Australian Dental Journal, December, 1975
36 1
Volume 20, No. 6
The properties of four dental cements L. Stwens, M.D.Sc.
Lecturer in Restorative Dentistry, University of Queensland
ABsrruCr--Tests on 4 cements showed no significant difference in the retentive property of zinc phosphate and polycarboxylate cements but significant differences existed between these and two zinc oxide eugenol (modified) materials. Polycarboxylate cement was the only material which fractured partially or wholly within the cement film and the zinc oxide eugenol modifications mostly adhered to the inlay. (Received for publication July. 1974. Revised August, 1975)
PART I
RETENTIVE PROPERTY introduction
In recent years crown and bridgework has formed an increasing proportion of dental practice. This increase is related to improvements in equipment, techniques, and materials. Associated with these improvements is the modification of existing dental cements and the development of new cements. The retentive property of four currently available cements was compared in a laboratory investigation. Observations were also made on the fracture of the joints made with these cements. Apart from biological considerations cements should be strong enough to withstand the forces of mastication and to retain a restoration in its preparation. The retentive property of cements has been investigated and compared by various workers in different ways. Because. of the effect of joint design in par-
ticular, and other effects in general, retentive property is peculiar to a particular joint. Joint design and retentive property are related; the strength of a joint of good design is less affected by the retentive property of a cement than a joint of poor design'. The design of the joint used in this experiment was easily and accurately reproducible. Zinc phosphate cement was introduced in 1878. Combinations of other materials, for example eugenol and fluoride, with this cement have been made mainly in attempts to improve its biological properties. These have been unsuccessful. Apart from improvements in manufacture which resulted in reduced particle size and elimination of arsenic, zinc phosphate cements are much the same today E.-Retention of inlays and crowns as a function of gcanetrical form. Brit. D.J., 103:11, 388-394 (Dec. 3) 1957.
1 Rosential,
Australian Dental Journal, December, 1975
362
as they were almost one hundred years ago. Nevertheless, zinc phosphate cement is widely used as an adhesive agent. Correct manipulation has been repeatedly stressed for obtaining optimum results with zinc phosphate cement2*3; other types of cements are less critical in this respect. Zinc oxide-eugenol cements were developed from zinc ’ oxychloride cements by substitution of the liquid with, first creosote, then eugenol. The first proprietary zinc oxide-eugenol product was “Pulpol”, introduced by Wessler in 1894. Similar mixtures were in use prior to this date”. Various additives have been combined with zinc oxideeugenol cement to improve its strength, for example, silica, alumina, rosin, dicalcium phosphate, polystyrene, polymethylmethacrylate, and ortho-ethoxybenzoic acids, 8.7.8.9. Improvement in strength resulted from the addition of polystyrene or polymethylmethacrylate to the liquid and more particularly the addition of ortho-ethoxybenzoic acid to the liquid together with silica or alumina to the powder. Polycarboxylate cement was introduced by Smith in 1968IO. It consists of a powder of zinc oxide and a liquid of an aqueous solution of polyacrylic acid. This cement has the unique property of being specifically adhesive to tooth structure, particularly enamel. This property of specific adhesion due to chelation of calcium ions on the tooth surface with carboxyl groups along the polyacrylic acid molecule can affect retentive property, to an extent, related to the design of the joint used and the amount of specific adhe-
I G r l i t h J R and Ware A. L.-An evaluation of dental ccme& Austral. D.J.’, 5 5 , 285-289 (Oct.) 1960. 3 Swartz, M. L.-Dental cements and restorative materials.
Zn, Dental Clinics of North America, Philadelphia, W. B. Sanders, March 1965 (pp. 169-183).
4 Luckie, S . 4 x i d e of zinc and eugenol. Items D. Int. 20,
490-491 (July) 1898. 5 Weiss. M. B.-Improved zinc oxide-eugenol cement. Illinois D.J., 2 7 ~ 4 261-271 , (Apr.) 1958. 6Roland. N., Kutscher, A. H. and Ayres, H. D.-EfTect of dicalcium phosphate on the crushing Strength of zinc oxide-eugenol cement, New York State D.J., 25:2. 84-86 (Feb.) 1959. 1 Brauer, G. M., Simon, L. and Sangemano, L.-Improved zinc oxide-eugenol ty cements. J.D. Res., 4 1 5 , 10961102 (Sept.-Oct.) l%? 8 Brauer G. M., McLau hlin R and Huget, E. F.-Aluminium oxide as a reinforciAg agent for zinc-oxide-eugenolo-ethoxyknzoic acid cements. J.D. Res., 47:4, 622-628 (July-Aug.) 1968. 9 Brauer. G. M.-A review of zinc oxide-eugenol type filling materials nd cements. Rev. Belge. Med. Dent., 20:3, 323-364, 1965. lOSmith, D . C.-A new dental cement. Brit. D.J., 125:9, 381-384 (Nov. 5 ) 1968. 11 Beech. D. R.-Improvement in the adhesion of polyacrylate cements to human dentine. Brit. D.J., 135:10, 442445 (Nov. 20) 1913.
sion obtained. Recent work is directed towards increasing specific adhesion to dentine by surface treatmentll. Fluoride can be incorporated into polycarboxylate cement as indicated by Smith and such a product is now available. It was felt that the comparison of retentive property of currently available cements when used in joints of a particular design might aid in the selection of a cement for joints of other designs. Materials end Methods
The cements selected were considered to be representative of those available: S.S. white Improved zinc phosphate cement (ZP); Fynal, a zinc oxide-eugenol cement having polymethylmethacrylate dissolved in the liquid eugenol 0; Opotow AJmnina, a zinc oxide-eugenolsrthoethoxybenzoic acid cement with alumina particles (0); Poly C, a polycarboxylate cement (P). 1. Comparison of retentive property Five tooth-inlay test pieces, as nearly identical as possible, were used; each one was cemented with the four cements in an order chosen at random and the test was repeated twice.
