There are well over 100,000 total hip replacements done per year in the U.S., and over 250,000 per year worldwide. Extending the service life of these implants is a high priority due to the large number of patients and the increasing use of joint replacement in younger patients. In an older population, total hip replacements usually outlast the patient, due to advanced age and the relatively low physical demands placed on the prosthesis. Younger, more active, and heavier patients often require revision of their total hip due to progressive implant loosening and the pain which results. Revised total hips do not last as long as the original, and a younger patient can sometimes require more than three revisions. In a standard total hip replacement, a steel prosthesis is fixed to the thigh bone using polymethyl methacrylate bone cement. The fatigue characteristics of the bone cement, and of the prosthesis-cement interface have been identified as the critical areas which initiate failure in this structure. In this study, the stresses within the cement and at the cement-metal interface have been studied using a large-scale linear finite element analysis. Also, to assess the effects of cement-prosthesis debonding, a 3D contact study was conducted which assessed the effect of several different areas of debonding between the prosthesis and the cement. The results of finite element analysis showed that the most likely sites for failure initiation are in the proximal antero-medial region and at the distal prosthesis tip. The loading situation which appeared to put the interface in the most danger for failure is that encountered in stair climbing. The stresses during normal walking did not seem as critical. Simulation of cement-metal debonding showed that a drastic increase in cement stresses occurred under both gait and stair climbing loads, and that stair climbing loads also produced much higher stresses with debonding. The effect of pores which often occur in bone cement was assessed using an analytical elasticity solution for a spherical void in an infinite medium, which allowed a calculation of the maximum tensile stress at the surface of a pore. These stresses were sufficient to initiate fractures near the distal tip of the implant in many cases, and near the proximal medial region of the implant in stair climbing. This study concurs to a remarkable degree with a study of well-functioning total hip replacements recovered at autopsy. The initiating failure events seen in the retrieved femoral components matched closely with the predicted areas of failure initiation. The conclusions of the study were that the cement-prosthesis interface should be strengthened, porosity in bone cement should be minimized, and that total hip patients should not use their prosthetic hip when climbing stairs.
ASJC Scopus subject areas
- Civil and Structural Engineering
- Modeling and Simulation
- Materials Science(all)
- Mechanical Engineering
- Computer Science Applications