- Email: [email protected]
The effect of impression volume and double-arch trays on the registration of maximum intercuspation
The effect of impression volume and double-arch trays on the registration of maximum intercuspation Sara M. Hahn,a Philip L. Millstein, DMD, MS,b Taru H. Kinnunen, MA, PhD,c and Robert F. Wright, DDSd Harvard School of Dental Medicine, Boston, Mass Statement of problem. The type of double-arch trays used may affect occlusion. Purpose. The purpose of this study was to determine what effect, if any, double-arch tray design and impression material volume had on the registration of maximum intercuspation (MI). Material and methods. Quadrant impressions were made on articulated fracture-resistant dental casts mounted in maximum intercuspation occlusion. Three types of sideless double-arch impression trays were used: First Bite with nylon webbing, Sultan’s 3-Way with double crosshatch webbing, and Premium’s 3-in-1 Tray with single crosshatch webbing. Vinyl polysiloxane impression material (Aquasil Ultra Rigid Fast Set) was distributed at 2 different volumes (5.4 ml and 8.3 ml), and 60 impressions were made (n=10). A weight of 1.2 kg was placed on the upper arm of an Artex articulator, ensuring complete closure. The impressions were allowed to polymerize for 5 minutes. After polymerization, specimens were placed on a light box, and a camera set at a fixed distance was used to capture the light transmission that was projected through the impression material. The camera transferred the information to an image analysis program (ImageJ). This system allowed the different amounts of light projected through the impression to be translated into a gray scale value (GSV), which was assigned a thickness value, in millimeters, of a specified occlusal contact area. To assess reliability of the experimental design, 10 control impressions were made by directly applying impression material onto the typodont. These were analyzed in the same manner as the impressions made with trays. A 2-way ANOVA comparing volume by tray type was used (α=.05). This was followed by a Tukey HSD test. Results. There was no main effect for volume of impression material (P=.71). Tray type was significantly different (P<.001). Impressions made with Sultan trays were significantly less accurate than impressions made with First Bite or Premium impression trays. The Premium tray type had the highest mean (SD) GSV, 179.8 (8.1), significantly higher than First Bite (164.7 (19.8); P<.001) or Sultan (82.8 (7.4); P<.001) trays. Conclusions. This in vitro study demonstrated that certain trays hinder closure into MI, which may negatively affect the accuracy of record making. (J Prosthet Dent 2009;102:362-367)
Clinical Implications
Not all sideless double-arch impression trays are created equal. Premium’s 3-in-1 tray and Dentsply’s First Bite trays provided a more accurate registration of the occlusal position than Sultan trays. The volume of impression material dispensed for a double-arch tray has no effect on the occlusal relationship.
Funded by the Office of Enrichment Programs, Harvard Medical School, as well as the Prosthodontics Research and Scholarship Fund and the Department of Restorative Dentistry and Biomaterial Sciences, Harvard School of Dental Medicine. DMD student. Lecturer, Restorative Dentistry and Biomaterials Sciences; private practice, Cambridge, Mass. c Assistant Professor, Department of Oral Health Policy and Epidemiology. d Associate Professor, Restorative Dentistry and Biomaterials Sciences; Director, Advanced Graduate Prosthodontics. a
b
The Journal of Prosthetic Dentistry
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December 2009 Dual-arch impression trays are used for fixed prosthodontics procedures.1 Some clinicians prefer sideless plastic trays, while others use metal trays with side walls.2 Proponents for the plastic trays claim they are easy to use, while others believe they are too flexible. The use of a sideless plastic tray provides an escape of impression material during impression making.1 The mesh designs of double-arch trays differ.3 Some are filamentous, while others are crosshatched. It appears that all meshes are made from a relatively impermeable nylon type of material that does not absorb fluids. A filamentous mesh is thin and adapts readily to occlusal surfaces. Crosshatched meshes can resist full closure because the mesh does not adapt as readily to the occlusal surfaces. The meshes are compressible to provide the closest possible contact, to afford accurate interocclusal registration. The impression material of choice when using dual-arch trays is vinyl polysiloxane, because it is clean, provides excellent surface detail, is accurate, and is easy to remove.4 Vinyl polysiloxane is a dual-mix rigid material that sets hard; the flexible tray becomes semirigid upon polymerization of the impression material. There may be some resistance to closure when using various impression trays of this type because they contain different impression support systems. The purpose of this study was to compare 3 tray systems and to determine whether closure into double-arch trays was affected by impression volume and/ or type of tray support, for example, webbing or mesh. The null hypothesis was that there would be no significant difference found between tray design and registration of maximum intercuspation.
