Structural interaction between bone and implants due to arthroplasty of the first metatarsophalangeal joint

Background: Currently, the metatarsophalangeal joint replacement through a restorative arthroplasty, where implants are used, is a viable invasive surgical medical procedure in the treatment of severe cases of osteoarthritis in this joint, better known as hallux rigidus. However, few things are known about the postoperative complications that implants can cause on the joint, like Swanson and Tornier implants.Research in this field can provide a valuable information that would help the specialist surgeon in the decision-making during the selection of the more suitable joint implant in each patient, as well as the redesign of the devices, to make them more efficient, durable and biocompatible with the human body. Methods: The aim of this work is to perform a structural biomechanical analysis of a restorative arthroplasty of the first metatarsophalangeal joint, and to analyze the interaction between bone and medical grade silicone implants. For that, a simulation of a foot with Swanson and Tornier joint implants were performed to evaluate the stress/strain distribution during a critical stage (toe-off). Results and conclusions: Principal stresses obtained for the first metatarsal with both implants suggest that failure is induced in this bone because, values exceed (up to 136.84% for Swanson model) the tensile strength reported for phalange trabecular bone, which may be related to osteolysis. Stress and strain values obtained in this work suggest that arthroplasty surgery with Swanson implant is more likely to cause postoperative complications versus Tornier implant

Numerical Assessment of the Structural Effects of Relative Sliding between Tissues in a Finite Element Model of the Foot

Penetration and shared nodes between muscles, tendons and the plantar aponeurosis mesh elements in finite element models of the foot may cause inappropriate structural behavior of the tissues. Penetration between tissues caused using separate mesh without motion constraints or contacts can change the loading direction because of an inadequate mesh displacement. Shared nodes between mesh elements create bonded areas in the model, causing progressive or complete loss of load transmitted by tissue. This paper compares by the finite element method the structural behavior of the foot model in cases where a shared mesh has been used versus a separated mesh with sliding contacts between some important tissues. A very detailed finite element model of the foot and ankle that simulates the muscles, tendons and plantar aponeurosis with real geometry has been used for the research. The analysis showed that the use of a separate mesh with sliding contacts and a better characterization of the mechanical behavior of the soft tissues increased the mean of the absolute values of stress by 83.3% and displacement by 17.4% compared with a shared mesh. These increases mean an improvement of muscle and tendon behavior in the foot model. Additionally, a better quantitative and qualitative distribution of plantar pressure was also observed.Fac. de Enfermería, Fisioterapia y PodologíaTRUEMinistry of Economy Government of SpainCONACYT, Mexicopu

VALIDACIÓN NUMÉRICA Y EXPERIMENTAL DE UN ARO DE FIJACIÓN EXTERNA ILIZAROV PARA FRACTURAS

La validación numérica de sistemas biomecánicos sigue siendo un tema de interés. Por lo anterior, este trabajo tiene como objetivo determinar y analizar el comportamiento mecánico del sistema de fijación externa circular Ilizarov (F.E.C) y su validación numérica por M.E.F con resultados de pruebas experimentales por extensometria basadas en ASTM F-1541-02 A.3 (Standard Specification and Test Methods for External Skeletal Fixation Device) [1], en donde previamente se fabricaron probetas en base a ASTM-E8M [3]. A través de este trabajo se obtuvieron el diagrama fuerza-deformación de la prueba experimental, por medio de un  sistema adquisitor, utilizando extensometria con strain-gages [2], y se compararon los resultados contra un análisis numérico del tipo multi-lineal isotrópico, Los resultados presentan una comparación entre los resultados por simulación numérica y experimental, se muestran  la rigidez, curvas de fuerza versus desplazamiento,  esfuerzo y fuerza versus deformación unitaria, en donde se obtuvo  el diagrama fuerza- desplazamiento, con una fuerza al punto de fluencia de 1800 N, con su respectivo desplazamiento de 3.4 mm, de lo que se calcula la rigidez del fijador y se obtiene el valor de 530 N/mm, con lo cual , se excede el criterio interno T.D.I, el cual define parámetros de rigidez y fuerza para la aceptación de dispositivos médicos, la simulación del aro y su validación son parte de las pruebas requeridas por la COFEPRIS, para la regulación sanitaria correspondiente a pruebas biomecánicas en dispositivos médicos.Este trabajo permitió observar los efectos estructurales del aro de fijación externa de manera experimental y la validación de su modelo numérico. Adicionalmente se lograron simular las no-linealidades por contacto en la región de unión del aro, obteniendo buenos resultados no solo cualitativos, si no también cuantitativos. Finalmente, los resultados obtenidos en este trabajo permitirán simular de manera confiable, el sistema completo de fijación externa circular Ilizarov y con ello tener un modelo numérico completo de este sistema para estudios e investigación posteriores.Palabra(s) Clave(s): F.E.C. Ilizarov, Aro, Strain-gage, Traumatología por osteosíntesis, Prueba de compresión, Elemento finito