Statement of problem
Pressure transmission and distribution under denture bases may be different depending on the material of the artificial denture teeth used.
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).
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).
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)
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 .
|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|
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 .
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.
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 .
|Source||Sum of Squares||df||Mean Square||F||P|
|Pressure transmission area|
|Maximum pressure transmission|
|Modulus of elasticity|
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.
|Type/Pressure||P<0.5 MPa (Low Range)||0.5≤P<2.5 MPa (Intermediate Range)||P≥2.5 MPa (High Range)|
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.
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.