Accuracy and dimensional stability hydrocolloid impression system Mathilde TRIKON,
of a combined
C. R. R. Peters, DDS, PhD,a and Annebeth Tieleman, DDSb Institute for Dental Clinical Research, University of Nijmegen, The Netherlands
The accuracy of a combined hydrocolloid impression system was studied as a function of time of pour. There was little change in dimension between posts for all time intervals studied. No statistically significant dilYerences were noted among the observation periods until 3 hours. The results indicate that for the impression system studied, a 3-hour pouring time can be used for transport to a commercial dental laboratory, provided the impressions are stored in 100% relative humidity. The hydrocolloid impression system tested resulted in a stone cast of slightly deviating dimensions compared with the master model. Therefore laboratory procedures should compensate for cement thickness, taking into account the minimal changes in dimensions of the die. (J PROSTHET DENT 1992;67:873-8.)
R
eversible and irreversible hydrocolloids are used as impression materials in restorative and prosthetic dentistry. The hydrocolloid systems have met the test of time, appear to be excellent impression materials, and are gaining popularity among c1inicians.l An impression technique that combines the well-known reversible agar-hydrocolloid and the irreversible alginate materials has been introduced to the dental profession. The benefits of this procedure have been noted by earlier investigators.2,3 At present. this technique is inexpensive, simple to use, and the clinical time required is short. A distinct advantage besides ease of manipulation (no special trays, no cooling system) is the unimportance of moisture control. However, there seems to be general agreement that the casts should be poured immediately after the impressions are removed from the mouth to obtain maximum accuracy. Two principal characteristics of impression materials are accuracy and dimensional stability. Accuracy can be evaluated in terms of horizontal and vertical changes from a master die. The accuracy of various combinations of hydrocolloid materials using different master dies has been evaluated.4-s Johnson and Craig9 found little difference in accuracy among agar hydrocolloids, although they reported distortion of the diameter of the stone die. The aim of the present study was to evaluate the comparative accuracy and stability of a hydrocolloid combination impression system as a function of pouring time. If delayed pour is allowed, one of the disadvantages of the use of hydrocolloid impression materials has been eliminated. A time lapse of 3 hours could be allowed to transport the hydrocolloid combination impression to the dental laboraPresented in part at the CED-IADR meeting in Bern, Switzerland. BAssociate Professor, Department of Cariology and Endodontology. bResearchAssociate, Department of Cariology and Endodontology. 1011134356 THE
JOURNAL
OF PROSTHETIC
DENTISTRY
tory. Such a practice has been criticized in the case of a hydrocolloid impression because of the lack of dimensional stability of the hydrocolloid in the air. It is a natural phenomenon for a gel to shrink by syneresis or evaporation of water when it is exposed to air. However, it might be expected that if the relative humidity of the air were kept at 100%) a satisfactory dimensional stability would result.lO When alginate materials were exposed to air in a relative humidity of 100 % , several earlier brands exhibited remarkably stable dimensions for 3 hours and longer.ll Recently, the hydrocolloid impression materials have been improved. Nonetheless, the current method to be employed with any type of reversible hydrocolloid impression material requires construction of the cast immediately after the impression is obtained. Hattori and Lacy12 reported on the effect of storage mode and time on alginate/ hydrocolloid impression materials. No significant differences were noted if the impressions were poured immediately or 1 hour later. Dies made from impressions stored longer than 1 hour were significantly smaller. A study by Dahl et a1.13concluded that clinically acceptable working casts could be derived if the impressions were poured within 3 hours. Moreover, four out of five of the material combinations tested could safely be used within 24 hours provided they were stored in 100% relative humidity. The present study was designed to investigate the dimensional accuracy and stability of a current, reversible/irreversible hydrocolloid impression system when stored for varying ity.
lengths
of time in 100% relative
MATERIALS
AND
METHODS
humid-
Reversible (Cavex Combiloid, Cavex Holland BV, Haarlem, The Netherlands) and irreversible (Alginate CA 37 Superior Pink, Cavex Holland BV) hydrocolloid combina873
PETERS
Hl
H2
H4
! H3
F
AND
TIELEMAN
Fig. 1. Schematic representation of stainless steel master model denoting the locations A, B, C, and D (horizontal measurements) and HI, H2, H3, and H4 (vertical measurements) for evaluation of dimensional accuracy.
