Microprocessor Knees through Different Eyes

The O&P industry has taken on several new descriptors with the introduction of new technologies during the last few decades — power, microprocessor, bionic — to transform from what traditionally was an industry of craftsmen and artisans toward a computerized era of technology. Practitioners are more capable now than ever before to restore the ability to move naturally with confidence.

In the October 15 issue of O&P Business News, we spoke to some of the market leaders in powered O&P technology to gauge how far the industry has come and where it is headed. For this article we turned to researchers and practitioners using microprocessor knee technologies, in practice and study, to see what these technologies have done clinically to enhance the lives of their users.

Transfemoral challenges

Challenges facing transfemoral amputees are especially daunting. With the loss of the knee joint, practitioners need to do more with less; reduce metabolic cost while also allowing for efficient and safe gait functions and address the concerns associated with negotiating uneven terrain, stairs, ramps or inclines.

Kevin Symms, BSc (Hons), CO, BOCPO, director of clinical operations for Hanger Orthopedic Group Inc. explained that higher levels of amputation raise a population’s risk of suffering from falls and stumbles.

Microprocessor knee
Image reprinted with permission of Freedom Innovations.

“The propensity to a fall is statistically greater for a transfemoral amputee than a transtibial. As well as ensuring patients achieve their rehabilitation goals, the ability to retain balance and stability is also important,” Symms told O&P Business News.

Hugh Herr, associate professor of media arts and sciences at MIT echoed Symms’ comments also adding that the amount of energy expended by an amputee to ambulate is higher than normal.

“Another challenge transfemoral amputees face is that they typically walk more slowly,” Herr said. “If you ask an amputee to walk at their normal, most comfortable speed, it is slower than a person with intact limbs.”

All of these challenges affect the confidence of the user which, Symms explained, only intensifies the probability that any of the misfortunes, a stumble or a fall, for example, might take place.

“If you’re in a situation where you perceive a fall is going to happen than you might, from a patient’s point of view, tilt your head down and look down at the ground,” he said. “If you can walk in a confident upstanding situation like any non-amputee typically would, observing the ground but not necessarily concentrating on it, then you will typically do better.”

Microprocessor advantages

For Symms, patient security and confidence go hand-in-hand and from his perspective, many microprocessor knees offer not only an increase in patient security but also address many of the concerns presented by his patients.

“Stumble recovery is a great asset and improves security for our users,” he said. “If a patient is in a stumble situation, the resistance to flexion in the knee is sufficient to allow the patient to recover, to retain or regain balance or stability. In most non-microprocessor knees where a flexion moment is initiated, it is difficult to reduce that flexion moment and would typically result in a fall.”

Jason Highsmith, PT, DPT, CP, FAAOP, assistant professor at the University of South Florida, College of Medicine, School of Physical Therapy & Rehabilitation Sciences, who has experience studying Otto Bock HealthCare’s C-Leg and Ossur’s Rheo Knee also offers that most microprocessor knees that participate in studies such as those he has conducted with the C-Leg “seem to minimize the number of falls and the number of stumbles.”

In his research with the C-Leg he has found that patients are able to select a faster walking speed but that this fact can present a new dilemma to users.

“Just because people are able to walk faster and do walk faster doesn’t mean that it’s more efficient,” Highsmith said. “Most amputees … tend to select their most efficient walking speed to conserve energy. Here is a component that comes along that allows people to walk faster but not necessarily more efficiently … It might fatigue you a little bit more to maintain that speed but at least the knee will enable you to do that. So that could be a benefit from a quality of life perspective but it doesn’t necessarily make you more efficient.”

Herr, who has studied variable-damper microprocessor knees explained that his findings show that patients experienced an increase in stability but still struggled with the metabolic energy necessary to ambulate.

