New theoretical model to measure pressure produced during impression procedure for complete dentures-Visual inspection of impression material flow.

OBJECTIVE: A theoretical model, based on fluid dynamics, was developed to measure impression pressure. The purpose of this study was to evaluate the validity of this theoretical model by comparing its theoretical analysis against actual pressure measurements conducted using an impression tray and edentulous oral mucosa analog embedded with pressure sensors. METHODS: In the theoretical model, a hollow tube was mounted onto an impression tray by penetrating through the tray. When force was applied to the tray, pressure was produced which then caused the impression material to flow into the hollow tube. Length of impression material which flowed into tube was denoted as l. In the calculation formula for theoretical model, pressure impulse I was expressed as a function of impression flow length l. For actual pressure measurements, four electric pressure sensors were embedded in an experimental edentulous arch. To visually observe and measure length of impression material flow, four transparent silicon tubes were mounted vertically at different positions on tray. During tray seating, impression material flowed into tubes and pressure which caused material flow movement was measured by the embedded sensor at each tube's position. RESULTS: Based on actual pressure measurements under one experimental condition, regression analysis of pressure data acquired from electric sensors yielded the formula, Y=0.056X²+0.124X. Based on theoretical analysis using a particular viscosity value, the numerical formula yielded was Y=0.057X², which resembled that of the regression formula. SIGNIFICANCE: Theoretical model presented in this paper augured well for clinical application as an easy and economical means to examine magnitude and distribution of impression pressure by measuring lengths of impression material flow in tubes fixed to impression tray.

Plasma treatment increased shear bond strength between heat cured acrylic resin and self-curing acrylic resin.

Self-curing acrylic resin is generally used for the repair of a fractured denture base. However, re-fracture of the repaired denture base resin often occurs because of poor bonding strength between the base resin and self-curing repair resin. The effect of plasma treatment on the shear bond strength between heat cured acrylic resin and the self-cured acrylic was examined. It was revealed that plasma irradiation is effective in increasing the shear bond strength. Plasma irradiation does not cause environmental pollution, as it does not require chemicals. It is a useful method to increase adhesive strength between heat cured acrylic resin and self-curing acrylic resin.

Visual observation of the dynamic flow of elastomer rubber impression material between the impression tray and oral mucosa while seating the impression tray.

The purpose of this study was to inspect visually, the dynamics of the impression flow at seating of the impression tray. The effects of the relief and the escape hole of the impression tray on the impression flow were also examined. Three types of the transparent impression tray (flat tray, relief tray and escape hole tray) were prepared. Transparent silicone polymer was put on the impression tray surface. Four drops of the dark blue silicone impression material was injected into the transparent silicone polymer on the impression tray. The impression tray was seated on the model of the denture-supporting mucosa. The movement of the four drops caused by the impression flow was visually recorded with the video camera and examined. The result for the flat tray showed that the impression material moved from inside to the outside. It was also shown that the speed of the moved impression material increased as the seating of the impression tray advanced. The results for the relief tray and the escape hole tray showed the effect of the relief and the escape hole prepared to the impression tray on the speed and the direction of the flow of the impression material.

Finite element analysis of the effect of the bucco-lingual position of artificial posterior teeth under occlusal force on the denture supporting bone of the edentulous patient.

To improve the quality of the complete denture prosthesis, the bucco-lingual position of the artificial posterior teeth must be determined with consideration of the shape of the maxillary and mandibular residual ridge and the relationship between them. The arrangement of posterior artificial teeth should be considered not only for the denture stability but also for the avoidance of high pressure on the supporting structures. A two-dimensional finite element method program to investigate the statics for the contour of the complete denture and the residual ridge was developed. With this program, the effect of the bucco-lingual position of the artificial posterior teeth under occlusal force on the denture supporting bone could be investigated.

Galvanic skin response of oral cancer patients during speech.

