Pressure transmission and distribution under impact load using artificial denture teeth made of different materials



Pressure transmission and distribution under impact load using artificial denture teeth made of different materials




Journal of Prosthetic Dentistry, 2009-11-01, Volume 102, Issue 5, Pages 319-327, Copyright © 2009 The Editorial Council of the Journal of Prosthetic Dentistry


Statement of problem

Pressure transmission and distribution under denture bases may be different depending on the material of the artificial denture teeth used.

Purpose

The purposes of this study were to evaluate pressure transmission and distribution under impact load using artificial denture teeth composed of different materials, and to examine the modulus of elasticity of the artificial denture teeth.

Material and methods

The denture base specimens with artificial denture teeth made of 4 different materials (acrylic resin, microfilled composite resin, nanocomposite resin, and ceramic) were evaluated. Pressure transmission, distribution, and maximum pressure (n=10) were observed with pressure-sensitive sheets under an impact load. Modulus of elasticity of the artificial denture teeth (n=10) was measured by using an ultramicroindentation system. Data were statistically analyzed with 1-way ANOVA, followed by Tukey HSD and Tamhane's multiple range post hoc tests (α=.05).

Results

Maximum pressure transmission observed from ceramic denture teeth was significantly higher than that of other groups ( P <.001). Nanocomposite resin denture teeth presented the lowest pressure transmission, whereas a localized stress transmission area was observed in the ceramic denture teeth group. Significant differences in the modulus of elasticity were observed among the 4 types of artificial denture teeth ( P <.001).

Conclusions

Pressure transmission and distribution varied among the denture tooth materials. Differences in the modulus of elasticity of each type of denture tooth were demonstrated. (J Prosthet Dent 2009;102:319-327)

Clinical Implications

Artificial denture teeth composed of different materials showed different amounts and patterns of pressure distribution. Choosing the appropriate denture tooth material may lessen the force transmitted to the supporting structures.

Pressure from a prosthesis is an important factor in increasing residual ridge resorption in denture wearers. Several authors have indicated that bone resorption was observed when high pressure was applied. Berg et al stated that to maintain normal blood circulation, a continuous mechanical pressure higher than 1.3 kPa should not be exerted to the denture-supporting tissues. Zarb et al mentioned that a continuous mechanical pressure of 1.3 kPa would compress soft tissues to the thickness of 95% of the tissue at rest. Matsuo et al investigated the intracellular response to hydraulic pressure in human periodontal ligament fibroblasts. The results suggested that fibroblasts started to respond to the pressure by increasing intracellular calcium at a threshold level of 27 to 68 g/cm 2 of pressure. According to these studies, dental prostheses should be fabricated with a goal of reducing the amount of force to the residual ridges. Choosing appropriate denture tooth materials is one of the crucial steps in prostheses fabrication that may reduce pressure and avoid stress concentration to the supporting tissues.

Artificial denture teeth should have the ability to resist impact force and transfer light and well distributed pressure to the supporting structures. Suzuki et al demonstrated that porcelain teeth showed poor impact resistance. Conventional acrylic resin teeth had superior impact resistance compared to highly crosslinked plastic teeth. These results were supported by Kawano et al, who measured the impact values of different types of artificial denture teeth. The results demonstrated that acrylic resin teeth presented excellent impact resistance and shock absorbability, whereas porcelain denture teeth revealed the highest impact values. However, both studies used measuring devices which showed only the impact values and not the pattern of impact force distribution.

Nanocomposite resins have been introduced as a material of choice for artificial denture teeth. Zheng et al studied the effects of nanoparticles on the performance of nanocomposite resins. The results revealed that nanoparticles serve as binding agents to modify the morphological structure of the epoxy resin. On the basis of this structure, numerous cavitation sites are created at the interface between the nanoparticles and the amorphous layer. When the impact occurs, the formed cavities will release the plastic constraint in the matrix and trigger large-scale plastic deformation. As a result, the fracture toughness of the matrix will be significantly improved. Moreover, the interfacial surfaces generated between polymer and nanoparticles also assist in absorbing stresses.

Several authors have attempted to record pressure under the denture base and to evaluate the amount and distribution of pressure transmission. Several techniques and measuring devices have been developed and widely used. Strain gauge and pressure transducer measurements are 2 of the most common pressure-recording methods. However, these 2 devices are only suitable for measuring pressure at specific sites. A pressure-sensitive sheet (Prescale Film; Fuji Photo Film Co, Ltd, Tokyo, Japan) has been developed and used as a pressure-detecting device for measuring occlusal pressure, occlusal force, and occlusal contact areas. When contact pressure is applied over the film, different shades of red color are developed which are correlated to the amount of pressure. This film may be considered one of the most useful devices for pressure measurement due to its simplicity and capability in detecting large pressure ranges and large distribution areas.

