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About 4 million people in the US are currently living with a total knee arthroplasty (TKA). That number is expected to increase over the next few decades as the population ages and as younger and younger patients are offered the procedure.

Survivorship for knee implants is between 90% and 95% at 10 years and between 80% and 85% at 20 years. But what if a patient lives longer than 20 years postoperatively? Is there a way to mitigate problems so that knee implants last longer?

Shahrzad “Sherry” Towfighian, PhD, believes there is, and she recently received a 5-year, $2.3 million grant from the National Institutes of Health’s National Institute of Arthritis and Musculoskeletal and Skin Diseases to continue research she began several years ago on sensors that can be used to monitor the status of knee implants.

Dr. Towfighian is an Associate Professor in the Department of Mechanical Engineering at Binghamton University’s Thomas J. Watson College of Engineering and Applied Science. Co-investigators on the project are Ryan Willing, PhD, from the University of Western Ontario, and Emre Salman, PhD, and Milutin Stanaćević, PhD, from Stony Brook University.

Monitoring Implant Performance

Total knee arthroplasty has a good track record of relieving the pain of osteoarthritis and restoring function of the joint. After surgery, many patients want to return to their previous level of activity.

That could put undue stress on the implant, depending on the activity. However, monitoring the knee from the outside yields limited data and generally prevents the surgeon from knowing that a problem is developing until the patient is symptomatic and may need a new implant.

With the sensors Dr. Towfighian and her collaborators have developed – which are designed to be incorporated into the knee implant – patients would use a smartphone app to collect data on implant performance in real time. This would allow the surgeon to continually monitor the load on the knee and intervene before the implant is damaged and a revision procedure is needed.

Solving an Energy Problem

Initially, Dr. Towfighian and her collaborators were challenged by the need for a long-term power source for the sensors. “You don’t want to use batteries, because batteries have a short life and you still would need surgery to replace [them],” she said.

“That doesn’t make any sense. You need to think of some sort of self-powered mechanism.”

The solution was to use a mechanism that harvests electrical energy from motion – in this case, walking – to power the sensors over the long term. The mechanism consists of 2 paper-thin plates that are 100 microns apart and that collect electricity from the friction of walking.

Dr. Salman designed the circuit and determined that it would need 4.6 microwatts to operate. Testing showed the average person’s walk will produce 20 to 70 microwatts of power, more than enough for the sensors. [1] This part of the research was complemented by work on the implant design and the package of the sensor by Dr. Willing. [2,3]

Next Research Steps

Another challenge Dr. Towfighian and her collaborators faced was ensuring that the self-powered mechanisms are biocompatible — for instance, not using lead or other materials that could cause damage to human tissue. Luckily, safe alternatives such as titanium and polyethylene can be used instead.

“So far, we have tested [the sensors] on a machine that simulates the walking motion. [4,5] The next phase is to test it on cadavers,” she said. “We need to simulate the motion for millions of cycles to see if it lasts and also to see how much data we can transfer in a 24-hour period.

“We will also need to investigate the load distribution and durability of the device. These are the things that need to be done before we move on to clinical studies.”

Dr. Towfighian and her collaborators are looking forward to this new research, which she hopes will have applications beyond knee implants to the wider field of flexible sensors.

“We have a very good team,” she said. “We’re all excited about the next phase of the project.”

References

1. Ibrahim A, Jain M, Salman E, Willing R, Towfighian S. A smart knee implant using triboelectric energy harvesters. Smart Mater Struct. 2019 Feb;28(2):025040.doi: 10.1088/1361-665X/aaf3f1. Epub 2019 Jan 25.
2. Jain M, Hossain NA, Towfighian S, Willing R, Stanaćević M, Salman E. Self-powered load sensing circuitry for total knee replacement. IEEE Sens J. 2021 Oct 15;21(20):22967-22975. doi: 10.1109/jsen.2021.3110241. Epub 2021 Sep 3.
3. Hossain NA, Yamomo GG, Willing R, Towfighian S. Effect of dielectric material and package stiffness on the power generation in a packaged triboelectric energy harvesting system for total knee replacement. J Biomech Eng. 2021 Oct 1;143(10):101009. doi: 10.1115/1.4051220.
4. Ibrahim A, Yamomo G, Willing R, Towfighian S. Parametric study of a triboelectric transducer in total knee replacement application. J Intell Mater Syst Struct. 2021 Jan;32(1):16-28. doi: 10.1177/1045389X20948581. Epub 2020 Aug 20.
5. Hossain NA, Yamomo GG, Willing R, Towfighian S. Characterization of a packaged triboelectric harvester under simulated gait loading for total knee replacement. IEEE ASME Trans Mechatron. 2021 Dec;26(6):2967-2976. doi: 10.1109/tmech.2021.3049327. Epub 2021 Jan 6.