tion impressions were made of a specially manufactured stainless steel model (Fig. 1). The stainless steel master die consisted of four posts, milled parallel at the surfaces to be measured, which provided eight standard measurements. A description of the actual standard model dimensions as derived from 15 measurements is included in Tables I and II. The accuracy of the impression material was assessed indirectly by measuring eight locations on improved stone casts recovered from impressions of the master model. A traveling microscope (type 060-366.002, Leitz, Wetzlar, Germany) capable of measuring 0.001 mm was used. To simulate oral conditions, the model was maintained at 32 + 2’ C during impressions. A custom resin tray without any perforations provided a uniform thickness of 3 mm for the impression materials. The borders of the tray conformed to the edge of the master die to achieve a positive and uniform seating. The thickness of the custom tray was 3 mm. The impression materials used meet IS0 specification No. 1564 (agar) and the ISO/DIS specification No. 1563 (alginate). The reversible hydrocolloid was supplied in sticks for syringe injection. The samples were prepared according to the manufacturer’s instructions. Alginate CA 37 Superior Pink is a dust-free normal-setting alginate. The amount of alginate powder was preweighed and the manufacturer’s recommended water/powder ratio was used. The alginate was mixed by hand at room temperature (23.0 f 2.0° C). The master die was prepared for the impression by washing it with a water spray. A syringe with hydrocolloid was removed from the storage bath and the material was syringed over the wet surfaces of the abutments of the stainless steel model. The tray with unset alginate was immediately positioned over the model and fully seated. After 5 minutes, the impression was removed with a single dislodging force (“snap-out” method). After they were rinsed, the impressions were immediately placed in a humidor before they were poured at the studied intervals. The experiments were done to investigate the variation 874
in accuracy as a function of the time of pour. The impressions were poured with dental stone (GC New Fujirock, G-C Dental Industrial Corp., Tokyo, Japan) after ?4, Y’z, 1, 2,3, and 4 hours. For the first series, three consecutive impressions were made at each of the six observation periods, resulting in 18 impressions and casts. The dental stone was used in proportions according to the instructions given by the manufacturer. The dental stone was mixed first by hand (15 seconds), vacuum-spatulated for 45 seconds, and poured into the impression on a vibrator. The impressions were placed in a humidor and the casts were allowed to set for 1 hour before separation. The second series consisted of 23 impressions that were kept in a humidor for 3 hours, followed by the same pouring procedure. After 24 hours, measurements were performed at all eight reference points. Each measurement was repeated five times and the mean of the five readings was used for the accuracy measurement of a particular dimension. The mean percent difference ([Q - x,]/ x, . 100 % ) of measured distances between the master (m) and the stone casts (c) was calculated at various times of pour. The relative differences are used for statistical analyses. The data were analyzed using one-way analysis of variance to determine differences between groups. To pinpoint the location of the significant differences, the multiple comparison test of Scheffe was used. All testing was performed at the 95% confidence level.
RESULTS Dimensional
change
Horizontal distances (A, B, C, and D), as well as the height at each post (Hl, H2, H3, and H4), were measured to yield information about the amount of change in distance. The locations are depicted in Fig. 1. Mean absolute dimensions and standard deviations for both the master die (n = 15) and the caste (n = 5) poured at various storage times are given in Tables I and II. From these ta-
JUNE
1992
VOLUME
67
NUMBER
6
HYDROCOLLOID
DIMENSIONAL
STABILITY
Table I. Mean values and standard deviations (SD) of measured horizontal dimensions in millimeters on the master (n = 15) and the stone casts (n = 5) for the various pouring times
Table II. Mean values and standard deviations (SD) of measured vertical dimensions in millimeters on the master (n = 15) and the stone casts (n = 5) for the various pouring times
Horizontal
Vertical
dimension
Time of pour
'Vi hr
Time of pour
kO.004 29.771 + 0.008 29.732 kO.008
15.065 -1-0.006 15.057 kO.007 15.091 ~0.010
kO.006 29.631 kO.007 29.601 kO.006
15.164 kO.002 15.123 kO.018 15.097 kO.006
% hr
29.679 f 0.005 29.671 kO.002 29.738 kO.011
15.063 ~0.003 15.093 kO.002 15.061 kO.002
29.617 + 0.006 29.581 kO.006 29.642 f 0.009
15.134 kO.004 15.093 kO.005 15.116 ~0.006
1 hr
29.732 kO.007 29.537 kO.005 29.752 +0.019
15.101 kO.006 15.086 *0.007 15.000 *0.002
29.767 +0.003 29.591 to.002 29.572 kO.007
2 hr
29.621 kO.014 29.790 f 0.009 29.716 kO.005
15.114 kO.003 15.096 kO.008 15.197 kO.003
3 hr
29.790 kO.020 29.716 kO.014 29.736 + 0.008
4 hr
Master (n = 15)
0.827 kO.011 0.800 kO.034 0.821 kO.034
0.702 kO.017 0.741 kO.017 0.724 kO.015
0.731 kO.020 0.760 kO.023 0.723 kO.024
kO.006 0.809 kO.026 0.784 kO.005
Yzhr
0.822 kO.018 0.794 kO.006 0.816 + 0.009
0.696 +0.008 0.730 kO.011 0.721 kO.014
0.720 +0.010 0.712 to.012 0.754 kO.004
0.783 kO.015 0.796 k 0.009 0.812 kO.008
15.168 kO.006 15.116 kO.005 15.080 kO.012
1 hr
0.824 kO.043 0.819 +0.013 0.829 kO.020
0.708 kO.014 0.746 kO.011 0.691 kO.017
0.722 kO.024 0.748 kO.013 0.704 k-o.023
0.827 kO.033 0.801 kO.006 0.817 kO.007
29.764 kO.006 29.566 +0.006 29.508 *0.004
15.040 kO.004 15.126 kO.004 15.049 *0.001
2 hr
0.714 kO.016 0.818 ~1~0.016 0.802 kO.007
0.820 + 0.009 0.704 kO.024 0.697 kO.022
0.772 kO.026 0.722 r0.022 0.727 kO.013
0.718 ~0.015 0.817 +0.018 0.790 kO.028
15.064 + 0.009 14.974 kO.006 15.047 kO.007
29.555 +0.006 29.495 +O.OlO 29.577 *0.003
15.151 kO.010 15.140 kO.012 15.076 LO.007
3 hr
0.829 kO.025 0.810 kO.012 0.817 kO.017
0.715 kO.006 0.720 +0.030 0.686 kO.011
0.717 +0.008 0.718 kO.020 0.731 kO.013
0.776 kO.035 0.771 kO.021 0.790 eo.015
29.615 + 0.009 29.651 kO.011 29.650 +0.018
14.977 ~~0.008 14.995 kO.007 14.946 ~0.016
29.512 *O.OlO 29.501 kO.004 29.506 +0.010
15.085 kO.014 15.090 ~0.006 15.016 kO.006
4 hr
0.800 kO.013 0.848 co.018 0.805 kO.020
0.690 kO.014 0.715 kO.020 0.690 +0.027
0.731 to.013 0.709 kO.013 0.731 kO.016
0.808 + 0.009 0.797 kO.022 0.775 kO.017
29.681 kO.004
15.043 kO.004
29.567 kO.004
15.106 kO.005
Master 15)
0.856 kO.016
0.743 kO.014
0.753 kO.017
0.817 kO.015
29.769
29.667
bles it may be concluded that the overall measurement error for horizontal dimensions was 0.009 mm per measurement. The vertical dimensions showed an overall measurement error of 0.019 mm per measurement. The analyses are based on the mean of 20 measurements. This results in a considerable reduction of measurement error.
