- Email: [email protected]
Dimensional stability of elastomeric impression materials in custom-made and stock trays
SCHNEIDER
AND
TAYLOR
Dimensional stability of elastomeric impression materials in custom-made and stock trays J, Valderhaug, I)cntal
Faculty.
Dr. Odont.,* University
of Oslo,
and F. Fl#ystrand*” and NIOM.
Scandinavian
R
ubber base impression materials are highly accurate and are widely used for making impressions in fixed prosthodontics.! ” Such materials are reported to he most stable when they have an even thickness of 2 to 4 mm, achieved with an acrylic resin custom-made impression tray.” ” However, the material may distort during setting. In addition, it has greater temperature dependence and other elastic properties compared with metal trays, which may jeopardize the good qualities of an impression material.‘. !I’ Finally, making custom trays is time consuming and costly. An alterna!ivc approach is the use of stock trays. Because little information exists concerning the reliability of this method, the present study assessedand compared the dimensional stability of impressions made in custommade acrylic resin trays and stock metallic trays. MATERIAL
AND
METHODS
Two metallic models of the upper jaw were produced. Model A had remaining teeth in the lateral segments with undercuts similar to those in a normal dentition. Model B had no intact teeth. Both models were supplied with standardized abutment teeth in the aren of the canines and first molars. The dimensions are shown in Fig. 1. The teeth were drilled from stainless steel with a taper of 10 degrees toward the flat
Institute
of Dental
Materials,
0+
Oslo,
Norway
Ab: 27996 +6 Bb: 34005 ,+6 +
0
Bd: 25265 I! 6
Bc: 25259 ,+ 6
Aa: 44978 It:6 Ba: 499922 6
0 +
Fig. 1. Schematic drawing of the abutment teeth showing distances in micrometers between center of crosses on occlusal surfaces. A = Brass model; B = aluminium model; a = distances between molars; b = distances between canines; c = distance between molar and canine on right ride; d = distance between molar and canine on left side.
occlusal surface and were fastened with screws. TWO grooves 5 pm wide were engraved at right angles in the center on the occlusal surfaces to provide points of reference that were easily discernible in a microscope (Nikon Profile Pro.jector, Nippon, Kogaku, Japan). Because of the restricted range of the projector (25 mm), additional reference points were located in the middle of the palate. Based on consecutive recordings of the distances on the models, the error of method was calculated to be within k6 Iurn (Fig. t ). The models and trays were OCTOBER
1984
VOLUME
52
NUMBER
4
CUSTOM
AND
STOCK
TRAYS
FOR IMPRESSION
STABILITY
Fig. 2. Photographs of A, brass model, and B, aluminium model, with corresponding trays mounted in jigs. Screws for removal of trays are seen on top of jigs.
Table I. Results from different Distance
Time (hours) 0 1 24 0 1 24 0 1 24 0 1 24
measurements
Impregum/stock
tray
+26
+27 +28 +22
+16 +20 +33
+10 +10
(in micrometers)
Impregumlcustom + 9
+24 +22 +15 +11 +9
+23 +42 +36 +16 +23 +34 +15 +24 +15
tray +26 +35 +27 -126 +17 +10 +12 +12
-11 -13
Xantoprenlstock
tray
+17 +25 +24
+24 +21 +14
-15
-12
+10 +17
-16
f27 t36 -41 +9 +21 -25
-16 -21 -35
-19 - 9 -11 -55
+10 +12
-22 -9
+9 +29 +20
Xantopren/custom
-13 -10 +19 +37
tray +10 +19 +12
-11
-19
Numbers represent deviation from distances measured on models. Each number is mean of four recordings made from impressions. Vacant spaces denote values within error of measurements (+-8pm).
