Análise de elementos finitos da distribui??o de tens?es equivalentes nos pinos de Schanz durante o uso do distrator femoral de fraturas ☆

摘要

Resumo Avaliar o comportamento do estresse mecanico e da deforma??o elástica exercida na transi??o rosca‐talo liso dos pinos de Schanz do distrator femoral de fraturas em montagens com diferentes distancias de ancoragem dos pinos na cortical óssea de entrada através de estudo da distribui??o e da localiza??o de tens?es no corpo de prova. Feita análise de elementos finitos 3 D para a avalia??o da distribui??o das tens?es equivalentes em um pino de Schanz fixado de modo bicortical e ortogonal a um osso tubular, em dois padr?es de montagem: (1) transi??o rosca‐talo liso distante 20 mm da ancoragem dos pinos de Schanz na cortical de entrada e (2) transi??o rosca‐talo liso distante 3 mm da ancoragem dos pinos de Schanz na cortical de entrada. Foram feitas simula??es e manteve‐se a mesma dire??o da carga e a mesma distancia do vetor for?a em rela??o ao centro do osso hipotético. A carga aplicada, sua dire??o e a distancia ao centro do osso foram constantes durante as simula??es para manter o momento fletor igualmente constante. Os cálculos apresentados demonstraram comportamento linear durante todo o experimento. Verificou‐se que o modelo com uma distancia de 20 mm entre a ancoragem dos pinos de Schanz na cortical óssea de entrada e a transi??o rosca‐talo liso reduziu o risco de ruptura ou fadiga do material durante a aplica??o de cargas estáticas constantes. Nesse modelo, as for?as máximas observadas foram superiores (350 Mpa). A análise do comportamento do estresse mecanico e da deforma??o elástica exercida na transi??o rosca‐talo liso dos pinos de Schanz do distrator femoral de fraturas mostrou que distancias maiores entre a ancoragem dos pinos na cortical óssea de entrada e a transi??o rosca‐talo liso dos pinos de Schanz permitem menor estresse mecanico na transi??o rosca‐talo liso e maior grau de deforma??o elástica do material e minimizam quebra ou fadiga. A distancia sugerida é de 20 mm. Abstract To evaluate the mechanical stress and elastic deformation exercised in the thread/shaft transition of Schanz screws in assemblies with different screw anchorage distances in the entrance to the bone cortex, through the distribution and location of tension in the samples. An analysis of 3 D finite elements was performed to evaluate the distribution of the equivalent stress (triple stress state) in a Schanz screw fixed bicortically and orthogonally to a tubular bone, using two mounting patterns: (1) thread/shaft transition located 20 mm from the anchorage of the Schanz screws in the entrance to the bone cortex and (2) thread/shaft transition located 3 mm from the anchorage of the Schanz screws in entrance to the bone cortex. The simulations were performed maintaining the same direction of loading and the same distance from the force vector in relation to the center of the hypothetical bone. The load applied, its direction, and the distance to the center of the bone were constant during the simulations in order to maintain the moment of flexion equally constant. The present calculations demonstrated linear behavior during the experiment. It was found that the model with a distance of 20 mm between the Schanz screws anchorage in the entrance to the bone cortex and the thread/shaft transition reduces the risk of breakage or fatigue of the material during the application of constant static loads; in this model. the maximum forces observed were higher (350 Mpa). The distance between the Schanz screws anchorage at the entrance to the bone cortex and the smooth thread/shaft transition of the screws used in a femoral distractor during acute distraction of a fracture must be farther from the entrance to the bone cortex, allowing greater degree of elastic deformation of the material, lower mechanical stress in the thread/shaft transition, and minimized breakage or fatigue. The suggested distance is 20 mm.