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Analysis of pressures produced during maxillary edentulous impression procedures
Completeden.tures
Analysis
of pressures
edentulous Richard
impression
P. Frank,
produced
during
maxillary
procedures
D.D.S., M.S.D.*
United, States Public Health Service Hospital, New Orleans, La.
impression pressures have been thought to be a primary cause of tissue displacement and, consequently, to be a factor in denture stability, efficiency, and comfort. Displacement of ridge and palatal tissues is often said to result in denture looseness and tissue irritation. Hence, numerous modifications of impression techniques have been suggested for control of impression pressures. A search of the literature, however, revealed little information about pressures produced during impression making. No study has been conducted to determine the effect of tray modifications or materials upon impression pressures. Therefore, the objective of this investigation was to find a method to measure the pressures exerted upon the edentulous maxillary residual ridge and the palate during impression procedures. It was necessary to design and to prove the reliability of instruments capable of measuring these pressures, and to develop a suitable technique for their use. The effects of impression tray modifications, various types of materials, and dentists upon these pressures were also studied, A review of the literature revealed that impression tray modifications differed widely, and that all were based upon empirical observations. Materials were compared on the basis of pressure measurements because it has been suggested that some types of materials result in much higher pressures against the tissues than do other materials. Finally, the manner in which a dentist makes a final impression may be more important than either the type of material or the type of tray design used regarding production or reduction of pressure. Tray modifications selected for study were the presence or absence of relief space and/or escape holes. The impression materials investigated were a zinc oxide and eugenol paste,+ a light-bodied thiokol rubber,$ and an irreversible hydrocolloids of Read before *Assistant $Coe-flo, $Coe-flex,
and Staff
Coe Laboratories, Impression
of Denture
Prosthetics
Prosthodontist, Inc., Chicago,
Coe Laboratories,
SD-P Elastic 400
The Academy
Chief
Inc., Chicago,
Cream, Dental
United
in Honolulu,
Hawaii.
States Public
Health
Ill. Ill.
Perfection
Co., Glendale,
Calif.
Service
Hospital.
Impre.s.sion
Fig. 1. The surfac :e of the analogue is a soft silicone have been drilled to receive the pressure gauges.
denture
linrr:
Molloplast
pressures
B. Two
401
hc
two different powder/water ratios. One ratio was that recommended by the manufacturer; the other ratio had 20 per cent more water. Pressures occurring at the completion of tray seating and at the time of the initial setting of the materials wt:~ measured in the palate and on the crest of the residual ridge. Because there would have been several variables during intraoral impression polocedures that would have tended to obscure the influence of the various factors upon the pressures produced, and in order to measure pressures exerted against the mucosal surface, an oral analogue was constructed so that precise measurements in thrl appropriate locations could be made in the laboratory. For this an average-&et1 maxillary arch was duplicated in acrylic resin having a covering of a soft siliron~~ rubber denture liner, Molloplast B% (Fig. 1) . MEASURING
APPARATUS
A pressure gauge was constructed of brass tubing with one end covered with a thin flexible rubber membrane and the other end connected to a polyethylene tube (Fig. 2). The gauge and connecting tubing were filled with distilled water, and the free end of the tubing was then connected to an unbonded wire strain gauge+ (Fig. 3) . Pressure applied to the rubber membrane caused water displacement in the closed system, and the water displacement was converted to an electrical signal b\ the strain gauge. The signal was amplified and recorded by an oscil1ograph.S Calibration of each strain gauge was done before and after each testing session with ;L mercury manometer. “Kostner
and Company,
+Model
P23AA,
Statham
$Model
150, Sanborn
Oberursel/Taunus, Instruments,
Company,
Germany.
Inc., Los Angeles, Calif.
Waltham,
Mass.
402
J. Pros. Dent. October, 1969
Frank
plug B adapted Fig. 2. The pressure gauge consists of rubber membrane A and watertight to the ends of brass tube C. Metal disk D enables the gauge to be bolted to the analogue E. Polyethylene tube F connects the gauge to a pressure transducer.
Fig. 3. The pressure
VALIDITY
transducer
consisted
of an unbonded
wire strain
gauge.