mounted in jigs providing identical direction of insertion and removal of the trays that contained the impression materials (Fig. 2). The part of the jig containing the impression could be removed by turning screws located on each side of the tray. The custom trays were made of Formatray (batch No. 1 4086, Kerr/Sybron Corp., Romulus, Mich.). Special care was taken to obtain an acrylic resin thickness of 3 mm. The models for making the trays included spacing that allowed a thickness of 2 to 4 mm of impression material. All custom-made trays were stored for at least 24 hours prior to use.9 Nonperforated chromium-plated brass trays (Martin, Ehricke, West Germany) of a size considered most appropriate (No. 3) were selected as stock trays. The elastomeric materials chosen for this study were Impregum (batch No. BG 053, Espe, GmbH, Seefeld, West Germany) and Xantopren (Bayer Dental, Leverkusen, West Germany) light body (batch No. 2730 F) THE JOURNAL
OF PROSTHETIC
DENTISTRY
and medium body (batch No. 3442 F). All materials were handled according to the manufacturers’ instructions and the trays were thoroughly coated with the recommended adhesives. The experimental design included the following: (1) two types of impression trays, (2) two types of impression materials, (3) two clinical situations, and (4) variable times for storing the impressions. Three impressions were made of each combination to provide a total of 24 impressions. In the impressions of model A, the distances a and b were measured; and in the impressions of model B, the distances a, b, c, and d were measured (Fig. 1). The distances were assessedfour times. The measurements were performed immediately after removal of the impression from the model, and after 1 and 24 hours. The impressions were made, stored, and measured in a laboratory under constant conditions (21” C and 50% humidity). The statistical analysis included calculation 515
VALDERHAUG
AND
FLdYSTRAND
MODEL B DISTANCEb IN JI : S.O. OHOUR U 1HOUR EB 24 HOURS
DISTANCEb IN p : S.D.
P +40I-
D OHDUR i7 1HDUR tZB24HOURS
+ 30l+20 + 10
..-
..
- 10 - 20 -30 IMPR CUSTOM TRAY
XANTOP STOCK TRAY
XANTOP CUSTOM TRAY
Fig. 3. Time-dependent dimensional stability of recorded distances A6 made with stock and custommade trays with Impregum and Xantopren based on several recordings.
-40
i
IMPR. STOCK TRAY
IMPR.
XANTOP STOCK TRAY
%ioM
XANTDP CUSTOM TRAY
Fig. 4. Time-dependent dimensional stability of recorded distances Bb made with stock and custom made trays with Impregum and Xantopren based on several recordings.
trays was 42%. All the measurements except one complied with the requirements for dimensional stability of rubber impression material in the American Dental Association (ADA) Specification No. 19.” Figs. 3 and 4 show the time-dependent dimensional stability of the recorded distances Ab and Bb with stock and custom-made trays and Impregum and Xantopren f
Ii,j Oh lb, 24,-
Oh Ih 24h ---
Oh Ih 2dh
IMPR. STOCK TRAY
IMPR. CUSTOM TRAY
XANTOP. STOCK TRAY
Oh I,, E4h
XANTOP. CUSTOM TRAY
Fig. 5. Distribution of recorded values when surveyed according to calculated level of dimensional stability of 0.03%~.
of the mean of all corresponding registrations and the Student t test. RESULTS The engraved reference points on the occlusal surfakes were easily recognized in all impressions. The error of method when the distances in the impressions were assessedwas calculated to be within +-8 pm. The results from the different measurements are shown in Table I. Sixty-four percent of the measurements from the impressions made in stock trays were the same as those in the models. The corresponding value for the measurements made in custom-made 51h
impression materials. These illustrations reflect the general tendency of the two-dimensional changes regis-
tered. There were no statistically significant differences between the distances recorded in impressions made with stock trays and those made with custom trays (p < .05). The calculated mean value for all 864 measurements differed 0.03% from the distances in the models. Fig. 5 shows the distribution of the recordings when surveyed according to this level. In this study no statistically significant difference was found in the linear dimensional stability of the two impression materials used (p < .05). DISCUSSION Several laboratory tests demonstrated the ability of elastomeric impression materials to reproduce details. The dimensional stability of these materials is considered to depend on the bulk of elastic material, that is, the distance from the inner surface of the tray to the surface of the impression.4.i - ’ Although several reports emphasize the importance of custom-made trays, many dentists use stock trays for fixed prosthoOCTOBER
1984
VOLUME
52
NUMBER
4
CUSTOM
AND
STOCK
TRAYS
FOR IMPRESSION
STABILITY
dontics with acceptable results.6 The need for stable and reproducible test conditions justified the present experimental design. Similar conditions could not be obtained clinically because of tooth mobility and difficulties in achieving identical insertion and removal of the impression trays. The two models were made to reproduce clinical situations. Impregum and Xantopren represent widely used impression materials.6 An objection to the method used may be that all measurements were restricted to the horizontal plane. No information was obtained regarding three-dimensional stability in the tested impressions. Three-dimensional studies have been performed by pouring stone into the impressions and making casts.2~“~‘2However this approach includes other variables such as expansion or contraction of the stone. The present study avoided such errors by direct inspection and measurement of the impressions. The small dimensional differences between polyether and silicone recorded in this study are in opposition to observations made by others and may justify the sample size.13,14 The impression trays were securely attached to the supporting jigs to eliminate possible elastic properties of the trays.6 The stability of the impressions highly exceeds the requirements in ADA Specification No. 19.” The dimensional stability in this study is the result not only of the dimensional stability of the impression materials themselves, but is also due to the bonding adhesives on nonperforated trays with limited elastic properties. SUMMARY
REFERENCES 1.