blach, Austria). The casts were made of epoxy resin die material, which is more resistant to abrasion and stronger than a gypsum die material. The casts were developed by P.K. Thomas in 1960. The methodology of using these casts was established by Hatzi et al in 2001.5 Impressions of the left posterior quadrant of the casts were made using plastic double-arch trays. The 2 independent variables were type of tray (from 3 different manufacturers) and volume of impression material. A sample size of 10 was used, yielding 60 impressions. Three common types of dualarch trays were used, and all were plastic and comparable in size: First Bite, with thin, gauze, nylon webbing (Dentsply Caulk, Milford, Del), 3-Way, with a nylon double crosshatch mesh (Sultan Dental Materials Ltd, Englewood, NJ), and Premium 3-in-1 Tray, made of a single crosshatch material (Premium Plus Dental Supplies, Inc, Brooklyn, NY) (Fig. 1). Using a vernier caliper (General Tools & Instruments Co, New York, NY), the thickness of Sultan’s 3-Way mesh was measured to be 0.25 mm, and both First Bite and 3-in-1 trays measured 0.10 mm. The dual-arch trays were placed to ensure that the casts mounted on the articulator could be closed into occlusion without any interference from the tray.6 Vinyl polysiloxane impression material (Aquasil Ultra Rigid Fast Set; Dentsply Caulk) was auto-
mixed and injected into a dual-arch tray using a disposable syringe tip. A dispensing gun (Dispensing Gun; Dentsply Caulk) was used to produce a constant volume with each pull of the trigger; the volume is not dependent on the force applied to the trigger. For the low volume group, 5 complete activations of the automix cartridge were used to dispense tray material into both sides of the dualarch tray. The first complete activation was not enough to bring the material beyond the disposable tip. Then, 2 more complete activations were dispensed on the maxillary side, and the remaining 2 complete activations were dispensed on the mandibular side. For the high volume group, 7 complete activations were used, 1 preliminary pull, 3 on the maxillary side, and 3 on the mandibular side. After each record was made, it was weighed with a balance scale (Vicon VIC-711 Precision Balance scale; Acculab Sartorius Group, Edgewood, NY) with 0.1-g readability. The mass of the appropriate dual-arch tray was subtracted, and the volume was calculated by dividing the mass by the density of the vinyl polysiloxane impression material (Aquasil Ultra Rigid Fast Set; Dentsply Caulk), which is 1.5 g/ml, as purported by the manufacturer. Five complete activations equaled 5.4 ±0.1 ml, and 7 complete activations equaled 8.3 ±0.1 ml. The dual-arch trays were placed onto the mandibular left quadrant, the articu-
MATERIAL AND METHODS Two impact-resistant casts were mounted in maximum intercuspal position on a semi-adjustable articulator (Artex AL; Amann Girrbach AG, Ko-
Hahn et al
1 Comparison of 3 dual-arch impression trays: Dentsply Caulk First Bite (left), Sultan 3-Way (middle), and Premium 3-in-1 Tray (right).
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2 Impression tray held in fixed position with articulated typodont in closed position (1.2-kg weight is not shown). Incisal pin was maintained in place.
3 Example of zero (left), low (middle), and high (right) volume. lator was closed, and a 1.2-kg weight was placed on top to simulate a constant occlusal force.7 The weight of 1.2 kg was chosen because of its availability and to consistently reproduce the same amount of force during closure. A clamp/jig was used so that the position of the impression tray was constant and reproducible between trials (Fig. 2). The impressions were removed 5 minutes after the articulator was closed (Fig. 3). The method of recording and measuring was as follows. A silicone record (Regisil Bite Registration Material; Dentsply Caulk) of contact, near contact, or noncontact was made.
