Dimensional bases
accuracy
and stability
of acrylic
resin dent
Robin Huggett, Alan Harrison, University
MSc, PhD,a Alkiviades Zissis, DDS,b BDS, PhD,e and Amanda Dennis, BSc, MScd of Bristol, Dental School, Bristol, England
Proponents of injection molding systems have claimed a number of benefits over conventional press-pack dough molding systems. The aim of this study was to evaluate a recently developed injection (dry heat) procedure of processing compared with press-pack dongh molding utilizing three curing cycles. The dimensional accuracy and stability of acrylic resin bases produced by the two molding procedures were compared. Dimensional changes were assessed over a period of 4 months using an optical comparator. The results demonstrate that baseplates produced by the injection molding procedure exhibit less shrinkage than those produced by the conventional press-pack procedures. (J PROSTHET DENT 1992;68:634-40.)
imensional accuracy measurements of the consplit-mold press-pack dough molding system are venti well documented and have been presented in a review.r Techniques using closed molding systems (pour resins and injection molding) have been used for many years, but have failed to achieve universal commercial acceptance.2-8 Proponents of injection molding systems claim the advantages of the elimination of flash and the consequent “raised bite” improved dimensional form and reduced porosity. Claims are also made for reduced production time and costs as well as reduced skin contact and vapor inhalation of the methyl methacrylate monomer. Several studies have compared dimensional accuracy and stability of injection and conventional processing of acrylic resins.5, 6,8-1QThis study evaluates a recently developed injection method of processing compared with the press-pack method. It also compares the dimensional accuracy and stability of acrylic resin denture bases produced by the two methods.
IAL
AND
METHODS
A brass master die was machined to simulate the shape of an edentulous arch in such a way that it could be used for either mandibular or maxillary jaw simulation. Symmetrically located index marks were incorporated by drilling small holes in specific locations; however, only those labeled A, B, C, were used in tlnis study. These were in the
Supported by a grant from the Bristol and Weston Health Authority Medical Research Committee. aLecturer, Department of Prosthetic Dentistry and Dental Care of the Elderly. bVisiting Lecturer. CProfessor, Department of Prosthetic Dentistry and Dental Care of the Elderly. dStatistics Consultant, Computkg Department.
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Fig. 1. The brass master die, permanent silicone elastomer mold, and a stone cast illustrating the index marks.
central incisor and left and right posterior regions (Fig. 1). A Dublisil permanent silicone elastomer mold (Panadent Ltd., London, England) was made of the master die. Forty casts were prepared by pouring Kaffir D (South Western Industrial Plasters, Chippenham, England) stone into the mold, ensuring that each cast was produced using the same powder-water ratio and construction technique. Following the conventional dental laboratory procedures, a 3 mm thick wax sheet was adapted and sealed to each cast. Acrylic resin baseplates were then constructed using each of two processing procedures.
Injection
molding
Ten baseplates were constructed by use of an injection molding system described by Knott? and by Huggett et OCTOBER1992
VOLUME68
NUMBER4
EVALUATION
OF MOLDING
SYSTEMS
Percent linear change of four curing methods
Fig. 2. Significance of the percentage of linear change of the four curing methods (MI to M4) using Tukey’s HSD test.
Table I. Analysis of variance Greenhouse
ss
SOWW
Between specimens Method 1 Error Within specimens Time Time x method 2 Error Dimension Method X dimension 3 Error Time x dimension Method X time X dimension 4 Error Total
10.96980 6.93993 0.56044 0.50427 4.38697 3.09413 1.94202 12.15919 0.05451 0.58726 7.53067 48.72919
DF
39 3 36 440 3 9 108 2 6 72 6 18 216 479
Geisser prob.
Huynh Feldt prob.
F
P
3.65660 0.19278
18.97
0.001
0.18681 0.05603 0.04062 1.54707 0.32367 0.16888 0.00909 0.03263 0.03486
4.60 1.38
0.0045 0.2063
0.0058 0.2127
0.0045 0.2063
9.16 1.92
0.0003 0.0897
0.0006 0.1025
0.0003 0.0921
0.26 0.94
0.9545 0.5360
0.9293 0.5236
0.9545 0.5361
MS
SS, Sum ofsquares;MS, mean square;prob,probability.
a1.12The polymer and monomer mix was introduced into a closed mold under a pressure of 6 atm at the “stringy” stage. The injection polymer was held under pressure during the thermostatically controlled polymerization process to ensure a flow of polymer to compensate for any contraction. The denture base material used was Meadway (Meadway Dental Supplies Ltd., Old Woking, England) poly (methyl methacrylate) heat-curing resin in a polymermonomer ratio of 3.2:1. Dry heat was used for polymerization using the curing cycle of room temperature to 100’ C in 1 hour and then holding for 30 minutes under 6 atm pressure. This procedure is coded IM.
