A titanium-alloy spike is now part of my femur.
The five-inch-long forged hunk of metal came off the assembly line in a Memphis, Tennessee, factory in December 2017. Manufactured by a company called Smith and Nephew, the component is a model called Anthology. It’s one of four pieces that comprise my artificial hip.
That part, called a stem, joined my body in April of last year, at NYU Langone Orthopedic Hospital in Manhattan. A surgeon cut off the boney ball at the top of my femur, reamed out the socket in the part of the pelvis where the hip joint is located (anatomically, the “acetabulum”), and installed that stem, plus three other pieces. Later that same day, I left the hospital and went home with those components as new members of the body I was born with in 1978.
I’m one of hundreds of thousands of people in the U.S. who receive a total hip replacement each year. As a young person—I was 39 when it was installed—I’m an outlier, but also somewhat hip to a trend. The age of artificial hip recipients is falling: In 2000, the average age was just over 66; in 2014, it was 64.9. The fastest growing group? It isn’t retirees, but rather people ages 55 to 64, says Matt Sloan, a surgical resident at the University of Pennsylvania medical school who has researched the procedure’s demographic trends.
I wanted a new hip because I’d been in pain for years. In 2010, when I was in my early 30s, I tore my labrum, which is ring-like cartilage in the joint. A painful arthroscopic surgery in 2011 to repair it failed to make my hip better, and I needed a “revision” surgery. In 2014, a doctor did everything he could to repair the joint, and gave me a tissue graft from a cadaver to fix my re-torn labrum. Ultimately, that operation also failed. I needed a new hip. It was the weakest point in my body—an arthritic joint that had to go.
Over the decades, the materials in artificial hips have improved enough that doctors are now confident putting them in younger patients like me. “Surgeons, in the past, were unwilling to do a total hip replacement on them, because they thought they might be starting a sequence of multiple operations,” says Dr. Lawrence Dorr, a professor of clinical orthopaedic surgery at the University of Southern California Keck School of Medicine. Now, “they know they can do a hip replacement, and if it’s very well done, there isn’t any reason it can’t go 30 years.”
A series of failures, breakthroughs, and incremental improvements throughout the 20th century have led to a prosthesis so refined that bone can literally grow into it.
Here’s how it works: The four parts of a modern artificial hip replace a big ball and socket joint. During the operation, after the surgeon cuts off that ball at the top of the femur—the thigh bone, if you’re singing along—they insert a titanium stem inside the hollowed-out bone, with a portion of it still protruding. Then, a ceramic head (the new ball), attaches to the top of the stem; mine was made by German company Caramtec, and has a pinkish hue. On the acetabular side, a titanium-alloy shell shaped like a hemispheric cup press-fits into the reamed-out socket, and then a durable plastic (technically, cross-linked ultra-high-molecular-weight polyethylene) liner fits inside that metal cup.
So that the metal can join with my skeleton, part of the stem and shell have a coating of a substance called commercially-pure titanium. Bone can fuze with this porous layer, no cement needed, joining the natural with the artificial. “Bone looks at this surface and kinda sees itself,” Dorr, who’s replaced about 7,000 hips and 5,000 knees, explains. A few weeks post-op, immature bone will have already started growing into the metal. “I never figured out why bone is so dumb,” Dorr lightheartedly says. Grow into the titanium? Sure, why not.
A disjointed journey
Titanium and polyethylene are, of course, relatively modern developments. In the 1940s and ‘50s, both the materials and the procedure were in their infancies.
One early artificial hip came from a surgeon named Austin Moore. But his version was just a half hip: a metal replacement on the femur side, no artificial socket. “None of them were very effective,” Dorr says. (One of Moore’s implants, circa 1950s, is shown in the photo at top.) Part of the problem was how doctors attached the implant to the femur. “You just kinda pounded the stems into the bone.” The implants could work loose. Plus, the prosthesis fit directly into the natural socket—metal against bone. Ouch. “It probably was only 30 to 40 percent successful,” Dorr estimates.
Moore’s material selection process was also fairly homespun. Legend has it, when deciding whether to make the implant out of cobalt chrome or stainless steel, he buried samples of both in his backyard. When he dug them back up, the steel had rust, but the cobalt chrome did not. (Moore even mounted one of his fake hips to his Chrysler as a hood ornament.) “I don’t think the FDA would pass that today,” quips Dorr.
By the 1960s, hip replacements began to take on their modern form, thanks largely to a British orthopedic surgeon named John Charnley. According to former Massachusetts General Hospital hip surgeon William Harris, Charnley had an “incredible single focus—nothing else in the world mattered except solving this problem: creating an artificial hip joint.”
Charnley’s choices weren’t all perfect, but he had the right idea about a couple key things. First, he used bone cement to glue a metal implant into the femur. “That gave pain relief and a strong leg,” Dorr, of USC, says, “people could walk on it without limping.” Second, his first hips—he did about 300 in total—had an artificial socket so that the metal prosthesis wasn’t rubbing directly against the bone.
