The dissolution mechanisms of silicate and glass-ionomer dental cements

The dissolutionmechanismsof silicate

and mass-ionomerdental cement AT.Kuhn Faculty of Science end Technology, Harrow College of Higher Educerion, Northwick Park, Harrow, HA1 3TP UK

Laboratory of the Government Chemist Cornwall House, (Received 18 June 1984: revised 15 April 1985)

WaterlooRoad,

London SE1 8XX UK

The mechanism of dissolution of two dental cements of the acid-base setting types (silicate and glassionomer) is considered. Dissolution is incongruent, probably because most of the leached species can derive both from the matrix (polysalt gel) and the partly reacted glass particles. The release occurs by means of three discrete mechanisms, surface wash-off, diffusion through pores and cracks or diffusion through the bulk. Such behaviour is shown to be capable of being modelled with extremely high goodnessof-fit values, using equations such as y = const + at” f bt. Analogies with research from the fields of geochemists and nueloar fuel storage am made and those systems obey similar relationships. The dental cement systems differ, however, in that their dissolution is to some extent reversible. This is explained in terms of formation of insoluble complexes, either by reaction of the constituent ions, or by replacement of OH-, for example, with F-. Keywords: Dental materials, cement

silicate, dissolution, glass-ionomer

The mechanism of dissolution of alumino-silicate dental cements (which are the acid-base setting reaction cements) continues to attract much attention, partly because of the adverse effects and partly because one component of the cements - fluoride - also leaches out and is known to have a welcome cariostatic action’. Of the many papers addressing one or both of these facets of the subject, few appear to have avoided the many experimental or theoretical pitfalls. Some of these pitfalls are considered here and the dissolution mechanism is examined.

dissolved species differs from that of the undissolved material. This situation is known as ‘incongruent dissolution’, and the behaviour of most dental cements falls into this category. Wilson and Batchelor’ have shown that the com~sition of the material leached out did not correspond to that of the parent cement and that while in the early stages, the sodium salts of phosphate and fluoride predominate, ultimately silica, sodium and fluoride at a much lower level are the main eluted species. Crisp et a/.6 found similar results for glass-ionomer cements.

THE CONCEPT OF DISSOLUTION

EXPERIMENTAL RESULTS ON THE DISSOLUTION DENTAL CEMENTS

A theoretical treatment of the dissolution of a solid in a dilute solution of itself, was formulated at the end of the last century by Noyes and Whitneg. By assuming that dissolution was controlled by diffusion from a thin layer of solution saturated with the solid they derived the equation: S f log, s-x

= C

Where r is time, S is the saturation con&x the cont. of the solid in solution at time f and C is a constant. Though the theory has been extended, it has remained essentially unchanged3*4. However, its application is restricted to a single phase i.e. dissolving in solution. A more complex situation arises when a solvent contains more than a single phase. Then, except in rare cases, the composition of

Confusion in the field arises from faulty or incomplete experimental data, thus dissolution of cements continues to be reported on the basis of weight loss of the cement sample. In the light of work by Crisp et al.” showing that glass-ionomer cements take up some 2% of their weight of water in approx. 5 d, and a similar finding7 in respect of the silicate cements, the weakness of this approach is selfevident. Moreover, individual dissolution rates of the components of the cements are aggregated and much valuable information is, thereby, lost. The best method is undoubtedly the individual analysis of each dissolving species. Apart from the work of Wilson’ this has scarcely been attempted. Before considering suitable models for the dissolution of the cements, it is appropriate to list the important 63 1985 B~e~o~h

