Long-term mechanical properties of glass ionomers

Dent Mater 10:78-82, March, 1994

Long-term mechanical properties of glass ionomers Sumita B. Mitra, Brant L. Kedrowski 3M Dental Products, St.Paul, Minnesota, USA

ABSTRACT Objective. Several methacrylate/glass ionomer hybrid materials are now available for clinical use as restorative filling materials. However, the long-term resistance of these materials to physical degradation in the humid oral condition is not known. The objective of this investigation was to determine the mechanical properties, e.g., ultimate compressive strength and diametral tensile strength, of several glass ionomer materials as a function of time after aging in water at oral temperature. Methods. Eight glass ionomer filling materials indicated for restorative or core build-up applications were studied. Three conventional glass ionomers, two metal-containing conventional glass ionomers and three methacrylate-modified systems were included in the study. Cured specimens of each were aged in distilled water at 37°C for 24 h, 1 wk, 4 wk, 12 wk, 24 wk and 52 wk. Results. Like the conventional glass ionomers, the methacrylate-modified glass ionomers of this study, with one exception, did not exhibit a decrease in compressive strength, modulus and diametral tensile strength as a result of prolonged storage in water at oral temperature. Some differences among the various groups were apparent. The compressive strengths of the conventional glass ionomers were lower than the methacrylate-modified system, except for one material, Fuji II (GC Dental Corp.), of the former group. A significant difference in the compressive strength was seen between the encapsulated and hand-mixed versions of the same commercial brand product. The compressive modulus was higher and the diametral tensile strength was lower for the conventional systems indicating that, as a group, these materials are more brittle than the methacrylate-modified hybrid ionomers. With the exception of VariGlass VLC (L.D. Caulk), most of the materials studied showed little decrease in mechanical properties after aging in water for 52 wk. Significance. These materials could, therefore, be indicated for use in applications where they are in contact with oral fluids under physiological conditions.

78 Mitra & Kedrowski/Long-term mechanical properties of glass ionomers

INTRODUCTION Many commercial glass ionomer products are now used in restorative and crown/core build-up applications. The ISO standard 9917 (1991) classifies these materials as type II glass ionomers. Since the development of the original glass ionomer cements (Wilson and Kent, 1971), these materials have undergone many improvements and modifications. Through advances in technology as well as improvements in placement techniques, conventional glass ionomers have provided clinically successful results (Wilson and McLean, 1988a). The combination of methacrylate technology with the conventional glass ionomer chemistry has increased the versatility of these materials and has made them essential components of the dental armamentarium (Croll and Killian, 1993). Several modified glass ionomer materials are now commercially available. In restorative applications, these materials are constantly exposed to oral fluids at physiological temperature. Some of these materials also release fluoride for extended time periods. The precursors to glass ionomers were the silicate cements, which were used for many of the same indications. However, the silicate materials were subject to clinical failure because of their high solubility in the oral environment and subsequent disintegration. There is now a substantial amount of clinical experience (Wilson and McLean, 1988b; Croll and Phillips, 1991) with the conventional glass ionomers which would indicate that long-term water solubility is not as serious a problem with these materials as it was with the silicate cements. However, there are few reports in the literature ofthe long-term mechanical properties of either the conventional or methacrylate-modified glass ionomers used as restorative filling materials. The objective of this investigation was to follow the ultimate compressive strengths and diametral tensile strengths of several type II glass ionomer materials, both the conventional systems, as well as the newer methacrylate-modifiedglass ionomers, after aging in water at oral temperature over extended time periods. MATERIALS AND METHODS Eight glass ionomer-based core build-up or restorative materials were evaluated in this investigation. The individual materials used, along with the respective manufacturers, batch numbers

