Comparison of the Power Knee and C-Leg during step-up and sit-to-stand tasks

Abstract

For U.S. military service members with transfemoral amputations there are different prosthetic knee systems available that function differently. For example the C-Leg^® (C-Leg, Otto Bock Healthcare, GmbH, Duderstadt, Germany) is a passive microprocessor knee, and the Power Knee™ (PK, Ossur, Reykjavík, Iceland) provides active positive power generation at the knee joint. This study examined both step-up and sit-to-stand tasks performed by service members using C-Leg and PK systems to determine if the addition of positive power generation to a prosthetic knee can improve symmetry and reduce impact to the remaining joints. For both tasks, average peak sagittal knee powers and vertical ground reaction forces (GRFs) were greater for the intact limb versus the amputated limb across PK and C-Leg groups. For the sit-to-stand task, peak knee power of the amputated limb was greater for PK users versus C-Leg users. Vertical GRFs of the intact limb were greater for the C-Leg versus the PK. The performance of the PK relative to the C-Leg during a STS task illustrated few differences between components and no effect on the intact limb.

Introduction

U.S. military service members who have sustained a transfemoral amputation as a result of their involvement in Operation Enduring Freedom (OEF) and Operation Iraqi Freedom (OIF) are a cohort of young adults, many of whom are capable of high function. These “tactical athletes” may be among those most likely to benefit from the expanded performance capabilities claimed by powered prostheses. Powered prostheses may also be advantageous for those with limited function or strength deficits.

The restoration of functional mobility in persons with transfemoral amputation has been limited in part by an absence of prosthetic knees that provide positive power generation to simulate the concentric function of the quadriceps. The Power Knee™ (PK, Ossur, Reykjavík, Iceland) represents the first commercial attempt to restore these functional characteristics. The PK technology purports to not only enhance safe and efficient level walking but also to further augment users’ capabilities during ambulation on stairs and inclines, as well as performance of transfer functions (sit-to-stand). Active propulsion may also help reduce compensatory loads on the non-amputated (intact) limb and prevent secondary injuries.

Secondary musculoskeletal disability in amputees may be related to excessive loading of musculoskeletal structures. Asymmetrical gait and compensatory actions by amputees may cause pain, specifically in lower extremity joints and the back. Multiple studies reported that 50–52% of amputees reported back pain and 19–25% reported that pain as severe [1], [2]. Another study reported increased forces on the intact limb at higher walking speeds [3]. This increased force could account for joint pain and degeneration, as well as development of osteoarthritis [2], [3], [4], [5], [6], [7], [8], [9].

Previous research has shown that a microprocessor knee is not only preferred but also more functional than a mechanical knee [9]. Studies have demonstrated increased performance descending stairs with the C-Leg (C-Leg, Otto Bock Healthcare, GmbH, Duderstadt, Germany) versus a mechanical knee [9], [10]. The C-Leg has also outperformed other microprocessor knees, offering greater functionality and safety, including decreased loading of the contralateral limb during stair and ramp ascent [11]. Published research testing the PK is sparse; however one study reported improved walking speed and step length [12]. A case study that focused on the task of standing presented reduced sound limb knee and hip moments during a sit-to-stand task while wearing the PK compared to wearing the C-Leg [13].

The sit-to-stand (STS) task is often used in clinical assessments to measure the functional level of a person [14]. It is considered “the most mechanically demanding functional task routinely undertaken during daily activities” [15]. Stair climbing is a functional task that poses a significant challenge to those with transfemoral amputation. These individuals commonly employ a “step to” pattern where they step up with the intact extremity and then bring the prosthetic up to that step. This is the result of absent quadriceps like function on the prosthetic knee. The step-up (SU) task has been used to simulate a stair ascent task [16], [17], and is also functionally relevant because it simulates the functional task of stepping up a curb. It has not however been used to study the biomechanical characteristics or adaptations of function for those with amputation.

The objective of this study was to examine if there are functional and clinically relevant differences among users of the PK compared with the C-Leg. The specific aim was to determine if the use of the either knee unit results in more normal and symmetrical kinematics and kinetics during SU and STS tasks. We hypothesized that (1) for the SU task, subjects would demonstrate improved symmetry in knee kinetics while using the PK as opposed to the C-Leg; and that (2) for the STS task, subjects would demonstrate improved symmetry in limb loading while using the PK as opposed to the C-Leg.