(i) Test piece The tooth-inlay test pieces were made as follows: extracted molar teeth were prepared by mounting in self-curing acrylic to allow attachment to both the Hounsfield Tensometer and to the vertical milling attachment of an engineering lathe. The teeth were kept wet with demineralized water during all stages of preparation and testing. Each tooth was held in the vertical milling attachment and the cusps ground flat. In succession a flat fissure end-cutting bur and a mounted diamond point (Horico No. 5 ) were used to block out a cavity and the final preparation completed with a mounted diamond point ( H d m No. 6) cutting to a final depth of 2.77 mm. The same diamond point was used to finish each cavity in an attempt to obtain equal surface roughness as well as equal dimensions. The Horico No. 6 mounted diamond point has an end diameter of 4.0 mm and a taper angle of 3.0 degrees. A localising notch was cut into the cavity at its cavo-surface angle using a tapered fissure bur (No. 700). (ii) Wax pattern A wax pattern was made in each of the cavities, to which a loop of wax was attached; a metal sprue was attached to the loop and the whole specimen was withdrawn and reproduced in Type B gold alloy. All inlays had a satisfactory accuracy of fit. The loop on the inlay allowed attachment to the Hounsfield Tensometer.
Australian Dental Journal, December, 1975
363
(iii) Cementation The powder-liquid ratios used for each cement were chosen after several trials and based on an estimation of a suitable clinical mix. The powders and liquids were dispensed by weight. The test piece parts were wiped with alcohol and TABLE 1
Results of ten tests on the retentive property of four dental cements Force in newtons (N)necessary to fracture tooth-inlay test joint Force (N)
Standard deviation
153.31 109.06 129.81 153.70
10.47 4.89 18.44 13.48
Ccment
ZP
F 0 PC
domly chosen. Each test piece was tested for retention by fracturing in tension using the Tensometer operated at a cross head speed of one sixteenth of an inch per minute; the 125 pound beam was used. The test pieces were attached so as to provide the best application of force with the least lateral or twist effect. The load that fractured the joint was recorded in pounds, corrected if necessary from the calibration certificate of the Tensometer and converted to newtons (N). The test piece parts were cleaned; the cavity by alternate soaking in demineralized water and gentle rubbing; the inlay by boiling in hydrochloric acid. Following this cleaning the test pieces were recemented and the retention tests repeated.
TABLE 2 Retention of tooth-inlay test pieces (five) joined (twice) by a dental cement (Raw data, in kg, coded by subtracting ten from each item)-anaZysis of variance. Source of sums of Variance DP estimates F P Variation squares
Cement (C) Test piece Q CxT Residual Total ** significant at the 1 p r cent NS = not significant
143.6770 0.9593 17.3131 18.3186 180.2680
3
4 12 20 39
47.8923 0.2398 1.4427 0.9159
52.290 0.262 1.575
** NS NS
level
DF = degrees of freedom F = varianw ratio P = probability
subjected to a short blast of air before cementation. T h e cements were mixed on a Jota slab at 21°C. The mixed cement was applied to both parts of the test piece in all cases except for Fpal cement where it was applied only to the inlay, in accordance with the manufacturer's instructions. Immediately after the cement was applied each test piece was assembled using light finger pressure. Three minutes after starting the mix a cementing weight of 1 kg was applied. Ten minutes after starting the mix each cemented test piece was transferred to an oven (37°C and 100 per cent relative humidity) until tested twenty-four (21) hours after the start of mixing (except for a period of approximately fifteen seconds removal for trimming of excess cement at the cavo-surface margin with a sharp chisel).
Retention tesfs Two tests were made using five tooth inlay specimens with each of the four cements ran-
Ten such tests were carried out for each cement. Results The results of the retention tests are given in Table 1. An analysis of variance shows a significant difference (at the one per cent level) between cements but not between test pieces; the interaction between cement and test piece was also not significant (Table 2). Duncan's New Multiple Range Test shows no significant difference between zinc phosphate and polycarboxylate cement, but significant differences (at the one per cent level) exist between the other cements. Discussion In general the thinner the film of adhesive the stronger the joint. This is considered to be due to the probability of fewer flaws occurring in a thin film. A very thin film may result in a
364
Australian Dental Journal, December, 1975
weak joint because of the contact of one adherend with the other through the film of cement12. Control of film thickness is difficult to obtain's. 14. Measurements were made of the test pieces before and after cementation and the difference in length determined. High standard deviations occurred in these measurements and an analysis of variance showed that the main sources of variation and their interaction were not significant. Film thickness tests, under a load of 1 kg, were carried out for each cement using the powderliquid ratio used in the retention tests. A significant difference existed between all cements. This indicates that seating an inlay into its cavity by hand does not ensure minimum film thickness between the inlay and its cavity. This arises most likely from binding on axial walls. The effect of other factors such as surface roughness would produce thicker filmsl5, but assuming uniform surface roughness and all other factors being equal for each test piece, a direct correlation would be expected between the results of the increase in length measurement and the film thickness measurements. There was no correlation between film thickness and retention. This is in conformity with the findings of Jorgensen and Esbensen16. Diametral tensile strength and compressive strength tests were carried out on the cements using the same powder-liquid ratios used in the retention tests. Significant differences existed between all the cements for both types of
strength. No correlation existed between retention and either compressive or diametral tensile strength. However, zinc phosphate cement had the greatest compressive strength and retentive property, and zinc oxide-eugenol with polymethylmethacrylate the least. That no definite relationship existed between retentive property and compressive strength is illustrated by zinc oxideeugenol-ortho-ethoxybenzoic acid cement having greater compressive strength but less retentive property than polycarboxylate cement. Due to the different natures of the cements it would be surprising if a definite relationship existed, although Jorgensen and Holst" found a direct relationship between compressive strength of different luting materials and retention. Whether or not any specific adhesion occurred between the polycarboxylate cement and tooth structure is not known. However, observations (X 9 magnification) of the fractured surfaces of test pieces cemented with polycarboxylate cement indicated that partial cohesive failure occurred within the film of cement in four out of ten tests. Cohesive failure within the film of cement could be dictated by factors other than specific adhesion; even so, it might be the tendency for cohesive failure that accounts for the retentive property of polycarboxylate cement and zinc phosphate cement not being significantly different, while significant differences existed between their strength properties; zinc phosphate greater in compressive strength and less in diametral tensile strength.