THE
JOURNAL
dimension
OF PROSTHETIC
DENTISTRY
(n =
0.799
The accuracy of the casts is expressed in terms of percent deviation from the values of the master model to facilitate comparison of the relative amounts of change. The accuracy of the impression material for the selected time intervals is graphically shown in Fig. 2 for horizontal dimensions and in Fig. 3 for vertical dimensions.
PETERS
Mean percent
03
horizontal
AND
TIELEMAN
deviation
094 033 -
l
-
-z
O,l 5 ..-Is 5
04
-
-0,l
-
P
-0,2 -
z
-0,3 6
02
15
-0.4 - * l
significantly
different
pairs
I 1
-03 0
2
3
4
Time of pour (hrs)
Fig. 2. Mean percent difference (n = 3) of measured horizontal distances at various times of pour I(‘/ to 4 hours).
Mean
O-’ 1
percent
vertical
deviation
1 -6
1% = 0.008
mm
-7 I
I
0
1
0: Time
pour
I
I
3
4
(hrs)
Fig. 3. Mean percent difference (n = 3) of measured vertical distances at various times of pour (1/4 to 4 hours).
The analysis of variance showed a significant difference between the groups at the various times of pour for horizontal dimensions. The 4-hour pouring time was most clearly different, although significant differences were calculated only between the ‘/4-hour group versus the 4-hour group and between the 2-hour group versus the 4-hour group. The mean percent deviation for vertical dimensions is depicted in Fig. 3. The analysis of variance showed no significant difference between the various times of pour. To relate percent change to absolute change in millimeters, a change of 0.1% equates to a change of approximately 0.030 mm for the horizontal distances A and C, whereas with respect to the height measurements, a change of 1% equates to an absolute change of 0.008 mm.
Accuracy
at 3-hour
pouring
time
For the second series a replication size of 23 casts using a 3-hour pouring time was investigated. The mean percent deviation of the S-hour casts was 0.01% (S.D. 0.12%) for 876
horizontal dimensions and -4.30% (SD. 1.27%) for vertical dimensions. Histograms of these data (Figs. 4 and 5) display the frequency distribution of casts according to the amount of deviation from the master die.
DISCUSSION Dimensional
stability
There is no agreement regarding the maximum acceptable dimensional change between the master model and the dental stone casts. Christensen14 reported that a range of marginal openings of 2 to 51 pm at the occlusal part was clinically acceptable, whereas at the gingival level the range of acceptable marginal openings was 34 to 119 pm (mean score of 74 pm). Assuming a value of 50 pm as the maximum acceptable discrepancy from the master model for a single unit, the combination impression system studied showed acceptable dimensional accuracy. Variations in dimensional accuracy did occur, but were of questionable clinical significance. The relationship of dimensional change by time for horJUNE
1992
VOLUME
67
NUMBER
6
HYDROCOLLOID
DIMENSIONAL
Horizontal
-0.2
-0.1
STABILITY
deviation at 3 hr pour
0
0.1
Deviation
0.2
1
6d
-Vertical
deviation at 3 hr pour-
0.3
(%) -6
Fig. 4. Histogram of the mean percent difference of mea-
-5
-4
Deviation
sured horizontal distances at 3-hour pouring time.
