- Email: [email protected]
silicone impression material
SCHRADER
3. 4. 5.
6. 7.
8. 9. IO.
Rosner D: Function, placement, and reproduction of hevels fox gold castings. J PROSTHETDENT 13:1160. 1963. Miller LL: Framework design in ceramo-metal restorations. Dent Clin North Am 21:699, 1977. Goctdacre CJ, Van Roekel NB, Dykema RW, Ullman RB: l‘he collarless metal-ceramic crown. J PROSTHET DENT 38~612, 1977. Toogood GD, Archibald JF: Technique for establishing porcelain margins. J PROSTHETDENT 403464, 1978. Schneider DM, Levi MS, Mori DF: Porcelain shouldw adaptation using direct refractory dies. J PRCSTHET DENT 36:583, 1976. McLean JW, Jeansonne EE, Bruggers HB, Lynn DB: A new metal-ceramic crown. J PROSTHETDENT 40~273, 1978. Vryonis P: A simplilied approach to [he ctrmplr~r porcelain margin. J PROSTHETDENT 42:592, 1979. Sozio RB, Riley EJ: A precision ceramic-metal resmra&n with a facial butted margin. J PROSTHETDENT 37:517, 1977.
ET AL
I I.
Prince Jo Donovan TE, Presswmd KG: ‘The all-pc~~uelain labi;~l margin for reramomelal rcsmrations: A new ctmwpi J PROSTHET DENT 50~793, 1983 12. Prinw J> Donovan T: ‘l‘he esthetic, metal-(cwlnic. marqin ,\ comparison of techniques. J PROSTHET DEN.I 50: I X5, 1983. 13. ,Johnsltrn JF, Mumford (;, Dykcma RW hi&~-n Pr.tctice in I)cnlal Ceramics. Philadelphia. 1967, M% Snundw~ ( :(I, pp IOT-126. 14. Phillips RW: Skinner‘s Scienrc 01 Drnul ~l.itrria/\, cd K. Philadelphia, 1982. WB Saunders (:o, pp iO3-i ill, lt’c~piiil r/Ypr<~\/,ill. DR. E. STEVEN DUKE I)EPAR~EN~~oF GENERAL DENTISTRY LYILFORD HALL, USAF MEDICAL CENTER sc;Lx; I.XKLAND
AFB, TX 78236
D. R. Davis, D.D.S.,* and J. S. PreMe, D,D.S.** University of California, School of Dentistry, San Francisco, Calif.
E
lastic impression materials have improved greatly in accuracy and in manipulation since the introduction of agar-base reversible hydrocolloid in 1929. The polysulfide, polyether, condensing silicone, and addition silicone impression materials are as accurate as reversibie hydrocolloids and are superior in the physical properties of strain in compression, set in compression, and strain at point of rupture. 1*2Unlike reversible hydrocolloids, they are generally hydrophobic; the presence of crevicular fluid or hemorrhage causes voids in the impressions, typically at gingival margins. Polyether impression material is an exception; it is somewhat hydrophilic, but water extracts the plasticizer and causes dimensional changes.‘, 3,4 A new impression material (Ultrafine, Buffalo Dental Corp., Brooklyn, N.Y.) has been introduced. The manufacturer states that the material is a combination of alginate (irreversible hydrocolioid) substitute and silicone that jains the hydrophilic properties of the hydrocolloids to the superior physical properties of the silicones. It is a single-stage material that must be used in a
*.\\sisrant Prufessrx. Department of Restorative Dentistry. **.\ssistan~ Clinical Professor, Department of ResmraGve Dentisu-y.
304
wet environment and is purprted to be s&&&&y accurate for use in clinical prep. The -g*,pq3e of this study was to evaluate the a~u~~~~~. and compare its accuracy with that of a F stage, condensing silicone impnestin m@&iart &SW+ pren-Optosil, Unitek, Monrovia, C&f.>. Previous studies of the accuracy of i ~4W+4al used both master casting teehhniq~es w&& rn*j dies.5-” Examination of basic phyai&l prq+ties has also been used to evaluate a~~uraeies of. ieon materials and to illustrate their ~~i~r~,unde~ a variety of circumstances.2,12S’4UiGtiunately the ircewersible hydrocolloid/silicone was incompatibIe with m&4. dies; the injection material consistently tore on remova& regardless of whether water or surfactast s~~~~-~~~~ manuFacturer was used to wet the. d&z: A @iit system was &&gmxl that wo&4 sg&44y‘G!kq, $I@ dentist, be of statistical rctlev~e, aad -be with the material tested.
