ixing of zinc phosphate cement for fixed ontic procedures Zaki Karl
A. Fakiha, BDS, MS,a Leonard F. Leinfelder, DDS, MSC
A. Mueninghoff,
DDS,b
and
Ring Saud University, School of Dentistry, Riyadh, Saudi Arabia, and University of Alabama, School of Dentistry, Birmingham, Ala. This study was done ta determine the efficacy of rapidly mixing zinc phosphate cement in conjunction with fixed prosthodontic procedures. Properties measured included rate of pH change and temperature increase, film thickness, and setting time. The rapid mix technique in conjunction with the frozen glass slab resulted in a faster elevation of the pH but slightly greater temperature rise and film thickness. As was expected, the rapid mixing procedure resulted in an increased working time but decreased setting. On the basis of this study, it appears that the rapid mixing technique in conjunction with a frozen glass slab is an acceptable technique for preparing zinc phosphate cement for fixed prosthodontic procedures. (5 PROSTHET DENT 1992$7:52-S.)
inc phosphate cement has been used as a luting agent by the dental profession for over a century. Although used for a number of purposes, this long-time, highly dependable material has been targeted primarily to cement crowns, fixed partial dentures, and orthodontic bands.r Although its composition has changed little over the past three decades, recent studies have suggested that there may be some advantage to modifying the method of mixing.2 Specifically, in conjunction with orthodontic appliances, it has been shown that rapid mixing on a subzero centigrade glass slab will modify certain physical characteristics. When mixed at approximately -10’ C, for example, the working time can be extended by as much as 300 % . At the same time, this technique will reduce the setting time by as much as 50 % .3 The long-term clinical behavior of zinc phosphate cement depends on the exactness with which it is manipulated. Minor alterations in powderiliquid ratios, the method of incorporating powder, and the temperature of the mixing slab may all contribute to a less-than-acceptable end product4 Because the frozen slab technique tends to eliminate some of these parameters as variable, orthodontists have generally accepted this manipulative method as a standard practice. The purpose of this study was to determine whether the rapid mixing frozen slab technique would be appropriate for use in fixed prosthodontic procedures.
aAssistant Professor,
Department
of Restorative
Dentistry,
King
Saud University, School of Dentistry. bAssociate Professor, Department of Restorative Dentistry, University of Alabama At Birmingham, School of Dentistry. “Professor, Department of Dental Materials, University of Alabama at Birmingham, School of Dentistry.
Table I. Methods, temperature, and cements used Mixing
technique
Glass slab temperature
go-Second mix
18” + 1°C
go-Second mix 30-Second mix
-20” t 2°C -2cP -t 2°C
Cement Flecks* Tenacinj Fleck’s Flecks Tenacin
*M&y Inc., Clifton Forge, Va. tL. D. Caulk Co., Milford, Del.
Particular emphasis was placed on pH, film thickness, and temperature rise as well as working and setting times.5-7
METHODS
AND MATERIAL
Two different commercially available zinc phosphate cements were used in this study. The various materials, manufacturers, temperatures, and times are presented in Table I. Two different mixing procedures were used. The first consisted of incorporating the powder into the liquid over a go-second period. The exact procedure was done in accordance with the directions specified in ADA specification No. 8. Both cements were mixed on the surface of a glass slab cooled to a temperature of 18” + 1’ 6. This condition served as a control. The second procedure consisted of mixing Flecks cement in the manner described except on a glass slab stored at -20’ k lo C. The third method involved both proprietary luting agents and consisted of incorporating all of the powder into the liquid at one time on a glass slab stored at -20’ rt 1“ C. The entire spatulation process was carried
ZIMC
PHOSPHATE
CEMENT
AND
q
FROZEN
SLAB
Conventional
(ADA)
Slow mix-Frozen % Rapid mix-Frozen
6 Conventional
slab
(ADA)
Slow mix-Frozen
slab
r) Rapid mix-Frozen
35 -z 0’ L
30
L”
25
r
slab slab
T
$ 20 Q, y’ 15 2 E IO E E Fleck’s
Tenacin
Fig. 1. Comparison of proportion of powder and liquid necessary to achieve mix of standard consistency (specimen diameter 30 * 1 mm).
On preparation of the test specimens in accordance to ADA specification No. 8, each cement was evaluated for film thickness, changing pH, temperature rise, and setting times.
Sample preparation The glass slab used for conventional spatulation was stored in a water bath at 18” + 1’ C for 30 minutes. Before the powder and liquid were dispensed, the slab surface was carefully dried with cotton fabric. The glass slabs to be used for the frozen mixing technique were sealed in polyethylene containers and stored in a freezer compartment for at least 12 hours at -20’ + lo C. In each case the powder was weighed on a torsion scale balanced to the nearest milligram. The liquid was dispensed from a 1 mm tuberculin syringe calibrated to the nearest 0.1 ml. A new syringe was used for each mix. At no time was an effort made to remove moisture that may have condensed onto the surface of the frozen slab. Regardless of the mixing technique, all preweighed powder was incorporated each time into the metered liquid. All powders and liquids were stored at a room temperature of 23’ + 2’ C. All mixing procedures were done at this temperature and at a relative humidity of 50% + 10%.
Testing
consistency
Specimens for the consistency test were generated by dispension of 0.5 + 0.01 ml of the mixed cement onto a glass plate. At this point, another thin glass plate was positioned over the freshly mixed cement. Next, a load of 120 gm was placed on top of the cover plate. The plate and the 120 gm load was applied 3 minutes from the beginning of spatulation. Ten minutes from the start of mixing, the major and minor diameters of the resulting cement disk were measured. The proportion of powder in the mix was adjusted until the average diameter measurement was 30 i 1 mm. The weight of powder needed for 0.5 ml of liquid was recorded as the testing consistency.
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5 0 Fleck’s
Tenacin
Fig. 2. Comparison of mixing technique on film thickness of both brands.