Severe speech difficulty is often caused after surgery of an oral cancer. Prosthetic treatment with a removable obturator prosthesis is generally provided for such patients. Although some speech ability is recovered with prosthetic treatment, patients sometimes complain of continued dissatisfaction with their speech. However, it is difficult to evaluate the dissatisfaction. Therefore, a new method for evaluation is desirable. In this study, such a new method using the galvanic skin response as the index for the dissatisfaction of the patient was developed, and its objectivity was investigated. Eleven patients with maxillary bone defects were selected. Prior to the evaluation, improvement of speech with the removable prosthesis was confirmed using the speech intelligibility test and the visual analogue scale. The electrical resistant value at pronunciation was measured with the measuring system composed with the apparatus (galvanic skin response (GSR) measuring apparatus), the personal computer program. The changes for the electrical resistant value after pronunciation were evaluated by calculating the decrease ratio at pronunciation [(the mean electrical resistance before pronunciation - the mean electrical resistance after pronunciation)/the mean electrical resistance before pronunciation]. This decrease ratio at pronunciation was defined as the index of the speech dissatisfaction of the subject. The mean values for the decrease ratio with prosthesis were significantly smaller than the values without prosthesis (P < 0.05). From the results of this study, it is suggested that the measurement of the electrical resistance change of the skin during speech could be a new method for evaluating the speech dissatisfaction of the post-oral-cancer patient.

In situ microsampling technique for identification of elements of a restoration with exposed metal to identify potential allergens.

STATEMENT OF PROBLEM: For clinical investigations of allergic reactions to dental materials, it is necessary to know the type of metal used in restorations in the mouth of the patient. However, an intraoral method of sampling the dental alloy without removing the metal restoration has not yet been established. PURPOSE: This study attempted to establish a clinically serviceable method for microsampling dental alloy. MATERIAL AND METHODS: A piece of silicone sampling tube is held on the surface of the metal restoration. This surface was ground with a carbide bur through the sampling tube, and the ground metal particles were then recovered from the inner surface of the tube. The recovered sample was subjected to energy-dispersive x-ray microanalysis. The percentage recovery of ground alloy by this microsampling method, which was the ratio of the recovered weight of metal to the total weight of the metal ground, was evaluated using 3 mm diameter x 1 mm thick disk-shaped specimens of Ag-Au-Pd and Ni-Cr alloys. RESULTS: Elemental analysis of the retrieved alloys identified the component elements in the alloy compositions. The percentage recovery by this microsampling method was 75.0% for Ag-Au-Pd alloy and 69.9% for Ni-Cr alloy, respectively. CONCLUSIONS: The microsampling method demonstrated a relatively high percentage recovery of the ground alloy particles. This microsampling method would be of great benefit clinically to patients who have allergic reactions to dental materials.

New telescopic crown design for removable partial dentures.

STATEMENT OF PROBLEM: Special technical skills and experiences are required to fabricate a telescopic crown to provide adequate retention with precise frictional retention ("conus friction force") between coping and telescope. It is also difficult to control clinically alterations of retentive forces after prolonged usage. PURPOSE: This study examined an innovative telescopic crown system for removable partial dentures that can be fabricated without special technical skill or experience and readily adjusted retentive forces. The retention of telescopic crowns was investigated after repeated insertion/separation tests. MATERIAL AND METHODS: Ten telescopic crowns were constructed to evaluate retentive forces. Each telescopic crown was adjusted to provide a retentive force of approximately 9.8 N. Each telescopic crown was then subjected to 10,000 insertion/separation cycles, and retentive force of each telescopic crown was recorded initially and after every 1,000 cycles. The retentive force of each telescopic crown was re-adjusted after these measurements to provide a retentive force of approximately 9.8 N. The retentive force was recorded a second time after each 1,000 insertion/separation cycles up to 10,000 cycles. RESULTS: Retention of the telescopic crowns gradually diminished, depending on the number of insertion/separation cycles. The mean retentive force after 10,000 insertion/separation cycles was over 2 N. All 10 telescopic crowns were re-adjusted to exert a retentive force of 9.8 N after the initial 10,000 cycles, and the retentive force also diminished on the second 10,000 cycles. CONCLUSIONS: Reduction of retention was dependent on insertion/separation cycles. This new telescopic crown with reduced retention could be easily readjusted. Readjusted retentive forces were at least equivalent to the initial retention.

Efficacy of tray adhesives for the adhesion of elastomer rubber impression materials to impression modeling plastics for border molding.