OF THE ANALOGUE
The validity of using an analogue was ascertained by comparing the results obtained clinically with those obtained in the laboratory under simulated clinical conditions. For this part of the study, the pressure gauges were mounted on the impression tray flush with the internal surface of the tray. The subject, whose maxillary arch the analogue duplicated, was recalled for this test. The amount of force applied to the tray by the dentist was monitored by an apparatus consisting of a soft, plastic, water-filled tube placed between the tray and an acrylic resin plate (Fig. 4). Force applied against the acrylic resin plate caused a
Imjression
Fig. 4. The apparatus
under
the tray was used to monitor
Fig. 5. The distance water was displaced applied
into the smaller
fbessures
the force applied
tube indicated
403
to the tram.
the amount
of f’o~ce
to the tray.
measurable displacement of the water from the tube into a smaller tube lying alongside a millimeter rule (Fig. 5). The distance the water was displaced into the small tube indicated the amount of force used. Force was applied to the tray until the subject indicated that at least one tray stop on each side was completely seated. Twenty-one irreversible hydrocolloid impressions were made in this mamrer, and the mean time and amount of force used in seating the tray were computed. The same procedure was then conducted on the analogue, using the mean time and force previously determined. The tray stops were found to be seated, and higher forc,es were recorded in the palatal area than in the ridge crest area when testing both rite, subject and the analogue. Incomplete seatina of the tray resulted when either the force or the time of force application was decreased. It was concluded that the analogue was an acceptable substitute for the purposes of this study. The pressure gauges were then mounted in the analogue flush with the mucosal surface for the remainr!t*r of the study.
Fig. 6. The motor and keys E.
has been altered
by the addition
I Ll
of microswitches
-----D ; C
A and B, bolt C, nut D,
u
--;
L!i!!L B A
Fig. 7. The relationship of motor, tray, pressure gauge, and analogue to each other is illustrated: A, motor; B, motor shaft; C, threaded shaft extension; D, nut; E, metal tray support; F, tray seating guide; G, tray; H, analogue; Z, pressure gauge; ,J, analogue support.
Volume 22 Number 4
Fig. 8. The over-all
Impression
p~txures
405
view of the apparatus.
SEATING THE TRAYS In order to minimize the variables present during manual seating of an impression tray, and thus to enable critical comparison of types of trays and materials, a motor* with an electronic feedback mechanism? added to maintain a constant speed was used to carry the loaded impression tray to place (Fig. 6) . A nut was screwed on the threaded shaft, and the nut was prevented from turning with the shaft by keying the nut in a slot on the motor frame. As the shaft revolved, therefore, the nut was raised or lowered, depending on the motor direction. The distance of travel of the nut was regulated by microswitches so that the degree of seating 01 the tray could be precisely duplicated. Four brass rods were attached to the periphery of the analogue support parallel analogue, and a metal plate, centered over the tray by the rods, was attached to to each other and to the motor shaft. An impression tray was then seated upon the the outer surface of the tray. The rods thus served as mechanical guides for seating the tray by the motor. The analogue was then fixed in position above the motor shaft so that complete seating of the tray coincided with the maximum rise of the nut on the motor shaft (Fig. 7). A water bath supplied water at body temperature *Model 4K862, Dayton Electric Manufacturing Company, Chicago, tSpeedia1, Lutron Electronics Company, Inc., Emmaus, Pa.
Ill.
406
Frank
J. Pros. Dent. October, 1969
to soak the analogue prior to making an impression, so that the setting times of the materials were consistent with those noted intraorally. The entire system is shown in Fig. 8. STANDARDIZED TRAYS A standardized method of construction of individual trays was adopted. A wax spacer was evenly adapted to a cast of the analogue except in the region of the posterior palatal seal. An irreversible hydrocolloid impression of the completed waxed cast was then made, and poured in dental stone. The resultant cast served as the master cast upon which all trays incorporating relief were constructed. Trays without relief or stops were all made directly upon a cast of the analogue. THE IMPRESSIONS A standard amount of each of the impression materials was used for each impression. The time used for mixing a material, and for loading a tray, remained constant during the study. After pressures had been measured, using trays with and without relief space and no holes, five escape holes were drilled equidistantly from each other and one quarter inch from the center of each pressure gauge with a No. 6 round bur. When testing of these trays was completed, many holes with about the same spacing as before were drilled throughout the tray. The final procedure undertaken was manually making impressions of the analogue in a clinical manner to determine the validity of laboratory results. The analogue was bolted to the roof of a clear plastic box and provided with an environment similar to that found intraorally regarding temperature and humidity. The tray design found to be most effective and the one found least effective in reduction of pressures when seated by the motor were tested. Each of three dentists made four impressions of the oral analogue with each tray design and with each of the four impression materials studied. The only instruction for impression making given to the dentists was to make the impressions in their usual manner. RESULTS Analysis of variance was used to statistically evaluate the effect of the variables on the pressures recorded. Duncan’s multiple range test1 was used to isolate the factor (s) responsible for the variation found in the analysis. Figs. 9 through 1% illustrate the results of the statistical tests. Initial pressures, mechanically produced. The highest pressures recorded with any material occurred when neither escape holes nor relief was used. Addition of relief to the tray resulted in 70 per cent as much pressure, while use of escape holes yielded 65 per cent as much pressure. Relief and escape holes together in trays resulted in only 45 per cent as much pressure as an unrelieved tray (Fig. 9). Differences in the amount of pressure received by the palate and ridge crest areas were not statistically significant with one exception. When trays with no relief or holes were used, the ridge crest received much more pressure than did the palate (Fig. 10). Major differences in pressure production were noted among the materials. The
Impression
pressures
407
5-
I OFig. 9. Initial space and/or
Holes
Relief
pressures during mechanical escape holes to the tray.