Modulus of elasticity (Young's modulus or elastic modulus) describes the relative stiffness or rigidity of a material. The modulus of elasticity of artificial denture teeth for removable prostheses may be a factor that influences pressure transmission and pressure distribution on the underlying alveolar residual ridges.

The purposes of the present study were to examine pressure transmission and distribution using simplified models with artificial denture teeth of different materials under impact load, and to evaluate the modulus of elasticity of each type of denture tooth. The null hypotheses were that there would be no differences in pressure transmission and distribution and in the modulus of elasticity of the artificial denture teeth.


Material and methods

Forty mandibular right and left first molars made of 4 different types of denture tooth materials (acrylic resin, AR; microfilled composite resin, MC; nanocomposite resin, NC; and ceramic, CR) were examined in this study ( Table I ). The denture base specimen was composed of the artificial tooth on denture base acrylic resin. Ten denture bases with each type of denture tooth, 15 × 15 mm and a thickness of 3 mm, were fabricated by using putty-type silicone impression material (Provil; Heraeus Kulzer GmbH, Hanau, Germany) as a mold. Melted wax was poured into the mold, and each denture tooth was lowered into the wax using a surveyor (Ney Surveyor Parallometer System; Dentplsy Ceramco, Burlington, NJ) to ensure that the occlusal surface was parallel to the base. All specimens were then invested in denture flasks (Hanau; Water Pik, Inc, Ft. Collins, Colo), followed by conventional packing procedures using heat-polymerizing acrylic resin (Lucitone 199; Dentsply Trubyte, York, Pa). Long polymerizing cycles were used for acrylic resin packing procedures. The temperature was slowly raised from room temperature to 73°C and held for 9 hours. After completion of the polymerizing cycles, the flasks were allowed to cool to room temperature before deflasking. All specimens were then removed from the denture flasks, and any flash was removed with a carbide bur (Abbott-Robinson HP Burs; Buffalo Dental Mfg Co, Syosset, NY). The basal surfaces of all specimens were polished using an automatic polishing machine (DPS 3200; Imptech, Boksburg, South Africa) with 0.05-μm-particle-sized aluminum oxide slurry (Leco Corp, St. Joseph, Mich) under slow speed and constant water irrigation. A total of 40 specimens were stored in water for 1 day before testing. The specimen illustration is shown in Figure 1 .

TABLE I
Artificial denture teeth used in present study
Type Brand Name Code Mold Manufacturer
Acrylic resin Resin Kyushi AR M30 Shofu, Inc, Kyoto, Japan
Microfilled composite resin Endura posterior MC M30 Shofu, Inc
Nanocomposite resin NC Veracia posterior NC M30 Shofu, Inc
Ceramic Ace Kyushi CR M30 Shofu, Inc

Schematic drawing of denture base specimen with denture tooth.
Fig. 1
Schematic drawing of denture base specimen with denture tooth.

Pressure transmission and pressure distribution were examined by 2 types of pressure-sensitive sheets (Prescale Film, LLW and LW; Fuji Photo Film Co, Ltd). The reliable measuring ranges of the pressure of LLW and LW were between 0.5 and 2.5 MPa and between 2.5 and 10 MPa, respectively. The system includes pressure-sensitive film and digital analysis software. The LLW pressure-sensitive film was used primarily to evaluate pressure distribution. The structure of the pressure-sensitive sheet consists of microencapsulated color-forming and color-developing materials. When contact pressure is applied to the film, a red color impression is formed in various densities according to the amount of pressure and pressure distribution. In the present study, the pressure was applied by dropping a weight freely onto each denture tooth specimen with the film placed underneath to measure the force transmission and distribution.

After the pressures were transferred onto the sheets, the sheets were scanned and analyzed by digital analysis software (FujiFilm Pressure Distribution Mapping System FPD-8010E, version 1.1; Fuji Photo Film Co, Ltd). Different amounts of pressure were shown as different colors, which were automatically designated by the software. Areas of no pressure were displayed as white. Pressure of less than 0.5 MPa was green, whereas pressure higher than 2.5 MPa was yellow. The pressure range of 0.5 to 2.5 MPa was seen as different shades of red, depending on the intensity of the pressure. However, if pressure over 2.5 MPa was observed, the LW sheet type was used to determine the maximum pressure transmission.

According to the manufacturer, the accuracy of Prescale film is ±10%. However, authors of a previous study stated that the accuracy of the color scanner for area and pressure measurements is within 0.5%, and the accuracy of Prescale film for area and pressure measurements is within 10% and 2%, respectively. Another study suggested that for the greatest accuracy and reliability, Prescale films should be analyzed within 8 hours of exposure to pressure. Thus, Prescale films were analyzed immediately after testing for maximum accuracy and reliability.