cylinders were cut with a diamond saw to provide specimens that were Material Description Manufacturer Batch No. 8 mm in height and 4 mm in diamFuji II conventional glass ionomer, hand mixed GC Dental Corp., 191201 eter. For diametral tensile strength Tokyo, Japan measurements, the cylindrical dimensions were 2 mm in height and Fuji Cap II conventionalglass ionomer, encapsulated GC Dental Corp. 010901 4 mm in diameter. Samples were stored in distilled water at 37°C for the time periods indicated prior to Fuji II LC methacrylate-modified glass ionomer GC Dental Corp. 081011 being tested. A sample size of 6 was used for each of the time periods, 24 h, 1 wk, 4 wk, 12 wk, 24 wk and Miracle Mix metal-containingglass ionomer GC Dental Corp. 240311 52 wk. The samples were tested with a universal testing machine (Instron Model 1123, Instron Corp., Ketac-Fil conventionalglass ionomer ESPE, MD04119t Canton, MA, USA) at a crosshead Oberbay, Germany speedofl mm/min. Statistical analysis was performed for each mateKetac-Silver metal-containingglass ionomer ESPE 375E27 rial among the specified time periods using a two-way analysis of variance (ANOVA) at p<0.05 and VariGlass VLC methacrylate-modified hybrid L.D. Caulk, 920226 Scheff6's test. Compressive moduMilford, DE, USA lus was obtained from a leastVitremer methacrylate-modified glass ionomer 3M Dental Products, Exp 151 squares straight line fit ofthe stressTri-cure St.Paul, MN, USA strain profile obtained during measurement of compressive strength values. and codes used, are shown in Table 1. Three of these materials, For the measurement of flexural strength, 25 mm x 2 mm Fuji II (hand-mixed version), Fuji Cap II and Ketac-Fil, are x 2 mm specimens were prepared using a Teflon mold according conventional glass ionomer systems. Two metal-containing glass to the procedure outlined in ISO specification 4049 (1988). The ionomers, Ketac-Silver and Miracle Mix, were also included. In samples were stored in distilled water at 37'~Cfor 24 h. A threeKetac-Silver, the conventional fluoroaluminosilicate glass is re- point loading test was employed with the bar-shaped specimen inforced by sintering with metal (McLean and Gasser, 1985) being supported at the two ends by rollers set at 20 mm apart. The while in Miracle Mix, alloy particles are added to the powder load was applied from the top by a central roller at the mid-point component (Simmons, 1983). Three methacrylate-modified sys- ofthe specimen with the universal testing machine at a crosshead tems, Fuji II LC, VariGlass VLC and an experimental glass speed of 1 mm/min. The flexural strength, in MPa, was calculated ionomer system (now available commercially as Vitremer Tri- from the equation: Cure Glass Ionomer System) were also studied. The last three s = 3FI / (2bd2) undergo curing upon exposure to visible light. However, there is considerable difference between these materials. The VariGlass VLC material does not undergo any setting in the absence of where F is the maximum force, in N, exerted on the specimen, light. There does not appear to be any appreciable acid-base glass 1 is the distance, in mm, between the supports, b is the breadth, ionomer reaction in this system. The Vitremer Tri-Cure glass in mm, ofthe specimen, and d is the depth, in mm, ofthe specimen, ionomer material is claimed to be a bulk filling material and to immediately prior to testing. have good mechanical properties in both the light-cured and RESULTS chemical-cured mode. Hence, for this material, the properties were measured both in the light-cured and chemical-cured mode. The mean values of compressive strengths in MPa and the The Fuji II LC material is indicated for use only as a light-curing respective standard deviations of the materials are reported in material with incremental filling. Table 2. The results of the statistical analysis at p< 0.05 are For the measurement of compressive strength and diametral indicated in the table. Within each material group, the values tensile strength, cured cylindrical specimens of each material that are not significantly different from each other are indicated. were prepared by syringing the cement mix (either hand-mixed At the 24 h time period, most of the conventional glass ionomers, or, in the case ofthe capsules, activated and triturated as specified except Fuji II (conventional, hand-mixed), exhibited lower comby the manufacturer) into cylindrical glass tubes. Silicone rubber pressive strength than the methacrylate-modified glass ionomers. plugs were inserted above and below the material and then Several materials exhibited an increase in compressive strength pressure applied to eliminate entrapped air bubbles. This was compared to the 24 h time period. Significant increases were followed by radial exposure of the cylinders to two dental curing noted for four of the conventional glass ionomers, Fuji Cap II, lights (Visilux 2, 3M Dental Products) for 80 s. The chemicalKetac-Fil, Ketac-Silver and Miracle Mix. Although an increase in cured samples were kept under compression without light irradi- value was seen for Fuji II (hand-mixed), this was not found to be ation for 5 min. These cylinders were then held at 37°C and 100% a significant change. Ofthe modified glass ionomers, the Vitremer RH for 1 h. For compressive strength measurements, the cured Tri-Cure material, in both the light-cured or chemical-cured TABLE 1: MATERIALS USED

