The Journal of Prosthetic Dentistry

275April 2008

Clinical ImplicationsThe 2-step injection technique evaluated yielded more accu-rate resultant casts as compared with the monophase, 1-step, and 2-step impression techniques.

Statement of problem. Dimensional accuracy when making impressions is crucial to the quality of fixed prosthodontic treatment, and the impression technique is a critical factor affecting this accuracy.

Purpose. The purpose of this in vitro study was to compare the dimensional accuracy of a monophase, 1- and 2-step putty/light-body, and a novel 2-step injection impression technique.

Material and methods. A stainless steel model with 2 abutment preparations was fabricated, and impressions were made 15 times with each technique. All impressions were made with an addition-reaction silicone impression material (Aquasil) and a stock perforated metal tray. The monophase impressions were made with regular body material. The 1-step putty/light-body impressions were made with simultaneous use of putty and light-body materials. The 2-step putty/light-body impressions were made with 2-mm-thick resin-prefabricated copings. The 2-step injection impres-sions were made with simultaneous use of putty and light-body materials. In this injection technique, after removing the preliminary impression, a hole was made through the polymerized material at each abutment edge, to coincide with holes present in the stock trays. Extra-light-body material was then added to the preliminary impression and further injected through the hole after reinsertion of the preliminary impression on the stainless steel model. The ac-curacy of the 4 different impression techniques was assessed by measuring 3 dimensions (intra- and interabutment) (5-µm accuracy) on stone casts poured from the impressions of the stainless steel model. The data were analyzed by 1-way ANOVA and Student-Newman-Keuls test (α=.05).

Results. The stone dies obtained with all the techniques had significantly larger dimensions as compared to those of the stainless steel model (P<.01). The order for highest to lowest deviation from the stainless steel model was: mono-phase, 1-step putty/light body, 2-step putty/light body, and 2-step injection. Significant differences among all of the groups for both absolute dimensions of the stone dies, and their percent deviations from the stainless steel model (P<.01), were noted.

Conclusions. The 2-step putty/light-body and 2-step injection techniques were the most dimensionally accurate im-pression methods in terms of resultant casts. (J Prosthet Dent 2008;99:274-281)

Dimensional accuracy of resultant casts made by a monophase, one-step and two-step, and a novel two-step putty/light-body impression technique: An in vitro study

Sergio Caputi, MD, DDS,a and Giuseppe Varvara, DDSb

University G. D’Annunzio, Chieti, Italy

aProfessor, Prosthetic Dentistry, Department of Oral Sciences. bProfessor, Prosthetic Dentistry, Department of Oral Sciences.

Impression techniques can be categorized as monophase or dual-phase. Techniques that use mono-phase materials are accomplished in a single-step procedure, using materi-als with a medium viscosity to allow the material itself to record the finer

details while avoiding the slumping of the material in the tray. Techniques that use dual-phase materials such as the putty and light-body wash method may be accomplished in 1 or 2 steps (1-step and 2-step putty/light-body techniques). The 1-step

putty/light-body technique requires less chair time. In the 2-step putty/light-body technique, the details are recorded by the light-body material, only. Moreover, further modifica-tions to these techniques include the use of a polyethylene spacer in the 2-

step technique1 or the injection of the light-body impression material onto the preparation before the reposition-ing of the tray.2

The problem of accuracy of im-pressions has been stressed recently in a clinical study, which reported that over 89% of the impressions investi-gated had 1 or more observable er-rors; thus, a more critical evaluation of impressions on the part of the dentist is recommended.3 Several factors can influence the quality of impressions, including the impression technique,4-8 the impression material,5,9 the bulk of material,10-12 and others.5,13,14 More-over, the lower the viscosity of the material, the greater the contraction after polymerization.15

It is known that addition-type silicones are among the most dimen-sionally accurate and stable materials available for impression making.3,5 Some studies have indicated that as impression materials have improved, the dimensional accuracy is influenced more by the technique used rather than by the material itself.4-8 However, other studies have indicated that the impression technique does not affect the dimensional accuracy of impres-sions.15,16

Despite a number of studies on the accuracy of impressions as related to the impression materials and/or the impression techniques, controversies remain. The types of impression tech-niques and the different protocols used to assess the accuracy of impres-sions could explain the contradictory results reported in the literature. For instance, Lee et al16 and Nissan et al6 used different quantitative analyses. Moreover, in the 1-step and 2-step techniques, only the light-body ma-terial should cover the entire prepa-ration, but this cannot always be ac-complished clinically.4,7

The 2-step putty/light-body tech-nique has been reported to be more accurate than the 1-step putty/light-body technique.2,4 With the 2-step technique, the impression with the light-body material is made after the putty has polymerized and contract-

ed. Therefore, any further contraction of the light-body material results in minimal dimensional change.6,17 The 1-step putty/light-body technique has also been criticized because of the uncontrolled bulk of the light-body material.10 By diminishing the volume of the polymerizing material at each stage, the final contraction will also be reduced, and the accuracy of the impression can be improved.12 There-fore, careful control of the bulk of the light-body impression material has been advocated because it affects the accuracies of the stone casts.18

Further improvement in dimen-sional accuracy may occur with a nov-el 2-step putty/light-body technique, in which the polymerization of the putty and the light-body material (as in the classical 1-step putty/light-body technique) is followed by the injection of extra-light-body material into the preparation through a hole in a metal stock tray. The greater compensation for the contraction of the first 2 ma-terials could enhance the dimensional accuracy of the impression.

The aim of this study was, there-fore, to evaluate the dimensional ac-

curacy of casts made using this new 2-step putty/light-body technique (herein referred to as the 2-step in-jection technique), as compared with monophase and conventional 1-step and 2-step putty/light-body (referred to as 1-step and 2-step, respectively) techniques. The null hypothesis was that no differences would exist in the dimensional accuracy of casts fabri-cated using these various techniques.

MATERIAL AND METHODS

A stainless steel model containing 2 complete-crown, tapered abutment preparations was made on a lathe according to the ANSI/ADA specifi-cations (8.015 mm in height, 6.330-mm and 8.450-mm base dimensions, with a 28.270-mm distance between the centers of the abutments) (Fig. 1). The dimensions of this stainless steel model were also recorded and report-ed in Table I. This was then used as the definitive standardized model for the comparison of the impression tech-niques in this study. The abutments were prepared with reference cross-grooves on occlusal and proximal sur-

1 Stainless steel model containing 2 abutments showing reference grooves.

Table I. Means and SDs of dimensions of stainless steel model (n=15)

Diameter

Height

Distance

Dimension (mm)

6.33

8.02

28.25

Mean

0.01

0.01

0.01

SD

0.16

0.10

0.02

ME%

Caputi and Varvara Caputi and Varvara

The Journal of Prosthetic Dentistry

275April 2008

Clinical ImplicationsThe 2-step injection technique evaluated yielded more accu-rate resultant casts as compared with the monophase, 1-step, and 2-step impression techniques.