378

Eiomaterials

1985, Vol6 November

OF

b Co

~Publishers) Ltd.0142-9612/85/060378-05$03.~

Dissolution of dental cements: A.T. Kuhn and A.D. Wilson

experimental observations relating to these materials and their dissolution. (1) So/&ion facrors. Dissolution rates (in near-neutral media) increase as a function of the volume of solution in which the cement is immersed and the frequency of its replacement with fresh solution. This has been found by Lesan etal.g. Tan eta/” have summarized published data on pH, presence of complexing acids, ionic strength etc. and their effect on silicate cement solubility. Various workersg-” have all shown that under certain circumstances, species dissolved in solution (fluoride, phosphate) may be taken up by the cement as well as released from it. It has also been shown that solution cont. of a given species leached from the cement may increase with time, then may slowly decrease, implying a re-uptake of some of the dissolved species by the solid cement. (2) Cement geometry. Many workersg*“,” have studied the effect of the surface area : volume ratio on the overall dissolution and fluoride release processes. Analysis of these data13 suggests that most of the dissolution stems from the outermost 0.5 mm layers. (3) Cement composition and morphology. Apart from water uptake, which has been considered above, Hornsby14 has shown that the set cements possess considerable porosity. Such evidence can be confirmed by dye-penetration methods15 and recently it was reported’3 that the depth of dye permeation is of the same order (< 1 mm) as the depth from which 70 % of leached species derive. Analysis of published data shows that the amount of dissolution does not usually exceed 0.1 % (by mass) of the original sample. Kuhn and Jones13 studied the concentration of five elements (Al, F, Si, P, Ca) in a silicate cement, after immersion for several months, as a function of depth from the surface. Though considerable variation in the concentration of each of these five elements was found over the 1000 steps taken at 10 urn intervals, there was no systematic concentration gradient from the centre of the sample to its outer surface.

SELECTION

OF A THEORETICAL

MODEL

What is the most appropriate theoretical model for dissolution processes in a system such as these cements? In order to answer this question, we have examined the literature relating to a number of systems in which slow dissolution of a solid is a primary feature. The areas of greatest relevance are: (a) time release of drugs from capsules; (b) weathering of rocks; (c) leaching of radioisotopes sealed into glassy matrices. All have some relevance to the dental cement system. The literature on time-release drug systems is admirably summarized by Kydoneus16 who classifies timerelease systems into monolithic types, membrane types and porous granular systems. The proven porous nature of the dental cements13’ l4 and also the absence of any measurable concentration gradient l3 of the diffusing species within the leached-out cement samples, indicates that the porous granular monolith is the model most closely resembling the dental cements. However, the only mathematical treatment provided by Kydoneus l6 is one assuming that diffusional loss occurs entirely from the pores, and requires a knowledge of pore size and tortuosity factors. There are parallels in geochemistry, where Hurd et a/l7 have found that finely divided silicate rocks dissolve in simulated seawater. However, this process is unidirectional.

What is not found is dissolution followed by re-absorption. There is, in the work of White and Classen”, a further concept namely the specific adsorption of species on to the surface of the finely divided silicates. The leaching of radioactive wastes from glassy or cement matrices provides another close parallel with dental cement dissolution. The first model to emerge from this body of work suggests that there are at least three simultaneously occurring processes to be considered: (i) surface erosion; (ii) dissolution from cracks and fissures; (iii) dissolution by solid-state diffusion from the bulk. Such a triple mechanism is depicted in Figure 7. Firstly, in the older dental literature on dissolution, e.g. of dental silicate cements, there are numerous references to loose surface debris at the surface of cements after exposure to solution. In the oral environment, this is probably rubbed away as it forms. Secondly, cracks (i.e. porosity) are known to exist and these must also play a role. Lastly, there is the bulk solid-state diffusion. However, recalling13 that no significant concentration gradient was found across a section of cement which had been exposed for many months to water, it is hard to apportion a major role to this process. An equation which attempts to ‘ncorporate most of these effects is: y = const + at”2 + bt

(2)

and a number of authors’g,23 have modelled the leaching of radioisotopes from glassy matrices using an equation of this general form. The reduction of any experimental data to the form of a valid specific equation is useful, to allow interpolation and extrapolation, however, if in addition some physical significance can be attached to the actual terms, then the achievement is much more useful. Thus, the timeindependent release of species adhering to the surface, has been linked to the constant in Equation 1. (Some authorslg refer to this as ‘wash-off’, others” see it as adsorbed material for which an isotherm can be formulated). The second term is the well known (time)“2 diffusional one, which relates to bulk diffusion of species in the solid state. The third term is based on a (time)’ related corrosion of the surface, i.e. network breakdown. This treatment was applied by Kuhn et a/.“, for the first time, to the dental cement dissolution process. To avoid problems of incongruent dissolution, the fluoride release data of Cranfield et a/.12, was used. The result is shown in Figure 2, for which the goodness-of-fit is 99.9 %. Clearly then, whatever the physical significance, this mathematical treatment gives the best fit over the longest range of times that has ever been reported. In fact, they12 reported data for samples of varying surface area: volume ratios, and all this

Figure 1 Scheme showing dissolution processes occuring in dentalcements. and the various routes by which these occur.