Pressure-pack
dough
molding
Thirty test plates were constructed by the press-pack procedure, and the following three water bath curing cycles were used (10 baseplates for each): (1) rapid curing cycleTHE
JOURNAL
OF PROSTHETIC
DENTISTRY
100’ C for 20 minutes, bench cooling for 10 minutes, and immersion in cold water (coded RC); (2) overnight curing cycle-room temperature to 70’ C in 1 hour held at 70“ C for 7 hours (coded OC); and (3) overnight curing cycle-room temperature to 70” C in 1 hour, held at 70’ C for 7 hours, followed by 100° C for 3 hours (coded oc+).
Method
of measurement
Measurements were taken across the dimensions AB, BC, CA (Fig. 1) with a Nikon Optical Comparator instrument (Nikon Profile Projector 6CT, Rank Precision Industries Ltd., Leicester, England) at 10 power magnification. For each dimension, 10 readings were taken and the mean value was calculated. The coefficient of variation for the repeated measurements never exceeded 0.04%. 635
HUGGETT
ET AL
Analysis of linear changewith time
0 bows
28 days
4 mos.
24 hours
Tl
T3
T4
T2
-0.3161
-0.3459
-0.3848
-0.4043
3. Significance HSD test.
Fig.
of the percentage of linear change with time (T1 to 7’4) using Tukey’s
Analysis of effect of dimension
Dimension
AB
CA
BC
Dl
D3
D2
-0.2696
-0.3533
-0.4655
Fip;. 4. Significance of the percentage of linear change of the three dimensions (01 to 03) us&g Tukey’s HSD test. -
Table
II. Distance AB: Percent change
Procedure IM
RC OC 0C-t
0 hr -0.044 -0.293 -0.258 -0.280
24
hr
-0.083 -0.493 -0.362 -0.371
28 days
4mo
+0.060 -0.545 -0.294 -0.242
-0.070 -0.441 -0.339 -0.255
IM, Room temperature to 100’ C in 1 hour, held for 30 minutes at 6 atm; RC (rapid cure), 100’ C for 20 minutes, bench-cooled for 10 minutes, immersed in cold water; OC (overnight cure), room temperature to 70” C in 1 hour, held at 70° C for 7 hours; OC+, ~oorn temperature to 70’ C in 1 hour, held at 70° C for 7 hours, then 100” C for 3 hours.
The measurements were taken on five occasions. These were: 1. On the stone cast (considered to be the starting, or zero point, for the dimensional change) 2. On the corresponding baseplate immediately after deflasking (0 hour)
636
3. After immersion in water at 37“ C for 24 hours 4. After immersion in water at 37” C for 28 days 5. After immersion in water at 37O C for 4 months The data obtained were analyzed using analysis of variance with repeated measures.
RESULTS The results of the percentage of change are summarized in Table I. All three main effects of method (M), time (T), and dimension (D) were significant (p < O.Ol), while none of the interactions was significant. Tukey’s HSD test was used to compare means and revealed that (Qa.s5,se = 3.79; MS error = 0.1928) method 1, the injection method, was different from methods 2,3, and 4, with method 1 showing less change than the other three methods. Method 3 was also significantly different from method 2, with method 2 showing the greatest change (Fig. 2). Analysis of the time effect (Fig. 3) revealed that time 1 was significantly different from times 4 and 2 (Qs.ss.res
OCTOBER
1992
VOLUME
68
NUMBER
4
EVALUATION
OF MOLDZNG
SYSTEMS
0.1 O-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -
-
0 hour
I
4
24 hours
26 day.8
1
4 months
Time m m
Fig.
Procedure procedure
IM OC
5. Percentage
m
of change in relation
RC
Procedure
00
to time for the dimension
24 h’oura
0 h’our
Procedure
26 day8
AB.
4 months
Time
Fig.
m
Procedure
IM
m
Procedure
OC
6. Percentage
m
of change in relation
= 3.68; MS error = 0.0406), time 2 showing the greatest change and time 1 showing the smallest change. An analysis of the effect of dimension is presented in Fig. 4. Dimensions 1 and 3 were significantly different from dimension 2 (Qe,e5,7s = 3.4; MS error = 0.1689), and there was no apparent reason for this uneven contraction. All 40 baseplates, under the conditions of measurement used in this study, showed a shrinkage for all three dimensions at each time period. The only exceptions were the injection molded baseplates, which showed a mean expansion for the AB dimension of 0.06 % after immersion in water for 28 days; this difference, however, was not significant as there were no significant interactions. For the dimension AB, in all procedures there was a trend toward uneven contraction after immersion in water (Table II and Fig. 5).