Film thickness Film thickness determinations were generated by the application of a 15 kg load onto the cement positioned between two thin glass plates. The differential in thickness of the two plates with and without the cement was used to identify film thickness. Two and one half minutes after mixing, the cement was immediately loaded into a plastic syringe. This procedure was used for all specimens regardless of the method used for incorporating the powder into the liquid. At this point, 0.5 ml of cement was dispensed onto a glass plate and covered by the second glass plate. Three minutes from the initiation of the mixing procedure, the load was placed over the entire glass plate. Ten minutes after start of the mixing procedure the combined thickness of the plates and cement was recorded.
pH Determinations A pH electrode was used to measure the surface acidity of the cement. The electrode consisted of a float sensor and coplanar reference junction. The reference junction was Ag/AgCl in a saturated KC1 gel. The electrode was attached to a pH meter, which in turn was connected through a 100to-l voltage reducer to a recorder. The pH specimens were shaped into small disks by placing them between two microscope cover glasses.Three minutes after initiation ofthe mixing procedure the specimens were placed in a petri dish lined with filter paper and stored in a controlled humidity of 100 % at 37’ C. The pH was measured at 3 and 36 minutes, 1 hour, 1 day, 2 days, and 1 week from the initiation of mixing. Three mixes were prepared for each test.
Temperature
rise
Temperature changes were determined by a platinumiridium thermocouple and a two-channel recorder. In each instance the thermocouple was embedded in the cement 2% minute after initiation of the mixing procedure. Recording of the temperatures was initiated 3 minutes after the start of spatulation. The thermocouple was connected 53
I Conventional(ADA)
Slowmix-Frozenslab
98
7: Rapid mix-Frozen slab
c
96 e a g4 zi 92 5 ; 90
9 Fleck’s
E 88 F
q
Tenacin
86 a2 / 0
I 50
I 100
150
/ 200
250
300
Time (seconds) Tenacin
Fleck’s
Fig. 3. Comparison of mixing technique on setting time.
Fig. 5. Time-temperature curve recorded from setting of Flecks and Tenacin cements in rapid mix method. Statistical
86 f 84 ' 0
I 50
100 Time
150
200
I 250
300
(seconds)
Fig. 4. Time-temperature curve recorded from setting of Flecks and Tenacin method.
cements in conventional
(ADA)
directly to a two-channel recorder. Three temperature recordings were carried out for each mixing procedure and cement evaluated.
Setting
time
Two and one half minutes from the start of mixing, the cement was loaded into a plastic ring positioned over a flat glass plate. Thirty seconds later the flat glass plate and the ring were placed in a 37” C chamber for measurements. A Gillmore needle weighing 453.6 gm with a flat-ended needle 1.06 mm in diameter was carefully lowered vertically onto the horizontal surface of the cement. This procedure was repeated at X-second intervals. The time of set was recorded as the number of minutes elapsed from the start of mixing to the time when the needle failed to make a perceptible indentation on the surface of the cement. This procedure was done three times for each cement and for each method.
analysis
Mean temperature, pH, and film thickness for both brands were considered independent and normally distributed. The two-tail t- test was used to compare film thickness because of the small sample size. The null hypothesis was rejected if the calculated p-value was greater than a 0.05 level of significance (type 1 error). For temperature rise and pH, a one-way analysis of variance (ANOVA) test with repeated measurement was used. The null hypothesis was rejected if the calculated p-value was greater than 0.05. In each of methods 1 and 3, both brands (Flecks and Tenacin) were compared with respect to their response to temperature and pH.
RESULTS Powder/liquid consistencies
ratios
for mixing
The amount of powder necessary to produce a consistency recommended by ADA specifications No. 8 was the same for both cements. Specifically, Flecks and Tenacin cements required 1.3 gm of powder for 0.5 ml of liquid to obtain a 30 t 1 mm disc. A considerable increase in powder was necessary to maintain the same consistency when mixed on a frozen glass slab (Fig. I). The amount of powder increase, however, was nearly the same, regardless of the method by which the powder was incorporated into the liquid on the frozen slab. With Flecks cement, 2.15 gm of powder was necessary to maintain the sameviscosity as when mixed on a frozen glass slab with the ADA technique for powder incorporation. This amount represents a 65 % increase in the powder over the conventional slab technique. The increase in powder for mixing on a frozen slab was independent of whether the powder was slowly incorporated or incorporated instantaneously. In the case of the Tenacin cement, the increase in powder was 61.5 % .
ZINC
PNOSPHATE
CEMENT
AND
FROZEN
SLAB
100 98 j-ie 96
c
=I 92 t;; ; 90
94 92 A Conventional (ADA) . Slow mix - Frozen slab H Rapid mix - Frozen slab
90 88 a4 1 0
50
I 100 150 200 Time (seconds)
250
300
6. Time-temperature curve recorded from setting of Flecks cement in conventional, slow-mix, and rapid-mix methods.
Fig.
When the ADA method was used for mixing the cement, no moisture appeared on the surface of the glass slab. However, a considerable amount of residue occurred when the frozen slab technique was used. No attempt was made to remove the contaminant prior to mixing procedures.
Film thickness The results of the tests measuring film thickness are shown in Fig. 2. The mean film thickness for both cements when mixed according to ADA specification NO. 8 was 26 pm. The standard deviations for Flecks and Tenacin cements were 3.6 and 2.9, respectively. This value approximates the maximum 25 km limit for the film thickness of zinc phosphate cement as described in Specification No. 8. When the same technique was used for incorporation of the powder on a frozen glass slab, a slight increase was observed. The mean film thickness for Flecks cement was 31.6 pm. When the powder was rapidly mixed into the liquid over a 30-second period, the mean film thickness for both Flecks and Tenacin cement was 30.3 pm. Standard deviations were 4 and 2.5, respectively. No significant difference could be found with respect to the technique used for mixing the two cements (p < 0.05).
Setting
time
The setting time measurements are presented in Fig. 3. The values obtained for mixing the two brands in accordance with ADA specification No. 8 are represented by the first bar graph in the series. Under this condition, the setting time for Flecks cement was 9 minutes and 30 seconds whereas that for Tenacin cement was 6 minutes and 35 seconds. These values represent the mean number for three trials. Mixing on the frozen glass slab, regardless of the rate at which the powder was incorporated, resulted in an appreciable decrease of setting time.
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96 i 94
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2
88
F
86
0 Conventional q
0
50
(ADA)
Rapid mix - Frozen slab
100 150 200 Time (seconds)
250
300
Fig. 7. Time-temperature curve recorded from setting of Tenacin cement in conventional and rapid-mixing method.