Dental Materials/March 1994 79

TABLE 2: COMPRESSIVESTRENGTHS(MPa)

Fuji II LC

Miracle Mix

Mean±S.D. Ketac-Fil

Ketac-Silver VariGlassVLC VitremerSC*

Vitremer Tri-cure**

Fuji II

Fuji Capll

24 h

210±23

156±21

203±12

128± 3

172± 6

170± 4

190±9

203±12

229+5

1 wk

234±32

201±15

214±2

150±12

202±17

191± 7

159±4

225± 7

256+4

4 wk

201±20

217±37

214± 7

166±13

202±16

208±17

173±5

243± 2

261+4

12wk

211+13

225±21

203± 6

165±11

194±17

185±20

175±7

245± 3

270+4

24 wk

234±16

210±12

213± 5

147±23

182±12

231±18

181±5

243± 4

261+6

52 wk

219±23

220±16

209±10

167±16

213±21

219±27

173±3

233± 8

253+5

Note: The vertical lines indicate values that were not significantly different from each other at p < 0.05. *specimens prepared using chemical-cured mode **specimens prepared using light-cured mode

TABLE 3: COMPRESSIVE MODULI (GPa) Mean+S.D. Ketac-Fil

Fuji II

Fuji Cap II

Fuji II LC

Miracle Mix

24 h

7.10+0.20

6.68+0.46

3.91+0.21

4.66+0.32

8.00+0.60 I

1 wk

7.90+0.29

ND

4.55+0.31

6.05+0.26

4 wk

8.20+0.29

8.14_+0.41 I

4.58_+0.41

12 wk

7.80+0.18

8.34+0.89

24 wk

8.10+0.19

52 wk

7.69+0.26

Vitremer Ketac-Silver VariGlass VLC Vitremer SC* Tri-cure** 6.39+0.40

4.43+0.40

4.75+0.46

5.35+0.20

9.10+0.66

I 7.24+0.20

2.45+0.14

4.48+0.22

5.80+0.09

6.27+0.10

7.66+0.58

6.22+0.42

3.19+0.20

6.68+0.33

6.97+0.25

4.00+0.18

6.58+0.28

7.52+0.24

7.66+0.28

3.75+0.32

6.30+0.34

6.47+0.17

9.72+0.46

4.33+0.16

6.69+0.32

7.93+0.48

7.72+0.27

4.20+0.14

6.83+0.34

7.10+0.26

10.3+0.66

3.90+0.12

7.24+0.80

8.83+0.32

18.55+0.87

3.28+0.18

6.90+0.28

6.80+0.41

Note: ND = not determined. The vertical lines indicate values that were not significantly different from each other at p < 0.05. *specimens prepared using chemical-cured mode **specimens prepared using light-cured mode

mode, showed a small but significant increase in compressive strength between the 24 h and 1 wk time periods. The Fuji II LC material did not show any change during this time period. VariGlass VLC was the only material that showed a significant drop in compressive strength between the 24 h and 1 wk time periods. An increase in strength was noted from the 1 wk to the 4 wk period although the value at 24 h was still significantly higher. No further change was noted for subsequent measurements up to the 52 wk time period. The compressive strengths of all the other materials remained relatively unchanged from the 1 wk to the 52 wk time period ofthis study. The values ofthe mean compressive moduli obtained during the measurement of compressive strength are shown in Table 3 along with the respective standard deviations. Most materials exhibited a slight increase in compressive modulus after long-term aging. Fuji II LC showed no change. As with the ultimate strength value, the compressive modulus ofVariGlass VLC decreased from the 24 h to the 1 wk time period. The modulus increased somewhat at the 4 wk measurement period and remained unchanged thereafter up to the 52 wk period. The flexural strengths at 24 h are reported in Table 4. 80 Mitra & Kedrowski/Long-term mechanical properties of glass ionomers

