Radiopacity Related to Composition of Restorative Glass-Ionomer Cements with the Use of Digital Images

Lígia Saraiva Bueno¹, Paulo Henrique Martins Fernandes², Rafael Menezes-Silva³, Ana Paula R Magalhães⁴, Joana Yumi Teruya Uchimura⁵, Maria Fidela de Lima Navarro², Renata C Pascotto⁵, John W Nicholson³, Sharanbir K Sidhu³, Ana Flávia Sanches Borges^2*

¹Clinical Research Manager at 3Shape, Copenhagen, Denmark
²Department of Operative Dentistry, Endodontics and Dental Materials, School of Dentistry, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil
³Faculty of the Central-West of São Paulo (FACOP), Bauru, São Paulo, Brazil
⁴Professor at University Alfredo Nasser, Aparecida de Goiânia and Coordinator of Operative Dentistry Specialization Course at Brazilian Dental Association, Goiânia, Goiás, Brazil
⁵ State University of Maringá, Maringá, Paraná, Brazil
6Institute of Dentistry, Barts and The London School of Medicine and Dentistry, Queen Mary University of London, Turner Street, London and Bluefield Centre for Biomaterials, 67-68 Hatton Garden, London, UK

*Correspondence author: Ana Flávia Sanches Borges, Department of Operative Dentistry, Endodontics and Dental Materials, School of Dentistry, Bauru School of Dentistry, University of São Paulo, Bauru, São Paulo, Brazil; E-mail: [email protected]

Citation: Bueno LS, et al. Radiopacity Related to Composition of Restorative Glass-Ionomer Cements with the Use of Digital Images. J Dental Health Oral Res. 2025;6(2):1-8.

Copyright^© 2025 by Bueno LS, et al. All rights reserved. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Table 1: Brands and composition of glass-ionomer cements tested.

The materials were handled according to manufacturers’ instructions. Six disc-shaped specimens (Ø =15×01 mm) of each material were prepared according to ISO 9917-1:2007 standards using metal molds with glass plates and polyester films on both sides, all held with a clamp to ensure the correct specimen thickness [15]. The whole assembly was placed in an incubator at 37^oC and after 30 minutes, the specimen was removed from mold. The thickness of each specimen was measured near its center using a screw micrometer (0,01 mm accuracy) and when necessary, the specimen was polished using 1200 grit abrasive paper until the specified thickness (1 ± 0.1 mm) was obtained. The specimens were returned to the incubator for storage in water of grade 3 (ISO 3696:1987) for 7 days. 

Radiopacity Test

For the radiopacity test, the specimens (n = 5) were positioned in an occlusal sized imaging plate alongside an aluminum step wedge designed with a thickness range from 0,5 mm to 5 mm in equally spaced steps of 0,5 mm according to ISO 9917-1:2007 requirements [15]. The specimens and the aluminum step wedge were irradiated in a dental X-ray unit at 70kVp/7mA for 0.23s at a distance of 400mm. Five images of each specimen were obtained, resulting in 25 images per group and 350 imagens in total.

After irradiation, the image plates were developed in VistaScan digital and the images obtained (JPEG format) were analyzed in the photographic densitometer of Image J (National Institutes of Health) to measure their mean gray values (MGV). Each group had 5 specimens and each specimen was measured five times in order to obtain more accurate radiopacity values. These values were converted into millimeters of aluminum (mmAl), using the following equation proposed by Lachowski, et al., [13].

where A is the material’s MGV; B is the MGV of the aluminum step wedge increment immediately below the material’s MGV; and C is the MGV of the aluminum step wedge increment immediately above the material’s MGV.

Energy-Dispersive X-ray Spectroscopy 

The specimens (n=1) of all groups were subjected to a superficial analysis of the elements Si, Ca, Na, Al, Sr, Ba, Zn and F (according to atomic number and composition) performed by a Scanning Electron Microscope equipped with an X-radiation detector.

Statistical Analysis

Data were statistically analyzed with SPSS for Windows v.19.0 (IBM Statistics, United States). The normal distribution and homogeneity of variances assumptions were checked for all variables using the Shapiro-Wilk test and the Levene test, respectively. As the assumptions were satisfied, data were subjected to ANOVA, followed by Tukey’s test (α=0.05), for individual comparisons.

