Lactic acid jet test: in v&w erosion rates of glass ionomer dental cements conta~ ra~opacif~g elements

LA. Williams, R.W. Bilbzton and G.T. Pearson department of Biomaierials, institute of D%ntal Surgery, 256-Gray’s Inn Road, London WCI X 8LD, UK

The lactic acid jet test erosion rates were measured for 13 radiopaque glass ionomer dental materials obtained from a number of manufacturing sources. The erosion rate was compared with that found for the non-radiopaque restorative from the same manufacturer to determine whether the addition of an extra element had affected the resistance to erosion. Six materials were not significantly affected, six showed a significant increase in erosion rate. Only one material showed a reduced erosion rate. Materials containing a high proportion of any additive could show an increased erosion rate. Glass ionomer cements with or without radiopaci~ing elements had low erosion rates compared with other dental materials.

Keywords: Dental materials, lactic acid jet test, erosion, glass ionomer cements, racfiopacity

Received 8 December 1992; accepted 19 January 1993

A previous publication by Billington ef al. * examined the erosion resistance of a range of glass ionomer materials using the lactic acid jet test. It was not possible to attribute differences in erosion resistance between materials to any specific factor, since no correlation could be found between erosion rate and the type of mix (powder/liquid or powder/water), the polymeric acid or the mechanical and physical properties. When materials were grouped by similar erosion rates the major influence was the manufacturer from whom the materials were obtained.

One class of glass ionomers which was not included in the previous study was that used as bases and liners. It may be argued that for these materials erosion resistance is of less significance clinically in the dental environment; however, they do have a requirement for radiopacity and because of this criterion may contain elements not found in other glass ionomers. These additions, if not inert or unreactive, may give rise to divalent or trivalent ions capable of reacting with the polyacid to form a matrix which differs from that derived from the calcium or aluminium ions normally present in a radiolucent glass. The radiopacifying element may be incorporated either as a component of the glass ‘as made’ or added to the powder blend during processing.

It was anticipated that a comparison of the erosion rates of a radiopaque glass ionomer with the non- radiopaque restorative glass ionomer from the same manufactu~r might provide an understanding of the processes cont~lling erosion resistance.

Correspondence to Mrs J.A. Williams.

MATERIALS AND METHODS

Thirteen materials were found which claimed radiopacity. This was normally not clearly defined by the manufactu~r in terms of mm Al/l mm cement, nor was the radio- pacifying element given. Analysis was carried out on all materials where the radiopacifying element was unknown.

The categories were as follows: [a) restorative materials: Fuji II Radiopaque; (b] core build/posterior restoratives: Miracle Mix, RGI Core-build, RGI Posterior Filling material, KetacSilver, Ceramcore p; (c) base/lining materials: RGI Bond, BaseLine, Shofu Base, Liv Lining, Shofu Liner, Ceramlin & and KetacBond.

Further details can be found in Bible 1 where materials are grouped by manufactu~r together with the non- radiopaque restorative cements also available from the same manufacturer which were used for comparative purposes.

The erosion resistance test followed the method described in detail by Billington et al.’ where the erosion rate was measured as weight loss per hour following erosion in a stream of 0.02 M lactic acid at 37’C for periods up to 24 h. The samples were mixed according to the manufacturer’s directions at powder/liquid ratios given in ?gble 2, Eight specimens per material were tested as in the previous study’.

