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Evaluation of dentures processed by different techniques
EVALUATION PROCESSED
F.A.
PEYTON,
University
of
OF DENTURES BY DIFFERENT
DSc., AND D.H. Michigan,
TECHNIQUES
ANTHONY,
D.M.D.,M.S.
School of Dentistry,
Ann Arbor,
Mich.
THE PAST 2.5 YEARS, since the introduction of the acrylic resin denture base material in 1937, there have been many suggested variations in the techniques of processing these materials, with several additional materials proposed for the same purpose. Often these variations have been introduced with a claim of superior properties, improved accuracy of fit, and greater dimensional stability in service. The full significance of these claims on the function and performance of the denture in service has not always been clear. Because of the advantages claimed for the various denture base materials and the modified denture processing methods which have been introduced within recent years, an over-all evaluation was considered to be desirable. The exactness of the duplication of the detnures of the final impression and the accuracy of fit of the dentures made of various materials were determined by means of a modified comparator.le3 This modified panographic comparator employed dial gauges to record the anterior, posterior, and lateral positions of the carriage, so that reference points on the master impression and the dentures could be relocated for measurement with the vertical dial gauge. Other investigators have used a variety of methods to determine the dimensional change during processing and subsequent warpage of dentures. The change in dimension may be reflected by the fit of the denture on a stone cast, formed from the bearing surface of the denture,4 simply by placing the dentures lightly on the master casts and observing the discrepanciesat the denture border,5 or by measuring the discrepancies at the posterior border while the denture is held in position.6 Others have employed metal pins and bench marks in the molar tooth region or in the posterior lateral borders of the dentures, then measuring any changes with the aid of micrometer microscopes.rpsIt has been pointed out that usually upper dentures placed lightly on a master cast will appear to contact the cast in the flange regions, will be deficient at the buccal borders, and will show the greatest discrepancy at the central portion of the posterior border .2 Lower dentures usually appear to fit least accurately along the posterior lingual borders in relation to the master cast. These methods all lack the advantage of registering the actual contour of the
D
URING
These studies were supported by contract No. DA-49-007-MD-860 between the Office of the Surgeon General, Department of the Army, and the University of Michigan. A portion of this report is from a thesis submitted by D. H. Anthony to the Horace H. Rackham School of Graduate Studies, University of Michigan, as a partial fulfillment for the Master of Science degree in Dental Materials, 1961. Presented before the Academy of Denture Prosthatics, Colorado Springs, Cola. 269
270
PEYTON
AND
ANTHONY
J. Pros. March-April,
Den. 1963
tissue-bearing surface of the denture, and of comparing this contour with the contour of a master impression, which is possible with the instrument employed in this study. One contour meter has been described which was used to evaluate the denture adaptation ,O but the instrument seems to lack some degree of convenience in repositioning and recording results. MATERIALS
STUDIED
Since it was one purpose of this study to compare different types of materials as well as techniques of processing, a wide range of denture base materials were included. Four different self-cured types of material were employed ; seven heatcured acrylic resin materials were used ; and three special injection products were studied, which represented an acrylic resin, a styrene, and a vinylacrylic type of material. Two chromium-cobalt alloys were cast to form full coverage upper and lower denture bases. One epoxy-type denture plastic was included in the study, as was a vulcanite denture material and an all-porcelain base denture for purposes of comparison of these two types of material. This represents, therefore, a total of 19 different products of 9 different basic types of materials or techniques of use. This group was thought to be representative of the type of materials and recommended techniques currently available. In this report, however, comparisons of only the 3 principal types of materials are presented, with the results reported being limited primarily to the maxillary dentures. The recommended directions of the manufacturer were employed for each denture material processed. In order to eliminate undesirable variables among the dentures of various materials, each stone cast was formed in the same silver plated upper or lower master impression. A consistent thickness was secured in the construction of waxed dentures by pouring molten wax into a rubber mold which held the porcelain teeth and stone cast in the correct relationship to one another. All laboratory procedures were performed under as nearly identical conditions as possible, yet consistent with the manufacturers’ recommended pr0cedure.l After the dentures were deflasked they were stored in water for 24 hours at room temperature before the measurements were recorded. This procedure simulated the interval of time before delivery to the patient. During all mounting and measuring operations, mouth temperature, approximately, was maintained by an electrical heating tape placed around and beneath the denture, since early measurements indicated that dentures tend to expand slightly and to fit more accurately when they are warmed to body temperature. RESULTS
OF MEASUREMENTS
The measurements made at each 0.025 inch interval across a selected ‘section of the upper or lower denture were plotted on a graph with similar related measurements from the master impression. These plotted measurements resulted in the development of a graph for the product when they were measured laterally through the second molar section. Typical graphs of different types of material were made and some have been presented in previous reports.lT2 Similar contours for the various dentures were recorded and reproduced in
Volun1e 13 Number 2
DENTURES
PROCESSFB
BY
DIFFERENT
TECHNIQUES
271
graph form for both the transverse and midline sections. It is not practical or possible to present all the graphs for comparison of the discrepancies of fit. For this reason, selected regions in the upper transverse section, the upper midline section, and the lower midline and transverse sections were used to determine the average discrepancies between the dentures and the master impression in the regions determined. The sum of the average discrepancies in these clinically significant regions was taken as the basis for arranging the dentures in the order of accuracy of fit. The smallest number of average discrepancies represented the most accurate dentures. This order of accuracy has been presented in detail in a previous report.’ Table I presents these average discrepancies of several types which separate the self-cured, heat-cured, and special injection-type dentures into well-defined groups. These represent the three principal types of denture plastics currently available. The porcelain base, epoxy, vulcanite, and cast chromium-cobalt denture bases, previously described in some detail are employed in relatively small numbers. Table II shows the discrepancies in fit of the upper denture in thousandths of an inch in the selected regions through the transverse section of the second molar region. This is a distinct comparison of the three principal types of plastic denture materials, relative to the areas in which the discrepancies exist. Similar discrepancies and differences of fit of the various materials were observed also from measurements in the midline section of the upper and the midline and transverse sections of the lower dentures.
TABLE
I.
TYPICAL
RANGE
OF CONTOUR
DISCREPANCIESOF MOLAR SECTION
UPPER
DENTURESTHROUGH
SECOND
---___ I Self-cured EPOXY
(Densene) type
Chromium-cobalt No. 1 Vulcanite Heat-cured (Densene 33) Chromium-cobalt No. 2 Special injection
EFFECT
OF
AGIXG
ON
DENTURE
TOTAL 53 60 70 1;: 144 17.5
CONTOUR
The changes which occurred during storage in water were relatively insignificant in practically all the dentures studied. On the basis of over-all fit, the changes in the general form of expansion were in proportion to the amount of shrinkage which took place during processing. The exceptions were in the vinyl type plastic, Luxene, and the polystyrene, Jectron, which may be related to physical properties of these two types of material. The self-cured dentures apparently contained few internal stresses as a result of the small amount of thermal shrinkage which occurred before they were removed from the molds. A certain amount of water was absorbed over a period of time and
272
PEYTON
TABLE
II.
