Dartmouth Engineer - The Magazine of Thayer School of EngineeringDartmouth Engineer - The Magazine of Thayer School of Engineering

Dartmouth Engineering MS Student Works to Build a Better Hip

By Anna Fiorentino
October 2013 • Thayer By Degrees: MS

Robert Cunkelman’s osteoarthritis diagnosis at age 35 inspired his son Ben Cunkelman Th'13, '14 to enter Thayer School’s Master of Science (MS) program and begin research on artificial hip replacements. Under Professors John Collier and Douglas Van Citters, Cunkelman is now trying to answer a question on the minds of many in the field of orthopedics today: How are the materials used in artificial hip implants wearing down within the body?

“My dad had two hip replacements four months apart,” says Cunkelman. “It was really empowering for me to see the positive change in his quality of life after the surgery.”

It was an anomaly that Cunkelman’s father had spent hours researching the benefits of different materials, and no coincidence that, like his son, Robert is an engineer. Most patients aren’t as thorough. As a result, some experience complications or need revision surgery which has a slightly lower success rate. “It’s like buying a house, because you’re going to live with it for a long time,” says Cunkelman.

One of the primary materials used in joint implants is called ultra-high molecular weight polyethylene (UHMWPE). Cunkelman is investigating how different generations of UHMWPE wear down within the body.

hip socket model
3D-rendering in Geomagic of a retrieved artificial hip implant.

To do this, he is using reverse engineering techniques to create models of retrievals—joint implants removed from patients for various reasons. “It’s like forensics,” says Cunkelman. “You can take a retrieval and hold it in your hand and see the part where the polyethylene wore and how the material moved over the period of time it was in the patient.”

Through high resolution scanning and mapping of the implant in 3D-rendering software called Geomagic, Cunkelman can begin to understand more about the variety of factors leading to premature wear of UHMWPE in the body.

Joint implant engineers and surgeons began exploring the use of polyethylene in the early 1970s. Since then, four generations of the material have been developed, each one less susceptible to wear and oxidation—when oxygen reacts with unpaired electrons called free radicals—than the last. The first generation polyethylene, used until 1995, was gamma sterilized in air and began to oxidize and degrade even before reaching the patient. The second generation was gamma sterilized and then barrier packaged against oxygen, and the third was cross-linked through exposure to radiation and thermally stabilized against further chemical changes.

“When you take polyethylene that hasn’t been cross-linked and you irradiate it and then subsequently re-melt it, you are left with a material that is more wear resistant and less susceptible to oxidation,” says Cunkelman. “On the other hand are older materials that haven’t been cross-linked. These tend to show much greater wear, which is easier for validation of my research.”

Before his current research, Cunkelman’s first-year MS research project had him analyzing the performance of fourth generation polyethylene which is infused with Vitamin E, an antioxidant, to stabilize the material from oxidation.

“Some think Vitamin E could solve one of the largest problems with polyethylene as an orthopedic material, but it’s still so novel that it’s hard to confidently declare it a success,” he says. There is more work to be done, says Cunkelman, who is thankful to be part of potentially life-changing research.

“To do work on a day-to-day basis with people in the lab like professors Collier and Van Citters, Dr. Michael Mayor and John and Barbara Currier, and see the inner workings of this industry and the science behind it, has been an amazing opportunity,” adds Cunkelman.

Tags: alumni, engineering in medicine, faculty, research, students

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