Temperature
rise
The temperature measurements are given in Figs. 4 through 7. The change in temperature during setting for the two cements mixed according to ADA specifications No. 8 are presented in Fig. 4. As can be seen, the temperature rose rapidly during the first 2 minutes and then leveled to a relatively flat plateau. Although the mean temperatures for both cements were somewhat different, the differences were not significant at any given time. The effect of mixing both cements on the frozen glass slab by rapid incorporation of the powder into the liquid is shown in Fig. 5. Both cements reached maximum temperatures at approximately the same time. At no time did the temperature for either cement rise above 100’ F (38” C). Almost immediately the temperature began to decrease until body temperature was obtained. The temperatures of the two cements were different at baseline (zero seconds). Jn the case of Tenacin cement, the temperature was 84OF (29’ C) and that for Flecks was 93” F (34O C). It should be noted that the mixed cement was already 3 minutes old at baseline. This time frame is the same as that used in mixing the cement according to ADA Specification No. 8. Because the reaction rate for Flecks cement is faster,8 the higher temperature at the end of 3 minutes is realistic. With the exception of the first 60 seconds, the differences in temperature measurements between the two cements were not statistically significant (p < 0.5). No appreciable difference in temperature was observed when the cement was mixed on a frozen glass slab by use of eit.her the traditional technique for powder incorporation or rapid incorporation. At the end of the 30 seconds, the differences in temperature observed between the two conditions was never more than 1’ F. The maximum temperature of the setting cements never increased to more than 2’ F above body temperature.
55
FAKIHA,
MUEKINGHQFF,
AND
LEINFELDER
frommixingthecement rapidlyonafrozenglassslab.In general, thedifferences in valuesweresignificantat almost all pointsof timeafter100minutes.ThepH effectsof all
fe
three methods used for mixing Flecks cement are given in Fig. 10. The pH values for Tenacin cement over 7 days for mixing methods 1 and 3 are given in Fig. 11.
5.5
DISCUSSION Cement consistency 4.
143
(a2 Time
(1OY (minutes)
(10)’
(10) 5
Fig. 8. Mean values for surface pH plotted as function of log of time (minutes) for Flecks and Tenacin cements in ADA method.
6.0
5 5.5 5.0
4.5
Fig. 9. Mean values for surface pH plotted as function of log of time (minutes) for Flecks and Tenacin cements in rapid-mix technique on frozen slab. Standard deviation is indicated.
of the setting
cement
Flecks and Tenacin cements alike exhibited a pH of near equal value at 12 minutes from the beginning of mixing by the conventional ADA method of spatulation. In both cases, the cements did not approximate neutrality (6.5) until after 7 days. No significant differences could be shown between the cements, regardless of when the measurements were obtained. A comparison between Figs. 8 and 9 revealed that mixing rapidly on the frozen slab accelerated the rate at which the pH reached neutrality. Finally, at the end of 1 week, the pH values for the frozen slab techniques for Flecks cement were 6.76 (rapid mix) and 6.70 (slow mix). The pH for the conventional mix was 6.33. Although no significant differences could be detected between the frozen slab tech-
(powder/liquid
ratio)
To maintain a constant viscosity of the cement mix, it was necessary to increase the powder/liquid ratios for both brands of cement when spatulating on a low-temperature glass slab. The extra powder was necessitated by the dilution caused by moisture condensation when the temperature of the mixing surface was reduced below the dew point. The retardation of the setting reaction also allows additional powder incorporation. This finding is in agreement with several other investigators who evaluated the use of the reduced temperature glass slab.8,9 Generally, the amount of powder increase required when the frozen glass slab was used was approximately 65 % over that when the cement was mixed conventionally. However, other authors have reported even higher percentages of powder increase, often approaching 90% to 95% .2,3 This higher powder/ liquid ratio resulted from the use of a more viscous mix, which is necessary for luting orthodontic bands. Such a viscosity, however, would be too high for crown cementation. The powder/liquid ratio obtained in this study was identical to that obtained by Palermo et a1.8for Flecks cement. They showed that 2.15 gm of powder was required for mixing 0.5 ml to generate a viscosity resulting in a disk of 30 f 1 mm. In the present study, the amount of powder necessary to generate a certain viscosity was exactly the same regardless of the rate at which the powder was incorporated into the liquid. This shows the similarity between the rapid and the slow mixing techniques using the frozen glass slab. Myers et a1.galso reported the same value of powder addition for Tenacin cement mixed on a frozen glass slab.
Film thickness The results of this study show somewhat higher values for the film thickness of both cements than that allowed in the ADA specification when mixed conventionally. Windele? and Palermo et al.8 also reported higher values of film thickness when cements were mixed conventionally. These values, however, were slightly higher than those generated in this study. The reduced-temperature mix generally results in an increase of 3 to 5 pm in film thickness. The increased film thickness caused by mixing the cements on a frozen glass slab was found to be 5 pm for
ZINC
PHOSPHATE
CEMENT
Temperature
AND
FROZEN
SLAB
rise
7.0
Temperature rise for the conventional mix was slightly lower in this study than that reported by NewmanlO and in major disagreement with that reported by Palermo et al.,s who reported a 4’ C increase above body temperature for Tenacin cement and 10’ C for Flecks cement mixed on a frozen glass slab. In this study both brands of cements exhibited the same rise in temperature. A frozen glass slab mix attained higher peak temperature than that produced by conventional mixing but at insignificant limits statistically and probably clinically. The major difference between methods of mixing was that, in the instance of a frozen glass slab, the peak temperature was attained sooner than that of conventional mixes. No difference was found between the slow mix or the rapid mix with the frozen glass slab technique. Between brands, it was noted that the initial temperature of Flecks cement was always higher than that of Tenacin cement by a mean difference of 4’ C. This indicates that the working time of Flecks cement after removal from the frozen glass slab was shorter than that of Tenacin cement. These results are in agreement with the finding of other investigators who reported widely varying working time among brands of zinc phosphate cement.g NewmanlO reported that when a specimen of freshly mixed cement was left on the frozen glass slab for 10 minutes before testing, temperature rise from the exothermic reaction was in an acceptable range of 2OC above body temperature. In this study, similar results were noted, but after only 3 minutes from the start of mixing.
/
6.5 6.0 I
5.5 CL
5.0 4.5 4.0
Time (minutes)
Fig. 10. Mean values for surface pH plotted as function of log of time (minutes) for Flecks cement in all three methods. Standard deviation is indicated.