The best choice of material for the socket, however, still needed some figuring out. Charnley first tried Teflon, or polytetrafluoroethylene (PTFE), but it wore down quickly as the metal prosthesis rubbed against it, creating little particles. The connection between bone and implant didn’t hold up. “A loose implant like that hurts more than arthritis,” Dorr says. “The particulate debris kind of acts like a poison to the bone.”
Eventually, Charnley landed on a plastic called high-density polyethylene. It didn’t wear down as rapidly as Teflon, which he confirmed by testing the materials in his own leg. “After nine months in situ, the two PTFE specimens are clearly palpable as nodules,” he wrote in a letter to the Lancet. “They are almost twice the volume of the original implant.” Polyethylene had no such problem. Though it’s unclear when Charnley did the self-experiment, he completed his in-vivo testing before he put the new plastic in patients, a practice he began in 1962.
But even though Charnley’s invention got the basic conceit right, polyethylene eventually started causing serious problems for patients. Like Teflon, it wore down—just much more slowly—setting off a chain reaction in the body. Macrophages, a part of the immune system, gobbled up the plastic bits, which in turn led another type of cell, osteoclasts, to eat up nearby bone. The result is a problem called osteolysis, in which an implant can loosen, and the bone around it can even break.
“[It] was disastrous for many thousands of people and seriously disrupted the lives of more than a million,” writes biographer Harris in Vanishing Bone: Conquering a Stealth Disease Caused by Total Hip Replacements. He goes on to describe a patient who, in 1980, less than a decade after her total hip replacement, felt her leg bone simply snap while she was walking.
During the ‘90s, three teams, including one led by Harris, all worked separately to find a better material. Harris in particular was motivated because he’d had to reoperate on patients who had received artificial hips. “I told them to have the operation, and then the damned thing failed,” he remembers. Those complicated operations, called revision surgeries, could last as long as 12 hours.
Eventually, the groups each earned patents for cross-linked polyethylene, a plastic that’s made more durable at the molecular level. It’s now in millions of modern artificial hips. Under an electron microscopic, regular ultra-high-molecular-weight polyethylene looks like long strands of simple molecules of carbon and hydrogen wrapped and entangled with each other, but not firmly connected. Cross-linking causes adjacent molecules to connect with one another via strong covalent bonds in many different places, toughening the material. After cross-linking, the plastic hip liner is like “one huge polyethylene molecule,” says Harry McKellop, an orthopedic and biomechanical engineer and former VP of research at the Orthopedic Institute for Children in Los Angeles.
For today’s patients, the difference is measurable. The new stuff provides a 90-to-95 percent reduction in annual wear. In one double-blind study in New Zealand, patients who’d had old polyethylene hips for 10 years experienced an annual wear rate of 0.27 millimeters, while those with the cross-linked stuff wore at an average of just 0.03 millimeters per year. A recent paper in the Lancet found that 58 percent of the hips in its study lasted 25 years. Bear in mind, however: That number includes implants that predate the newest material, so the survival rate of modern models will likely rise; “Everything we’ve seen to date suggests that they are doing better,” says study author Jonathan Evans.
But, “the holy grail is the hundred-year poly,” says Roy Davidovitch, the surgeon at NYU Langone Orthopedic Hospital who did my hip replacement. “If you could do that, you could basically put in one hip replacement, and hopefully that will be it, and you could do it on younger and younger patients.”
Getting more hip
Today, a surgery that began with hundreds of failures is routine. In 2014—the most recent year for which data is available—370,770 people in the United States got a new hip. That number is increasing steadily: According to one recent study, by the year 2030, an estimated 635,000 people will receive a new artificial hip every year in the U.S.
It’s common, but is still major surgery. “You’re ripping out a big segment of the body, and replacing it mechanically—that is a massive assault on the human body,” Mass General’s Harris reflects. “And yet it has an extraordinary success rate.”
While decades of incremental improvement have zeroed in on the right materials and operative techniques for gifting patients with a synthetic joint, there’s still work to be done to perfect it. Today, one of the biggest problems with artificial hips is that they’re easier to dislocate than natural ones. Another is infection—a human-made hip has no blood flow, so bacteria can accumulate on it. Researchers are working on a fix, says Michael Alexiades, an orthopedic surgeon at the Hospital for Special Surgery in New York. One strategy is to “coat the implant with an antibiotic that’s bonded to the metal,” he says, which then can be released locally under the right circumstances. “That’s still in very early stages.”
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Ultimately, everyone is trapped in their body until they die. But when body parts start to fail, if it’s the right part, you can get a new one: Doctors can also swap out knees, shoulders, elbows, ankles, wrists, even the discs in your spine.
My new hip was installed in me in the morning in the springtime, and I went home later that day. It’s my third hip surgery, but the first I feel actually worked. It was a more dramatic operation than an arthroscopic one, but the recovery has been much easier. I’ve been in physical therapy for about 11 months, and, while I’m not totally pain free, my joint finally feels better. I’m glad that the nature-given part of me is gone. I’m glad the metal and plastic and ceramic is there in its place. For the first time in years, I have hope for my hip.