The goal of this proposed project is to gather community-based data from the K4-level Transfemoral Amputee (TFA) population to aid in evidence-based prescription of powered prosthetic knees (i.e., choosing the right device to maximize the benefit for each patient). The investigators envision that this Level 1 submission will transition into a larger follow-on Level 2 trial that will explore a larger spectrum of patient populations (K2-K4), as well as testing additional Power Knees currently in development that are expected to become commercialized in the near future. The investigators intend to use this Level 2 trial data to guide the implementation of effective prescriptions towards those that can benefit most from a given device and limit prescription to those who would not see benefit in order to ensure the most judicious use of Department of Defense (DoD) and Veteran's Affairs healthcare dollars. The findings will also be shared with the research community to help drive the design of future devices by identifying what features and functions are most beneficial to which patient populations when the devices are used outside of the laboratory. In summary, more community-based data on how powered prosthetic knees compare with MPKs is needed to allow for improved clinical decision making and clinical outcomes.

The goal of this proposed project is to gather community-based data from the K4-level Transfemoral Amputee (TFA) population to aid in evidence-based prescription of powered prosthetic knees (i.e., choosing the right device to maximize the benefit for each patient). The investigators envision that this Level 1 submission will transition into a larger follow-on Level 2 trial that will explore a larger spectrum of patient populations (K2-K4), as well as testing additional Power Knees currently in development that are expected to become commercialized in the near future. The investigators intend to use this Level 2 trial data to guide the implementation of effective prescriptions towards those that can benefit most from a given device and limit prescription to those who would not see benefit in order to ensure the most judicious use of Department of Defense (DoD) and Veteran's Affairs healthcare dollars. The findings will also be shared with the research community to help drive the design of future devices by identifying what features and functions are most beneficial to which patient populations when the devices are used outside of the laboratory. In summary, more community-based data on how powered prosthetic knees compare with MPKs is needed to allow for improved clinical decision making and clinical outcomes.

This study will focus on investigating the tradeoffs between the commercially available Ossur Power Knee and traditional MPKs to identify the prosthetic knee that is best suited for K4 users during different tasks. The investigators will assess measured, observed, and self-reported outcomes achieved through real-world use of the prosthetic knees.

The results of this study will (1) provide initial data to inform the evidence-based prescription and use of powered knee technologies for K4-level individuals with sound scientific data, (2) provide pilot data for a larger clinical trial to further inform the evidence-based prescription of powered knee technologies to other K-levels to maximize patient benefit while optimizing device cost and thereby influencing clinical practice and informing policy decisions, and (3) provide data that can be disseminated to other researchers to inform the design of future powered prosthetic knees by understanding the needs of users and identifying the features and functions that are most valuable to this patient population as well as the capabilities and limitations of current devices.

The primary outcome measures for each aim are outlined as follows:

• [Mobility Primary Outcome]: daily number of steps taken
• [Safety Primary Outcome]: Activities-Specific Balance Scale
• [Wellbeing Primary Outcome]: PEQ-Well-Being (PEQ-WB)

The exploratory data collected in the in-clinic and take-home portions of this study will include number of steps and number of self-reported falls, and a series of standard outcomes measures, such as: 6MWT, SAI, RAI, PROMIS-PF short form, PLUS-M, TUG, ABC, RPE, PROMIS-Fat, fall count, PROMIS-29, ODI, PEQ-WB, and PROMIS-APSRA short form.

The investigators plan to perform a pilot longitudinal randomized crossover study to compare the powered knee and passive microprocessor knees in individuals with unilateral transfemoral amputation. The two interventions, or conditions, to be tested are: (A) Ossur PK and (B) a standard MPK. For this study the investigators will recruit individuals who are currently MPK users; therefore, for the MPK condition, they will wear their usual prosthesis (e.g. Otto Bock C-leg, Össur Rheo, etc) as they represent the current standard of care. This will be recorded as a potential confounder in post-hoc analysis. Each participant will be provided with prosthetic knees for the powered knee condition.

