Microprocessor-Controlled Knees: Evolution of a Game Changer

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By Miki Fairley

Photograph courtesy of Ottobock.

Since their arrival on the U.S. prosthetic stage in 1999 with the Ottobock C-Leg®, microprocessor-controlled knees (MPKs) have arguably been one of the most significant advancements in transfemoral clinical care in prosthetic history. The C-Leg has become the most scientifically researched prosthetic device ever, with more than 40 published research studies, and more than 40,000 users worldwide. Since that first breakthrough device, other manufacturers have joined the MPK marketplace, which now offers a plethora of choices to consumers, clinicians, and prescribing physicians.

Not only has MPK technology achieved the anticipated mobility benefits, research and user feedback has revealed additional positive outcomes impacting the overall health and well-being of patients, including increased stability and confidence, reduced cognitive burden, higher quality of life, and expanded activity levels. Several studies have even suggested that swing-and-stance MPKs may actually elevate some users' functional abilities to the next higher K-level when compared with nonmicroprocessor-controlled knees (NMPKs).

Doors Could Open to Wider World

"The relevance of [findings regarding improved K-level status] is that subjects who were previously considered unable to traverse more than low-level environmental barriers (MFCL [Medicare Functional Classification Level]-2), were able to achieve unlimited community ambulation and could walk with variable cadence (i.e., both level 3 traits), after exposure to the MPK," according to a systematic literature review, "Outcomes Associated with the Use of Microprocessor-Controlled Prosthetic Knees among Individuals with Unilateral Transfemoral Limb Loss: A Systematic Review," by Andrew B. Sawers, PhD, CPO, and Brian J. Hafner, PhD (the American Academy of Orthotists and Prosthetists State-of-the-Science Conference (the Academy's SSC) Proceedings: Microprocessor-Controlled Knees, October 2013) .

The prospect that MPK technology may be able to improve less functional users' abilities "challenges certain clinical and reimbursement paradigms," since historically the recommendation for patients with lower ambulatory potential has been to use less expensive, simpler prostheses, with more complex and expensive technology considered only for the most active individuals, the authors noted. One common requirement for MPK prescription and reimbursement is the ability to walk with variable cadence. However, some studies suggest that it is the MPK that allows certain patients to achieve variable cadence. "Thus, these patients may be denied access to the very technology that may allow them to qualify for it," Sawers and Hafner pointed out. "Though it is premature to suggest that this paradox exists based upon the existing evidence, these findings merit further investigation."

Photograph courtesy of Ottobock.

Fear of falling is one of the most common fears people with a transfemoral amputation face, especially on uneven terrain, in crowds, or with unexpected need for quick movement, but the stability of MPKs may meet that challenge. "Current published studies have indicated that individuals wearing a microprocessor knee have an 88.1 percent increase in confidence and security and 88.4 percent improvement in gait and maneuverability when compared to a passive or nonmicroprocessor knee," Dale Berry, CP, FAAOP, vice president, clinical operations, Hanger Clinic, headquartered in Austin, Texas, points out.

Sawers and Hafner's literature review indicates that, when compared to NMPKs, swing-and-stance MPKs significantly decrease the number of subject-reported stumbles and falls and increase balance confidence. Frequency of falls decreases by as much as 64 percent with use of MPKs. For MFCL-2 subjects in one study, "an even more impressive 80 percent decrease was noted."

Individuals' biomechanical responses to physical perturbations with the NMPKs they wore originally as compared with the swing-and-stance MPKs they wore later similarly show improvements in standing and walking balance using MPKs. "[These results] may have may have long-term healthcare and cost implications by helping to prevent falls and resulting injuries."


