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Comparison of the Power Knee and C-Leg during step-up and sit-to-stand tasks
Gait & Posture 38 (2013) 397–402
Contents lists available at SciVerse ScienceDirect
Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Comparison of the Power Knee and C-Leg during step-up and sit-to-stand tasks Erik. J. Wolf a,*, Vanessa Q. Everding a,1, Alison A. Linberg a,2, Joseph M. Czerniecki b,c,3, COL Jeffrey M. Gambel a,4 a
Walter Reed National Military Medical Center, Department of Orthopaedics and Rehabilitation, Bethesda, MD, United States Seattle Veterans Administration Medical Center, Seattle, WA, United States c Department of Rehabilitation Medicine, University of Washington, Seattle, WA, United States b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 July 2012 Received in revised form 2 January 2013 Accepted 3 January 2013
For U.S. military service members with transfemoral amputations there are different prosthetic knee systems available that function differently. For example the C-Leg1 (C-Leg, Otto Bock Healthcare, GmbH, Duderstadt, Germany) is a passive microprocessor knee, and the Power KneeTM (PK, Ossur, Reykjavı´k, 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. Published by Elsevier B.V.
Keywords: Transfemoral Amputation Prosthesis Kinetics Biomechanics Power
1. 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.
* Corresponding author at: Walter Reed National Military Medical Center, America Building #19, Room B320, 8901 Rockville Pike, Bethesda, MD 20889, United States. Tel.: +1 301 400 2082. E-mail addresses: [email protected], [email protected] (Erik. J. Wolf), [email protected] (V.Q. Everding), [email protected] (A.A. Linberg), [email protected] (J.M. Czerniecki), [email protected] (C.J.M. Gambel). 1 CAREN Operator, Walter Reed National Military Medical Center, America Building #19, Room B328, United States. 2 DoD-VA Extremity Trauma and Amputation Center of Excellence (EACE), Walter Reed National Military Medical Center, America Building #19, Room B315, 8901 Rockville Pike, Bethesda, MD 20889, United States. 3 Regional Amputation Center, VA Puget Sound Health Care System, 1660 S. Columbian Way, Seattle, WA 98108, United States. 4 Department of Orthopaedics and Rehabilitation, Walter Reed National Military Medical Center, America Building #19, Room 1604, 8901 Rockville Pike, Bethesda, MD 20889, United States. 0966-6362/$ – see front matter . Published by Elsevier B.V. http://dx.doi.org/10.1016/j.gaitpost.2013.01.007
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 KneeTM (PK, Ossur, Reykjavı´k, 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 [2–9]. 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 CLeg has also outperformed other microprocessor knees, offering
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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. 1.1. General and specific aims 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 CLeg; 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. 2. Methods Approval to conduct this study was granted by the Institutional Review Board. Ten service members with unilateral transfemoral amputations and 10 non-injured controls were recruited to participate in this study. Inclusion criteria for subjects included a comfortable total surface bearing suction seal socket as a part of an existing C-Leg prosthetic system; independence as a community ambulator without an assistive device other than a prosthesis; and no contralateral limb injuries or co-morbidities that significantly affected gait, joint range of motion, or limb muscle activity. A crossover study design was used to evaluate differences between knees. Following informed consent, the first five subjects were organized into the PK user group (Group A). The second five were organized into the C-Leg user group (Group B). Group A subjects were fit with a custom PK prosthetic system and given sixweeks of training which taught users to navigate inclines/declines and stairs, transfer from a sitting to a standing position, and walk on level ground. The training was carried out by a physical therapist that had expertise in use of the PK. Subjects in Group B began in their existing C-Leg prosthetic system and received six weeks of C-Leg specific training with a physical therapist on the same basic tasks as Group A. This six-week period allowed participants to become familiar with the prosthetic systems and allowed for observation of the impact of the tested knee technologies on mobility. At the end of the six-week period, data were collected from each subject, in their assigned prosthetic system. Participants from Group A were returned to their C-Leg prosthetic system and participants from Group B were fit with custom PK fitting prosthetic knee systems. Training specific to each knee resumed for both groups for another six weeks. Data were again collected at the end of these six weeks from each subject in their assigned prosthetic system. Four platforms, each with a height of 20 cm, were used for the SU task (Fig. 1). Step Platforms 1, 3, and 4 were arranged around Step Platform 2, which was placed on an instrumented force platform, to provide an even surface for the subject. Step Platforms 1, 3, and 4 were not instrumented. All subjects were instructed to begin with one foot on Force Plate 1 and the other foot on the floor beside it. They were then asked to initiate a single upward step from Force Plate 1 onto Step Platform 2. The trailing foot followed onto Step Platforms 1 or 3, depending on the side of the trailing foot. The subject was asked to continue with a step onto Step Platform 4. The motion was completed when both of the subject’s feet were on Step Platform 4. This motion was repeated five times initiating with the right leg, and five times initiating
Fig. 1. Example of platform configuration for a right step up. The black footprints indicate foot placement; L indicates a left foot placement and R indicates a right.