The record was a 3-dimensional (3-D) representation of the specific cusp/ fossa relationship. A silicone calibration grid was also made at the time of record making.8 The calibration grid was formed by placing a small amount of silicone on the lower plate of a press (made by the second author), the upper plate of which contained a small sphere with a diameter of three eighths of an inch. The press was closed until contact was made between the sphere and plate. The press provided a uniformly circular silicone grid that was used for calibration purposes.8 The grid and record were illuminated by placing them on
The Journal of Prosthetic Dentistry
a direct current light box (Fostec; Schott North America, Inc, Southbridge, Mass) and photographed at a fixed distance of 8 inches (Fig. 4). Direct current was used because it provides a constant emission of light. If the records were placed on the light box at varying angles, this would potentially cause variation in light transmission. It was estimated through geometric formulas that if the angle was less than 25 degrees, the uncertainty would be less than 10% of the range of interest.5 Excessive material from the interocclusal records was removed to minimize placement error.5 The gray scale digital camera (CFW-1308M; Scion Corp, Frederick, Md) was set at a fixed distance of 8 inches and captured the light transmitted through the impression material. The camera transferred this information to an image analysis program (ImageJ; NIH, Bethesda, Md).8 This system allows the different amounts of light projected through the impression to be translated into a gray scale value which can be assigned a thickness value. In this study, the values of the picture elements (pixels) were determined by the optical density of the impression material.8 The effective thickness assigned to a pixel corresponded to the thickness at the center of the pixel.8 An image was made of the calibration grid on the light box in the same manner as for the impressions. A thin rectangular box was selected across the diameter of the circle; the ends of the rectangular box were dark gray, and the center, from which point most of the light was transmitted, was white. At any point along the rectangle, a pixel could be assigned a gray scale value (GSV), which signified how black or white that pixel was. Using sphere geometry, the thickness of the calibration grid was determined and was correlated to its assigned GSV. This allowed for the determination of the corresponding thickness of any GSV. One way to describe a computerized record is to describe it numerically. The mesiolingual cusp on the max-
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December 2009
4 Calibration grid (left) and impression (right) photographed on light box. 800 700
Thickness (µm)
600 500 400 300 200 100 0
0
50
100
150
200
250
Gray Scale Value 5 Relationship between gray scale value and corresponding thickness. illary left first molar was highlighted with a customized standard circular template for all 60 records. Through the image analysis software (ImageJ; NIH), a customized 3-mm-diameter circle was used for each specimen. The same circle was applied to each specimen so that the exact same number of pixels was analyzed. The circle was centered over the mesiolingual cusp using radiolucent landmarks that were visible on every image captured. Then, those regions of interest (ROIs) were analyzed for an average of all pixels. The range was 15 (black) to 255 (white). All of the ROIs were represented by a number, which was lower if less light was transmitted (a record with more noncontact areas). As described above, the use of the grid established the relationship between
Hahn et al
transmitted light (GSV) and thickness of the impression material8 (Fig. 5). To ensure reliability of the record-making technique, 10 control specimens of impression material dispensed directly onto the articulated typodont were collected. Two pulls were distributed onto 3 occlusal surfaces from the mandibular second premolar to the second molar. The same customized circular template captured the mesiolingual cusp of the maxillary first molar for analysis. To assess for variability between groups, each type of tray was placed on the light box without any impression material, and the light transmission was measured. A 2-way analysis of variance (ANOVA) was used to test main and interaction effects (tray (3) by volume (2)), and Tukey Honestly
Significant Difference (HSD) post hoc test was used to evaluate significant differences among groups.
RESULTS The control group’s regions of interest (ROIs) were measured, and the mean was 248.9 ±2.4 gray scale units. This indicated that the contact point was close to the maximum white gray scale value, which was 255 GSV. The purpose of the control group was to assess reliability of the entire apparatus, from impressions to image analysis. For this reason, it was not included in the statistical analysis with the impressions with trays. When the trays without impression material were measured for light transmission, all of them were 255 GSV. The photos
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Volume 102 Issue 6 200
Light (Gray Scale Units)
180
Low volume High volume
160 140 120 100 80 60 40 20 0
First Bite
Sultan
Premium
Tray Type 6 Mean light in gray scale units transmitted for each tray at 2 different volumes. were made at the same light box intensity as the impressions with trays. The results from the 3 (tray) by 2 (volume) between-subjects ANOVA include a significant main effect for the tray F (2, 54) = 323.6; P<.001. No significant main effect was found for volume F (1, 54) = 0.15; or tray by volume interaction F (2, 54) =1.8. The Tukey HSD test for equal sample sizes was used to examine the significant differences among tray types. The Premium tray type had the highest mean (SD) GSV, 179.8 (8.1), significantly higher than First Bite (164.7 (19.8); P<.001) or Sultan (82.8 (7.4); P<.001) trays. The Sultan tray had the lowest gray scale value, which was significantly lower (P<.001) than First Bite (Fig. 6). Figure 5 depicts the relationship between gray scale value and thickness of the contact point, measured in micrometers. First Bite was 250-300 μm, Sultan was 430530 μm, and Premium was 250-300 μm. The mean (SD) control measured 248.9 (2.4) gray scale units, which translates to being less than 200 μm thick in the same region of interest. First Bite and Premium were at least 50 μm thicker, and Sultan was more than 200 μm thicker than an impression with no tray.