DAW® Industries, San Diego, California, U.S. distributor for Teh Lin Prosthetic & Orthopaedic, New Taipei City, Taiwan, revealed the new Stealth™ MPK, which utilizes triplanar linear acelerometers to accurately evaluate all positions and functions for greater safety and amputee control.
Chas A Blatchford & Sons, headquartered in Basingstoke, England, is developing an intelligent prosthesis integrating the ORION knee with the microprocessor-controlled elán foot, for expected release in 2015, according to Saeed Zahedi, PhD, Blatchford Clinical Services' technical director. The new product, Linx®, uses sensors from the elán and ORION to input information to the master controller for decision making. The Linx uses more sensor density and a new output strategy for actuator controls of both the ORION and elán, Zahedi says. Blatchford's U.S. branch, Endolite North America, is located in Miamisburg, Ohio.
Freedom Innovations, Irvine, California, will release a new version of the Plié knee this year featuring better water resistance, easier programming, and enhanced durability, according to Hogni Fridriksson, director of marketing and clinical services.
Nabtesco, Tokyo, Japan, is unveiling its third-generation knee, the Allux®, later this year, according to Leslie Roberts, MSS, CP, clinical and technical service manager for Nabtesco Proteor - USA, Muskego, Wisconsin. A polycentric stance-and-swing phase hydraulic MPK, the water-resistant knee will allow shower use and be programmable for various sports and activities. Nabtesco Proteor - USA is a partnership formed by Nabtesco and Proteor, Saint-Apollinaire, France.
Össur, Reykjavik, Iceland (Össur Americas, Foothill Ranch, California), launched its third-generation MPK, the RHEO KNEE® 3, featuring advanced kinematic sensor technology to increase stance and standing stability. It thus expands the user profile to include the full range of K3 users, including low K3 users who typically received hydraulic-controlled microprocessor devices in the past, according to Kim De Roy, CPO, PT, Össur Americas' vice president of sales and marketing and education for prosthetics.
Ottobock, Duderstadt, Germany (Ottobock USA, Minneapolis, Minnesota), developed its latest MPK, the X3, in cooperation with the U.S. military. Completely waterproof, the X3 can switch into a larger swing phase angle for running and other sports; advanced technology includes an accelerometer and gyroscope to help prevent stumbles and falls. The knee enables walking backward, crossing obstacles with more natural movement, and climbing stairs step-over-step without excessive compensating movements; a mute mode can silence vibration and beep signals.

Clinicians often note that microprocessor control's sophisticated software algorithms reduce cognitive burden-users don't need to literally "watch their step," freeing up their minds for other things. This may allow them to be more active, spend more time with family and friends, increase employment options, and improve quality of life overall.

Even within the K2 classification, increased mobility can improve cardiopulmonary health. In one promising case study, a geriatric patient with a K2 knee was walking with a low cardiac capacity that would normally contraindicate ambulation. "Following microprocessor-controlled knee accommodation and physical therapy training, he reached a higher level within K2 to the point he could care for his disabled wife," says M. Jason Highsmith, PT, DPT, PhD, CP, FAAOP, assistant professor, and codirector of the Center for Neuromuscular Research at the University of South Florida Health Morsani College of Medicine, Tampa. "So if a higher-level component along with physical therapy training enables a patient to fulfill more roles in their life, then indication in lower functioning patients should be explored further."

Technology Leaps Forward

In the 15 years since the C-Leg was introduced, MPK technology has been taking advantage of the increasing sophistication of software; the fast evolution of microprocessors, which have been doubling in power about every 18 months without increasing the cost; better sensor technologies; and longer-lived lithium-ion battery technology. A recent scientific report notes that software algorithms have improved by up to 100 times over the last two decades. MPK manufacturers have also been improving and miniaturizing hydraulic, electric, and magnetic controls for better performance. One power-assisted MPK to help users replace lost muscle power is currently clinically available, and others may soon follow. Highsmith says, "A huge amount of power assist can be built in, but if a patient is not comfortable with the technology, he [or she] may not use it. Well-trained users would probably benefit, but the literature is still emerging and there are reimbursement concerns. The other approach, such as being used in Ottobock's Genium®, is capitalizing on the hydraulics already in place in yielding knees. The difference is that the hydraulics are there in the event of a safety compromise, but are out of the way when patients want to use their own muscular power-basically this approach helps users train their own muscles and joints, but the hydraulic safety functions are there if needed."