6.5 6.0 I P
0 Conventional
5.0 4.5
(IO)’
Setting
time
Kendzior et a1.2reported a significant increase in working time with the use of frozen slab mixes. Furthermore, they noted that the mix remained in a manageable state nearly seven times as long as room-temperature mixes. The powder/liquid ratio was based on a consistency test of 26 + I mm. The frozen slab technique also provided an excellent means for extending the working time of zinc phosphate cement when the cement was allowed to remain on the frozen slab. This result occurred because the decreased temperature of the slab retards the reaction of the cement, causing a slow reaction. Any increase in the temperature of the mixed cement accelerates the reaction. In the test design, there was a 30-second interval between the removal of the cement from the slab and its placement in the plastic ring and then in the 37” C chamber for testing. Removal of the cement from the slab, transfer to a room-temperature casting and then to 37’ C will accelerate the reaction and result in a shortened setting time. The decrease in setting time is one of the advantages of this technique because it decreases the possibility for contamination of the cement by oral fluids during setting. Namiki et al.li found that the setting time for three brands of cement included in their study ranged from 9 to
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(ADA)
R Rapid mix - Frozen slab
w Time (minutes)
mf’
(10)”
Fig. 11. Mean values for surface pH plotted as function of log of time (minutes) for Tenacin cement in conventional (ADA) method and rapid mix-frozen slab technique. Standard deviation is indicated.
12 minutes when the cements were mixed conventionally. When mixed under the same condition in this study, Flecks cement required 7 minutes to set after removal from the slab. When mixed on the frozen slab, both slow and rapid mix required a total of 2 minutes and 20 secon.ds.This type of rapid setting could result in an incomplete seating of the prosthesis. Of interest, Tenacin cement did not exhibit the same difference when mixed conventionally on the frozen slab. Specifically, this cement required slightly more than 4 minutes after removal from the slab when mixed conventionally. When mixed on a frozen glass slab by use of the rapid mixing technique, 2 minutes and 50 seconds were required.
pH of setting
cement
The pH of the frozen slab mixes generally was approximately one half pH unit higher than in those mixed on a
57
FAKIHA,MUENINGHOFF,AND LEINFELDER
room-temperature slab. In this study, the elevated pH observed in cements mixed on a frozen glass slab can be related to an increased powder/liquid ratio. It has been reported that the pH of zinc phosphate cement is relatively higher in vivo than in vitro.12 Furthermore, the rate of change of the pH appeared to be directly related to the pH of the saliva of the individual. The higher the pH of the saliva the more rapid was the rise in the pH of the cement. With the positive difference in the increase of pH when the frozen glass slab is used, the hazard of the acidity of zinc phosphate cement in vivo should decrease.
Within the limits of this study, the following conclusions can be made. 1. The frozen glass slab mixing technique increased the temperature of the reacting cement more than the conventional mixing technique. This difference in temperature rise however, was not significant. 2. The frozen glass slab generated a faster increase in pH as compared with mixing on a 23’ C surface (ADA technique) . 3. Rapid or conventional mixing of zinc phosphate cement on a frozen glass slab resulted in a slight increase in film thickness. 4. Rapid mixing of zinc phosphate cement on a frozen glass slab permitted the incorporation of more powder while maintaining a constant consistency. 5. Rapid mixing of zinc phosphate cement on a frozen glass slab increased the working time and decreased the setting time. 6. Rapid mixing on a frozen slab surface appears to be a satisfactory method for fixed prosthodontic procedures.
Bound
volumes
available
Strength and solubility dressed in this study.
are important factors not ad-
REFERENCES 1. Craig RG, Peyton FA. Restorative dental materials. 7th ed. St Louis; CV Mosby Co, 1985;166-77. 2. Kendzior GM, Leinfelder KF, Hershey HG. The effect of cold temperature mixing on the properties of zinc phosphate cement. Angle Orthod 1976;46:345-50. 3. Shepherd WB, Leinfelder KF, Hershey HG. The effect of mixing method, slab temperature, and humidity on the properties of zinc phosphate and zinc silicophosphate cement. Angle Ortbod 1978;48:21926. 4. Norman RD, Swarm ML, Phillips RW, Sears CR. Properties of cements mixed from liquids with altered water content. J PROSTHET DENT
1970;24:410-18. 5. Eames WB, Munroe SD, Roan JD, O’Neal 53. Proportioning and mixing of cements: a comparison of working times. Oper Dent 1977;2:97-104.
6. Windeler AS. Powder enrichment effect on film thickness of zinc phosD~~~1979;42:299-303. phate cement. 3 PROSTHET 7. Plant CB, Jones DW, DarveII BW. The heat evolved and temperatures attained during the setting of restorative materials. Br Dent J 1974; 137:233-S. 8. Palermo JJ, Leinfelder KF, Holland GA. Evaluation of the frozen slab DENT 1982; technique for cementing cast restorations. J PROST~ET
48~555-61. 9. Myers CL, Drake JT, Brantley WA. A comparison of properties of zinc phosphate cements mixed on room temperature and frozen slabs. J
PROSTHETDENT~~~~;~OZ~O~-~~. 10. Newman SM. Frozen slab technique for mixing zinc phosphate cement for cast restorations. J PROSTHET DENT1980;43:46-9. 11. Namiki Y, Ikemi T, Ueda I. Setting time of dental cements for lutingthe relation between the exothermic peak and the mixing time. J Oral Sci 1985;11:237-48. 12. Virmani R, Norman RD, Swarm ML, Phillips RW. The pH of setting cements. III, In vivo. J PROSTHET DENT1970;23:66-72. Reprint
requests to:
DR.LEONARD A.MUENINGHOFF DEPARTMENTOFRESTORATIVEDENTISTRY UNIVERSITYOFALABAMA,SCHOOLOFDENT~STRY BIRMINGHAM, AL 35294
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Bound volumes of THE JOURNAL OF PROSTHETIC DENTISTRY are available to subscribers (only) for the 1991 issues from the publisher at a cost of $50.00 ($63.00 international) for Vol. 65 (January-June) and Vol. 66 (July-December). Shipping charges are included. Each bound volume contains a subject and author index, and all advertising is removed. Copies are shipped within 30 days after publication of the last issue in the volume. The binding is durable buckram with the journal name, volume number, and year stamped in gold on the spine. Volumes 63 and 64 are also available. Payment must accompany all orders. Contact Mosby-Year Book, Inc., Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63x46-3318, USA, phone (800) 3254177, ext. 4351. Subscriptions must be in force to qualify. in place of a regular JOURNAL subscription.
