Shikaigaku

Strain gauge experiments showed that the strain arising in the mandible when occlusal forces were imparted on an implant bridge were less than those when the same forces were imparted on the teeth of controls or on natural teeth bridges. In addition, it was found that the strain was about the same regardless of where occlusal pressure was applied. Also, the direction of the principal strain was essentially the same regardless of where it was measured. In other words, it was clear that occlusion on the implant bridge did not concentrate large stresses in the mandible.
In contrast, when occlusal forces were imparted on the natural teeth bridge, large strains arose in the mandible, and the direction of the principal strain varied depending on the region where it was measured. In other words, it was found that a strong, complex torsion arose in the mandible when occlusal forces were imparted on the natural teeth bridge.
Holographic interferometry experiments showed that when a load is imparted on the implant bridge, stresses may concentrate in the mesial region of the pontic, causing a fracture here, or, as a result of displacement of the implant and natural tooth abutments, stresses may arise around both abutments, resulting in bone resorption.

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I analyzed masticatory forces in three dimensions using a transducer for measuring these forces and their characteristics. The transducer was placed on the lower first molar. Forces were measured using chewing gum of three hardnesses in one experiment and for three different conditions of occlusal contact in another. In the latter, masticatory forces were measured assuming 100% for the value of occlusal contact with tripodization, or what is called A, B, C contact.
The results obtained were as follows :
1) The resultant force increased with increases in the hardness of the chewing gum. The greatest increase occurred with vertical occlusal force.
2) Vertical and resultant forces decreased when there were fewer contact points, although lateral forces increased. Large unanticipated forces occurred in the buccal and lingual directions.

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Minimizing internal stress that occurs in heat cured resin is important in obtaining an accurate denture. I used a strain gauge and temperature sensor to investigate the relationship of strain and temperature during the curing of various thicknesses and shapes of resin exposed to different curing methods. The influence of flasking plaster was also investigated.
The following conclusions were obtained.
1) The behavior of strain was similar for a particular curing method, although the values for the strain differed.
2) The behavior of the strain and internal temperature were characteristic of the curing method.
3) The compressive strain was less in plaster than in dental stone when curing was by boiling water.
The strain was also less after deflasking. Strain was very dependent on thickness and shape of the specimen, as well as changes in temperature. Flasking plaster was also important. The system developed for this experiment was useful in measuring the strain and internal temperature in the heat cured resin.

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We investigated the effects of multiple fluoride applications on the surface structure of synthetic hydroxyapatite (HAp) using X-ray photoelectron spectroscopy (ESCA). Sodium fluoride (NaF, pH7.0) and acidulated phosphate fluoride (APF, pH3.6) were prepared to concentrations of 100, 1,000 and 9,000ppm fluoride. HAp was treated for four minuxes in each NaF and APF solution, washed and dried. The HAp samples were retreated three times at one hour intervals in the same way. Untreated HAp and calicum fluoride (CaF₂) were employed as controls. ESCA analysis was conducted with wide and narrow scanning, and the relative concentration investigated for the surface and the fourth layer (10.5nm) of the fluoride treated HAp.
F_1s2 and Ca_2p binding energy showed that the type of uptake fluoride was different between the surface and fourth layer depending on the pH, concentration, and application time. For the 9,000ppm F APF-HAp, CaF₂ was produced on the surface and in the fourth layer for all fluoride applications. For the 100 and 1,000ppm F APF-HAp, only FHAp formed on the surface, even as the level of fluoride in the applications increased. However, CaF₂ was seen in the fourth layer after four fluoride applications. On the other hand, when NaF was applied, FHAp formed on the surface for all fluoride applications and CaF₂ formed in the fourth layer at the second and fourth applications. The relative concentration of fluoride in the HAp indicated that fluoride uptake increased with increasing fluoride concentration in the application media, and wi h an increase in the number of applications. Multiple fluoride applications of low fluoride concentration produced CaF₂ in the deeper layers of HAp.

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