DAW Industries

Popularity Grows

Patients and prosthetists have increasingly embraced MPKs. "There certainly has been a huge patient demand," says Gary Berke, MS, CP, FAAOP, adjunct clinical assistant professor, Department of Orthopaedic Surgery, Stanford University, California, and owner of Berke Prosthetics, Redwood City, California. "The quality of the knees themselves has gone up, and there's been more acceptance from insurers based on the literature that we have, so it has become much more a standard of care than it was 15 years ago."

"Microprocessor knees have become the standard of care in the prosthetic community," says Berry. "Clinical studies and patient experience and outcomes have overwhelmingly validated the numerous benefits and advantages of the microprocessor knee to provide optimum stability, function, and prosthetic reliability." Berry says that, according to published Medicare utilization data, less than 1 percent of Medicare reimbursed transfemoral prostheses were MPKs in 2000, but in 2011, more than 30 percent of transfemoral prostheses include an MPK, according to the same reporting parameters.

Which Patients Are MPK Candidates?

Berke, referring to the published Academy SSC findings on MPKs, says, "It seems, clinically, that the optimal candidate is someone who would benefit from the advantages of microprocessor control-including decreased cognitive load and improved safety. We are beginning to obtain studies that better demonstrate the benefits of MPKs in those two areas. It makes sense for higher-level K2 patients (who can handle the weight and complexity of the device) through the mid- and high-level K3 patients (who put some demand on the knee but not so much that the impact of their activity, sport, or water use would preclude an MPK) to be fitted with this technology. So we believe the evidence supports optimal candidates as being high K2 through mid- and high-level K3."


Mark Geil, PhD, associate professor in the Department of Kinesiology and Health, Georgia State University, Atlanta, noted in his paper published in the SSC proceedings on MPKs, "Recommendations for Research on Microprocessor Knees," that two main domains received much attention in the conference: research to improve outcome measures relevant to MPKs, and research to predict candidacy for MPKs. Among other recommendations, participants urged researchers to develop valid and reliable outcome tools that can predict which people will be successful with MPKs. "The prescription criteria for MPKs should be better informed by this research, including specific indications and contraindications. It is equally important to understand who is a candidate for an MPK and who is not a candidate."

Clinicians note that mechanical knees may be the more appropriate choice for some patients, since individual functional needs, goals, circumstances, financial resources, and preferences vary widely.

MPKs Evolve

During the 1970s and 1980s, Massachusetts Institute of Technology (MIT) Professor Woodie Flowers, PhD, and graduate students began developing a computer-controlled prosthetic knee. "Refrigerator-sized computers operated a tethered knee in a preview of things to come," said David Boone, CP, MPH, PhD, in a December 2008 article, "Looking Ahead at Computer- Controlled Knees," inMotion. "All the basic ideas were there, even if suitable microprocessors were not."


In 1989, Kobe Steel, Tokyo, Japan, demonstrated the first clinical application of microprocessor control to prosthetic knee mechanisms. Chas A Blatchford & Sons, headquartered in Basingstoke, England, obtained the license and commercialized the knee, marketing the first MPK in the world, the Endolite Intelligent Prosthesis (IP) in 1993 ("Perceived Stability, Function, and Satisfaction among Transfemoral Amputees Using Microprocessor and Nonmicroprocessor Controlled Prosthetic Knees: A Multicenter Survey," Dale Berry, CP, FAAOP, et al., Journal of Prosthetics and Orthotics, January 2009).