The order these conditions are tested in will be randomized across subjects. Block randomization using a previously developed custom software based on random number generation will be used to equally distribute the subjects into the 2 possible intervention orders (AB or BA). For each condition, the subject will be trained on and acclimated to the device during the first month of the take-home period, and then will continue to wear it at home after training for a total of three months per condition. There may be seasonal effects due to the timing of when the various test conditions are administered, however, the investigators plan to offset these effects with condition randomization, multiple site testing, and staggered start times.

Testing will consist of test sessions, training sessions, at-home wear of the device, and functional and self-report assessment at the end of each intervention condition. If the subject's current foot is not compatible with the Power Knee, they will be provided with a commercially available prosthetic foot for the study. In this case, the subject will be given an initial acclimation period of 1 month on the new foot before starting interventional testing.

After receiving IRB and HRPO (US Army Human Research Protection Office) approval, subjects will be recruited and screened for eligibility. If they are eligible and decide to participate, they will be scheduled for their first site visit. They will either be consented over the phone before their first site visit or at the first site visit. If over the phone, study staff will use the telephone script to confirm eligibility and guide the subject through consent form details. Subjects will be provided the informed consent form to read through entirely before signing, and are encouraged to ask questions at any time.

Prior to each condition, each subject will be fit and aligned on the knee by a trained certified prosthetist. An activity monitor (such as the GeneActiv™) will be worn by the subject. As the investigators have done in prior studies, the investigators will use the activity monitor to track the number of steps the subjects take in each condition. Each subject will wear the same foot for all study periods for comparability between the two knee conditions.

The subject will wear the knee being tested at home for three months. During the first month of each condition, subjects will be given time to acclimate to the knee being tested. Before and after each condition, the subject will complete a battery of outcomes and self-report surveys. Subjects will participate in at least two, but up to eight, 1 hour training sessions per knee. At a minimum, each subject will receive one training session to learn how to properly use each device for a variety of daily activities and a second session to demonstrate retention of the learned skills. The subject will continue training until they show retention of the understanding and/or proficiency of the device functions or until they have received eight training sessions, whichever comes first. After 8 training sessions, they will have completed all of the available training and will therefore continue with the study at their current proficiency level. For the powered knee condition, the investigators will work with trained and experienced clinicians, consulting prosthetic knee manufacturers as needed, to develop the training protocol and determine proficiency. At the last training session, the subject will complete a subset of the outcomes and self-report measures in order to collect and compare data from before and after the subject gets trained on each knee condition.

After the training visits are completed, subjects will be asked to continue to wear the knee full time for the remainder of the three month take-home period. Through Hanger's current patient care system, the investigators will have the ability to push out a digital to the patient's phone asking them whether they have experienced a recent fall. If they respond positively that they had a fall, follow up questions can be asked to get more specific details about the fall. After three months of at-home wear, the subject will return for the second test visit. During that visit, functional tests and self-report surveys will be administered. At the completion of the second test visit (Period 1), the subject will then be fit and aligned on the prosthetic knee condition for Period 2. The training, at home wear, and in-clinic assessments will be repeated for the next knee condition.

Inclusion Criteria:

• Are at least 18 years old
• Unilateral transfemoral prosthesis user (limb absence between the knee and hip)
• Current user of a microprocessor-controlled knee (MPK)
• Have adequate clearance between distal end and ground for necessary knee and foot components
• Medicare Functional Classification Level (K-Level): 4
• Socket-Comfort Score: 6 or above to ensure adequate socket fit
• PLUS-M T-score of 55 or above
• Six months or more experience on a prosthesis
• Body weight between 50kg and 116kg (110lbs - 256lbs)

Exclusion Criteria:

• Present injuries to residual limb or contralateral leg affecting functional ability
• Socket issues/changes in the last 6 weeks
• Users with bone-anchored implants

Subjects can be excluded at the discretion of the investigator for other unforeseen disqualifying criteria (such as specific cognitive issues, etc.). Subjects in this study will not be discriminated by sex/gender or race/ethnicity.

Using a knee that the subject is unfamiliar with may increase the risk of falling. Therefore, pregnant women should not participate in the study and will be screened by self-disclosure.