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DENTISTRY
Procedure
RC
Procedure
00
to time for the dimension
Table III. Procedure IM RC oc oc+
BC.
Distance BC: Percent change 0 hr
24 hr
28 days
4mo
-0.184 -0.453 -0.403 -0.623
-0.218 -0.603 -0.499 -0.659
-0.284 -0.555 -0.454 -0.555
-0.257 -0.602 -0.438 -0.660
Abbreviations as in Table II.
Table III and Fig. 6 show that all the procedures displayed similar behavior in the BC dimension-that is, shrinkage after deflasking and further shrinkage after immersion in water at all time periods. Again, the injection molded baseplate (IM) exhibited less contraction than all other
637
HUGGETT
ET AL
-0.4
-0.6
4 months
26 days
24 hours
0 hour
Time m
Procedure
D
Fig.
Procedure
IM
m
Procedure
RC
OC
m
Procedure
00
7. Percentage of change in relation to time for the dimension CA.
0 -0.2 -0.4 -0.6 -0.8 -1 - 1.2 -1.4
u
I 24 hours
I
0 hour
/ 28 days
/ 4 months
Time m
Procedure
IM
0
Procedure
OC
&@
Procedure
RC
Procedure
OC+
Fig. 8. Percentage of change in relation to time for the area ABC.
Table
IV.
Procedure IM RC oc oc+
Distance CA: Percent change 0 hr
24 hr
28 days
4mo
-0.119 -0.602 -0.234 -0.300
-0.061 -0.814 -0.300 -0.386
-0.131 -0.641 -0.271 -0.239
-0.194 -0.585 -0.415 -0.360
Abbreviations as in Table II.
baseplates and for all time periods. The third considered dimension, CA (Table IV and Fig. 7), exhibited the same changes. Again, the injection molded baseplates exhibited the least shrinkage.
638
There was a range of changes from -0.284 % to +0.060 % for the injection molded baseplates (IM), from -0.293% to -0.814% for the rapid curing procedure (RC), from -0.234% to -0.499% for the overnight procedure (OC), and from -0.239 % to -0.660 % for the overnight procedure with a terminal boil (OC+). Since there were no significant interactions, the various methods may be considered to have similar effects over time and dimension. Further consideration was given to the changes in the area limited between the index marks (A,B, and C). This area, defined ABC (A), is a triangle with irregular sides. Using the mathematical formula A = $S(S - A)(S - B) (S - C), where S = A + B + C, and A, B, and C = length of the sides of the triangle, the changes in the area can be de-
OCTOBER
1992
VOLUME
68
NUMBER
4
EVALUATION
OF MOLDING
SYSTEMS
Area change of four curing methods
I
IM
oc
oc+
RC
Ml
M3
M4
M2
-0.278
-0.671
-0.771
-1.114
Fig. 9. Significance of the percentage of area change of the four curing methods (Ml to M3) using Tukey’s HSD test.
Analysis of area change with time
0 hours
28 days
4 mos.
24 hours
Tl
T3
T4
T2
-0.626
-0.698
-0.738
-0.816
10. Significance of the percentage of area change with time (77 to T4) using Tukey’s
Fig.
HSD test.
Table
Analysis
V.
of variance Greenhouse
Source
Between specimens Method 1 Error Within specimens Time Time X method 2 Error Total
ss
13.46259 9.66226 0.70982 0.31495 6.17332
DF
38 3 35 117 3 9 105 155
F
P
4.48753 0.27606
16.26
0.001
0.23661 0.03499 0.05879 30.34917
4.02 0.60
0.0094 0.7985
MS
Geisser prob.
0.0118 0.7837
Huynh Feldt prob.
0.0094 0.7985
Abbreviations as in Table I.
termined.13 Table V shows that both the main effects of method and time are significant (p < O.Ol), with a nonsignificant method by time interaction (it was not possible to determine the area of one of these specimens).