The impact drop test used in the present study was modified from the study by Kawano et al. In that study, an accelerometer was used to evaluate only the impact values, while in the present study, pressure-sensitive sheets were used to evaluate the amounts, transmission areas, and distribution of the pressure underneath the denture bases. According to results from previous studies, the mean maximum occlusal force for complete denture wearers was approximately or under 100 N. Therefore, 100 N was chosen as the impact load used in the present study. A glass tube was set perpendicular to the long axis and central fossa of the denture tooth specimen. A small piece of the LW pressure-sensitive sheet was put over the occlusal surface of each type of denture tooth specimen to measure the impact force at the contact sites. The mass and height of the drop test were adjusted to achieve a 100-N impact force at the central fossa. A fixed mass of 15 g and height of 20 cm were found to be appropriate, and, therefore, used in the present study.

In the impact drop test procedures, pressure-sensitive sheets were placed underneath the acrylic resin denture bases, over the flat surface. A 15-g mass with a 5-mm-diameter ball tip was released and allowed to fall to the central fossa of the artificial tooth specimens. The impact load testing apparatus is shown in Figure 2 .

Impact load testing apparatus.
Fig. 2
Impact load testing apparatus.

Modulus of elasticity at the surfaces of the artificial denture teeth was determined after load testing by an ultramicroindentation system (UMIS 2000; CSIRO, Lindfield, Australia). The load applied was composed of 25 incremental loading steps with a delay of 0.1 seconds at every increment. The maximum force applied was 25 mN. The modulus of elasticity can be automatically calculated from the load-displacement curve instead of visual measurement of the indentation impression. Ten specimens of each type of denture tooth were evaluated for the modulus of elasticity. Three indentations were performed on the enamel layer of each specimen. The distance between each indentation was 100 μm to prevent indentation overlapping. The overall average value for each material was obtained to represent the modulus of elasticity of each type of denture tooth.

Pressure transmission area, maximum pressure transmission, and modulus of elasticity were analyzed using 1-way analysis of variance (ANOVA) (α=.05). Tukey's HSD (Honestly Significant Difference) post hoc test was used to compare means of the maximum pressure transmission among groups. For the pressure transmission area and modulus of elasticity, the robust tests of equality of means and Tamhane's post hoc test were used, as equal variances could not be assumed.


Results

Pressure-sensitive sheets obtained from artificial denture teeth made of different materials showed different amounts of force and different patterns of force distribution ( Fig. 3 ). Pressure transmission was divided into 3 group ranges: low (pressure of less than 0.5 MPa), intermediate (pressure range between 0.5 to 2.5 MPa), and high (pressure higher than 2.5 MPa), as designated by the software. The total pressure transmission area was calculated from the sum of the 3 group ranges. The results of the ANOVA for each of the 3 measured parameters are shown in Table II .

Representative pressure-sensitive sheets from all test groups.
Fig. 3
Representative pressure-sensitive sheets from all test groups.

TABLE II
One-way ANON/A results for pressure transmission area, maximum pressure transmission, and modulus of elasticity
Source Sum of Squares df Mean Square F P
Pressure transmission area
Between groups 19133 3 6378 32 <.001
Within groups 7115 36 198
Total 26248 39
Maximum pressure transmission
Between groups 32 3 11 109 <.001
Within groups 4 36 0.098
Total 36 39
Modulus of elasticity
Between groups 34649 3 11550 762 <.001
Within groups 545 36 15
Total 35195 39

Percent pressure transmission areas calculated from the total pressure transmission areas for each type of artificial denture tooth are shown in Table III . NC and CR transferred more than half of the pressure in the low range (62.15% and 57.54%, respectively). In contrast, MC showed more than half of the pressure in the intermediate range (61.63%). AR demonstrated about half in the low and half in the intermediate range. Different pressure transmission areas of the different pressure ranges after software analysis are shown in Figure 4 . A 0.64% pressure transmission area above 2.5 MPa was observed only in the CR group, whereas no pressure above 2.5 MPa was found in the other groups.

TABLE III
Percent pressure transmission areas calculated from total pressure transmission areas for all specimens in different pressure ranges
Type/Pressure P<0.5 MPa (Low Range) 0.5≤P<2.5 MPa (Intermediate Range) P≥2.5 MPa (High Range)
AR 46.54% 53.46% 0.00%
MC 38.37% 61.63% 0.00%
NC 62.15% 37.85% 0.00%
CR 57.54% 41.82% 0.64%

Representative pressure sheets after analysis by software in 3 different pressure ranges (high, intermediate, and low pressure ranges).
Fig. 4
Representative pressure sheets after analysis by software in 3 different pressure ranges (high, intermediate, and low pressure ranges).