TRANSVERSE
SECTION
AND
CONTOUROF
J. Pros. Den. March-April, 1963
ANTHONY UPPER
DENTURESIN~ECOND
MOLAR
REGIONS
DISCREPANCIESINTHOUSANDTHSOFINCHES MATER1AL*
-1-i
2
1
3
/
4
j
5
/
6
/
7
(
8
1
9
ToTAL
Self-Cured Heat-Cured Special Injection *Typical
examples
of type.
a slight amount of expansion occurred. The changes were only a few thousandths of an inch in most regions and the over-all effect was to allow a further seating of the denture on the cast. The discrepancies in contour ‘of representative groups of dentures of the three principal types of plastic materials when measured at various times are shown in Table III. The first values are given for the transverse section of upper dentures after they were deflasked and stored in water for 24 hours. The second measurements were obtained after the dentures had been stored in water for 8 months, and the third set after storage for 1 year and 8 months, for the self-cured and heatcured types. Each number listed in the chart represents the average of the discrepancies measured in the selected regions of 4 dentures of the same materials, processedin as nearly an identical manner as possible. In the transverse section, the largest discrepancies which occurred consistently in the heat-cured dentures were located at the lateral borders. The possibility of lateral shrinkage during processing was indicated by the relatively small discrepancies recorded in the middle of the flanges (regions Nos. 2 and 8) at the time of the first measurements. With aging, however, the discrepancies improved in these regions, with the indication that the dentures had expanded in the lateral dimension. The accuracy of fit seemedto have improved in the palatal region at the time of the second measurement, but after further aging the values were similar to those recorded originally. Similar observations resulted when measurements were made in the midline section of the denture. The special injection group of dentures produced varied results after storage in water, possibly because of the different plastics employed. The Tilon plastic,
Valume Number
13 2
DENTURES
PROCESSED
BY
DIFFERENT
273
TECHNIQUES
which is an acrylic gel, is processed at a higher heat than usual (278” F.). The excess heat, plus injection, may explain the higher processing shrinkage of these dentures. More changes occurred during storage of these dentures than with other acrylic resins, and the expansion resulted in an improvement of fit which eventually approached that of the more conventional heat-cured dentures. The Jectron dentures displayed a relatively high rate of molding shrinkage, possibly due to the high temperature (410” F.) required to soften the polystyrene material for injection, and the large cooling range after hardening. The low water sorption rate of the polystyrene may he the reason for the exceptional dimensional stability of these dentures, which may be considered to he the most stable plastic dentures evaluated. The Luxene dentures represent a blend of acrylic and vinyl plastics, and the high shrinkage during processing may be related to the tendency of the vinyl plastics to harden at a higher temperature as they cool from the softened state, even though the heat applied is not excessive (210” F.). After storage in water, these dentures did not expand as did the other plastic dentures. The low rate of water sorption of the material is possibly related to their relative stabi1it.y.
TABLE
III.
EFFECTOFAGINGON
CONTOURSOF
MAXILLARY
DENTURES
(Average of 4 Dentures)
-_____ -~~
DISCREPANCIES AGE
Hea$cyf
(Hy-Pro
1
j
2)
14
.: 6
:: 7
p
8 mo.
15
(
6
17
18
j
9
ToTAL
9 i
10
9
13 8
7 9
11
9
A 3
:
21 19
10
29
12
31
1:
lo”
.: 5
15
5
4 2 5
3” 9
::
79 58 a2
10 8 8
34 25 SO
27 35
175 181
88)
8 m;. 20 mo. Speciai
14
OF INCHES
Lucitone)
8 mo. 20 mo. .Self-y4txt (Acralile
3
INTHOUSANDTHS
-
4 7 (Jectron)
ii
2
: 6
:
; 5
4 5
27 26
21 lb
::
274 INFLUENCE
PEYTON
OF REPAIR
ON
DENTURE
AND
ANTHONY
J. Pros. March-April,
Den. 1963
CONTOUR
The fracture of a denture from any cause is often embarrassing to the dentist and is considered by most patients to be an emergency requiring prompt attention. The dentist, therefore, must be prepared to make repairs in as short a time as is practical and without altering the properties or fit of the denture. For many years the repairs have been made with heat-cured plastic in much the same manner as the original denture was processed, with the observation that frequently the re-cured denture showed varying degrees of warpage. When it was repaired at lower temperatures (165” F.) for longer periods of time, few changes in contour occurred. The self-cured plastics have become popular for denture repairs not only because of their convenience, but because of the fewer changes in contour. This study was undertaken to determine more precisely the changes of contour which occur when both heat-cured and self-cured dentures were repaired with either heat-curing or self-curing plastic material. The method of study consisted of : (1) recording the contours in selected regions of 4 heat-cured and 4 self-cured maxillary dentures which had been stored in water for approximately 20 months, (2) fracturing each of the dentures under load along the midline, (3) reassembling the pieces of the dentures and remeasuring the contours to insure accurate alignment, (4) repairing 2 dentures of each type by a self-curing method, (5) repairing 2 of each type by a heat-curing method, and (6) repeating the measurements of contour. A comparison of the data from these measurements should permit an evaluation of the effect of these methods of repair on the fit of the 2 types of dentures. FRACTURE
OF THE
DENTURES
ALONG
MIDLINE
After the contours of the dentures had been recorded, each denture was fractured along the midline. To ensure this type of fracture, the notch in the region of the labial frenum was extended l/s inch farther with a Carborundum disk, and another notch was prepared at the posterior border of each denture at the midline. A compressive load was applied with a testing machine through a tip which was tapered to a rounded edge l/16 inch wide and l/2 inch long. Each denture was placed with the teeth downward on a steel platform and the rounded edge of the load applicator was placed along the midline of the tissue surface of the denture at a distance vs inch from the posterior border. The porcelain teeth were cushioned by a single layer of felt which permitted a small lateral movement of the teeth and prevented the fracture of individual teeth as the dentures were bent downward under load. The application of the downward force along the midline of the inner surface of the dentures was designed to be equivalent to an upward force on the teeth on both sides in conjunction with an unyielding support in the central portion of the palate. Although this method was simple and convenient, it was not intended to be a standard compression test for maxillary dentures. The data for the loads at the initial cracking and the final separation of the pieces were recorded only as a matter of interest in order to compare the relative strength of this series of similar heatand self-cured dentures.
Volume Nnmber
13 2
DENTURES
PROCESSED
BY
DIFFERENT
27.5
TECHNIQUES
The rate of loading was set for 0.065 inch per minute. The loads which were recorded at the initial and the final break are listed for the heat-cured and selfcured dentures in Table IV. Both the heat-cured and self-cured dentures resisted an average force of approximately 139 pounds before the initial break occurred along the posterior portion of the midline. Considerable additional force was required in each instance to complete the fracture in the front. Although this was not intended to be a standard test for the strength of dentures, these values suggest that there is probably little difference in the strength of heat- and self-cured dentures when they are properly processed by controlled methods. TABLE
IV.
LOAD
REQUIRED
TOFRACTURE
DENTURES
---~ -.-_-___ ___
DENTURE
NO.
Heat-cured 107 108 110 111
sezf-“lr’7”d 118 119 120
INITIAL
BREAK
(LB.)
FINAL
BREAK
I
150 12.5 120 160
225 225 220 275
150 140 140 125
260
METHODSOFREPAIR
Heat-cured.--An occlusal index of dental stone was made for each denture before it was fractured to assist in the reassembly of the pieces in their correct relationship. After the fracture had occurred, the pieces were placed together by hand and held in position with sticky wax. Little difficulty was experienced in reassemblingthe parts and the stone index was required only to check the alignment. In order to eliminate the possibility of changes of contour which might have occurred as the result of misalignment of the parts, each denture, after it has been reassembled, was warmed to 100” F. and remounted on the comparator in the same manner as it was mounted before it was fractured. Differences of only 0.001 inch were permitted in measurements in selected regions before the denture was accepted for repair. Since no wax had been used in the fracture region, it was not necessary to apply heat to the denture. Warpage by procedures such as heating the flask or by pouring boiling water over the region of the joint, thereby, was eliminated. The full length of the fracture was widened slightly and prepared in a rounded “V” form with a fissure bur mounted in the handpiece of a laboratory dental engine. This method of preparation was chosen in order to ensure that the pieces of the denture would remain firmly in place on the cast without the change of their relative position, which might occur if they were removed from the cast for the preparation of the joint.