PART II
OBSERVATIONS ON THE FRACTURE OF JOINTS Materials and Methods
Two experiments were performed. Firstly, twenty tooth-inlay test pieces were arranged randomly into four sets of five; each set of five was used to test one cement four times. Secondly, five tooth-inlay test pieces were used; each one was cemented twice with the four cements in an order chosen at random, and for both, tooth-inlay test pieces were prepared in the same manner as those for the tests concerning retentive property. The fractured joints were examined under (X 9) magnification.
The test piece parts were cemented, stored at 37°C and 100 per cent relative humidity for twenty-four hours and fractured in tension on the Hounsfield Tensometer. The method of manufacture, cementing, storing and fracturing was the same for each test piece. The powder-liquid ratios used were determined after several trials and were of a clinically acceptable consistency. Results
When fracture occurred at or near to an interface adherent particles were observed on the
Australian Dental Journal, December, 1975 adherend involved in the interface; this occurred with all cements in both experimental designs. An estimate was made of the percentage of the surface area of the inlay, and of the cavity, that was covered with a film of adherent cement. Tables 3 and 4 show the estimates for each experimental design. A check was also made to determine whether or not the fracture occurred within the film. This was done by examining both surfaces to see if the adherent film was present in matching areas. Five types of fracture were observed:1. Failure at or near the cement-tooth interface; 2. Failure at or near the cement-inlay interface; 3. Failure at or near the cement-tooth interface together with failure at the cement-inlay interface, and across but not within the film of cement; 4. Failure wholly within the film of cement; 5. Failure within the film of cement in some places, together with failure at either interface. Tables 5 and 6 show the frequency with which these types of failures occurred for each experimental design for each cement. In the first experimental design fractures of both the eugenol-containing cements were mostly Type 1 and resulted in the cement film being adherent to the inlay. Zinc phosphate cement fractures tended to Type 3. Polycarboxylate cement did not fracture predominantly in any one way and was the only cement that showed fracture either wholly or partially within the film; five fractures occurred entirely within the film and four showed evidence of fracture within the film in some places. The pattern of the types of fractures that occurred with the second experimental design appear the same except that no fracture was observed wholly within a film of M. G.-Principles of adhesive retention and adhesive restorative materials. J.A.D.A.. 67:3. 382-391 (Sept.) 1963. 13 Bowen, R. L.-Adhesive bonding of various materials to hard tooth tissue. I. Method of determininp bond strenpth. J.D. Res., 44:4, 690-695 (July-Aug.) 1965. 14 Bikerman, J. J.-Science of adhesive joints. In Adhesion and Cohesion. Edit. P. Weiss. London, El&vier, 1962 (PP. 36-45). 15 Smith, B. G. N.-The effect of the surface roughness of prepared dentine on the retention of castings. J. Pros. D . , 23:2, 187-198 (Feb.) 1970. 16 Jorgensen, K. D . , and Esbensen, A . L.-The relationship between the film thickness of zinc phosphate cement and the retention of veneer crowns. Acta. Odont. Scand., 26:3, 169-175 (Aug.) 1968. 11 Jorgensen, K. D. and Holst. K.-The relationship between the retention of cemented veneer crowns and the crushing stren th of cements. Acta. Odont. Scand., 25:4, 355-359 (Dee? 1967. 12 Buoncore.
365 polycarboxylate cement. Discussion
A general concept of joint fracture is that the energy which causes the failure is consumed by plastic or elastic deformation of the adherends and adhesive until the joint commences to fracture at a point where local stress exceeds local strengt"g. Any one of a number of factors can operate singly or in combination t o affect the strength of a joint, and how a joined unit fractures is of importanceao. 11 in the consideration of joint strength. Joints made with zinc phosphate and zinc oxideeugenol type cements described in the experiment rely on mechanical interlocking at each interface. Those made with polycarboxylate cement are mechanically adhesive at the cement-gold interface but may be specifically adhesive at the cement-tooth structure interfaceaa. This specific adhesion is due to primary bonding although it has been suggested that because of improved wetting secondary bonding might be responsible=. 23. Differences in the manner of fracture would be expected to occur if specific adhesion at one interface did occur. In this event the film of cement would tend to fracture cohesively within the film not at a specifically adhesive interface. In the experiments this appeared to occur; polycarboxylate cement was the only cement that left a definite matching film on each adherend. This did not occur consistently, which supports the contention that the occurrence of specific adhesion to dentine is not certainn. The fact that fracture did not occur wholly within the film of polycarboxylate cement when the second experimental design was used may have been due to the prior use of the other cements interfering with the obtaining of specific adhesion; this prior use may have an adhesive effect. The areas of cement interfaces are unequal; the smaller area being at the cement-inlay interface. If all other conditions are equal, fracture in tension would be expected to start at or near the interface having the smaller area; the stress
Salomon, G.-Adhesion. In. Adhesion and Adhesives. Amsterdam, Elsevier, Vol. 1, 2nd ed., 1965 (p. 6). ID Bikerman, J. J.-Science of adhesive joints. In Adhesion and Cohesion. Edit. Weiss, P. London, E1slvir.r. 1962 (pp. 36-45). 20 Eley, D. D . and Tabor, D.-Fundamentals of adhesive joints. In, Adhesion. Edit. Eley. D . D . London, Oxford University Press. 1961 (PP. 1-18). 21 Reinhart, F . W.-Survey of adhesion and types of bonds involved. In. Adhesion and Adhesives Fundamentals and Practice. London, Society of Chemical Industry, 1954 (pp. 9-15). 22 Grieve, A . R.-A study of dental cements. Brit. D.J., 127:9, 405-410 (Nov. 4) 1969. 18
-
3 66
Australian Dental Journal, December, 1975 TABLE 3
Frequency of percentage of area of cavity covered with adherent cement after fracture of tooth-inlay test pieces (four sets of five) joined (five times) by a dental cement Cement
Percentage area covered
ZP F 0 PC
0-10
11-20
21-30
3140
1
1 3 2
0 1 0 2
3 0 2 3
21 20 4(5*)
* Fractures wholly
1
41-50
61-70
71-80
0 0 0
51-60
5
14
1
81-90
0
0
0
0 0
1
0
1
0 0
0
1
1
2
91-100
0 0 0 5(5*)
within the film of cement.