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DENTISTRY
From Table VI and Fig. 8, it can be seen that the changes that occurred were minimal in the injection molding procedure for all four periods of time (range of changes, -0.234 % to -0.319%). The changes that occurred with the
639
HUGGETT
Table
VP. Area ABC: Percent change 24 hr
28 days
4mo
-0.269
-0.319
IM RC
-0.234 -0.971
-1.274
-1.125
-1.088
oc
-0.590
-0.716
-0.673
-0.707
OW
-0.671
-0.933
-0.683
-0.797
Abbreviations
-0.289
as in Table II.
rapid curing procedure (RC) were the largest (-0.971% to -1.274%). The changes that occurred with both overnight curing cycles OC (-0.590% to -0.716%) and OC+ (-0.671% to -0.933 %) were smaller than those of the rapid cure procedure. Tukey’s HSD test shows similar results. Method 1 was again significantly different from methods 2, 3, and 4 (Qa,os,es= 3.85; MS error = 0.2761), and method 2 was significantly different from methods 3 and 4. Method 1 showed the smallest change, and method 2 showed the greatest change (Fig. 9). Fig. 10 shows that results similar to the above were found with the time effect; time 1 was significantly different from time 2 (Qo.os,1os= 3.68; MS error = 0.0588).
The results demonstrate that the injection molding procedure used produced baseplates that exhibited statistically significantly less contraction than the three conventional processing cycles with the press-pack procedure. There have not been any other reports concerning the dimensional stability of baseplates produced by the injection molding procedure described; however, the findings of other studies concerning other injection methods and using the same measuring and testing processes are noteworthy. Goodkind and Schulte3 reported that there were no significant changes in dimensional stability of bases produced by either the pour technique or with conventional autopolymerizing resin. Murphy et a1.,gAnderson et al.,1° and Freilich et a1.14reported that the SR-Ivocap injection system (Ivoclar-Vivadent, Ltd., Lekester, England) exhibited less polymerization shrinkage than a conventional press-pack system. Therefore from this and previous reports it would seem that the injection molding procedure
640
ET AL
has the advantage, when comparing dimensional accuracy and stability, over the conventional press-pack method. CONCLUSION With a newly developed injection molding procedure (Isomate Accord system ‘I, I’), it has been shown that baseplates can be produced that exhibit less shrinkage than those produced by the conventional press-pack procedures. These findings were statistically significant, and are in agreement with those of previous studies. We thank Mr. N. J. Knott for the useof the IsomateAccord inmolding system.
jection
REFERENCES 1. Zissis A, Huggett R, Harrison A. Measurement methods used for the determination of dimensional accuracy and stability of denture base materials-a review. J Dent 1991;19:199-206. 2. Pryor WJ. Injection molding of plastics for dentures. J Am Dent Assoc 1942;29:1400-8. 3. Goodkind A, Schulte J. Dimensional accuracy of pour resin and conventional processing of denture base acrylic resin. J PXOSTHETDENT 1970;24:662-8. 4. Lyon FF, Anderson JN. The Lyon injector. Dent Tech 1979;32:4-8. 5. Garfunkel E. Evaluation of dimensional change in complete dentures processed by injection-pressing and pack-press technique. d PROSTHET
DENT 1983;50:757-61. 6. MacGregor AR, Graham J, Stafford GD, Huggett R. Recent experiences with denture base polymers. J Dent 1984;12:146-57. 7. Dukes BS, Fields H, Olson JW, Scheetz JP. A laboratory study of changes in vertical dimension using a compression molding and a pour resin technique. J PROSTHETDENT 1985;53:667-9. 8. Stafford GD, Huggett R, MacGregor AR, Graham J. The use of nylon as a denture base material. J Dent 1986;14:18-22. 9. Murphy WM, Bates JF, Huggett R, Bright R. A comparative study of three denture base materials. Br Dent 3 1982;152:263-73. 10. Anderson J, Schulte J, Arnold T. Dimensional stability of injection and conventional processing of denture base acrylic resin. 3 PROSTHETDENT
1988;60:394-8. 11. Knott NJ. Isomate Accord systems. Oxford, U.K.: Proceedings Br Sot Study Prosthet Dent, 1985. 12. Huggett R, Bates JF, Knott NJ. A comparison of some properties of denture base acrylic resins polymerized by dry and wet curing systems. Quintessence Dent Tech 1987;11:265-9. 13. DicksonLE. Plane trigonometry. New York: ChelseaPubl, 1975;129-34. 14. Freilich S, Dirckx JJJ, Goodacre CJ, Swartz ML, Andres CJ. Moire topography for measuring the dimensional accuracy of resin complete denture bases. Int J Prosthodont 1989;2:272-9.
