Finite Element Analysis and Mechanical Testing of External Fixator Designs

Abstract

Experimental and theoretical stress analysis methods are used to evaluate the mechanical behaviour of external jixation devices as load-bearing structures. For the experimental part, a modular assembly was fabricated from which unilateral and bilateral jixators of different design conjigurations were assembled and tested under various loading conditions. A reflective photoelasticity technique was used to study the eflect of frame configuration on the stress patterns generated around the pin-bone interface. Finite element models of each design were also generated using three-dimensional beam and shell elements. Spring elements were used to model the pinlsidebar clamp. It is shown that close correspondence between the experimental and theoretical methods of investigation is obtained when theflexibility of the pinfside-bar clamp is taken into account. It is also shown that a unilateral design, modified by attaching a second side-bar to the first and connecting them by means of a semicircular component, can achieve some of the structural advantages of bilateraljixators without the clinical disadvantage of transfixing pins.

Finite Element Analysis of Osteosynthesis Screw Fixation in the Bone Stock: An Appropriate Method for Automatic Screw Modelling

“…Structural elements like beams are a common method to discretise slender structures, e.g. in external fixation systems with satisfying accuracy [28] , [29] . The described method can therefore be used to create screws parameterised only by few parameters, e.g.…”

Re-design of a uniplanar, monolateral external fixator

“…The experimental results in terms of the x-axis normal elastic strain, preload force and mean coefficient of friction are reported in Table 3. The mean coefficient of friction can be calculated by solving equation (2) for m m .…”

“…4,5 Uniplanar fixators are called monolateral if one side-bar is connected to the bone fragments using half-pins 6,7 or bilateral if they employ one side-bar on each side of the limb, attached by pins that transfix the bone. 2 Many studies are available in the literature that deal with the structural analysis of external fixators. Nonetheless, the vast majority of them are concerned with the evaluation of the stiffness characteristics of the fixator device, [8][9][10][11][12][13] while some focus on the fatigue or wear properties of its components.…”

“…External fixation is the treatment of choice in the case of (1) open fractures, especially if soft tissue treatments have to be performed, 1 and (2) limb lengthening, in which a bone diaphysis is distracted by extension of the side-bars of the fixator frame. 2 In the literature, classification schemes are available for external fixation devices: according to Behrens, 3 a distinction can be made between uniplanar and multiplanar fixator frames. Uniplanar devices have side-bars in the same plane as the bone fragments, whereas multiplanar external fixators have connection members that surround the limb.…”