Neither
Both
tray seating were reduced
by the addition
of relief
I 0
Ridge
Palate
NO RELIEF
Ridge
Palate
RELIEF
Fig. 10. Initial pressures against the crest of the ridge and the palate were nearly identical when relieved trays were seated mechanically. The ridge crest received much more pressure than did the palate when unrelieved trays were used.
more viscous mix of irreversible hydrocolloid caused significantly higher pressures than any other material. The rubber and the thinner mix of irreversible hydrocol.loid were not significantly different from each other in pressure production, but did result in 66 per cent as much pressure as the thicker irreversible hydrocolloid. The use of the zinc oxide and eugenol paste caused the least pressure: 43 per cent of tha.t of the thick irreversible hydrocolloid and 65 per cent of that of either the rubber or the thinner mix of irreversible hydrocolloid (Fig. 11) . End pressures, mechanically produced. The highest pressures were recorded when an unrelieved tray was used. The placement of escape holes or relief in the
408
J. Pros. Dent. October, 1969
Frank
0
Alginate (Regular)
Fig. 11. Major differences mechanical tray seating.
Fig.
12. Adding
relief
Rubber
Alginate (Thin)
were noted among the materials
to the tray
resulted
in much
smaller
relative
Zn OE to initial
end pressures
pressures during
during mechanical
tray seating. tray resulted in 16 per cent as much pressure. Combining escape holes with a relief space resulted in only 5 per cent of the pressure associated with an unrelieved tray (Fig. 12). No statistically significant difference in pressure gauge sites relative to the amount of pressure recorded was found except when an unrelieved tray was used. Under that condition twice as much pressure was recorded over the ridge crest as in the palatal vault (Fig. 13) . Differences in pressures among the materials tested were not pronounced. The highest pressures were recorded when using rubber or the thicker mix of irreversible hydrocolloid in trays with no escape holes or relief space, followed by the zinc oxide and eugenol paste or thinner mix of irreversible hydrocolloid in the same type of tray. No statistically significant differences among materials were found relative to
Volume 22 Number 4
Imj!ression
NO RELIEF Fig. 13. Little difference mechanical tray seating
NO RELIEF
409
RELIEF
was found between the ridge crest and palatal except when an unrelieved tray was used.
Fig. 14. Significant differences in end pressures among chanical seating only when unrelieved trays were used.
pressuwr
end pressures during
RELIEF the materials
resulted
during
mc-
pressure when trays with either escape holes, relief space, or both were used. In no instance did the pressure fall below zero (Fig. 14) . Initial pressures, manually produced. The pressures caused by the three dentists ranged from a little less than one half pound per square inch to 4v4 pounds per square inch. No two dentists produced the same degree of pressure while making an impression. The same relationship of mean pressures was found with manual tray seating as before regarding the presence or absence of tray relief and the type of material, as shown in Figs. 15 and 16. One of the dentists also made the specified number of impressions with the vari-
410
J. Pros. Dent. October. 1969
Frank
RELIEF NO RELIEF Fig. 15. Initial pressures exerted against the palate and the ridge crest during manual tray seating were nearly the same except when unrelieved trays were used.
(Regular) Fig. 16. The same order of materials manually
as that
found
during
(Thin)
ranked by initial pressures was found mechanical tray seating.
with
trays seated
ous materials that resulted in no statistically significant difference attributable to the type of material used. Differences between pressures with the various materials were more pronounced when the impressions were made by the other two dentists. End pressures, manually produced. Pressures ranged from a negative one quarter pound per square inch to a positive 1 pound per square inch. Negative pressures were often produced by all of the dentists. The negative pressures occurred almost exclusively while using the relieved trays, and were evenly divided between the ridge crest gauge and the palatal gauge. Impressions made with the thick irreversible hydrocolloid, however, seldom resulted in the occurrence of negative pressure.