58
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JANUARY1992 VOLUME67 NUMBER1
A. Fakiha, BDS, MS,a Leonard F. Leinfelder, DDS, MSC
A. Mueninghoff,
DDS,b
and
Ring Saud University, School of Dentistry, Riyadh, Saudi Arabia, and University of Alabama, School of Dentistry, Birmingham, Ala. This study was done ta determine the efficacy of rapidly mixing zinc phosphate cement in conjunction with fixed prosthodontic procedures. Properties measured included rate of pH change and temperature increase, film thickness, and setting time. The rapid mix technique in conjunction with the frozen glass slab resulted in a faster elevation of the pH but slightly greater temperature rise and film thickness. As was expected, the rapid mixing procedure resulted in an increased working time but decreased setting. On the basis of this study, it appears that the rapid mixing technique in conjunction with a frozen glass slab is an acceptable technique for preparing zinc phosphate cement for fixed prosthodontic procedures. (5 PROSTHET DENT 1992$7:52-S.)
inc phosphate cement has been used as a luting agent by the dental profession for over a century. Although used for a number of purposes, this long-time, highly dependable material has been targeted primarily to cement crowns, fixed partial dentures, and orthodontic bands.r Although its composition has changed little over the past three decades, recent studies have suggested that there may be some advantage to modifying the method of mixing.2 Specifically, in conjunction with orthodontic appliances, it has been shown that rapid mixing on a subzero centigrade glass slab will modify certain physical characteristics. When mixed at approximately -10’ C, for example, the working time can be extended by as much as 300 % . At the same time, this technique will reduce the setting time by as much as 50 % .3 The long-term clinical behavior of zinc phosphate cement depends on the exactness with which it is manipulated. Minor alterations in powderiliquid ratios, the method of incorporating powder, and the temperature of the mixing slab may all contribute to a less-than-acceptable end product4 Because the frozen slab technique tends to eliminate some of these parameters as variable, orthodontists have generally accepted this manipulative method as a standard practice. The purpose of this study was to determine whether the rapid mixing frozen slab technique would be appropriate for use in fixed prosthodontic procedures.
aAssistant Professor,
Department
of Restorative
Dentistry,
King
Saud University, School of Dentistry. bAssociate Professor, Department of Restorative Dentistry, University of Alabama At Birmingham, School of Dentistry. “Professor, Department of Dental Materials, University of Alabama at Birmingham, School of Dentistry.
Table I. Methods, temperature, and cements used Mixing
technique
Glass slab temperature
go-Second mix
18” + 1°C
go-Second mix 30-Second mix
-20” t 2°C -2cP -t 2°C
Cement Flecks* Tenacinj Fleck’s Flecks Tenacin
*M&y Inc., Clifton Forge, Va. tL. D. Caulk Co., Milford, Del.
Particular emphasis was placed on pH, film thickness, and temperature rise as well as working and setting times.5-7
METHODS
AND MATERIAL
Two different commercially available zinc phosphate cements were used in this study. The various materials, manufacturers, temperatures, and times are presented in Table I. Two different mixing procedures were used. The first consisted of incorporating the powder into the liquid over a go-second period. The exact procedure was done in accordance with the directions specified in ADA specification No. 8. Both cements were mixed on the surface of a glass slab cooled to a temperature of 18” + 1’ 6. This condition served as a control. The second procedure consisted of mixing Flecks cement in the manner described except on a glass slab stored at -20’ k lo C. The third method involved both proprietary luting agents and consisted of incorporating all of the powder into the liquid at one time on a glass slab stored at -20’ rt 1“ C. The entire spatulation process was carried
ZIMC
PHOSPHATE
CEMENT
AND
q
FROZEN
SLAB
Conventional
(ADA)
Slow mix-Frozen % Rapid mix-Frozen
6 Conventional
slab
(ADA)
Slow mix-Frozen
slab
r) Rapid mix-Frozen
35 -z 0’ L
30
L”
25
r
slab slab
T
$ 20 Q, y’ 15 2 E IO E E Fleck’s
Tenacin
Fig. 1. Comparison of proportion of powder and liquid necessary to achieve mix of standard consistency (specimen diameter 30 * 1 mm).
On preparation of the test specimens in accordance to ADA specification No. 8, each cement was evaluated for film thickness, changing pH, temperature rise, and setting times.
Sample preparation The glass slab used for conventional spatulation was stored in a water bath at 18” + 1’ C for 30 minutes. Before the powder and liquid were dispensed, the slab surface was carefully dried with cotton fabric. The glass slabs to be used for the frozen mixing technique were sealed in polyethylene containers and stored in a freezer compartment for at least 12 hours at -20’ + lo C. In each case the powder was weighed on a torsion scale balanced to the nearest milligram. The liquid was dispensed from a 1 mm tuberculin syringe calibrated to the nearest 0.1 ml. A new syringe was used for each mix. At no time was an effort made to remove moisture that may have condensed onto the surface of the frozen slab. Regardless of the mixing technique, all preweighed powder was incorporated each time into the metered liquid. All powders and liquids were stored at a room temperature of 23’ + 2’ C. All mixing procedures were done at this temperature and at a relative humidity of 50% + 10%.
Testing
consistency
Specimens for the consistency test were generated by dispension of 0.5 + 0.01 ml of the mixed cement onto a glass plate. At this point, another thin glass plate was positioned over the freshly mixed cement. Next, a load of 120 gm was placed on top of the cover plate. The plate and the 120 gm load was applied 3 minutes from the beginning of spatulation. Ten minutes from the start of mixing, the major and minor diameters of the resulting cement disk were measured. The proportion of powder in the mix was adjusted until the average diameter measurement was 30 i 1 mm. The weight of powder needed for 0.5 ml of liquid was recorded as the testing consistency.
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JOURNAL
OF PROSTHETIC
DENTISTRY
5 0 Fleck’s
Tenacin
Fig. 2. Comparison of mixing technique on film thickness of both brands.