DISCUSSION Interestingly, the thickness of vinyl polysiloxane impression material was significantly different among the 3 tray types. Therefore, the null hypothesis was rejected. All of the trays were sideless and comparable in size. The only differences between trays were the mesh design and mesh thickness. The thicker meshes were more stiff and resisted complete closure. Such shifts in closure may affect accuracy in the transfer to an articulator. The region of interest analyzed was the mesiolingual cusp of the first molar. First Bite and Premium both measured 250-300 μm for this area. Sultan was thicker, at 430-530 μm. Complete closure did not occur with any of the trays when compared to the control. The aim of dual-arch trays is to be as thin as possible to allow intimate interocclusal contact. The double crosshatched webbing significantly hindered closure. Increasing the volume by 3 ml did not affect closure. The null hypothesis stated that no difference was expected between the low volume group and high volume group. Therefore, the null hypothesis was not rejected with regard to volume. Assuming that the 1.2-kg weight was applied at a con-
The Journal of Prosthetic Dentistry
stant rate for each trial, closure would be dependent on the viscosity of the material. The next step would be to examine the differences between vinyl polysiloxane impression materials with different viscosities to determine if heavy-body materials hinder interocclusal contact more than mediumbody materials. Although there was no difference in vitro, there may be a difference clinically, because patients occlude with different amounts of force. The mesh is compressible, and, when coupled with the presence of saliva and different closure forces, the results may be different. Different trays were compared in this study because, to date, it is unknown how trays differ once impression material and closure force are added. This study could be improved by using a micrometer screw to measure the exact thickness of the meshes studied. In addition, thin articulating paper should be used to ensure that the incisal pin contacts the table after each impression is made. Furthermore, a wider range of weights should be used to resemble forces that are clinically applicable. Finally, to further validate the results, stone casts made from the impressions should be evaluated for accuracy.
CONCLUSIONS This in vitro study compared the effect of impression material volume and tray design on closure of mounted casts. Sultan’s thicker double crosshatch mesh significantly hindered closure, more than the tray designs of First Bite or Premium trays. Increasing the amount of impression material did not hinder interocclusal closure.
REFERENCES 1. Breeding LC, Dixon DL. Accuracy of casts generated from dual-arch impressions. J Prosthet Dent 2000;84:403-7. 2. Kaplowitz GJ. Trouble-shooting dual arch impressions II. J Am Dent Assoc 1997;128:1277-81.
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December 2009 3. Wilson EG, Werrin SR. Double arch impressions for simplified restorative dentistry. J Prosthet Dent 1983;49:198-202. 4. Parker MH, Cameron SM, Hughbanks JC, Reid DE. Comparison of occlusal contacts in maximum intercuspation for two impression techniques. J Prosthet Dent 1997;78:255-9. 5. Hatzi P, Millstein P, Maya A. Determining the accuracy of articulator interchangeability and hinge axis reproducibility. J Prosthet Dent 2001;85:236-45.
6. Ceyhan J, Johnson G, Lepe X. The effect of tray selection, viscosity of impression material and sequence of pour on the accuracy of dies made from dual-arch impressions. J Prosthet Dent 2003;90:143-9. 7. Davis RD, Schwartz RS. Dual-arch and custom tray impression accuracy. Am J Dent 1991;4:89-92. 8. Millstein PL. A method to determine occlusal contact and noncontact areas: preliminary report. J Prosthet Dent 1984;52:10610.
Corresponding author: Sara Hahn Harvard School of Dental Medicine 188 Longwood Ave Boston, MA 02115 Fax: 617-432-0901 E-mail: [email protected] Acknowledgements The authors thank Dentsply Caulk for providing impression materials and Deborah Lukey for her technical assistance. Copyright © 2009 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Changes in patient and FDP profiles following the introduction of osseointegrated implant dentistry in a prosthodontic practice Walton TR. Int J Prosthodont 2009;22:127–35. Purpose: This paper describes the effects of implant-related dentistry on patient profiles and the types of fixed dental prostheses provided in clinical practice. Materials and Methods: All implant- and tooth-supported prostheses provided in a prosthodontic practice between 1984 and 2007 were tabulated. Incidence was analyzed in relation to gender, age at time of prostheses insertion, and prostheses type. Results: Tooth-supported single crowns (TSCs) and tooth-supported fixed dental prostheses (TFDPs) were involved in 97% of treatments requiring fixed dental prostheses from 1984 to 1991. From 1991 to 2007, however, a marked increase in the number of implants restored directly corresponded with a decrease in the number of TFDPs so that by 2007, implant-supported fixed dental prostheses (IFDPs) accounted for 81% of all tooth replacements. Between 1984 and 2007 the incidence of TFDPs was 61% in females and 39% in males, whereas the incidence of IFDPs was 55% in females and 45% in males. IFDPs were also involved in 35% of restorations in patients under 31 years of age and TFDPs in 19%. In the under-21 years age group, IFDPs were more common in females (9%) than males (4%), but in the 21 to 30 years age group they were more common in males (21%) than females (13%). There was a decrease in three-unit TFDPs, in TFDPs with four or more pontics and those not satisfying Ante’s Law, and in teeth used that had been subjectively assessed to have an unfavorable 10-year prognosis at the time of prosthesis insertion. Conclusions: The incorporation of osseointegrated implant dentistry into a clinical practice has resulted in changes in the patient profile and type of fixed dental prostheses provided, including a decrease in the use of TFDPs; an increase in the referral of patients under 31 years of age; a decrease in three-unit, long-span, and complex TFDPs; and a decrease in tooth abutments assessed to be structurally or biologically compromised. Reprinted with permission of Quintessence Publishing.