MATB?JAL
AND M
A master model was prepared to sSgbte tions for a three-unit f&d pa&& de&+@. diblrlar hu-man molars w (L.D. CauIk Co., Mil&nd MARCH
1986
VOLUME
55
:I$*. Qhn NUMBER
3
HYDROPHILIC
IRREVERSIBLE
HYDROCOLLOIDISILICONE
IMPRESSION
Fig. 1. Prepared teeth. Note undercuts in teeth adjacent to preparations.
edentulous space. Tray stops of the same resin were added to the model to provide positive, reproducible seating for impression trays. A diamond bur was used to prepare the complete crown abutments. The teeth adjacent to the abutments were not prepared and were positioned to form gingival undercuts of about 1 mm, roughly equivalent to a periodontally compromised dentition (Fig. 1). Threaded 0.017-inch TMS pins (Whaledent, New York, N.Y.) were inserted at four approximately equidistant points in the occlusal surface of each abutment so that only the rounded heads of the pins protruded (Fig. 2). These served as points for measurements of occlusal widths and span length. The pins were assigned letters for later reference. Except during application of the injection material and seating of trays, the master model was stored in a humidor at room temperature (21” + 1” C). The humidor consisted of a polyethylene container, a cotton towel saturated with water covering the floor, and a tightly fitting polyethylene lid.
Master casting Palladium-silver alloy complete crown retainers were made by direct waxing on the abutments and centrifugal casting technique. Windows were prepared in the facial aspects of the retainers to allow assessmentof the degree of seating on the casts that were to be made. A T-shaped piece of the same alloy was cast to fit between and across the occlusal surfaces of the retainers and was soldered to them. The master casting was placed on the master model, and the fit was verified by the absence of noticeable rocking of the unit and visual assessment without magnification of the castings’ adaptation to the abutments at the facial windows and facial and lingual margins. Examination of the facial and lingual margins of the castings under X15 magnification showed them to be open. This was caused by overexpansion of the copings. However, since all judgments and comparisons of fit were made by assessment without magnification, THE JOURNAL
OF PROSTHETIC
DENTISTRY
MATERIAL
Fig. 2. Pins embedded in preparations. Note tray stops.
Fig. 3. Master casting is fit onto preparations.
the lack of marginal integrity does not affect the results or conclusions. The interproximal margins were not included in the evaluation because it was impossible to achieve good wax adaptation (Fig. 3).
Impressions Five impressions were made with the condensing silicone and irreversible hydrocolloid/silicone impression materials. Rigid metal partial impression trays (RimLock, L.D. Caulk Co.) were used throughout to minimize flexion and distortion of the impression material. Condensing silicone. Trays were customized with extra heavy-bodied tray material before making the final impression. Tray adhesive supplied by the manufacturer was used to coat the trays. Hollow wax thimbles 1 mm thick axially and occlusally were adapted to the abutments to provide a uniform space, because uniformity of thickness and distribution of impression material affects accuracy.5*9,15-” They- were removed after the customized trays were made. The base tray material was mixed with the catalyst, and the filled tray was seated firmly onto the model and allowed to set. Setting was determined by rebound when pressed with a blunt instrument. An appropriate amount of catalyst was mixed for 30 seconds in a small plastic bowl with the necessary 305
DAVIS
AND
PREBLE
Meatswements
Fig. 4. Diagram of preparations shows points between which measurements were made. Dotted lines represent gingival margins.
Fig. 5. Occlusogingival distortion reseating dies in master casting.