Statement of problem. Dimensional accuracy when making impressions is crucial to the quality of fixed prosthodontic treatment, and the impression technique is a critical factor affecting this accuracy.

Purpose. The purpose of this in vitro study was to compare the dimensional accuracy of a monophase, 1- and 2-step putty/light-body, and a novel 2-step injection impression technique.

Material and methods. A stainless steel model with 2 abutment preparations was fabricated, and impressions were made 15 times with each technique. All impressions were made with an addition-reaction silicone impression material (Aquasil) and a stock perforated metal tray. The monophase impressions were made with regular body material. The 1-step putty/light-body impressions were made with simultaneous use of putty and light-body materials. The 2-step putty/light-body impressions were made with 2-mm-thick resin-prefabricated copings. The 2-step injection impres-sions were made with simultaneous use of putty and light-body materials. In this injection technique, after removing the preliminary impression, a hole was made through the polymerized material at each abutment edge, to coincide with holes present in the stock trays. Extra-light-body material was then added to the preliminary impression and further injected through the hole after reinsertion of the preliminary impression on the stainless steel model. The ac-curacy of the 4 different impression techniques was assessed by measuring 3 dimensions (intra- and interabutment) (5-µm accuracy) on stone casts poured from the impressions of the stainless steel model. The data were analyzed by 1-way ANOVA and Student-Newman-Keuls test (α=.05).

Results. The stone dies obtained with all the techniques had significantly larger dimensions as compared to those of the stainless steel model (P<.01). The order for highest to lowest deviation from the stainless steel model was: mono-phase, 1-step putty/light body, 2-step putty/light body, and 2-step injection. Significant differences among all of the groups for both absolute dimensions of the stone dies, and their percent deviations from the stainless steel model (P<.01), were noted.

Conclusions. The 2-step putty/light-body and 2-step injection techniques were the most dimensionally accurate im-pression methods in terms of resultant casts. (J Prosthet Dent 2008;99:274-281)

Dimensional accuracy of resultant casts made by a monophase, one-step and two-step, and a novel two-step putty/light-body impression technique: An in vitro study

Sergio Caputi, MD, DDS,a and Giuseppe Varvara, DDSb

University G. D’Annunzio, Chieti, Italy

aProfessor, Prosthetic Dentistry, Department of Oral Sciences. bProfessor, Prosthetic Dentistry, Department of Oral Sciences.

Impression techniques can be categorized as monophase or dual-phase. Techniques that use mono-phase materials are accomplished in a single-step procedure, using materi-als with a medium viscosity to allow the material itself to record the finer

details while avoiding the slumping of the material in the tray. Techniques that use dual-phase materials such as the putty and light-body wash method may be accomplished in 1 or 2 steps (1-step and 2-step putty/light-body techniques). The 1-step

putty/light-body technique requires less chair time. In the 2-step putty/light-body technique, the details are recorded by the light-body material, only. Moreover, further modifica-tions to these techniques include the use of a polyethylene spacer in the 2-

step technique1 or the injection of the light-body impression material onto the preparation before the reposition-ing of the tray.2

The problem of accuracy of im-pressions has been stressed recently in a clinical study, which reported that over 89% of the impressions investi-gated had 1 or more observable er-rors; thus, a more critical evaluation of impressions on the part of the dentist is recommended.3 Several factors can influence the quality of impressions, including the impression technique,4-8 the impression material,5,9 the bulk of material,10-12 and others.5,13,14 More-over, the lower the viscosity of the material, the greater the contraction after polymerization.15

It is known that addition-type silicones are among the most dimen-sionally accurate and stable materials available for impression making.3,5 Some studies have indicated that as impression materials have improved, the dimensional accuracy is influenced more by the technique used rather than by the material itself.4-8 However, other studies have indicated that the impression technique does not affect the dimensional accuracy of impres-sions.15,16

Despite a number of studies on the accuracy of impressions as related to the impression materials and/or the impression techniques, controversies remain. The types of impression tech-niques and the different protocols used to assess the accuracy of impres-sions could explain the contradictory results reported in the literature. For instance, Lee et al16 and Nissan et al6 used different quantitative analyses. Moreover, in the 1-step and 2-step techniques, only the light-body ma-terial should cover the entire prepa-ration, but this cannot always be ac-complished clinically.4,7

The 2-step putty/light-body tech-nique has been reported to be more accurate than the 1-step putty/light-body technique.2,4 With the 2-step technique, the impression with the light-body material is made after the putty has polymerized and contract-

ed. Therefore, any further contraction of the light-body material results in minimal dimensional change.6,17 The 1-step putty/light-body technique has also been criticized because of the uncontrolled bulk of the light-body material.10 By diminishing the volume of the polymerizing material at each stage, the final contraction will also be reduced, and the accuracy of the impression can be improved.12 There-fore, careful control of the bulk of the light-body impression material has been advocated because it affects the accuracies of the stone casts.18

Further improvement in dimen-sional accuracy may occur with a nov-el 2-step putty/light-body technique, in which the polymerization of the putty and the light-body material (as in the classical 1-step putty/light-body technique) is followed by the injection of extra-light-body material into the preparation through a hole in a metal stock tray. The greater compensation for the contraction of the first 2 ma-terials could enhance the dimensional accuracy of the impression.

The aim of this study was, there-fore, to evaluate the dimensional ac-

curacy of casts made using this new 2-step putty/light-body technique (herein referred to as the 2-step in-jection technique), as compared with monophase and conventional 1-step and 2-step putty/light-body (referred to as 1-step and 2-step, respectively) techniques. The null hypothesis was that no differences would exist in the dimensional accuracy of casts fabri-cated using these various techniques.

MATERIAL AND METHODS

A stainless steel model containing 2 complete-crown, tapered abutment preparations was made on a lathe according to the ANSI/ADA specifi-cations (8.015 mm in height, 6.330-mm and 8.450-mm base dimensions, with a 28.270-mm distance between the centers of the abutments) (Fig. 1). The dimensions of this stainless steel model were also recorded and report-ed in Table I. This was then used as the definitive standardized model for the comparison of the impression tech-niques in this study. The abutments were prepared with reference cross-grooves on occlusal and proximal sur-

1 Stainless steel model containing 2 abutments showing reference grooves.

Table I. Means and SDs of dimensions of stainless steel model (n=15)

Diameter

Height

Distance

Dimension (mm)

