic restorations as a result of poor cutting ability, but due to bioactivity, it can increase the durability of prosthetic restoration [3, 14, 15]. The major threat to permanent prosthetic restorations is secondary caries. Coating a layer of bioactive glass on the surface of the pillar may protect against secondary caries after the degradation of prosthetic cement. The bioactive glass enriched with niobium, as reported by Carvahlo et al. [14], does not affect the adhesive strength generated by composite cement, but due to high bioactivity, it may protect the pillars. The addition of niobium in the structure allows for better strength, bioactivity, and opacity to the radia¬tion.
Another field, in which air abrasion with bioactive glass can be used, is implantology. Various techniques to increase the surface available for osseointegration are widely used in the manufacture of implants, including acid etching, laser processing, or air abrasion. Nowadays, the most common method of increasing the implant surface area is sandblasting with aluminum oxide. However, alumina is not a substance that promotes osseointegration. By anchoring itself to the surface of the implant, it constitutes contamination that inhibits the proliferation of osteoblasts on the implant material. Despite cleaning by various methods, it is not possible to eliminate all alumina particles from the surface subjected to abrasion [41]. The solution to this problem may be the use of bioactive glass in the process of production. Bioactive glasses are commonly known as osteoconductive materials used in the process of guided bone regeneration. The application of bioactive glass on the surface of an implant has a significant effect on wettability and surface-free energy, which enhances osteoblasts’ prolife¬ration ability [22, 29]. Air abrasion using bioactive glass causes the formation of an apatite within 6-24 hours after incubation in a TRIS-buffered solution environment that contains no Ca2+ and PO43– in comparison with tissue fluid [10, 25].
However, the action of bioactive glass is not limited to improving osseointegration. According to Abushahba et al. [22, 26, 31], this material is perfect for the treatment of peri-implantitis. The proven activity against many pathogens and modification of the surface preventing their subsequent invasion, produce favorable conditions for the maintenance of implant. The addition of zinc ions to the bioactive glass allows for the reduction of tissue solubility, and thus does not cause such an intense increase in pH, which turns into the formation of a more tissue-friendly environment without reducing the antimicrobial effect.

BIOACTIVE GLASS IN ORTHODONTICS

Treatment with fixed orthodontic appliances, apart from numerous advantages, has a significant disadvantage: after finishing the treatment, it is easy to damage the enamel when removing the adhesive from the enamel surface. Air abrasion may be helpful in this aspect. After using an abrasive blaster, regardless of the selected abrasive, the surface is uniformly matte and easier to polish, unlike the bur, which creates numerous grooves on the surface. Due to the limited cutting ability, bioactive glass, unlike aluminum oxide, causes minimal damage to healthy enamel in the air abrasion technique [7, 13]. Lower-hardness bioactive glass that removes orthodontic adhesive with minimal enamel loss is currently developing. Taha et al. [25] developed fluoride-containing bioactive glass, with increased sodium content to 30 mol%.Vickers hardness of novel bioactive glass was 350 in comparison with 472 of commercially available SylcTM. The use of this experimental abrasive resulted in the fact that at the cost of doubling the working time, the roughness after removal of orthodontic adhesive was comparable with the initial roughness before bonding orthodontic brackets.

CONCLUSIONS

Bioactive glass is a widely used material in denti¬stry. Its’ antibacterial and bioactive properties, combined with sufficient hardness, make this material successfully used in the air abrasion technique. Further research and the introduction of different hardness of glasses into the market (hard ones used for cutting tissues and soft ones used for polishing) are necessary for this material to become more popular as an abrasive. With the wide introduction of this material into dental procedures, further new applications of bioactive glass air abrasion can be discovered.

CONFLICT OF INTEREST

The authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

References