Purpose: The purpose of this study was to survey the production of titanium dentures in Tsurumi University Dental Hospital and to study the actual use of titanium dentures in our university.
Methods: The survey was based on the metal base dentures produced at Tsurumi University Dental Hospital during an 11-year period between April 1995 and March 2006, with relation to the types of metals used in the dentures, types of dentures, dental condition, and the number of artificial teeth.
Results: 1871 metal base dentures were produced in the study period. The dentures consisted of 1290 cobalt-chromium alloy dentures (68.9%), 350 titanium dentures (18.7%) and 231 platinum-added gold alloy dentures (12.3%). Within the titanium denture groups, 132 were complete dentures (37.7%), 55 were complete overdentures (15.7%) and 163 were removable partial dentures (46.6%). JIS type 3 pure titanium was used in the titanium base of 252 dentures (72.0%). The average number of artificial teeth was 10.9, which was the highest among the three types of metal bases, exhibiting significant differences (P < 0.01).
Conclusion: Approximately 32 titanium dentures per year on average were delivered at our university and they were applied to almost edentulous arch compared with the other metal base dentures.

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Purpose: The purpose of the present study was to obtain fundamental data for application of the cast-on method by evaluating the effect of mold temperatures on the interface between primary and secondary castings in detail.
Methods: Silver-palladium-gold alloy (Ag-Pd-Au), type-4 gold alloy (Type4), and chromium-cobalt alloy (Cr-Co) were used in the present study. A polished flat, square metal plate, 10.0 mm × 10.0 mm × 1.0 mm, was used as the primary casting. Awax pattern, 2.0 mm in diameter and 2.0 mm thick, was prepared for the secondary casting and invested together with the primary casting. The mold was heated at 600, 700 and 800 °C for Ag-Pd-Au and Type4, and 700, 800 and 900 °C for Cr-Co. After casting, the mold was embedded and sectioned. The cross-section was observed using a scanning electron microscope (SEM) and analyzed using energy dispersiveX-ray spectroscopy (EDS).The gaps between the primary and secondary castings were analyzed for each alloy by analysis of variance and Tukey's honestly significant difference test. The significance levelwas set at 0.05. The heated primary casting without the secondary casting was examined using a thin film X-ray diffractometer (XRD).
Results: Gaps were observed between the primary and secondary castings in all examined conditions. The primary casting surface was covered with oxide layers such as CuO and Cr₂O₃, and became rough with an increase of the mold temperature.
Conclusions: The results suggested that the cast-on method was influenced by the mold temperature.

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Purpose: This study aimed to examine the relationship between oral dryness and medical treatment (medication, radiotherapy) for patients with maxillofacial prostheses.
Methods: Sixteen patients with maxillofacial prostheses were examined according to the following procedure. For evaluating oral dryness, oral moisture was measured at the lingual mucosa and the right and left buccal mucosa using a Moisture Checker Mucus®. The mean value of each measurement was recorded as the moisture value (%). A value of 29% was used as a reference value to divide patients into a normal group and an oral dryness group. Patients with a moisture value of more than 29% were classified as normal. The clinical histories of the patients (including their current medication use and radiotherapy history-dose of radiation and period after radiation) were surveyed from their clinical records. Patients rated themselves on their feelings for oral dryness using a faces scale.
Results: The moisture value was significantly lower in the oral dryness group than the normal group. There was a significant difference in moisture values between the number of patients with and without radiotherapy, but not patients with and without medications. The moisture value had a significant negative correlation with the dose of radiation and a positive one with the period after radiation. The score of faces scale showed a significant negative correlation with moisture value.
Conclusion: The patients with maxillofacial prostheses who had undergone radiotherapy need moisture retention because of oral dryness depending on the dose of radiation and the period after radiation.

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Purpose: The purpose of this study was to compare the muscle activity of jaw-closing muscles during chewing of a wax cube used for mixing ability test with that of peanuts used for masticatory performance test.
Methods: Twenty subjects with complete dentitions (mean age 24.1 years) were directed to perform a mixing ability test and a masticatory performance test. Surface electromyograms (EMGs) were recorded from the masseter and temporalis muscles on right and left sides. Muscle work, maximal voltage and duration were measured on the integrated EMG signal in each muscle. In addition, muscle work was normalized with EMG activity during maximum voluntary clenching at centric occlusion. The mean of the 2nd to 5th cycle in each parameter was employed for statistical analysis.
Results: Repeated measures one-way multiple analysis of variance found a significant effect of chewing test on muscle work, normalized muscle work and maximal voltage (P < 0.002), but no significant effect on duration (P = 0.07). Muscle work, normalized muscle work and maximal voltage for the mixing ability test were significantly less than for the masticatory performance test in all muscles (paired t-test, P < 0.006).
Conclusion: The results of this study suggest that muscle activity of jaw-closing muscles in the mixing ability test is less than that in the masticatory performance test using peanuts as test food.