Hahn et al
Clinical Implications
Not all sideless double-arch impression trays are created equal. Premium’s 3-in-1 tray and Dentsply’s First Bite trays provided a more accurate registration of the occlusal position than Sultan trays. The volume of impression material dispensed for a double-arch tray has no effect on the occlusal relationship.
Funded by the Office of Enrichment Programs, Harvard Medical School, as well as the Prosthodontics Research and Scholarship Fund and the Department of Restorative Dentistry and Biomaterial Sciences, Harvard School of Dental Medicine. DMD student. Lecturer, Restorative Dentistry and Biomaterials Sciences; private practice, Cambridge, Mass. c Assistant Professor, Department of Oral Health Policy and Epidemiology. d Associate Professor, Restorative Dentistry and Biomaterials Sciences; Director, Advanced Graduate Prosthodontics. a
b
The Journal of Prosthetic Dentistry
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December 2009 Dual-arch impression trays are used for fixed prosthodontics procedures.1 Some clinicians prefer sideless plastic trays, while others use metal trays with side walls.2 Proponents for the plastic trays claim they are easy to use, while others believe they are too flexible. The use of a sideless plastic tray provides an escape of impression material during impression making.1 The mesh designs of double-arch trays differ.3 Some are filamentous, while others are crosshatched. It appears that all meshes are made from a relatively impermeable nylon type of material that does not absorb fluids. A filamentous mesh is thin and adapts readily to occlusal surfaces. Crosshatched meshes can resist full closure because the mesh does not adapt as readily to the occlusal surfaces. The meshes are compressible to provide the closest possible contact, to afford accurate interocclusal registration. The impression material of choice when using dual-arch trays is vinyl polysiloxane, because it is clean, provides excellent surface detail, is accurate, and is easy to remove.4 Vinyl polysiloxane is a dual-mix rigid material that sets hard; the flexible tray becomes semirigid upon polymerization of the impression material. There may be some resistance to closure when using various impression trays of this type because they contain different impression support systems. The purpose of this study was to compare 3 tray systems and to determine whether closure into double-arch trays was affected by impression volume and/ or type of tray support, for example, webbing or mesh. The null hypothesis was that there would be no significant difference found between tray design and registration of maximum intercuspation.