Film thickness Film thickness determinations were generated by the application of a 15 kg load onto the cement positioned between two thin glass plates. The differential in thickness of the two plates with and without the cement was used to identify film thickness. Two and one half minutes after mixing, the cement was immediately loaded into a plastic syringe. This procedure was used for all specimens regardless of the method used for incorporating the powder into the liquid. At this point, 0.5 ml of cement was dispensed onto a glass plate and covered by the second glass plate. Three minutes from the initiation of the mixing procedure, the load was placed over the entire glass plate. Ten minutes after start of the mixing procedure the combined thickness of the plates and cement was recorded.
pH Determinations A pH electrode was used to measure the surface acidity of the cement. The electrode consisted of a float sensor and coplanar reference junction. The reference junction was Ag/AgCl in a saturated KC1 gel. The electrode was attached to a pH meter, which in turn was connected through a 100to-l voltage reducer to a recorder. The pH specimens were shaped into small disks by placing them between two microscope cover glasses.Three minutes after initiation ofthe mixing procedure the specimens were placed in a petri dish lined with filter paper and stored in a controlled humidity of 100 % at 37’ C. The pH was measured at 3 and 36 minutes, 1 hour, 1 day, 2 days, and 1 week from the initiation of mixing. Three mixes were prepared for each test.
Temperature
rise
Temperature changes were determined by a platinumiridium thermocouple and a two-channel recorder. In each instance the thermocouple was embedded in the cement 2% minute after initiation of the mixing procedure. Recording of the temperatures was initiated 3 minutes after the start of spatulation. The thermocouple was connected 53
I Conventional(ADA)
Slowmix-Frozenslab
98
7: Rapid mix-Frozen slab
c
96 e a g4 zi 92 5 ; 90
9 Fleck’s
E 88 F
q
Tenacin
86 a2 / 0
I 50
I 100
150
/ 200
250
300
Time (seconds) Tenacin
Fleck’s
Fig. 3. Comparison of mixing technique on setting time.
Fig. 5. Time-temperature curve recorded from setting of Flecks and Tenacin cements in rapid mix method. Statistical
86 f 84 ' 0
I 50
100 Time
150
200
I 250
300
(seconds)
Fig. 4. Time-temperature curve recorded from setting of Flecks and Tenacin method.
cements in conventional
(ADA)
directly to a two-channel recorder. Three temperature recordings were carried out for each mixing procedure and cement evaluated.
Setting
time
Two and one half minutes from the start of mixing, the cement was loaded into a plastic ring positioned over a flat glass plate. Thirty seconds later the flat glass plate and the ring were placed in a 37” C chamber for measurements. A Gillmore needle weighing 453.6 gm with a flat-ended needle 1.06 mm in diameter was carefully lowered vertically onto the horizontal surface of the cement. This procedure was repeated at X-second intervals. The time of set was recorded as the number of minutes elapsed from the start of mixing to the time when the needle failed to make a perceptible indentation on the surface of the cement. This procedure was done three times for each cement and for each method.
analysis
Mean temperature, pH, and film thickness for both brands were considered independent and normally distributed. The two-tail t- test was used to compare film thickness because of the small sample size. The null hypothesis was rejected if the calculated p-value was greater than a 0.05 level of significance (type 1 error). For temperature rise and pH, a one-way analysis of variance (ANOVA) test with repeated measurement was used. The null hypothesis was rejected if the calculated p-value was greater than 0.05. In each of methods 1 and 3, both brands (Flecks and Tenacin) were compared with respect to their response to temperature and pH.
RESULTS Powder/liquid consistencies
ratios
for mixing
The amount of powder necessary to produce a consistency recommended by ADA specifications No. 8 was the same for both cements. Specifically, Flecks and Tenacin cements required 1.3 gm of powder for 0.5 ml of liquid to obtain a 30 t 1 mm disc. A considerable increase in powder was necessary to maintain the same consistency when mixed on a frozen glass slab (Fig. I). The amount of powder increase, however, was nearly the same, regardless of the method by which the powder was incorporated into the liquid on the frozen slab. With Flecks cement, 2.15 gm of powder was necessary to maintain the sameviscosity as when mixed on a frozen glass slab with the ADA technique for powder incorporation. This amount represents a 65 % increase in the powder over the conventional slab technique. The increase in powder for mixing on a frozen slab was independent of whether the powder was slowly incorporated or incorporated instantaneously. In the case of the Tenacin cement, the increase in powder was 61.5 % .
ZINC
PNOSPHATE
CEMENT
AND
FROZEN
SLAB
100 98 j-ie 96
c
=I 92 t;; ; 90
94 92 A Conventional (ADA) . Slow mix - Frozen slab H Rapid mix - Frozen slab
90 88 a4 1 0
50
I 100 150 200 Time (seconds)
250
300
6. Time-temperature curve recorded from setting of Flecks cement in conventional, slow-mix, and rapid-mix methods.
Fig.
When the ADA method was used for mixing the cement, no moisture appeared on the surface of the glass slab. However, a considerable amount of residue occurred when the frozen slab technique was used. No attempt was made to remove the contaminant prior to mixing procedures.
Film thickness The results of the tests measuring film thickness are shown in Fig. 2. The mean film thickness for both cements when mixed according to ADA specification NO. 8 was 26 pm. The standard deviations for Flecks and Tenacin cements were 3.6 and 2.9, respectively. This value approximates the maximum 25 km limit for the film thickness of zinc phosphate cement as described in Specification No. 8. When the same technique was used for incorporation of the powder on a frozen glass slab, a slight increase was observed. The mean film thickness for Flecks cement was 31.6 pm. When the powder was rapidly mixed into the liquid over a 30-second period, the mean film thickness for both Flecks and Tenacin cement was 30.3 pm. Standard deviations were 4 and 2.5, respectively. No significant difference could be found with respect to the technique used for mixing the two cements (p < 0.05).
Setting
time
The setting time measurements are presented in Fig. 3. The values obtained for mixing the two brands in accordance with ADA specification No. 8 are represented by the first bar graph in the series. Under this condition, the setting time for Flecks cement was 9 minutes and 30 seconds whereas that for Tenacin cement was 6 minutes and 35 seconds. These values represent the mean number for three trials. Mixing on the frozen glass slab, regardless of the rate at which the powder was incorporated, resulted in an appreciable decrease of setting time.
THE
JOURNAL
96 i 94
OF PROSTHETIC
DENTISTRY
2
88
F
86
0 Conventional q
0
50
(ADA)
Rapid mix - Frozen slab
100 150 200 Time (seconds)
250
300
Fig. 7. Time-temperature curve recorded from setting of Tenacin cement in conventional and rapid-mixing method.