Microprocessor Knees: An Overview

SLK Self Learning Knee
Manufacturer: DAW Industries
Approximate Weight: 934 grams
Power Source: Rechargeable lithium-ion battery lasts up to six months depending on activity level
Intended User: K3, K4 amputees

Smart Adaptive Knee
Manufacturer: Endolite
Approximate Weight: 3 pounds
Power Source: On-board batteries last up to 14 days on a single charge.
Intended User: K3/4 amputees

SmartIP Microprocessor Knee
Manufacturer: Endolite
Approximate Weight: 2 pounds, 11 ounces
Power Source: Battery life expectancy 9-12 months under normal usage.
Intended User: K3 amputees

Plié Microprocessor Knee
Manufacturer: Freedom Innovations
Approximate Weight: 2.3 pounds
Power Source: One lithium-ion battery will provide approximately 36 hours of typical use
Intended User: K3, K4 amputees

Rheo Knee
Manufacturer: Ossur
Approximate Weight: 3.59 pounds
Power Source: Single lithium-ion battery facilitates 36 hours of constant use
Intended User: K3 amputees

Manufacturer: Otto Bock HealthCare
Approximate Weight: 1143 grams
Power Source: Rechargeable lithium-ion batteries hold 40-45 hours of power
Intended User: K3, K4 amputees

Compact Microprocessor Knee
Manufacturer: Otto Bock HealthCare
Approximate Weight: 1215 grams
Power Source: Rechargeable lithium-ion batteries hold 40-45 hours of power
Intended User: K2/3 amputees

4-Bar Power Knee
Manufacturer: Trulife
Approximate Weight: 2.2 pounds
Power Source: Battery that lasts up to 2 years depending on activity level
Intended User: K3, K4 amputees

Fusion Power Knee
Manufacturer: Trulife
Approximate Weight: 3 pounds
Power Source: Battery that lasts up to 2 years depending on activity level
Intended User: K3, K4 amputees

Single Axis Power Knee
Manufacturer: Trulife
Approximate Weight: 2.4 pounds
Power Source: Battery that lasts up to 2 years depending on activity level
Intended User: K3, K4 amputees


Anecdotal and empirical research

Research in the field of microprocessor technology has been ongoing for nearly a decade. Jason Kahle, CPO, LPO, staff prosthetist for Westcoast Brace and Limb, began researching microprocessor knees out of pure skepticism.

“My main concern was they were going to make amputees lazy walkers because the computer would take over motor function,” Kahle said. “The other aspect of it was that when we started this study insurance companies were still calling these experimental and weren’t going to pay for them.”

A lot of time has passed since his initial study in 2003-2004 which looked at the C-Leg to measure the effectiveness of the stance phase feature of the device.

“That is the function that made me skeptical of decreasing muscle function because the way an amputee controls stumbles and falls otherwise is with her or his muscles in conjunction with whatever mechanical function is available on a knee,” Kahle said. “From our study we found the two most important performance outcomes were the stumbles and falls. We found it decreased stumbles by 59% and decreased falls by 64%.”

As part of his study, patients were asked to complete a prosthetic evaluation questionnaire which addressed questions regarding walking on slippery surfaces and walking within tight spaces. Patients reported much better results with the C-Leg than with a mechanical knee.

Herr conducted a study comparing two microprocessor controlled variable-damper knees, the Rheo Knee and the C-Leg, to a passive knee focusing on moderate walking speeds on level ground to measure the benefit of the variable-damper knee.

“We found a modest difference in terms of … the metabolic energy required to walk,” Herr said. “I believe we found with both variable damper knees … that people were able to walk in a more repeatable fashion. If you imagine the footholds that people select when they walk — with the variable damper knees, amputees seemed to be able to select a consistent foothold from step to step. I believe there is also an enhanced smoothness in how the lower leg swings forward in the swing phase.”

Fusion Power Knee 4-Bar Power Knee Single Axis Power Knee
Trulife’s current microprocessor knee technology— From left: Fusion Power Knee, 4-Bar Power Knee, Single Axis Power Knee.

Individual comfort

While Kahle saw vast improvements within his study regarding stumbles, falls and walking speeds, he, along with many others, is quick to remind practitioners and researchers that this technology is not for everyone. Our sources agree that it is not possible to pigeonhole any one population as microprocessor device candidates as they are each created with different ends to similar means in mind.

Highsmith reported success with a range of populations from higher functioning community ambulators to a lower functioning transition group.