Reprint requests to: DR.ROBIN HUCGETT DENTAL SCHOOL LOWERMAUDLINST. BRISTOL BSl2LY ENGLAM)
OCTOBER
1992
VOLUME
68
NUMBER
4
accuracy
and stability
of acrylic
resin dent
Robin Huggett, Alan Harrison, University
MSc, PhD,a Alkiviades Zissis, DDS,b BDS, PhD,e and Amanda Dennis, BSc, MScd of Bristol, Dental School, Bristol, England
Proponents of injection molding systems have claimed a number of benefits over conventional press-pack dough molding systems. The aim of this study was to evaluate a recently developed injection (dry heat) procedure of processing compared with press-pack dongh molding utilizing three curing cycles. The dimensional accuracy and stability of acrylic resin bases produced by the two molding procedures were compared. Dimensional changes were assessed over a period of 4 months using an optical comparator. The results demonstrate that baseplates produced by the injection molding procedure exhibit less shrinkage than those produced by the conventional press-pack procedures. (J PROSTHET DENT 1992;68:634-40.)
imensional accuracy measurements of the consplit-mold press-pack dough molding system are venti well documented and have been presented in a review.r Techniques using closed molding systems (pour resins and injection molding) have been used for many years, but have failed to achieve universal commercial acceptance.2-8 Proponents of injection molding systems claim the advantages of the elimination of flash and the consequent “raised bite” improved dimensional form and reduced porosity. Claims are also made for reduced production time and costs as well as reduced skin contact and vapor inhalation of the methyl methacrylate monomer. Several studies have compared dimensional accuracy and stability of injection and conventional processing of acrylic resins.5, 6,8-1QThis study evaluates a recently developed injection method of processing compared with the press-pack method. It also compares the dimensional accuracy and stability of acrylic resin denture bases produced by the two methods.
IAL
AND
METHODS
A brass master die was machined to simulate the shape of an edentulous arch in such a way that it could be used for either mandibular or maxillary jaw simulation. Symmetrically located index marks were incorporated by drilling small holes in specific locations; however, only those labeled A, B, C, were used in tlnis study. These were in the
Supported by a grant from the Bristol and Weston Health Authority Medical Research Committee. aLecturer, Department of Prosthetic Dentistry and Dental Care of the Elderly. bVisiting Lecturer. CProfessor, Department of Prosthetic Dentistry and Dental Care of the Elderly. dStatistics Consultant, Computkg Department.
IO/l/37922
634
Fig. 1. The brass master die, permanent silicone elastomer mold, and a stone cast illustrating the index marks.
central incisor and left and right posterior regions (Fig. 1). A Dublisil permanent silicone elastomer mold (Panadent Ltd., London, England) was made of the master die. Forty casts were prepared by pouring Kaffir D (South Western Industrial Plasters, Chippenham, England) stone into the mold, ensuring that each cast was produced using the same powder-water ratio and construction technique. Following the conventional dental laboratory procedures, a 3 mm thick wax sheet was adapted and sealed to each cast. Acrylic resin baseplates were then constructed using each of two processing procedures.
Injection
molding
Ten baseplates were constructed by use of an injection molding system described by Knott? and by Huggett et OCTOBER1992
VOLUME68
NUMBER4
EVALUATION
OF MOLDING
SYSTEMS
Percent linear change of four curing methods
Fig. 2. Significance of the percentage of linear change of the four curing methods (MI to M4) using Tukey’s HSD test.
Table I. Analysis of variance Greenhouse
ss
SOWW
Between specimens Method 1 Error Within specimens Time Time x method 2 Error Dimension Method X dimension 3 Error Time x dimension Method X time X dimension 4 Error Total
10.96980 6.93993 0.56044 0.50427 4.38697 3.09413 1.94202 12.15919 0.05451 0.58726 7.53067 48.72919
DF
39 3 36 440 3 9 108 2 6 72 6 18 216 479
Geisser prob.
Huynh Feldt prob.
F
P
3.65660 0.19278
18.97
0.001
0.18681 0.05603 0.04062 1.54707 0.32367 0.16888 0.00909 0.03263 0.03486
4.60 1.38
0.0045 0.2063
0.0058 0.2127
0.0045 0.2063
9.16 1.92
0.0003 0.0897
0.0006 0.1025
0.0003 0.0921
0.26 0.94
0.9545 0.5360
0.9293 0.5236
0.9545 0.5361
MS
SS, Sum ofsquares;MS, mean square;prob,probability.
a1.12The polymer and monomer mix was introduced into a closed mold under a pressure of 6 atm at the “stringy” stage. The injection polymer was held under pressure during the thermostatically controlled polymerization process to ensure a flow of polymer to compensate for any contraction. The denture base material used was Meadway (Meadway Dental Supplies Ltd., Old Woking, England) poly (methyl methacrylate) heat-curing resin in a polymermonomer ratio of 3.2:1. Dry heat was used for polymerization using the curing cycle of room temperature to 100’ C in 1 hour and then holding for 30 minutes under 6 atm pressure. This procedure is coded IM.