is demonstrated by
amount of light-bodied base injection material. Sufficient injection material to cover all surfaces was applied to the abutments. The remainder of the injection material was placed in the customized tray, which was then seated firmly on the master model. Irreversible hydrocolloid/silicone. In accordance with the manufacturer’s instructions and because the irreversible hydrocolloid/silicone is a single-stage impression material, neither customized trays nor tray adhesive was used. Equal portions of base and catalyst of the injection material were placed on a mixing tablet, spatulated until streak free, and placed in a syringe. Equal portions of base and catalyst of the tray material were then spatulated in a rubber mixing bowl and placed in the tray. Sufficient injection material to cover all surfaces was applied to the abutments, and the tray was firmly seated. Excess impression material was removed as it was expressed from the trays. AI1 impressions were allowed to set for 10 minutes at room temperature in the humidor then removed from the model in one quick, vertical motion. Casts were immediately poured with a Densite stone (Die Keen, Columbus Dental, Columbus, Ohio) mixed at the recommended water-powder ratio. The stone casts were allowed to set for 45 minutes in the humidor, removed from the impressions, and allowed to stand for at least 24 hours at ambient temperature and humidity before they were trimmed and measured. 306
Measurements were made between the lettered points (Fig. 4) with a comparator microscope (Gaertner Scientific Co., Chicago, III.) (SD + 0.005 mmj and a handheld micrometer (Mitutoyo, Japan). The comparator microscope was used to measure distances between points A-B, G-H, and A-H. Measurements were made from the most distant edges of the pins, from the outer edge of one pin to the outer edge of another, whether involving the occlusal width of the abutments or the entire span length. Because of lack of reproducibility of measurements of the buccolingual dimensions at the occlusal (C-D and I-J) with the comparator microscope, the micrometer was used for all buccolingual dimensions. Five measurements were made for each lettered distance on the master model, and three measurements for each lettered distance were made on each cast. All measurements were made by one investigator. Evaluation of fit. After measurement, the master casting was placed on the stone casts with light finger pressure and rated by one investigator as to completeness of fit encompassing (1) the presence or absence of space occlusally between the casting windows and the casts and (2) obvious visible discrepancies at the facial and lingual margins. The casts were then marked on a lateral side with a straight mesiodistal line and sectioned between the abutments with a hand saw. The individual dies were then seated in the master casting and rated again according to the same criteria. Differences in line direction or cast orientation were noted. For example, if an impression distorted along the span in a buccolingual direction, one section of the cast would move buccally or lingually with respect to the other. If the impression distorted in an occlusogingival direction, one lateral line would move occlusally or gingivally with respect to the other. The measurements were tabulated; and the ranges? means, and standard deviations calculated for the master model and the casts. The results were compared statistically with a one-sample t-test to compare the means of casts of each of the impression materials to the measurements of the master model, and a two-sample t-test was used to compare casts from the two impression materials. RESULTS The means of measurements of casts from the condensing silicone and irreversible h~r~il~id/s~~eo~e were compared with the means of measurements of the master model. With the condensing silicone, si@ficant differences (p < 0.05) were noted in di~nsion~between points A-B, C-D, K-L, and A-H. With the irrepteraige hydrocolloid/silicone, significant d%Fences (jb < .J%) were noted between pc&ts A-B, -E--F, I-J, and A-N. When means of measurements of the casts from the two MARCH
1986
VOLUME
55
NUMBER
3
HYDROPHILIC
IRREVERSIBLE
HYDROCOLLOID/SILICONE
Table I. Comparison of measurements from impression material Master
IMPRESSION
in millimeters Condensing
model
MATERIAL
between the master model and casts made Irreversible
silicone
hydrocolloidlsilicone
Points
Mean
SD
Mean
SD
A’
Mean
SD
A*
A-B C-D E-F G-H
7.966 10.490 8.814 7.227 8.941 8.001 24.561
0.006 0.025 0.041 0.005 0.025 0.033 0.008
7.933 10.363 8.839 7.231 8.890 7.976 24.611
0.015 0.025 0.051 0.021 0.058 0.024 0.029
+0.027 -0.127 +0.025 +0.004 -0.051 -0.025 +0.050
8.048 10.439 8.915 7.259 8.992 8.001 24.668
0.009 0.061 0.053 0.030 0.043 0.022 0.003
+0.082 -0.051 +0.101 to.032 +0.051 0.0 to.106
1-J K-L A-H
*A = Dill’erence in millimeters belween cast measurement and master model measurement.