2.
3.
4.
5.
6.
7.
8.
OF PROSTHETIC
DENTISTRY
Bergman, B., Olsson, K. A., and Stenberg, ‘I’.: Dimensional stability of a rubber impression material. Swed Dent J 6~559, 1972. Eames, W. B., Wallace, S. W., Suway, B. S., and Rogers, L. B.: Accuracy and dimensional stability of elastorreric impression materials. J PR~STHET DENT 42:159, 1979. Lacy, A. M., Fukui, H., Bellman, T., and Jendresen, M. D.: Time-dependent accuracy of elastomer impression materials. Part II: Polyether, polysulfides, and polyvinylsiloxane. J PROSTHET DENY 45:329, 1981. McCabe, J. F., and Storer, R.: Elastomeric impression materials. The measurement of some properties relevant to clinical practice. Br Dent J 73:73, 1980. Reisbick, M. H., and Matyas, J.: The accuracy of highly filled elastomeric impression materials. J PRCSTHE~ DENT 33:67, 1975. Shillingburg, H. T., Hatch, R.A., Keenan. M. R., and Hemphill, M. W.: Impression materials and techniques used for cast restorations in eight states. J Am Dent Assoc 100~696, 1980. Ciesco, J. N., Malone, W. F. P., Sandrik. J. L., and Mazur, B.: Comparison of elastomeric impression materials used in fixed prosthodontics. J PROSTHET DENT 45:89, 198:. Eames, W. B., Sieweke, J. C., Wallace, S. W., and Rogers, L. B.: Elastomeric impression materials: Effect of bulk on accuracy. J PROSTHET
9. 10.
Elastomeric impression materials for fixed prosthodontics are considered most stable when they have an even thickness of 2 to 4 mm. To obtain this, a custom-made impression tray is recommended. The purpose of the present study was to compare the stability of impressions made in custom trays and trays made of chromium-plated brass. The impression materials chosen were polyether and silicone. Two master models of the upper jaw were made of metal. The canines and first molars represented abutment teeth with flat occlusal surfaces. An engraved cross on each surface made it possible to measure in a microscope the distances between the abutment teeth on the models and in the impressions. The accuracy of the method was within +8 pm. Twelve standardized impressions were made with each impression material
THE JOURNAL
in the two types of trays. The distances between the abutment teeth were measured immediately on removal of the impression, and after 1 and 24 hours. Although ample amount of impression material (2 to 9 mm) was allowed, the linear dimensional stability of the impressions made in sfock trays was not inferior to the stability of impressions made in custom-made trays.
11.
12.
13. 14.
Re,brmt JAKOB
DENT
41:304,
1979.
Phillips, R. W.: Skinner’s Science of Dental Materials, ed 7. Chicago, 1973, W. B. Saunders Co., pp 178216. Rehberg, H. J.: The impression tray: An important factor in impression precision. Int Dent J 27:146, 1977. Revised American Dental Association Specification No. 19 for non-aqueous elastomeric dental impression materials. J Am Dent Assoc 94:71, 1977. Stauffer, J. P., Meyer, J. M., and Nally, J. N.: Accuracy of six elastic impression materials used for complete-arch fixed partial dentures. J PROSTHET DENT 35:407, 1976. Craig, R. G.: A review of properties of rubber impression materials. J Mich Dent Assoc 59:254, 1977. Phillips, R. W.: Skinner’s Science of Dental Materials, ed 7. Chicago, 1973, W. B. Saunders Co., pp 136-156.
requests to: VALnERHAUG
DENTAL FACULTY UNIVERSITY OF OSLO Box 1109 BLINDERN OSLO
3, NORWAY
517
AND
TAYLOR
Dimensional stability of elastomeric impression materials in custom-made and stock trays J, Valderhaug, I)cntal
Faculty.