Biomaterials

r‘985. Vol6 November

379

Dissolution of dental cemenrs: AT. Kuhn and A 0, Wilson

10

0

20

Time

3

(d)

Figure 2 Application of Equation 2 to the data in Reference 12 for fluoride release over 30 d. Goodness-of-fir = 99.7%. F- cont. in mol X lo4 per 50 ml. (Equation of best f&: F(x) = 223 + 239x’” - 0.118x).

data was fitted to the same type of equation. The results are shown in Table 1. Following this successful application of the method, a more severe test was the fitting of the same equation to data obtained by Wilson et a/.” (reported in this issue) in which the dissolution of a dental glass-ionomer cement was followed for 598 d. The goodness-of-fit for F, Na and SiOzreleased from discs and cylinders is excellent. Wilson et al. 22 have found that equally good fits to this type of data can be obtained with an equation of the form: y = A + Bt”2 - C expent

(3)

but bearing in mind that Equation3 has four ratherthan three constants for curve fitting this is not surprising. Generally, the simpler form is to be preferred. However, there is one exception in the phosphate leach from silicate cements which is most accurately represented by this equation. The 1 - exp-“’ term in this equation is that of the Noyes and Whitney2 equation which describes the dissolution of a solid in a dilute solution of itself and is particulary appropriate for representing the solution of soluble sodium acid phosphates contained in freshly set dental silicate cements. Apart from the remarkable goodness-of-fit (shown in Tab/e 1 and Reference 21) it is interesting that all the ‘b’ coefficients are negative. This would seem to contradict the idea of apportioning to each term in Equation 2 a specific part of the mechanism. In fact other workers have also observed negative coefficients in their treatment of leach data. Thus

Table 1

Cuwe-fining rhe dara’ of Reference 12

Sample thickness

Constant 2.2 1.6 -0.097 0.64

1 mm 3mm 5mm 10mm

a

b

Goodness-of-fit (%)

2.4 3.3 6.4 5.7

-0.12 -0.18 - 0.41 - 0.28

99.98** 99.96 99.96 99.96

*Data ftied over a 28 d period. **Shown in Figure 2.

380

Biomarerikis

1985, Vol6 November

Heimerl and Heine23 comment ‘, . . sometimes a negative sign of the linear t term is obtained. This suggests that a fast leach process is followed by a slower one.. .‘. A further important point (as Table I and Reference 21 show) is that when t = 0, y, the amount of released species, is the constant term in eq. 2, and is not zero. Previously, data has been presented to suggest that the dissolution-time plot passes through the origin. The computer data shows clearly that this is not so, and thus supports the concept of an initial ‘wash-off’ of species contained at the surface. The one negative value for the 5 mm data in Tab/e 1 (although very small) is somewhat baffling. Two of the three factors described in Figure 1. are accounted for by Equation 2. What of leaching from cracks or fissures, which, as we have seen, undoubtedly exist? Perez et a1.24,in studying the leaching of radioactive species, set up a model based on discs of borosilicate glass, which were clamped together, but with wires inserted between certain discs to create artificial cracks. They rightly distinguish between two extreme cases, namely diffusion through solution in the crack being either (i) faster or (ii) slower, than the leaching process. Once again, incongruent dissolution was observed. Their work shows that, although the cracks represent the major part of the accessible surface area, their contribution to the leaching process is considerably less. They also suggest that the ratio of the various components dissolving from the cracks is not the same as from the outer surface, perhaps because pH and ionic strength of solution will be different in such cracks.

CHEMISTRY

OF DISSOLUTION

Too little emphasis has been placed on the chemistry underlying the dissolution mechanism of dental cements of the silicate or glass-ionomer types. Certain inherent problems in elucidating mechanisms should be recognized from the outset. Among these are:

(4

PI

most of the elements which leach out are present in the set cement in more than a single phase. Thus leached F- will derive both from unreacted glass particles and from, for example, fluoroalumino-phosphates in the matrix. It may well be that at least for a time, one source of such leachants will dominate; some workersg* lo*25 have demonstrated that not only are fluoride and phosphate ions released from cements into solution, but that they can also be taken up by them. This reversibility does not appear to have been discussed previously in the literature.