impressions materials were compared, significant differences were found between all points except G-H (Table I). The greatest difference between the master model and all casts was in the span length, A-H. The mean of A-H for the condensing silicone was longer than that of the master model by 0.05 + 0.029 mm, and that of the irreversible hydrocolloid/silicone was longer by 0.106 +0.003 mm. When the master casting was placed on casts made from the condensing silicone, one of the five casts fit the master casting completely (to the same degree as the master model), three were judged to be close to fitting, and one would not seat even two thirds of the way. When the master casting was placed on casts made from the irreversible hydrocolloid/silicone, none of the casts seated completely and none were considered close to fitting. The one cast’from the condensing silicone that fit the master casting completely differed from the master model in span length A-H by only 0.02%. The other four casts from the condensing silicone were longer than the span of the master model by 0.15% to 0.34%. When the individual dies were placed in the master casting, all 10 cast abutments from the condensing silicone fit well as viewed through the facial windows, as did nine of 10 cast abutments from the irreversible hydrocolloid/silicone. All 10 of the facial margins and nine of the lingual margins of dies from the condensing silicone were judged acceptable; one die was elongated lingually, and the margin was gingival to the casting. Only four of the possible 20 margins of dies made from the irreversible hydrocolloid/silicone were acceptable. The unacceptable margins were open and gingival to the margin of the master casting as was seen in one unacceptable margin of the condensing silicone. In nearly all casesin which sectioning of the casts and seating of the dies produced changes in line direction, distortion occurred in an occlusogingival direction; the lateral line extending from one abutment was seen to tip downward at the end of the cast and upward toward the center as if rotating about a buccolingual axis (Fig. 5). THE JOURNAL
OF PROSTHETIC
DENTISTRY
Two sectioned casts from the condensing silicone impression material shifted buccolingually. The one condensing silicone cast that seated completely exhibited no discernible shift in line direction or cast orientation. DISCUSSION The irreversible hydrocolloid/silicone was less accurate, although more reproducible, than the condensing silicone; casts from the irreversible hydrocolloid/silicone were more distorted and fit the master casting with less frequency than did casts from the.condensing silicone. However, others have indicated that successful single crowns and single inlays could be made with the irreversible hydrocolloid/silicone impression material.18 As noted in previous studies, undercuts seriously affect the accuracy of impression materials? lo,‘X 15,I9 Other factors affecting withdrawal of impression material from prepared teeth are friction, gravity, and contraction of impression material during the setting reaction.*, I3 Distortion of impressions as they are withdrawn from undercuts is related to strain in compression, the change in length per unit length caused by compression, the set in compression, the amount a material does not elastically recover after compression, and strain at point of rupture.‘) ’ Manufacturer’s values for strain in compression and set in compression for the injection material of the condensing silicone were within the limits for elastomers as listed by the American Dental Association (ADA) specifications No. 11 for hydrocolloids and No. 19 for nonaqueous elastomers. The strain in compression value for the injection material of the irreversible hydrocolloid/silicone does not meet the requirements of ADA specification No. 11 (18.7% versus the ADA requirements of from 4% to 1,%).I8 The set in compression value was not available for comparison. However, the tray material of the irreversible hydrocolloid/silicone is within the limits of ADA specification No. 19 and could possibly withstand the forces of withdrawal from undercuts without excessive distortion. Clearly, stretching without recovery occurred with both materials on withdrawal from the master model 307
and was more pronounced in the irreversible hydrocollaid/silicone than in the condensing silicone. In general, the buccolingual dimensions C-D, E-F, I-J, and K-L were smaller than those of the master model. One study showed that the condensing silicone recoils into the impression space after withdrawal from undercuts.“ Unfortunately, the micrometer was limited in accuracy to only +O.OOl inch (kO.025 mm). Because of the limited accuracy and subsequent large standard deviations, the buccolingual measurements were less meaningful than they might have been. The observed fit of the dies to the master casting at the facial and lingual margins is probably a better judgment of fit. In an effort to reduce the number of possible variables leading to dimensional changes, all manipulations of the materials were made at a uniform room temperature rather than making the impressions at intraoral temperatures and then pouring and storing them at room temperature. Thermal contraction would have introduced potential errors for which we had no correction factors. Although the irreversible hydrocolloid/silicone was kept in the humidor at room temperature while the impression material was setting and during setting of the stone, it is possible that some of the dimensional changes occurred because of syneresis.“,?’ Most of the mesiodistal changes were positive (0.043 to 0.106 mm), which compares favorably to the changes seen when impression materials shrink in a tray that tightly retains the material.+ I” CONCLUSION Under test conditions of fixed partial denture preparations, the presence of undercuts and minimum angle to the preparations, the irreversible hydrocolloid/silicone impression material was found to be inaccurate when casts were compared with the master model or with casts made from a condensing silicone impression material.
I -1
15
I (1 I-. IX. I’).
?(I.
?I.