TABLE 4
Frequency of percentage of area of cavity covered wtih adherent cement after fracture of foothinlay test pieces (five) joined (twice) by a dental cement Cement
Percentage atea covered 11-20
21-30
3140
41-50
51-60
61-70
71-80
81-90
91-100
0 10
0 0 1
0 0 0
0 0
1 0 0
2
1
1
0
1
6 0 0 3
0 0 0 1
2 0 0
0
0 0 0 0
1
9
0 0 0
0-10
ZP F 0 PC
1
TABLE 5
Frequency of type of fracture of tooth-inlay test pieces (four sets of five) joined (five times) by a dental cement Type of fracture
Cement
ZP F 0
Pc “1” 6.
2
3.
“3” “4” “5”
“1”
“2”
“3”
“4”
“5”
2 21 20
3
20
0
4
0
5 7
0 0 0
0 0 0
5
4
5
4
= fracture = fracture = fracture = =
at or near the cement-tooth interface. at or near the cement-inlay interface. at or near both interfaces, with fracture across (not within) the film of cement. fracture wholly within the film of cement. fracture partially within the film of cement.
TABLE 6
Frequency of type of fracture of tooth inlay test pieces (five) ioined (twice) by a dental cement ~
~~
Type of fracture
Cement
ZP F 0 “1”
“2” “3” “4”
“5”
PC = fracture
= fracture = fracture = fracture = fracture
“1”
“2”
“3”
“4”
“5”
0 10 9 0
2 0
8
0
0
0
1
0
0 0
1
5
0
0
4
0
at or near the cement-tooth interface. at or near the cement-inlay interface. at or near both interfaces. with fracture across (not within) the film of cemetnt. wholly within the film of cement. partially within the film of cement.
would be more concentrated at this interface. Fracture at the cement-inlay interface was observed with zinc phosphate and polycarboxylate cement frequently, only once with the zinc oxideeugenol-ortho-ethoxybenzoicacid cement, and not
at all with the zinc oxideeugenol polymethylmethacrylate cement. It is possible that the surface layer of all cements at the interface of cement-tooth structure was weaker than that at the cement-inlay interface
367
Australian Dental Journal, December, 1975 possibly because contamination by moisture was likely at the cement-tooth structure interface. This would be so despite both surfaces being wiped with alcohol and dried with a short blast of air before cementation. Polycarboxylate cement, being hydrophillic, is less likely to be affected in this way than the other cements. The matrices of the cements form the interface with the adherends. The matrices of the eugenolcontaining cements are considered to be weaker than the matrix of zinc phosphate cement, based on the consideration that the former is bound by secondary bonds, the latter by primary bondP. 14. The matrix of polycarboxylate cement can be specifically adhesive; bound by primary bonds, chelation, to calcium ions on the surface of the tooth structure. The matrix of zinc phosphate cement, being acidic, could act as an etching agent and promote better mechanical adhesion to tooth tissue than the other cements. It is suggested that the eugenolcontaining cements possessed sufficiently weak adhesion to tooth structure and a sufficiently weak surface layer adjacent to the tooth structure for fracture to occur at the cement-tooth structure interface. The polymeric nature of polycarboxylate cement 13 Orin!, A. A., Greener, E. H. and Meshii, M.-Hi& resoluation microscopy of dental cements. Austral. D.J., 13:4, 29s-301 @us.) 1968. 24 Anderson, J. N.-Applied dental materia!s. Oxford, Blackwell, 4th ed., 1972 @. 291). I Bryant, R. W.-Strenpth of adhesive joints. Nature, 202: 4937, 1087-1088 (JUIIC13th) 1964.
suggests that it might absorb more energy, at the same rate of strain, before fracture than the other cements. Joints are strain rate dependentl5. This would be a factor affecting retentive property. Smith described set polycarboxylate cement as a network or gel-like structurelo. Such a structure would tend to allow fracture within the film because of plastic and/or elastic deformation producing greater strain within rather than at the boundaries of the film of cement. This could affect the manner of fracture of the polycarboxylate cement. The zinc oxide eugenol cements fractured at the cement tooth interface whilst the zinc phosphate and polycarboxylate fractured at either or within or across the cement. Condwion
Under the conditions of the experiment, zinc phosphate cement and polycarboxylate cement proved to be equally superior to the other cements with respect to retentive property. Under other conditions, for example a change in test piece design, a different result might occur. In practice, properties other than retentive property have to be considered and a cement should be chosen after a careful assessment of both mechanical and biological factors. Such an assessment may call for the use of any of the types of cement investigated. Dental School, University of Queensland, Turbot Street, Brisbane, Qland. 4000.