Results

The results for radiopacity are shown in Table 2 and Fig. 1. The highest radiopacity values were registered for FJ2, IS, GL9 and CR (p>0.05). The lowest value was recorded for MX which was different to the other groups (p<0.05). The other groups, KM, EF, RV, IP, IM, VM, VD, VF, BG, GM, IZ, IG and GI presented intermediate values between them. ISO 9917-1 establishes that for a material to be described as radiopaque by its manufacturer, the radiopacity must be at least equivalent to that of aluminum of the same thickness when tested in accordance with ISO 9917-1 requirements. Since ISO 9917-1 specifies specimens with a thickness of 1 mm, the minimum radiopacity should therefore be equivalent to 1 mm of aluminum (1 mmAl). Considering this reference, the GICs RV, EF, KM, CR, GL9, IS and FJ2 achieved the minimum radiopacity value determined by the ISO standard and they are correctly labeled as radiopaque in its package inserts.

Table 2: Mean values and standard deviation (DV) of radiopacity for the fourteen brands of glass-ionomer cements.

Figure 1: Mean values and standard deviation (DV) of radiopacity for the fourteen brands of glass-ionomer cements.

The results of the EDX analyses of the material with higher radiopacity are presented in Fig. 2. Strontium concentrations were highest for the materials FJ2, IS and GL9 and are 13.49, 12.23 and 13.35 weight percent respectively. The element zinc was found in many brands as GI, VM, EF, IP, but the higher value was observed for CR (11.26%), which also presented a lower amount of strontium (6.26%). Likewise, BG, VM and VD also presented some amount of barium (more than 5%). The Fig. 2 demonstrates EDX analysis and the correspondent elements. In all cases, data are expressed in terms of normalized mass percentage of oxides of the elements with atomic number higher than 10, with special attention to the elements zinc, barium and strontium.

Figure 2: EDX analysis graphics showing major amount of the element Sr for the materials FJ2 (A), IS (B), GL9 (C) and CR (D).

Discussion

In the present study, radiopacity showed variable results among restorative glass ionomer cements. Unfortunately, it is difficult to compare our results with literature data, since many studies evaluate radiopacity results among different types of restorative materials [1,17,18]. There is not a recognized ideal value for radiopacity, although many authors discuss it. A tendency to strive for the highest achievable radiopacity was seen but the possible “Match effect” would be a concern, with enhancement of the contrast between a light and a dark area and consequent inadequate diagnosis of marginal areas [3,19]. Thus, the radiopacity considered acceptable for restorative materials is when it slightly exceeds the enamel radiopacity [3,13,17,18,20]. As seen in Fig. 1, no material presented radiopacity to give rise to the “Match effect” levels.

It is known that 1 mm of dentin corresponds to approximately the equivalent radiopacity of 1 mm of Al used in the step wedge [21]. The International Organization for Standardization (ISO) for water-based cements determines that a material should contain at least 1 mmAl in order for manufacturers to label it as radiopaque. According to Bouschlicher, et al., even when the radiopacity value complies with ISO standards, it may not be enough to detect small defects and the restoration limits [22]. The density of dentin should be the least to assure that the material would not be mistaken for carious dentin radiographically [13]. The results found shows that seven out of the fourteen materials tested have less than 1mmAl, which means that the manufacturer of those brands should not label those materials as radiopaque, however IP, VM, VF and BG are labeled as radiopaque materials in its package inserts. That information is concerning, once glass-ionomer cements are widely used as restorative materials, especially in the primary dentition [23] and for temporary restorations, where conservative removal requires radiographic visualization.

Lack of information on composition by manufacturers led to the need of surface analysis through Energy-dispersive X-ray spectroscopy (EDX), in order to explain the results found. There is only one study in the literature reporting results of radiopacity associated with surface analysis by X-ray energy dispersive, although it was carried out with composite resins, instead of glass-ionomer cements [24]. Earlier glass-ionomer cements were radiolucent, limiting their use as restorative materials. Thus, the need for more radiopaque cements led to the addition of other components. Molecular structure influences the material radiodensity, i.e. the relative inability of radiation to pass through a particular material [18]. As seen in Table 1, the powder composition of conventional glass-ionomer cements is basically aluminosilicate glass. Atomic numbers of aluminum, silicon and calcium are 13, 14 and 20 respectively and are considerably low. Accordingly, incorporation of aluminosilicate glass alone leads to radiolucent glass-ionomer cements [13,14,25]. Studies shows that incorporation of elements with high atomic number is associated with high radiopacity and the presence of the elements zinc, strontium and barium (atomic numbers 30, 38 and 56 respectively) is attributed to that improvement [24,26]. From those, barium and strontium are the most used today (both in group II on the Periodic Table). This inclusion is usually done with fillers as zinc oxide, strontium oxide and barium sulphate to the glass particles [10].