The element(s) responsible for radiopacity in each material were identified by electron diffraction analysis by X-rays (EDAX) on discs of mixed cement in a scanning electron microscope, Energy peaks are emitted in the form of X-rays following the electron beam scan. Their

8 1993 Butteworth-Heinemann Ltd Biomaterials 1993, Vol. 14 No. 7 0142-9612/93/070551-05

552 The lactic acid jet test: J.A. Williams et al.

Table 1 The materials used

Manufacturer Material Batch number P/L ratio

Shofu Inc., Kyoto, Japan

De Trey, Konstanz, Germany

ESPE Fabrik Pharmazeutische, Seefeld/Oberlay, Germany

Schein Rexodent Ltd, Southall, Middlesex, UK

G-C Dental Industrial Corp., Habashi-ku, Tokyo, Japan

PSP Dental Manufacturing Ltd, Belvedere, Kent, UK

Shofu Base

Shofu Liner

Shofu Type II

BaseLine ChemFil II KetacSilver KetacBond KetacFil RGI CoreBuild

RGI Reinforced RGI Bond

RGI Filling Miracle Mix

Liv Lining

Fuji II Radiopaque Fuji II

CeramLin p CeramCore B CeramFil p

P 048701 L 128701 P 028612 L 018610 P 048701 L 318708 880827 880584 N 300 P 0013/L 0014 0045 P DO68808 L 8048806 A 038921 P A 078801 L D 088803 C 078812 P 140392 L 310391 P 250461 L 210461 P 020382 L 290381 P 030651 L 290751 48606 028610 118509

2.611

1.511

3.1611

6.411 6.811 Capsule 3.411 Capsule 6.011

11.0/l 5.411

7.0/l 5.011

1.2/l

2.711

2.411

5.411 5.9/l 6.8/l

position on the energy scale can then be used to identify the elements present on the surface of the cement. The number of counts (either on a linear or logarithmic scale) is an indication of the amount of element present. Examples are given in Figures l-3 for Shofu Lining, KetacBond and BaseLine.

The radiopacity of each material had been reported previously by Williams and Billington’. No quantifiable figures were established for the amount of radiopacifying element but for materials containing the same element it may be assumed that those with a higher radiopacity have more of the element present.

RESULTS

The radiopacifying elements were limited to a few elements, the most commonly used being zinc. Five

Energy 0 CO”M 0

Figure 2 EDAX plot for KetacBond cement.

CAL” tazn ca

i I . . .; .-:. Y-e

.- ,y:*.;... *

v,~‘,,~.‘. ., ‘..__,_ .”

.‘.,rp*4’ v

Energy 0

Figure 3 EDAX plot for BaseLine cement. Figure 1 EDAX plot for Shofu Lining cement.

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The tactic acid jet test: J.A. WiNiams et al. 553

materials contained this element: Ceramlin p < Ceramcore p < Liv Lining < RGI Bond < Shofu Lining, with Sbofu Lining containing the most zinc.

A siIver~tin alloy was identified in RGI Core Build (RGI CBU], RGI Posterior Filling (RGI R] and Miracle Mix (EM), as was silver in KetacSilver (KS), S~untium appeared in both BaseLine (BL) and Fuji II Radiopaque [FUR]. Shofu Base {SB) contained barium and KetacBond fi(B] contained lanthanum.

The erosion rates are set out in Tabltl2 and F&we 4, grouped by manufacturer together with the radiopacifying element and the degree of radiopacity, The group with the highest erosion rate contains CeramLin /3 (CL /3],

CeramCore /3 (CC p) and CeramFii J? (CF @] with rates of weight loss of 1.2-1.4 mg/h. This group was followed by KetacSilver and KetacFiI (KF] at 0.9 mg/h althou~ KetacBond fKB], from ESPE (Oberhy, Germany], had one of the lowest erosion rates at 0.29 mg/h, The next two groups had similar rates apart from Shofu Lining (SL] with the exceptionally high erosion rate of 46mglh. Shofu Base together with BaseLine and ChemFB II {CII] had rates of 0.3-0.8 mgfh. Liv Lining (Lining) also eroded at a similar rate, 0.54 mg/h. Slightly lower erosion rates of 0.2-0.3 mglh were shown by the remaining materials, RGI Bond, Reinforced, Core Build and Filling, Miracle Mix, Fuji II and Fuji II Radiopaque.

