Smart Knee Implants Show Promise in Post-op Care

Jacob Ziegler, MD, an orthopedic surgeon at the Mayo Clinic Health System’s Mankato, MN, center, is one of the first in the country to implant a “smart knee” device into patients who need total knee replacement; its utility for some patients, he believes, is clear.

The device, the Persona IQ, manufactured through a partnership of Warsaw, IN.-based Zimmer Biomet and Vancouver, B.C.-based Canary Medical, combines Zimmer’s Persona knee with Canary’s proprietary implantable canturio te tibial extension sensor technology. The sensor technology consists of two separate sensors; the primary sensor is an inertial measuring unit (IMU) that captures three-dimensional accelerations and gyroscopic measurements.

The secondary sensor is a pedometer that captures a patient’s steps and assists in turning the device on and off. Data measured include range of motion, step count, walking speed, and other gait metrics. Through an Internet-connected base station located within six feet of the patient’s sleeping area, the system transmits the daily kinematic data, which is encrypted both at rest and in transit, to his or her care team; no personally identifiable data is sent. The system, which received de novo approval in 2021, is now being rolled out by surgeons throughout the U.S. For Ziegler, the technology fits the bill for two cohorts in particular.

Related:FDA Clears Zimmer Biomet's Smart Knee Implant

“One of the things that came up in our conversations with the Zimmer [Biomet] team was how to serve a couple of very special populations in some ways unique to Mayo,” Ziegler said. “One is a very rural population in Minnesota that has, potentially, a lot of travel challenges – in winter, on the wrong day a 30-minute drive can become a three-hour drive. In Minnesota, northern Iowa, and Wisconsin there are patients for whom travel to even their local clinic may be still two to three hours on a nice day, let alone in winter.

“Then also, particularly at our Rochester center, it’s a destination medical center and people travel significant distances, and this is an opportunity for us to keep tabs on their recovery despite geographical barriers.”

Quantification breakthroughs

Another attractive element of the new implants for Ziegler is the ability to refer to more quantified data in assessing a patient’s recovery status.

“There’s interest in more detailed quantification of recovery data,” he said. “Ultimately, what matters is are they happy with their outcome. That’s the most critical thing. But then how do you determine who will be happy and what makes them happy?”

Related:Zimmer Biomet Launches Personalized Shoulder Implant

Current standards of practice often entail a nurse or physician asking patients how they feel via a questionnaire-type approach, but more data-intensive assessments are hard to come by: “That requires them to come in and be seen and it’s a fairly intense aspect of their recovery,” Ziegler said. “They need to be examined by a doctor or physical therapist. You just can’t get that level of fidelity with questionnaires.”

Ziegler, who estimated he has done about 25 Persona IQ procedures, said he has already noticed much more granular data from patients who have received the device.

“The level of fidelity is so much more finely detailed compared to me or my nurse calling them up and asking them how their knee is bending.”

However, it is still too early in the game for either the manufacturers, other researchers, or front-line clinicians like Ziegler to declare a definitive clinical benefit to the smart knee implant over older technology.

“The objective kinematic data generated by the CTE with CHIRP System are not intended to support clinical decision-making and have not been shown to provide any clinical benefit,” Zimmer Biomet said in supporting documents about the implant. But Ziegler said that disclaimer may be just a factor of time and the small number of Persona IQ’s already in use.

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“My simple answer is that it is too new for us to know that answer for sure,” he said. “From a pure clinically significant, very scientifically rigorous standpoint, it’s way too early to be able to say what’s a game changer and what is not.

“Earlier detection of somebody who is struggling in their early recovery and asking us to intervene will take a lot of numbers and that is a challenge we have in orthopedics. Cardiologists can get 100,000 patients’ data with ease. We have a harder time doing that. But the first likely potential benefit will be range of motion data in the first few months of recovery.”

Jim Lancaster, president of reconstruction at Zimmer, said the company is bullish on proving the benefits of the new devices.

“Persona IQ is a first-to-world technology and as such the data collected by Persona IQ has never existed before now,” Lancaster said in an email to MD+DI. “So, it stands to reason that no clinical benefit can be attributed to it – yet. We are actively investing in the research that will unlock our understanding of the full clinical benefits of Persona IQ.”

Infection detection

Though initial indications for the device do not include post-surgical infection detection, Ziegler said it might be possible to eventually have enough aggregate data to help pinpoint possible cases.

“That is something I’ve discussed with the Zimmer and Canary folks,” he said. “I think its certainly possible. Whether or not it will be able to get the fidelity to differentiate an early aseptic or non-infection loosening pattern from an infection pattern, I’m not sure. I think they may be able to. They think they’ll be able to.

“I think what we’ll see is that well-bonded ones which develop an infection will probably develop a fairly abrupt pattern change in their gait cycle. With enough volume and numbers it will get there. The hard part on that is infection rates are about 1% nationally, so you’ll need data from several thousand patients to clinically prove it and then tease out the difference between the two.”

Battery life, end-of-life signals & new research

At the other end of the post-operative spectrum, Zimmer and Canary have engineered the implant to send progressively less data: for the first year, data is collected and transmitted daily, whereas by Year 3, it is collected and transferred only for 36 consecutive days beginning on the surgery’s anniversary date. Though the device’s battery is listed as lasting 10 years, it could go longer. Whether it lasts long enough to signal that an implant is nearing the end of its useful life is an open question.

However, university researchers are already working on knee implants that would require no battery or stem that holds the circuitry and power (Ziegler estimates stemmed implants are suitable for only about 30% of his patients).

Shahrzad Towfighian, professor of mechanical engineering at Binghamton University in the State University of New York system, is leading a team that includes colleagues at SUNY’s Stony Brook campus and the University of Western Ontario in designing a smart knee implant that powers itself and needs no stem. The project received a $2.3 million, five-year grant from the National Institutes of Health in 2022. The team has already created a prototype, using triboelectric and piezoelectric principles, with sufficient power to make a working circuit.

“There is still a lot of testing that needs to be done to make it ready for commercialization,” Towfighian said. “But we have shown a working prototype. It can provide about 5 microwatts of power and our collaborators at Stony Brook have shown they can build a circuit that can be as low as 5 microwatts and process the data.”

The next steps in testing the device will entail testing it in cadaver limbs, but Towfighian said there are several factors that could broaden the market beyond stemmed implants that rely on accelerometers for readouts.

“If you can think of the area of the knee divided into four quadrants, we can see if any of the quadrants are receiving more force than the others,” she said. “That can be an early indication of loosening of the device just by comparing the forces in each quadrant, through different voltage readouts – because voltage is proportional to the force so you can detect loosening. You can’t get that from accelerometers in the Persona IQ.”

Lancaster said Zimmer is indeed aware of research such as Towfighian’s team is conducting and welcomes advances throughout the community.

“Our research teams are aware of the state-of-the-art technologies that can provide power to technologies like our Persona IQ implant,” he said. “The existing, battery-based approach remains the preferred approach for our smart implant program, but our research teams continue to follow these evolving technologies and will continually consider optimizing our program with them as they develop.”