REFERENCES
308
MARCH
1986
VOI.tJMI!
55
NUMBER
3
3. 4. 5.
6. 7.
8. 9. IO.
Rosner D: Function, placement, and reproduction of hevels fox gold castings. J PROSTHETDENT 13:1160. 1963. Miller LL: Framework design in ceramo-metal restorations. Dent Clin North Am 21:699, 1977. Goctdacre CJ, Van Roekel NB, Dykema RW, Ullman RB: l‘he collarless metal-ceramic crown. J PROSTHET DENT 38~612, 1977. Toogood GD, Archibald JF: Technique for establishing porcelain margins. J PROSTHETDENT 403464, 1978. Schneider DM, Levi MS, Mori DF: Porcelain shouldw adaptation using direct refractory dies. J PRCSTHET DENT 36:583, 1976. McLean JW, Jeansonne EE, Bruggers HB, Lynn DB: A new metal-ceramic crown. J PROSTHETDENT 40~273, 1978. Vryonis P: A simplilied approach to [he ctrmplr~r porcelain margin. J PROSTHETDENT 42:592, 1979. Sozio RB, Riley EJ: A precision ceramic-metal resmra&n with a facial butted margin. J PROSTHETDENT 37:517, 1977.
ET AL
I I.
Prince Jo Donovan TE, Presswmd KG: ‘The all-pc~~uelain labi;~l margin for reramomelal rcsmrations: A new ctmwpi J PROSTHET DENT 50~793, 1983 12. Prinw J> Donovan T: ‘l‘he esthetic, metal-(cwlnic. marqin ,\ comparison of techniques. J PROSTHET DEN.I 50: I X5, 1983. 13. ,Johnsltrn JF, Mumford (;, Dykcma RW hi&~-n Pr.tctice in I)cnlal Ceramics. Philadelphia. 1967, M% Snundw~ ( :(I, pp IOT-126. 14. Phillips RW: Skinner‘s Scienrc 01 Drnul ~l.itrria/\, cd K. Philadelphia, 1982. WB Saunders (:o, pp iO3-i ill, lt’c~piiil r/Ypr<~\/,ill. DR. E. STEVEN DUKE I)EPAR~EN~~oF GENERAL DENTISTRY LYILFORD HALL, USAF MEDICAL CENTER sc;Lx; I.XKLAND
AFB, TX 78236
D. R. Davis, D.D.S.,* and J. S. PreMe, D,D.S.** University of California, School of Dentistry, San Francisco, Calif.
E
lastic impression materials have improved greatly in accuracy and in manipulation since the introduction of agar-base reversible hydrocolloid in 1929. The polysulfide, polyether, condensing silicone, and addition silicone impression materials are as accurate as reversibie hydrocolloids and are superior in the physical properties of strain in compression, set in compression, and strain at point of rupture. 1*2Unlike reversible hydrocolloids, they are generally hydrophobic; the presence of crevicular fluid or hemorrhage causes voids in the impressions, typically at gingival margins. Polyether impression material is an exception; it is somewhat hydrophilic, but water extracts the plasticizer and causes dimensional changes.‘, 3,4 A new impression material (Ultrafine, Buffalo Dental Corp., Brooklyn, N.Y.) has been introduced. The manufacturer states that the material is a combination of alginate (irreversible hydrocolioid) substitute and silicone that jains the hydrophilic properties of the hydrocolloids to the superior physical properties of the silicones. It is a single-stage material that must be used in a
*.\\sisrant Prufessrx. Department of Restorative Dentistry. **.\ssistan~ Clinical Professor, Department of ResmraGve Dentisu-y.
304
wet environment and is purprted to be s&&&&y accurate for use in clinical prep. The -g*,pq3e of this study was to evaluate the a~u~~~~~. and compare its accuracy with that of a F stage, condensing silicone impnestin m@&iart &SW+ pren-Optosil, Unitek, Monrovia, C&f.>. Previous studies of the accuracy of i ~4W+4al used both master casting teehhniq~es w&& rn*j dies.5-” Examination of basic phyai&l prq+ties has also been used to evaluate a~~uraeies of. ieon materials and to illustrate their ~~i~r~,unde~ a variety of circumstances.2,12S’4UiGtiunately the ircewersible hydrocolloid/silicone was incompatibIe with m&4. dies; the injection material consistently tore on remova& regardless of whether water or surfactast s~~~~-~~~~ manuFacturer was used to wet the. d&z: A @iit system was &&gmxl that wo&4 sg&44y‘G!kq, $I@ dentist, be of statistical rctlev~e, aad -be with the material tested.
