Biomechanical Study of Rigidity in the External Fixators

Ihn, Joo Chul; Ahn, Myun Whan; Seo, Jae Sung; Lee, Jong Hyeung; Lee, Il Key; Song, Jung Il; Kang, Young Hoon

Abstract

The finite element method was utilized to find the rigidity of the device in changing the parameters, such as geometrical configuration, material property, and loading condition in an external fixation. In the experimental analysis, a universal testing machine(UTM) was used to evaluate the rigidity of EBI, Monofixator and Hoffmann under the different type of lodings, such as axial compression, bending and torsion in changing the parameters. The result was compared with those of the experimental work. The results were as follows: 1) In standard condition, the yielding loads in axial compression were 85Kg in the EBI, 55Kg in the Hoffmann, and 100Kg in the Monofixator, The rigidity figures were 2930N/cm in the EBI, 764N/cm in the Hoffmann, and 2391M/cm in the Monofixator. Z) The experimental method was adapted to figure out the effect of parametric change on the stiffness of 3 different external fixation apparatus(Monofixator,EBI, Hoffmann). A Monofixator with a parametric change of pin diameter from 6mm to 5mm indicated 40% reduction of stiffness in axial compression and 60% reduction of stiffness in torsion. Also, in the case of the change of pin group separation from 20cm to 15cm it showed 64% increase of stiffness in, AP bending. EBI with the increase of the distance between pin and aluminum bar to 8cm showed 32% decrement of stiffness in axial compression and 38% decrement of stiffness in AP bending. The stiffness of the Hoffmann device was remarkable according to the number of the pin used and the distance between the pin and the aluminum bar. 3) Concerning the difference between the experimental analysis and finite element method in a Monofixator, the results of the experimental analysis were 10% to 37% lower than those of the finite element method. The reason is thought to be caused by complete binding. The obtained data shed light on the future guide-lines and future alterations of the design factors in external fixation device. Approprate use of the obtained information is desirable for the optimal,treatment of the fractured long bone.