36 1
Volume 20, No. 6
The properties of four dental cements L. Stwens, M.D.Sc.
Lecturer in Restorative Dentistry, University of Queensland
ABsrruCr--Tests on 4 cements showed no significant difference in the retentive property of zinc phosphate and polycarboxylate cements but significant differences existed between these and two zinc oxide eugenol (modified) materials. Polycarboxylate cement was the only material which fractured partially or wholly within the cement film and the zinc oxide eugenol modifications mostly adhered to the inlay. (Received for publication July. 1974. Revised August, 1975)
PART I
RETENTIVE PROPERTY introduction
In recent years crown and bridgework has formed an increasing proportion of dental practice. This increase is related to improvements in equipment, techniques, and materials. Associated with these improvements is the modification of existing dental cements and the development of new cements. The retentive property of four currently available cements was compared in a laboratory investigation. Observations were also made on the fracture of the joints made with these cements. Apart from biological considerations cements should be strong enough to withstand the forces of mastication and to retain a restoration in its preparation. The retentive property of cements has been investigated and compared by various workers in different ways. Because. of the effect of joint design in par-
ticular, and other effects in general, retentive property is peculiar to a particular joint. Joint design and retentive property are related; the strength of a joint of good design is less affected by the retentive property of a cement than a joint of poor design'. The design of the joint used in this experiment was easily and accurately reproducible. Zinc phosphate cement was introduced in 1878. Combinations of other materials, for example eugenol and fluoride, with this cement have been made mainly in attempts to improve its biological properties. These have been unsuccessful. Apart from improvements in manufacture which resulted in reduced particle size and elimination of arsenic, zinc phosphate cements are much the same today E.-Retention of inlays and crowns as a function of gcanetrical form. Brit. D.J., 103:11, 388-394 (Dec. 3) 1957.
1 Rosential,
Australian Dental Journal, December, 1975
362
as they were almost one hundred years ago. Nevertheless, zinc phosphate cement is widely used as an adhesive agent. Correct manipulation has been repeatedly stressed for obtaining optimum results with zinc phosphate cement2*3; other types of cements are less critical in this respect. Zinc oxide-eugenol cements were developed from zinc ’ oxychloride cements by substitution of the liquid with, first creosote, then eugenol. The first proprietary zinc oxide-eugenol product was “Pulpol”, introduced by Wessler in 1894. Similar mixtures were in use prior to this date”. Various additives have been combined with zinc oxideeugenol cement to improve its strength, for example, silica, alumina, rosin, dicalcium phosphate, polystyrene, polymethylmethacrylate, and ortho-ethoxybenzoic acids, 8.7.8.9. Improvement in strength resulted from the addition of polystyrene or polymethylmethacrylate to the liquid and more particularly the addition of ortho-ethoxybenzoic acid to the liquid together with silica or alumina to the powder. Polycarboxylate cement was introduced by Smith in 1968IO. It consists of a powder of zinc oxide and a liquid of an aqueous solution of polyacrylic acid. This cement has the unique property of being specifically adhesive to tooth structure, particularly enamel. This property of specific adhesion due to chelation of calcium ions on the tooth surface with carboxyl groups along the polyacrylic acid molecule can affect retentive property, to an extent, related to the design of the joint used and the amount of specific adhe-
I G r l i t h J R and Ware A. L.-An evaluation of dental ccme& Austral. D.J.’, 5 5 , 285-289 (Oct.) 1960. 3 Swartz, M. L.-Dental cements and restorative materials.
Zn, Dental Clinics of North America, Philadelphia, W. B. Sanders, March 1965 (pp. 169-183).
4 Luckie, S . 4 x i d e of zinc and eugenol. Items D. Int. 20,
490-491 (July) 1898. 5 Weiss. M. B.-Improved zinc oxide-eugenol cement. Illinois D.J., 2 7 ~ 4 261-271 , (Apr.) 1958. 6Roland. N., Kutscher, A. H. and Ayres, H. D.-EfTect of dicalcium phosphate on the crushing Strength of zinc oxide-eugenol cement, New York State D.J., 25:2. 84-86 (Feb.) 1959. 1 Brauer, G. M., Simon, L. and Sangemano, L.-Improved zinc oxide-eugenol ty cements. J.D. Res., 4 1 5 , 10961102 (Sept.-Oct.) l%? 8 Brauer G. M., McLau hlin R and Huget, E. F.-Aluminium oxide as a reinforciAg agent for zinc-oxide-eugenolo-ethoxyknzoic acid cements. J.D. Res., 47:4, 622-628 (July-Aug.) 1968. 9 Brauer. G. M.-A review of zinc oxide-eugenol type filling materials nd cements. Rev. Belge. Med. Dent., 20:3, 323-364, 1965. lOSmith, D . C.-A new dental cement. Brit. D.J., 125:9, 381-384 (Nov. 5 ) 1968. 11 Beech. D. R.-Improvement in the adhesion of polyacrylate cements to human dentine. Brit. D.J., 135:10, 442445 (Nov. 20) 1913.
sion obtained. Recent work is directed towards increasing specific adhesion to dentine by surface treatmentll. Fluoride can be incorporated into polycarboxylate cement as indicated by Smith and such a product is now available. It was felt that the comparison of retentive property of currently available cements when used in joints of a particular design might aid in the selection of a cement for joints of other designs. Materials end Methods
The cements selected were considered to be representative of those available: S.S. white Improved zinc phosphate cement (ZP); Fynal, a zinc oxide-eugenol cement having polymethylmethacrylate dissolved in the liquid eugenol 0; Opotow AJmnina, a zinc oxide-eugenolsrthoethoxybenzoic acid cement with alumina particles (0); Poly C, a polycarboxylate cement (P). 1. Comparison of retentive property Five tooth-inlay test pieces, as nearly identical as possible, were used; each one was cemented with the four cements in an order chosen at random and the test was repeated twice.