276
PEYTON
AND
ANTHONY
J. Pros. March-April,
Den. 1963
Heat-cured plastic (Hy-Pro Lucitone) was packed into the prepared joint after the exposed plastic was painted with monomer, and trial packing was completed in the conventional manner. A separator of the alginate type (Al-tote) was applied to the upper portion of the mold. When the flask had been closed in the flask press, it was set aside for 15 minutes before it was placed in a Hanau curing unit with the temperature of the bath at 169” F. for a period of 8 hours. The flask was permitted to cool to room temperature in the bath before deflasking was completed. The areas which had been repaired then were smoothed with plastic trimmers, but not polished, and the dentures were-stored in water at room temperature for 24 hours before the contours were measured. Self-cured.-The method employed for the self-cured repairs was similar in most respects to the method outlined for the heat-cured repairs. The assembly of the pieces of the dentures, pouring of the models, half-flasking, full-flasking, and the preparation of the joints were performed in the same manner. The material chosen for the repairs was Acralite 88 self-curing denture plastic, and the separator for the molds was Microphane, supplied by the same manufacturer. The edges of the joints to be repaired were painted with the monomer and the packing of the plastic was begun immediately when the “dough” stage was reached. Trial packing was finished in a short time and the flasks were permitted to remain in a flask press overnight to ensure complete curing. After the deflasking operation, the excess plastic was trimmed from the area of the repairs and the dentures were placed in water at room temperature for 24 hours before the contours were measured. CONTOUR
OF HEAT-CURED
DENTURES
AFTER
REPAIRS
Heat-cured Repairs.-The two dentures, Nos. 110 and 111, were selected for the evaluation of the effect of heat-cured repairs on heat-cured dentures. The data from the measurements of the contours of these 2 dentures were averaged in each region and, after the repairs had been completed, the contours were remeasured in the same regions for comparison. The data obtained from the selected regions in the transverse sections were recorded in Table V. The regions which were selected have been indicated on the contours drawn above the data. The changes of contour which occurred in each region are shown below the data. The negative signs indicate the loss of fit, measured at right angles to the denture surfaces. It became evident from the data that the heat-cured repairs caused a loss of ‘fit of the heat-cured dentures in spite of the controlled technique and the minimum amount of heat which was applied. Distortion occurred which would not permit the dentures to be seated closely on the cast, and in spite of repeated efforts ato mount the dentures so that the discrepancies at the lateral borders were nearly the same, the discrepancies were always a little greater on the left side than on the right. The sum of the average discrepancies in the transverse section was 94 before the repairs and after the repairs it had increased to 169. The difference between these values is another indication of the relatively large changes which resulted from repairs made by the heat-curing method.
z!ze~‘2”
DENTURES
PROCESSFdl
BY
DIFFERENT
277
TECHNIQUES
Self-cured Repairs.-The data obtained from the measurements of the contours of the remaining heat-cured dentures, Nos. 107 and 108, were averaged for comparison with the data from the same regions after the self-cured repairs. The discrepancies in the transverse sections, measured before and after repair, were recorded in Table V and the changes were shown below the data. It is apparent that few changes of contour occurred as a result of the selfcured repairs. A slight expansion of the flanges may have taken place which permitted a slightly closer fit in the palatal region. The sum of the discrepancies measured in the transverse section changed from 67 to 59, which was a further indication that practically no changes had occurred. V. CHANGESIN
TABLE
CONTOUR
OF HEAT-CURED FROM REPAIRS
DISCREPANCIESIN
) Heal-cured Before
1121
3
14
MAXILLARY
THOUSANDTHS
1.5
16
DENTURES
RESULTING
OF INCHES
17
I
181
9
IToTAL
repairs
After Change
1s
0
13 -“;
::
-6
Self-cy&epaks After Change
; 0
-1
;
$1
5”
5” +1
-10 7 +3”
-8 10 t-2
:: -16
+2
;
-3
5
12
2
:2” $1
94 169
67 59
CONTOUROFSELF-CUREDDENTURESAFTERREPAIRS
Heat-cured Repairs.-Two of the self-cured dentures, Nos. 117 and 120, were selected and the data for the contours in the selected regions were averaged for comparison with the data obtained after heat-cured repairs. The data obtained from the measurements and the amount of the changes which occurred were recorded in Table VI. It is evident from the table that the changes in the contours were small in both the transverse and midline sections. It would be difficult to conclude that measurable changes occurred when the accuracy of the method of measurement is taken into account. The sum of the discrepancies before fracture was 47 and after repair it
278
PEYTON
AND
J. Pros. March-April,
ANTHONY
Den. 1963
was 56. A large part of this change was recorded in one region, however, and the difference between these values is probably not significant. Self-cured Repairs.-The data for the contours of the remaining self-cured dentures, Nos. 118 and 119, were averaged for comparison with the data obtained after the self-cured repairs. The average discrepancies in the selected regions, which were determined both before and after the repairs, were recorded in Table VI. The data in this table indicate that practically no measurable changes have occurred as the result of self-cured repairs. The small change in the values of the total discrepancies in the transverse section, which changed from 53 to 59, was additional evidence of the few changes that were recorded. PROCESSING
TIME
STUDIES
The time required for each operation of the more important denture processing methods was recorded by means of a stop watch or taken from instruction manuals. Timing was done continuously during each operation such as half-flasking, fullflasking, hardening time of stone, and boil out or dig out after curing, and was repeated numerous times ; the resulting values were averaged. In this manner the basic times of the operations, which were common to many of the different techniques, were established. There were differences noted whether stone or plaster was used for half-flasking, or if a simple or double pour was used for full-flasking. The application of the separator and packing the molds varied and was determined separately for each technique. The curing time was clearly specified for each as well. TABLE
VI.
CHANGESIN
CONTOUROF
SELF-CURED FROM REPAIRS
MAXILLARY
DISCREPANCIESINTHOUSANDTHS (1
/2/
Heat-c;c~~~r&airs After Change
(4
/
:
7
7 ::
RESULTING
OF INCHES j
6
/
7
I
(81
9
3
-2’
% 0
-3
2::
5
5
7 2;
Se~-~.;~rrdEpairs After Change
3
DENTURES
5 -:
+1
2 -2
5
2
: 0
+3
ii 0
+3
:: 0
iToTAL
47 56
4
+z
i +3
+i
:.
1: -3
:;
Volunte Number
13 2
DENTURES
PROCESSED
BY
DIFFERENT
279
TECHNIQUES
Since one denture is rarely processed singly, the time intervals were recorded for each operation when they were performed in groups of four. This time interval was then divided by four to obtain the time intervals for a single operation under conditions of efficient operation. In addition to the time required for various operations and the total processing times, the intervals during certain steps, which did not require work by the laboratory technicians, were listed and totaled to determine the free time. This time presumably would be available for performing operations on other dentures in various stages of processing. It differs little for either one denture or a group, since it occurs when stone is hardening or during curing operations. From the total processing time and the amount of free time the actual working time was determined by subtraction. The working time was considered to be imTABLE
Jectron 1. 2. 3. 4. 2.
VII.
DENTURE
Luxene
MIN.