-3
-2
-1
(%)
Fig. 5. Histogram of the mean percent difference of measured vertical distances at 3-hour pouring time. izontal model locations can be seen in Fig. 2. The 1/ -hour to 3-hour groups did not differ at all. Although the 4-hour data were not significantly different from each group as calculated by ScheffL’s test, they still indicated a step between casts of the ‘/4-hour to 3-hour pours and the 4-hour pours. SchefE’s test can .be considered as rather conservative. Significances are underestimated by this procedure. The 3-hour pouring time can be regarded as a borderline situation. Thus this time interval was selected for study in the second series of experiments. The mean horizontal dimensions among the posts changed for each time of pour for the l/4-hour to 3-hour periods (Fig. 2). The range of change for these time intervals was +0.41% to -0.09%. For example, this change equates to a maximum change of +0.12 mm for the long span dimensions A and C. It is likely that the periodontal ligament may accommodate a fixed partial denture that is longer by this magnitude. On seating a four-unit fixed partial denture, each abutment tooth would be displaced 0.060 mm. Since Hellie et a1.15reported a tooth displacement of 0.090 mm caused by wedging before amalgam placement, this magnitude seems clinically insignificant and can be accommodated by the periodontal ligament. The relationship of dimensional change by time for vertical model locations can be seen in Fig. 3. The data show a large variation, even at. the short-term pouring intervals. The amount of change for the 1/ to 3-hour time intervals shown in the present study ranged from -4.79% to -1.61% . This change equates to a range of -0.038 to -0.013 mm for the measured height dimensions. However, the short distances measured and the variation of the data do not allow strong conclusions with respect to the vertical dimensions. Models were shorter because the vertical component of contraction was in a direction toward the occlusal portion of the posts where the impression adheres to the tray. It is desirable to minimize the decrease in height because a shorter model will produce a casting that is short THE
JOURNAL
OF PROSTHETIC
DENTISTRY
at the margins. The parallel sides of the posts may also have contributed to some distortion of the impression material when the impression was removed from the posts. Future research on dimensional stability will include a more realistic master model of the human dentition to study clinical intra-arch distances as well as the height and circumference of full crown preparations.ls
Accuracy
at 3-hour
pouring
time
The frequency distribution of the relative amount of change of the 23 replicas poured at 3 hours is graphically shown in Figs. 4 and 5. These graphs reveal at a glance the shape of the distribution and the spread of the individual scores, and the location and frequency of the extreme scores. Most of the horizontal measurements (Fig. 4) were between a 0.09 % and 0.19 % deviation from the master die and show a distribution with a maximum between -0.05% and 0.00% amount of change (-0.015 to 0.00 mm for the long span dimensions A and C). The vertical measurements, depicted in Fig. 5, showed the same tendency as noted before. The maximum amount of change of this distribution is located at approximately -4 % . This equates to an absolute decrease of -0.032 mm for the studied height dimensions. When compared with the standard, the produced casts showed an overall minimal change in horizontal dimensions (increase as well as decrease) as well as a minimal decrease for the vertical dimensions. However, even the increased dimensions will not supply sufficient space for the luting cement. To date many impression materials require additional die relief or mold expansion to facilitate the fit of castings. The use of these additional techniques is supported by both the need to compensate for cement thickness as well as to allow for the changes in dimension found in this study. 877
PETERS
CONCLUSION A delay of up to 3 hours in pouring the impressions had little effect on the change in distance between posts. No statistically significant difference was found between the G-hour to 3-hour pouring time (p > 0.05). The 4-hour pouring time was shown to be significantly different from two of the studied intervals. Thus storage of the impressions for up to 3 hours in 100 % relative humidity does not affect the accuracy significantly. The changes were within clinically acceptable limit,s and sufficient dimensional stability was demonstrated up to a delayed pour of 3 hours. Although this study only investigated dimensional stability as related to a stainless steel master die, it appears likely that the 3-hour time lapse before pouring the impression can be used for routine transportation of the impression to a commercial laboratory. Future evaluations will include clinical master model replications using various disinfection methods as well as measurements made directly on the impression materials using laser optics. We acknowledgethe technical assistanceof Mrs. A. F. M. Leydekkers-Goversand the statistical adviceof Dr. M. A. van ‘t Hof and Dr. J. Rudney (University of Minnesota).
AND
TIELEMAN
5. Herring HW, Tames MA, Zardiackas LD. Comparison of the dimensional accuracy of a combined reversible/irreversible hydrocolloid impression system with other commonly used impression materials. J PROSTHET DENT 1984,52:795-g. 6. Appleby DC, Smith W, Lontz JF, et al. Combined reversible/irreversible hydrocolloid impression systems: comparative analysis. J PROSTHET DENT 1985;54:627-32. 7. Odman PA, Jemt TM. Accuracy of impression materials in a semi-clinical model. Dent Mater 1988$64-‘7. 8. Supowitz ML, Schnell RJ, Dykema RW, et al. Dimensional accuracy of combined reversible and irreversible hydrocolloid impression materials. J PROSTHET DENT 198&59:404-g. 9. Johnson GH, Craig RG. Accuracy and bond strength of combination agar/alginate hydrocolloid impression materials. 3 PROSTHET DENT 1986,55:1-6. 10. Skinner EW, Cooper EN, Beck FE. Reversible and irreversible hydrocolloid impression materials. J Am Dent Assoc 1950;40:196-207. 11. Skinner EW, Porn& CE. Dimensional stability of alginate impression materials. J Am Dent Assoc 1946;33:1253-60. 12. Hattory H, Lacy A. Effect of storage mode and time on alginate/ hydrocolloid impression materials [Abstract]. J Dent Res 1985;64:243. 13. D&l BL, Dymbe B, Valderhaug J. Bonding properties and dimensional stability of hydrocolloid impression systems in fixed prosthodontics. J PROSTHET DENT 1985;53:796-800. 14. Christensen GJ. Marginal fit of gold inlay castings. J PROSTHET DENT 196&X297-305. 15. Hellie CM, Charbeneau GT, Craig RG, et al. Quantitative evaluation of proximal tooth movement effected by wedging: a pilot study. J PROSTHET DENT 1985;53:335-41. 16. Johnson GH, Chellis KD, Gordon GE. Dimensional stability and detail reproduction of disinfected alginate and elastomeric impressions [Abstract]. J Dent Res 1940,69:368.
REFERENCES Reprint
1. Reed HV. Reversible agar agar hydrocolloid. Quintessence Int 1990,21:225-g. 2. Schwartz JR. The use of the hydrocolloids or alginates as impression materials for indirect or indirect/-direct inlay construction procedure. Dent Items Int 1951;73:379-.89. 3. Skinner EW, Hoblit NE. A study of the accuracy of hydrocolloid impressions. J PROSTHET DENT 1956;6:80-6. 4. Appleby DC, Pameyer CH, Boffa J. The combined reversible hydrocollaid/irreversible hydrocolloid impression system. J PROSTHET DENT 1980,44:27-35.