Table 2 Erosion rates, radiopacity and radiopaque elements found for the materials tested and grouped by manufacturer

Material Radiopaqus Radiopacity Mean erosion rate Significance and element” (mm Al/l mm cement) (mg/h) change

CeramLin /3 CeramCore p CeramFil p KetacSilver KetacBond KetacFil Shofu Liner Shofu Base Shofu It BaseLine ChemFif II RGt CoreBuild RGI Reinforced RGI Bond RGI Filling Liv Lining Miracle Mix Radiopaque Fuji Ii Fuji II

Zinc, (copper) Zinc, (copper)

Silver, (titanium) Lanthanum

Zinc Barium, (zinc)

Strontium

Silver altoy Silver alloy Zinc

Zinc Silver alloy Strontium

1.6 1.39 NS 2.2 1.39 NS

1.22 0.88 S-

::: 0.29 HS- 0.92

4.5 45.8 HS+ 1.5 0.49 NS

0.51 2.0 0.63 S-l-

0.33 3.1 0.33 NS 3.4 0.28 NS 2.6 0.24 HS+

0.18 t.8 0.54 HS+ 7.6 0.30 s+ 2.1 0.15 NS

0.24

NS, not significant; S, significant P < 0.05; KS, highly significant P < 0.01 ‘Elements found in small quantities are given in parentheses.

CC Corp. Rexodent De Tfey Shofu ESPE PSP

Figure4 Erosion rates of glass ionomer dental cements, grouped by manufacturer.

Biomat~rials 1933, Vof, 14 No. 7

554

The data were examined using the Wilcoxon test, a non-parametric test being necessary since eight samples were insufficient to determine whether a normal distri- bution existed. This enabled differences between materials from each manufacturer to be quantified as to the degree of significance between the erosion rates. These are shown in lsbles 2 and 3 where the materials are grouped by radiopacifying element.

DISCUSSION

It appears that adding a radiopacifying element rarely improves the erosion rate. Nine out of 13 materials listed showed an increase, although this was highly significant in only three cases. These three materials, Shofu Liner, Liv Lining and RGI Bond, used zinc as the radiopacifying element. The other two materials also with zinc showed no significant increase but had high erosion rates for glass ionomers. EDAX analysis showed Shofu Liner to contain low amounts of calcium and silica. It also had a very high radiopacity value of 4.5 mm Al/l mm cement and may therefore contain a high proportion of zinc compared with glass. The erosion rate is high for a glass ionomer, possibly atypically so, since even ASPA, the first glass ionomer, had a lower erosion rate of 2.8 mgfh. The erosion rate of Shofu Liner is in fact more typical of that seen for zinc polycarboxylates as measured by Wilson et aIs3 and Williams et a14. It may be that zinc, a divalent ion, may either produce a weaker cross-link than that normally formed between the trivalent aluminium ion and divalent calcium ion present in the glass and polymeric acid or that an excess of zinc disrupts the cross-linking process in some way. It is not known how much, if any, of the additive has taken part in the cross-linking reaction. Liv Lining and RGI Bond which appeared to contain less zinc still had low erosion rates but were, however, higher than their corresponding restorative.

The presence of other divalent ions, strontium and barium, did not produce highly significant changes in erosion rate but the only added trivalent ion, that of lanthanum, did produce a highly significant reduction in the erosion rate of KetacBond compared with KetacSilver or KetacFil. A study carried out by Wall and Drenan’

The lactic acid jet test: J.A. Wiiliams et al.

which measured the ability of calcium, strontium and barium ions to combine with polyacrylic acid indicated a difference between these ions but was unable to account for this in terms of simple ionic equilibria. Calcium was found to be the least efficient and barium the most efficient ion. Since neither zinc nor lanthanum was used it is difficult to extend this theory to the current study other than to observe that the change from calcium to barium had less effect upon the erosion rate than did a change of st~ntium. This is the reverse of what is expected if the binding efficiency effect is a controlling ‘zature. Incorporation of metal, either as alloy or silver, had little effect and these may simply act as inert fillers. The metal-containing materials are said by the manu- facturers to be an admixture of metal and glass particles, while KetacSilver, a cermet, is made from a fused silver/ glass frit. Only the latter showed a significant decrease in erosion rate and was, in actual terms, still relatively high.

