Cartilage is a tough but flexible tissue that plays a crucial role in protecting joints, providing cushioning as people move.
It’s very effective at dissipating energy—but if enough force is applied, cartilage will fracture. Cracks in the tissue can lead to osteoarthritis, a common and incurable disease in which cartilage wears down around a joint.
Now, collaborative research from the labs of two University of Wisconsin-Madison mechanical engineers is revealing new details about how cartilage’s energy-dissipation mechanisms influence fracture in the tissue.
In the new study published in May 4, 2021 in the journal Scientific Reports, the researchers also provided an estimate of the critical energy release rate of cartilage, which is a proxy for the toughness of the material. Their findings could eventually lead to more effective treatments for osteoarthritis, as well as inform the design of synthetic soft materials with superior toughness.
“The critical energy release rate is a useful benchmark value that can enable more accurate modeling of cartilage. It also allows us to compare cartilage to other materials in terms of toughness,” says Corinne Henak, an assistant professor of mechanical engineering at UW-Madison who conducted the research with Associate Professor Melih Eriten.
In the study, Guebum Han, a recent PhD graduate and the paper’s first author, conducted well-controlled experiments to investigate how fracture occurs in intact, healthy cartilage and in unhealthy, degraded cartilage under varying conditions. The degraded cartilage simulated the early stages of osteoarthritis.
Han applied loading to the cartilage samples (from pig patella) at a number of different loading rates, ranging from very fast to very slow, to induce fracture. “We found an interesting link between rate-dependent cartilage failure and cartilage dissipation degrees because we were able to conduct both failure and relaxation tests at similar microscale lengths thanks to our microindentation approach,” Han says.
The researchers discovered that the cartilage’s energy-dissipation mechanisms played a critical role in delaying fracture.
“We showed that cartilage has an excellent ability to protect against fracture by actually diffusing localized stresses from loading across larger surface areas. Various energy-dissipation mechanisms accompanying this process also toughen the tissue,” Eriten says.
These mechanisms take a certain amount of time to effectively dissipate energy, so Han also examined multiple timescales together with the loading rates in his experiments.
“If the loading is applied really fast, then none of these dissipation processes will have time to occur,” Henak says. “But if the loading is very slow, then all of those dissipation mechanisms have time to occur and accommodate the localized loading, delaying the formation of cracks in the cartilage.”
The researchers also measured the length of the cracks that formed during the experiments. They found that cracks with sizes of less than 100 microns do not have a tendency to grow larger in cartilage. “So cartilage can take many small cracks before it fails catastrophically, and that was one of the key findings that came out of this study,” Eriten says. “This shows that cartilage is quite tough and tolerant to small flaws.”
Uraching Chowdhury, a PhD student in mechanical engineering, was a co-author on the paper. This research was supported by funding from the National Science Foundation (CMMI-DCSD-1662456).
Author: Adam Malecek