6.33

8.02

28.25

Mean

0.01

0.01

0.01

SD

0.16

0.10

0.02

ME%

Caputi and Varvara Caputi and Varvara

276 Volume 99 Issue 4

The Journal of Prosthetic Dentistry

277April 2008

2 Stainless steel model incorporated into acrylic resin device.

faces for reference measurements (Fig. 1), and all of the impressions were made in stock perforated metal trays (size 6; ASA Dental, Bozzano, Italy). The stainless steel model was manual-ly incorporated into an autopolymer-izing acrylic resin device (Lang Den-tal, Wheeling, Ill) and fixed on a base (Zeiser, Hemmingen, Germany) (Fig. 2). The acrylic resin device was then prepared to allow the reproducible positioning of the tray on the stainless steel model (Fig. 2).15 Impressions of the stainless steel model were made 15 times for each of the 4 techniques, monophase, 1-step, 2-step, and 2-step injection. Impressions were made with addition-reaction silicone im-pression materials (Aquasil; Dentsply Intl, York, Pa). The viscosities of the materials used were: soft putty (Lot 0405000002, 2007.06, ISO 4823, Type 1, Cat. A, very high); regular body (Lot 0408001734, 2007.11, ISO 4823, Type 3, Cat. A, low); light body (Lot 0408001289, 2007.9, ISO 4823, Type 3, Cat. A, low); and extra-light body (Lot 0405000010, 2007.07, ISO 4823, Type 3, Cat. A, low). The soft putty material was mixed with finger-tips for 30 seconds until the color was uniform, and all of the other materi-als were dispensed with an automatic mixing syringe (AutoMix; Dentsply Intl) The polymerization times used for each material were double those recommended by the manufacturer to compensate for the impressions being made at room temperature (20°C) in-stead of at mouth temperature.19 No tray adhesive was used, but it was not necessary with the perforated metal trays.

The monophase impressions were made using an impression material that was dispensed with the auto-matic mixing syringe; the impression was allowed to polymerize on the stainless steel model for 12 minutes. The 1-step impressions were made with the putty and light-body mate-rials simultaneously, and the impres-sions were allowed to polymerize on the stainless steel model for 12 min-utes. The 2-step impressions were

made by using prefabricated 2-mm-thick acrylic resin copings (DuraLay; Reliance Dental Mfg Co, Worth, Ill) placed on each abutment to create a uniform and optimal space for the light-body material.10 The dimensions of these copings were standardized by using a caliper (#500-181-21; Roder Electronics, Torino, Italy). The pre-liminary putty impressions were made first and allowed to polymerize for 12 minutes. In the second step, the acryl-ic resin copings were removed and the wash material was added. The im-pressions were reinserted and evalu-ated for positioning on the abutment until firm contact was made with the border of the tray, and they were al-lowed to polymerize on the stainless steel model for 12 more minutes. The 2-step injection impressions (Fig. 3) were made with putty and light-body materials simultaneously (as in the classical 1-step technique) (Fig. 3, A) and then allowed to polymerize on the stainless steel model for 12 min-utes (Fig. 3, B). The preliminary im-pressions were then removed from the stainless steel model, and in the vicin-ity of each abutment’s most coronal end (edge), a hole was made through the polymerized material with a car-bide bur (187 023; Komet, Lemgo, Germany) (Fig. 3, C), which coincid-ed with 1 of the holes present in the stock tray (Figs. 3, D and 4). There-after, the selected holes on the stock tray were labeled with ink to facilitate their identification. During the proce-

dure, a thin layer of interdental mate-rial was manually removed from the impression with the same carbide bur (Fig. 3, D). Extra-light-body material was then added to the preliminary im-pression (Fig. 3, E), which was imme-diately reinserted onto the stainless steel model (Fig. 3, F). Further extra-light-body material was placed in a sy-ringe (Elastomer; 3M ESPE, Seefeld, Germany) and injected through the holes marked on the abutments (Figs. 3, F and 5). The trays were held down during this procedure to prevent them from lifting off the model. These im-pressions were allowed to polymerize for 12 minutes.

All impressions were stored at room temperature (20°C) for 1 hour before pouring in improved type IV stone (Lot 0608291 2008 09, GC Fujirock EP; GC Europe NV, Leuven, Belgium) mixed, according to the manufacturer’s instructions, with a water/powder ratio of 10 ml/100 g. The improved stone was first mixed by hand to incorporate the water for 10 seconds and then mixed mechani-cally under vacuum for 20 seconds (Smart-nix; AmannGirrbach AG, Ko-blach, Austria). All of the mixes were vibrated (Top; Dentalfarm, Torino, It-aly) into the impressions and allowed to polymerize for 1 hour before being separated from the impressions.

Three different dimensions (Fig. 6) were measured on the stainless steel model at room temperature (control) and on the stone casts from each of the

3 Diagram of 2-step injection procedure. A, Putty and light-body materials in tray. B, Polymerization of impression material on stainless steel model. C, Hole on preliminary impressions made by carbide bur. D, Manual removal of thin layer of interdental material from impression with same carbide bur. E, Addition of extra-light-body material to pre-liminary impression with syringe. F, Reinsertion onto stainless steel model and injection of additional extra-light-body material through hole.

4 Hole made in impression to correspond with hole present in stock tray.

5 Injection of extra-light-body material through hole marked on abutments.

6 Diagram of stainless steel model displaying 2 abutments (1 and 2, respectively), corresponding mean intraabutment (diameter and height, 6.33 and 8.02 mm, respec-tively), and interabutment dimensions (28.25 mm) made after 15 replications.

Caputi and Varvara Caputi and Varvara

276 Volume 99 Issue 4

The Journal of Prosthetic Dentistry

277April 2008

2 Stainless steel model incorporated into acrylic resin device.

faces for reference measurements (Fig. 1), and all of the impressions were made in stock perforated metal trays (size 6; ASA Dental, Bozzano, Italy). The stainless steel model was manual-ly incorporated into an autopolymer-izing acrylic resin device (Lang Den-tal, Wheeling, Ill) and fixed on a base (Zeiser, Hemmingen, Germany) (Fig. 2). The acrylic resin device was then prepared to allow the reproducible positioning of the tray on the stainless steel model (Fig. 2).15 Impressions of the stainless steel model were made 15 times for each of the 4 techniques, monophase, 1-step, 2-step, and 2-step injection. Impressions were made with addition-reaction silicone im-pression materials (Aquasil; Dentsply Intl, York, Pa). The viscosities of the materials used were: soft putty (Lot 0405000002, 2007.06, ISO 4823, Type 1, Cat. A, very high); regular body (Lot 0408001734, 2007.11, ISO 4823, Type 3, Cat. A, low); light body (Lot 0408001289, 2007.9, ISO 4823, Type 3, Cat. A, low); and extra-light body (Lot 0405000010, 2007.07, ISO 4823, Type 3, Cat. A, low). The soft putty material was mixed with finger-tips for 30 seconds until the color was uniform, and all of the other materi-als were dispensed with an automatic mixing syringe (AutoMix; Dentsply Intl) The polymerization times used for each material were double those recommended by the manufacturer to compensate for the impressions being made at room temperature (20°C) in-stead of at mouth temperature.19 No tray adhesive was used, but it was not necessary with the perforated metal trays.

