ABSTRACT
The aim of this study was to use photoelastic models to analyze the distribution of stress caused by the incidence of loads on a mandibular distal extension removable partial denture, both on the abutment teeth and on differently shaped residual ridges: distal ascending, descending-ascending, horizontal and distal descending. The best type of retainer and location of the rest on the last abutment tooth were determined for the different types of ridge. Four models were made from photoelastic resin (PL-1 for the teeth and PL-2 for the alveolar ridge), one for each kind of ridge. For each model, 4 removable partial dentures (RPD) were made (16 RPD altogether): T-bar retainer and distal rest, T-bar retainer and mesial rest, circumferential retainer and distal rest, and circumferential retainer and mesial rest. The models were placed on a circular polariscope and a 100 N axial load (point load) was applied to premolars and molars of the RPD. The formation of photoelastic bands was photographed for qualitative analysis. Results showed that the horizontal ridge had better distribution of stress, while the distal descending ridge had greater concentration of stress. The circumferential retainer had greater areas of stress for all types of ridges except the horizontal ridge, where there was no influence related to retainer type. The distribution of stress was similar among the different types of ridges when the rest was mesial or distal to the last abutment tooth, except for the distal descending ridge, where there was greater concentration of stress when the rest was located distally to the last abutment tooth. Thus, it may be concluded that (1) the situation was least favorable for the distal descending ridge and most favorable for the horizontal ridge, (2) the T-bar retainer had more favorable stress distribution, except when the ridge was horizontal, in which case there was no influence in relation to the type of retainer, (3) the location of the rest showed similar behavior in all except the distal descending ridge.
O objetivo da pesquisa foi analisar, atraves de modelos fotoelasticos, a distribuicao das tensoes formadas pela incidencia de cargas sobre a protese parcial removivel de extremidade livre mandibular, nos dentes suporte e no rebordo residual nas formas: ascendente distal, descendente-ascendente, horizontal e descendente distal. Nos diferentes tipos de rebordo foi verificado o melhor tipo de grampo e a localizacao do apoio no ultimo dente suporte da extremidade livre. Foram construidos 4 modelos de resina fotoelastica (PL-1 para os dentes e PL-2 para o rebordo alveolar), um modelo para cada tipo de rebordo. Para cada modelo foram confeccionados 4 proteses parciais removiveis (PPR)(total de 16 PPR): com grampo em barra T e apoio na face distal, com grampo em barra T e conector na mesial, grampo circunferencial e apoio na face distal e grampo circunferencial e apoyo na face mesial. Os modelos foram posicionados em um polariscopio circular e aplicado carga pontuais e axiais de 100 N nos pre-molares e molares das proteses. As franjas formadas foram fotografadas para analise. Os resultados foram analisados de uma forma qualitativa e podemos observar que o rebordo horizontal apresentou uma melhor distribuicao das tensoes, enquanto o rebordo descendente distal apresentou as maiores concentracoes de tensoes. O grampo circunferencial apresentou as maiores formacoes de tensoes em todos os tipos de rebordo, exceto no rebordo horizontal, onde nao houve influencia quanto ao tipo de grampo. Nao houve diferenca significante entre os tipos de rebordo ao posicionar o apoio na mesial ou distal do ultimo dente suporte, com excecao feita ao rebordo descendente distal, onde se acentuou a concentracao de tensoes ao posicionar o apoio na distal do ultimo dente. Desse modo, podemos concluir que (1) o rebordo mais desfavoravel foi o descendente distal e o mais favoravel o horizontal, (2) o grampo em barra T apresentou distribuicao de tensoes mais favoraveis, com excecao do rebordo horizontal, onde nao houve influencia quanto ao tipo de grampo, (3) e que a localizacao do apoio apresentou comportamentos semelhantes, a nao ser no rebordo descendente distal.