MATB?JAL
AND M
A master model was prepared to sSgbte tions for a three-unit f&d pa&& de&+@. diblrlar hu-man molars w (L.D. CauIk Co., Mil&nd MARCH
1986
VOLUME
55
:I$*. Qhn NUMBER
3
HYDROPHILIC
IRREVERSIBLE
HYDROCOLLOIDISILICONE
IMPRESSION
Fig. 1. Prepared teeth. Note undercuts in teeth adjacent to preparations.
edentulous space. Tray stops of the same resin were added to the model to provide positive, reproducible seating for impression trays. A diamond bur was used to prepare the complete crown abutments. The teeth adjacent to the abutments were not prepared and were positioned to form gingival undercuts of about 1 mm, roughly equivalent to a periodontally compromised dentition (Fig. 1). Threaded 0.017-inch TMS pins (Whaledent, New York, N.Y.) were inserted at four approximately equidistant points in the occlusal surface of each abutment so that only the rounded heads of the pins protruded (Fig. 2). These served as points for measurements of occlusal widths and span length. The pins were assigned letters for later reference. Except during application of the injection material and seating of trays, the master model was stored in a humidor at room temperature (21” + 1” C). The humidor consisted of a polyethylene container, a cotton towel saturated with water covering the floor, and a tightly fitting polyethylene lid.
Master casting Palladium-silver alloy complete crown retainers were made by direct waxing on the abutments and centrifugal casting technique. Windows were prepared in the facial aspects of the retainers to allow assessmentof the degree of seating on the casts that were to be made. A T-shaped piece of the same alloy was cast to fit between and across the occlusal surfaces of the retainers and was soldered to them. The master casting was placed on the master model, and the fit was verified by the absence of noticeable rocking of the unit and visual assessment without magnification of the castings’ adaptation to the abutments at the facial windows and facial and lingual margins. Examination of the facial and lingual margins of the castings under X15 magnification showed them to be open. This was caused by overexpansion of the copings. However, since all judgments and comparisons of fit were made by assessment without magnification, THE JOURNAL
OF PROSTHETIC
DENTISTRY
MATERIAL
Fig. 2. Pins embedded in preparations. Note tray stops.
Fig. 3. Master casting is fit onto preparations.
the lack of marginal integrity does not affect the results or conclusions. The interproximal margins were not included in the evaluation because it was impossible to achieve good wax adaptation (Fig. 3).
Impressions Five impressions were made with the condensing silicone and irreversible hydrocolloid/silicone impression materials. Rigid metal partial impression trays (RimLock, L.D. Caulk Co.) were used throughout to minimize flexion and distortion of the impression material. Condensing silicone. Trays were customized with extra heavy-bodied tray material before making the final impression. Tray adhesive supplied by the manufacturer was used to coat the trays. Hollow wax thimbles 1 mm thick axially and occlusally were adapted to the abutments to provide a uniform space, because uniformity of thickness and distribution of impression material affects accuracy.5*9,15-” They- were removed after the customized trays were made. The base tray material was mixed with the catalyst, and the filled tray was seated firmly onto the model and allowed to set. Setting was determined by rebound when pressed with a blunt instrument. An appropriate amount of catalyst was mixed for 30 seconds in a small plastic bowl with the necessary 305
DAVIS
AND
PREBLE
Meatswements
Fig. 4. Diagram of preparations shows points between which measurements were made. Dotted lines represent gingival margins.
Fig. 5. Occlusogingival distortion reseating dies in master casting.