The monophase impressions were made using an impression material that was dispensed with the auto-matic mixing syringe; the impression was allowed to polymerize on the stainless steel model for 12 minutes. The 1-step impressions were made with the putty and light-body mate-rials simultaneously, and the impres-sions were allowed to polymerize on the stainless steel model for 12 min-utes. The 2-step impressions were

made by using prefabricated 2-mm-thick acrylic resin copings (DuraLay; Reliance Dental Mfg Co, Worth, Ill) placed on each abutment to create a uniform and optimal space for the light-body material.10 The dimensions of these copings were standardized by using a caliper (#500-181-21; Roder Electronics, Torino, Italy). The pre-liminary putty impressions were made first and allowed to polymerize for 12 minutes. In the second step, the acryl-ic resin copings were removed and the wash material was added. The im-pressions were reinserted and evalu-ated for positioning on the abutment until firm contact was made with the border of the tray, and they were al-lowed to polymerize on the stainless steel model for 12 more minutes. The 2-step injection impressions (Fig. 3) were made with putty and light-body materials simultaneously (as in the classical 1-step technique) (Fig. 3, A) and then allowed to polymerize on the stainless steel model for 12 min-utes (Fig. 3, B). The preliminary im-pressions were then removed from the stainless steel model, and in the vicin-ity of each abutment’s most coronal end (edge), a hole was made through the polymerized material with a car-bide bur (187 023; Komet, Lemgo, Germany) (Fig. 3, C), which coincid-ed with 1 of the holes present in the stock tray (Figs. 3, D and 4). There-after, the selected holes on the stock tray were labeled with ink to facilitate their identification. During the proce-

dure, a thin layer of interdental mate-rial was manually removed from the impression with the same carbide bur (Fig. 3, D). Extra-light-body material was then added to the preliminary im-pression (Fig. 3, E), which was imme-diately reinserted onto the stainless steel model (Fig. 3, F). Further extra-light-body material was placed in a sy-ringe (Elastomer; 3M ESPE, Seefeld, Germany) and injected through the holes marked on the abutments (Figs. 3, F and 5). The trays were held down during this procedure to prevent them from lifting off the model. These im-pressions were allowed to polymerize for 12 minutes.

All impressions were stored at room temperature (20°C) for 1 hour before pouring in improved type IV stone (Lot 0608291 2008 09, GC Fujirock EP; GC Europe NV, Leuven, Belgium) mixed, according to the manufacturer’s instructions, with a water/powder ratio of 10 ml/100 g. The improved stone was first mixed by hand to incorporate the water for 10 seconds and then mixed mechani-cally under vacuum for 20 seconds (Smart-nix; AmannGirrbach AG, Ko-blach, Austria). All of the mixes were vibrated (Top; Dentalfarm, Torino, It-aly) into the impressions and allowed to polymerize for 1 hour before being separated from the impressions.

Three different dimensions (Fig. 6) were measured on the stainless steel model at room temperature (control) and on the stone casts from each of the

3 Diagram of 2-step injection procedure. A, Putty and light-body materials in tray. B, Polymerization of impression material on stainless steel model. C, Hole on preliminary impressions made by carbide bur. D, Manual removal of thin layer of interdental material from impression with same carbide bur. E, Addition of extra-light-body material to pre-liminary impression with syringe. F, Reinsertion onto stainless steel model and injection of additional extra-light-body material through hole.

4 Hole made in impression to correspond with hole present in stock tray.

5 Injection of extra-light-body material through hole marked on abutments.

6 Diagram of stainless steel model displaying 2 abutments (1 and 2, respectively), corresponding mean intraabutment (diameter and height, 6.33 and 8.02 mm, respec-tively), and interabutment dimensions (28.25 mm) made after 15 replications.

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impression techniques: the diameter of abutment number 1, the height of abutment number 1, and the distance between the centers of the abutments determined by the crossing of the grooves. All of these measurements were made with a scanner (PIZCA 3-D, model PIX-4; Roland Europe SpA, Acquaviva Picena, Italy). The opera-tion of this scanner is based on mul-tiple-touch points, with the use of a piezo-controlled sensor with 5-µm ac-curacy (Roland Active Piezo Sensor; Roland Europe SpA). All of the stone casts were measured 48 hours after retrieval from the impressions, and all measurements were made by the same operator.

For each of the 3 dimensions on the stainless steel model, the mea-surements were made 15 times. The coefficients of variability were calcu-lated by dividing the standard devia-tions for each dimension on the stain-less steel model by the corresponding

mean values and multiplying them by 100. In the same way, the measure-ments on the stone casts for each di-mension were repeated 3 times, and the corresponding mean values were considered as the statistical units. For each dimension, the differences be-tween the mean values of the stone casts (MSC) and the mean values of the stainless steel model (MSSM), divided by the mean of the stainless steel model and multiplied by 100, were expressed as the percentage de-viations from the stainless steel model for each test group, as follows:

Percent deviation = [(MSC – MSSM)/MSSM)] × 100

For each dimension, 2 separate 1-way analyses of variance (ANOVAs) were used to assess the significance of the differences in both the absolute di-mensional measurements and in their corresponding percent deviations from the stainless steel model among all of the test groups. Subsequently,

Diameter

Height

Distance

Dimension (mm)

6.53 (0.04)

8.11 (0.01)

28.38 (0.02)

Monophase

6.50 (0.03)

8.08 (0.01)

28.36 (0.02)

1-Step

6.44 (0.02)

8.07 (0.01)

28.334 (0.01)

2-Step

6.41 (0.02)

8.07 (0.01)

28.30 (0.01)

2-Step Injection

multiple pairwise comparisons were performed between the test groups by using the Student-Newman-Keuls test (α=.05).

RESULTS

Table I lists the mean values for each of the 15 measurements for each of the dimensions on the stainless steel model and the corresponding stan-dard deviations and percent errors. The percent errors were low, ranging from 0.02% to 0.16%, and they were greater for the smaller dimensions. Table II displays the absolute mean values and corresponding standard deviations for each of the dimensions of the stone casts, according to the impression techniques. All cast di-mensions were greater than those for the stainless steel model. The 1-way ANOVAs (Table III) revealed that all of the dimensions were significantly different among the impression tech-

Table II. Mean dimensions (SD) of stone casts according to impression technique (n=15)

Table III. One-way ANOVAs for dimensions of stone casts according to impression technique (n=15)

Diameter

Height

Distance

Differences are significant at P<.05

4

70

74

4

70

74

4

70

74

df

0.367

0.046

0.413

0.057

0.005

0.062

0.153

0.015

0.168

0.092

0.001

0.014

0.000

0.038

0.000

Squares SquareSum of Mean

140.695

202.526

179.592

FDimension

Between groups

Within groups

Total

Between groups

Within groups

Total

Between groups

Within groups

Total

Source

<.01

<.01

<.01

P

niques (and among all the pairwise comparisons). Moreover, all of the pairwise comparisons between the impression techniques showed signifi-cant differences.

