Failure Analysis of a Ti6Al4V Screw Used in a RASL Procedure

Abstract

A failure analysis investigation was performed on a Ti6Al4V medical implant screw that failed after 1 month of surgical implantation. Light microscopy, scanning electron microscopy, and finite element analysis (FEA) techniques were utilized to characterize the mode(s) of failure and fracture surfaces. Complete fracture of the screw was observed near the transition from the tapered thread at the back of the screw, to the flute portion towards the front of the screw. Fractographic analysis revealed significant fretting and flattening of the threads, indicating lack of fixation to the bone, and micromotion of the screw during implant life. Off-axis, reverse bending macroscopic beach marks, as well as microscopic fatigue striations were found on opposite sides of the annular fracture surface. These features suggest intermittent cyclic bending led to crack initiation, and propagation of crack fronts originating along circumference of the tapered thread transition region. Microvoid coalescence (overload features) were also found on the fracture surface, as well as secondary cracks along the screw circumference within the thread root. FEA was used to show the development of localized stress concentrations within the thread root radius at the taper transition region, consistent with observed crack initiation areas. The fracture morphology suggests that premature failure occurred because of interacting factors including: a lack of fixation between bone and screw, micromotion of the screw threads against the bone, and reverse bending fatigue within the tapered to flute transition region. These conditions may have been exacerbated by the use of an improper screw diameter or inadequate installation torque.