The total pressure transmission areas, measured from pressure-sensitive sheets of all types of artificial denture teeth, are shown in Figure 5 . The total denture base area calculated by the size of the denture base specimen was 225 mm 2 (15 × 15 mm). Means of the total pressure transmission areas observed on the sheets ranged from 144.9 ±17.8 mm 2 (CR group) to 201.3 ±7.3 mm 2 (MC group). The areas of colors developed on the sheet represented the pressure transmission areas. MC exhibited the highest pressure transmission area, which was significantly different from the NC and CR groups ( P =.002 and P <.001, respectively). AR also demonstrated a higher pressure transmission area than NC and CR, but was not significantly different from the MC group. NC presented a significantly higher pressure transmission area compared to the CR group.

Total pressure transmission areas (mm 2 ) of all types of artificial denture teeth (horizontal bars show standard deviation; bars with same letter are not significantly different at P <.05).
Fig. 5
Total pressure transmission areas (mm
2 ) of all types of artificial denture teeth (horizontal bars show standard deviation; bars with same letter are not significantly different at
P <.05).

The maximum pressure transmission that appeared on the pressure-sensitive sheets is shown in Figure 6 . The highest maximum pressure was observed in CR, and was statistically different as compared to the other tooth materials ( P <.001). NC showed significantly less maximum pressure than MC ( P <.05); however, it was not significantly different from the AR group. No statistical differences were found between AR and MC. Comparisons of the modulus of elasticity of each type of denture tooth are shown in Figure 7 . The mean modulus of elasticity of CR (72.24 GPa) was significantly higher than the other denture teeth ( P <.001). Tamhane's post hoc test showed significant differences of the modulus of elasticity among the artificial denture teeth.

Maximum pressure transmission observed on pressure-sensitive sheets (horizontal bars show standard deviation; bars with same letter are not significantly different at P <.05).
Fig. 6
Maximum pressure transmission observed on pressure-sensitive sheets (horizontal bars show standard deviation; bars with same letter are not significantly different at
P <.05).

Modulus of elasticity of all types of artificial denture teeth (horizontal bars show standard deviation; bars with same letter are not significantly different at P <.05).
Fig. 7
Modulus of elasticity of all types of artificial denture teeth (horizontal bars show standard deviation; bars with same letter are not significantly different at
P <.05).