Table IV shows the percent devia-tions from the stainless steel model for each dimension, according to the im-pression technique. All the differences among the impression techniques were significant, as demonstrated by the 1-way ANOVAs (Table V). As for the mean dimensions, all the pairwise comparisons between the impression techniques showed significant differ-ences. In general, the greatest percent-age deviations for all techniques were seen for the intraabutment dimen-sions, with the diameter showing the largest deviation, irrespective of the technique. Of note, the monophase and 2-step injection techniques yield-

ed the highest and lowest variations, respectively, for all of the dimensions. Furthermore, in terms of accuracy, the 1-step and 2-step techniques per-formed better than the monophase, but worse than the 2-step injection technique.

DISCUSSION

In the present study, the accuracy of 4 different impression techniques was investigated, and the results support rejection of the null hypothesis. For all groups, significantly larger dimen-sions were observed when compared with the stainless steel model. This observation may also be explained by an expansion of stone material, although the casts were measured 48 hours after the retrieval from the impression. The concept that a simi-

lar expansion rate is expected for all specimens would avoid any bias in the comparisons of the accuracy of each impression technique. However, while the results from the monophase and 1-step techniques were fairly similar for each dimension (although still significantly different), the 2-step and 2-step injection techniques produced the best results in terms of dimension-al accuracy. In particular, the 2-step injection technique was seen to be significantly more accurate than the 2-step technique (Tables II and IV).

This in vitro study suggests that the impression technique can be a significant factor in determining the accuracy of impressions. Here, to re-duce the number of factors that could have influenced the outcome, stock trays were used for all of the groups, and the same operator made all im-

Table IV. Mean percent deviations (SD) of dimensions of stone casts from those of stain-less steel model, according to impression technique (n=15)

Table V. One-way ANOVAs for percent deviations of dimensions of stone casts from those of stainless steel model, according to impression technique (n=15)

Diameter

Height

Distance

Dimension (mm)

3.09 (0.66)

1.06 (0.14)

0.45 (0.08)

Monophase

2.73 (0.40)

0.76 (0.09)

0.36 (0.06)

1-Step

1.69 (0.33)

0.67 (0.10)

0.29 (0.04)

2-Step

1.25 (0.29)

0.58 (0.09)

0.15 (0.03)

2-Step Injection

Diameter

Height

Distance

Differences are significant at P<.05

3

56

59

3

56

59

3

56

59

df

33.575

11.022

44.597

1.958

0.630

2.588

0.716

0.182

0.897

11.192

0.197

0.653

0.011

0.239

0.003

Squares SquareSum of Mean

56.862

58.018

73.522

FDimension

Between groups

Within groups

Total

Between groups

Within groups

Total

Between groups

Within groups

Total

Source

<.01

<.01

<.01

P

Caputi and Varvara Caputi and Varvara

278 Volume 99 Issue 4

The Journal of Prosthetic Dentistry

279April 2008

impression techniques: the diameter of abutment number 1, the height of abutment number 1, and the distance between the centers of the abutments determined by the crossing of the grooves. All of these measurements were made with a scanner (PIZCA 3-D, model PIX-4; Roland Europe SpA, Acquaviva Picena, Italy). The opera-tion of this scanner is based on mul-tiple-touch points, with the use of a piezo-controlled sensor with 5-µm ac-curacy (Roland Active Piezo Sensor; Roland Europe SpA). All of the stone casts were measured 48 hours after retrieval from the impressions, and all measurements were made by the same operator.

For each of the 3 dimensions on the stainless steel model, the mea-surements were made 15 times. The coefficients of variability were calcu-lated by dividing the standard devia-tions for each dimension on the stain-less steel model by the corresponding

mean values and multiplying them by 100. In the same way, the measure-ments on the stone casts for each di-mension were repeated 3 times, and the corresponding mean values were considered as the statistical units. For each dimension, the differences be-tween the mean values of the stone casts (MSC) and the mean values of the stainless steel model (MSSM), divided by the mean of the stainless steel model and multiplied by 100, were expressed as the percentage de-viations from the stainless steel model for each test group, as follows:

Percent deviation = [(MSC – MSSM)/MSSM)] × 100

For each dimension, 2 separate 1-way analyses of variance (ANOVAs) were used to assess the significance of the differences in both the absolute di-mensional measurements and in their corresponding percent deviations from the stainless steel model among all of the test groups. Subsequently,

Diameter

Height

Distance

Dimension (mm)

6.53 (0.04)

8.11 (0.01)

28.38 (0.02)

Monophase

6.50 (0.03)

8.08 (0.01)

28.36 (0.02)

1-Step

6.44 (0.02)

8.07 (0.01)

28.334 (0.01)

2-Step

6.41 (0.02)

8.07 (0.01)

28.30 (0.01)

2-Step Injection

multiple pairwise comparisons were performed between the test groups by using the Student-Newman-Keuls test (α=.05).

RESULTS

Table I lists the mean values for each of the 15 measurements for each of the dimensions on the stainless steel model and the corresponding stan-dard deviations and percent errors. The percent errors were low, ranging from 0.02% to 0.16%, and they were greater for the smaller dimensions. Table II displays the absolute mean values and corresponding standard deviations for each of the dimensions of the stone casts, according to the impression techniques. All cast di-mensions were greater than those for the stainless steel model. The 1-way ANOVAs (Table III) revealed that all of the dimensions were significantly different among the impression tech-

Table II. Mean dimensions (SD) of stone casts according to impression technique (n=15)

Table III. One-way ANOVAs for dimensions of stone casts according to impression technique (n=15)

Diameter

Height

Distance

Differences are significant at P<.05

4

70

74

4

70

74

4

70

74

df

0.367

0.046

0.413

0.057

0.005

0.062

0.153

0.015

0.168

0.092

0.001

0.014

0.000

0.038

0.000

Squares SquareSum of Mean

140.695

202.526

179.592

FDimension

Between groups

Within groups

Total

Between groups

Within groups

Total

Between groups

Within groups

Total

Source

<.01

<.01

<.01

P

niques (and among all the pairwise comparisons). Moreover, all of the pairwise comparisons between the impression techniques showed signifi-cant differences.

Table IV shows the percent devia-tions from the stainless steel model for each dimension, according to the im-pression technique. All the differences among the impression techniques were significant, as demonstrated by the 1-way ANOVAs (Table V). As for the mean dimensions, all the pairwise comparisons between the impression techniques showed significant differ-ences. In general, the greatest percent-age deviations for all techniques were seen for the intraabutment dimen-sions, with the diameter showing the largest deviation, irrespective of the technique. Of note, the monophase and 2-step injection techniques yield-

ed the highest and lowest variations, respectively, for all of the dimensions. Furthermore, in terms of accuracy, the 1-step and 2-step techniques per-formed better than the monophase, but worse than the 2-step injection technique.