Pressure-pack
dough
molding
Thirty test plates were constructed by the press-pack procedure, and the following three water bath curing cycles were used (10 baseplates for each): (1) rapid curing cycleTHE
JOURNAL
OF PROSTHETIC
DENTISTRY
100’ C for 20 minutes, bench cooling for 10 minutes, and immersion in cold water (coded RC); (2) overnight curing cycle-room temperature to 70’ C in 1 hour held at 70“ C for 7 hours (coded OC); and (3) overnight curing cycle-room temperature to 70” C in 1 hour, held at 70’ C for 7 hours, followed by 100° C for 3 hours (coded oc+).
Method
of measurement
Measurements were taken across the dimensions AB, BC, CA (Fig. 1) with a Nikon Optical Comparator instrument (Nikon Profile Projector 6CT, Rank Precision Industries Ltd., Leicester, England) at 10 power magnification. For each dimension, 10 readings were taken and the mean value was calculated. The coefficient of variation for the repeated measurements never exceeded 0.04%. 635
HUGGETT
ET AL
Analysis of linear changewith time
0 bows
28 days
4 mos.
24 hours
Tl
T3
T4
T2
-0.3161
-0.3459
-0.3848
-0.4043
3. Significance HSD test.
Fig.
of the percentage of linear change with time (T1 to 7’4) using Tukey’s
Analysis of effect of dimension
Dimension
AB
CA
BC
Dl
D3
D2
-0.2696
-0.3533
-0.4655
Fip;. 4. Significance of the percentage of linear change of the three dimensions (01 to 03) us&g Tukey’s HSD test. -
Table
II. Distance AB: Percent change
Procedure IM
RC OC 0C-t
0 hr -0.044 -0.293 -0.258 -0.280
24
hr
-0.083 -0.493 -0.362 -0.371
28 days
4mo
+0.060 -0.545 -0.294 -0.242
-0.070 -0.441 -0.339 -0.255
IM, Room temperature to 100’ C in 1 hour, held for 30 minutes at 6 atm; RC (rapid cure), 100’ C for 20 minutes, bench-cooled for 10 minutes, immersed in cold water; OC (overnight cure), room temperature to 70” C in 1 hour, held at 70° C for 7 hours; OC+, ~oorn temperature to 70’ C in 1 hour, held at 70° C for 7 hours, then 100” C for 3 hours.
The measurements were taken on five occasions. These were: 1. On the stone cast (considered to be the starting, or zero point, for the dimensional change) 2. On the corresponding baseplate immediately after deflasking (0 hour)
636
3. After immersion in water at 37“ C for 24 hours 4. After immersion in water at 37” C for 28 days 5. After immersion in water at 37O C for 4 months The data obtained were analyzed using analysis of variance with repeated measures.
RESULTS The results of the percentage of change are summarized in Table I. All three main effects of method (M), time (T), and dimension (D) were significant (p < O.Ol), while none of the interactions was significant. Tukey’s HSD test was used to compare means and revealed that (Qa.s5,se = 3.79; MS error = 0.1928) method 1, the injection method, was different from methods 2,3, and 4, with method 1 showing less change than the other three methods. Method 3 was also significantly different from method 2, with method 2 showing the greatest change (Fig. 2). Analysis of the time effect (Fig. 3) revealed that time 1 was significantly different from times 4 and 2 (Qs.ss.res
OCTOBER
1992
VOLUME
68
NUMBER
4
EVALUATION
OF MOLDZNG
SYSTEMS
0.1 O-0.1 -0.2 -0.3 -0.4 -0.5 -0.6 -
-
0 hour
I
4
24 hours
26 day.8
1
4 months
Time m m
Fig.
Procedure procedure
IM OC
5. Percentage
m
of change in relation
RC
Procedure
00
to time for the dimension
24 h’oura
0 h’our
Procedure
26 day8
AB.