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Pressure transmission and distribution under impact load using artificial denture teeth made of different materials Thitima Phunthikaphadr DDS , Hidekazu Takahashi DDS, PhD and Mansuang Arksornnukit DDS, MS, PhD Journal of Prosthetic Dentistry, 2009-11-01, Volume 102, Issue 5, Pages 319-327, Copyright © 2009 The Editorial Council of the Journal of Prosthetic Dentistry Statement of problem Pressure transmission and distribution under denture bases may be different depending on the material of the artificial denture teeth used. Purpose The purposes of this study were to evaluate pressure transmission and distribution under impact load using artificial denture teeth composed of different materials, and to examine the modulus of elasticity of the artificial denture teeth. Material and methods The denture base specimens with artificial denture teeth made of 4 different materials (acrylic resin, microfilled composite resin, nanocomposite resin, and ceramic) were evaluated. Pressure transmission, distribution, and maximum pressure (n=10) were observed with pressure-sensitive sheets under an impact load. Modulus of elasticity of the artificial denture teeth (n=10) was measured by using an ultramicroindentation system. Data were statistically analyzed with 1-way ANOVA, followed by Tukey HSD and Tamhane's multiple range post hoc tests (α=.05). Results Maximum pressure transmission observed from ceramic denture teeth was significantly higher than that of other groups ( P <.001). Nanocomposite resin denture teeth presented the lowest pressure transmission, whereas a localized stress transmission area was observed in the ceramic denture teeth group. Significant differences in the modulus of elasticity were observed among the 4 types of artificial denture teeth ( P <.001). Conclusions Pressure transmission and distribution varied among the denture tooth materials. Differences in the modulus of elasticity of each type of denture tooth were demonstrated. (J Prosthet Dent 2009;102:319-327) Clinical Implications Artificial denture teeth composed of different materials showed different amounts and patterns of pressure distribution. Choosing the appropriate denture tooth material may lessen the force transmitted to the supporting structures. Pressure from a prosthesis is an important factor in increasing residual ridge resorption in denture wearers. Several authors have indicated that bone resorption was observed when high pressure was applied. Berg et al stated that to maintain normal blood circulation, a continuous mechanical pressure higher than 1.3 kPa should not be exerted to the denture-supporting tissues. Zarb et al mentioned that a continuous mechanical pressure of 1.3 kPa would compress soft tissues to the thickness of 95% of the tissue at rest. Matsuo et al investigated the intracellular response to hydraulic pressure in human periodontal ligament fibroblasts. The results suggested that fibroblasts started to respond to the pressure by increasing intracellular calcium at a threshold level of 27 to 68 g/cm 2 of pressure. According to these studies, dental prostheses should be fabricated with a goal of reducing the amount of force to the residual ridges. Choosing appropriate denture tooth materials is one of the crucial steps in prostheses fabrication that may reduce pressure and avoid stress concentration to the supporting tissues. Artificial denture teeth should have the ability to resist impact force and transfer light and well distributed pressure to the supporting structures. Suzuki et al demonstrated that porcelain teeth showed poor impact resistance. Conventional acrylic resin teeth had superior impact resistance compared to highly crosslinked plastic teeth. These results were supported by Kawano et al, who measured the impact values of different types of artificial denture teeth. The results demonstrated that acrylic resin teeth presented excellent impact resistance and shock absorbability, whereas porcelain denture teeth revealed the highest impact values. However, both studies used measuring devices which showed only the impact values and not the pattern of impact force distribution. Nanocomposite resins have been introduced as a material of choice for artificial denture teeth. Zheng et al studied the effects of nanoparticles on the performance of nanocomposite resins. The results revealed that nanoparticles serve as binding agents to modify the morphological structure of the epoxy resin. On the basis of this structure, numerous cavitation sites are created at the interface between the nanoparticles and the amorphous layer. When the impact occurs, the formed cavities will release the plastic constraint in the matrix and trigger large-scale plastic deformation. As a result, the fracture toughness of the matrix will be significantly improved. Moreover, the interfacial surfaces generated between polymer and nanoparticles also assist in absorbing stresses. Several authors have attempted to record pressure under the denture base and to evaluate the amount and distribution of pressure transmission. Several techniques and measuring devices have been developed and widely used. Strain gauge and pressure transducer measurements are 2 of the most common pressure-recording methods. However, these 2 devices are only suitable for measuring pressure at specific sites. A pressure-sensitive sheet (Prescale Film; Fuji Photo Film Co, Ltd, Tokyo, Japan) has been developed and used as a pressure-detecting device for measuring occlusal pressure, occlusal force, and occlusal contact areas. When contact pressure is applied over the film, different shades of red color are developed which are correlated to the amount of pressure. This film may be considered one of the most useful devices for pressure measurement due to its simplicity and capability in detecting large pressure ranges and large distribution areas. Modulus of elasticity (Young's modulus or elastic modulus) describes the relative stiffness or rigidity of a material. The modulus of elasticity of artificial denture teeth for removable prostheses may be a factor that influences pressure transmission and pressure distribution on the underlying alveolar residual ridges. The purposes of the present study were to examine pressure transmission and