878
requests
to:
DR. M. C. R. B. PETERS TRIKON, INSTITUTE FOR DENTAL CLINICAL RESEARCH UNIVERSITY OF NIJMECEN P.O. Box 9101 6500 HB NIJMEGEN THE NETHERLANDS
JUNE
1992
VOLUME
67
NUMBER
6
of a combined
C. R. R. Peters, DDS, PhD,a and Annebeth Tieleman, DDSb Institute for Dental Clinical Research, University of Nijmegen, The Netherlands
The accuracy of a combined hydrocolloid impression system was studied as a function of time of pour. There was little change in dimension between posts for all time intervals studied. No statistically significant dilYerences were noted among the observation periods until 3 hours. The results indicate that for the impression system studied, a 3-hour pouring time can be used for transport to a commercial dental laboratory, provided the impressions are stored in 100% relative humidity. The hydrocolloid impression system tested resulted in a stone cast of slightly deviating dimensions compared with the master model. Therefore laboratory procedures should compensate for cement thickness, taking into account the minimal changes in dimensions of the die. (J PROSTHET DENT 1992;67:873-8.)
R
eversible and irreversible hydrocolloids are used as impression materials in restorative and prosthetic dentistry. The hydrocolloid systems have met the test of time, appear to be excellent impression materials, and are gaining popularity among c1inicians.l An impression technique that combines the well-known reversible agar-hydrocolloid and the irreversible alginate materials has been introduced to the dental profession. The benefits of this procedure have been noted by earlier investigators.2,3 At present. this technique is inexpensive, simple to use, and the clinical time required is short. A distinct advantage besides ease of manipulation (no special trays, no cooling system) is the unimportance of moisture control. However, there seems to be general agreement that the casts should be poured immediately after the impressions are removed from the mouth to obtain maximum accuracy. Two principal characteristics of impression materials are accuracy and dimensional stability. Accuracy can be evaluated in terms of horizontal and vertical changes from a master die. The accuracy of various combinations of hydrocolloid materials using different master dies has been evaluated.4-s Johnson and Craig9 found little difference in accuracy among agar hydrocolloids, although they reported distortion of the diameter of the stone die. The aim of the present study was to evaluate the comparative accuracy and stability of a hydrocolloid combination impression system as a function of pouring time. If delayed pour is allowed, one of the disadvantages of the use of hydrocolloid impression materials has been eliminated. A time lapse of 3 hours could be allowed to transport the hydrocolloid combination impression to the dental laboraPresented in part at the CED-IADR meeting in Bern, Switzerland. BAssociate Professor, Department of Cariology and Endodontology. bResearchAssociate, Department of Cariology and Endodontology. 1011134356 THE
JOURNAL
OF PROSTHETIC
DENTISTRY
tory. Such a practice has been criticized in the case of a hydrocolloid impression because of the lack of dimensional stability of the hydrocolloid in the air. It is a natural phenomenon for a gel to shrink by syneresis or evaporation of water when it is exposed to air. However, it might be expected that if the relative humidity of the air were kept at 100%) a satisfactory dimensional stability would result.lO When alginate materials were exposed to air in a relative humidity of 100 % , several earlier brands exhibited remarkably stable dimensions for 3 hours and longer.ll Recently, the hydrocolloid impression materials have been improved. Nonetheless, the current method to be employed with any type of reversible hydrocolloid impression material requires construction of the cast immediately after the impression is obtained. Hattori and Lacy12 reported on the effect of storage mode and time on alginate/ hydrocolloid impression materials. No significant differences were noted if the impressions were poured immediately or 1 hour later. Dies made from impressions stored longer than 1 hour were significantly smaller. A study by Dahl et a1.13concluded that clinically acceptable working casts could be derived if the impressions were poured within 3 hours. Moreover, four out of five of the material combinations tested could safely be used within 24 hours provided they were stored in 100% relative humidity. The present study was designed to investigate the dimensional accuracy and stability of a current, reversible/irreversible hydrocolloid impression system when stored for varying ity.
lengths
of time in 100% relative
MATERIALS
AND
METHODS
humid-
Reversible (Cavex Combiloid, Cavex Holland BV, Haarlem, The Netherlands) and irreversible (Alginate CA 37 Superior Pink, Cavex Holland BV) hydrocolloid combina873
PETERS
Hl
H2
H4
! H3
F
AND
TIELEMAN
Fig. 1. Schematic representation of stainless steel master model denoting the locations A, B, C, and D (horizontal measurements) and HI, H2, H3, and H4 (vertical measurements) for evaluation of dimensional accuracy.