4 months
Time
Fig.
m
Procedure
IM
m
Procedure
OC
6. Percentage
m
of change in relation
= 3.68; MS error = 0.0406), time 2 showing the greatest change and time 1 showing the smallest change. An analysis of the effect of dimension is presented in Fig. 4. Dimensions 1 and 3 were significantly different from dimension 2 (Qe,e5,7s = 3.4; MS error = 0.1689), and there was no apparent reason for this uneven contraction. All 40 baseplates, under the conditions of measurement used in this study, showed a shrinkage for all three dimensions at each time period. The only exceptions were the injection molded baseplates, which showed a mean expansion for the AB dimension of 0.06 % after immersion in water for 28 days; this difference, however, was not significant as there were no significant interactions. For the dimension AB, in all procedures there was a trend toward uneven contraction after immersion in water (Table II and Fig. 5).
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Procedure
RC
Procedure
00
to time for the dimension
Table III. Procedure IM RC oc oc+
BC.
Distance BC: Percent change 0 hr
24 hr
28 days
4mo
-0.184 -0.453 -0.403 -0.623
-0.218 -0.603 -0.499 -0.659
-0.284 -0.555 -0.454 -0.555
-0.257 -0.602 -0.438 -0.660
Abbreviations as in Table II.
Table III and Fig. 6 show that all the procedures displayed similar behavior in the BC dimension-that is, shrinkage after deflasking and further shrinkage after immersion in water at all time periods. Again, the injection molded baseplate (IM) exhibited less contraction than all other
637
HUGGETT
ET AL
-0.4
-0.6
4 months
26 days
24 hours
0 hour
Time m
Procedure
D
Fig.
Procedure
IM
m
Procedure
RC
OC
m
Procedure
00
7. Percentage of change in relation to time for the dimension CA.
0 -0.2 -0.4 -0.6 -0.8 -1 - 1.2 -1.4
u
I 24 hours
I
0 hour
/ 28 days
/ 4 months
Time m
Procedure
IM
0
Procedure
OC
&@
Procedure
RC
Procedure
OC+
Fig. 8. Percentage of change in relation to time for the area ABC.
Table
IV.
Procedure IM RC oc oc+
Distance CA: Percent change 0 hr
24 hr
28 days
4mo
-0.119 -0.602 -0.234 -0.300
-0.061 -0.814 -0.300 -0.386
-0.131 -0.641 -0.271 -0.239
-0.194 -0.585 -0.415 -0.360
Abbreviations as in Table II.
baseplates and for all time periods. The third considered dimension, CA (Table IV and Fig. 7), exhibited the same changes. Again, the injection molded baseplates exhibited the least shrinkage.
638
There was a range of changes from -0.284 % to +0.060 % for the injection molded baseplates (IM), from -0.293% to -0.814% for the rapid curing procedure (RC), from -0.234% to -0.499% for the overnight procedure (OC), and from -0.239 % to -0.660 % for the overnight procedure with a terminal boil (OC+). Since there were no significant interactions, the various methods may be considered to have similar effects over time and dimension. Further consideration was given to the changes in the area limited between the index marks (A,B, and C). This area, defined ABC (A), is a triangle with irregular sides. Using the mathematical formula A = $S(S - A)(S - B) (S - C), where S = A + B + C, and A, B, and C = length of the sides of the triangle, the changes in the area can be de-
OCTOBER
1992
VOLUME
68
NUMBER
4
EVALUATION
OF MOLDING
SYSTEMS
Area change of four curing methods
I
IM
oc
oc+
RC
Ml
M3
M4
M2
-0.278
-0.671
-0.771
-1.114
Fig. 9. Significance of the percentage of area change of the four curing methods (Ml to M3) using Tukey’s HSD test.
Analysis of area change with time
0 hours
28 days
4 mos.
24 hours
Tl
T3
T4
T2
-0.626
-0.698
-0.738
-0.816
10. Significance of the percentage of area change with time (77 to T4) using Tukey’s
Fig.
HSD test.
Table
Analysis
V.
of variance Greenhouse
Source
Between specimens Method 1 Error Within specimens Time Time X method 2 Error Total
ss
13.46259 9.66226 0.70982 0.31495 6.17332
DF
38 3 35 117 3 9 105 155
F
P
4.48753 0.27606
16.26
0.001
0.23661 0.03499 0.05879 30.34917
4.02 0.60
0.0094 0.7985
MS
Geisser prob.
0.0118 0.7837
Huynh Feldt prob.
0.0094 0.7985
Abbreviations as in Table I.
termined.13 Table V shows that both the main effects of method and time are significant (p < O.Ol), with a nonsignificant method by time interaction (it was not possible to determine the area of one of these specimens).