Kahle has found that the microprocessor knees are most beneficial in a high K2 or low K3 level patient.

“It seems like the market for microprocessor knees is that of higher functioning community ambulatory amputees,” Highsmith said offering that other factors might exist that might cause that amputee to choose something different.

Ultimately patient comfort and security are most important, according to Symms who has experience with one 82-year-old patient who was successfully fitted with a Compact stance control microprocessor knee, showing that geriatrics too, can benefit from microprocessor technology.

“Occasionally you will get an amputee who loves their passive knee,” Herr said. “They don’t see much benefit from the variable damper knees but I think that’s the exception.”

Kahle’s research revealed that many young female transfemoral amputees chose to reject the microprocessor knee used in his study because they could not wear high-heeled shoes or because the unit was heavier therefore causing a level of discomfort that they were not willing to endure.

“At the end of the day we can make our best guess as to who is going to be a candidate but you really don’t know until you try them,” Kahle said. “I’ve had them go both ways with people who I didn’t think would be candidates but I tried it because they requested it and they became successful users of the technology and I’ve had the opposite where I thought they were going to be good users, tried it and they weren’t good users.”

Gauging patient interest and candidacy as well as listening to their needs are a critical in implementing microprocessor technology.

Most times, the inclusion of a microprocessor knee component is met with positive response, according to Symms.

“Patients are able to carry out simple activities of daily living and typically use less effort and are able to do things … that they perhaps found difficulty with [before],” he said. “Patient responses are overwhelmingly positive.”

Endolite's Smart Adaptive Knee DAW Industries' SLK Knee Otto Bock HealthCare's C-Leg Otto Bock HealthCare's Compact Knee Ossur's Rheo Knee
Microprocessors across the market— From left: Endolite’s Smart Adaptive Knee, DAW Industries’ SLK Knee, Otto Bock HealthCare’s C-Leg, Otto Bock HealthCare’s Compact Knee, Ossur’s Rheo Knee.

The next wave

Before the future of this technology can be considered, Highsmith offers that currently technologies should all undergo the same critical and rigorous testing to support the reports being anecdotally shared throughout the industry.

“Anecdotally we have heard that some knees work better in some populations,” Highsmith said. “There are some knees, for example, that anecdotally work better in a higher functioning population and some might work better in a lower functioning group. What we found in one study of one knee does not automatically mean that is true in all knees or in all populations.”

Patient and practitioner feedback proves that the individual is so critical when considering the efficacy of a microprocessor knee, or any prosthetic component. While some sources reported, through their own observation as well as patient-offered information, concerns with these devices and specific areas where they hope the evolution of technology will make improvements, some have not had similar experiences. Among those interests: battery life, weight and cosmetic options.

“This is constant evolution,” Highsmith said. “Manufacturers are building better batteries so they can last longer. Hopefully they’ll continue to make [the devices] lighter and then the biggest things for people would be cost. Hopefully as more of them get out there, cost will come down.”

Kahle also has fielded questions regarding weight and size and adds that in the future he would like to see more incorporation between the knee component and the foot component.

“I think that is where things are heading– toward the incorporation of the knee and the foot,” he said.

The overall goal is to mimic the movement of the human knee, which these recent strides have become closer to reaching. Herr recognizes the challenges that such an imitation presents.

“The human knee can behave like a spring, like a damper, and can provide motive thrusting. Thus, for a prosthetic knee to truly mimic human knee behaviors, it would have to operate across all these mechanical domains,” he said.

The strides made by the profession so far shed a positive light on the future of the microprocessor technology field, however challenges still remain.

“Our goal is to present manufacturers with the challenge to further improve [technology] and provide a means to return amputees to even better levels of functionality approaching [levels] prior to amputation and ideally equaling [that level],” Symms said.

For more information:

Jennifer Hoydicz is the managing editor of O&P Business News.

Editor’s Note:
This story includes a small representative sample of individuals, companies and products. O&P Business News does not intend to promote individual companies or their products, nor to achieve an industry-wide consensus on the issue. Individuals contacted in developing this story were randomly selected.

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