Nearly one million Americans will have hip or knee replacement surgery this year. They’re among the fastest growing procedures in medicine.
For most people, the implants function just fine, but sometimes, that artificial knee or hip needs taken out …
And in this week’s Exploradio, we investigate what those discarded devices can tell us.
Each year, orthopedic surgeon Dr. Matthew Kraay implants around 400 artificial hips and knees in his patients at University Hospitals, and he says, each year, for one reason or another, around 150 implants are taken out of people.
Those discarded parts end up in his very clean and very new looking lab.
Spread before us on a bench are metal knee joints caked with bone-like glue, plastic hip sockets, and dagger shaped metal inserts all recently removed from people's legs.
In an adjoining room, Kraay opens a bin filled with more used parts, "they’re all catalogued, stored in a bag…”
He has thousands more stored on shelves and in super-cold freezers representing more than 20 years’ worth of discarded artificial hips and knees.
“I started this when I was a resident and it’s just grown and grown and grown,” Kraay said.
Kraay is a surgeon.
He also has a degree in mechanical engineering, and, sometimes, he’s a medical device detective.
Digging for patterns of failure
Any implant can fail, says Kraay, but when he sees two or three similar cases, he says, it’s time to start digging.
“We don’t know what’s going on until we start to see patterns.”
Patterns that can be revealed by examining devices collected in Kraay’s archive.
For example, some artificial hips are being removed because of corrosion of the metal stem that holds the ball part of the ball-and-socket joint.
“What I’m doing now is, I’m going back," Kraay said.
He's pulling all of his cases from the archive that might have shown the same issue known as taper corrosion.
Kraay says this problem peaked around eight to ten years ago, "which maybe shows there was a change in the metallurgy.”
Metal and plastic in the body
Metal corrosion in the body is a bad thing.
The FDA in 2012 recommended that doctors stop using metal-on-metal hip devices because tiny bits of metal were wearing off, poisoning surrounding tissue.
That’s led to more reliance on another key component of artificial hips and knees, plastic.
Clare Rimnac is an engineering professor at Case Western Reserve University and a long-time collaborator with Dr. Kraay.
She uses the machines in her lab to test the used plastic parts that Kraay pulls out of patients.
That includes her beloved Instron axial torsion servohydraulic testing system.
"This bad boy allows us to pull, and compress, and twist, and we can pull and compress and twist at the same time,” says Rimnac.
Back in her office, she shows me a used hip joint taken out of a patient. “This is a good example of the kind of wear you can get with the plastic," she says, "do you feel those ridges?”
The deep grooves inside the artificial hip socket are hard to miss, and obviously not a good thing.
Rimnac says manufacturers altered the plastic formulation to avoid this kind of wear, but those changes then made it more brittle.
“This is a quintessential engineering conundrum," says Rimnac, "you make one modification to improve one of your design considerations, durability and wear, and you make another one of your design considerations, less good.”
Some device makers responded by making the plastic thicker to prevent cracking, but that means less room for the hip socket, and a greater chance of dislocations.
Forward, with an eye on the past
For orthopedic surgeon Dr. Matthew Kraay, there’s a continual interplay of providing his patients with the best technology and making sure that technology won’t cause problems down the road.
“Every couple of years we find a problem that shouldn’t be there," says Kraay, "and is it related to subtle change in the design, a subtle change in the material, things like that.”
Ideally, Kraay says, an implant should last a lifetime. We’re not there yet, but he says new devices are likely to last 20 years or more.
The implant archive he and Clare Rimnac created is now part of a 15 member national consortium of discarded devices.
Kraay says that’s important because the way forward often means taking a hard look back.
Support for Exploradio is provided by the Burton D Morgan Foundation.