distribution using simplified models with artificial denture teeth of different materials under impact load, and to evaluate the modulus of elasticity of each type of denture tooth. The null hypotheses were that there would be no differences in pressure transmission and distribution and in the modulus of elasticity of the artificial denture teeth. Material and methods Forty mandibular right and left first molars made of 4 different types of denture tooth materials (acrylic resin, AR; microfilled composite resin, MC; nanocomposite resin, NC; and ceramic, CR) were examined in this study ( Table I ). The denture base specimen was composed of the artificial tooth on denture base acrylic resin. Ten denture bases with each type of denture tooth, 15 × 15 mm and a thickness of 3 mm, were fabricated by using putty-type silicone impression material (Provil; Heraeus Kulzer GmbH, Hanau, Germany) as a mold. Melted wax was poured into the mold, and each denture tooth was lowered into the wax using a surveyor (Ney Surveyor Parallometer System; Dentplsy Ceramco, Burlington, NJ) to ensure that the occlusal surface was parallel to the base. All specimens were then invested in denture flasks (Hanau; Water Pik, Inc, Ft. Collins, Colo), followed by conventional packing procedures using heat-polymerizing acrylic resin (Lucitone 199; Dentsply Trubyte, York, Pa). Long polymerizing cycles were used for acrylic resin packing procedures. The temperature was slowly raised from room temperature to 73°C and held for 9 hours. After completion of the polymerizing cycles, the flasks were allowed to cool to room temperature before deflasking. All specimens were then removed from the denture flasks, and any flash was removed with a carbide bur (Abbott-Robinson HP Burs; Buffalo Dental Mfg Co, Syosset, NY). The basal surfaces of all specimens were polished using an automatic polishing machine (DPS 3200; Imptech, Boksburg, South Africa) with 0.05-μm-particle-sized aluminum oxide slurry (Leco Corp, St. Joseph, Mich) under slow speed and constant water irrigation. A total of 40 specimens were stored in water for 1 day before testing. The specimen illustration is shown in Figure 1 . TABLE I Artificial denture teeth used in present study Type Brand Name Code Mold Manufacturer Acrylic resin Resin Kyushi AR M30 Shofu, Inc, Kyoto, Japan Microfilled composite resin Endura posterior MC M30 Shofu, Inc Nanocomposite resin NC Veracia posterior NC M30 Shofu, Inc Ceramic Ace Kyushi CR M30 Shofu, Inc Fig. 1 Schematic drawing of denture base specimen with denture tooth. Pressure transmission and pressure distribution were examined by 2 types of pressure-sensitive sheets (Prescale Film, LLW and LW; Fuji Photo Film Co, Ltd). The reliable measuring ranges of the pressure of LLW and LW were between 0.5 and 2.5 MPa and between 2.5 and 10 MPa, respectively. The system includes pressure-sensitive film and digital analysis software. The LLW pressure-sensitive film was used primarily to evaluate pressure distribution. The structure of the pressure-sensitive sheet consists of microencapsulated color-forming and color-developing materials. When contact pressure is applied to the film, a red color impression is formed in various densities according to the amount of pressure and pressure distribution. In the present study, the pressure was applied by dropping a weight freely onto each denture tooth specimen with the film placed underneath to measure the force transmission and distribution. After the pressures were transferred onto the sheets, the sheets were scanned and analyzed by digital analysis software (FujiFilm Pressure Distribution Mapping System FPD-8010E, version 1.1; Fuji Photo Film Co, Ltd). Different amounts of pressure were shown as different colors, which were automatically designated by the software. Areas of no pressure were displayed as white. Pressure of less than 0.5 MPa was green, whereas pressure higher than 2.5 MPa was yellow. The pressure range of 0.5 to 2.5 MPa was seen as different shades of red, depending on the intensity of the pressure. However, if pressure over 2.5 MPa was observed, the LW sheet type was used to determine the maximum pressure transmission. According to the manufacturer, the accuracy of Prescale film is ±10%. However, authors of a previous study stated that the accuracy of the color scanner for area and pressure measurements is within 0.5%, and the accuracy of Prescale film for area and pressure measurements is within 10% and 2%, respectively. Another study suggested that for the greatest accuracy and reliability, Prescale films should be analyzed within 8 hours of exposure to pressure. Thus, Prescale films were analyzed immediately after testing for maximum accuracy and reliability. The impact drop test used in the present study was modified from the study by Kawano et al. In that study, an accelerometer was used to evaluate only the impact values, while in the present study, pressure-sensitive sheets were used to evaluate the amounts, transmission areas, and distribution of the pressure underneath the denture bases. According to results from previous studies, the mean maximum occlusal force for complete denture wearers was approximately or under 100 N. Therefore, 100 N was chosen as the impact load used in the present study. A glass tube was set perpendicular to the long axis and central fossa of the denture tooth specimen. A small piece of the LW pressure-sensitive sheet was put over the occlusal surface of each type of denture tooth specimen to measure the impact force at the contact sites. The mass and height of the drop test were adjusted to achieve a 100-N impact force at the central fossa. A fixed mass of 15 g and height of 20 cm were found to be appropriate, and, therefore, used in the present study. In the impact drop test procedures, pressure-sensitive sheets were placed underneath the acrylic resin denture bases, over the flat surface. A 15-g mass with a 5-mm-diameter ball tip was released and allowed to fall to the central fossa of the artificial tooth specimens. The impact load testing apparatus is shown in Figure 2 . Fig. 2 Impact load testing apparatus. Modulus of elasticity at the surfaces of the artificial denture teeth was determined after load testing by an ultramicroindentation system (UMIS 2000; CSIRO, Lindfield, Australia). The