DISCUSSION

In the present study, the accuracy of 4 different impression techniques was investigated, and the results support rejection of the null hypothesis. For all groups, significantly larger dimen-sions were observed when compared with the stainless steel model. This observation may also be explained by an expansion of stone material, although the casts were measured 48 hours after the retrieval from the impression. The concept that a simi-

lar expansion rate is expected for all specimens would avoid any bias in the comparisons of the accuracy of each impression technique. However, while the results from the monophase and 1-step techniques were fairly similar for each dimension (although still significantly different), the 2-step and 2-step injection techniques produced the best results in terms of dimension-al accuracy. In particular, the 2-step injection technique was seen to be significantly more accurate than the 2-step technique (Tables II and IV).

This in vitro study suggests that the impression technique can be a significant factor in determining the accuracy of impressions. Here, to re-duce the number of factors that could have influenced the outcome, stock trays were used for all of the groups, and the same operator made all im-

Table IV. Mean percent deviations (SD) of dimensions of stone casts from those of stain-less steel model, according to impression technique (n=15)

Table V. One-way ANOVAs for percent deviations of dimensions of stone casts from those of stainless steel model, according to impression technique (n=15)

Diameter

Height

Distance

Dimension (mm)

3.09 (0.66)

1.06 (0.14)

0.45 (0.08)

Monophase

2.73 (0.40)

0.76 (0.09)

0.36 (0.06)

1-Step

1.69 (0.33)

0.67 (0.10)

0.29 (0.04)

2-Step

1.25 (0.29)

0.58 (0.09)

0.15 (0.03)

2-Step Injection

Diameter

Height

Distance

Differences are significant at P<.05

3

56

59

3

56

59

3

56

59

df

33.575

11.022

44.597

1.958

0.630

2.588

0.716

0.182

0.897

11.192

0.197

0.653

0.011

0.239

0.003

Squares SquareSum of Mean

56.862

58.018

73.522

FDimension

Between groups

Within groups

Total

Between groups

Within groups

Total

Between groups

Within groups

Total

Source

<.01

<.01

<.01

P

Caputi and Varvara Caputi and Varvara

280 Volume 99 Issue 4

The Journal of Prosthetic Dentistry

281April 2008

pressions. Moreover, the same mate-rials were used for all of the 3 putty/light-body techniques considered. Nevertheless, there were noted differ-ences in terms of the accuracy among the different techniques, thus, contra-dicting previous reports.15,16

The monophase technique is the easiest to perform, but it has been reported to be the worst in terms of dimensional accuracy and surface defects, as compared to putty/light-body techniques, because of the rela-tively high viscosity and reduced flow of the material used.2 The 1-step tech-nique has the advantages of simplicity and reasonable economy; however, in this technique, the putty tends to push the light-body wash off the pre-pared tooth, and, thus, critical areas, such as the finish line, can be covered by the putty, which cannot record de-tails to a satisfactory level.7,9 For this reason, even in studies in which the 1-step technique has been seen to be as accurate as the 2-step technique, concerns have been raised about sur-face defects when using the 1-step technique.15 Another difficulty with the 1-step technique is that once the light body material is on the prepara-tion, the putty needs to be brought into position and seated. During this critical phase, the patient’s tongue or the elevated floor of the mouth can re-move the light-body material from the tooth. The 2-step technique allows these problems to be overcome, but it can be associated with the creation of an occlusal step on adjacent teeth, as some light-body material may spread along the occlusal surfaces during the reseating of the putty. Moreover, the light-body material is displaced during this second step, and this can generate distortions that may result in reduced dimensional accuracy of the impression. It has also been reported that the wash material can displace the preliminary putty impression dur-ing the seating, and, afterwards, an elastic recovery of the putty can oc-cur upon removal of the impression, resulting in a tendency towards larger interabutment distances.15 To address

these concerns, the 2-step injection technique allows the displacement of soft tissues, such as the tongue, dur-ing the first seating of the putty and wash materials, while in the second step, the extra-light-body material re-cords all of the finer details without being compressed.

In the present study, the percent deviations from the stainless steel model ranged from 0.15% to 3.09% across the various techniques. In some instances, these deviations are greater than those of Hung et al4 and Idris et al,15 but similar to those of Nissan et al,6 and all of the dimensions on the stone casts (in each group) were greater than the corresponding di-mensions of the stainless steel model. Furthermore, in each group, the high-est and lowest percent deviations from the stainless steel model were seen for the intraabutment and inter-abutment dimensions, respectively (Table II). It is possible, however, that the greater contraction of the impres-sion material towards the walls of the trays might have primarily affected the regions with the smaller amounts of the impression material per wall sur-face, such as in the areas surrounding the abutments. Moreover, the greater percent deviations for the intraabut-ment dimensions were also related to their calculation on the basis of be-ing much smaller dimensions, as has been reported by Idris et al.15 When comparing the dimensions of the stone casts with the corresponding stainless steel model, previous studies have reported smaller vertical dimen-sions and greater horizontal dimen-sions.6,12,15 This was explained by the contraction of the impression mate-rial towards the tray walls. However, in a similar study, Hung et al4 did not report such a clear distinction in the dimensional changes between the ver-tical and horizontal measurements. The differences in the protocols and materials used among these studies, such as the application of an adhe-sive on the tray,15 may explain these discrepancies. Therefore, any clinical application based on the results of

different studies should strictly follow the experimental protocols used to achieve the same outcome.

The best results in the present study were seen for the 2-step injec-tion technique, which could be relat-ed to the reduced bulk wash obtained through the use of the extra-light-body material. Indeed, a bulk wash of 1-2 mm has been reported to perform significantly better than a bulk wash of 4 or 6 mm, in terms of dimensional accuracy.10 However, in using the clas-sic 2-step technique, the expected bulk wash might be greater than with the extra-light-body material used in the 2-step injection technique.

The use of light- and heavy-bodied automixed impression materials used in a 1-stage technique is common in clinical practice, and, since this uses a secondary material that is not as vis-cous as putty, further studies should include this technique as well. Finally, further in vitro and in vivo studies are warranted to fully explore the relative merits of the 2-step injection tech-nique, in particular, regarding the formation of surface defects and per-formance in an oral environment that includes saliva.

CONCLUSIONS

Within the limitations of this in vitro study, it can be concluded that different impression techniques affect the dimensional accuracy of the resul-tant casts, and, in particular:

1. The monophase technique yielded the lowest cast accuracy in every dimension considered, as com-pared to all of the other techniques (P<.01).

2. The 1-step technique yielded more accurate casts as compared to the monophase technique (P<.01), but less accurate casts than those from the 2-step and injection 2-step techniques, in every dimension con-sidered, as compared to all of the other techniques (P<.01).

3. The 2-step and injection 2-step techniques produced the most ac-curate casts (P<.01), with the latter

performing better than the former in every dimension considered (P<.01).