Half-flask (stone) Sprue Full-flask (stone) Stone harden ’ g;znout and separator
TIMES
PROCESSING
MIN.
13
1. Half-flask
ii 30
2. 3. 4. i,
13
Full-flask Stone harden Boil out Fulerator and pack
6: 7: Inject a. Cool 9. Dig out Total Free time 1Vorking
7. Cool 8. Dig out
2; 3
time
Working
Tilon 1. 2. 3. 4. 5. ;:
1.5
3 196 16.5
Total Free time
141 110 31
Half-flask (stone) Sprue . Full-flask (stone) Stone harden Boil out Fyerator and pack
3;: 6 35 90
time
31
Self-cured 1. Half-flask 2. Full-flask 3. Stone harden 4. Boil out i. perrator and
MIN. 10
3: pack
7. Dig out 8. Couol 9. Dig out
Total Free time
Total Free time Lliorking
164 120 time
Working
44 Heat-cured 1. Half-flask (plaster) 2. Full-flask (stone) 3. Stone harden 4. Boil out 5. Separator and pack 6. Cure 7. Cool 8. Dig out Total Free time U’orking
time
MIN. 10
3;: 6 10
180 30 3 273 240 33
time
213 180 33
PEYTON
280
AND
ANTHONY
portant, because it showed the amount of activity required by the technician and is, therefore, indicative of the relative number of dentures which would be processed if operations were continuous and new dentures were started frequently. A chart of all the data from these time studies was assembled and is shown in Table VII. According to this basis of comparison, the heat-cured technique consumed the greatest amount of time, mostly because of the combined curing and cooling time of 3$4 hours. The self-curing technique generally involves a minimum curing time of 2g hours. The mixing and packing operations are essentially the same for both the heat- and self-cured methods. The Tilon technique requires the greatest amount of working time because of the use of stone for half-flasking, sprues which must be attached before full-flasking, and the use of a double pour for this latter operation. The curing and cooling is accomplished in only 90 minutes. The Luxene technique requires somewhat less total time than the more conventional compression molding methods, but approximately the same working time is involved. The use of stone for half-flasking requires slight additional time, and more delays are caused by drying the separator film under infra-red lamps and by the procedure for filling the molds with the gel. The Jectron technique was considered to be the most efficient method studied. The actual working time per denture is the same with this method as most of the others, but considerable time is saved in the injection process. Before this operation, however, placing the inlet and outlet sprues after half-flasking consumes extra time and the molds must be dried in an infra-red oven for 60 minutes after the wax boil-out operation. DISCUSSION
AND
SUMMARY
Studies of the accuracy of fit of dentures produced of various materials and by methods suitable for large scale production have resulted in separation into three well-defined groups. The most accurate dentures were invariably of the self-cured type. When stored in water for an extended period, they also changed relatively little, apparently as the result of fewer stresses formed during cooling in the molds. The heat-cured dentures did not fit quite as well, but were considered good. The changes during storage were minor and tended to improve rather than detract from the initial fit. The special injection group comprised of Jectron, Tilon, and Luxene, was similar in regard to fit and was slightly less accurate as a group. The clinical importance of the relatively small discrepancies of fit found in all of the dentures that were studied has not been determined with any degree of certainty. It is thought that all that were described would be clinically acceptable. It is known that the retention of a denture is dependent upon the distance between the denture surface and the tissues, as well as upon the capillary forces and the area covered.lO The oral tissues have been shown to possess remarkable properties of adaptation,ll but this does not mean that they are normal and healthy under conditions which require changes of a millimeter or more. The evaluation of denture processing methods did not yield as well-defined results as the accuracy studies, since more factors were involved. In general, it has
ic%E‘2”
DENTURES
PROCESSED
BY
DIFFERENT
TECHNIQUES
281
been determined that the self-curing method offers the simplest method and involves the least amount of equipment. It is, therefore, more adaptable to varied conditions and may be employed where technicians do not have special training. There are relatively few opportunities for serious errors with this method other than packing the plastic into molds insufficiently hardened, using excessive molding forces, or delayed trial packing. The self-curing materials should find wide application for repairs and relining, since the application of heat causes warping of plastic dentures. The heat-cured method is being employed satisfactorily in large laboratory installations at present. The total processing time is relatively long, but since the actual working time is the same as in other methods, it is nearly as efficient for continuous production. The opportunities for error are similar to those listed for the self-cured method and there is an additional hazard in the control of the heat and time of the curing cycle. The special injection methods require more highly trained personnel, and the equipment is more complicated and expensive. The contours of 4 heat-cured and 4 self-cured maxillary dentures, which had been stored in water for approximately 20 months, were measured and the discrepancies were compared with previous measurements obtained 24 hours after deflasking and after storage in water for S months. These data indicated that remarkably few changes of contour occurred in the dentures of either type of plastic. Each of the dentures was fractured along the midline before repair, and the average load required to fracture the heat and self-cured dentures at the time of fracture was found to be the same. One pair of heat-cured dentures was repaired by a heat-curing method and the other pair by a self-curing method. A pair of self-cured dentures was repaired by the heat-curing method and the remaining pair by the self-curing method. The contours of each of the dentures were remeasured after the repairs and the data for the discrepancies were compared with the data obtained from the same dentures before the repairs. The heat-cured dentures exhibited considerable changes in contour after they had been repaired by the heat-curing method, but relatively few changes resulted from the self-curing repairs. The self-cured dentures exhibited practically no changes during repairs by either method. REFERENCES
1. Anthony! D. H., and Peyton, F. A.: Evaluating Dimensional Accuracy of Denture Bases wtth a Modified Comparator, J. PROS. DEN. 9:683-692, 1959. 2. Anthony, D. H., and Peyton, F. A.: Dimensional Accuracy of Various Denture-base Materials, J. PROS. DEN. 12:67-81, 1962. 3. Rupp, N. W., Dickson, G., Lawson, M. E., Jr., and Sweeney, W. T.: A Method for Measuring the Mucosal Surface Contours of Impressions, Casts, and Dentures, J.A.D.A. 54:24-32, 1957. 4. Souder, W., and Paffenbarger, G. C.: Physical Properties of Dental Materials, Washington, 1942, Circular C433, U. S. Government Printing Office, p. 167. 5. Tuckfield, W. J., Warner, H. K., and Guerin, B. C.: Acrylic Resins in Dentistry, Part II. Australian D. J. 47:1-25, 1943. 6. Skinner, E. W., and Cooper! E. N.: Physical Properties of Denture Resins: Curing Shrinkage and Water Sorptron, J.A.D.A. 30:1845-1852, 1943. 7. Mowery, W. E., Burns, C. L., Dickson, G., and Sweeney, W. T.: Dimensional Stability of Denture Base Resins, J.A.D.A. 57:345-353, 1958.
282
PEYTON
AND
ANTHONY
J. Pros. Den.
March-April,
1963
8. Woelfel, J. B., Paffenbarger, G. C., and Sweeney, W. T.: Dimensional Changes Occurring in Dentures During Processmg, J.A.D.A. 61:413-430, 1960. 9. Ryge, G., and Fairhurst, C. W.: The Contour Meter: An Apparatus for Comparison of Mucosal Surface Contour of Impressions, Models, and Dentures, J. PROS. DEN. 9:676-682, 1959. 10. Craig, R. G., Berry, G. C., and Peyton, F. A.: Physical Factors Related to Denture Retention, J. PROS. DEN. 10:459-467,1960. 11. Lytle, R. B.: The Management of Abused Oral Tissues in Complete Denture Construction, J. PROS. DEN. 727-42, 1957. UNIVERSITY OF MICHIGAN SCHOOL OF DENTISTRY ANN ARBOR, MICH.