Temperature
rise
The temperature measurements are given in Figs. 4 through 7. The change in temperature during setting for the two cements mixed according to ADA specifications No. 8 are presented in Fig. 4. As can be seen, the temperature rose rapidly during the first 2 minutes and then leveled to a relatively flat plateau. Although the mean temperatures for both cements were somewhat different, the differences were not significant at any given time. The effect of mixing both cements on the frozen glass slab by rapid incorporation of the powder into the liquid is shown in Fig. 5. Both cements reached maximum temperatures at approximately the same time. At no time did the temperature for either cement rise above 100’ F (38” C). Almost immediately the temperature began to decrease until body temperature was obtained. The temperatures of the two cements were different at baseline (zero seconds). Jn the case of Tenacin cement, the temperature was 84OF (29’ C) and that for Flecks was 93” F (34O C). It should be noted that the mixed cement was already 3 minutes old at baseline. This time frame is the same as that used in mixing the cement according to ADA Specification No. 8. Because the reaction rate for Flecks cement is faster,8 the higher temperature at the end of 3 minutes is realistic. With the exception of the first 60 seconds, the differences in temperature measurements between the two cements were not statistically significant (p < 0.5). No appreciable difference in temperature was observed when the cement was mixed on a frozen glass slab by use of eit.her the traditional technique for powder incorporation or rapid incorporation. At the end of the 30 seconds, the differences in temperature observed between the two conditions was never more than 1’ F. The maximum temperature of the setting cements never increased to more than 2’ F above body temperature.
55
FAKIHA,
MUEKINGHQFF,
AND
LEINFELDER
frommixingthecement rapidlyonafrozenglassslab.In general, thedifferences in valuesweresignificantat almost all pointsof timeafter100minutes.ThepH effectsof all
fe
three methods used for mixing Flecks cement are given in Fig. 10. The pH values for Tenacin cement over 7 days for mixing methods 1 and 3 are given in Fig. 11.
5.5
DISCUSSION Cement consistency 4.
143
(a2 Time
(1OY (minutes)
(10)’
(10) 5
Fig. 8. Mean values for surface pH plotted as function of log of time (minutes) for Flecks and Tenacin cements in ADA method.
6.0
5 5.5 5.0
4.5
Fig. 9. Mean values for surface pH plotted as function of log of time (minutes) for Flecks and Tenacin cements in rapid-mix technique on frozen slab. Standard deviation is indicated.
of the setting
cement
Flecks and Tenacin cements alike exhibited a pH of near equal value at 12 minutes from the beginning of mixing by the conventional ADA method of spatulation. In both cases, the cements did not approximate neutrality (6.5) until after 7 days. No significant differences could be shown between the cements, regardless of when the measurements were obtained. A comparison between Figs. 8 and 9 revealed that mixing rapidly on the frozen slab accelerated the rate at which the pH reached neutrality. Finally, at the end of 1 week, the pH values for the frozen slab techniques for Flecks cement were 6.76 (rapid mix) and 6.70 (slow mix). The pH for the conventional mix was 6.33. Although no significant differences could be detected between the frozen slab tech-
(powder/liquid
ratio)
To maintain a constant viscosity of the cement mix, it was necessary to increase the powder/liquid ratios for both brands of cement when spatulating on a low-temperature glass slab. The extra powder was necessitated by the dilution caused by moisture condensation when the temperature of the mixing surface was reduced below the dew point. The retardation of the setting reaction also allows additional powder incorporation. This finding is in agreement with several other investigators who evaluated the use of the reduced temperature glass slab.8,9 Generally, the amount of powder increase required when the frozen glass slab was used was approximately 65 % over that when the cement was mixed conventionally. However, other authors have reported even higher percentages of powder increase, often approaching 90% to 95% .2,3 This higher powder/ liquid ratio resulted from the use of a more viscous mix, which is necessary for luting orthodontic bands. Such a viscosity, however, would be too high for crown cementation. The powder/liquid ratio obtained in this study was identical to that obtained by Palermo et a1.8for Flecks cement. They showed that 2.15 gm of powder was required for mixing 0.5 ml to generate a viscosity resulting in a disk of 30 f 1 mm. In the present study, the amount of powder necessary to generate a certain viscosity was exactly the same regardless of the rate at which the powder was incorporated into the liquid. This shows the similarity between the rapid and the slow mixing techniques using the frozen glass slab. Myers et a1.galso reported the same value of powder addition for Tenacin cement mixed on a frozen glass slab.
Film thickness The results of this study show somewhat higher values for the film thickness of both cements than that allowed in the ADA specification when mixed conventionally. Windele? and Palermo et al.8 also reported higher values of film thickness when cements were mixed conventionally. These values, however, were slightly higher than those generated in this study. The reduced-temperature mix generally results in an increase of 3 to 5 pm in film thickness. The increased film thickness caused by mixing the cements on a frozen glass slab was found to be 5 pm for
ZINC
PHOSPHATE
CEMENT
Temperature
AND
FROZEN
SLAB
rise
7.0
Temperature rise for the conventional mix was slightly lower in this study than that reported by NewmanlO and in major disagreement with that reported by Palermo et al.,s who reported a 4’ C increase above body temperature for Tenacin cement and 10’ C for Flecks cement mixed on a frozen glass slab. In this study both brands of cements exhibited the same rise in temperature. A frozen glass slab mix attained higher peak temperature than that produced by conventional mixing but at insignificant limits statistically and probably clinically. The major difference between methods of mixing was that, in the instance of a frozen glass slab, the peak temperature was attained sooner than that of conventional mixes. No difference was found between the slow mix or the rapid mix with the frozen glass slab technique. Between brands, it was noted that the initial temperature of Flecks cement was always higher than that of Tenacin cement by a mean difference of 4’ C. This indicates that the working time of Flecks cement after removal from the frozen glass slab was shorter than that of Tenacin cement. These results are in agreement with the finding of other investigators who reported widely varying working time among brands of zinc phosphate cement.g NewmanlO reported that when a specimen of freshly mixed cement was left on the frozen glass slab for 10 minutes before testing, temperature rise from the exothermic reaction was in an acceptable range of 2OC above body temperature. In this study, similar results were noted, but after only 3 minutes from the start of mixing.
/
6.5 6.0 I
5.5 CL
5.0 4.5 4.0
Time (minutes)
Fig. 10. Mean values for surface pH plotted as function of log of time (minutes) for Flecks cement in all three methods. Standard deviation is indicated.
