Experimental and numerical failure analysis of Thompson hip prosthesis

BackgroundThere are different surgical procedures to fix femoral head-neck fractures, such as total hip arthroplasty and hemiarthroplasty. Under optimal conditions, the inserted prosthesis should last a long time, possibly a lifetime, with only trace amounts of wear each year due to the friction between the femur bone and the prosthesis stem. However, in some cases, failure due to excessive wear, creep, fatigue, corrosion, etc., can occur.ObjectiveThis study investigates the failure causes of a Thompson hip prosthesis both experimentally and numerically.MethodsThe stem of the prosthesis, which is inserted into the femur, broke inside a patient's body over time. Following the removal surgery, the severely damaged and fractured stem was examined to determine the root causes of the failure. For this purpose, fractographic examination of the fractured surfaces was conducted with scanning electron microscope (SEM). Microstructural analyses were performed using optical microscope, and the chemical composition of the prosthesis was analyzed with energy dispersive spectroscopy (EDS). Vickers hardness (HV30) test was conducted on the femoral stem. Additionally, finite element method (FEM) was applied to estimate the forces acting on the prosthesis.ResultsIt was observed that fatigue cracks initiated and propagated from the outer surface of the femoral stem at locations very close to the maximum von Mises stresses observed in the FEM analysis. However, the load magnitudes in the FEM analysis were not high enough to initiate any cracks.ConclusionIt is considered that crack initiation occurred due to material defects from the prosthesis manufacturing process, and the cyclic loading during body motion propagated these cracks. Ultimately, the fracture of the Thompson hip prosthesis occurred.