Dr. Odont.,* University
of Oslo,
and F. Fl#ystrand*” and NIOM.
Scandinavian
R
ubber base impression materials are highly accurate and are widely used for making impressions in fixed prosthodontics.! ” Such materials are reported to he most stable when they have an even thickness of 2 to 4 mm, achieved with an acrylic resin custom-made impression tray.” ” However, the material may distort during setting. In addition, it has greater temperature dependence and other elastic properties compared with metal trays, which may jeopardize the good qualities of an impression material.‘. !I’ Finally, making custom trays is time consuming and costly. An alterna!ivc approach is the use of stock trays. Because little information exists concerning the reliability of this method, the present study assessedand compared the dimensional stability of impressions made in custommade acrylic resin trays and stock metallic trays. MATERIAL
AND
METHODS
Two metallic models of the upper jaw were produced. Model A had remaining teeth in the lateral segments with undercuts similar to those in a normal dentition. Model B had no intact teeth. Both models were supplied with standardized abutment teeth in the aren of the canines and first molars. The dimensions are shown in Fig. 1. The teeth were drilled from stainless steel with a taper of 10 degrees toward the flat
Institute
of Dental
Materials,
0+
Oslo,
Norway
Ab: 27996 +6 Bb: 34005 ,+6 +
0
Bd: 25265 I! 6
Bc: 25259 ,+ 6
Aa: 44978 It:6 Ba: 499922 6
0 +
Fig. 1. Schematic drawing of the abutment teeth showing distances in micrometers between center of crosses on occlusal surfaces. A = Brass model; B = aluminium model; a = distances between molars; b = distances between canines; c = distance between molar and canine on right ride; d = distance between molar and canine on left side.
occlusal surface and were fastened with screws. TWO grooves 5 pm wide were engraved at right angles in the center on the occlusal surfaces to provide points of reference that were easily discernible in a microscope (Nikon Profile Pro.jector, Nippon, Kogaku, Japan). Because of the restricted range of the projector (25 mm), additional reference points were located in the middle of the palate. Based on consecutive recordings of the distances on the models, the error of method was calculated to be within k6 Iurn (Fig. t ). The models and trays were OCTOBER
1984
VOLUME
52
NUMBER
4
CUSTOM
AND
STOCK
TRAYS
FOR IMPRESSION
STABILITY
Fig. 2. Photographs of A, brass model, and B, aluminium model, with corresponding trays mounted in jigs. Screws for removal of trays are seen on top of jigs.
Table I. Results from different Distance
Time (hours) 0 1 24 0 1 24 0 1 24 0 1 24
measurements
Impregum/stock
tray
+26
+27 +28 +22
+16 +20 +33
+10 +10
(in micrometers)
Impregumlcustom + 9
+24 +22 +15 +11 +9
+23 +42 +36 +16 +23 +34 +15 +24 +15
tray +26 +35 +27 -126 +17 +10 +12 +12
-11 -13
Xantoprenlstock
tray
+17 +25 +24
+24 +21 +14
-15
-12
+10 +17
-16
f27 t36 -41 +9 +21 -25
-16 -21 -35
-19 - 9 -11 -55
+10 +12
-22 -9
+9 +29 +20
Xantopren/custom
-13 -10 +19 +37
tray +10 +19 +12
-11
-19
Numbers represent deviation from distances measured on models. Each number is mean of four recordings made from impressions. Vacant spaces denote values within error of measurements (+-8pm).