TABLE 4: FLEXURALSTRENGTHSAT 24 h (MPa) Material

Flexural Strength

Fuji II

14.2+1.2 I

Fuji Cap II

20.5+2.5

Fuji II LC

56.6+3.8

Miracle Mix

10.6+1.3 I

Ketac-Fil

12.2+2.21

Ketac-Silver

26.9+2.1

VariGlass VLC

20.3+3.8 II

Vitremer Tri-Cure

61.7+4.1

II

Note: Vertical lines of similar notation indicate values that were not significantly different p < 0.05.

TABLE5: DIAMETRALTENSILE STRENGTHS (MPa) Mean+S.D. Miracle Mix Ketac-Fil

Vitremer Ketac-Silver

VariGlass VLC Vitremer SC* Tri-cure**

Fuji II

Fuji Cap II

Fuji II LC

24 h

16.3_+0.9

7.9+1.6

40.7+0.5

7.0+0.9

15.4+1.7

14.1-+1.4

25.9_+1.1

35.6_+0.5 J

40.9+2.6

1 wk

14.5-+2.6

15.0+2.5

41.2-+1.4

8.9+1~3

18.2+0.6

12.6+0.3

26.6+0.8

33.9_+1.5

44.8+1.5

4 wk

13.7+2.4

14.1+3.8

41.6+1.5

8.1+0.7

16.2+3.1

13.7+1.9

27.7+1.0

41.4_+1.8

45.4+0.9

12 wk

15.8+2.3

13.1+2.2

35.4+3.4

14.3+2.3

22.4+2.3

16.6+2.5

28.0+0.6

38.0-+3.2

49.0+0.8

24 wk

18.8_+0.7

21.0_+1.6

40.0+2.1

10.4+0.7

21.0+2.6

17.5+2.0

27.2+1.1

40.8-+4.1

48.7+1.8

52 wk

18.5_+2.2

16.1_+1.0

40.0+1.1

10.2+0.7

20.3+2.1

16.5+1.5

27.8-+0.7

40.3-+0.9

46.3+2.5

Note: the vertical lines indicate values that were not significantly different p < 0.05.

The mean values of the diametral tensile strengths and the standard deviations are shown in Table 5 along with the results of the statistical analysis. Within each material group, vertical lines were used to indicate values which were not significantly different from each other. None of the materials showed a decrease in diametral tensile strength from the initial time of measurement (24 h) to the 52 wk time. Small, but significant, increases in value were obtained at the extended time periods for several of the materials, whereas for others these values were relatively unaffected. DISCUSSION The results of this one-year in vitro aging study support the clinical observation that long-term water solubility of conventional glass ionomers is not as serious a problem as it was with the silicate cements. Although important differences do exist between materials, none of the conventional glass ionomer restoratives exhibited a decline in the values of compressive strength, compressive modulus or diametral tensile strength of cured samples of the materials after 52 wk of storage in water at 37°C. However, reports of long-term studies, both in vitro and in vivo, of the more recently available methacrylate-modified glass ionomers are quite limited. A few reports have been published on light-cured glass ionomers used as liners and bases. However, these reports were not in agreement. A report by Nicholson et al. (1992) indicates that the compressive strengths oftwo light-cured glass ionomer liners were unchanged over a 90 d period of storage in air but decreased significantly after storage in water. However, Lewis et al. (1992) reported that the strength of these materials did not decrease (some actually increased significantly) upon storage in water at 37°C for 90 d. The results of Mitra (1991) also did not show any significant decrease in the properties of a lightcured glass ionomer liner/base upon storage in water for up to 7 mon in water at 37°C. A three-year clinical study (Powell et al., 1992) showed that the same light-cured glass ionomer liner/base material had excellent clinical performance. All of the foregoing reports are limited to applications of the methacrylate modified glass ionomers as liners and bases in which these materials are seldom exposed to oral fluids. However, a report by Croll (1991) indicated that when one such material was used as an interim restoration exposed to oral fluids, the clinical performance was quite good after 2 y. It might, therefore, be expected that the methacrylate-modified type II glass ionomer materials should