with the left leg. Rest intervals were taken as needed on an individual basis to avoid fatigue effects. For the STS task, subjects were instructed to sit on a stable, backless, armless stool placed adjacent to two force plates. The height of the seat was adjusted to match the height of the center of rotation of the intact knee such that a 908 angle was formed between the thigh and the intact shank. Subjects were instructed to place one foot on each force plate and then to rise to a standing position with their hands placed on their hips. Fig. 2 displays a snapshot of (a) a test subject and (b) a control subject performing the STS task. Subjects were asked to repeat this task eight times. Rest intervals were taken as needed on an individual basis to avoid fatigue effects. A 23 camera Vicon motion capture system (Vicon, Oxford, UK) with two instrumented force plates (AMTI Corp, Watertown, MA) was used to capture lower body and trunk kinematics along with ground reaction forces for both tasks. Kinematic and kinetic data were simultaneously collected at 120 Hz and 1200 Hz respectively using Vicon Nexus software (Vicon, Oxford, UK) and processed using Visual 3D (C-Motion, Germantown, MD). For the SU task, the capture interval began when the initiating foot left Force Plate 1 and ended when that same foot left Force Plate 2. For the STS task, the capture interval began with the initiation of standing and ended with its completion. Initiation was defined by forward motion of the trunk, and completion was defined by the first instance of fully upright posture [18]. Kinematic and kinetic data were filtered with a bi-directionally passed, 2nd order, Butterworth filter at 6 Hz and 50 Hz, respectively. Average peak sagittal joint powers (hip, knee, and ankle), as well as vertical GRFs were compared for the amputated and intact limbs of the PK and C-Leg groups. Peak data were extracted between the times when the subject broke contact with the stool until the prosthetic knee reached full extension. This end point was used as a time when the subject had completed the standing action and had begun to assume a standing posture. Joint powers were examined to provide clinical significance and to directly assess the design intentions of the powered prosthesis. Symmetry in these measures, between the amputated and intact limbs of the PK and C-Leg groups, was calculated using the symmetry index (Eq. (1)) [19]. This provided a symmetry index ranging from 200 to 200 where a value of zero represents perfect symmetry. SI ¼
intact affected 100 0:5 ðintact þ affectedÞ
(1)
A one-factor repeated measures analysis of variance (ANOVA) was performed to determine if there were limb (intact vs. amputated), knee device (PK vs. C-Leg), and/ or limb by device interaction effects. Where an interaction effect was significant, a post hoc paired samples t-test was used to determine whether there were differences between knee devices within each limb. Paired samples t-tests were
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Fig. 2. Image of a test (a) and control (b) subject performing the sit-to-stand task. The test subject is wearing his C-Leg in this example. The dashed line connects the markers on the spine (C7, T4, Left and Right Posterior Iliac Spine, and Super-sacral) to illustrate the orientation of the trunk. Other markers used to define the full body model were removed from the control subject prior to the photo. (For interpretation of references to colour in the text, the reader is referred to the web version of this article.)
performed on the symmetry measures. Due to the exploratory nature of this study, and the small number of subjects, results were considered statistically significant for p < 0.1, which is standard for exploratory studies [20–22]. Time to complete the STS task was determined from initiation and completion events marked at the first forward motion of the trunk and the final extension of the trunk/hips to reach a neutral standing posture. These definitions were selected to provide a consistent outcome between participants. This measure is important because kinetics could be affected when comparing subjects who stand at different rates. Statistical significance for time measurements were determined by a paired samples t-test performed between PK and C-Leg conditions. Differences from controls were evaluated using an independent samples t-test with a Bonferroni correction for multiple statistical tests. Results of these tests were considered statistically significant for p < 0.033.
3. Results Demographic data for all subjects are presented in Table 1. Eight subjects and 10 controls successfully completed the testing. Three subjects withdrew from the study due to unforeseen personal time conflicts. Data from these subjects were not included due to the crossover design of the study. With approval from the IRB, an additional subject was recruited. 3.1. Step-up task When subjects were asked to perform the SU task, no significant interactions between limb and device were observed. Comparing between limbs, peak sagittal knee powers, ankle powers, and vertical GRFs (Table 2) were greater for the intact limb versus the amputated limb across PK and C-Leg groups (pKnee < 0.001, pAnkle = 0.003, pGRF = 0.047). No significant differences were seen at the hip with regard to peak power generation.