(i) Test piece The tooth-inlay test pieces were made as follows: extracted molar teeth were prepared by mounting in self-curing acrylic to allow attachment to both the Hounsfield Tensometer and to the vertical milling attachment of an engineering lathe. The teeth were kept wet with demineralized water during all stages of preparation and testing. Each tooth was held in the vertical milling attachment and the cusps ground flat. In succession a flat fissure end-cutting bur and a mounted diamond point (Horico No. 5 ) were used to block out a cavity and the final preparation completed with a mounted diamond point ( H d m No. 6) cutting to a final depth of 2.77 mm. The same diamond point was used to finish each cavity in an attempt to obtain equal surface roughness as well as equal dimensions. The Horico No. 6 mounted diamond point has an end diameter of 4.0 mm and a taper angle of 3.0 degrees. A localising notch was cut into the cavity at its cavo-surface angle using a tapered fissure bur (No. 700). (ii) Wax pattern A wax pattern was made in each of the cavities, to which a loop of wax was attached; a metal sprue was attached to the loop and the whole specimen was withdrawn and reproduced in Type B gold alloy. All inlays had a satisfactory accuracy of fit. The loop on the inlay allowed attachment to the Hounsfield Tensometer.
Australian Dental Journal, December, 1975
363
(iii) Cementation The powder-liquid ratios used for each cement were chosen after several trials and based on an estimation of a suitable clinical mix. The powders and liquids were dispensed by weight. The test piece parts were wiped with alcohol and TABLE 1
Results of ten tests on the retentive property of four dental cements Force in newtons (N)necessary to fracture tooth-inlay test joint Force (N)
Standard deviation
153.31 109.06 129.81 153.70
10.47 4.89 18.44 13.48
Ccment
ZP
F 0 PC
domly chosen. Each test piece was tested for retention by fracturing in tension using the Tensometer operated at a cross head speed of one sixteenth of an inch per minute; the 125 pound beam was used. The test pieces were attached so as to provide the best application of force with the least lateral or twist effect. The load that fractured the joint was recorded in pounds, corrected if necessary from the calibration certificate of the Tensometer and converted to newtons (N). The test piece parts were cleaned; the cavity by alternate soaking in demineralized water and gentle rubbing; the inlay by boiling in hydrochloric acid. Following this cleaning the test pieces were recemented and the retention tests repeated.
TABLE 2 Retention of tooth-inlay test pieces (five) joined (twice) by a dental cement (Raw data, in kg, coded by subtracting ten from each item)-anaZysis of variance. Source of sums of Variance DP estimates F P Variation squares
Cement (C) Test piece Q CxT Residual Total ** significant at the 1 p r cent NS = not significant
143.6770 0.9593 17.3131 18.3186 180.2680
3
4 12 20 39
47.8923 0.2398 1.4427 0.9159
52.290 0.262 1.575
** NS NS
level
DF = degrees of freedom F = varianw ratio P = probability
subjected to a short blast of air before cementation. T h e cements were mixed on a Jota slab at 21°C. The mixed cement was applied to both parts of the test piece in all cases except for Fpal cement where it was applied only to the inlay, in accordance with the manufacturer's instructions. Immediately after the cement was applied each test piece was assembled using light finger pressure. Three minutes after starting the mix a cementing weight of 1 kg was applied. Ten minutes after starting the mix each cemented test piece was transferred to an oven (37°C and 100 per cent relative humidity) until tested twenty-four (21) hours after the start of mixing (except for a period of approximately fifteen seconds removal for trimming of excess cement at the cavo-surface margin with a sharp chisel).
Retention tesfs Two tests were made using five tooth inlay specimens with each of the four cements ran-
Ten such tests were carried out for each cement. Results The results of the retention tests are given in Table 1. An analysis of variance shows a significant difference (at the one per cent level) between cements but not between test pieces; the interaction between cement and test piece was also not significant (Table 2). Duncan's New Multiple Range Test shows no significant difference between zinc phosphate and polycarboxylate cement, but significant differences (at the one per cent level) exist between the other cements. Discussion In general the thinner the film of adhesive the stronger the joint. This is considered to be due to the probability of fewer flaws occurring in a thin film. A very thin film may result in a
364
Australian Dental Journal, December, 1975
weak joint because of the contact of one adherend with the other through the film of cement12. Control of film thickness is difficult to obtain's. 14. Measurements were made of the test pieces before and after cementation and the difference in length determined. High standard deviations occurred in these measurements and an analysis of variance showed that the main sources of variation and their interaction were not significant. Film thickness tests, under a load of 1 kg, were carried out for each cement using the powderliquid ratio used in the retention tests. A significant difference existed between all cements. This indicates that seating an inlay into its cavity by hand does not ensure minimum film thickness between the inlay and its cavity. This arises most likely from binding on axial walls. The effect of other factors such as surface roughness would produce thicker filmsl5, but assuming uniform surface roughness and all other factors being equal for each test piece, a direct correlation would be expected between the results of the increase in length measurement and the film thickness measurements. There was no correlation between film thickness and retention. This is in conformity with the findings of Jorgensen and Esbensen16. Diametral tensile strength and compressive strength tests were carried out on the cements using the same powder-liquid ratios used in the retention tests. Significant differences existed between all the cements for both types of
strength. No correlation existed between retention and either compressive or diametral tensile strength. However, zinc phosphate cement had the greatest compressive strength and retentive property, and zinc oxide-eugenol with polymethylmethacrylate the least. That no definite relationship existed between retentive property and compressive strength is illustrated by zinc oxideeugenol-ortho-ethoxybenzoic acid cement having greater compressive strength but less retentive property than polycarboxylate cement. Due to the different natures of the cements it would be surprising if a definite relationship existed, although Jorgensen and Holst" found a direct relationship between compressive strength of different luting materials and retention. Whether or not any specific adhesion occurred between the polycarboxylate cement and tooth structure is not known. However, observations (X 9 magnification) of the fractured surfaces of test pieces cemented with polycarboxylate cement indicated that partial cohesive failure occurred within the film of cement in four out of ten tests. Cohesive failure within the film of cement could be dictated by factors other than specific adhesion; even so, it might be the tendency for cohesive failure that accounts for the retentive property of polycarboxylate cement and zinc phosphate cement not being significantly different, while significant differences existed between their strength properties; zinc phosphate greater in compressive strength and less in diametral tensile strength.