F.A.
PEYTON,
University
of
OF DENTURES BY DIFFERENT
DSc., AND D.H. Michigan,
TECHNIQUES
ANTHONY,
D.M.D.,M.S.
School of Dentistry,
Ann Arbor,
Mich.
THE PAST 2.5 YEARS, since the introduction of the acrylic resin denture base material in 1937, there have been many suggested variations in the techniques of processing these materials, with several additional materials proposed for the same purpose. Often these variations have been introduced with a claim of superior properties, improved accuracy of fit, and greater dimensional stability in service. The full significance of these claims on the function and performance of the denture in service has not always been clear. Because of the advantages claimed for the various denture base materials and the modified denture processing methods which have been introduced within recent years, an over-all evaluation was considered to be desirable. The exactness of the duplication of the detnures of the final impression and the accuracy of fit of the dentures made of various materials were determined by means of a modified comparator.le3 This modified panographic comparator employed dial gauges to record the anterior, posterior, and lateral positions of the carriage, so that reference points on the master impression and the dentures could be relocated for measurement with the vertical dial gauge. Other investigators have used a variety of methods to determine the dimensional change during processing and subsequent warpage of dentures. The change in dimension may be reflected by the fit of the denture on a stone cast, formed from the bearing surface of the denture,4 simply by placing the dentures lightly on the master casts and observing the discrepanciesat the denture border,5 or by measuring the discrepancies at the posterior border while the denture is held in position.6 Others have employed metal pins and bench marks in the molar tooth region or in the posterior lateral borders of the dentures, then measuring any changes with the aid of micrometer microscopes.rpsIt has been pointed out that usually upper dentures placed lightly on a master cast will appear to contact the cast in the flange regions, will be deficient at the buccal borders, and will show the greatest discrepancy at the central portion of the posterior border .2 Lower dentures usually appear to fit least accurately along the posterior lingual borders in relation to the master cast. These methods all lack the advantage of registering the actual contour of the
D
URING
These studies were supported by contract No. DA-49-007-MD-860 between the Office of the Surgeon General, Department of the Army, and the University of Michigan. A portion of this report is from a thesis submitted by D. H. Anthony to the Horace H. Rackham School of Graduate Studies, University of Michigan, as a partial fulfillment for the Master of Science degree in Dental Materials, 1961. Presented before the Academy of Denture Prosthatics, Colorado Springs, Cola. 269
270
PEYTON
AND
ANTHONY
J. Pros. March-April,
Den. 1963
tissue-bearing surface of the denture, and of comparing this contour with the contour of a master impression, which is possible with the instrument employed in this study. One contour meter has been described which was used to evaluate the denture adaptation ,O but the instrument seems to lack some degree of convenience in repositioning and recording results. MATERIALS
STUDIED
Since it was one purpose of this study to compare different types of materials as well as techniques of processing, a wide range of denture base materials were included. Four different self-cured types of material were employed ; seven heatcured acrylic resin materials were used ; and three special injection products were studied, which represented an acrylic resin, a styrene, and a vinylacrylic type of material. Two chromium-cobalt alloys were cast to form full coverage upper and lower denture bases. One epoxy-type denture plastic was included in the study, as was a vulcanite denture material and an all-porcelain base denture for purposes of comparison of these two types of material. This represents, therefore, a total of 19 different products of 9 different basic types of materials or techniques of use. This group was thought to be representative of the type of materials and recommended techniques currently available. In this report, however, comparisons of only the 3 principal types of materials are presented, with the results reported being limited primarily to the maxillary dentures. The recommended directions of the manufacturer were employed for each denture material processed. In order to eliminate undesirable variables among the dentures of various materials, each stone cast was formed in the same silver plated upper or lower master impression. A consistent thickness was secured in the construction of waxed dentures by pouring molten wax into a rubber mold which held the porcelain teeth and stone cast in the correct relationship to one another. All laboratory procedures were performed under as nearly identical conditions as possible, yet consistent with the manufacturers’ recommended pr0cedure.l After the dentures were deflasked they were stored in water for 24 hours at room temperature before the measurements were recorded. This procedure simulated the interval of time before delivery to the patient. During all mounting and measuring operations, mouth temperature, approximately, was maintained by an electrical heating tape placed around and beneath the denture, since early measurements indicated that dentures tend to expand slightly and to fit more accurately when they are warmed to body temperature. RESULTS
OF MEASUREMENTS
The measurements made at each 0.025 inch interval across a selected ‘section of the upper or lower denture were plotted on a graph with similar related measurements from the master impression. These plotted measurements resulted in the development of a graph for the product when they were measured laterally through the second molar section. Typical graphs of different types of material were made and some have been presented in previous reports.lT2 Similar contours for the various dentures were recorded and reproduced in
Volun1e 13 Number 2
DENTURES
PROCESSFB
BY
DIFFERENT
TECHNIQUES
271
graph form for both the transverse and midline sections. It is not practical or possible to present all the graphs for comparison of the discrepancies of fit. For this reason, selected regions in the upper transverse section, the upper midline section, and the lower midline and transverse sections were used to determine the average discrepancies between the dentures and the master impression in the regions determined. The sum of the average discrepancies in these clinically significant regions was taken as the basis for arranging the dentures in the order of accuracy of fit. The smallest number of average discrepancies represented the most accurate dentures. This order of accuracy has been presented in detail in a previous report.’ Table I presents these average discrepancies of several types which separate the self-cured, heat-cured, and special injection-type dentures into well-defined groups. These represent the three principal types of denture plastics currently available. The porcelain base, epoxy, vulcanite, and cast chromium-cobalt denture bases, previously described in some detail are employed in relatively small numbers. Table II shows the discrepancies in fit of the upper denture in thousandths of an inch in the selected regions through the transverse section of the second molar region. This is a distinct comparison of the three principal types of plastic denture materials, relative to the areas in which the discrepancies exist. Similar discrepancies and differences of fit of the various materials were observed also from measurements in the midline section of the upper and the midline and transverse sections of the lower dentures.
TABLE
I.
TYPICAL
RANGE
OF CONTOUR
DISCREPANCIESOF MOLAR SECTION
UPPER
DENTURESTHROUGH
SECOND
---___ I Self-cured EPOXY
(Densene) type
Chromium-cobalt No. 1 Vulcanite Heat-cured (Densene 33) Chromium-cobalt No. 2 Special injection
EFFECT
OF
AGIXG
ON
DENTURE
TOTAL 53 60 70 1;: 144 17.5
CONTOUR
The changes which occurred during storage in water were relatively insignificant in practically all the dentures studied. On the basis of over-all fit, the changes in the general form of expansion were in proportion to the amount of shrinkage which took place during processing. The exceptions were in the vinyl type plastic, Luxene, and the polystyrene, Jectron, which may be related to physical properties of these two types of material. The self-cured dentures apparently contained few internal stresses as a result of the small amount of thermal shrinkage which occurred before they were removed from the molds. A certain amount of water was absorbed over a period of time and
272
PEYTON
TABLE
II.
