Reliable evaluation and grading of students' clinical and laboratory performance continue to be a problem for dental education.
In this article, 31 samples of tooth carving product selected from 138 whole product at random were evaluated under 5-grading-system and reliability of the rating grades, especially interrater variance, was studied. Furthermore, in order to enhance the reliability, two modified rating methods were presented. The results were as follows:
1. When rating was left to raters' criteria, large interrater variance came out in assigning carving product to each grade.
2. Correlation coefficient between 10 raters' grades for the same product varied from 0.20 to 0.88 (mean 0.60).
3. Carving product evaluated with large interrater variance had always an inharmonious part which deranged the whole integrity and was generally above average-ranked one. On the contrary, carving product evaluated with consistency was very poor one.
4. When rating was repeated within the same raters, the rating grades were comparatively reliable; 0.64-0.94 (mean 0.79) of correletion coefficient.
5. In the viewpoint of reliability of rating grades, it was recommendable in the rating process to classify the carving product into two or three groups.
6. When approximate numbers of carving product at each grade were provided on the assumption that carving skill of the dental students indicates normal distribution, the reliability of rating grades was improved; 0.43-0.85 (mean 0.71) of correlation coefficient.

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[in Japanese]

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As it is well known, the connection between the artificial teeth and metal plate in cast metal plate dentures is made by a resin material which is also used for the alveolar part of the denture for esthetic reasons. The bonding mechanism between alloy and resin involves a mechanical union between the two which requires an undercut or any other retentive device that can strengthen the force of adhesion.
However, in dentures used for a long time, the finishing line (the line formed by the connection of alloy and resin) will develop a crevice, with alloy and resin separating due to external forces such as masticatory pressure. Also, discoloration results and the crevice may develop bacterial contamination. All of these are a part of the problem which exists in the connection of resin and metal surface in the denture.
Considering the problem, the author tried a method of etching the metal to strengthen the mechanical union between metal and resin and to prevent their separation.
Three alloys were used in these experiments: Co-Cr, Ni-Cr, and Ni-Co-Cr.
The resin used was a heat-curing type known an URBAN.
The results of experiments were as follows:
1. Bonding strength between the etched surface and resin: Surface roughness increased as the etching time was prolonged. An increase in tensile strength was also noted with increasing etching time. For example, the Co-Cr alloy tested at 201 ± 14 kg/cm² (max. value after 120 minutes); Ni-Cr, 101 ± 9 kg/cm² (max. value after 120 minutes); and Ni-Co-Cr, 202 ± 10 kg/cm² (after 30 minutes etching).
2. SEM observations and EPMA and ESCA analysis; Different surface conditions of the three alloys were revealed after the etching process. Only the chromium component of the various alloys remained after the etching process. Acid treatment of the three alloys revealed differing resistance to corrosion by these alloys.
3. Inhibition of dye penetration: The specimens were soaked in a 0.25% basic fuchsin solution for 2 weeks. With Co-Cr and Ni-Co-Cr alloys, the dye did not penetrate, while in the case of the Ni-Cr alloy, there was some staining.
4. Effect of thermal cycling: The tensile strength value after 18 cyclings of the specimen was 167 kg/cm² for the Co-Cr alloy; 68 kg/cm² for the Ni-Cr alloy; and 198 kg/cm² for the Ni-Co-Cr alloy. The results indicated a decreasing bonding strength with increasing numbers of thermal cycles.
5. SEM photographs showed different shaped, corroded areas of 50 μm deepth in the connection region of each specimen. The Co-Cr and Ni-Co-Cr alloys had columnar-shaped corrosion areas, whereas the corrosion sites of the Ni-Cr alloy had the appearance of smooth hills and valley.
6. Clinial application: The effect of etching the metal plates in dentures was tested in 16 cases. After 2 1/2 years, the etched portion of the dentures showed a better inhibition of dye penetration than the control (non-etched) parts. This suggests that etching may be of value in clinical dentistry.

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It is known that patients who ceased to wear their dentures less than 5 years reached to more than 60%. Denture failure due to the loss of the abutment tooth or caries accounted for about 20% of the number. Abrasion of the abutment tooth with clasp is thought to be another reason for cease to wearing dentures.
Considering these data, the author undertook a study of plating on the human teeth as a means of caries prevention. The method of Electroless Nickel plated surface is acidresistant, wear-resistant, very hard, and heat-resistant.
Since these properties were desired for the tooth surface, the author decided to examine the possible application of Nickel plating for dental practice.
Following results were obtained:
1. The possible active agent for the Catalyzing (=Catalysis stage) was PdCl₂ (2g/l) at pH 4.
2. The thickness of the plating film increased with plating time and reached a plateau after 10 minutes.
3. Surface roughness increased due to the etching procedure prior to Nickel plating. Although Nickel plating for 60 minutes reduced the roughness, the plated tooth surface was not so smooth as the original surface.
4. Surface hardness of the tooth decreased from the normal value of 380 Hv to about 260 Hv following etching. Five minutes after plating, the hardness returned to normal and reached to plateau at 450 Hv by 10 minutes.
5. The force of adhesion between the plating film and the teeth was approximately 140-160 kg/cm².
6. After 3 hours of thermal shock testing between 4°C and 80°C (30 cycles), no specimen showed signs of “blistering”, “peeling off” or “cracking”.
7. The acid-resistance test was carried out in 1/1, 000 N and 1/100 N Hydrochloric acid. The plated film was not affected, whereas the control tooth surface was corroded.
8. In the wear-resistance test, the gloss of the plated tooth surface increased, but the tooth surface was not exposed after 20, 000 toothbrush strokes. But when a Co-Cr alloy plate was applied instead of brush (with a load of 2kg), the tooth surface appeared as a small round after 100 strokes followed by rapid expansion in size.
9. The interface between the plating film and the teeth was analyzed by SEM (Scanning electron microscope), EPMA (Electron probe X-ray microanalyser), ESCA (Electron spectroscopy for chemical analyser) and following observation were obtained.
At first, the inter-rod substance of surface enamel was corroded due to the etching process. Followed by dissolution of residual substance by the low pH during the Catalyzing step. The plated film was appeared as a globular granules of 1 μm in size and was intimately associated with the tooth surface when observed by SEM. Tin and palladium attached to the tooth during the Catalyzing step were observed by EPMA in a very small quantities.
After Nickel plating, tin and palladium were still present in the deeply etched areas. The Ca/P ratio was decreased due to the presence of phosphorous in the Nickel plating solution. These were ascertained with ESCA analysis of plating film.
From the above results, it appeares that the Electroless Nickel plating method will be used as a means of caries prevention and may have application in dental practice.

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Prior to taking the impression for a definitive removable prostheses on the partially edentulous patients with partial maxill- or mandiblectomy, not a small numbers of remaining teeth are usually found to be restored. Most of these patients have opening defects on the palate or are unable to close pharyngeal opening at will. Consequently, cooling water from the air turbine may run down in these defects and cause patient's sudden choke forcing prosthodontist to stop the treatment at each occasion. Such sudden reactions of the patients may also bring about the secondary accident.
We have developed the temporary obturator and found it suitable for practical application for such patient to protect these clinical complications. This newly developed appliance is quite useful also to protect accidental swallowing of tried-in metal crown or inlay and to broaden the working field by dislocating tongue and cheek.
The temporary obturator is made of the Erkopress system using acetobutylene cellurose sheet of 0.6 mm thick. Since the ordinary casts can be used, extra time for preparing the obtulator is quite less.

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