is demonstrated by
amount of light-bodied base injection material. Sufficient injection material to cover all surfaces was applied to the abutments. The remainder of the injection material was placed in the customized tray, which was then seated firmly on the master model. Irreversible hydrocolloid/silicone. In accordance with the manufacturer’s instructions and because the irreversible hydrocolloid/silicone is a single-stage impression material, neither customized trays nor tray adhesive was used. Equal portions of base and catalyst of the injection material were placed on a mixing tablet, spatulated until streak free, and placed in a syringe. Equal portions of base and catalyst of the tray material were then spatulated in a rubber mixing bowl and placed in the tray. Sufficient injection material to cover all surfaces was applied to the abutments, and the tray was firmly seated. Excess impression material was removed as it was expressed from the trays. AI1 impressions were allowed to set for 10 minutes at room temperature in the humidor then removed from the model in one quick, vertical motion. Casts were immediately poured with a Densite stone (Die Keen, Columbus Dental, Columbus, Ohio) mixed at the recommended water-powder ratio. The stone casts were allowed to set for 45 minutes in the humidor, removed from the impressions, and allowed to stand for at least 24 hours at ambient temperature and humidity before they were trimmed and measured. 306
Measurements were made between the lettered points (Fig. 4) with a comparator microscope (Gaertner Scientific Co., Chicago, III.) (SD + 0.005 mmj and a handheld micrometer (Mitutoyo, Japan). The comparator microscope was used to measure distances between points A-B, G-H, and A-H. Measurements were made from the most distant edges of the pins, from the outer edge of one pin to the outer edge of another, whether involving the occlusal width of the abutments or the entire span length. Because of lack of reproducibility of measurements of the buccolingual dimensions at the occlusal (C-D and I-J) with the comparator microscope, the micrometer was used for all buccolingual dimensions. Five measurements were made for each lettered distance on the master model, and three measurements for each lettered distance were made on each cast. All measurements were made by one investigator. Evaluation of fit. After measurement, the master casting was placed on the stone casts with light finger pressure and rated by one investigator as to completeness of fit encompassing (1) the presence or absence of space occlusally between the casting windows and the casts and (2) obvious visible discrepancies at the facial and lingual margins. The casts were then marked on a lateral side with a straight mesiodistal line and sectioned between the abutments with a hand saw. The individual dies were then seated in the master casting and rated again according to the same criteria. Differences in line direction or cast orientation were noted. For example, if an impression distorted along the span in a buccolingual direction, one section of the cast would move buccally or lingually with respect to the other. If the impression distorted in an occlusogingival direction, one lateral line would move occlusally or gingivally with respect to the other. The measurements were tabulated; and the ranges? means, and standard deviations calculated for the master model and the casts. The results were compared statistically with a one-sample t-test to compare the means of casts of each of the impression materials to the measurements of the master model, and a two-sample t-test was used to compare casts from the two impression materials. RESULTS The means of measurements of casts from the condensing silicone and irreversible h~r~il~id/s~~eo~e were compared with the means of measurements of the master model. With the condensing silicone, si@ficant differences (p < 0.05) were noted in di~nsion~between points A-B, C-D, K-L, and A-H. With the irrepteraige hydrocolloid/silicone, significant d%Fences (jb < .J%) were noted between pc&ts A-B, -E--F, I-J, and A-N. When means of measurements of the casts from the two MARCH
1986
VOLUME
55
NUMBER
3
HYDROPHILIC
IRREVERSIBLE
HYDROCOLLOID/SILICONE
Table I. Comparison of measurements from impression material Master
IMPRESSION
in millimeters Condensing
model
MATERIAL
between the master model and casts made Irreversible
silicone
hydrocolloidlsilicone
Points
Mean
SD
Mean
SD
A’
Mean
SD
A*
A-B C-D E-F G-H
7.966 10.490 8.814 7.227 8.941 8.001 24.561
0.006 0.025 0.041 0.005 0.025 0.033 0.008
7.933 10.363 8.839 7.231 8.890 7.976 24.611
0.015 0.025 0.051 0.021 0.058 0.024 0.029
+0.027 -0.127 +0.025 +0.004 -0.051 -0.025 +0.050
8.048 10.439 8.915 7.259 8.992 8.001 24.668
0.009 0.061 0.053 0.030 0.043 0.022 0.003
+0.082 -0.051 +0.101 to.032 +0.051 0.0 to.106
1-J K-L A-H
*A = Dill’erence in millimeters belween cast measurement and master model measurement.
