Failure Analysis of the Wire Fixation Bolts of an Ilizarov External Fixator

Abstract

The Ilizarov method is a treatment used to recover the functionality of the upper and lower limbs of patients who have lost bone tissues due to fractures or/and infections caused by accidents or congenital problems. This technique consists of assembling a device on the patient’s limb through which the bone is manipulated gradually. The device consists of external rings connected to each other by threaded rods and fixed to the bone by small diameter steel wires. These wires fasten the bone in an axial cross through a central axis perpendicular to the plane of the bone end and they are tightened to the rings by fixation bolts to provide assembly stability. The proper operation of the device depends mainly on the wire tension, which, in turn, depends on the torque applied to the bolts that hold the wires in place. The failure of these wire fixation bolts may compromise the Ilizarov frame function. In this study, four slotted bolts from a series of slotted bolts fractured during the assembly of Ilizarov fixator in clinical practice were analyzed and the upper limit for tightening these bolts was investigated. The results show that the failures occurred through a combination of mechanical and microstructural factors, including stress concentrators at the upper region of the screw, a high level of microstructural inclusions, and poor surface finish. The mechanical behavior of the bolts under loads was analyzed using the finite element method with the purpose of correlating it with the failure of the bolts.