STATEMENT OF PROBLEM: Tray adhesive, which is used for the adhesion of elastomer rubber impression materials to a custom resin tray, lowers the retention of the impression materials to the impression modeling plastics, as some ingredients of tray adhesive make the impression modeling plastic soft and tacky. PURPOSE: The efficacy of tray adhesive, which is used for the adhesion of elastomer rubber impression materials to a custom resin tray, on the adhesion between elastomer rubber impression material and impression modeling plastic was investigated. MATERIAL AND METHODS: Four silicone rubber impression materials (two addition reaction types and two condensation reaction types), two polysulfide rubber impression materials, and one impression modeling plastic were used in this study. Tensile strength between elastomer rubber impression material and impression modeling plastic with or without the application of tray adhesive was evaluated. RESULTS: Although tray adhesives for both addition reaction type and both condensation reaction type of silicone impression materials and one tray adhesive for polysulfide rubber impression material increased the tensile strength between the impression material and impression modeling plastic, one tray adhesive for polysulfide rubber impression material decreased the tensile strength when sufficient drying time was not applied.

Relationship between size of denture foundation area and resorption of alveolar ridge in the edentulous mandible.

The relationship between the size of the denture foundation area and the resorption of the alveolar ridge was investigated in 55 edentulous subjects. The denture foundation area was recorded using a modelling compound impression technique with border moulding. Both sides of each edentulous mandible were examined separately, making a total of 110 experimental sides in the study. On a stone cast made from each impression, the size of of the superficial denture foundation area and of the projected denture foundation area on the tentative plane of occlusion of each anatomical zone were measured. The vertical height of the alveolar ridge at the lateral incisor and first molar region was also evaluated. The size of the superficial denture foundation area in the antero-lingual and postero-lingual zones showed no significant correlation with the degree of alveolar ridge resorption. The size of the projected denture foundation area on the tentative plane of occlusion in the anterior section showed negative significant correlation with the degree of alveolar ridge resorption. The size of the projected denture foundation area on the tentative plane of occlusion in the posterior section showed no significant correlation with the degree of alveolar ridge resorption.

The effect of the slope inclination of buccal polished surface of mandibular denture base on food retention.

The accumulation of food debris in the buccal vestibule can inhibit normal mastication and appears to be a factor that can cause patient discomfort. This study focused on the effect of the slope inclination of the buccal polished surface of the mandibular denture base on the ability to expel test-food particles from the buccal vestibule. It was revealed that an inclination of the slope steeper than 80 degrees markedly inhibits the normal ability to expel foods from the vestibule. It was revealed that the most appropriate inclination of the buccal slope is approximately 60 degrees or 70 degrees. These results could be utilized for constructing a denture prosthesis.

The effect of the mandibular buccal denture base surface on food retention.

The accumulation of food debris in the buccal vestibule surrounding a mandibular denture flange was studied. This investigation focused on the effect of the thickness, physiologic molding, and slope of the buccal flange of denture base on the patient's ability to expel test-food particles from the area. Experimental denture bases were placed in the mandibular buccal vestibule of subjects having a complete natural dentition. Only the slope of the polished surface of buccal flange was found to have a significant positive correlation with the ability to expel test food from the buccal vestibule.

Lateral forces exerted on the abutment tooth of complete mandibular overdentures.

The purpose of this study was to analyse the magnitude and direction of lateral forces exerted on the abutment tooth of a complete mandibular overdenture. A newly developed transducer was used for measuring forces. It was small enough to be embedded in the lower denture base, and could detect the magnitude and direction of forces. Five subjects, 38 to 77 years of age, with an edentulous maxilla and one or two canines remaining in the mandible were selected for this experiment. An upper complete denture and a lower complete overdenture with only one abutment tooth were fabricated for each subject. Forces exerted on the abutment tooth were measured during tapping and chewing of kamaboko (a Japanese cooked fish paste with soft consistency) and also peanuts, a few days and about 20 days after the denture insertion. The following results were obtained: (i) the average magnitude of lateral forces ranged from 0.5 to 2.0 kgf, and there were significant differences of the mean value between subjects; (ii) lateral forces were mainly observed on an imaginary line drawn through the experimental tooth (lower canine) and the opposite retromolar pad. There were no significant differences between the first and the second measurement.