The materials GL9, FJ2 and IS, according to EDX analysis, presented the highest content of strontium (more than 10% weight) and also presented the highest radiopacity values among all materials tested. It corroborates other literature findings, which conclude that radiopacity increases linearly with the strontium content [10]. CR was the unique material presenting a high quantity of zinc in the EDX analysis (more than 10%) and is among the few materials with adequate radiopacity values. Thus, the most viable explanation for the high value of radiopacity for CR is the high amount of zinc found in the analysis along with the content of strontium, superior to other materials. Some materials, such as IP and VF, presented some amount of Sr on the surface (less than 10%), but low values of radiopacity. This suggests that just the presence of Sr alone is not enough to increase radiopacity; there must be a minimal amount necessary to produce satisfactory radiopacity results. The presence of zinc may influence this property, but the authors suggest that it is associated to the simultaneous presence of strontium. Despite the high atomic number, all Ba-containing cements tested were radiolucent, which suggests that addition of barium (BG, VM) probably had no influence on radiodensity or was not high enough to have an influence on this property.

Radiopacity should not be enhanced at the expense of other properties or characteristics [24]. Many fillers added to the glass particles produce “opaque” cements or have the tendency to increase erosion rate whereas incorporation of considerable amounts of heavy metal oxides may reduce the resistance to chemical degradation by the presence of large ions disrupting the network matrix [24,27]. The consequences are weakened materials, increased wear rates with time and notably lower success rate of restorations. From a biological aspect, studies have shown that elution of Sr was not cytotoxic for human dental pulp cells also considering the relatively small amount that is present in restorative materials composition and that is released from that [28-30]. In fact, strontium demonstrated to have a positive response accelerating bone ingrowth around implants in rats [31]. There are still few data on cytotoxicity for barium and zinc in glass ionomer cements, although studies concluded that both lead to cell necrosis only in high concentrations [32,33]. Besides, glasses with high Sr concentration (equivalent to 40% SrO) are associated with poor wear resistance [24]. Regarding translucency, one study showed a reduction of 7% or more in optical opacity for all five Sr containing cements after 24hrs [10].

The present study used standardized tests to reduce the risk of bias, yet in-vitro studies do not reproduce oral environment perfectly. Thus, further research should concentrate on clinical outcomes to validate and provide reliability for the results found. More studies on chemical analysis and cytotoxicity of metal oxides should be performed to better understand and support the indication of Sr-containing cements over other cements.

Conclusion

It was concluded that from 14 brands tested 7 of them did not achieve the minimum radiopacity value to the manufacturers label the product as a radiopaque material, but 4 of those GICs are labeled as radiopaque in its package inserts.

Conflict of Interest

The authors declare no conflict of interest.

Funding

None.