TRANSVERSE
SECTION
AND
CONTOUROF
J. Pros. Den. March-April, 1963
ANTHONY UPPER
DENTURESIN~ECOND
MOLAR
REGIONS
DISCREPANCIESINTHOUSANDTHSOFINCHES MATER1AL*
-1-i
2
1
3
/
4
j
5
/
6
/
7
(
8
1
9
ToTAL
Self-Cured Heat-Cured Special Injection *Typical
examples
of type.
a slight amount of expansion occurred. The changes were only a few thousandths of an inch in most regions and the over-all effect was to allow a further seating of the denture on the cast. The discrepancies in contour ‘of representative groups of dentures of the three principal types of plastic materials when measured at various times are shown in Table III. The first values are given for the transverse section of upper dentures after they were deflasked and stored in water for 24 hours. The second measurements were obtained after the dentures had been stored in water for 8 months, and the third set after storage for 1 year and 8 months, for the self-cured and heatcured types. Each number listed in the chart represents the average of the discrepancies measured in the selected regions of 4 dentures of the same materials, processedin as nearly an identical manner as possible. In the transverse section, the largest discrepancies which occurred consistently in the heat-cured dentures were located at the lateral borders. The possibility of lateral shrinkage during processing was indicated by the relatively small discrepancies recorded in the middle of the flanges (regions Nos. 2 and 8) at the time of the first measurements. With aging, however, the discrepancies improved in these regions, with the indication that the dentures had expanded in the lateral dimension. The accuracy of fit seemedto have improved in the palatal region at the time of the second measurement, but after further aging the values were similar to those recorded originally. Similar observations resulted when measurements were made in the midline section of the denture. The special injection group of dentures produced varied results after storage in water, possibly because of the different plastics employed. The Tilon plastic,
Valume Number
13 2
DENTURES
PROCESSED
BY
DIFFERENT
273
TECHNIQUES
which is an acrylic gel, is processed at a higher heat than usual (278” F.). The excess heat, plus injection, may explain the higher processing shrinkage of these dentures. More changes occurred during storage of these dentures than with other acrylic resins, and the expansion resulted in an improvement of fit which eventually approached that of the more conventional heat-cured dentures. The Jectron dentures displayed a relatively high rate of molding shrinkage, possibly due to the high temperature (410” F.) required to soften the polystyrene material for injection, and the large cooling range after hardening. The low water sorption rate of the polystyrene may he the reason for the exceptional dimensional stability of these dentures, which may be considered to he the most stable plastic dentures evaluated. The Luxene dentures represent a blend of acrylic and vinyl plastics, and the high shrinkage during processing may be related to the tendency of the vinyl plastics to harden at a higher temperature as they cool from the softened state, even though the heat applied is not excessive (210” F.). After storage in water, these dentures did not expand as did the other plastic dentures. The low rate of water sorption of the material is possibly related to their relative stabi1it.y.
TABLE
III.
EFFECTOFAGINGON
CONTOURSOF
MAXILLARY
DENTURES
(Average of 4 Dentures)
-_____ -~~
DISCREPANCIES AGE
Hea$cyf
(Hy-Pro
1
j
2)
14
.: 6
:: 7
p
8 mo.
15
(
6
17
18
j
9
ToTAL
9 i
10
9
13 8
7 9
11
9
A 3
:
21 19
10
29
12
31
1:
lo”
.: 5
15
5
4 2 5
3” 9
::
79 58 a2
10 8 8
34 25 SO
27 35
175 181
88)
8 m;. 20 mo. Speciai
14
OF INCHES
Lucitone)
8 mo. 20 mo. .Self-y4txt (Acralile
3
INTHOUSANDTHS
-
4 7 (Jectron)
ii
2
: 6
:
; 5
4 5
27 26
21 lb
::
274 INFLUENCE
PEYTON
OF REPAIR
ON
DENTURE
AND
ANTHONY
J. Pros. March-April,
Den. 1963
CONTOUR
The fracture of a denture from any cause is often embarrassing to the dentist and is considered by most patients to be an emergency requiring prompt attention. The dentist, therefore, must be prepared to make repairs in as short a time as is practical and without altering the properties or fit of the denture. For many years the repairs have been made with heat-cured plastic in much the same manner as the original denture was processed, with the observation that frequently the re-cured denture showed varying degrees of warpage. When it was repaired at lower temperatures (165” F.) for longer periods of time, few changes in contour occurred. The self-cured plastics have become popular for denture repairs not only because of their convenience, but because of the fewer changes in contour. This study was undertaken to determine more precisely the changes of contour which occur when both heat-cured and self-cured dentures were repaired with either heat-curing or self-curing plastic material. The method of study consisted of : (1) recording the contours in selected regions of 4 heat-cured and 4 self-cured maxillary dentures which had been stored in water for approximately 20 months, (2) fracturing each of the dentures under load along the midline, (3) reassembling the pieces of the dentures and remeasuring the contours to insure accurate alignment, (4) repairing 2 dentures of each type by a self-curing method, (5) repairing 2 of each type by a heat-curing method, and (6) repeating the measurements of contour. A comparison of the data from these measurements should permit an evaluation of the effect of these methods of repair on the fit of the 2 types of dentures. FRACTURE
OF THE
DENTURES
ALONG
MIDLINE
After the contours of the dentures had been recorded, each denture was fractured along the midline. To ensure this type of fracture, the notch in the region of the labial frenum was extended l/s inch farther with a Carborundum disk, and another notch was prepared at the posterior border of each denture at the midline. A compressive load was applied with a testing machine through a tip which was tapered to a rounded edge l/16 inch wide and l/2 inch long. Each denture was placed with the teeth downward on a steel platform and the rounded edge of the load applicator was placed along the midline of the tissue surface of the denture at a distance vs inch from the posterior border. The porcelain teeth were cushioned by a single layer of felt which permitted a small lateral movement of the teeth and prevented the fracture of individual teeth as the dentures were bent downward under load. The application of the downward force along the midline of the inner surface of the dentures was designed to be equivalent to an upward force on the teeth on both sides in conjunction with an unyielding support in the central portion of the palate. Although this method was simple and convenient, it was not intended to be a standard compression test for maxillary dentures. The data for the loads at the initial cracking and the final separation of the pieces were recorded only as a matter of interest in order to compare the relative strength of this series of similar heatand self-cured dentures.
Volume Nnmber
13 2
DENTURES
PROCESSED
BY
DIFFERENT
27.5
TECHNIQUES
The rate of loading was set for 0.065 inch per minute. The loads which were recorded at the initial and the final break are listed for the heat-cured and selfcured dentures in Table IV. Both the heat-cured and self-cured dentures resisted an average force of approximately 139 pounds before the initial break occurred along the posterior portion of the midline. Considerable additional force was required in each instance to complete the fracture in the front. Although this was not intended to be a standard test for the strength of dentures, these values suggest that there is probably little difference in the strength of heat- and self-cured dentures when they are properly processed by controlled methods. TABLE
IV.
LOAD
REQUIRED
TOFRACTURE
DENTURES
---~ -.-_-___ ___
DENTURE
NO.
Heat-cured 107 108 110 111
sezf-“lr’7”d 118 119 120
INITIAL
BREAK
(LB.)
FINAL
BREAK
I
150 12.5 120 160
225 225 220 275
150 140 140 125
260
METHODSOFREPAIR
Heat-cured.--An occlusal index of dental stone was made for each denture before it was fractured to assist in the reassembly of the pieces in their correct relationship. After the fracture had occurred, the pieces were placed together by hand and held in position with sticky wax. Little difficulty was experienced in reassemblingthe parts and the stone index was required only to check the alignment. In order to eliminate the possibility of changes of contour which might have occurred as the result of misalignment of the parts, each denture, after it has been reassembled, was warmed to 100” F. and remounted on the comparator in the same manner as it was mounted before it was fractured. Differences of only 0.001 inch were permitted in measurements in selected regions before the denture was accepted for repair. Since no wax had been used in the fracture region, it was not necessary to apply heat to the denture. Warpage by procedures such as heating the flask or by pouring boiling water over the region of the joint, thereby, was eliminated. The full length of the fracture was widened slightly and prepared in a rounded “V” form with a fissure bur mounted in the handpiece of a laboratory dental engine. This method of preparation was chosen in order to ensure that the pieces of the denture would remain firmly in place on the cast without the change of their relative position, which might occur if they were removed from the cast for the preparation of the joint.