Impression 1.5
-
1.0
-
pressures
411
; 4.0.5
-
0 -0.25’
Ridge
Palate
NO RELIEF
Ridge Palate RELIEF
Fig. 17. The highest end pressures during manually made impressions were found over the ridge crest when trays with no relief were used. Negative pressures were often recorded while using trays with relief space and escape holes.
I .5-
l.O; QL 0.5 -
(15%) Rubber (Regular)
(Thin )
Fig. 18. Differences in end pressures relative to the type of material used were not statistically significant except for the thick mix of irreversible hydrocolloid during manual seating.
The highest pressures were recorded over the ridge crest while impressions were made with an unrelieved tray; 54 per cent as much pressure was recorded in the palate. The differences in pressures between the ridge and palate were not statiscitally significant while impressions were made with relieved trays (Fig. 17) . Two of the dentists produced higher pressures with the unrelieved trays than with the relieved ones, but the third dentist produced essentially equal pressures with either type of tray. No statistically significant differences were seen in pressures relative to the type of impression material with any of the dentists with one exception. The use of the
412
Frank
.I. Pros. Dent. October, 1969
thick mix of irreversible hydrocolloid resulted in significantly higher pressures for two of the dentists than did the use of the other materials (Fig. 18). DISCUSSION
In 1925 Stansbery2 claimed that higher pressures would be found in the region of the palatal vault than elsewhere when a maxillary impression was made. His statement was based upon the results seen when impression compound was compressed between two blocks of plaster, one of which was perforated: a larger amount of compound was forced into the central perforations than into the peripheral ones. Douglas, Bates, and Wilson3 mounted a variable capacitance transducer on an impression tray and measured the forces occurring while intraoral impressions were made with various brands of zinc oxide and eugenol paste. Forces arising in the palatal vault area could not be directly compared with those occurring elsewhere in the arch since one transducer had been located over the center of the palatal vault in one subject, and over the palatal slope of the ridge in another subject. They believed that the higher values of impression pressures found in the first subject may have been due to the more central location of the gauge. In the present study, a direct comparison of pressures in the ridge and palatal areas has been possible, and the results in general do not confirm previous beliefs. Instances in which trays with relief were used showed no statistically significant difference in pressure against the palate and the ridge crest. Only when trays with no relief were used was a significant difference in pressures found. Negative pressures were recorded during impressions made by the dentists, and were probably due to the dentist inadequately supporting the tray against gravity while waiting for the initial set of the impression material to occur. Any change in pressure near the beginning of the initial set due to tray movement could result in an inaccurate impression. That tissues covering an edentulous ridge can be displaced by an impression procedure has been graphically demonstrated by Woelfel.4 Different tissue contours resulted when impressions were made of one edentulous maxillary arch with five different materials. Tissue displacement occurred to a variable extent with all materials, although the largest displacement occurred over the ridge crest when using the rubber impression material. That finding is consistent with the results of this study. A most important aspect of this study has been the finding that pressure can, in fact, be selectively applied or reduced. Demonstration of the capability of several methods to alter impression pressures should encourage the dentist to modify his technique to be more in accordance with the soft tissue conditions presented by each patient. It is reasonable to assume a reduction in pressure would result in less displacement of the soft tissue. Reduced pressure would be most likely achieved with the use of a zinc oxide and eugenol paste material in a tray constructed with relief space and escape holes, and gently carried to place. SUMMARY
A method for measuring pressures produced during edentulous impression procedures has been described. An oral analogue was constructed, and an electric
Impression
pressures
413
motor was used to carry loaded impression trays to place to minimize variables. Impressions of the analogue were then made by dentists in a clinical manner to establish the validity of the laboratory results. The influence of several impression materials and tray modifications upon these pressures was determined. CONCLUSION
It was concluded from this investigation that impression pressures can be controlled by tray design and material selection. I wish to thank United States Public
the Educational and Research Foundation of Prosthodontics Health Service for the generous support given to this study.
and the
References Ann Arbor, 1957, Edwards Brothers, 1. Li, J. C. R.: Introduction to Statistical Inference, Inc., p. 238. 2. Stansbery, C. J.: The Negative Pressure Method of Impression Taking, J. A. D. A. 12: 438-445, 1925. 3. Douglas, W. H., Bates, J. F., and Wilson, H. J.: A Study of Zinc Oxide-Eugenol Type Impression Pastes, Brit. D. J. 116: 34-36, 1964. 4. Woelfel, J. B.: Contour Variations in Impressions of One Edentulous Patient, J. PROS. DENT. 12: 225-254, 1962. UNITED STATES PUBLXC HEALTH SERVICE HOSPITAL 210 STATE ST. NEW ORLEANS, LA. 70118
Analysis
of pressures
edentulous Richard
impression
P. Frank,
produced
during
maxillary
procedures
D.D.S., M.S.D.*
United, States Public Health Service Hospital, New Orleans, La.