REFERENCES

1. Schoenrock GA. The laminar impression technique. J Prosthet Dent 1989;62:392-5.

2. Millar BJ, Dunne SM, Robinson PB. In vitro study of the number of surface defects in monophase and two-phase addition silicone impressions. J Prosthet Dent 1998;80:32-5.

3. Samet N, Shohat M, Livny A, Weiss EI. A clinical evaluation of fixed partial denture impressions. J Prosthet Dent 2005;94:112-7.

4. Hung SH, Purk JH, Tira DE, Eick JD. Accu-racy of one-step versus two-step putty wash addition silicone impression technique. J Prosthet Dent 1992;67:583-9.

5. Chen SY, Liang WM, Chen FN. Factors af-fecting the accuracy of elastometric impres-sion materials. J Dent 2004;32:603-9.

6. Nissan J, Laufer BZ, Brosh T, Assif D. Accu-racy of three polyvinyl siloxane putty-wash impression techniques. J Prosthet Dent 2000;83:161-5.

7. Morgano SM, Milot P, Ducharme P, Rose L. Ability of various impression materials to produce duplicate dies from successive impressions. J Prosthet Dent 1995;73:333-40.

8. Craig RG. Review of dental impression materials. Adv Dent Res 1988;2:51-64.

9. Chee WW, Donovan TE. Fine detail reproduction of very high viscosity poly (vinyl siloxane) impression materials. Int J Prosthodont 1989;2:368-70.

10.Nissan J, Gross M, Shifman A, Assif D. Effect of wash bulk on the accuracy of polyvinyl siloxane putty-wash impressions. J Oral Rehabil 2002;29:357-61.

11.Chee WW, Donovan TE. Polyvinyl siloxane impression materials: a review of prop-erties and techniques. J Prosthet Dent 1992;68:728-32.

12.Lewinstein I. The ratio between vertical and horizontal changes of impressions. J Oral Rehabil 1993;20:107-14.

13.Boulton JL, Gage JP, Vincent PF, Basford KE. A laboratory study of dimensional changes for three elastomeric impression materials using custom and stock trays. Aust Dent J 1996;41:398-404.

14.Carrotte PV, Johnson A, Winstanley RB. The influence of the impression tray on the accuracy of impressions for crown and bridge work--an investigation and review. Br Dent J 1998;185:580-5.

15.Idris B, Houston F, Claffey N. Comparison of the dimensional accuracy of one- and two-step techniques with the use of putty/wash addition silicone impression materi-als. J Prosthet Dent 1995;74:535-41.

16.Lee IK, DeLong R, Pintado MR, Malik R. Evaluation of factors affecting the accuracy

of impressions using quantitative surface analysis. Oper Dent 1995;20:246-52.

17.Eames WB, Sieweke JC, Wallace SW, Rog-ers LB. Elastomeric impression materials: effect of bulk on accuracy. J Prosthet Dent 1979;41:304-7.

18.de Araujo PA, Jorgensen KD. Effect of ma-terial bulk and undercuts on the accuracy of impression materials. J Prosthet Dent 1985;54:791-4.

19.Johnson GH, Craig RG. Accuracy of four types of rubber impression materials com-pared with time of pour and a repeat pour of models. J Prosthet Dent 1985;53:484-90.

Corresponding author:Dr Sergio Caputi Department of Oral Sciences University G. D’AnnunzioVia dei Vestini 31, 66013 ChietiITALYFax: 39-0871-3554148E-mail: s.caputi@unich.it

AcknowledgementsThe authors thank Dr Christopher P. Berrie for a critical reading of the manuscript.

Copyright © 2008 by the Editorial Council for The Journal of Prosthetic Dentistry.

Caputi and Varvara Caputi and Varvara

Access to The Journal of Prosthetic Dentistry Online is reserved for print subscribers!

Full-text access to The Journal of Prosthetic Dentistry Online is available for all print subscribers. To activate your individu-al online subscription, please visit The Journal of Prosthetic Dentistry Online. Point your browser to http://www.journals.elsevierhealth.com/periodicals/ympr/home, follow the prompts to activate online access here, and follow the instruc-tions. To activate your account, you will need your subscriber account number, which you can find on your mailing label (note: the number of digits in your subscriber account number varies from 6 to 10). See the example below in which the subscriber account number has been circled.

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*********AUTO**SCH 3-DIGIT 001

280 Volume 99 Issue 4

The Journal of Prosthetic Dentistry

281April 2008

pressions. Moreover, the same mate-rials were used for all of the 3 putty/light-body techniques considered. Nevertheless, there were noted differ-ences in terms of the accuracy among the different techniques, thus, contra-dicting previous reports.15,16

The monophase technique is the easiest to perform, but it has been reported to be the worst in terms of dimensional accuracy and surface defects, as compared to putty/light-body techniques, because of the rela-tively high viscosity and reduced flow of the material used.2 The 1-step tech-nique has the advantages of simplicity and reasonable economy; however, in this technique, the putty tends to push the light-body wash off the pre-pared tooth, and, thus, critical areas, such as the finish line, can be covered by the putty, which cannot record de-tails to a satisfactory level.7,9 For this reason, even in studies in which the 1-step technique has been seen to be as accurate as the 2-step technique, concerns have been raised about sur-face defects when using the 1-step technique.15 Another difficulty with the 1-step technique is that once the light body material is on the prepara-tion, the putty needs to be brought into position and seated. During this critical phase, the patient’s tongue or the elevated floor of the mouth can re-move the light-body material from the tooth. The 2-step technique allows these problems to be overcome, but it can be associated with the creation of an occlusal step on adjacent teeth, as some light-body material may spread along the occlusal surfaces during the reseating of the putty. Moreover, the light-body material is displaced during this second step, and this can generate distortions that may result in reduced dimensional accuracy of the impression. It has also been reported that the wash material can displace the preliminary putty impression dur-ing the seating, and, afterwards, an elastic recovery of the putty can oc-cur upon removal of the impression, resulting in a tendency towards larger interabutment distances.15 To address

these concerns, the 2-step injection technique allows the displacement of soft tissues, such as the tongue, dur-ing the first seating of the putty and wash materials, while in the second step, the extra-light-body material re-cords all of the finer details without being compressed.

