Failure analysis of a modular revision total HIP arthroplasty femoral stem fractured in vivo

An implant fracture is an unusual reason for total hip arthroplasty (THA) revision; however, it may cause serious clinical complications. The stress in femoral stems may increase considerably with the loss of bone support around the implant, leading to implant failure. This paper investigates the main reasons for the in vivo fracture of a Ti6Al4V revision femoral ZMR stem. Various methods were used: examination of the patient's by radiographies; optical emission spectrometry; and Scanning Electron Microscopy (SEM). The titanium (Ti) alloy was also characterized through optical microscopy and a Vickers hardness test was carried out. A finite element analysis (FEA) was conducted to evaluate the stress distribution occurring in the femoral stem revision under loading. The chemical composition and microstructure of the prosthesis did not reveal any non-conformity. The SEM analysis revealed fretting damage near the crack initiation site and the fractured surfaces exhibited characteristic ratchet marks, poorly formed striations and a small final fracture area. The FEA showed a stress concentration located at the proximal-distal taper junction, with a smaller diameter, coinciding with the crack initiation site. The conclusion was that fatigue failure was due to a lack of bone sustainability that increased the stress, associated with design characteristics that caused a localized concentration of stress, and these were potentialized by a fretting mechanism, leading to the failure.