276
PEYTON
AND
ANTHONY
J. Pros. March-April,
Den. 1963
Heat-cured plastic (Hy-Pro Lucitone) was packed into the prepared joint after the exposed plastic was painted with monomer, and trial packing was completed in the conventional manner. A separator of the alginate type (Al-tote) was applied to the upper portion of the mold. When the flask had been closed in the flask press, it was set aside for 15 minutes before it was placed in a Hanau curing unit with the temperature of the bath at 169” F. for a period of 8 hours. The flask was permitted to cool to room temperature in the bath before deflasking was completed. The areas which had been repaired then were smoothed with plastic trimmers, but not polished, and the dentures were-stored in water at room temperature for 24 hours before the contours were measured. Self-cured.-The method employed for the self-cured repairs was similar in most respects to the method outlined for the heat-cured repairs. The assembly of the pieces of the dentures, pouring of the models, half-flasking, full-flasking, and the preparation of the joints were performed in the same manner. The material chosen for the repairs was Acralite 88 self-curing denture plastic, and the separator for the molds was Microphane, supplied by the same manufacturer. The edges of the joints to be repaired were painted with the monomer and the packing of the plastic was begun immediately when the “dough” stage was reached. Trial packing was finished in a short time and the flasks were permitted to remain in a flask press overnight to ensure complete curing. After the deflasking operation, the excess plastic was trimmed from the area of the repairs and the dentures were placed in water at room temperature for 24 hours before the contours were measured. CONTOUR
OF HEAT-CURED
DENTURES
AFTER
REPAIRS
Heat-cured Repairs.-The two dentures, Nos. 110 and 111, were selected for the evaluation of the effect of heat-cured repairs on heat-cured dentures. The data from the measurements of the contours of these 2 dentures were averaged in each region and, after the repairs had been completed, the contours were remeasured in the same regions for comparison. The data obtained from the selected regions in the transverse sections were recorded in Table V. The regions which were selected have been indicated on the contours drawn above the data. The changes of contour which occurred in each region are shown below the data. The negative signs indicate the loss of fit, measured at right angles to the denture surfaces. It became evident from the data that the heat-cured repairs caused a loss of ‘fit of the heat-cured dentures in spite of the controlled technique and the minimum amount of heat which was applied. Distortion occurred which would not permit the dentures to be seated closely on the cast, and in spite of repeated efforts ato mount the dentures so that the discrepancies at the lateral borders were nearly the same, the discrepancies were always a little greater on the left side than on the right. The sum of the average discrepancies in the transverse section was 94 before the repairs and after the repairs it had increased to 169. The difference between these values is another indication of the relatively large changes which resulted from repairs made by the heat-curing method.
z!ze~‘2”
DENTURES
PROCESSFdl
BY
DIFFERENT
277
TECHNIQUES
Self-cured Repairs.-The data obtained from the measurements of the contours of the remaining heat-cured dentures, Nos. 107 and 108, were averaged for comparison with the data from the same regions after the self-cured repairs. The discrepancies in the transverse sections, measured before and after repair, were recorded in Table V and the changes were shown below the data. It is apparent that few changes of contour occurred as a result of the selfcured repairs. A slight expansion of the flanges may have taken place which permitted a slightly closer fit in the palatal region. The sum of the discrepancies measured in the transverse section changed from 67 to 59, which was a further indication that practically no changes had occurred. V. CHANGESIN
TABLE
CONTOUR
OF HEAT-CURED FROM REPAIRS
DISCREPANCIESIN
) Heal-cured Before
1121
3
14
MAXILLARY
THOUSANDTHS
1.5
16
DENTURES
RESULTING
OF INCHES
17
I
181
9
IToTAL
repairs
After Change
1s
0
13 -“;
::
-6
Self-cy&epaks After Change
; 0
-1
;
$1
5”
5” +1
-10 7 +3”
-8 10 t-2
:: -16
+2
;
-3
5
12
2
:2” $1
94 169
67 59
CONTOUROFSELF-CUREDDENTURESAFTERREPAIRS
Heat-cured Repairs.-Two of the self-cured dentures, Nos. 117 and 120, were selected and the data for the contours in the selected regions were averaged for comparison with the data obtained after heat-cured repairs. The data obtained from the measurements and the amount of the changes which occurred were recorded in Table VI. It is evident from the table that the changes in the contours were small in both the transverse and midline sections. It would be difficult to conclude that measurable changes occurred when the accuracy of the method of measurement is taken into account. The sum of the discrepancies before fracture was 47 and after repair it
278
PEYTON
AND
J. Pros. March-April,
ANTHONY
Den. 1963
was 56. A large part of this change was recorded in one region, however, and the difference between these values is probably not significant. Self-cured Repairs.-The data for the contours of the remaining self-cured dentures, Nos. 118 and 119, were averaged for comparison with the data obtained after the self-cured repairs. The average discrepancies in the selected regions, which were determined both before and after the repairs, were recorded in Table VI. The data in this table indicate that practically no measurable changes have occurred as the result of self-cured repairs. The small change in the values of the total discrepancies in the transverse section, which changed from 53 to 59, was additional evidence of the few changes that were recorded. PROCESSING
TIME
STUDIES
The time required for each operation of the more important denture processing methods was recorded by means of a stop watch or taken from instruction manuals. Timing was done continuously during each operation such as half-flasking, fullflasking, hardening time of stone, and boil out or dig out after curing, and was repeated numerous times ; the resulting values were averaged. In this manner the basic times of the operations, which were common to many of the different techniques, were established. There were differences noted whether stone or plaster was used for half-flasking, or if a simple or double pour was used for full-flasking. The application of the separator and packing the molds varied and was determined separately for each technique. The curing time was clearly specified for each as well. TABLE
VI.
CHANGESIN
CONTOUROF
SELF-CURED FROM REPAIRS
MAXILLARY
DISCREPANCIESINTHOUSANDTHS (1
/2/
Heat-c;c~~~r&airs After Change
(4
/
:
7
7 ::
RESULTING
OF INCHES j
6
/
7
I
(81
9
3
-2’
% 0
-3
2::
5
5
7 2;
Se~-~.;~rrdEpairs After Change
3
DENTURES
5 -:
+1
2 -2
5
2
: 0
+3
ii 0
+3
:: 0
iToTAL
47 56
4
+z
i +3
+i
:.
1: -3
:;
Volunte Number
13 2
DENTURES
PROCESSED
BY
DIFFERENT
279
TECHNIQUES
Since one denture is rarely processed singly, the time intervals were recorded for each operation when they were performed in groups of four. This time interval was then divided by four to obtain the time intervals for a single operation under conditions of efficient operation. In addition to the time required for various operations and the total processing times, the intervals during certain steps, which did not require work by the laboratory technicians, were listed and totaled to determine the free time. This time presumably would be available for performing operations on other dentures in various stages of processing. It differs little for either one denture or a group, since it occurs when stone is hardening or during curing operations. From the total processing time and the amount of free time the actual working time was determined by subtraction. The working time was considered to be imTABLE
Jectron 1. 2. 3. 4. 2.