As with zinc, it is possible that large amounts of added metal introduce a weakening of the matrix. Miracle Mix with a higher proportion of metal than RGI Core Build and Reinforced Filling Material showed a significant increase in erosion rate (P < 0.05). The latter two cements exhibited a slight, but not significant, increase.

BaseLine is stated by the manufacturer to be a strontium aluminosilicate glass and it is believed that Fuji II Radiopaque also uses a strontium glass. One showed an increase and the other a decrease in erosion rate so that for this element no discernible pattern was seen. The assumption that each manufacturer will use the same material for a restorative and retain the same basic chemical system for other products may be unjustified. It is made only in the knowledge that each appears to keep to the same polymeric acid and thus to a particular type of chemistry for the range of cements they produce. The evidence of this study that the factor most strongly influencing the rate of erosion still appears to be the manufacturer would support this.

CONGLUSIONS

It would seem that while conferring the benefits of radiopacity, the addition of other elements will have a

Table 3 The erosion rates, radiopacity and radiopaque elements results grouped by radiopaque element

Material ~~~e~que Radiopacity Erosion rate (mm Al/l mm cement) (mgih)

~~~~~~ and

Shofu Liner CeramLin fi CeramCore B Liv Lining RGI Bond KetacBond BaseLine Fuji Ii Radiopaque Shofu Bay KetacSilver RGI CoreBuild RGI Reinforced Miracle Mix

Zinc Zinc Zinc Zinc Zinc Lanthanum Strontium Strontium Barium Silver Silver alloy Silver alloy Silver alloy

4.5 1.6 2.2

:.: 2:1 2.0 2.1

::“5 3.1 3.4 7.6

45.8 1.39 1.39 0.54 0.24 0.29 0.63 0.15 0.49 0.88 0.33 0.28 0.30

HS+ NS NS HS+ HS+ HS- s+ NS

;S NS

s”s

NS, not significant; S, significant P < 0.05; HS, highly significant P < 0.01.

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The lactic acid jet test: J.A. Williams et at. 555

tendency to increase the erosion rate. The only element associated with a decreased erosion rate was lanthanum,

REFERENCES

Erosion of materials still appears to be manufacturer 1 related. The range of erosion rates may vary quite widely, there being a factor of app~ximateIy six between the highest and lowest rates.

Class ionomers even with increases in erosion rates 2

still had fow erosion rates and therefore good erosion resistance compared with other types of water-based dental cements.

3

ACKNOWLEDGEMENT 4

Thanks are due to Mr C, H&and, Physics Department, Sussex University, Falmer, Sussex, for providing the 5 EDAX measurements,

Bilfington, R.W., Wifliams, J.A. and Pearson, G.J., in vitro erosion of 20 commercial glass ionomer cements measurer3 using the lactic acid jet test, ~ioma~er~a~s 1992, X3, 543-547 Williams, J.A. and 3il~ington, R.W., The radiopaci~ of glass ionomer dental materials, f. &a1 Rehab. 199O,lT, 245-248 Wilson, A.D., Groffman, D.&f., Powis, I3.R. and Scott, R.P., AR evaluation of the significance of the impinging jet method for measuring the acid erosion of dental cements, Biomaterials 1986, 7, 55-60 Williams, J.A., Billington, R.W. and Pearaon, G.J., The effect of maturation on in v&r0 erosion of glass ionomer and other dental cements, Br. Dani. J. 1992, 173, 349-342 Wall, F.T. and Drenan, J.W., Gelation of palyacrylic acid by divalent cations, J, PoJym. Sci. 1951, 7, k&88

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