A Biomechanical Evaluation of the Ossur Power Knee in Persons With Transfemoral Amputation

The anatomical knee is a complex, polycentric joint characterized by a large range of motion in the sagittal plane and limited mobility in the coronal and transverse planes. The sagittal plane motion is used for progression in stance phase, and limb clearance and advancement in swing. Limb prostheses are intended to restore function and cosmesis in persons with limb loss. The complexity and function of prosthetic components have advanced significantly as technology has improved, but a state-of-the-art artificial limb is still a relatively poor substitute for an anatomical one. Microprocessor-controlled knees (MPKs) do not typically utilize motors to power joint rotation, but they automatically adjust resistance or damping in the joint to improve swing- and/or stance-phase control as appropriate for the user during gait. The Ossur Power Knee, which was first introduced in 2006, is the only MPK that uses a motor to provide active power generation during walking and other activities involving knee flexion and extension. Last year, Ossur released the 3rd generation of its Power Knee and appears to have addressed many of the previous shortcomings in terms of reduced weight, less noise, lower cost and longer battery life. Therefore, the newest version of the Power Knee warrants a fresh evaluation since it is, in essence, substantially different in design and function from previous models.

The purpose of this proposed investigation is to perform biomechanical evaluations of the new Ossur Power Knee during walking and other activities by transfemoral prosthesis users. Using a cross-over experimental design, approximately 20 unilateral, transfemoral amputee subjects (10 men and 10 women) will be evaluated in the Jesse Brown VAMC Motion Analysis Research Laboratory (MARL) while wearing the Ossur Power Knee and Ossur Rheo XC, which is a passive MPK device. Furthermore, the Ossur Power Knee may offer distinct advantages to female prosthesis users, so mobility will be compared between men and women to determine if differences exist on the basis of sex. The specific aims and hypotheses of this project are:

Aim 1: To compare the effects of the Ossur Power Knee and Ossur Rheo XC on the gait of unilateral, transfemoral prosthesis users during level walking. Kinematic and kinetic data will be collected as subjects walk in the MARL. Hypothesis--the active knee flexion and extension provided by the Ossur Power Knee will (1) improve walking performance, and (2) reduce metabolic energy cost during ambulation.

Aim 2: To compare the effects of the Ossur Power Knee and Ossur Rheo XC on stairs, slopes and during sit-to-stand/stand-to-sit activities of unilateral, transfemoral prosthesis users. Kinematic data will be collected as subjects perform these activities. Hypothesis--subjects will have improved performance when using the Ossur Power Knee.

Aim 3: To analyze and compare gender specific data between subjects using the Ossur Power Knee and the Ossur Rheo XC. Hypothesis--male and female subjects will demonstrate substantially different abilities to use the two knee components, indicating that gender specific components should be further investigated and developed. Research focusing on the unique prosthetic needs of women Veterans is currently a special emphasis area of the VA RR&D Service.

Subjects will also be administered questionnaires and provided at-home diaries to document their perceptions of comfort, exertion and stability while using the different prosthetic knee units.

The repeated-measures, cross-over protocol design enables each subject to serve as their own control, and it will be determined whether statistically significant changes occur in their performance measures. A two-way mixed-methods ANOVA will be used to assess the main and interaction effects of sex (male, female) and knee type (Ossur Power Knee, Rheo XC). As a conservative approach, a Bonferroni correction will be used to account for the family-wise Type I error rate because multiple outcomes will be used to address each hypothesis.

Increased understanding about how the Ossur Power Knee affects the abilities of transfemoral prosthesis users will facilitate appropriate component selection by prosthetists and ultimately improve quality of life for prosthesis users.

Inclusion criteria

• Unilateral, transfemoral amputation (any etiology).
• Age from 45-75 years.
• Residual limb length classified as standard (i.e., medium) to long.
• Prosthesis user for at least 1 year prior to enrolling in the study.
• K3 or K4 level ambulator.
• Good sensation on their residual limb(s) (upon routine clinical examination).
• Good skin integrity upon visual inspection.
• Does not require the use of assistive devices to walk for short distances.
• Not currently taking medications that are known to affect balance or gait.
• Presents with good socket fit based upon a standard assessment by the study prosthetist.

Exclusion criteria

• Bilateral amputations
• Individuals with a knee disarticulation