blach, Austria). The casts were made of epoxy resin die material, which is more resistant to abrasion and stronger than a gypsum die material. The casts were developed by P.K. Thomas in 1960. The methodology of using these casts was established by Hatzi et al in 2001.5 Impressions of the left posterior quadrant of the casts were made using plastic double-arch trays. The 2 independent variables were type of tray (from 3 different manufacturers) and volume of impression material. A sample size of 10 was used, yielding 60 impressions. Three common types of dualarch trays were used, and all were plastic and comparable in size: First Bite, with thin, gauze, nylon webbing (Dentsply Caulk, Milford, Del), 3-Way, with a nylon double crosshatch mesh (Sultan Dental Materials Ltd, Englewood, NJ), and Premium 3-in-1 Tray, made of a single crosshatch material (Premium Plus Dental Supplies, Inc, Brooklyn, NY) (Fig. 1). Using a vernier caliper (General Tools & Instruments Co, New York, NY), the thickness of Sultan’s 3-Way mesh was measured to be 0.25 mm, and both First Bite and 3-in-1 trays measured 0.10 mm. The dual-arch trays were placed to ensure that the casts mounted on the articulator could be closed into occlusion without any interference from the tray.6 Vinyl polysiloxane impression material (Aquasil Ultra Rigid Fast Set; Dentsply Caulk) was auto-
mixed and injected into a dual-arch tray using a disposable syringe tip. A dispensing gun (Dispensing Gun; Dentsply Caulk) was used to produce a constant volume with each pull of the trigger; the volume is not dependent on the force applied to the trigger. For the low volume group, 5 complete activations of the automix cartridge were used to dispense tray material into both sides of the dualarch tray. The first complete activation was not enough to bring the material beyond the disposable tip. Then, 2 more complete activations were dispensed on the maxillary side, and the remaining 2 complete activations were dispensed on the mandibular side. For the high volume group, 7 complete activations were used, 1 preliminary pull, 3 on the maxillary side, and 3 on the mandibular side. After each record was made, it was weighed with a balance scale (Vicon VIC-711 Precision Balance scale; Acculab Sartorius Group, Edgewood, NY) with 0.1-g readability. The mass of the appropriate dual-arch tray was subtracted, and the volume was calculated by dividing the mass by the density of the vinyl polysiloxane impression material (Aquasil Ultra Rigid Fast Set; Dentsply Caulk), which is 1.5 g/ml, as purported by the manufacturer. Five complete activations equaled 5.4 ±0.1 ml, and 7 complete activations equaled 8.3 ±0.1 ml. The dual-arch trays were placed onto the mandibular left quadrant, the articu-
MATERIAL AND METHODS Two impact-resistant casts were mounted in maximum intercuspal position on a semi-adjustable articulator (Artex AL; Amann Girrbach AG, Ko-
Hahn et al
1 Comparison of 3 dual-arch impression trays: Dentsply Caulk First Bite (left), Sultan 3-Way (middle), and Premium 3-in-1 Tray (right).
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2 Impression tray held in fixed position with articulated typodont in closed position (1.2-kg weight is not shown). Incisal pin was maintained in place.
3 Example of zero (left), low (middle), and high (right) volume. lator was closed, and a 1.2-kg weight was placed on top to simulate a constant occlusal force.7 The weight of 1.2 kg was chosen because of its availability and to consistently reproduce the same amount of force during closure. A clamp/jig was used so that the position of the impression tray was constant and reproducible between trials (Fig. 2). The impressions were removed 5 minutes after the articulator was closed (Fig. 3). The method of recording and measuring was as follows. A silicone record (Regisil Bite Registration Material; Dentsply Caulk) of contact, near contact, or noncontact was made.
The record was a 3-dimensional (3-D) representation of the specific cusp/ fossa relationship. A silicone calibration grid was also made at the time of record making.8 The calibration grid was formed by placing a small amount of silicone on the lower plate of a press (made by the second author), the upper plate of which contained a small sphere with a diameter of three eighths of an inch. The press was closed until contact was made between the sphere and plate. The press provided a uniformly circular silicone grid that was used for calibration purposes.8 The grid and record were illuminated by placing them on
The Journal of Prosthetic Dentistry
a direct current light box (Fostec; Schott North America, Inc, Southbridge, Mass) and photographed at a fixed distance of 8 inches (Fig. 4). Direct current was used because it provides a constant emission of light. If the records were placed on the light box at varying angles, this would potentially cause variation in light transmission. It was estimated through geometric formulas that if the angle was less than 25 degrees, the uncertainty would be less than 10% of the range of interest.5 Excessive material from the interocclusal records was removed to minimize placement error.5 The gray scale digital camera (CFW-1308M; Scion Corp, Frederick, Md) was set at a fixed distance of 8 inches and captured the light transmitted through the impression material. The camera transferred this information to an image analysis program (ImageJ; NIH, Bethesda, Md).8 This system allows the different amounts of light projected through the impression to be translated into a gray scale value which can be assigned a thickness value. In this study, the values of the picture elements (pixels) were determined by the optical density of the impression material.8 The effective thickness assigned to a pixel corresponded to the thickness at the center of the pixel.8 An image was made of the calibration grid on the light box in the same manner as for the impressions. A thin rectangular box was selected across the diameter of the circle; the ends of the rectangular box were dark gray, and the center, from which point most of the light was transmitted, was white. At any point along the rectangle, a pixel could be assigned a gray scale value (GSV), which signified how black or white that pixel was. Using sphere geometry, the thickness of the calibration grid was determined and was correlated to its assigned GSV. This allowed for the determination of the corresponding thickness of any GSV. One way to describe a computerized record is to describe it numerically. The mesiolingual cusp on the max-