load applied was composed of 25 incremental loading steps with a delay of 0.1 seconds at every increment. The maximum force applied was 25 mN. The modulus of elasticity can be automatically calculated from the load-displacement curve instead of visual measurement of the indentation impression. Ten specimens of each type of denture tooth were evaluated for the modulus of elasticity. Three indentations were performed on the enamel layer of each specimen. The distance between each indentation was 100 μm to prevent indentation overlapping. The overall average value for each material was obtained to represent the modulus of elasticity of each type of denture tooth. Pressure transmission area, maximum pressure transmission, and modulus of elasticity were analyzed using 1-way analysis of variance (ANOVA) (α=.05). Tukey's HSD (Honestly Significant Difference) post hoc test was used to compare means of the maximum pressure transmission among groups. For the pressure transmission area and modulus of elasticity, the robust tests of equality of means and Tamhane's post hoc test were used, as equal variances could not be assumed. Results Pressure-sensitive sheets obtained from artificial denture teeth made of different materials showed different amounts of force and different patterns of force distribution ( Fig. 3 ). Pressure transmission was divided into 3 group ranges: low (pressure of less than 0.5 MPa), intermediate (pressure range between 0.5 to 2.5 MPa), and high (pressure higher than 2.5 MPa), as designated by the software. The total pressure transmission area was calculated from the sum of the 3 group ranges. The results of the ANOVA for each of the 3 measured parameters are shown in Table II . Fig. 3 Representative pressure-sensitive sheets from all test groups. TABLE II One-way ANON/A results for pressure transmission area, maximum pressure transmission, and modulus of elasticity Source Sum of Squares df Mean Square F P Pressure transmission area Between groups 19133 3 6378 32 <.001 Within groups 7115 36 198 Total 26248 39 Maximum pressure transmission Between groups 32 3 11 109 <.001 Within groups 4 36 0.098 Total 36 39 Modulus of elasticity Between groups 34649 3 11550 762 <.001 Within groups 545 36 15 Total 35195 39 Percent pressure transmission areas calculated from the total pressure transmission areas for each type of artificial denture tooth are shown in Table III . NC and CR transferred more than half of the pressure in the low range (62.15% and 57.54%, respectively). In contrast, MC showed more than half of the pressure in the intermediate range (61.63%). AR demonstrated about half in the low and half in the intermediate range. Different pressure transmission areas of the different pressure ranges after software analysis are shown in Figure 4 . A 0.64% pressure transmission area above 2.5 MPa was observed only in the CR group, whereas no pressure above 2.5 MPa was found in the other groups. TABLE III Percent pressure transmission areas calculated from total pressure transmission areas for all specimens in different pressure ranges Type/Pressure P<0.5 MPa (Low Range) 0.5≤P<2.5 MPa (Intermediate Range) P≥2.5 MPa (High Range) AR 46.54% 53.46% 0.00% MC 38.37% 61.63% 0.00% NC 62.15% 37.85% 0.00% CR 57.54% 41.82% 0.64% Fig. 4 Representative pressure sheets after analysis by software in 3 different pressure ranges (high, intermediate, and low pressure ranges). The total pressure transmission areas, measured from pressure-sensitive sheets of all types of artificial denture teeth, are shown in Figure 5 . The total denture base area calculated by the size of the denture base specimen was 225 mm 2 (15 × 15 mm). Means of the total pressure transmission areas observed on the sheets ranged from 144.9 ±17.8 mm 2 (CR group) to 201.3 ±7.3 mm 2 (MC group). The areas of colors developed on the sheet represented the pressure transmission areas. MC exhibited the highest pressure transmission area, which was significantly different from the NC and CR groups ( P =.002 and P <.001, respectively). AR also demonstrated a higher pressure transmission area than NC and CR, but was not significantly different from the MC group. NC presented a significantly higher pressure transmission area compared to the CR group. Fig. 5 Total pressure transmission areas (mm 2 ) of all types of artificial denture teeth (horizontal bars show standard deviation; bars with same letter are not significantly different at P <.05). The maximum pressure transmission that appeared on the pressure-sensitive sheets is shown in Figure 6 . The highest maximum pressure was observed in CR, and was statistically different as compared to the other tooth materials ( P <.001). NC showed significantly less maximum pressure than MC ( P <.05); however, it was not significantly different from the AR group. No statistical differences were found between AR and MC. Comparisons of the modulus of elasticity of each type of denture tooth are shown in Figure 7 . The mean modulus of elasticity of CR (72.24 GPa) was significantly higher than the other denture teeth ( P <.001). Tamhane's post hoc test showed significant differences of the modulus of elasticity among the artificial denture teeth. Fig. 6 Maximum pressure transmission observed on pressure-sensitive sheets (horizontal bars show standard deviation; bars with same letter are not significantly different at P <.05). Fig. 7 Modulus of elasticity of all types of artificial denture teeth (horizontal bars show standard deviation; bars with same letter are not significantly different at P <.05). Discussion The results showed differences in pressure transmission and distribution with artificial denture teeth made with different materials and differences in the modulus of elasticity of each type of denture tooth; therefore, the null hypotheses were rejected. Artificial denture teeth used in the present study were selected from the same mold and manufacturer in an attempt to minimize the effect of the size and shape of the specimens. In addition, the same type of pressure-sensitive sheet was used in all tests; therefore, pressure transmission areas between each type of denture tooth could be compared. However, only in the CR group, which demonstrated pressure above 2.5 MPa, was a higher pressure range sheet (LW type) used to determine the maximum