Mechanical metamaterials offer new hope to orthopedic patients and their doctors by nearly eliminating degradation and damage to the hip socket.
The man limps into his orthopedist’s office. His hip hurts when he walks, he can barely get up from a chair, and he’s tired of the pain. After all these years with arthritis, it’s time, his doctor says, and the man agrees. Soon he’s getting a total hip replacement.
This scenario plays out more than 300,000 times a year in the United States, making the operation one of the most common surgical procedures in the country. Now, engineers in the Netherlands have made a hip implant that could fit better, last longer, and prevent replacement surgeries. The implants are the first to incorporate mechanical metamaterials— exotic materials that have mechanical properties no conventional material ever had.
Such implants are a ways off, but they’re part of what could eventually be used to replace bone lost to osteoporosis, bone cancer, or injury, says Amir Zadpoor, the Anton von Leeuenhoek professor of biomaterials and tissue biomechanics at the Delft University of Technology in the Netherlands.
“We are making materials with impossible properties to solve problems for patients with difficult-to-treat skeletal diseases,” Zadpoor says.
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The idea grew from a discovery biologists made in the mid-2000s, Zadpoor says. They learned that physics and mechanical properties determined the behavior of cells and tissues as much as chemistry does. This is especially true for bone, which begins to weaken unless it’s subjected to the correct mechanical forces.
Improper mechanical forces have been a big problem for patients who have undergone total hip replacement, which is one of the most common orthopedic surgeries. Hips are ball-and-socket joints, with the ball sitting on a stem that angles slightly off at the end of the thigh bone, or femur. This ball fits precisely into a socket in the pelvic bone, and it rotates freely to give the hip flexibility.
In a total hip replacement, a surgeon replaces the diseased femur stem and head with a metal or ceramic femur head, and installs metal, plastic or ceramic lining to the socket to help the hip rotate.
Unfortunately, the mechanical properties of conventional hip prostheses differ from those of healthy bone, and the altered mechanics can cause implants to fail. That’s because each time the hip bends, one side of the implant compresses into the hip socket, which is good for the remaining bone in the socket. But the other side pulls away, removing mechanical pressure and creating a gap. Over time, tiny metal particles from the prosthesis accumulate in this gap, bone becomes inflamed, it degrades, worsening the implant’s fit, causing more pain, and ultimately causing the hip implant to fail.
To solve this problem. Zadpoor designed a hybrid implant from two different mechanical metamaterials.
Since they were discovered in the 1990s, metamaterials have astounded observers with unprecedented properties. There are optical metamaterials that can bend light around an object and make it invisible, and acoustic metamaterials that can bend flames and levitate objects.
Zadpoor took advantage of a mechanical metamaterial with an equally strange property. Most conventional objects, such as a rubber band, get thinner when they’re stretched. But auxetic mechanical metamaterials get thicker. To envision how this works, imagine pulling a bowtie from its ends, which causes the middle portion to get thicker. Convert that two-dimensional example into three dimensions, and you have a honeycomb that gets thicker when it’s stretched.
To design a hip implant with the proper mechanical properties, Zadpoor’s team used computational geometry to design a hybrid femur stem that combined an auxetic metamaterial with a second, similar mechanical metamaterial that behaves like a normal material. They then used selective laser printing of a biomedical grade titanium alloy to fabricate six slightly different prototypes of the hybrid implant. The resulting hybrid femur stems, like real femur stems, were shaped like the head of a golf putter.
In the laboratory, the researchers subjected the hybrid implants to the same sort of forces they’d see in the body, and conducted full-field strain measurements to see how they responded. The two best prototypes delivered force evenly on both sides, they reported earlier this year in the journal Materials Horizons. In the body, this should let them fit snugly into the hip pocket, which could prevent bone loss and make the implant last longer, ideally for a lifetime.
“Attaching hip implants has been a very difficult problem for a very long time,” says regenerative biologist David Dean of Ohio State University. Having the implant press evenly against bone “could be a very important property,” he said.
“I’m intrigued and I’m rooting for him,” Dean says.
Dan Ferber is an independent writer.
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