The clinical effects of the Össur Power Knee with phase-based and default control during sitting, standing, and walking.

Background: A lack of evidence of compelling clinical benefits is a key factor limiting the adoption of commercialized powered robotic knee prostheses into mainstream clinical practice. Previous studies have demonstrated mixed results, potentially due to a combination of limitations in prosthetic hardware, control algorithms, and testing methodologies.

Methods: We investigated the clinical effects of a commercialized robotic knee prosthesis (the latest generation Össur Power Knee^TM) with n=7 above-knee amputee participants. Participants with both higher (K4) and lower mobility (K3) completed a series of experiments including repeated sitting and standing, a stand, walk, sit shuttle test, and fast walking on a treadmill. We tested both standard (ÖSSR) and novel (HKIC) control policies and compared the resulting clinical metrics to those found with the users' prescribed passive prostheses. Our experiments were physically demanding, which could help elucidate the potential benefits of powered knees.

Results: The clinical effects of the Power Knee varied with mobility level and the control policy used. The phase-based controller often produced stronger walking and sit/stand improvements for the higher mobility group compared to the default controller, though it also presented a steeper learning curve and reduced walk-to-sit transition speed. Conversely, the default control policy was perceived as easier to master but was less assistive to the higher mobility group and produced slower sit/stand cycles. Lower mobility participants experienced improvements in standing speed (HKIC: [Formula: see text]% faster, [Formula: see text]; ÖSSR: [Formula: see text]% faster, [Formula: see text]), inter-limb ground reaction force symmetry (HKIC: [Formula: see text], [Formula: see text]; ÖSSR: [Formula: see text], [Formula: see text]), and inter-limb peak knee moment symmetry (HKIC: [Formula: see text], [Formula: see text]; ÖSSR: [Formula: see text], [Formula: see text]) during sit-to-stand tasks relative to their passive prostheses. In contrast, higher mobility participants benefited less in sit/stand but showed improvements while walking including increased toe clearance (HKIC: [Formula: see text] mm, [Formula: see text]; ÖSSR: [Formula: see text] mm, [Formula: see text]), greater early stance knee flexion (HKIC: [Formula: see text], [Formula: see text]; ÖSSR: [Formula: see text], [Formula: see text]), and, for the HKIC policy, a reduced swing-phase peak hip flexion moment (HKIC: [Formula: see text] Nm/kg/(m/s), [Formula: see text]). Despite these biomechanical improvements and qualitative reports of reduced effort, neither control policy produced significant benefits in endurance or repeated task performance compared to the passive condition. Sit-to-stand cycle count in the lower mobility group was unchanged (HKIC: [Formula: see text], ÖSSR: [Formula: see text]), and it was reduced in the higher mobility group with the ÖSSR condition ([Formula: see text] fewer, [Formula: see text]). In the shuttle walk test, laps completed by higher mobility users decreased with HKIC ([Formula: see text] fewer, [Formula: see text]), and no significant differences were found for lower mobility users. No significant changes in fast walking distance or speed were observed across conditions.

Conclusions: The latest generation Power Knee can create clinical improvements in walking and sit/stand behaviors compared to passive (microprocessor) knees, though the effects are sensitive to the user's mobility level and the Power Knee's control policy. However, these improvements did not directly translate to improved functional performance or endurance. Some negative effects of the Power Knee were also observed including reduced agility, slower transitions, and thermal limitations, though some of these limitations could potentially be addressed through future control innovations or with more thorough acclimation. The observed benefits motivate future longitudinal studies to investigate the clinical effects of robotic knees compared to passive (microprocessor) knees in real-world settings and to elucidate how they could be best utilized in clinical practice.

Trial registration: The experimental protocol was approved by the University of Michigan Institutional Review Board (HUM00230065) on February 9th, 2024. The trial is registered with the National Institutes of Health under ClinicalTrials.gov ID NCT06138977.