In the early 1990s, Kelly James, an engineer at the University of Alberta, Edmonton, Canada, developed the C-Leg, the first leg with microprocessor-controlled swing and stance phases. Buying the rights from the university, he traveled around the world to interest prosthetic manufacturers in his invention ("A Leg Up," by Isabelle Gallant, U of A Engineer, Spring 2011). However, he didn't receive any commercial interest until German manufacturer Ottobock bought the patent in 1992 and launched the groundbreaking technology.

Other companies have since joined the marketplace with their own MPK versions and continue the technological advances.

Reimbursement Issues

As has been the case in other areas of O&P, reimbursement issues and gaps in research are impediments to technological advances and patient access. "Patients' acceptance and desire for these products are probably at an alltime high. We want to provide safety and functionality to our patients, but payers often allow providers to only fit components that may actually decrease the patient's safety and functionality," says Leslie Roberts, CP, MSS, clinical and technical service manager, Nabtesco Proteor - USA, Muskego, Wisconsin. However, she believes advancements in technology will continue. "There are too many extremely bright engineers with great ideas." But Roberts fears that only patients who can pay out of pocket will have access to higherlevel technology, "furthering the gap between 'haves' and 'have nots.'"


Since technology costs are rising, and Medicare and other payers are demanding savings, the provision of MPKs may fall under even more intense scrutiny regarding medical necessity and benefits not available in lower-cost devices, Berke says. "We need to justify the cost by showing that the technology saves money, whether by preventing more falls, keeping people from fracturing a hip, keeping them healthier and out of the hospital, enabling them to continue employment, being able to do more activities of daily living, and caring for their families. If insurers pay for the technology, they'll want to be able to save money somewhere else."

Should K-levels Be Updated?

In his summary of the Academy's published SSC recommendations on MPKs, Geil writes,"Candidacy for an MPK is often based on K-level, but these are broad classification bins that cover a range of function. Because some research...has noted the potential for some patients classified at K2 to benefit from an MPK, future research should consider a more descriptive functional cutoff for MPK candidacy."

"Although the functional K-level criteria developed by Medicare in the 1980s have been helpful in identifying the activity levels and subsequent component selection, they are debatably outdated now due to the number of components and introduction of technology over the past 30 years," Berry says. He points out specifically the wide range of ages and activities that fall into the K3 level. A more robust and accurate functional evaluation method would better identify individual patients' needs in order to establish appropriate components and technology, Berry believes. He adds that many national insurance payers have adopted Hanger's Patient Assessment Validation Evaluation Test (PAVET™).

Freedom Innovations

Future Directions

The 2013 SSC proceedings clarified future directions in research and urged development of an MPK classification system to differentiate between the three basic types of MPKs (swing only, stance only, both stance and swing) and highlight functional outcome differences between MPKs in each category. A preliminary classification system was proposed, and SSC participants encouraged the manufacturing, clinical, and research communities to work together to formulate a classification system to better differentiate MPK functions and allow appropriate comparison for improved function and research.

On the technology side, neural control, whether by myoelectric, brain-computer interface, or other modes, looms in the future as MPKs will play a part in robotic and neurally-controlled prosthetic systems.

Research and development continues in this area, which has already seen striking advances in upper-limb prosthetics. For example, Zac Vawter achieved a prosthetic milestone when he used the world's first neurally-controlled, powered prosthetic limb to climb 103 floors of Chicago's Willis Tower in 2012. The bionic leg was developed at the Vanderbilt University Center for Intelligent Mechatronics, Nashville, Tennessee, directed by Michael Goldfarb, PhD. The Rehabilitation Institute of Chicago, Illinois, developed the neural controls, using targeted muscle reinnervation surgery. Vanderbilt has licensed its powered transfemoral prosthesis to Freedom Innovations, Irvine, California, for commercialization. "Many exciting technologies are being developed that may give patients the opportunity to use neural-control prosthetics in the future," says Hogni Fridriksson, Freedom's director of marketing and clinical services.

Miki Fairley is a freelance writer based in southwest Colorado. She can be contacted via e-mail at