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Purpose: The purpose of this study was to investigate the effect of the hardness of resilient materials applied to the female parts of specialized ball attachments, and the diameter of ball abutments on the retention of an implant overdenture.
Methods: A cast chrome-cobalt framework was mounted onto a maxillary edentulous model. Eight different ball abutments, with diameters of 2.5, 2.75, 3.0 and 3.25 mm, and with undercut (UC) of 0.5 and 1.0 mm, which was the difference between the ball diameter and the neck diameter, were screwed into implants which were connected to resilient materials with four different degrees of hardness (58, 68, 78 and 90) conforming to ISO7619. A universal testing machine was used to test the 32 different attachments (crosshead speed 3 mm/min). Three tests of 10 consecutive pull separations were done in both vertical and oblique directions. After 10 consecutive pull separations, the inner surfaces of the resilient materials were examined by a scanning electron microscope (SEM). The measured retentive forces were analyzed statistically using ANOVA and Tukey's tests.
Results: Both the hardness of the resilient materials and the diameter of the ball abutments significantly affected the retentive force (P < 0.05).
Conclusions: The designs with the diameter of the ball abutments ranging from 2.75 to 3.0 mm and UC of 0.5 mm, and with the hardness of the resilient materials ranging from 68 to 78, were considered to be suitable for this attachment system.

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Purpose: The purpose of this study was to investigate oral environmental risk factors involved in caries incidence in the elderly. We investigated the relationship between the oral environment factors of the elderly with both fixed prostheses and removable prostheses at baseline and at follow-up and examined time-course changes of each oral environmental factor by prosthesis type.
Materials and methods: The subject group consisted 11 elderly patients with fixed prostheses and 11 who wore removable prostheses. We examined oral environmental factors by saliva tests. Five oral environmental factors were examined: the stimulated salivary flow rate, buffering capacity, and the counts of mutans streptococci (SM), lactobacilli (LB), and Candida (CA). We compared these factors for subjects with fixed prostheses and those wearing removable prostheses at baseline and at follow-up. Furthermore, 3-year changes in the factors of each oral environment were compared and evaluated.
Results: Significant differences were observed between the two groups in the salivary microbial counts of SM and LB at baseline and at follow-up. The LB counts increased in the Denture group during the 3-year period and significant differences were noted.
Conclusions: We found that fixed prostheses are less cariogenic, and removable prostheses cause an increase in the cariogenic bacterial count. Regarding time-course changes by the type of prosthesis, the LB count tended to increase in the subjects with removable prostheses. The risk of caries due to a fixed prosthesis may be lower than that of removable prostheses.

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Purpose: The purpose of this study was to characterize the relation between stimulus intensity, masseteric exteroceptive suppression (ES), and sensory perception as a possible quantitative or qualitative tool in the diagnosis of the masticatory system for prosthodontic treatment.
Methods: From 12 males and 12 females (22-41 years), surface EMG was recorded from the left masseter muscle. Visual feedback allowed the subjects to maintain a constant clenching level of 30% maximal voluntary contraction. Thirteen fixed stimulus intensities from 5 to 35 mA in 2.5 mA increments were applied at random. A computer program calculated the duration of ES, based on the 50% level of the mean pre-stimulus EMG. The stimulation intensity at which early (ES1) or late (ES2) exteroceptive suppressions appeared first and at which subjects answered as being painful were defined as the reflex threshold (RT1 or RT2) and pain threshold (PT), respectively. The relation between sensory perception and ES was evaluated using stimulus-response (S-R) curve analysis.
Results: Significant differences were found between PTand RT1, and between RT1 and RT2 (P < 0.001). After S-R curve analysis, the maximum duration of ES1 and ES2 was estimated at 9.6 ms and 33.5 ms, and appearance was rated as 6.2 and 4.0 on numeric rating scale (NRS), saturation was 7.6 and 6.9, and slope was 7.0 and 11.2, respectively.
Conclusions: All parameters for changes in ES following increasing incremental stimulus intensity could be analyzed. The construction of S-R curves in relation to sensory perception and ES parameters characterized the difference between ES1 and ES2.

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Purpose: The purpose of this study was to develop design criteria for palatal straps according to palatal shape.
Methods: The palatal shapes of 60 dentate maxillary stone casts were measured using a contact-type three-dimensional measuring instrument. Based on the average of the 60 palatal shapes, 5 three-dimensional finite element models (FE models) were constructed: a basic FE model, a wide palatal FE model, a narrow palatal FE model, a deep palatal FE model and a shallow palatal FE model. The FE models were simulated unilateral free-end saddle dentures with the maxillary left second premolar and first and second molars missing. The FE models included a denture base, two rests and a palatal strap. The mechanical properties of Co-Cr alloy, Ti-6Al-7Nb alloy and type IV gold alloy were given to the FE models. A vertical load of 50 N was loaded onto the occlusal surface of the left first molar. Vertical displacement of the distal edge of the denture base was then compared between models.
Results: Compared to the basic FE model, displacement in the wide FE palatal model and shallow palatal FE model was larger, and displacement in the narrow palatal FE model was smaller. The difference in displacement between the deep palatal FE model and basic FE model was small.
Conclusions: With a wide and shallow palate, it is necessary to design a reinforced palatal strap.With a narrow palate, the anteroposterior width of the palatal strap can be narrower. With a deep palate, it is not necessary to change the palatal strap design according to the palatal shape.

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