mounted in jigs providing identical direction of insertion and removal of the trays that contained the impression materials (Fig. 2). The part of the jig containing the impression could be removed by turning screws located on each side of the tray. The custom trays were made of Formatray (batch No. 1 4086, Kerr/Sybron Corp., Romulus, Mich.). Special care was taken to obtain an acrylic resin thickness of 3 mm. The models for making the trays included spacing that allowed a thickness of 2 to 4 mm of impression material. All custom-made trays were stored for at least 24 hours prior to use.9 Nonperforated chromium-plated brass trays (Martin, Ehricke, West Germany) of a size considered most appropriate (No. 3) were selected as stock trays. The elastomeric materials chosen for this study were Impregum (batch No. BG 053, Espe, GmbH, Seefeld, West Germany) and Xantopren (Bayer Dental, Leverkusen, West Germany) light body (batch No. 2730 F) THE JOURNAL
OF PROSTHETIC
DENTISTRY
and medium body (batch No. 3442 F). All materials were handled according to the manufacturers’ instructions and the trays were thoroughly coated with the recommended adhesives. The experimental design included the following: (1) two types of impression trays, (2) two types of impression materials, (3) two clinical situations, and (4) variable times for storing the impressions. Three impressions were made of each combination to provide a total of 24 impressions. In the impressions of model A, the distances a and b were measured; and in the impressions of model B, the distances a, b, c, and d were measured (Fig. 1). The distances were assessedfour times. The measurements were performed immediately after removal of the impression from the model, and after 1 and 24 hours. The impressions were made, stored, and measured in a laboratory under constant conditions (21” C and 50% humidity). The statistical analysis included calculation 515
VALDERHAUG
AND
FLdYSTRAND
MODEL B DISTANCEb IN JI : S.O. OHOUR U 1HOUR EB 24 HOURS
DISTANCEb IN p : S.D.
P +40I-
D OHDUR i7 1HDUR tZB24HOURS
+ 30l+20 + 10
..-
..
- 10 - 20 -30 IMPR CUSTOM TRAY
XANTOP STOCK TRAY
XANTOP CUSTOM TRAY
Fig. 3. Time-dependent dimensional stability of recorded distances A6 made with stock and custommade trays with Impregum and Xantopren based on several recordings.
-40
i
IMPR. STOCK TRAY
IMPR.
XANTOP STOCK TRAY
%ioM
XANTDP CUSTOM TRAY
Fig. 4. Time-dependent dimensional stability of recorded distances Bb made with stock and custom made trays with Impregum and Xantopren based on several recordings.
trays was 42%. All the measurements except one complied with the requirements for dimensional stability of rubber impression material in the American Dental Association (ADA) Specification No. 19.” Figs. 3 and 4 show the time-dependent dimensional stability of the recorded distances Ab and Bb with stock and custom-made trays and Impregum and Xantopren f
Ii,j Oh lb, 24,-
Oh Ih 24h ---
Oh Ih 2dh
IMPR. STOCK TRAY
IMPR. CUSTOM TRAY
XANTOP. STOCK TRAY
Oh I,, E4h
XANTOP. CUSTOM TRAY
Fig. 5. Distribution of recorded values when surveyed according to calculated level of dimensional stability of 0.03%~.
of the mean of all corresponding registrations and the Student t test. RESULTS The engraved reference points on the occlusal surfakes were easily recognized in all impressions. The error of method when the distances in the impressions were assessedwas calculated to be within +-8 pm. The results from the different measurements are shown in Table I. Sixty-four percent of the measurements from the impressions made in stock trays were the same as those in the models. The corresponding value for the measurements made in custom-made 51h
impression materials. These illustrations reflect the general tendency of the two-dimensional changes regis-
tered. There were no statistically significant differences between the distances recorded in impressions made with stock trays and those made with custom trays (p < .05). The calculated mean value for all 864 measurements differed 0.03% from the distances in the models. Fig. 5 shows the distribution of the recordings when surveyed according to this level. In this study no statistically significant difference was found in the linear dimensional stability of the two impression materials used (p < .05). DISCUSSION Several laboratory tests demonstrated the ability of elastomeric impression materials to reproduce details. The dimensional stability of these materials is considered to depend on the bulk of elastic material, that is, the distance from the inner surface of the tray to the surface of the impression.4.i - ’ Although several reports emphasize the importance of custom-made trays, many dentists use stock trays for fixed prosthoOCTOBER
1984
VOLUME
52
NUMBER
4
CUSTOM
AND
STOCK
TRAYS
FOR IMPRESSION
STABILITY
dontics with acceptable results.6 The need for stable and reproducible test conditions justified the present experimental design. Similar conditions could not be obtained clinically because of tooth mobility and difficulties in achieving identical insertion and removal of the impression trays. The two models were made to reproduce clinical situations. Impregum and Xantopren represent widely used impression materials.6 An objection to the method used may be that all measurements were restricted to the horizontal plane. No information was obtained regarding three-dimensional stability in the tested impressions. Three-dimensional studies have been performed by pouring stone into the impressions and making casts.2~“~‘2However this approach includes other variables such as expansion or contraction of the stone. The present study avoided such errors by direct inspection and measurement of the impressions. The small dimensional differences between polyether and silicone recorded in this study are in opposition to observations made by others and may justify the sample size.13,14 The impression trays were securely attached to the supporting jigs to eliminate possible elastic properties of the trays.6 The stability of the impressions highly exceeds the requirements in ADA Specification No. 19.” The dimensional stability in this study is the result not only of the dimensional stability of the impression materials themselves, but is also due to the bonding adhesives on nonperforated trays with limited elastic properties. SUMMARY
REFERENCES 1.
