Orthopaedic surgeon Robert Klapper literally wrote the book about how to heal your knees. "And I'm going to write another book," he says, "It's called stop exercising—you're killing yourself."
Klapper is joking, of course. But as he chats with me, on a break between surgeries at Cedars-Sinai Medical Center in Los Angeles where he is director of the Joint Replacement Program, what he isn't joking about is the wear and tear on knee joints that happens with age and sports.
One particularly knee-knackering activity for humans is jumping. And while medical advances have been made by studying knee care and repair in humans, some researchers are venturing further afield: At the Queensland University of Technology, for instance, researcher Tonima Ali is studying the knees of the kangaroo, to see what its inner workings may reveal.
It's not the first time non-human animals have been looked to for inspiration in human clinical care. But the four-legged animals already studied, such as cows, sheep, goats, dogs, and pigs, "have a movement gait that is different than that of humans," Ali says. That's what drew her to kangaroos as two-legged hoppers.
In her ongoing research, Ali has focused on the kangaroo's articular cartilage. That's the smooth, white connective tissue that cushions the bone ends where femur and tibia bones meet, allowing them to articulate with little friction while bearing weight.
Klapper speaks of the stuff with awe. "This material is almost frictionless," he explains. "The coefficient of friction of your joints moving against each other is less than two ice cubes rubbing together," he says. But cartilage in the knee can be damaged by age and overuse, often without us realizing since it's a tissue not supplied with nerves.
Also poorly supplied with blood, articular cartilage doesn't heal well on its own. Its repair by surgeons and or treatment by physical therapists has also proved challenging. So in an animal that hops along at speeds that can exceed 40 kilometers per hour and whose knees endure constant hard landings, Ali was curious about the kangaroo knee's evolutionary solution.
Cartilage can be tricky to image, with the directional orientation of cells providing some technical headaches. So Ali used an MRI trick called the magic angle effect to see how collagen is distributed in the kangaroo knee. In humans, the 3D structure of articular cartilage includes cells called chondrocytes embedded in a thick fluid matrix of water, stringy collagen fibers, and spongy proteins called proteoglycans. For a visual, Klapper says, picture spaghetti: The pasta is the collagen and the spaghetti sauce is the matrix.
In the kangaroo's landing, a lot of downward force is absorbed by the cartilage of the tibia or shinbone. Not surprisingly, Ali and her collaborators found the cartilage coating the tibia to be much thicker in kangaroos than in humans.
They also found a feature not seen in human knees. In the middle of the tibia's glass-like hyaline cartilage, there appears to be a fibrous pad of cartilage that is partially deformable. When the kangaroo hops, she explains, this cartilage pad is momentarily deformed, and then bounces back to its original shape. "That allows it to distribute the stress more effectively than a rigid structure," she says.
With this knowledge, Ali hopes to do biomechanical testing on this proof of concept. Her eventual hope is that the kangaroo knee-collagen structure may inspire new designs for tissue engineering.
Asked what he thinks of the idea of gleaning knee repair clues from kangaroos, Klapper, who was not involved in the research, says, "It's great." He suggests the researchers don't stop with cartilage, and also look at kangaroo knee ligaments, bone shape, and muscles to see what other odd anatomical tricks this bouncy antipodean might have inside its knees.