The first two of these studiesg* lo were concerned to demonstrate simply that such effects occurred and used somewhat extreme conditions to make their point. That similar effects can take place under much less extreme conditions, is shown by the following data taken from Reference 25. In this work, a cement was made using Super Syntrex glass powder and a liquid containing phosphoric acid, aluminium and zinc to form discs (20 mm diam. X 1.5 mm ht) which were then placed in 50 ml of the given solution for 24 h at 37°C. The concentrations of leached-out species are shown in Table 2. Thus while the more extreme conditions (higher concentrations) used by Lesan etaLg and Tan et al.” actually caused reversal of the ionic release, the data in Table 2 shows that even at very low cont., phosphate ions in solution can repress the

Table 3 L&solution of silicate cement*

Table2 Solubiility of a silicate cement’

Eluted species** SOlUtiOil

Na

PO4

SiO,

Water t&5000 M/2500 M/2000 ~A000 ~/2500 ~ZOOO

1.26 0.74 0.26 0.36 1.55 1.66 t .66

2.67 1.71 0.91 0.75 1.7% 1.16 0.63

0.40 0.34 0.36 0.3% 0.37 0.41 0.41

NaHzP04 NaH,W, NaH$Q4 NH4H~~4 NH,H,PQ NH~H~~~

*From References 25. **Expressed as mg/disc of cement.

release of this ion from the cement. However, the difference between the sodium and ammonium phosphates in respect of Na+ release is notable and suggests ion-exchange effects. In a further series of experiments, the same type of cement disc was immersed for 24 h in water (preliminary leach), then for 6 d in water (fu~her leach) after which it was transferred to a solution for one more day. This elaborate sequence (always at 37°C) allowed the pattern of leaching with time to be foliowed. However special interest attached to the leach behaviour in the final (6 + 1) d sequence, and the release (and uptake) of materials are recorded in Table 3. While data in Table2 showed how dissolution could be retarded by species in sotution, Table3 shows an actual reversal of the process, again under far iess extreme c~d~ti~s - and therefore of more clinical relevance -than the results quoted in References 9 and 10 but like the work of Lesan et al.9 there is a clear trend which is dependent on concentration in solution. (C) the glass alone, forming the basis of silicete or glassionomer cements is capable of leaching ions in contact with water. This has been demonstrated already26, 2f, These observations appear to preclude a simple mechanism in which soiubility is an equilibrium between the same chemical species in the sotid and dissolved in solution, i.e. a mechanism governed by the solubility product. Such mechanisms do operate in the dissolution of minerals in water’ ‘. However, the reversible nature of dissolution followed by re-uptake of the same species, demands an alternative explanation. It is known, mainly from the work of Wilson et a1.28, that an acid-base reaction forms the basis of setting of the cements. In these reactions, soluble forms e.g. of phosphate, are progressjve~y converted to ‘insoluble* forms”. 30. If the overall process is:

Then these reactions may proceed at a diminishing rate for months if not years 30,31. Chemical analysis does not suffice to follow such very slow reactions, where the fraction of unreacted species after many weeks is extremely smalt. However, the steady change of physical properties, together with other methods such as electrical conductivity32 do testify to long-term continuing reaction. Similar processes occur in respect of f~uorid~ontain~ng species, although the chemistry here is less certain. The complex f~uoroa~uminate cation is soluble and this could react with phosphate to form the species: A12(OH)3P04 - corresponding to the mineral Augite, which has in fact been found in dentat silicate cements**. Since OH- exchanges with fluoride, these ions

N&LpC34

Na

*4

sio,

0.04 0.40 - 1.33 - 2.15 - 3.64

0.95 I.01 l.O? 1.03

(Gmc.) 0 (water) hlf20 000

0.40

M/j

0.1%

0 000

M/5000 M/3333 M/2000

-0.17 - 0.62 - 1.61

-

1.OJ

*For details of regime sea text and Reference 26. **In mg/disc over 7 d. Negative valws indicate uptake from solutw.