tion impressions were made of a specially manufactured stainless steel model (Fig. 1). The stainless steel master die consisted of four posts, milled parallel at the surfaces to be measured, which provided eight standard measurements. A description of the actual standard model dimensions as derived from 15 measurements is included in Tables I and II. The accuracy of the impression material was assessed indirectly by measuring eight locations on improved stone casts recovered from impressions of the master model. A traveling microscope (type 060-366.002, Leitz, Wetzlar, Germany) capable of measuring 0.001 mm was used. To simulate oral conditions, the model was maintained at 32 + 2’ C during impressions. A custom resin tray without any perforations provided a uniform thickness of 3 mm for the impression materials. The borders of the tray conformed to the edge of the master die to achieve a positive and uniform seating. The thickness of the custom tray was 3 mm. The impression materials used meet IS0 specification No. 1564 (agar) and the ISO/DIS specification No. 1563 (alginate). The reversible hydrocolloid was supplied in sticks for syringe injection. The samples were prepared according to the manufacturer’s instructions. Alginate CA 37 Superior Pink is a dust-free normal-setting alginate. The amount of alginate powder was preweighed and the manufacturer’s recommended water/powder ratio was used. The alginate was mixed by hand at room temperature (23.0 f 2.0° C). The master die was prepared for the impression by washing it with a water spray. A syringe with hydrocolloid was removed from the storage bath and the material was syringed over the wet surfaces of the abutments of the stainless steel model. The tray with unset alginate was immediately positioned over the model and fully seated. After 5 minutes, the impression was removed with a single dislodging force (“snap-out” method). After they were rinsed, the impressions were immediately placed in a humidor before they were poured at the studied intervals. The experiments were done to investigate the variation 874
in accuracy as a function of the time of pour. The impressions were poured with dental stone (GC New Fujirock, G-C Dental Industrial Corp., Tokyo, Japan) after ?4, Y’z, 1, 2,3, and 4 hours. For the first series, three consecutive impressions were made at each of the six observation periods, resulting in 18 impressions and casts. The dental stone was used in proportions according to the instructions given by the manufacturer. The dental stone was mixed first by hand (15 seconds), vacuum-spatulated for 45 seconds, and poured into the impression on a vibrator. The impressions were placed in a humidor and the casts were allowed to set for 1 hour before separation. The second series consisted of 23 impressions that were kept in a humidor for 3 hours, followed by the same pouring procedure. After 24 hours, measurements were performed at all eight reference points. Each measurement was repeated five times and the mean of the five readings was used for the accuracy measurement of a particular dimension. The mean percent difference ([Q - x,]/ x, . 100 % ) of measured distances between the master (m) and the stone casts (c) was calculated at various times of pour. The relative differences are used for statistical analyses. The data were analyzed using one-way analysis of variance to determine differences between groups. To pinpoint the location of the significant differences, the multiple comparison test of Scheffe was used. All testing was performed at the 95% confidence level.
RESULTS Dimensional
change
Horizontal distances (A, B, C, and D), as well as the height at each post (Hl, H2, H3, and H4), were measured to yield information about the amount of change in distance. The locations are depicted in Fig. 1. Mean absolute dimensions and standard deviations for both the master die (n = 15) and the caste (n = 5) poured at various storage times are given in Tables I and II. From these ta-
JUNE
1992
VOLUME
67
NUMBER
6
HYDROCOLLOID
DIMENSIONAL
STABILITY
Table I. Mean values and standard deviations (SD) of measured horizontal dimensions in millimeters on the master (n = 15) and the stone casts (n = 5) for the various pouring times
Table II. Mean values and standard deviations (SD) of measured vertical dimensions in millimeters on the master (n = 15) and the stone casts (n = 5) for the various pouring times
Horizontal
Vertical
dimension
Time of pour
'Vi hr
Time of pour
kO.004 29.771 + 0.008 29.732 kO.008
15.065 -1-0.006 15.057 kO.007 15.091 ~0.010
kO.006 29.631 kO.007 29.601 kO.006
15.164 kO.002 15.123 kO.018 15.097 kO.006
% hr
29.679 f 0.005 29.671 kO.002 29.738 kO.011
15.063 ~0.003 15.093 kO.002 15.061 kO.002
29.617 + 0.006 29.581 kO.006 29.642 f 0.009
15.134 kO.004 15.093 kO.005 15.116 ~0.006
1 hr
29.732 kO.007 29.537 kO.005 29.752 +0.019
15.101 kO.006 15.086 *0.007 15.000 *0.002
29.767 +0.003 29.591 to.002 29.572 kO.007
2 hr
29.621 kO.014 29.790 f 0.009 29.716 kO.005
15.114 kO.003 15.096 kO.008 15.197 kO.003
3 hr
29.790 kO.020 29.716 kO.014 29.736 + 0.008
4 hr
Master (n = 15)
0.827 kO.011 0.800 kO.034 0.821 kO.034
0.702 kO.017 0.741 kO.017 0.724 kO.015
0.731 kO.020 0.760 kO.023 0.723 kO.024
kO.006 0.809 kO.026 0.784 kO.005
Yzhr
0.822 kO.018 0.794 kO.006 0.816 + 0.009
0.696 +0.008 0.730 kO.011 0.721 kO.014
0.720 +0.010 0.712 to.012 0.754 kO.004
0.783 kO.015 0.796 k 0.009 0.812 kO.008
15.168 kO.006 15.116 kO.005 15.080 kO.012
1 hr
0.824 kO.043 0.819 +0.013 0.829 kO.020
0.708 kO.014 0.746 kO.011 0.691 kO.017
0.722 kO.024 0.748 kO.013 0.704 k-o.023
0.827 kO.033 0.801 kO.006 0.817 kO.007
29.764 kO.006 29.566 +0.006 29.508 *0.004
15.040 kO.004 15.126 kO.004 15.049 *0.001
2 hr
0.714 kO.016 0.818 ~1~0.016 0.802 kO.007
0.820 + 0.009 0.704 kO.024 0.697 kO.022
0.772 kO.026 0.722 r0.022 0.727 kO.013
0.718 ~0.015 0.817 +0.018 0.790 kO.028
15.064 + 0.009 14.974 kO.006 15.047 kO.007
29.555 +0.006 29.495 +O.OlO 29.577 *0.003
15.151 kO.010 15.140 kO.012 15.076 LO.007
3 hr
0.829 kO.025 0.810 kO.012 0.817 kO.017
0.715 kO.006 0.720 +0.030 0.686 kO.011
0.717 +0.008 0.718 kO.020 0.731 kO.013
0.776 kO.035 0.771 kO.021 0.790 eo.015
29.615 + 0.009 29.651 kO.011 29.650 +0.018
14.977 ~~0.008 14.995 kO.007 14.946 ~0.016
29.512 *O.OlO 29.501 kO.004 29.506 +0.010
15.085 kO.014 15.090 ~0.006 15.016 kO.006
4 hr
0.800 kO.013 0.848 co.018 0.805 kO.020
0.690 kO.014 0.715 kO.020 0.690 +0.027
0.731 to.013 0.709 kO.013 0.731 kO.016
0.808 + 0.009 0.797 kO.022 0.775 kO.017
29.681 kO.004
15.043 kO.004
29.567 kO.004
15.106 kO.005
Master 15)
0.856 kO.016
0.743 kO.014
0.753 kO.017
0.817 kO.015
29.769
29.667
bles it may be concluded that the overall measurement error for horizontal dimensions was 0.009 mm per measurement. The vertical dimensions showed an overall measurement error of 0.019 mm per measurement. The analyses are based on the mean of 20 measurements. This results in a considerable reduction of measurement error.