impression pressures have been thought to be a primary cause of tissue displacement and, consequently, to be a factor in denture stability, efficiency, and comfort. Displacement of ridge and palatal tissues is often said to result in denture looseness and tissue irritation. Hence, numerous modifications of impression techniques have been suggested for control of impression pressures. A search of the literature, however, revealed little information about pressures produced during impression making. No study has been conducted to determine the effect of tray modifications or materials upon impression pressures. Therefore, the objective of this investigation was to find a method to measure the pressures exerted upon the edentulous maxillary residual ridge and the palate during impression procedures. It was necessary to design and to prove the reliability of instruments capable of measuring these pressures, and to develop a suitable technique for their use. The effects of impression tray modifications, various types of materials, and dentists upon these pressures were also studied, A review of the literature revealed that impression tray modifications differed widely, and that all were based upon empirical observations. Materials were compared on the basis of pressure measurements because it has been suggested that some types of materials result in much higher pressures against the tissues than do other materials. Finally, the manner in which a dentist makes a final impression may be more important than either the type of material or the type of tray design used regarding production or reduction of pressure. Tray modifications selected for study were the presence or absence of relief space and/or escape holes. The impression materials investigated were a zinc oxide and eugenol paste,+ a light-bodied thiokol rubber,$ and an irreversible hydrocolloids of Read before *Assistant $Coe-flo, $Coe-flex,
and Staff
Coe Laboratories, Impression
of Denture
Prosthetics
Prosthodontist, Inc., Chicago,
Coe Laboratories,
SD-P Elastic 400
The Academy
Chief
Inc., Chicago,
Cream, Dental
United
in Honolulu,
Hawaii.
States Public
Health
Ill. Ill.
Perfection
Co., Glendale,
Calif.
Service
Hospital.
Impre.s.sion
Fig. 1. The surfac :e of the analogue is a soft silicone have been drilled to receive the pressure gauges.
denture
linrr:
Molloplast
pressures
B. Two
401
hc
two different powder/water ratios. One ratio was that recommended by the manufacturer; the other ratio had 20 per cent more water. Pressures occurring at the completion of tray seating and at the time of the initial setting of the materials wt:~ measured in the palate and on the crest of the residual ridge. Because there would have been several variables during intraoral impression polocedures that would have tended to obscure the influence of the various factors upon the pressures produced, and in order to measure pressures exerted against the mucosal surface, an oral analogue was constructed so that precise measurements in thrl appropriate locations could be made in the laboratory. For this an average-&et1 maxillary arch was duplicated in acrylic resin having a covering of a soft siliron~~ rubber denture liner, Molloplast B% (Fig. 1) . MEASURING
APPARATUS
A pressure gauge was constructed of brass tubing with one end covered with a thin flexible rubber membrane and the other end connected to a polyethylene tube (Fig. 2). The gauge and connecting tubing were filled with distilled water, and the free end of the tubing was then connected to an unbonded wire strain gauge+ (Fig. 3) . Pressure applied to the rubber membrane caused water displacement in the closed system, and the water displacement was converted to an electrical signal b\ the strain gauge. The signal was amplified and recorded by an oscil1ograph.S Calibration of each strain gauge was done before and after each testing session with ;L mercury manometer. “Kostner
and Company,
+Model
P23AA,
Statham
$Model
150, Sanborn
Oberursel/Taunus, Instruments,
Company,
Germany.
Inc., Los Angeles, Calif.
Waltham,
Mass.
402
J. Pros. Dent. October, 1969
Frank
plug B adapted Fig. 2. The pressure gauge consists of rubber membrane A and watertight to the ends of brass tube C. Metal disk D enables the gauge to be bolted to the analogue E. Polyethylene tube F connects the gauge to a pressure transducer.
Fig. 3. The pressure
VALIDITY
transducer
consisted
of an unbonded
wire strain
gauge.
