Research Article: Bimodal ankle-foot prosthesis for enhanced standing stability

Date Published: September 26, 2018

Publisher: Public Library of Science

Author(s): Sara R. Koehler-McNicholas, Billie C. Savvas Slater, Karl Koester, Eric A. Nickel, John E. Ferguson, Andrew H. Hansen, Alena Grabowski.


Previous work suggests that to restore postural stability for individuals with lower-limb amputation, ankle-foot prostheses should be designed with a flat effective rocker shape for standing. However, most commercially available ankle-foot prostheses are designed with a curved effective rocker shape for walking. To address the demands of both standing and walking, we designed a novel bimodal ankle-foot prosthesis that can accommodate both functional modes using a rigid foot plate and an ankle that can lock and unlock. The primary objective of this study was to determine if the bimodal ankle-foot system could improve various aspects of standing balance (static, dynamic, and functional) and mobility in a group of Veterans with lower-limb amputation (n = 18). Standing balance was assessed while subjects completed a series of tests on a NeuroCom Clinical Research System (NeuroCom, a Division of Natus, Clackamas, OR), including a Sensory Organization Test, a Limits of Stability Test, and a modified Motor Control Test. Few statistically significant differences were observed between the locked and unlocked ankle conditions while subjects completed these tests. However, in the absence of visual feedback, the locked bimodal ankle appeared to improve static balance in a group of experienced lower-limb prosthesis users whose PLUS-M mobility rating was higher than approximately 73% of the sample population used to develop the PLUS-M survey. Given the statistically significant increase in mean equilibrium scores between the unlocked and locked conditions (p = 0.004), future testing of this system should focus on new amputees and lower mobility users (e.g., Medicare Functional Classification Level K1 and K2 prosthesis users). Furthermore, commercial implementation of the bimodal ankle-foot system should include a robust control system that can automatically switch between modes based on the user’s activity.

Partial Text

Postural stability, defined as the ability to control the body center of mass (COM) within a given base of support, is essential to many activities of daily living [1]. Among healthy individuals, a combination of physiological systems (i.e., visual, vestibular, and somatosensory) are used to maintain postural stability. Together, these systems regulate motor control strategies used to limit body COM movements (i.e., static postural stability), voluntarily shift the body COM within the base of support (i.e., dynamic postural stability), and control the body COM in the presence of a perturbation (i.e., functional postural stability). When one or more of these physiological systems are compromised, the capacity for individuals to appropriately control their body COM is diminished. For example, numerous studies have reported that somatosensory losses associated with a lower-limb amputation often lead to poor balance and balance confidence [2–4], resulting in significant barriers to community participation, health outcomes, and quality of life. The capacity for lower-limb prostheses to restore postural stability is therefore an important aspect of rehabilitation among this population.

Previous work suggests that two distinct functional modes are needed for ankle-foot prostheses to restore both effective walking mobility and stable standing balance [5]. To address the demands of standing and walking in individuals with lower-limb amputation, our group designed a novel bimodal ankle-foot prosthesis that can accommodate both functional modes using a rigid foot plate and an ankle that can lock (resulting in a flat effective rocker shape for standing) and unlock (resulting in a curved effective rocker shape for walking). The goal of this study was to examine the effect of the bimodal ankle-foot system on balance and mobility in a group of Veterans with unilateral transtibial amputation. Overall, we expected that subjects would exhibit an improvement in static, dynamic, and functional balance when using the bimodal ankle in the locked mode compared to the unlocked mode and that the unlocked bimodal ankle would perform similarly to the user’s usual prosthetic ankle-foot system during functional walking tasks.




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