having very similar sizes, there seems no reason why a fluoroaugite should not be formed by reactions between AIF*‘, AIF,+ and H2P04-, HP042-, e.g.: 2AIF2+ + HPOh2- = A12F3(P0,) + WF and unce again a more soluble ~uorjde containing species is converted into a less soluble one. Thus the m-adsorption of species from solution by aged water-leached cements can be seen in terms of the foliowing steps: 0) (II) (I!{) (IV)

setting cement contains H2P0,- and HPOa2ions; these ions are removed by (a) reaction with glass and (b) teaching into solution; phosphate cont. in gel decreases, then falls below phosphate cone. in solution; phosphate diffuses from solution back into gel matrix until concentrations are the same in both phases.

A similar type of mechanism can be envisaged for fluoride ions. An intriguing recent finding by Pederson et af.33 retates to the leaching of nuciear waste glass, and the effect on this process of the container material in which it is held. They found the rate of leaching when held in lead containers was two orders of magnitude iower than that from ptfe vessels. While the metat lead is not retevant here, they also found that a one order of magnitude reduction was obtained from an aluminum container (their glass was Al-free). The explanation of these observations remains somewhat s~culat~ve but (foijowing ~olchanov and Fr~khid’Kho3~) the authors postulate the adsorption of aluminium ions on sites of the lowest energy, which are the sites of the most intensive glass corrosion. In the in v&o dental context, such atuminium ions could be derived from the glass itself. Furthermore, it is difficult to see what other stabilizing species could adsorb onto the glassy corrosion sites of the dental cements.

CONCLUSION in conclusion, we feel it is necessary to envisage the dental cement dissolution process as being the sum of several totailydifferentformsof action. Most laborato~studies have focussed (perhaps mjsguidedly) on the initial period when the diffusional component seems to be the most important, together with a time-independent ‘wash-off’, Beyond this, lies a region where the dissolution rate is proportional to (time)‘. On the other hand long-term studies, such as those reported by Causton35 or Swartz and PhiiIips3” catl for thought in the choice of solution change regime.

Dissolution of dental cements: AT. Kuhn and

AD.

Wilson

REFERENCES 1

2

9 10 11

12 13

14 15

16 17

18

382

19

Wilson, A.D. and Kent, 8.E.. The glass-ionomer cement-a new translucent dental cement, J. &I/. Chem. Biotechnol. 1971, 21, 313 Noyes. AA. and Whitney, W.R., Ueber die Auflosungsgeschwindigkeit von festen Stoffen in ihren eigenen Losungen, Phys. Chemie 1897, 23,689-692 Mueller, R., Ein Gemeinsam Ausdruckes fur die Losungsgeschwindigket eines festen Korpers, Acta fhysicochem. URSS 1936,4, 48 l-493 Moelwyn-Hughes, E.A., Textbook of Physical Chemistry, (2nd Edn), Fergamon Press, Oxford, 196 1, pp 1215 et seq. Wilson, AD. and 8atchelor. R.F., Dental silicate cements: I. The chemistry of erosion, ./. Dent. Res. 1967,46, 1075-l 085 Crisp, S., Lewis, 8.G. and Wilson AD., Glass-ionomer cements: Chemistry of erosion, J. Dent. Res. 1976, 55, 1032-1041 Davies, E.H., Kuhn, AT.and Tan, WK.. Water uptake and loss in silicate cements, J. Mater. Sci. 1982, 17. 3263-3267 Wilson, A.D., Specification test for the solubility and disintegration of dental cements: A critical evaluation of its meaning, J. Dent. Res 1976, 55.721-729 Lesan, W., Kuhn, A.T. and Setchell, D.J., Further factors affecting the solubility of silicate cements, J. Mater. Sci Letts 1983, 2, 191-l 94 Tan, WK. Kuhn, AT. and Winter, G.B., The dissolution rates of dental silicate cement, Biomaterials 1982, 3, 136-l 44 Levinkind, M., Fluoride release: An in vitro investigation on glassionomer and silicate cements, M9c project Thesis, University of London, 1982 Cranfield, M., Kuhn, A.T. and Winter, G.B., Fluoride release from glassionomer cements, J. Dent. 1982, 10, 333-341 Kuhn, AT. and Jones, M.P. A model for the dissolution and fluoride release from dental cements, Biomat. Med. Dev. Art. Org. 1982,lO (4). 281-293 Hornsby, P.R., A study of the formation and properties of ionic polymer cements, PhD Thesis, Brunei University, 1977 Paffenbarger, G.C., Schoonover, I.C. and Souder. W. Dental silicate cements: Physical and chemical properties and a specification, J. Amer. Dent. Ass. 1938, 26. 32-87 Kydoneus, A.F. Controlled Release Technologies, CRC Press, Ohio, 1980 Hurd, D.C.. Fraleyl, C. and Fugate, J. K., Silica apparent solubilities and rates of dissolution and precipitation in aqueous systems, ACS Symp. Ser. 1979, 93, (Chemical Modelling in Aqueous Systems), 413445 White, A.F. and Classen, H.C., Dissolution kinetics of silicate rocks application of solute modelling,ACS Symp. Ser. 1979,93, (Chemical Modelling in Aqueous Systems), 447-477