OF THE ANALOGUE
The validity of using an analogue was ascertained by comparing the results obtained clinically with those obtained in the laboratory under simulated clinical conditions. For this part of the study, the pressure gauges were mounted on the impression tray flush with the internal surface of the tray. The subject, whose maxillary arch the analogue duplicated, was recalled for this test. The amount of force applied to the tray by the dentist was monitored by an apparatus consisting of a soft, plastic, water-filled tube placed between the tray and an acrylic resin plate (Fig. 4). Force applied against the acrylic resin plate caused a
Imjression
Fig. 4. The apparatus
under
the tray was used to monitor
Fig. 5. The distance water was displaced applied
into the smaller
fbessures
the force applied
tube indicated
403
to the tram.
the amount
of f’o~ce
to the tray.
measurable displacement of the water from the tube into a smaller tube lying alongside a millimeter rule (Fig. 5). The distance the water was displaced into the small tube indicated the amount of force used. Force was applied to the tray until the subject indicated that at least one tray stop on each side was completely seated. Twenty-one irreversible hydrocolloid impressions were made in this mamrer, and the mean time and amount of force used in seating the tray were computed. The same procedure was then conducted on the analogue, using the mean time and force previously determined. The tray stops were found to be seated, and higher forc,es were recorded in the palatal area than in the ridge crest area when testing both rite, subject and the analogue. Incomplete seatina of the tray resulted when either the force or the time of force application was decreased. It was concluded that the analogue was an acceptable substitute for the purposes of this study. The pressure gauges were then mounted in the analogue flush with the mucosal surface for the remainr!t*r of the study.
Fig. 6. The motor and keys E.
has been altered
by the addition
I Ll
of microswitches
-----D ; C
A and B, bolt C, nut D,
u
--;
L!i!!L B A
Fig. 7. The relationship of motor, tray, pressure gauge, and analogue to each other is illustrated: A, motor; B, motor shaft; C, threaded shaft extension; D, nut; E, metal tray support; F, tray seating guide; G, tray; H, analogue; Z, pressure gauge; ,J, analogue support.
Volume 22 Number 4
Fig. 8. The over-all
Impression
p~txures
405
view of the apparatus.
SEATING THE TRAYS In order to minimize the variables present during manual seating of an impression tray, and thus to enable critical comparison of types of trays and materials, a motor* with an electronic feedback mechanism? added to maintain a constant speed was used to carry the loaded impression tray to place (Fig. 6) . A nut was screwed on the threaded shaft, and the nut was prevented from turning with the shaft by keying the nut in a slot on the motor frame. As the shaft revolved, therefore, the nut was raised or lowered, depending on the motor direction. The distance of travel of the nut was regulated by microswitches so that the degree of seating 01 the tray could be precisely duplicated. Four brass rods were attached to the periphery of the analogue support parallel analogue, and a metal plate, centered over the tray by the rods, was attached to to each other and to the motor shaft. An impression tray was then seated upon the the outer surface of the tray. The rods thus served as mechanical guides for seating the tray by the motor. The analogue was then fixed in position above the motor shaft so that complete seating of the tray coincided with the maximum rise of the nut on the motor shaft (Fig. 7). A water bath supplied water at body temperature *Model 4K862, Dayton Electric Manufacturing Company, Chicago, tSpeedia1, Lutron Electronics Company, Inc., Emmaus, Pa.
Ill.
406
Frank
J. Pros. Dent. October, 1969
to soak the analogue prior to making an impression, so that the setting times of the materials were consistent with those noted intraorally. The entire system is shown in Fig. 8. STANDARDIZED TRAYS A standardized method of construction of individual trays was adopted. A wax spacer was evenly adapted to a cast of the analogue except in the region of the posterior palatal seal. An irreversible hydrocolloid impression of the completed waxed cast was then made, and poured in dental stone. The resultant cast served as the master cast upon which all trays incorporating relief were constructed. Trays without relief or stops were all made directly upon a cast of the analogue. THE IMPRESSIONS A standard amount of each of the impression materials was used for each impression. The time used for mixing a material, and for loading a tray, remained constant during the study. After pressures had been measured, using trays with and without relief space and no holes, five escape holes were drilled equidistantly from each other and one quarter inch from the center of each pressure gauge with a No. 6 round bur. When testing of these trays was completed, many holes with about the same spacing as before were drilled throughout the tray. The final procedure undertaken was manually making impressions of the analogue in a clinical manner to determine the validity of laboratory results. The analogue was bolted to the roof of a clear plastic box and provided with an environment similar to that found intraorally regarding temperature and humidity. The tray design found to be most effective and the one found least effective in reduction of pressures when seated by the motor were tested. Each of three dentists made four impressions of the oral analogue with each tray design and with each of the four impression materials studied. The only instruction for impression making given to the dentists was to make the impressions in their usual manner. RESULTS Analysis of variance was used to statistically evaluate the effect of the variables on the pressures recorded. Duncan’s multiple range test1 was used to isolate the factor (s) responsible for the variation found in the analysis. Figs. 9 through 1% illustrate the results of the statistical tests. Initial pressures, mechanically produced. The highest pressures recorded with any material occurred when neither escape holes nor relief was used. Addition of relief to the tray resulted in 70 per cent as much pressure, while use of escape holes yielded 65 per cent as much pressure. Relief and escape holes together in trays resulted in only 45 per cent as much pressure as an unrelieved tray (Fig. 9). Differences in the amount of pressure received by the palate and ridge crest areas were not statistically significant with one exception. When trays with no relief or holes were used, the ridge crest received much more pressure than did the palate (Fig. 10). Major differences in pressure production were noted among the materials. The
Impression
pressures
407
5-
I OFig. 9. Initial space and/or
Holes
Relief
pressures during mechanical escape holes to the tray.