THE
JOURNAL
dimension
OF PROSTHETIC
DENTISTRY
(n =
0.799
The accuracy of the casts is expressed in terms of percent deviation from the values of the master model to facilitate comparison of the relative amounts of change. The accuracy of the impression material for the selected time intervals is graphically shown in Fig. 2 for horizontal dimensions and in Fig. 3 for vertical dimensions.
PETERS
Mean percent
03
horizontal
AND
TIELEMAN
deviation
094 033 -
l
-
-z
O,l 5 ..-Is 5
04
-
-0,l
-
P
-0,2 -
z
-0,3 6
02
15
-0.4 - * l
significantly
different
pairs
I 1
-03 0
2
3
4
Time of pour (hrs)
Fig. 2. Mean percent difference (n = 3) of measured horizontal distances at various times of pour I(‘/ to 4 hours).
Mean
O-’ 1
percent
vertical
deviation
1 -6
1% = 0.008
mm
-7 I
I
0
1
0: Time
pour
I
I
3
4
(hrs)
Fig. 3. Mean percent difference (n = 3) of measured vertical distances at various times of pour (1/4 to 4 hours).
The analysis of variance showed a significant difference between the groups at the various times of pour for horizontal dimensions. The 4-hour pouring time was most clearly different, although significant differences were calculated only between the ‘/4-hour group versus the 4-hour group and between the 2-hour group versus the 4-hour group. The mean percent deviation for vertical dimensions is depicted in Fig. 3. The analysis of variance showed no significant difference between the various times of pour. To relate percent change to absolute change in millimeters, a change of 0.1% equates to a change of approximately 0.030 mm for the horizontal distances A and C, whereas with respect to the height measurements, a change of 1% equates to an absolute change of 0.008 mm.
Accuracy
at 3-hour
pouring
time
For the second series a replication size of 23 casts using a 3-hour pouring time was investigated. The mean percent deviation of the S-hour casts was 0.01% (S.D. 0.12%) for 876
horizontal dimensions and -4.30% (SD. 1.27%) for vertical dimensions. Histograms of these data (Figs. 4 and 5) display the frequency distribution of casts according to the amount of deviation from the master die.
DISCUSSION Dimensional
stability
There is no agreement regarding the maximum acceptable dimensional change between the master model and the dental stone casts. Christensen14 reported that a range of marginal openings of 2 to 51 pm at the occlusal part was clinically acceptable, whereas at the gingival level the range of acceptable marginal openings was 34 to 119 pm (mean score of 74 pm). Assuming a value of 50 pm as the maximum acceptable discrepancy from the master model for a single unit, the combination impression system studied showed acceptable dimensional accuracy. Variations in dimensional accuracy did occur, but were of questionable clinical significance. The relationship of dimensional change by time for horJUNE
1992
VOLUME
67
NUMBER
6
HYDROCOLLOID
DIMENSIONAL
Horizontal
-0.2
-0.1
STABILITY
deviation at 3 hr pour
0
0.1
Deviation
0.2
1
6d
-Vertical
deviation at 3 hr pour-
0.3
(%) -6
Fig. 4. Histogram of the mean percent difference of mea-
-5
-4
Deviation
sured horizontal distances at 3-hour pouring time.
-3
-2
-1
(%)
Fig. 5. Histogram of the mean percent difference of measured vertical distances at 3-hour pouring time. izontal model locations can be seen in Fig. 2. The 1/ -hour to 3-hour groups did not differ at all. Although the 4-hour data were not significantly different from each group as calculated by ScheffL’s test, they still indicated a step between casts of the ‘/4-hour to 3-hour pours and the 4-hour pours. SchefE’s test can .be considered as rather conservative. Significances are underestimated by this procedure. The 3-hour pouring time can be regarded as a borderline situation. Thus this time interval was selected for study in the second series of experiments. The mean horizontal dimensions among the posts changed for each time of pour for the l/4-hour to 3-hour periods (Fig. 2). The range of change for these time intervals was +0.41% to -0.09%. For example, this change equates to a maximum change of +0.12 mm for the long span dimensions A and C. It is likely that the periodontal ligament may accommodate a fixed partial denture that is longer by this magnitude. On seating a four-unit fixed partial denture, each abutment tooth would be displaced 0.060 mm. Since Hellie et a1.15reported a tooth displacement of 0.090 mm caused by wedging before amalgam placement, this magnitude seems clinically insignificant and can be accommodated by the periodontal ligament. The relationship of dimensional change by time for vertical model locations can be seen in Fig. 3. The data show a large variation, even at. the short-term pouring intervals. The amount of change for the 1/ to 3-hour time intervals shown in the present study ranged from -4.79% to -1.61% . This change equates to a range of -0.038 to -0.013 mm for the measured height dimensions. However, the short distances measured and the variation of the data do not allow strong conclusions with respect to the vertical dimensions. Models were shorter because the vertical component of contraction was in a direction toward the occlusal portion of the posts where the impression adheres to the tray. It is desirable to minimize the decrease in height because a shorter model will produce a casting that is short THE
JOURNAL
OF PROSTHETIC
DENTISTRY
