Jeff Renaud

Orthopedic implants are designed to restore movement and relieve pain, offering patients a second or even third chance at mobility. A hip or knee replacement is often framed as a durable fix, engineered to last for years, sometimes decades, inside the human body. And modern hip and knee implants exceed expectations, providing long service time and significantly improved life quality for the vast majority of patients. But durability inside biology, especially the human body, is never static once a foreign material is introduced. It responds continuously, at every scale. “Having an implant in your body is going to change your body chemistry,” said Western University chemistry professor Yolanda Hedberg. “You might not get the corrosion and wear damage some people experience, but your body chemistry is constantly changing even if you don’t have any pain or other symptoms.” A study, recently published in npj Materials Degradation (Nature Research), describes a project at Western and London Health Sciences Centre Research Institute (LHSCRI) that examined more than 240 retrieved hip and knee implant components to understand how and why they degrade over time. Researchers, including Hedberg, Western chemistry postdoctoral associate Saman Nikpour and Schulich School of Medicine & Dentistry professor and LHSCRI scientist Matthew Teeter, combined optical microscopy with scanning electron microscopy and spectroscopy (scientific techniques used to analyze structure and composition of materials) to assign a score quantifying surface damage and identifying corrosion patterns. Infection prior to or at surgery was correlated with higher damage scores of the trunnion (connecting point on an artificial hip where the top of the metal stem connects to the artificial ball) while inflammatory arthritis and cemented hip implants correlated with lower damage scores. What emerged was a layered picture of mechanical and chemical change unfolding together inside the body. For both hip and knee implants, the dominant process of degradation was mechanically assisted corrosion, known as tribocorrosion. This type of corrosion occurs when movement and chemistry act on the same surface at the same time, accelerating damage beyond what either process would cause alone. Teeter, director of Western’s Bone and Joint Institute, leads the Implant Retrieval Laboratory at LHSCRI, home to Canada’s largest collection of failed hip, knee, shoulder and dental implants. “Collaboration is what lets us go deeper, into the mechanisms behind failure rather than just its consequences,” said Teeter, medical biophysics professor. “When you can connect implant design, materials and patient biology, you give manufacturers the evidence to build better devices and surgeons the insight to make genuinely patient-specific choices.”  

Proteins defend and damage

Most implants rely on a thin protective oxide layer to remain stable inside the body. That layer forms naturally on metals such as titanium and acts as a barrier between the metal and surrounding biological fluids. “All hip and knee implants on the market today are constructed with passive metals like titanium, so they rely on a surface oxide protecting them from oxidizing,” said Hedberg. “That protection depends on stability at the surface – but movement inside the joint introduces repeated disruption.” Once that protective layer is disturbed, the surface becomes reactive. Body fluids interact directly with exposed metal and corrosion processes begin almost immediately. The system then rebuilds the oxide layer, only to have it disrupted again with continued motion. “The mechanical wear is destroying the surface oxide,” said Hedberg. “It actually only takes milliseconds. Every step by the person with the implant destroys the oxide temporarily, and then we get a huge chemical response. After that, the oxide is reformed, and the process begins again.” This repeating cycle creates a dynamic surface environment where damage and repair occur together at microscopic scales. Biology adds another layer of complexity. As soon as an implant is placed in the body, proteins from surrounding fluids coat its surface. That coating influences how cells and tissues respond. “The proteins adhering to the surface determine how the body reacts,” said Hedberg. “The proteins act like the language of the body, or the mode of communication.” Depending on which proteins dominate the surface of the implant, outcomes can shift toward bone integration, inflammatory response or bacterial colonization. In some cases, proteins contribute to protective behaviour. In others, they contribute to degradation processes.

Canada’s largest collection of failed implants

Patient factors played a measurable role in how damage developed. Higher body weight, increased body mass index and longer surgical implantation times were associated with higher damage scores. Certain clinical conditions, including infection at the time of surgery, were also linked to increased surface degradation in specific implant regions. Other factors, such as inflammatory arthritis and quality and quantity of acrylic bone cement used for stabilizing hip implants also impacted damage scores. These patterns highlight the variability of implant performance across individuals and over time. And that variability is one reason retrieval science has become central to the field. Laboratory testing provides controlled conditions, but it cannot reproduce the full range of mechanical forces, immune responses and biological differences present across patient populations. “Each patient is different. Retrieved implants offer us a living record of how materials behave after years inside the body,” said Hedberg. “And this doesn’t mean you should be scared to get a hip or knee implant. In most cases, hip or knee replacements significantly improve life quality and health for a long time. Our work just ensures that even more people can enjoy this improvement in the future.” Hedberg, Teeter, Nikpour and their collaborators analyze retrieved implants, collected expertly during revision surgeries at London Health Sciences Centre (LHSC), and cross reference their findings with retrieval networks across multiple countries, including Canada, Australia, United States and Slovenia, building datasets that connect surface damage with clinical histories. “The human body is one of the most complex environments a material can experience,” said Nikpour. “In many ways, the corrosion and wear we see in retrieved implants resemble degradation in industrial systems, but here the surface is exposed to a constantly changing mix of mechanical loading, body chemistry, proteins and inflammatory responses.” Learn more about how Western is optimizing health for all.