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
From Table VI and Fig. 8, it can be seen that the changes that occurred were minimal in the injection molding procedure for all four periods of time (range of changes, -0.234 % to -0.319%). The changes that occurred with the
639
HUGGETT
Table
VP. Area ABC: Percent change 24 hr
28 days
4mo
-0.269
-0.319
IM RC
-0.234 -0.971
-1.274
-1.125
-1.088
oc
-0.590
-0.716
-0.673
-0.707
OW
-0.671
-0.933
-0.683
-0.797
Abbreviations
-0.289
as in Table II.
rapid curing procedure (RC) were the largest (-0.971% to -1.274%). The changes that occurred with both overnight curing cycles OC (-0.590% to -0.716%) and OC+ (-0.671% to -0.933 %) were smaller than those of the rapid cure procedure. Tukey’s HSD test shows similar results. Method 1 was again significantly different from methods 2, 3, and 4 (Qa,os,es= 3.85; MS error = 0.2761), and method 2 was significantly different from methods 3 and 4. Method 1 showed the smallest change, and method 2 showed the greatest change (Fig. 9). Fig. 10 shows that results similar to the above were found with the time effect; time 1 was significantly different from time 2 (Qo.os,1os= 3.68; MS error = 0.0588).
The results demonstrate that the injection molding procedure used produced baseplates that exhibited statistically significantly less contraction than the three conventional processing cycles with the press-pack procedure. There have not been any other reports concerning the dimensional stability of baseplates produced by the injection molding procedure described; however, the findings of other studies concerning other injection methods and using the same measuring and testing processes are noteworthy. Goodkind and Schulte3 reported that there were no significant changes in dimensional stability of bases produced by either the pour technique or with conventional autopolymerizing resin. Murphy et a1.,gAnderson et al.,1° and Freilich et a1.14reported that the SR-Ivocap injection system (Ivoclar-Vivadent, Ltd., Lekester, England) exhibited less polymerization shrinkage than a conventional press-pack system. Therefore from this and previous reports it would seem that the injection molding procedure
640
ET AL
has the advantage, when comparing dimensional accuracy and stability, over the conventional press-pack method. CONCLUSION With a newly developed injection molding procedure (Isomate Accord system ‘I, I’), it has been shown that baseplates can be produced that exhibit less shrinkage than those produced by the conventional press-pack procedures. These findings were statistically significant, and are in agreement with those of previous studies. We thank Mr. N. J. Knott for the useof the IsomateAccord inmolding system.
jection
REFERENCES 1. Zissis A, Huggett R, Harrison A. Measurement methods used for the determination of dimensional accuracy and stability of denture base materials-a review. J Dent 1991;19:199-206. 2. Pryor WJ. Injection molding of plastics for dentures. J Am Dent Assoc 1942;29:1400-8. 3. Goodkind A, Schulte J. Dimensional accuracy of pour resin and conventional processing of denture base acrylic resin. J PXOSTHETDENT 1970;24:662-8. 4. Lyon FF, Anderson JN. The Lyon injector. Dent Tech 1979;32:4-8. 5. Garfunkel E. Evaluation of dimensional change in complete dentures processed by injection-pressing and pack-press technique. d PROSTHET
DENT 1983;50:757-61. 6. MacGregor AR, Graham J, Stafford GD, Huggett R. Recent experiences with denture base polymers. J Dent 1984;12:146-57. 7. Dukes BS, Fields H, Olson JW, Scheetz JP. A laboratory study of changes in vertical dimension using a compression molding and a pour resin technique. J PROSTHETDENT 1985;53:667-9. 8. Stafford GD, Huggett R, MacGregor AR, Graham J. The use of nylon as a denture base material. J Dent 1986;14:18-22. 9. Murphy WM, Bates JF, Huggett R, Bright R. A comparative study of three denture base materials. Br Dent 3 1982;152:263-73. 10. Anderson J, Schulte J, Arnold T. Dimensional stability of injection and conventional processing of denture base acrylic resin. 3 PROSTHETDENT
1988;60:394-8. 11. Knott NJ. Isomate Accord systems. Oxford, U.K.: Proceedings Br Sot Study Prosthet Dent, 1985. 12. Huggett R, Bates JF, Knott NJ. A comparison of some properties of denture base acrylic resins polymerized by dry and wet curing systems. Quintessence Dent Tech 1987;11:265-9. 13. DicksonLE. Plane trigonometry. New York: ChelseaPubl, 1975;129-34. 14. Freilich S, Dirckx JJJ, Goodacre CJ, Swartz ML, Andres CJ. Moire topography for measuring the dimensional accuracy of resin complete denture bases. Int J Prosthodont 1989;2:272-9.
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OCTOBER
1992
VOLUME
68
NUMBER
4