Reference

1. Pekkan G. Radiopacity of dental materials: An overview. Avicenna J Dent Res. 2016;8:8.
2. Shah PMM, Sidhu SK, Chong BS, Ford TRP. Radiopacity of resin-modified glass ionomer liners and 2 bases. J Prosthet Dent. 1997;77:239-42.
3. Espelid I, Tveit AB, Erickson RL, Keck SC, Glasspoole EA. Radiopacity of restorations and detection of secondary caries. Dent Mater. 1991;7:114-7.
4. The desirability of using radiopaque plastics in dentistry: A status report. J Am Dent Assoc. 1981;102:347-9.
5. Alomari Q, Al-Saiegh F, Qudeimat M, Omar R. Recurrent caries at crown margins: Making a decision on treatment. Med Princ Pract. 2009;18:187-92.
6. Aldabeeb D, Alofi A, Alfaran R, Salam A, Alsuhaibani L, Alanazi M. The psychological impact of anterior teeth caries and treatment needs among adults at King Saud University Clinics in Riyadh, Saudi Arabia. Cureus. 2024;16:e68115.
7. Kastenbom L, Falsen A, Larsson P, Sunnegårdh-Grönberg K, Davidson T. Costs and health-related quality of life in relation to caries. BMC Oral Health. 2019;19:187.
8. Türkün L, Kanik Ö. A prospective six-year clinical study evaluating reinforced glass ionomer cements with resin coating on posterior teeth: quo vadis? Oper Dent. 2016;41:587-98.
9. Guggenberger R, May R, Stefan KP. New trends in glass-ionomer chemistry. Biomaterials. 1998;19:479-83.
10. Shahid S, Hassan U, Billington RW, Hill RG, Anderson P. Glass ionomer cements: Effect of strontium substitution on esthetics, radiopacity and fluoride release. Dent Mater. 2014;30:308-13.
11. Gurgan S, Kutuk ZB, Ergin E, Oztas SS, Cakir FY. Clinical performance of a glass ionomer restorative system: A 6-year evaluation. Clin Oral Investig. 2017;21:2335-43.
12. Söderholm KJ, Zigan M, Ragan M, Fischlschweiger W, Bergman M. Hydrolytic degradation of dental composites. J Dent Res. 1984;63:1248-54.
13. Prévost AP, Forest D, Tanguay R, DeGrandmont P. Radiopacity of glass ionomer dental materials. Oral Surg Oral Med Oral Pathol. 1990;70:231-5.
14. Tsuge T. Radiopacity of conventional, resin-modified glass ionomer and resin-based luting materials. J Oral Sci. 2009;51:223-30.
15. International Organization for Standardization. Dentistry-Water-based cements-Part 1: Powder/liquid acid-base cements. ISO 9917-1:2007.
16. Lachowski K, Botta S, Lascala C, Matos A, Sobral M. Study of the radio-opacity of base and liner dental materials using a digital radiography system. Dentomaxillofac Radiol. 2013;42:20120153.
17. Hitij T, Fidler A. Radiopacity of dental restorative materials. Clin Oral Investig. 2013;17:1167-77.
18. Salzedas LMP, Louzada MJQ, de Oliveira Filho AB. Radiopacity of restorative materials using digital images. J Appl Oral Sci. 2006;14:147-52.
19. Goshima T, Goshima Y. Optimum radiopacity of composite inlay materials and cements. Oral Surg Oral Med Oral Pathol. 1991;72:257-60.
20. Sidhu SK, Shah PM, Chong BS, Pitt Ford TR. Radiopacity of resin-modified glass-ionomer restorative cements. Quintessence Int. 1996;27:639-43.
21. Devito KL, Ortega AI, Haiter-Neto F. Radiopacity of calcium hydroxide cement compared with human tooth structure. J Appl Oral Sci. 2004;12:290-3.
22. Bouschlicher MR, Cobb DS, Boyer DB. Radiopacity of compomers, flowable and conventional resin composites for posterior restorations. Oper Dent. 1999;24:20-5.
23. García-Godoy F. Resin-based composites and compomers in primary molars. Dent Clin North Am. 2000;44:541-70.
24. Watts DC. Radiopacity vs. composition of some barium and strontium glass composites. J Dent. 1987;15:38-43.
25. Williams JA, Billington RW. The radiopacity of glass ionomer dental materials. J Oral Rehabil. 1990;17:245-8.
26. Bowen RL, Cleek GW. A new series of X-ray-opaque reinforcing fillers for composite materials. J Dent Res. 1972;51:177-82.
27. Williams JA, Billington RW, Pearson GJ. Lactic acid jet test: In-vitro erosion rates of glass ionomer dental cements containing radiopacifying elements. Biomaterials. 1993;14:551-5.
28. Kanjevac T, Milovanovic M, Volarevic V, Lukic ML, Arsenijevic N, Markovic D, et al. Cytotoxic effects of glass ionomer cements on human dental pulp stem cells correlate with fluoride release. Med Chem. 2012;8:40-5.
29. Schmalz G, Thonemann B, Riedel M, Elderton RJ. Biological and clinical investigations of a glass ionomer base material. Dent Mater. 1994;10:304-13.
30. Chen S. Glass ionomer cements with improved bioactive and antibacterial properties. Uppsala: Uppsala University. 2016.
31. Andersen OZ, Offermanns V, Sillassen M, Almtoft KP, Andersen IH, Sørensen S, et al. Accelerated bone ingrowth by local delivery of strontium from surface functionalized titanium implants. Biomaterials. 2013;34:5883-90.
32. Brauer DS, Gentleman E, Farrar DF, Stevens MM, Hill RG. Benefits and drawbacks of zinc in glass ionomer bone cements. Biomed Mater. 2011;6:045007.
33. Loza K, Föhring I, Bünger J, Westphal GA, Köller M, Epple M, et al. Barium sulfate micro- and nanoparticles as bioinert reference material in particle toxicology. Nanotoxicology. 2016;10:1492-502.