Dynamic Stability and Cognitive Burden: Are We Properly Evaluating the Benefits of Microprocessor Knees and Feet?

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By Seth O’Brien, CP, FAAOP

As microprocessor-controlled knees (MPK) and feet (MPF) are established as the standard of care for people with lower-limb amputations, it is crucial that the clinical benefits of the devices be accurately described and fully understood. MPKs produce significant improvements to hip, knee, and ankle kinetic symmetry compared to non-MPKs, normalizing gait and reducing the risk of osteoarthritis.1-3 MPF allow for adaptation to uneven or sloped terrain, increasing stability, and reducing peak pressures within the socket.4 However, the ability of MPKs and MPF to prevent falls, and fall-related injuries, is perhaps the most important clinical benefit. In 2013, Kahle et al. reported non-MPK users have an 82 percent probability of falling per year, while MPK users have only a 26 percent probability.5 In 2017, Liu et al. completed analysis that suggested non-MPK users were at much greater risk for injury than MPK users.

More specifically, the non-MPK incident rate of minor injurious falls increased 488 percent; the incident rate for major injurious falls increased 473 percent; and the incident rate of fall-related deaths increased 467 percent.6 While improvements in outcomes relating to gait symmetry, fall prevention, and stability for microprocessor devices have been established, relatively new measures may add to their clinical impact. Because lower-limb prostheses are used in constantly changing environments on dynamic human bodies, it is important to measure and assess how prostheses influence the entire human body. Two concepts that could quantify additional benefits of microprocessor devices are dynamic stability and cognitive burden.

Dynamic Stability
The concept of dynamic stability is not new to researchers but may be unfamiliar to most clinicians. Our ability to remain stable during ambulation is challenged constantly by our environment. Uneven terrain, ramps, curbs, and obstacles require the human body to react as a system to maintain dynamic stability. Considering whole-body dynamics when assessing a prosthesis is crucial. Too often, the focus is limited to local joint mechanics, which may limit the understanding of how the prosthetic device is interacting with the dynamic body. As clinicians, we may improve patient outcomes by focusing on dynamic stability and how it is affected by the prescribed prosthesis.

One measure of dynamic stability can be described as the regulation of whole-body angular momentum (). Analysis of is useful in identifying coordination strategies that may increase the likelihood of a fall. When investigating the effects of a powered prosthesis on gait in 2017, Pickle et al. found that although the powered prosthesis "increased the mechanical power generated by the prosthetic ankle during push-off, this resulted in relatively small changes in H."7 This study highlighted the fact that simply restoring ankle biomechanics may not significantly improve whole-body dynamics. 

A second approach to studying dynamic stability is known as the Lyapunov exponent (l), or the stride-to-stride fluctuations that occur over a series of steps. The prosthetic limb has been shown to have a greater l about the ankle than the sound side limb.8 Decreased l has been strongly correlated to the patient's prosthesis preference.9 Furthermore, l has been used to identify fall-prone individuals, and there is growing evidence that this measure may be able to estimate the probability of falling.10-12

Cognitive Burden
The amount of brain activity required to complete a specific task is often referred to as the cognitive burden of said task. People with amputations lack the sensory feedback and balance strategies that are fundamental components of mobility—specifically at the knee and/or ankle depending on amputation level. This is observed clinically in amputees who feel the need to focus on each step to make sure they don't fall. The relationship between cognitive processes and prosthetic use is one that should be explored further to fully understand the implications of prosthetic knee and foot selection.

Research surrounding cognitive burden may utilize performance-based techniques requiring a dual-task (such as a Stroop test) or measurement techniques (such as optical brain imaging). The Stroop test will be further discussed in the following section. Optical brain imaging has been used to show reduced cortical brain activity during ambulation with MPKs compared to non-MPKs.13 Additionally, reduced cognitive burden and energy expenditure with MPKs may be due to the ability of the knee to dynamically adjust to uneven terrain compared to non-MPKs. Improved stability and user confidence have also been reported with MPKs.14

The limited research around dynamic stability and cognitive burden in amputees has demonstrated strong clinical benefits for MPKs. However, further studies are needed to see if the same benefits are present with MPF (including powered and passive devices).

Pilot Study
In the summer of 2017, Arizona State University Professor Thurmon Lockhart, PhD, and Jeffrey Ward, PhD, director of research and development at SpringActive, a human locomotion technology company, collected pilot data on measuring dynamic stability between a powered plantarflexion MPF (the College Park Odyssey Power Ankle) and a traditional energy-storing foot (non-MPF). In their pilot study, one 65kg subject walked on the GRAIL treadmill while motion-capture and force-plate data were collected. The subject walked for three minutes at a fixed, comfortable pace comparing the non-MPF to the Odyssey Power Ankle. The subject performed these trials on +/- 5-degree slopes, level ground, and level ground with a dual task. During the dual-task three-minute level ground walking session, additional cognitive loading was placed on the subject by performing a Stroop test.

In the Stroop test, the subject must say the color of a word but not the name of the word. For example, the word "white" may appear on the screen in a blue font. The subject should respond with the correct verbal answer, "blue." The Stroop effect refers to a phenomenon in which it is easier to say the color of a word if it matches the semantic meaning of the word.

This testing found significant improvements to dynamic stability with the Odyssey Power Ankle for +/- 5-degree inclines and Stroop testing (Table 1). The Odyssey Power Ankle used in the pilot study has a control system that adapts to the user's walking dynamics in real time. Per Ward, "One interpretation of these results is that this adaptability is important across multiple environments. Specifically, the evidence of the Stroop testing may indicate that the MPF is lowering the cognitive burden of gait by adapting to the distracted user in real-time, improving stability and reducing fall risk. As both the Lyapunov exponent (l) and whole-body angular momentum () are measures of dynamic stability, we would expect similar improvements in (). This hypothesis should be verified with further testing, and more information is certainly needed to determine the underlining mechanisms for improving walking stability."

Seth O'Brien, CP, FAAOP, is the clinical director of Artificial Limb Specialists' Mesa, Arizona, clinic location, He is a member of the American Academy of Orthotists and Prosthetists (the Academy) Lower Limb Society.

Academy Society Spotlight is a presentation of clinical content by the Societies of the Academy in partnership with  The O&P EDGE.


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