In the present study, the percent deviations from the stainless steel model ranged from 0.15% to 3.09% across the various techniques. In some instances, these deviations are greater than those of Hung et al4 and Idris et al,15 but similar to those of Nissan et al,6 and all of the dimensions on the stone casts (in each group) were greater than the corresponding di-mensions of the stainless steel model. Furthermore, in each group, the high-est and lowest percent deviations from the stainless steel model were seen for the intraabutment and inter-abutment dimensions, respectively (Table II). It is possible, however, that the greater contraction of the impres-sion material towards the walls of the trays might have primarily affected the regions with the smaller amounts of the impression material per wall sur-face, such as in the areas surrounding the abutments. Moreover, the greater percent deviations for the intraabut-ment dimensions were also related to their calculation on the basis of be-ing much smaller dimensions, as has been reported by Idris et al.15 When comparing the dimensions of the stone casts with the corresponding stainless steel model, previous studies have reported smaller vertical dimen-sions and greater horizontal dimen-sions.6,12,15 This was explained by the contraction of the impression mate-rial towards the tray walls. However, in a similar study, Hung et al4 did not report such a clear distinction in the dimensional changes between the ver-tical and horizontal measurements. The differences in the protocols and materials used among these studies, such as the application of an adhe-sive on the tray,15 may explain these discrepancies. Therefore, any clinical application based on the results of

different studies should strictly follow the experimental protocols used to achieve the same outcome.

The best results in the present study were seen for the 2-step injec-tion technique, which could be relat-ed to the reduced bulk wash obtained through the use of the extra-light-body material. Indeed, a bulk wash of 1-2 mm has been reported to perform significantly better than a bulk wash of 4 or 6 mm, in terms of dimensional accuracy.10 However, in using the clas-sic 2-step technique, the expected bulk wash might be greater than with the extra-light-body material used in the 2-step injection technique.

The use of light- and heavy-bodied automixed impression materials used in a 1-stage technique is common in clinical practice, and, since this uses a secondary material that is not as vis-cous as putty, further studies should include this technique as well. Finally, further in vitro and in vivo studies are warranted to fully explore the relative merits of the 2-step injection tech-nique, in particular, regarding the formation of surface defects and per-formance in an oral environment that includes saliva.

CONCLUSIONS

Within the limitations of this in vitro study, it can be concluded that different impression techniques affect the dimensional accuracy of the resul-tant casts, and, in particular:

1. The monophase technique yielded the lowest cast accuracy in every dimension considered, as com-pared to all of the other techniques (P<.01).

2. The 1-step technique yielded more accurate casts as compared to the monophase technique (P<.01), but less accurate casts than those from the 2-step and injection 2-step techniques, in every dimension con-sidered, as compared to all of the other techniques (P<.01).

3. The 2-step and injection 2-step techniques produced the most ac-curate casts (P<.01), with the latter

performing better than the former in every dimension considered (P<.01).

REFERENCES

1. Schoenrock GA. The laminar impression technique. J Prosthet Dent 1989;62:392-5.

2. Millar BJ, Dunne SM, Robinson PB. In vitro study of the number of surface defects in monophase and two-phase addition silicone impressions. J Prosthet Dent 1998;80:32-5.

3. Samet N, Shohat M, Livny A, Weiss EI. A clinical evaluation of fixed partial denture impressions. J Prosthet Dent 2005;94:112-7.

4. Hung SH, Purk JH, Tira DE, Eick JD. Accu-racy of one-step versus two-step putty wash addition silicone impression technique. J Prosthet Dent 1992;67:583-9.

5. Chen SY, Liang WM, Chen FN. Factors af-fecting the accuracy of elastometric impres-sion materials. J Dent 2004;32:603-9.

6. Nissan J, Laufer BZ, Brosh T, Assif D. Accu-racy of three polyvinyl siloxane putty-wash impression techniques. J Prosthet Dent 2000;83:161-5.

7. Morgano SM, Milot P, Ducharme P, Rose L. Ability of various impression materials to produce duplicate dies from successive impressions. J Prosthet Dent 1995;73:333-40.

8. Craig RG. Review of dental impression materials. Adv Dent Res 1988;2:51-64.

9. Chee WW, Donovan TE. Fine detail reproduction of very high viscosity poly (vinyl siloxane) impression materials. Int J Prosthodont 1989;2:368-70.

10.Nissan J, Gross M, Shifman A, Assif D. Effect of wash bulk on the accuracy of polyvinyl siloxane putty-wash impressions. J Oral Rehabil 2002;29:357-61.

11.Chee WW, Donovan TE. Polyvinyl siloxane impression materials: a review of prop-erties and techniques. J Prosthet Dent 1992;68:728-32.

12.Lewinstein I. The ratio between vertical and horizontal changes of impressions. J Oral Rehabil 1993;20:107-14.

13.Boulton JL, Gage JP, Vincent PF, Basford KE. A laboratory study of dimensional changes for three elastomeric impression materials using custom and stock trays. Aust Dent J 1996;41:398-404.

14.Carrotte PV, Johnson A, Winstanley RB. The influence of the impression tray on the accuracy of impressions for crown and bridge work--an investigation and review. Br Dent J 1998;185:580-5.

15.Idris B, Houston F, Claffey N. Comparison of the dimensional accuracy of one- and two-step techniques with the use of putty/wash addition silicone impression materi-als. J Prosthet Dent 1995;74:535-41.

16.Lee IK, DeLong R, Pintado MR, Malik R. Evaluation of factors affecting the accuracy

of impressions using quantitative surface analysis. Oper Dent 1995;20:246-52.

17.Eames WB, Sieweke JC, Wallace SW, Rog-ers LB. Elastomeric impression materials: effect of bulk on accuracy. J Prosthet Dent 1979;41:304-7.

18.de Araujo PA, Jorgensen KD. Effect of ma-terial bulk and undercuts on the accuracy of impression materials. J Prosthet Dent 1985;54:791-4.

19.Johnson GH, Craig RG. Accuracy of four types of rubber impression materials com-pared with time of pour and a repeat pour of models. J Prosthet Dent 1985;53:484-90.

Corresponding author:Dr Sergio Caputi Department of Oral Sciences University G. D’AnnunzioVia dei Vestini 31, 66013 ChietiITALYFax: 39-0871-3554148E-mail: s.caputi@unich.it

AcknowledgementsThe authors thank Dr Christopher P. Berrie for a critical reading of the manuscript.

Copyright © 2008 by the Editorial Council for The Journal of Prosthetic Dentistry.

Caputi and Varvara Caputi and Varvara

Access to The Journal of Prosthetic Dentistry Online is reserved for print subscribers!

Full-text access to The Journal of Prosthetic Dentistry Online is available for all print subscribers. To activate your individu-al online subscription, please visit The Journal of Prosthetic Dentistry Online. Point your browser to http://www.journals.elsevierhealth.com/periodicals/ympr/home, follow the prompts to activate online access here, and follow the instruc-tions. To activate your account, you will need your subscriber account number, which you can find on your mailing label (note: the number of digits in your subscriber account number varies from 6 to 10). See the example below in which the subscriber account number has been circled.

Personal subscriptions to The Journal of Prosthetic Dentistry Online are for individual use only and may not be trans-ferred. Use of The Journal of Prosthetic Dentistry Online is subject to agreement to the terms and conditions as indicated online.

This is your subscription account number

Sample mailing label

1 V97-3 J010 12345678-9J. H. DOE531 MAIN STCENTER CITY, NY 10001-001

*********AUTO**SCH 3-DIGIT 001