Improving Total Knee Implants by Discovering Fatigue Mechanisms
In This Article
- A multifactorial process including particulate debris, delamination and oxidation all threaten implant integrity of total knee arthroplasty (TKA), leading to revisions and poor patient outcomes
- The development of advanced materials at the Massachusetts General Hospital Harris Orthopaedics Laboratory is leading to improvements in TKA wear rates and performance
- Highly crosslinked polyethylene (HXLPE) in TKA is one such advance yielding promising clinical results
- In TKA, a research study shows a lower 10-year rate of revision when using highly crosslinked polyethylene
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Particulate debris has been recognized for decades as a major cause of component loosening in joint implants. The result is limited longevity and clinical performance, and a negative impact on patients’ quality of life. However, comprehensive solutions are emerging to the multifaceted processes of implant wear, fatigue and failure.
Since 1999, Massachusetts General Hospital’s Harris Orthopaedics Laboratory in the Department of Orthopaedics has pioneered wear-resistant materials such as ultrahigh molecular weight polyethylene (UHMWPE) and highly crosslinked polyethylene (HXLPE). The Harris Lab’s ongoing investigations seek to improve TKA and reduce wear through developing new insights into oxidation-related fatigue mechanisms.
The clinical goals of this work on knee implant technology include:
- Improving durability and performance
- Reducing the TKA revision rate due to wear and delamination damage
- Reducing the prevalence and severity of periprosthetic osteolysis
- Improving oxidative resistance of UHMWPE through the discovery of new material processing methods
Improving Outcomes in TKA
In 2001, Harris Lab researchers published the results of a study in the Journal of Arthroplasty on a cross-linking method for UHMWPE, which is more wear-resistant for load-bearing applications in total joints. The study’s authors included Orhun Muratoglu, PhD, director of the Harris Lab, Charles Bragdon, PhD, associate director of Clinical Studies at the Harris Lab, and Director Emeritus W. H. Harris, MD, DSc. Since then, more than eight million highly crosslinked UHMWPE implants have been implanted into humans in the United States.
In a 2015 outcome study published in Acta Orthopaedica, Dr. Muratoglu and colleagues showed a lower rate of revision for crosslinked polyethylene in total knee arthroplasty (TKA), a finding that appears to be prosthesis-specific for TKA. The lower rate was most evident in patients greater than 65 years old, producing a reduction in revisions for implant loosening and osteolysis.
In both hip and knee implantation, debris generated consists of small particles of polyethylene less than one micron in size, provoking a biological response that leads to periprosthetic osteolysis. However, in addition to particulate debris, a second problem emerges with knee implantation: delamination damage (Figure 1). When small fractures form on a knee implant, delamination damage produces larger chunks of debris greater than one micron in size. As with the small particle wear, the delamination damage also threatens the implant’s integrity through fatigue-induced cracks, which leads to breakage in the implant.
Figure 1: DELAMINATION DAMAGE
An example of a retrieved polyethylene knee component from a revision surgery. Loss of material from delamination and fatigue are evident on both sides of the bearing surface. The areas of continued sub-surface cracking will eventually lead to more delamination.
Dr. Muratoglu and his team have recently developed the second generation of highly crosslinked polyethylene by stabilizing it with vitamin E, which not only resists the formation of small particles but also substantially increases the resistance to delamination.
“By crosslinking the polyethylene, we can substantially reduce the wear rate, because we reduce the number of particles,” explains Dr. Muratoglu. “The goal of our new materials technology is to reduce both the problem of particulate debris and delamination damage from larger chunks. We are trying to control the destructive processes on all levels to improve TKA implant performance.”
“These advances have changed the practice and long-term success of joint replacement worldwide,” adds Andrew A. Freiberg, MD, chief of the Center for Hip & Knee Replacement at the Mass General Department of Orthopaedics. “Preventing bone loss and implant loosening, as well as the need for revision surgery, have made joint replacement an option for young patients. This has dramatically extended implant life span for all.”
3 Forms of Oxidation in TKA Implants
Controlling oxidation is key to reducing particle wear and delamination damage. Traditionally, gamma radiation has been the focus as the sole cause of oxidation. A 1996 Biomaterials study by Dr. Harris and colleagues linked fatigue in the first generation of polyethylene knee implants to oxidation related to gamma radiation used to sterilize implant surfaces.
Subsequent investigations by Dr. Muratoglu’s team discovered that the shelf-life period of these implants led to oxidation, and that the creation of long-lived free radicals weakened implants. An additional 2011 study in the Journal of Biomedical Materials Research by Dr. Muratoglu and Ebru Oral, PhD, associate director of Biomaterials at the Harris Lab, suggested altered fabrication methods to eliminate free radicals.
Dr. Muratoglu and his team have identified two additional mechanisms of oxidation that lead to implant failure: squalene and cyclic loading.
In 2012, Dr. Muratoglu, Dr. Oral and colleagues reported in the Journal of Biomedical Materials Research the polyethylene implant’s absorption of lipids from the lubricating synovial fluids as a source of oxidation. A pre-cursor of cholesterol known as squalene is particularly important. When it diffuses into the implant, it can cause severe oxidation of UHMWPE and increase delaminations.
Understanding this provides another means for protecting implants from fatigue. The 2014 Journal of Arthroplasty report by Dr. Muratoglu’s team supports the addition of vitamin E to UHMWPE to reduce oxidation caused by squalene. The vitamin E-infused knee implant has been in clinical use for over seven years, and explanted components show excellent resistance to in vivo oxidation.
Cyclic loading is another oxidation mechanism the team is investigating. In cyclic loading, every step taken by a person with a TKA loads and unloads the joint. This dynamic input can generate free radicals, and when combined with diffused squalene from the synovial fluid, can further increase the oxidation damage of the components. Dr. Muratoglu says implant material design needs to defend against all possible oxidation mechanisms.
“Understanding fatigue processes will help us design new and better materials and implants—and a better kind of implant can help patients regain and enjoy long-term function,” concludes Dr. Muratoglu.
Learn more about the Center for Hip & Knee Replacement