6.5 6.0 I P
0 Conventional
5.0 4.5
(IO)’
Setting
time
Kendzior et a1.2reported a significant increase in working time with the use of frozen slab mixes. Furthermore, they noted that the mix remained in a manageable state nearly seven times as long as room-temperature mixes. The powder/liquid ratio was based on a consistency test of 26 + I mm. The frozen slab technique also provided an excellent means for extending the working time of zinc phosphate cement when the cement was allowed to remain on the frozen slab. This result occurred because the decreased temperature of the slab retards the reaction of the cement, causing a slow reaction. Any increase in the temperature of the mixed cement accelerates the reaction. In the test design, there was a 30-second interval between the removal of the cement from the slab and its placement in the plastic ring and then in the 37” C chamber for testing. Removal of the cement from the slab, transfer to a room-temperature casting and then to 37’ C will accelerate the reaction and result in a shortened setting time. The decrease in setting time is one of the advantages of this technique because it decreases the possibility for contamination of the cement by oral fluids during setting. Namiki et al.li found that the setting time for three brands of cement included in their study ranged from 9 to
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
(ADA)
R Rapid mix - Frozen slab
w Time (minutes)
mf’
(10)”
Fig. 11. Mean values for surface pH plotted as function of log of time (minutes) for Tenacin cement in conventional (ADA) method and rapid mix-frozen slab technique. Standard deviation is indicated.
12 minutes when the cements were mixed conventionally. When mixed under the same condition in this study, Flecks cement required 7 minutes to set after removal from the slab. When mixed on the frozen slab, both slow and rapid mix required a total of 2 minutes and 20 secon.ds.This type of rapid setting could result in an incomplete seating of the prosthesis. Of interest, Tenacin cement did not exhibit the same difference when mixed conventionally on the frozen slab. Specifically, this cement required slightly more than 4 minutes after removal from the slab when mixed conventionally. When mixed on a frozen glass slab by use of the rapid mixing technique, 2 minutes and 50 seconds were required.
pH of setting
cement
The pH of the frozen slab mixes generally was approximately one half pH unit higher than in those mixed on a
57
FAKIHA,MUENINGHOFF,AND LEINFELDER
room-temperature slab. In this study, the elevated pH observed in cements mixed on a frozen glass slab can be related to an increased powder/liquid ratio. It has been reported that the pH of zinc phosphate cement is relatively higher in vivo than in vitro.12 Furthermore, the rate of change of the pH appeared to be directly related to the pH of the saliva of the individual. The higher the pH of the saliva the more rapid was the rise in the pH of the cement. With the positive difference in the increase of pH when the frozen glass slab is used, the hazard of the acidity of zinc phosphate cement in vivo should decrease.
Within the limits of this study, the following conclusions can be made. 1. The frozen glass slab mixing technique increased the temperature of the reacting cement more than the conventional mixing technique. This difference in temperature rise however, was not significant. 2. The frozen glass slab generated a faster increase in pH as compared with mixing on a 23’ C surface (ADA technique) . 3. Rapid or conventional mixing of zinc phosphate cement on a frozen glass slab resulted in a slight increase in film thickness. 4. Rapid mixing of zinc phosphate cement on a frozen glass slab permitted the incorporation of more powder while maintaining a constant consistency. 5. Rapid mixing of zinc phosphate cement on a frozen glass slab increased the working time and decreased the setting time. 6. Rapid mixing on a frozen slab surface appears to be a satisfactory method for fixed prosthodontic procedures.
Bound
volumes
available
Strength and solubility dressed in this study.
are important factors not ad-
REFERENCES 1. Craig RG, Peyton FA. Restorative dental materials. 7th ed. St Louis; CV Mosby Co, 1985;166-77. 2. Kendzior GM, Leinfelder KF, Hershey HG. The effect of cold temperature mixing on the properties of zinc phosphate cement. Angle Orthod 1976;46:345-50. 3. Shepherd WB, Leinfelder KF, Hershey HG. The effect of mixing method, slab temperature, and humidity on the properties of zinc phosphate and zinc silicophosphate cement. Angle Ortbod 1978;48:21926. 4. Norman RD, Swarm ML, Phillips RW, Sears CR. Properties of cements mixed from liquids with altered water content. J PROSTHET DENT
1970;24:410-18. 5. Eames WB, Munroe SD, Roan JD, O’Neal 53. Proportioning and mixing of cements: a comparison of working times. Oper Dent 1977;2:97-104.
6. Windeler AS. Powder enrichment effect on film thickness of zinc phosD~~~1979;42:299-303. phate cement. 3 PROSTHET 7. Plant CB, Jones DW, DarveII BW. The heat evolved and temperatures attained during the setting of restorative materials. Br Dent J 1974; 137:233-S. 8. Palermo JJ, Leinfelder KF, Holland GA. Evaluation of the frozen slab DENT 1982; technique for cementing cast restorations. J PROST~ET
48~555-61. 9. Myers CL, Drake JT, Brantley WA. A comparison of properties of zinc phosphate cements mixed on room temperature and frozen slabs. J
PROSTHETDENT~~~~;~OZ~O~-~~. 10. Newman SM. Frozen slab technique for mixing zinc phosphate cement for cast restorations. J PROSTHET DENT1980;43:46-9. 11. Namiki Y, Ikemi T, Ueda I. Setting time of dental cements for lutingthe relation between the exothermic peak and the mixing time. J Oral Sci 1985;11:237-48. 12. Virmani R, Norman RD, Swarm ML, Phillips RW. The pH of setting cements. III, In vivo. J PROSTHET DENT1970;23:66-72. Reprint
requests to:
DR.LEONARD A.MUENINGHOFF DEPARTMENTOFRESTORATIVEDENTISTRY UNIVERSITYOFALABAMA,SCHOOLOFDENT~STRY BIRMINGHAM, AL 35294
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Bound volumes of THE JOURNAL OF PROSTHETIC DENTISTRY are available to subscribers (only) for the 1991 issues from the publisher at a cost of $50.00 ($63.00 international) for Vol. 65 (January-June) and Vol. 66 (July-December). Shipping charges are included. Each bound volume contains a subject and author index, and all advertising is removed. Copies are shipped within 30 days after publication of the last issue in the volume. The binding is durable buckram with the journal name, volume number, and year stamped in gold on the spine. Volumes 63 and 64 are also available. Payment must accompany all orders. Contact Mosby-Year Book, Inc., Subscription Services, 11830 Westline Industrial Drive, St. Louis, MO 63x46-3318, USA, phone (800) 3254177, ext. 4351. Subscriptions must be in force to qualify. in place of a regular JOURNAL subscription.
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JANUARY1992 VOLUME67 NUMBER1