Biomaterials

1985, \/o/6

November

20

21

22 23

24 25

26

27 28

29

30

31 32 33

34

35

36

Lee, D.J. and Brown, D.J., Factors affecting the leachability of caesium and strontium from cemented simulated evaporator wastes, Report R 7467, Technology Division, AERE AEA, Winfriih, Dorset, UK (and references therein), August 198 1 Kuhn, A.T., Winter, G.B. and Davies, E.H., Dissolution and fluoride release from silicate and glass-ionomer cements, J. Dent. Res. 1982, 61, 555 (IADR Abstr. no. 173) Wilson, AD.,Kuhn, A.T. and G&man, D.M., The release of fluoride and other chemical species from a glass-ionomer cement, Biomaterials 1985,6,431-433 Wilson, A.D., Crisp, S. and Lewis, B.G., The aqueous erosion of silicophosphate cements, J. Dent. 1982, 10, 187-l 97 Heimerl, W. and Heine, H., Solubilities of leachants in radioactive wastes, in Management of Radioactive Wastes from Fuel Processing OECD 1973, Proc. Symp., Paris 1972 (D 73-21146) pp 536-551 Rrez, J.M. and Westsik, J. H., Effects of cracks on glass leaching, Nud and Chem. Waste Management 1981, 2, 165-l 68 Kent, 8.E.. Lewis, B.G. and Wilson, AD., Silicate, phosphate and sodium exchange on dental silicate cements, Laboratory of the Government Chemist Report 1967 Kent, 8.E.. Stockwell. J.A. and Wilson, A.D., The glass structureextraction studies, Laboratory of the Government Chemist Report 1969 Kuhn, AT., (unpublished observations) Wilson, AD.,Kent, B.E., Clinton, D. and Miller, R.P., The formation and microstructure of dental silicate cements, J. Mater. Sci 1972,7, 220238 D’Neill, I.K.. Presser. H.J., Richards, C.P. and Wilson, AD., NMR spectroscopyofdental materials. I. Studies in phosphate bonded liquids, J. Biomed. Mater. Res. 1982, 16. 39-49 Crisp, S., Lewis, B.G. and Wilson, A.D., Characterization of glass-ionomer cements. I. Long-term hardness and compressive strength, J. Dent. 1976,4. 162-l 66 Paddon, J. M. and Wilson, AD.,Stress relaxation studies on dental materials. I. Dental cements, J. Dent. i 976, 4, 183-l 89 Wilson, A.D. and Kent, B.E., Dental silicate cements. V. Electrical conductivity, J. Dent. Res. 1968,47.463-470 Buckwalter. C.Q and F’ederson, L.R., Inhibition of nuclear waste glass leaching by chemisorption, J. Amer. Ceram. Sot. 1982, 65, 43 l436 Molchanov, V.S. and Prikhid’Kho, N.E., Corrosion of silicate glass by alkaline solutions. Ill. /zv Akad Nauk SSR Oid khim-Naok 1958, 7, 801-808 Causton, B.E., The physico-mechanical consequences of exposing glass-ionomer cements to water during setting, Biomaterials 198 1, 2(2), 112-l 15 Swartz, M.L., Phillips, R.W. and Clark, H.E., Long-term release from certain cements, J. Dent. Res. 1983, 62, 191 (IADR Abstr. no 173)