Hahn et al
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December 2009
4 Calibration grid (left) and impression (right) photographed on light box. 800 700
Thickness (µm)
600 500 400 300 200 100 0
0
50
100
150
200
250
Gray Scale Value 5 Relationship between gray scale value and corresponding thickness. illary left first molar was highlighted with a customized standard circular template for all 60 records. Through the image analysis software (ImageJ; NIH), a customized 3-mm-diameter circle was used for each specimen. The same circle was applied to each specimen so that the exact same number of pixels was analyzed. The circle was centered over the mesiolingual cusp using radiolucent landmarks that were visible on every image captured. Then, those regions of interest (ROIs) were analyzed for an average of all pixels. The range was 15 (black) to 255 (white). All of the ROIs were represented by a number, which was lower if less light was transmitted (a record with more noncontact areas). As described above, the use of the grid established the relationship between
Hahn et al
transmitted light (GSV) and thickness of the impression material8 (Fig. 5). To ensure reliability of the record-making technique, 10 control specimens of impression material dispensed directly onto the articulated typodont were collected. Two pulls were distributed onto 3 occlusal surfaces from the mandibular second premolar to the second molar. The same customized circular template captured the mesiolingual cusp of the maxillary first molar for analysis. To assess for variability between groups, each type of tray was placed on the light box without any impression material, and the light transmission was measured. A 2-way analysis of variance (ANOVA) was used to test main and interaction effects (tray (3) by volume (2)), and Tukey Honestly
Significant Difference (HSD) post hoc test was used to evaluate significant differences among groups.
RESULTS The control group’s regions of interest (ROIs) were measured, and the mean was 248.9 ±2.4 gray scale units. This indicated that the contact point was close to the maximum white gray scale value, which was 255 GSV. The purpose of the control group was to assess reliability of the entire apparatus, from impressions to image analysis. For this reason, it was not included in the statistical analysis with the impressions with trays. When the trays without impression material were measured for light transmission, all of them were 255 GSV. The photos
366
Volume 102 Issue 6 200
Light (Gray Scale Units)
180
Low volume High volume
160 140 120 100 80 60 40 20 0
First Bite
Sultan
Premium
Tray Type 6 Mean light in gray scale units transmitted for each tray at 2 different volumes. were made at the same light box intensity as the impressions with trays. The results from the 3 (tray) by 2 (volume) between-subjects ANOVA include a significant main effect for the tray F (2, 54) = 323.6; P<.001. No significant main effect was found for volume F (1, 54) = 0.15; or tray by volume interaction F (2, 54) =1.8. The Tukey HSD test for equal sample sizes was used to examine the significant differences among tray types. The Premium tray type had the highest mean (SD) GSV, 179.8 (8.1), significantly higher than First Bite (164.7 (19.8); P<.001) or Sultan (82.8 (7.4); P<.001) trays. The Sultan tray had the lowest gray scale value, which was significantly lower (P<.001) than First Bite (Fig. 6). Figure 5 depicts the relationship between gray scale value and thickness of the contact point, measured in micrometers. First Bite was 250-300 μm, Sultan was 430530 μm, and Premium was 250-300 μm. The mean (SD) control measured 248.9 (2.4) gray scale units, which translates to being less than 200 μm thick in the same region of interest. First Bite and Premium were at least 50 μm thicker, and Sultan was more than 200 μm thicker than an impression with no tray.
DISCUSSION Interestingly, the thickness of vinyl polysiloxane impression material was significantly different among the 3 tray types. Therefore, the null hypothesis was rejected. All of the trays were sideless and comparable in size. The only differences between trays were the mesh design and mesh thickness. The thicker meshes were more stiff and resisted complete closure. Such shifts in closure may affect accuracy in the transfer to an articulator. The region of interest analyzed was the mesiolingual cusp of the first molar. First Bite and Premium both measured 250-300 μm for this area. Sultan was thicker, at 430-530 μm. Complete closure did not occur with any of the trays when compared to the control. The aim of dual-arch trays is to be as thin as possible to allow intimate interocclusal contact. The double crosshatched webbing significantly hindered closure. Increasing the volume by 3 ml did not affect closure. The null hypothesis stated that no difference was expected between the low volume group and high volume group. Therefore, the null hypothesis was not rejected with regard to volume. Assuming that the 1.2-kg weight was applied at a con-
The Journal of Prosthetic Dentistry
stant rate for each trial, closure would be dependent on the viscosity of the material. The next step would be to examine the differences between vinyl polysiloxane impression materials with different viscosities to determine if heavy-body materials hinder interocclusal contact more than mediumbody materials. Although there was no difference in vitro, there may be a difference clinically, because patients occlude with different amounts of force. The mesh is compressible, and, when coupled with the presence of saliva and different closure forces, the results may be different. Different trays were compared in this study because, to date, it is unknown how trays differ once impression material and closure force are added. This study could be improved by using a micrometer screw to measure the exact thickness of the meshes studied. In addition, thin articulating paper should be used to ensure that the incisal pin contacts the table after each impression is made. Furthermore, a wider range of weights should be used to resemble forces that are clinically applicable. Finally, to further validate the results, stone casts made from the impressions should be evaluated for accuracy.
