Tesis profesional presentada por
Licenciatura en Ingeniería Mecánica. Departamento de Ingeniería Mecánica. Escuela de Ingeniería, Universidad de las Américas Puebla.
Presidente: Mtro. Juan Carlos Cisneros
Vocal y Director: Dr. Sudhakara Katapadi Vadiraja
Secretario: M.C. Cosme Gómez Pineda
Cholula, Puebla, México a 2 de mayo de 2005.
The study of biological systems is not something new, for many years scientist have been researching about the behavior of human and animal body. These studies gave birth to biomechanics, which is the study of the structure and function of biological systems using the method of mechanics. Biomechanics is closely related to mathematical concepts that support it and give a better approach to medical research.
The main objective of this study is to analyze the performance of vitallium screw in bone fixation and the mechanical properties that the material has in order to be selected as a biomaterial.
Vitallium has an excellent balance between biocompatibility and mechanical properties, having an average hardness of 36 HRC, and impact toughness of 76.46 Nm and a fatigue endurance limit around 720 MPa.
Human body is also a mechanism and a structure, since transmits movement and is subjected to forces. All the elements present in human being exert and are subjected to external and internal forces, this is why is very difficult make a stress strain analysis because of the infinite number of constrains, known and unknown forces and displacements and it is needed to make many assumptions in order to have a better approximation to real conditions.
This study is mostly interested in the stress distribution during the human gait. The gait cycle has two main phases; the stand phase and the swing phase, the stages considered in this study are the full foot strike, toe off and mid swing stages. The forces applied in the model are the results of the wear on knee joint during the gait by ISO-14242-1.4 in a model drawn in Pro-E.
For the case of this study the fracture is a superior metafisis of tibia and fixed with one lag orthopedic screw of 40 mm length. To make the analysis were considered theoretical properties of vitallium such as Young´s modulus, Poisson´s ratio and coefficient of thermal expansion.
In the analysis of the FEM analysis the important thing to notice is that any of the parts of the joint is even close to a fracture stress that according to studies is of 260GPa, having the knee a maximum stress of 210 MPa. The two stages in the stance phase have almost the same behavior, but with also some differences because of the differences in axial force and moment, the mid swing stage provides information about same distribution but dramatically lower stresses.
In the analysis of the behavior of the vitallium element used for fixation the result is a maximum 7 MPa stress in the firsts threads of the bottom of the screw, while the yield shear stress for vitallium is 25 MPa, concluding that it will not have plastic deformation. Finally the design a new screw is not only a mechanics matter, bur also are involved medical considerations. This is why as much as it could be done a suggested design of a new screw, this design would not be anatomically correct for the application.
Mendoza Kirsch, E. E. 2005. Evaluation of mechanical properties and investigation of stress distribution using FEM in knee joint with and without vitallium alloy implant. Tesis Licenciatura. Ingeniería Mecánica. Departamento de Ingeniería Mecánica, Escuela de Ingeniería, Universidad de las Américas Puebla. Mayo. Derechos Reservados © 2005.