VII.
DENTURE
Luxene
MIN.
Half-flask (stone) Sprue Full-flask (stone) Stone harden ’ g;znout and separator
TIMES
PROCESSING
MIN.
13
1. Half-flask
ii 30
2. 3. 4. i,
13
Full-flask Stone harden Boil out Fulerator and pack
6: 7: Inject a. Cool 9. Dig out Total Free time 1Vorking
7. Cool 8. Dig out
2; 3
time
Working
Tilon 1. 2. 3. 4. 5. ;:
1.5
3 196 16.5
Total Free time
141 110 31
Half-flask (stone) Sprue . Full-flask (stone) Stone harden Boil out Fyerator and pack
3;: 6 35 90
time
31
Self-cured 1. Half-flask 2. Full-flask 3. Stone harden 4. Boil out i. perrator and
MIN. 10
3: pack
7. Dig out 8. Couol 9. Dig out
Total Free time
Total Free time Lliorking
164 120 time
Working
44 Heat-cured 1. Half-flask (plaster) 2. Full-flask (stone) 3. Stone harden 4. Boil out 5. Separator and pack 6. Cure 7. Cool 8. Dig out Total Free time U’orking
time
MIN. 10
3;: 6 10
180 30 3 273 240 33
time
213 180 33
PEYTON
280
AND
ANTHONY
portant, because it showed the amount of activity required by the technician and is, therefore, indicative of the relative number of dentures which would be processed if operations were continuous and new dentures were started frequently. A chart of all the data from these time studies was assembled and is shown in Table VII. According to this basis of comparison, the heat-cured technique consumed the greatest amount of time, mostly because of the combined curing and cooling time of 3$4 hours. The self-curing technique generally involves a minimum curing time of 2g hours. The mixing and packing operations are essentially the same for both the heat- and self-cured methods. The Tilon technique requires the greatest amount of working time because of the use of stone for half-flasking, sprues which must be attached before full-flasking, and the use of a double pour for this latter operation. The curing and cooling is accomplished in only 90 minutes. The Luxene technique requires somewhat less total time than the more conventional compression molding methods, but approximately the same working time is involved. The use of stone for half-flasking requires slight additional time, and more delays are caused by drying the separator film under infra-red lamps and by the procedure for filling the molds with the gel. The Jectron technique was considered to be the most efficient method studied. The actual working time per denture is the same with this method as most of the others, but considerable time is saved in the injection process. Before this operation, however, placing the inlet and outlet sprues after half-flasking consumes extra time and the molds must be dried in an infra-red oven for 60 minutes after the wax boil-out operation. DISCUSSION
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SUMMARY
Studies of the accuracy of fit of dentures produced of various materials and by methods suitable for large scale production have resulted in separation into three well-defined groups. The most accurate dentures were invariably of the self-cured type. When stored in water for an extended period, they also changed relatively little, apparently as the result of fewer stresses formed during cooling in the molds. The heat-cured dentures did not fit quite as well, but were considered good. The changes during storage were minor and tended to improve rather than detract from the initial fit. The special injection group comprised of Jectron, Tilon, and Luxene, was similar in regard to fit and was slightly less accurate as a group. The clinical importance of the relatively small discrepancies of fit found in all of the dentures that were studied has not been determined with any degree of certainty. It is thought that all that were described would be clinically acceptable. It is known that the retention of a denture is dependent upon the distance between the denture surface and the tissues, as well as upon the capillary forces and the area covered.lO The oral tissues have been shown to possess remarkable properties of adaptation,ll but this does not mean that they are normal and healthy under conditions which require changes of a millimeter or more. The evaluation of denture processing methods did not yield as well-defined results as the accuracy studies, since more factors were involved. In general, it has
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been determined that the self-curing method offers the simplest method and involves the least amount of equipment. It is, therefore, more adaptable to varied conditions and may be employed where technicians do not have special training. There are relatively few opportunities for serious errors with this method other than packing the plastic into molds insufficiently hardened, using excessive molding forces, or delayed trial packing. The self-curing materials should find wide application for repairs and relining, since the application of heat causes warping of plastic dentures. The heat-cured method is being employed satisfactorily in large laboratory installations at present. The total processing time is relatively long, but since the actual working time is the same as in other methods, it is nearly as efficient for continuous production. The opportunities for error are similar to those listed for the self-cured method and there is an additional hazard in the control of the heat and time of the curing cycle. The special injection methods require more highly trained personnel, and the equipment is more complicated and expensive. The contours of 4 heat-cured and 4 self-cured maxillary dentures, which had been stored in water for approximately 20 months, were measured and the discrepancies were compared with previous measurements obtained 24 hours after deflasking and after storage in water for S months. These data indicated that remarkably few changes of contour occurred in the dentures of either type of plastic. Each of the dentures was fractured along the midline before repair, and the average load required to fracture the heat and self-cured dentures at the time of fracture was found to be the same. One pair of heat-cured dentures was repaired by a heat-curing method and the other pair by a self-curing method. A pair of self-cured dentures was repaired by the heat-curing method and the remaining pair by the self-curing method. The contours of each of the dentures were remeasured after the repairs and the data for the discrepancies were compared with the data obtained from the same dentures before the repairs. The heat-cured dentures exhibited considerable changes in contour after they had been repaired by the heat-curing method, but relatively few changes resulted from the self-curing repairs. The self-cured dentures exhibited practically no changes during repairs by either method. REFERENCES
1. Anthony! D. H., and Peyton, F. A.: Evaluating Dimensional Accuracy of Denture Bases wtth a Modified Comparator, J. PROS. DEN. 9:683-692, 1959. 2. Anthony, D. H., and Peyton, F. A.: Dimensional Accuracy of Various Denture-base Materials, J. PROS. DEN. 12:67-81, 1962. 3. Rupp, N. W., Dickson, G., Lawson, M. E., Jr., and Sweeney, W. T.: A Method for Measuring the Mucosal Surface Contours of Impressions, Casts, and Dentures, J.A.D.A. 54:24-32, 1957. 4. Souder, W., and Paffenbarger, G. C.: Physical Properties of Dental Materials, Washington, 1942, Circular C433, U. S. Government Printing Office, p. 167. 5. Tuckfield, W. J., Warner, H. K., and Guerin, B. C.: Acrylic Resins in Dentistry, Part II. Australian D. J. 47:1-25, 1943. 6. Skinner, E. W., and Cooper! E. N.: Physical Properties of Denture Resins: Curing Shrinkage and Water Sorptron, J.A.D.A. 30:1845-1852, 1943. 7. Mowery, W. E., Burns, C. L., Dickson, G., and Sweeney, W. T.: Dimensional Stability of Denture Base Resins, J.A.D.A. 57:345-353, 1958.
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8. Woelfel, J. B., Paffenbarger, G. C., and Sweeney, W. T.: Dimensional Changes Occurring in Dentures During Processmg, J.A.D.A. 61:413-430, 1960. 9. Ryge, G., and Fairhurst, C. W.: The Contour Meter: An Apparatus for Comparison of Mucosal Surface Contour of Impressions, Models, and Dentures, J. PROS. DEN. 9:676-682, 1959. 10. Craig, R. G., Berry, G. C., and Peyton, F. A.: Physical Factors Related to Denture Retention, J. PROS. DEN. 10:459-467,1960. 11. Lytle, R. B.: The Management of Abused Oral Tissues in Complete Denture Construction, J. PROS. DEN. 727-42, 1957. UNIVERSITY OF MICHIGAN SCHOOL OF DENTISTRY ANN ARBOR, MICH.