PART II
OBSERVATIONS ON THE FRACTURE OF JOINTS Materials and Methods
Two experiments were performed. Firstly, twenty tooth-inlay test pieces were arranged randomly into four sets of five; each set of five was used to test one cement four times. Secondly, five tooth-inlay test pieces were used; each one was cemented twice with the four cements in an order chosen at random, and for both, tooth-inlay test pieces were prepared in the same manner as those for the tests concerning retentive property. The fractured joints were examined under (X 9) magnification.
The test piece parts were cemented, stored at 37°C and 100 per cent relative humidity for twenty-four hours and fractured in tension on the Hounsfield Tensometer. The method of manufacture, cementing, storing and fracturing was the same for each test piece. The powder-liquid ratios used were determined after several trials and were of a clinically acceptable consistency. Results
When fracture occurred at or near to an interface adherent particles were observed on the
Australian Dental Journal, December, 1975 adherend involved in the interface; this occurred with all cements in both experimental designs. An estimate was made of the percentage of the surface area of the inlay, and of the cavity, that was covered with a film of adherent cement. Tables 3 and 4 show the estimates for each experimental design. A check was also made to determine whether or not the fracture occurred within the film. This was done by examining both surfaces to see if the adherent film was present in matching areas. Five types of fracture were observed:1. Failure at or near the cement-tooth interface; 2. Failure at or near the cement-inlay interface; 3. Failure at or near the cement-tooth interface together with failure at the cement-inlay interface, and across but not within the film of cement; 4. Failure wholly within the film of cement; 5. Failure within the film of cement in some places, together with failure at either interface. Tables 5 and 6 show the frequency with which these types of failures occurred for each experimental design for each cement. In the first experimental design fractures of both the eugenol-containing cements were mostly Type 1 and resulted in the cement film being adherent to the inlay. Zinc phosphate cement fractures tended to Type 3. Polycarboxylate cement did not fracture predominantly in any one way and was the only cement that showed fracture either wholly or partially within the film; five fractures occurred entirely within the film and four showed evidence of fracture within the film in some places. The pattern of the types of fractures that occurred with the second experimental design appear the same except that no fracture was observed wholly within a film of M. G.-Principles of adhesive retention and adhesive restorative materials. J.A.D.A.. 67:3. 382-391 (Sept.) 1963. 13 Bowen, R. L.-Adhesive bonding of various materials to hard tooth tissue. I. Method of determininp bond strenpth. J.D. Res., 44:4, 690-695 (July-Aug.) 1965. 14 Bikerman, J. J.-Science of adhesive joints. In Adhesion and Cohesion. Edit. P. Weiss. London, El&vier, 1962 (PP. 36-45). 15 Smith, B. G. N.-The effect of the surface roughness of prepared dentine on the retention of castings. J. Pros. D . , 23:2, 187-198 (Feb.) 1970. 16 Jorgensen, K. D . , and Esbensen, A . L.-The relationship between the film thickness of zinc phosphate cement and the retention of veneer crowns. Acta. Odont. Scand., 26:3, 169-175 (Aug.) 1968. 11 Jorgensen, K. D. and Holst. K.-The relationship between the retention of cemented veneer crowns and the crushing stren th of cements. Acta. Odont. Scand., 25:4, 355-359 (Dee? 1967. 12 Buoncore.
365 polycarboxylate cement. Discussion
A general concept of joint fracture is that the energy which causes the failure is consumed by plastic or elastic deformation of the adherends and adhesive until the joint commences to fracture at a point where local stress exceeds local strengt"g. Any one of a number of factors can operate singly or in combination t o affect the strength of a joint, and how a joined unit fractures is of importanceao. 11 in the consideration of joint strength. Joints made with zinc phosphate and zinc oxideeugenol type cements described in the experiment rely on mechanical interlocking at each interface. Those made with polycarboxylate cement are mechanically adhesive at the cement-gold interface but may be specifically adhesive at the cement-tooth structure interfaceaa. This specific adhesion is due to primary bonding although it has been suggested that because of improved wetting secondary bonding might be responsible=. 23. Differences in the manner of fracture would be expected to occur if specific adhesion at one interface did occur. In this event the film of cement would tend to fracture cohesively within the film not at a specifically adhesive interface. In the experiments this appeared to occur; polycarboxylate cement was the only cement that left a definite matching film on each adherend. This did not occur consistently, which supports the contention that the occurrence of specific adhesion to dentine is not certainn. The fact that fracture did not occur wholly within the film of polycarboxylate cement when the second experimental design was used may have been due to the prior use of the other cements interfering with the obtaining of specific adhesion; this prior use may have an adhesive effect. The areas of cement interfaces are unequal; the smaller area being at the cement-inlay interface. If all other conditions are equal, fracture in tension would be expected to start at or near the interface having the smaller area; the stress
Salomon, G.-Adhesion. In. Adhesion and Adhesives. Amsterdam, Elsevier, Vol. 1, 2nd ed., 1965 (p. 6). ID Bikerman, J. J.-Science of adhesive joints. In Adhesion and Cohesion. Edit. Weiss, P. London, E1slvir.r. 1962 (pp. 36-45). 20 Eley, D. D . and Tabor, D.-Fundamentals of adhesive joints. In, Adhesion. Edit. Eley. D . D . London, Oxford University Press. 1961 (PP. 1-18). 21 Reinhart, F . W.-Survey of adhesion and types of bonds involved. In. Adhesion and Adhesives Fundamentals and Practice. London, Society of Chemical Industry, 1954 (pp. 9-15). 22 Grieve, A . R.-A study of dental cements. Brit. D.J., 127:9, 405-410 (Nov. 4) 1969. 18
-
3 66
Australian Dental Journal, December, 1975 TABLE 3
Frequency of percentage of area of cavity covered with adherent cement after fracture of tooth-inlay test pieces (four sets of five) joined (five times) by a dental cement Cement
Percentage area covered
ZP F 0 PC
0-10
11-20
21-30
3140
1
1 3 2
0 1 0 2
3 0 2 3
21 20 4(5*)
* Fractures wholly
1
41-50
61-70
71-80
0 0 0
51-60
5
14
1
81-90
0
0
0
0 0
1
0
1
0 0
0
1
1
2
91-100
0 0 0 5(5*)
within the film of cement.