2.
3.
4.
5.
6.
7.
8.
OF PROSTHETIC
DENTISTRY
Bergman, B., Olsson, K. A., and Stenberg, ‘I’.: Dimensional stability of a rubber impression material. Swed Dent J 6~559, 1972. Eames, W. B., Wallace, S. W., Suway, B. S., and Rogers, L. B.: Accuracy and dimensional stability of elastorreric impression materials. J PR~STHET DENT 42:159, 1979. Lacy, A. M., Fukui, H., Bellman, T., and Jendresen, M. D.: Time-dependent accuracy of elastomer impression materials. Part II: Polyether, polysulfides, and polyvinylsiloxane. J PROSTHET DENY 45:329, 1981. McCabe, J. F., and Storer, R.: Elastomeric impression materials. The measurement of some properties relevant to clinical practice. Br Dent J 73:73, 1980. Reisbick, M. H., and Matyas, J.: The accuracy of highly filled elastomeric impression materials. J PRCSTHE~ DENT 33:67, 1975. Shillingburg, H. T., Hatch, R.A., Keenan. M. R., and Hemphill, M. W.: Impression materials and techniques used for cast restorations in eight states. J Am Dent Assoc 100~696, 1980. Ciesco, J. N., Malone, W. F. P., Sandrik. J. L., and Mazur, B.: Comparison of elastomeric impression materials used in fixed prosthodontics. J PROSTHET DENT 45:89, 198:. Eames, W. B., Sieweke, J. C., Wallace, S. W., and Rogers, L. B.: Elastomeric impression materials: Effect of bulk on accuracy. J PROSTHET
9. 10.
Elastomeric impression materials for fixed prosthodontics are considered most stable when they have an even thickness of 2 to 4 mm. To obtain this, a custom-made impression tray is recommended. The purpose of the present study was to compare the stability of impressions made in custom trays and trays made of chromium-plated brass. The impression materials chosen were polyether and silicone. Two master models of the upper jaw were made of metal. The canines and first molars represented abutment teeth with flat occlusal surfaces. An engraved cross on each surface made it possible to measure in a microscope the distances between the abutment teeth on the models and in the impressions. The accuracy of the method was within +8 pm. Twelve standardized impressions were made with each impression material
THE JOURNAL
in the two types of trays. The distances between the abutment teeth were measured immediately on removal of the impression, and after 1 and 24 hours. Although ample amount of impression material (2 to 9 mm) was allowed, the linear dimensional stability of the impressions made in sfock trays was not inferior to the stability of impressions made in custom-made trays.
11.
12.
13. 14.
Re,brmt JAKOB
DENT
41:304,
1979.
Phillips, R. W.: Skinner’s Science of Dental Materials, ed 7. Chicago, 1973, W. B. Saunders Co., pp 178216. Rehberg, H. J.: The impression tray: An important factor in impression precision. Int Dent J 27:146, 1977. Revised American Dental Association Specification No. 19 for non-aqueous elastomeric dental impression materials. J Am Dent Assoc 94:71, 1977. Stauffer, J. P., Meyer, J. M., and Nally, J. N.: Accuracy of six elastic impression materials used for complete-arch fixed partial dentures. J PROSTHET DENT 35:407, 1976. Craig, R. G.: A review of properties of rubber impression materials. J Mich Dent Assoc 59:254, 1977. Phillips, R. W.: Skinner’s Science of Dental Materials, ed 7. Chicago, 1973, W. B. Saunders Co., pp 136-156.
requests to: VALnERHAUG
DENTAL FACULTY UNIVERSITY OF OSLO Box 1109 BLINDERN OSLO
3, NORWAY
517