pressure transmission. A nanoindentation technique was chosen to evaluate the modulus of elasticity of each type of denture tooth. This technique is simple and reproducible for measuring mechanical properties of small specimens. Therefore, it was used for the modulus of elasticity measurement in the present study. Differences in pressure transmission and distribution under the impact force obtained from pressure-sensitive sheets were clearly demonstrated in the present study. When impact load occurred, pressure would be transmitted through the denture teeth and denture base layer before being transferred to the residual ridges. Low pressure with wide distribution would be preferable, as there would be less force and better pressure distribution to the supporting structures. Denture base fabrication in the present study was controlled to be the same in every specimen, and the size and shape of denture teeth used were equalized. Therefore, the results compared only the different tooth materials. Regarding pressure transmission, both NC and CR specimens demonstrated pressure primarily in the low range (62.15% and 57.54%, respectively) ( Table III ) and significantly smaller pressure transmission areas than MC and AR ( Fig. 5 ). Kawano et al suggested that impact values obtained from porcelain teeth with polymethyl methacrylate (PMMA) resin may be less than porcelain teeth alone because there is no bond between porcelain teeth and PMMA resin, so the impact may be dissipated at the interface of the porcelain and PMMA. The results from the present study seem to support this explanation. However, when pressure distribution was considered in the present study, a high stress concentration area in the CR group was observed. Such stress areas may result from a thin denture base support area. The results from the present study suggest that adequate thickness of the denture base resin might act as a shock absorber during impact when porcelain denture teeth are selected for removable prostheses. Moreover, when interarch space is limited, porcelain teeth might not be an appropriate choice because it would be difficult to provide adequate thickness for the PMMA denture base resin. The MC group exhibited larger pressure transmission areas compared to the NC group ( Fig. 5 ). MC showed most of the pressure in the intermediate range (61.63%) ( Table III ) and also presented significantly higher maximum pressure transmission compared to NC ( Fig. 6 ). This may be a result of the high modulus of elasticity of the MC tooth material. The role of the modulus of elasticity of denture tooth material is relevant because it shows the ability of the material to flex. Denture teeth with a lower modulus of elasticity may flex and absorb the impact energy from the impact force and transfer less pressure to the underlying structures. In the present study, the highest maximum pressure transmission was observed in the CR group, which may have resulted from the modulus of elasticity of the CR teeth, which was highest (72.24 GPa) ( Fig. 7 ). CR teeth were more rigid and less flexible; therefore, they did not absorb much pressure, and the higher forces were transferred onto the pressure-sensitive sheets. A similar trend was found in the MC group. The MC displayed the second highest pressure transmission and maximum pressure transfer and the second highest modulus of elasticity. It is interesting to note that NC showed a higher modulus of elasticity than AR, but NC still presented the lowest maximum pressure transmission. As reported by Zheng et al, numerous cavitations at the interfaces between nanoparticles and the resin matrix assisted in absorbing the impact. The impact energy was dissipated through these interfaces, so as to increase the impact strength of the nanocomposite resin. Pressure refers to the force per unit area. The pressure observed in the present study was divided into 3 ranges: less than 0.5 MPa (5.1 kgf/cm 2 ), 0.5 to 2.5 MPa (5.1-25.5 kgf/cm 2 ), and more than 2.5 MPa (>25.5 kgf/cm 2 ). Matsuo et al suggested that a pressure of 27 to 68 g/cm 2 caused fibroblasts to increase the intracellular calcium which, in turn, initiated the alveolar bone remodeling. Berg et al reported that, to keep blood circulation normal, continuous mechanical pressure higher than 1.3 kPa should not be transferred to the denture-supporting tissues. The maximum pressure observed in the present study was much higher than these pressures. However, the size of the denture base specimen was only 225 mm 2 ; thus, it had a limited area to absorb the pressure. Therefore, maximum extension of the denture bases within the anatomical and physiological limits is recommended to increase the supporting areas and minimize the pressure. Pressure transmission to the denture base in removable prostheses should be equally distributed to avoid high stress concentration areas which may result in discomfort, pain, inflammation, and bone resorption. From the results of the present study, NC appeared to be a favorable artificial denture tooth material because it displayed the best results within the measured parameters. NC presented well distributed pressure, the lowest pressure transmission, and the lowest maximum pressure transfer. CR should be avoided when limited interarch space is presented. A limitation of the present study was that pressure transmission and distribution were evaluated in vitro and under simplified models with only a vertical load application. Therefore, future research should be aimed towards multidirectional force application with conditions similar to those of the oral environment. Conclusions Within the limitations of the present study, the following conclusions were drawn. 1. Maximum pressure transmission with ceramic denture teeth was significantly higher as compared to all other groups ( P <.001). 2. Significant differences in modulus of elasticity were observed among all 4 types of denture teeth ( P <.001). Ceramic denture teeth demonstrated significantly highest values, followed by microfilled composite resin, nanocomposite resin, and acrylic resin ( P <.001) denture teeth. Acknowledgements The authors thank Shofu, Inc, Kyoto, Japan, for their generous donation of the artificial denture teeth used in the study. 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