impressions materials were compared, significant differences were found between all points except G-H (Table I). The greatest difference between the master model and all casts was in the span length, A-H. The mean of A-H for the condensing silicone was longer than that of the master model by 0.05 + 0.029 mm, and that of the irreversible hydrocolloid/silicone was longer by 0.106 +0.003 mm. When the master casting was placed on casts made from the condensing silicone, one of the five casts fit the master casting completely (to the same degree as the master model), three were judged to be close to fitting, and one would not seat even two thirds of the way. When the master casting was placed on casts made from the irreversible hydrocolloid/silicone, none of the casts seated completely and none were considered close to fitting. The one cast’from the condensing silicone that fit the master casting completely differed from the master model in span length A-H by only 0.02%. The other four casts from the condensing silicone were longer than the span of the master model by 0.15% to 0.34%. When the individual dies were placed in the master casting, all 10 cast abutments from the condensing silicone fit well as viewed through the facial windows, as did nine of 10 cast abutments from the irreversible hydrocolloid/silicone. All 10 of the facial margins and nine of the lingual margins of dies from the condensing silicone were judged acceptable; one die was elongated lingually, and the margin was gingival to the casting. Only four of the possible 20 margins of dies made from the irreversible hydrocolloid/silicone were acceptable. The unacceptable margins were open and gingival to the margin of the master casting as was seen in one unacceptable margin of the condensing silicone. In nearly all casesin which sectioning of the casts and seating of the dies produced changes in line direction, distortion occurred in an occlusogingival direction; the lateral line extending from one abutment was seen to tip downward at the end of the cast and upward toward the center as if rotating about a buccolingual axis (Fig. 5). THE JOURNAL
OF PROSTHETIC
DENTISTRY
Two sectioned casts from the condensing silicone impression material shifted buccolingually. The one condensing silicone cast that seated completely exhibited no discernible shift in line direction or cast orientation. DISCUSSION The irreversible hydrocolloid/silicone was less accurate, although more reproducible, than the condensing silicone; casts from the irreversible hydrocolloid/silicone were more distorted and fit the master casting with less frequency than did casts from the.condensing silicone. However, others have indicated that successful single crowns and single inlays could be made with the irreversible hydrocolloid/silicone impression material.18 As noted in previous studies, undercuts seriously affect the accuracy of impression materials? lo,‘X 15,I9 Other factors affecting withdrawal of impression material from prepared teeth are friction, gravity, and contraction of impression material during the setting reaction.*, I3 Distortion of impressions as they are withdrawn from undercuts is related to strain in compression, the change in length per unit length caused by compression, the set in compression, the amount a material does not elastically recover after compression, and strain at point of rupture.‘) ’ Manufacturer’s values for strain in compression and set in compression for the injection material of the condensing silicone were within the limits for elastomers as listed by the American Dental Association (ADA) specifications No. 11 for hydrocolloids and No. 19 for nonaqueous elastomers. The strain in compression value for the injection material of the irreversible hydrocolloid/silicone does not meet the requirements of ADA specification No. 11 (18.7% versus the ADA requirements of from 4% to 1,%).I8 The set in compression value was not available for comparison. However, the tray material of the irreversible hydrocolloid/silicone is within the limits of ADA specification No. 19 and could possibly withstand the forces of withdrawal from undercuts without excessive distortion. Clearly, stretching without recovery occurred with both materials on withdrawal from the master model 307
and was more pronounced in the irreversible hydrocollaid/silicone than in the condensing silicone. In general, the buccolingual dimensions C-D, E-F, I-J, and K-L were smaller than those of the master model. One study showed that the condensing silicone recoils into the impression space after withdrawal from undercuts.“ Unfortunately, the micrometer was limited in accuracy to only +O.OOl inch (kO.025 mm). Because of the limited accuracy and subsequent large standard deviations, the buccolingual measurements were less meaningful than they might have been. The observed fit of the dies to the master casting at the facial and lingual margins is probably a better judgment of fit. In an effort to reduce the number of possible variables leading to dimensional changes, all manipulations of the materials were made at a uniform room temperature rather than making the impressions at intraoral temperatures and then pouring and storing them at room temperature. Thermal contraction would have introduced potential errors for which we had no correction factors. Although the irreversible hydrocolloid/silicone was kept in the humidor at room temperature while the impression material was setting and during setting of the stone, it is possible that some of the dimensional changes occurred because of syneresis.“,?’ Most of the mesiodistal changes were positive (0.043 to 0.106 mm), which compares favorably to the changes seen when impression materials shrink in a tray that tightly retains the material.+ I” CONCLUSION Under test conditions of fixed partial denture preparations, the presence of undercuts and minimum angle to the preparations, the irreversible hydrocolloid/silicone impression material was found to be inaccurate when casts were compared with the master model or with casts made from a condensing silicone impression material.
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15
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REFERENCES
308
MARCH
1986
VOI.tJMI!
55
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