Failure analysis of retrieved stainless steel Exeter hip implants: A fractographic and corrosion perspective

Abstract

In this study, three fractured stainless steel Exeter (REX 734) hip joint implants were analyzed to identify the underlying failure mechanisms. These cemented hip stems with a polished surface finish failed earlier than their expected service life, indicated by sudden pain in the patients, necessitating revision surgeries. Optical and scanning electron microscopy were used to analyze the explanted hip stem fracture surfaces. The fractograms exhibited three distinctive zones; (1) crack initiation at the anterolateral part of the stem, (2) crack propagation zone with beach marks and striations, and (3) final fracture zone. Fatigue was the dominant failure mechanism. The anterolateral surface of the implants close to the fracture initiation site showed discoloration and surface alteration, possibly due to corrosion, and the presence of extrusions and intrusions formed by slip band accumulation, causing stress concentration and crack initiation. Also, elongated and sharp-edged phases enriched in niobium were found at the crack initiation site and throughout the microstructure. Medical radiographs indicated a lack of cement fixation at the proximal part of the stems, causing implant instability and ingress of body fluids, which potentially led to corrosion reactions. The crack propagation zone decreased in relative size with higher patient weight and BMI. Overall, this study suggests the possibility of corrosion contributions and a role of niobium-rich precipitates in the crack initiation and fatigue failure.