at the margins. The parallel sides of the posts may also have contributed to some distortion of the impression material when the impression was removed from the posts. Future research on dimensional stability will include a more realistic master model of the human dentition to study clinical intra-arch distances as well as the height and circumference of full crown preparations.ls
Accuracy
at 3-hour
pouring
time
The frequency distribution of the relative amount of change of the 23 replicas poured at 3 hours is graphically shown in Figs. 4 and 5. These graphs reveal at a glance the shape of the distribution and the spread of the individual scores, and the location and frequency of the extreme scores. Most of the horizontal measurements (Fig. 4) were between a 0.09 % and 0.19 % deviation from the master die and show a distribution with a maximum between -0.05% and 0.00% amount of change (-0.015 to 0.00 mm for the long span dimensions A and C). The vertical measurements, depicted in Fig. 5, showed the same tendency as noted before. The maximum amount of change of this distribution is located at approximately -4 % . This equates to an absolute decrease of -0.032 mm for the studied height dimensions. When compared with the standard, the produced casts showed an overall minimal change in horizontal dimensions (increase as well as decrease) as well as a minimal decrease for the vertical dimensions. However, even the increased dimensions will not supply sufficient space for the luting cement. To date many impression materials require additional die relief or mold expansion to facilitate the fit of castings. The use of these additional techniques is supported by both the need to compensate for cement thickness as well as to allow for the changes in dimension found in this study. 877
PETERS
CONCLUSION A delay of up to 3 hours in pouring the impressions had little effect on the change in distance between posts. No statistically significant difference was found between the G-hour to 3-hour pouring time (p > 0.05). The 4-hour pouring time was shown to be significantly different from two of the studied intervals. Thus storage of the impressions for up to 3 hours in 100 % relative humidity does not affect the accuracy significantly. The changes were within clinically acceptable limit,s and sufficient dimensional stability was demonstrated up to a delayed pour of 3 hours. Although this study only investigated dimensional stability as related to a stainless steel master die, it appears likely that the 3-hour time lapse before pouring the impression can be used for routine transportation of the impression to a commercial laboratory. Future evaluations will include clinical master model replications using various disinfection methods as well as measurements made directly on the impression materials using laser optics. We acknowledgethe technical assistanceof Mrs. A. F. M. Leydekkers-Goversand the statistical adviceof Dr. M. A. van ‘t Hof and Dr. J. Rudney (University of Minnesota).
AND
TIELEMAN
5. Herring HW, Tames MA, Zardiackas LD. Comparison of the dimensional accuracy of a combined reversible/irreversible hydrocolloid impression system with other commonly used impression materials. J PROSTHET DENT 1984,52:795-g. 6. Appleby DC, Smith W, Lontz JF, et al. Combined reversible/irreversible hydrocolloid impression systems: comparative analysis. J PROSTHET DENT 1985;54:627-32. 7. Odman PA, Jemt TM. Accuracy of impression materials in a semi-clinical model. Dent Mater 1988$64-‘7. 8. Supowitz ML, Schnell RJ, Dykema RW, et al. Dimensional accuracy of combined reversible and irreversible hydrocolloid impression materials. J PROSTHET DENT 198&59:404-g. 9. Johnson GH, Craig RG. Accuracy and bond strength of combination agar/alginate hydrocolloid impression materials. 3 PROSTHET DENT 1986,55:1-6. 10. Skinner EW, Cooper EN, Beck FE. Reversible and irreversible hydrocolloid impression materials. J Am Dent Assoc 1950;40:196-207. 11. Skinner EW, Porn& CE. Dimensional stability of alginate impression materials. J Am Dent Assoc 1946;33:1253-60. 12. Hattory H, Lacy A. Effect of storage mode and time on alginate/ hydrocolloid impression materials [Abstract]. J Dent Res 1985;64:243. 13. D&l BL, Dymbe B, Valderhaug J. Bonding properties and dimensional stability of hydrocolloid impression systems in fixed prosthodontics. J PROSTHET DENT 1985;53:796-800. 14. Christensen GJ. Marginal fit of gold inlay castings. J PROSTHET DENT 196&X297-305. 15. Hellie CM, Charbeneau GT, Craig RG, et al. Quantitative evaluation of proximal tooth movement effected by wedging: a pilot study. J PROSTHET DENT 1985;53:335-41. 16. Johnson GH, Chellis KD, Gordon GE. Dimensional stability and detail reproduction of disinfected alginate and elastomeric impressions [Abstract]. J Dent Res 1940,69:368.
REFERENCES Reprint
1. Reed HV. Reversible agar agar hydrocolloid. Quintessence Int 1990,21:225-g. 2. Schwartz JR. The use of the hydrocolloids or alginates as impression materials for indirect or indirect/-direct inlay construction procedure. Dent Items Int 1951;73:379-.89. 3. Skinner EW, Hoblit NE. A study of the accuracy of hydrocolloid impressions. J PROSTHET DENT 1956;6:80-6. 4. Appleby DC, Pameyer CH, Boffa J. The combined reversible hydrocollaid/irreversible hydrocolloid impression system. J PROSTHET DENT 1980,44:27-35.
878
requests
to:
DR. M. C. R. B. PETERS TRIKON, INSTITUTE FOR DENTAL CLINICAL RESEARCH UNIVERSITY OF NIJMECEN P.O. Box 9101 6500 HB NIJMEGEN THE NETHERLANDS
JUNE
1992
VOLUME
67
NUMBER
6