also have long-term physical integrity. The in vitro results of this long-term study would, for the most part, seem to support this hypothesis for these newer materials. In this study, the mechanical strengths of the materials were assessed by several different methods. Compressive strength is often used as a measure of the ability of a material to withstand the forces of mastication. In this test, a complex stress pattern is developed. Hence, this parameter does not have a fundamental meaning, since theoretically, a material can fail only by the separation of planes of atoms (i.e., tensile failure), or by the slipping of planes of atoms (i.e., shear failure). In spite of this limitation, the maintenance of compressive strength under prolonged aging is an indication of the mechanical integrity of a material. Direct measure of tensile strength is more valid. For dental materials undergoing brittle fracture, the diametral tensile test is often carried out because of its relative simplicity and reproducibility of results. It is an indirect tensile test in which a disk of the material is compressed diametrically until fracture occurs. The tensile stress is directly proportional to the load applied in compression. A limitation of the test is that, if the specimen deforms significantly before failure, the data may not be valid. Since, in this study, little deformation was noted for the specimens before failure occurred, the diametral tensile test could be regarded as being a valid measure of the relative brittleness of the materials. The flexural strength of materials in the transverse mode has been suggested as an alternative way of measuring the brittleness. The method employed here consisted of a three-point loading test. It may be regarded as being representative of a clinical situation which arises due to the forces exerted by the opposing cusp. The results of the compressive strength measurements of the methacrylate-modified glass ionomer indicate some differences among these materials. The reason for the decline in compressive strength of VariGlass VLC at the 1 wk period compared to the initial measurement time is not clear, particularly since the value increased somewhat at the 4 wk period and remained constant thereafter. A parallel change was reflected in the compressive modulus value, which indicates the resistance to deformation rather than the ultimate strength. None of the other materials, i.e., Vitremer Tri-Cure glass ionomer, in either the light-cured or chemical-cured mode, and Fuji II LC showed significant decreases in compressive strength or modulus during the 52 wk storage of these materials in water at 37°C. The results of the diametral

Dental Materials/March 1994

81

tensile strength measurements further corroborate the resistance to long-term deterioration in physical properties ofboth the conventional and methacrylate-modifiedglass ionomers. Some important differences are apparent among the conventional glass ionomers and the modified glass ionomers. The compressive strengths of the conventional systems were in general lower than those of the methacrylate-modified hybrid systems. The only exception was Fuji II hand-mixed conventional glass ionomer; the compressive strength of this material was not statistically differentfrom several ofthe modified systems. It was interesting to see that the encapsulated version of this product, Fuji Cap II, displayed considerably lower compressive strength (as well as diametral tensile) values during the early measurement periods of this study. The diametral tensile strengths ofthe conventional glass ionomers were, in general, lower than those of two of the modified glass ionomers, Vitremer Tri-Cure and Fuji II LC, with VariGlass VLC exhibiting intermediate values. The compressive moduli of the materials, as shown in Table 3, also indicate differences among materials. As mentioned earlier, modulus is obtained from the slope ofthe stress-strain curves and is indicative of the stiffness of materials. The higher modulus values of the conventional glass ionomers, coupled with the low diametral tensile strengths as well as the early flexural strengths, indicated that as a group, these materials would be more prone to brittle fracture than their methacrylate-modified counterparts. Of the modified systems, the Vitremer Tri-Cure glass ionomer material (in both light-cured and chemical-curedmodes) showed a progressive increase in compressive modulus, but the diametral tensile strength was maintained, indicating that there was a progressive resistance to deformationwithout an attending increase in brittleness. The maintenance ofphysical properties in water over the prolonged periods of this investigation would indicate that most of the newer methacrylate-modified glass ionomers tested in this study could be used in applications where they are in contact with oral fluids. Received November 29, 1993 / Accepted January 18, 1994 Address correspondence and reprint requests to: Sumita B. Mitra 3M Dental Products Division 260-2B-13, 3M Center St. Paul, MN 55144-1000 USA

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