[Basic experiments on denture rigidity at various types of palatal bars]

The purpose of this study was to investigate the rigidity of removable partial denture with a palatal bar. A metal model with a Kennedy Class II-modification 1, was fabricated as a simulated dental arch. The model palate was adjustable to three different radii (17.5 mm, 20.0 mm and 22.5 mm) on the frontal section.

These were bending palatal bars and preformed plastic patterns. One of the bending palatal bars was 3.4 mm wide 1.4 mm thick, the other was 3.1 mm wide and 1.1 mm thick. Two other bars, 5.0 mm wide, 1.7 mm thick and 5.0 mm wide, 1.5 mm thick, were cast in Ticonium p-100 alloy from the plastic pattern.

The materials were fitted to the model and rigidity was measured by loading at various parts equivalent to the distal extension base. The experiments were performed with a 1 mm space between the frames and the model and again with a 1 mm thick sheet of mercaptan rubber-base impression material between them.

1. Cast palatal bars showed a smaller deflection than the bending palatal bars. 2. The bending palatal bars showed a large deflection of the distal extension base. So, care should be exercised when fitting a bending palatalbar in a distal extension case because of the incompressibility of the mucous membrane. 3. Bar deflection was only slightly different for the three types of palate. 4. Deflection of the palatal bar and distal extension base was smaller when the bar was thicker and wider. 5. The amounts of palatal bar and base deflection with the 1.0 mm space was in proportion to the load. 6. With the rubber-base sheet between, the amounts of deflection was not proportional to the load, and the amount of deflection under a 30 kg load was almost twice that of a 5 kg load.