[Effect of the rest seat location alteration on distribution of forces in the distal extended abutment: a finite element method analysis]

In designing extension type removable partial denture [R.P.D], there is lots of controversy regarding the suitable location of rest seat of the terminal abutment. Discussions and Controversies are, mostly focused on minimizing lateral stress placed on terminal abutments. The purpose of this study was to assess the rate of stress placed on abutment teeth with different rest seat placements by a finite element method analysis. The skull of a 60 years old man was CT-Scanned with 0.5mm sections and data were used to make a 3.D model of the mandible. Then a partial denture was designed on this model, once with a distal rest on terminal abutment, and then with a mesial rest. Occlusal force was applied on the saddle, and stress was measured with a finite element method using "cosmos works 2003" soft ware. In both conditions, the highest stress concentration was observed on the distal surfaces of the abutment teeth, and the lowest stress on buccal cusps and roots of the teeth. The rest seat itself was the only place in which the observed stress was different in two conditions. The distal rest seat received more stress than the mesial rest seat. In both conditions the underlying bone received a moderate amount of stress, increasing in the distal part. Displacing the rest did not alter the stress applied on the abutment teeth. So it may be claimed that this factor could be neglected when determining the place of terminal rest of extension type RPD

[Effect of deflasking time on dimenstional stability of two heat-cured acrylic resins]

Dimensional changes of denture base acrylic resins are one of the most frequent phenomena that occur during the production of the dentures in different steps of curing and different times of deflasking. Measurement of the effect of different deflasking times on dimensional stability of acrylic resin can reduce the dimensional changes of these materials. The objective of this study was to compare these dimensional changes in different times of deflasking in two different acrylic resins [Acropars and Meliodent]. In this experimental study, the dimensional changes of two heat-cured poly methyl metacrylate resins [Meliodent and Acropars] were studied in different deflasking times. The initial model included one steel sheet approximately 3 mm thick, cut in the form of a dumbbell and fifty specimens of the same piece were regularly flasked and processed. Acrylic specimens, in different bench cooling times [4 hours, 12, 24, 48, 5 days] were taken out and measured in 4 areas with an accuracy of 0.01 mm using a digital caliper, subsequently, the results were studied using the One Way Anova method. Dimensional changes in two types of acrylic resins [Meliodent and Acropars] showed significant differences after 5 days deflasking [P=0.000], and had the least dimensional changes. No significant differences were observed between the average dimensional changes in reference points. Keeping Meliodent acrylic resin for at least 4 hours and Acropars acrylic resin for at least 12 hours after processing can reduce dimensional changes significantly. Iranian specimens of Acropars acrylic resin even show smaller dimensional changes than German specimens of Meliodent acrylic resin. After 5 days, deflasking time has the least dimensional changes in both acrylic resins