TABLE 4
Frequency of percentage of area of cavity covered wtih adherent cement after fracture of foothinlay test pieces (five) joined (twice) by a dental cement Cement
Percentage atea covered 11-20
21-30
3140
41-50
51-60
61-70
71-80
81-90
91-100
0 10
0 0 1
0 0 0
0 0
1 0 0
2
1
1
0
1
6 0 0 3
0 0 0 1
2 0 0
0
0 0 0 0
1
9
0 0 0
0-10
ZP F 0 PC
1
TABLE 5
Frequency of type of fracture of tooth-inlay test pieces (four sets of five) joined (five times) by a dental cement Type of fracture
Cement
ZP F 0
Pc “1” 6.
2
3.
“3” “4” “5”
“1”
“2”
“3”
“4”
“5”
2 21 20
3
20
0
4
0
5 7
0 0 0
0 0 0
5
4
5
4
= fracture = fracture = fracture = =
at or near the cement-tooth interface. at or near the cement-inlay interface. at or near both interfaces, with fracture across (not within) the film of cement. fracture wholly within the film of cement. fracture partially within the film of cement.
TABLE 6
Frequency of type of fracture of tooth inlay test pieces (five) ioined (twice) by a dental cement ~
~~
Type of fracture
Cement
ZP F 0 “1”
“2” “3” “4”
“5”
PC = fracture
= fracture = fracture = fracture = fracture
“1”
“2”
“3”
“4”
“5”
0 10 9 0
2 0
8
0
0
0
1
0
0 0
1
5
0
0
4
0
at or near the cement-tooth interface. at or near the cement-inlay interface. at or near both interfaces. with fracture across (not within) the film of cemetnt. wholly within the film of cement. partially within the film of cement.
would be more concentrated at this interface. Fracture at the cement-inlay interface was observed with zinc phosphate and polycarboxylate cement frequently, only once with the zinc oxideeugenol-ortho-ethoxybenzoicacid cement, and not
at all with the zinc oxideeugenol polymethylmethacrylate cement. It is possible that the surface layer of all cements at the interface of cement-tooth structure was weaker than that at the cement-inlay interface
367
Australian Dental Journal, December, 1975 possibly because contamination by moisture was likely at the cement-tooth structure interface. This would be so despite both surfaces being wiped with alcohol and dried with a short blast of air before cementation. Polycarboxylate cement, being hydrophillic, is less likely to be affected in this way than the other cements. The matrices of the cements form the interface with the adherends. The matrices of the eugenolcontaining cements are considered to be weaker than the matrix of zinc phosphate cement, based on the consideration that the former is bound by secondary bonds, the latter by primary bondP. 14. The matrix of polycarboxylate cement can be specifically adhesive; bound by primary bonds, chelation, to calcium ions on the surface of the tooth structure. The matrix of zinc phosphate cement, being acidic, could act as an etching agent and promote better mechanical adhesion to tooth tissue than the other cements. It is suggested that the eugenolcontaining cements possessed sufficiently weak adhesion to tooth structure and a sufficiently weak surface layer adjacent to the tooth structure for fracture to occur at the cement-tooth structure interface. The polymeric nature of polycarboxylate cement 13 Orin!, A. A., Greener, E. H. and Meshii, M.-Hi& resoluation microscopy of dental cements. Austral. D.J., 13:4, 29s-301 @us.) 1968. 24 Anderson, J. N.-Applied dental materia!s. Oxford, Blackwell, 4th ed., 1972 @. 291). I Bryant, R. W.-Strenpth of adhesive joints. Nature, 202: 4937, 1087-1088 (JUIIC13th) 1964.
suggests that it might absorb more energy, at the same rate of strain, before fracture than the other cements. Joints are strain rate dependentl5. This would be a factor affecting retentive property. Smith described set polycarboxylate cement as a network or gel-like structurelo. Such a structure would tend to allow fracture within the film because of plastic and/or elastic deformation producing greater strain within rather than at the boundaries of the film of cement. This could affect the manner of fracture of the polycarboxylate cement. The zinc oxide eugenol cements fractured at the cement tooth interface whilst the zinc phosphate and polycarboxylate fractured at either or within or across the cement. Condwion
Under the conditions of the experiment, zinc phosphate cement and polycarboxylate cement proved to be equally superior to the other cements with respect to retentive property. Under other conditions, for example a change in test piece design, a different result might occur. In practice, properties other than retentive property have to be considered and a cement should be chosen after a careful assessment of both mechanical and biological factors. Such an assessment may call for the use of any of the types of cement investigated. Dental School, University of Queensland, Turbot Street, Brisbane, Qland. 4000.