CONCLUSIONS This in vitro study compared the effect of impression material volume and tray design on closure of mounted casts. Sultan’s thicker double crosshatch mesh significantly hindered closure, more than the tray designs of First Bite or Premium trays. Increasing the amount of impression material did not hinder interocclusal closure.
REFERENCES 1. Breeding LC, Dixon DL. Accuracy of casts generated from dual-arch impressions. J Prosthet Dent 2000;84:403-7. 2. Kaplowitz GJ. Trouble-shooting dual arch impressions II. J Am Dent Assoc 1997;128:1277-81.
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December 2009 3. Wilson EG, Werrin SR. Double arch impressions for simplified restorative dentistry. J Prosthet Dent 1983;49:198-202. 4. Parker MH, Cameron SM, Hughbanks JC, Reid DE. Comparison of occlusal contacts in maximum intercuspation for two impression techniques. J Prosthet Dent 1997;78:255-9. 5. Hatzi P, Millstein P, Maya A. Determining the accuracy of articulator interchangeability and hinge axis reproducibility. J Prosthet Dent 2001;85:236-45.
6. Ceyhan J, Johnson G, Lepe X. The effect of tray selection, viscosity of impression material and sequence of pour on the accuracy of dies made from dual-arch impressions. J Prosthet Dent 2003;90:143-9. 7. Davis RD, Schwartz RS. Dual-arch and custom tray impression accuracy. Am J Dent 1991;4:89-92. 8. Millstein PL. A method to determine occlusal contact and noncontact areas: preliminary report. J Prosthet Dent 1984;52:10610.
Corresponding author: Sara Hahn Harvard School of Dental Medicine 188 Longwood Ave Boston, MA 02115 Fax: 617-432-0901 E-mail: [email protected] Acknowledgements The authors thank Dentsply Caulk for providing impression materials and Deborah Lukey for her technical assistance. Copyright © 2009 by the Editorial Council for The Journal of Prosthetic Dentistry.
Noteworthy Abstracts of the Current Literature Changes in patient and FDP profiles following the introduction of osseointegrated implant dentistry in a prosthodontic practice Walton TR. Int J Prosthodont 2009;22:127–35. Purpose: This paper describes the effects of implant-related dentistry on patient profiles and the types of fixed dental prostheses provided in clinical practice. Materials and Methods: All implant- and tooth-supported prostheses provided in a prosthodontic practice between 1984 and 2007 were tabulated. Incidence was analyzed in relation to gender, age at time of prostheses insertion, and prostheses type. Results: Tooth-supported single crowns (TSCs) and tooth-supported fixed dental prostheses (TFDPs) were involved in 97% of treatments requiring fixed dental prostheses from 1984 to 1991. From 1991 to 2007, however, a marked increase in the number of implants restored directly corresponded with a decrease in the number of TFDPs so that by 2007, implant-supported fixed dental prostheses (IFDPs) accounted for 81% of all tooth replacements. Between 1984 and 2007 the incidence of TFDPs was 61% in females and 39% in males, whereas the incidence of IFDPs was 55% in females and 45% in males. IFDPs were also involved in 35% of restorations in patients under 31 years of age and TFDPs in 19%. In the under-21 years age group, IFDPs were more common in females (9%) than males (4%), but in the 21 to 30 years age group they were more common in males (21%) than females (13%). There was a decrease in three-unit TFDPs, in TFDPs with four or more pontics and those not satisfying Ante’s Law, and in teeth used that had been subjectively assessed to have an unfavorable 10-year prognosis at the time of prosthesis insertion. Conclusions: The incorporation of osseointegrated implant dentistry into a clinical practice has resulted in changes in the patient profile and type of fixed dental prostheses provided, including a decrease in the use of TFDPs; an increase in the referral of patients under 31 years of age; a decrease in three-unit, long-span, and complex TFDPs; and a decrease in tooth abutments assessed to be structurally or biologically compromised. Reprinted with permission of Quintessence Publishing.
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