Neither
Both
tray seating were reduced
by the addition
of relief
I 0
Ridge
Palate
NO RELIEF
Ridge
Palate
RELIEF
Fig. 10. Initial pressures against the crest of the ridge and the palate were nearly identical when relieved trays were seated mechanically. The ridge crest received much more pressure than did the palate when unrelieved trays were used.
more viscous mix of irreversible hydrocolloid caused significantly higher pressures than any other material. The rubber and the thinner mix of irreversible hydrocol.loid were not significantly different from each other in pressure production, but did result in 66 per cent as much pressure as the thicker irreversible hydrocolloid. The use of the zinc oxide and eugenol paste caused the least pressure: 43 per cent of tha.t of the thick irreversible hydrocolloid and 65 per cent of that of either the rubber or the thinner mix of irreversible hydrocolloid (Fig. 11) . End pressures, mechanically produced. The highest pressures were recorded when an unrelieved tray was used. The placement of escape holes or relief in the
408
J. Pros. Dent. October, 1969
Frank
0
Alginate (Regular)
Fig. 11. Major differences mechanical tray seating.
Fig.
12. Adding
relief
Rubber
Alginate (Thin)
were noted among the materials
to the tray
resulted
in much
smaller
relative
Zn OE to initial
end pressures
pressures during
during mechanical
tray seating. tray resulted in 16 per cent as much pressure. Combining escape holes with a relief space resulted in only 5 per cent of the pressure associated with an unrelieved tray (Fig. 12). No statistically significant difference in pressure gauge sites relative to the amount of pressure recorded was found except when an unrelieved tray was used. Under that condition twice as much pressure was recorded over the ridge crest as in the palatal vault (Fig. 13) . Differences in pressures among the materials tested were not pronounced. The highest pressures were recorded when using rubber or the thicker mix of irreversible hydrocolloid in trays with no escape holes or relief space, followed by the zinc oxide and eugenol paste or thinner mix of irreversible hydrocolloid in the same type of tray. No statistically significant differences among materials were found relative to
Volume 22 Number 4
Imj!ression
NO RELIEF Fig. 13. Little difference mechanical tray seating
NO RELIEF
409
RELIEF
was found between the ridge crest and palatal except when an unrelieved tray was used.
Fig. 14. Significant differences in end pressures among chanical seating only when unrelieved trays were used.
pressuwr
end pressures during
RELIEF the materials
resulted
during
mc-
pressure when trays with either escape holes, relief space, or both were used. In no instance did the pressure fall below zero (Fig. 14) . Initial pressures, manually produced. The pressures caused by the three dentists ranged from a little less than one half pound per square inch to 4v4 pounds per square inch. No two dentists produced the same degree of pressure while making an impression. The same relationship of mean pressures was found with manual tray seating as before regarding the presence or absence of tray relief and the type of material, as shown in Figs. 15 and 16. One of the dentists also made the specified number of impressions with the vari-
410
J. Pros. Dent. October. 1969
Frank
RELIEF NO RELIEF Fig. 15. Initial pressures exerted against the palate and the ridge crest during manual tray seating were nearly the same except when unrelieved trays were used.
(Regular) Fig. 16. The same order of materials manually
as that
found
during
(Thin)
ranked by initial pressures was found mechanical tray seating.
with
trays seated
ous materials that resulted in no statistically significant difference attributable to the type of material used. Differences between pressures with the various materials were more pronounced when the impressions were made by the other two dentists. End pressures, manually produced. Pressures ranged from a negative one quarter pound per square inch to a positive 1 pound per square inch. Negative pressures were often produced by all of the dentists. The negative pressures occurred almost exclusively while using the relieved trays, and were evenly divided between the ridge crest gauge and the palatal gauge. Impressions made with the thick irreversible hydrocolloid, however, seldom resulted in the occurrence of negative pressure.
