Revolutionizing Hip Replacement

If a researcher’s productivity can be measured by the number of articles he publishes, and the value of his research can be measured by the number of times those articles are cited, then Bill Harris is one of the most productive and valuable researchers the field of orthopedics has ever seen.

Harris’s publications over the course of his 60-year career exceed 600, and he is the author of the number one cited paper in the annals of orthopedic surgery. According to an article in the Journal of Bone and Joint Surgery, his 1969 paper on arthritis of the hip has been cited more than 1,786 times (as of 2011, when the article appeared).

Now 91, Harris made it his life’s work to enhance total hip replacement, a procedure pioneered by British surgeon John Charnley shortly after Harris completed his specialty training at Boston Children’s Hospital and Massachusetts General Hospital.

While Harris idolized Charnley, he realized Charnley’s technique “was far from a bed of roses” and set out to solve its various hitches—in the process sparing patients years of physical and financial setbacks and often even saving their lives. He did most of this work at Massachusetts General, where he led a lab and was chief of adult reconstructive surgery for three decades, and as a faculty member at Harvard Medical School, where he holds an emeritus endowed professorship today.

Harris details many of his endeavors, primarily focusing on his career-defining discovery of a mystifying condition called periprosthetic osteolysis, in his 2017 book, Vanishing Bone: Conquering a Stealth Disease Caused by Total Hip Replacements (Oxford University Press). He talked about his long career in a recent conversation with Haverford magazine:

Why hip surgery?

I chose hip surgery before total hip replacement existed because very few surgeons were operating on hips—and even fewer were doing it well. After Sir John Charnley introduced total hip replacement, we saw enormous positives but enormous negatives, too. I was up for the challenge of taking a very demanding procedure and making it better. Hip surgery’s long recovery period also fostered deep, intimate interactions with patients, and I found those relationships very rewarding.

What were the earliest problems you tackled?

Alarmingly, two percent of hip replacement patients were dying due to pulmonary emboli. Hip surgery often caused blood clots to form in the legs, and those clots could become lethal if they migrated to the lungs. Making an effort to fix a condition that doesn’t kill anyone— arthritis—and as a result killing one out of every 50 patients was unacceptable. After one of my own patients died in my arms from a pulmonary embolus, I vowed to see that this would never happen again. I initiated the first effort to prevent fatal pulmonary emboli following hip surgery by administering just the right amount of blood thinner to protect against clots without causing excessive bleeding. That practice brought the rate of fatal pulmonary emboli in hip surgery patients down from 20 in 1,000 [surgeries] to three in 1,000.

How did you get involved with implant design?

From the start of my career, I saw room for improvement in both the design of the various implants available and with the techniques of applying them. One major problem was the “bone cement” used to connect the implant to a patient’s skeleton. The fixation would loosen over time, causing joint dislocation. I co-designed the first successful cementless hip socket, which allowed the body’s bone to grow into pores in the implant and lock it in place biologically. This design is now universal.

You're best recognized for your role in identifying and curing periprosthetic osteolysis, the condition alluded to in your book title. Can you decode that?

With hip replacement patients, we started seeing severe bone erosion happening directly adjacent to their prostheses. People were returning years after their surgeries in extreme pain because their thigh bones were being eaten away. Early on, we thought bone cement was the culprit, but the destruction continued after we did away with the cement. It took more than 30 years to unravel what in the world this disease was.

How did you figure it out?

I asked patients to agree to give their implants back to me after they died. We used scanning electron microscopy to study what happened to the polyethylene plastic after years in the body and observed drastic changes. Initially, the material was amorphous—long, thin polymers jumbled haphazardly like spaghetti in a bowl. But the repetitive motion of walking was reorganizing the spaghetti into alignment and wearing the material down, causing tiny bits of plastic to shed. The body’s natural defense mechanisms didn’t know how to destroy these foreign particles, and in a panic, the immune system inadvertently activated cells called osteoclasts, which actually break down bones.

How were you able to stop that from happening?

I changed my lab from a biomechanics lab designing implants to a biomaterials lab looking for a better plastic that would not release those tiny particles. My friend Ed Merrill, a plastics engineer at MIT, suggested we use energy in the form of electron beams to crosslink the molecules, or fix them permanently into their positions. It was trial and error—we added too much energy and melted the plastic or set it on fire, and once even caused it to explode—but eventually we got it just right. It was a long process through FDA approval and to market, but in 1998, the first artificial hips using our highly cross-linked polyethylene were used with patients. Today, nearly eight million people walk on crosslinked polyethylene, and among them there has not been a single re-operation required because of this bone destruction. The disease is gone.

What have your advances meant in terms of economics?

Dislocation and bone erosion surrounding a hip prosthesis were the major reasons patients used to need repeat hip replacements, and now the number of repeat operations is way down. Just in the U.S., we used to spend $2.5 billion per year in repeat operations, but we’ve since cut that number in half. Cross-linked polyethylene has saved the healthcare industry more than $1 billion a year through the elimination of periprosthetic osteolysis. Throughout my career, I was motivated by saving lives, not money—but the financial implications have been an added bonus.

How is retirement treating you?

I spend a lot of time with my four kids and five grandchildren—but I’m still on staff at the lab and just submitted a paper last week, so even though I don’t operate anymore, technically I’m not retired!