Researchers at Case Western Reserve University and Exponent, an engineering and science-consulting firm in Philadelphia, are working to improve the wear and performance of artificial joints made of ultra-high molecular weight polyethylene.
The team are employing high-power computing techniques, advanced mathematical models, and load-bearing tests to investigate the performance of the plastic in total joint replacements. Their aim is to uncover how the damage progresses in the plastic.
‘Total hip replacement (THR) and total knee replacement (TKR) are orthopaedic success stories, ‘ said Clare Rimnac, director of the Musculoskeletal Mechanics and Materials Laboratories at CWRU and associate professor of mechanical and aerospace engineering at the Case School of Engineering. ‘Unfortunately, in some cases these joints wear out more rapidly than we would like,’ she added.
‘Instead of implanting the joint and waiting to see what activities cause the material to fail, we’re developing computer simulated models to give us that information,’ Rimnac said. ‘The ultimate goal of our research is to make it possible to predict the performance of new implant designs before they are implanted into patients.’
A normally active person will load and unload a hip or knee joint by flexing and extending it between one million and three million times per year, Rimnac said. The force across the joint may vary from three times a person’s body weight when simply standing to more than six times body weight when climbing and descending stairs and jogging.
When the plastic is damaged, particles are released into the body, which may cause bone loss. Then, instead of having an implant that is well fixed into the bone, the reaction to the debris leads to gaps in the bone which compromise the mechanical and structural integrity of the implant.
Using a mechanical testing machine, Rimnac’s lab subjects sample pieces of the polyethylene to different loading conditions until the sample fails by fracture. The research team then studies how the samples deform and how cracks form and grow, developing a model of the plastic’s behaviour.
They are conducting this research on several new formulations of the polyethylene that recently have been introduced in total hip replacements and that are being implanted into patients.
‘Although these new polyethylene materials are already in clinical use, our goal is to provide better computational simulations of these joint replacements to identify potential design concerns associated with these new materials and to, in general, develop design strategies for improving the long-term performance of total joint replacements,’ said Rimnac.
Rimnac noted that these new formulations of polyethylene are of great clinical interest because research suggests that they are more resistant to wear than previous polyethylene formulations. With these new polyethylene materials, the long-term performance of implants may be extended because fewer wear particles will be released and there should be less bone loss.