Impression 1.5
-
1.0
-
pressures
411
; 4.0.5
-
0 -0.25’
Ridge
Palate
NO RELIEF
Ridge Palate RELIEF
Fig. 17. The highest end pressures during manually made impressions were found over the ridge crest when trays with no relief were used. Negative pressures were often recorded while using trays with relief space and escape holes.
I .5-
l.O; QL 0.5 -
(15%) Rubber (Regular)
(Thin )
Fig. 18. Differences in end pressures relative to the type of material used were not statistically significant except for the thick mix of irreversible hydrocolloid during manual seating.
The highest pressures were recorded over the ridge crest while impressions were made with an unrelieved tray; 54 per cent as much pressure was recorded in the palate. The differences in pressures between the ridge and palate were not statiscitally significant while impressions were made with relieved trays (Fig. 17) . Two of the dentists produced higher pressures with the unrelieved trays than with the relieved ones, but the third dentist produced essentially equal pressures with either type of tray. No statistically significant differences were seen in pressures relative to the type of impression material with any of the dentists with one exception. The use of the
412
Frank
.I. Pros. Dent. October, 1969
thick mix of irreversible hydrocolloid resulted in significantly higher pressures for two of the dentists than did the use of the other materials (Fig. 18). DISCUSSION
In 1925 Stansbery2 claimed that higher pressures would be found in the region of the palatal vault than elsewhere when a maxillary impression was made. His statement was based upon the results seen when impression compound was compressed between two blocks of plaster, one of which was perforated: a larger amount of compound was forced into the central perforations than into the peripheral ones. Douglas, Bates, and Wilson3 mounted a variable capacitance transducer on an impression tray and measured the forces occurring while intraoral impressions were made with various brands of zinc oxide and eugenol paste. Forces arising in the palatal vault area could not be directly compared with those occurring elsewhere in the arch since one transducer had been located over the center of the palatal vault in one subject, and over the palatal slope of the ridge in another subject. They believed that the higher values of impression pressures found in the first subject may have been due to the more central location of the gauge. In the present study, a direct comparison of pressures in the ridge and palatal areas has been possible, and the results in general do not confirm previous beliefs. Instances in which trays with relief were used showed no statistically significant difference in pressure against the palate and the ridge crest. Only when trays with no relief were used was a significant difference in pressures found. Negative pressures were recorded during impressions made by the dentists, and were probably due to the dentist inadequately supporting the tray against gravity while waiting for the initial set of the impression material to occur. Any change in pressure near the beginning of the initial set due to tray movement could result in an inaccurate impression. That tissues covering an edentulous ridge can be displaced by an impression procedure has been graphically demonstrated by Woelfel.4 Different tissue contours resulted when impressions were made of one edentulous maxillary arch with five different materials. Tissue displacement occurred to a variable extent with all materials, although the largest displacement occurred over the ridge crest when using the rubber impression material. That finding is consistent with the results of this study. A most important aspect of this study has been the finding that pressure can, in fact, be selectively applied or reduced. Demonstration of the capability of several methods to alter impression pressures should encourage the dentist to modify his technique to be more in accordance with the soft tissue conditions presented by each patient. It is reasonable to assume a reduction in pressure would result in less displacement of the soft tissue. Reduced pressure would be most likely achieved with the use of a zinc oxide and eugenol paste material in a tray constructed with relief space and escape holes, and gently carried to place. SUMMARY
A method for measuring pressures produced during edentulous impression procedures has been described. An oral analogue was constructed, and an electric
Impression
pressures
413
motor was used to carry loaded impression trays to place to minimize variables. Impressions of the analogue were then made by dentists in a clinical manner to establish the validity of the laboratory results. The influence of several impression materials and tray modifications upon these pressures was determined. CONCLUSION
It was concluded from this investigation that impression pressures can be controlled by tray design and material selection. I wish to thank United States Public
the Educational and Research Foundation of Prosthodontics Health Service for the generous support given to this study.
and the
References Ann Arbor, 1957, Edwards Brothers, 1. Li, J. C. R.: Introduction to Statistical Inference, Inc., p. 238. 2. Stansbery, C. J.: The Negative Pressure Method of Impression Taking, J. A. D. A. 12: 438-445, 1925. 3. Douglas, W. H., Bates, J. F., and Wilson, H. J.: A Study of Zinc Oxide-Eugenol Type Impression Pastes, Brit. D. J. 116: 34-36, 1964. 4. Woelfel, J. B.: Contour Variations in Impressions of One Edentulous Patient, J. PROS. DENT. 12: 225-254, 1962. UNITED STATES PUBLXC HEALTH SERVICE HOSPITAL 210 STATE ST. NEW ORLEANS, LA. 70118