A PASSIVE STATE SIMULATION OF AN ANAL SPHINCTER USING SIMMECHANICS

Nowadays, different mechanical artificial sphincters can be found implanted in human beings, trying to overcome a deficiency in the performance of the natural one. However, they do not take into account the natural anal sphincter's (AS) dimensions, and autonomous response; they also lack in basic contraction and relaxation properties. In this paper, by addressing the AS behavior, an AS model designed with Matlab/SimMechanics is shown. The model comprises bodies of concentrated mass interconnected by springs. The mass-spring system is arranged in concentric rings where every concentrated mass is interconnected by a spring. Each spring takes specific stiffness, which varies with length, in accordance to an experimental curve. The system described can be loaded or unloaded, describing then the muscle behavior. Each element that forms the model of rings is subject to displacements caused by forces of traction and compression, when a radial force is applied from the center towards the inner ring. The springs of the inner ring experience forces of traction, whereas the springs that connect the body of the inner ring with the outer ring perpendicularly are submitted to compression forces. The data used in the proposed model corresponded to dimensions of the humanAS: width, height, rigidity, stress, tension, basically obtaining an initial deformation behavior according to the sphincter in the passive state. The model remained stable with some mechanical oscillations due to the elastic elements; by modifying one of the parameters, the behavior became unstable and unmanageable. It was verified that it is a sensitive model when modifying the initial conditions that the concrete data requires in case of reproducing the sphincter muscle with particular dimensions.