3.2. Sit-to-stand task Fig. 3 shows representative data from one subject performing the sit-to-stand task. There was an interaction between limb and knee device for both knee power (ppower = 0.011) and vertical GRFs (pGRF = 0.057) during the STS task. The post hoc analysis revealed that for the amputated limb, generated peak knee power was greater for the PK versus the C-Leg (ppower = 0.003). The intact limb peak vertical GRFs were lower in the PK versus the C-Leg (pGRF = 0.078). Peak sagittal hip power, knee power, and vertical GRFs (Table 2) were again greater for the intact limb versus the amputated limb across PK and C-Leg groups (pHip < 0.001, pKnee < 0.001, pGRF < 0.001). Observation during testing revealed that test subjects moved their trunk toward their intact limb during the STS task. An additional analysis of the trunk was conducted to quantify this anomaly using the same statistical methods for assessing the time to complete STS. Using dotted (red) lines to connect the markers on the spine, Fig. 2 illustrates this difference in the STS task just after subjects broke contact with the bench. Figs. 4 and 5 present additional sit-to-stand data, showing the three-dimensional position of the trunk, and range of the mediolateral (ML) and vertical (VT) displacement of the trunk, respectively. Ranges of motion in the anteroposterior (AP) and VT directions were not significantly different between controls (AP = 12.4 1.8 cm; VT = 2.2 1.2 cm) and subjects using either the PK (AP = 12.3 2.8 cm; VT = 3.5 1.4 cm) or the C-Leg (AP = 12.4 2.2 cm; VT = 3.6 1.4 cm), while the range of motion
Table 1 Subject demographics (mean SD). Subjects
Height (m)
Weight (kg)
Age (yrs)
Time since amputation (yrs)
Controls (n = 10) TFA using Cleg (n = 8) TFA using PK (n = 8)
1.83 0.08 1.79 0.06 1.79 0.06
86.3 11.1 80.0 11.8 82.3 11.2
27.4 7.8 28.4 6.6 28.4 6.6
NA 1.8 1.5 1.7 1.5
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Table 2 Kinetic data for step-up task and sit-to-stand (mean SD). Peak ground reaction force is body weight normalized and therefore unitless. Step up
Peak vertical ground reaction force
Peak ankle power (W/kg)
Peak knee power (W/kg)
Peak hip power (W/kg)
Control Power knee intact C-Leg intact Power knee affected C-Leg affected Power knee symmetry C-Leg symmetry
1.02 0.03 0.94 0.32a 1.19 0.21b 0.46 0.31a 0.47 0.19b 81.6 53.8 90.1 33.4
0.69 0.25 0.76 0.44a 0.75 0.27b 0.22 0.25a 0.18 0.09b 119.9 71.6 119.2 46.9
2.11 0.56 2.09 0.98a 2.37 1.02b 0.24 0.23a 0.11 0.07b 156.2 41.1 179.3 17.5
2.08 1.39 2.69 3.59 2.62 1.58 1.65 1.51 1.75 0.64 32.1 52.7 30.8 53.8
Sit to stand
Peak vertical ground reaction force
Peak ankle power (W/kg)
Peak knee power (W/kg)
Peak hip power (W/kg)
Control Power knee intact C-Leg intact Power knee affected C-Leg affected Power knee symmetry C-Leg symmetry
0.50 0.04 0.81 0.11a,d 0.87 0.08b,d 0.23 0.05a 0.21 0.06b 109.0 22.3 117.5 0.04
0.16 0.04 0.13 0.11 0.12 0.07 0.12 0.06 0.07 0.04 1.5 72.5e 62.3 74.6e
1.31 0.29 2.22 0.67a 2.49 0.73b 0.56 0.28a,c 0.09 0.08b,c 116.6 37.4e 187.7 9.4e
0.97 0.4 2.53 0.62a 2.61 0.87b 0.54 0.32 0.76 0.69b 132.75 30.5 125.1 52.0
Statistical differences (p < 0.1) are denoted by a PK intact versus PK affected. b C-Leg intact versus C-Leg affected. c PK affected versus C-Leg affected. d PK intact versus C-Leg intact. e PK Symmetry versus C-Leg symmetry.
in the ML direction was significantly different between controls and subjects using both knees. Increased motion in all three planes, particularly the mediolateral, contributed to increased trunk displacement for the PK and C-Leg conditions. There was no significant difference in time to complete the STS task between the subjects using either the PK (1.96 0.27 s) or C-Leg (1.92 0.40 s), and the controls (1.71 0.18 s). This result confirms that the kinetic results are comparable between subjects and conditions. 4. Discussion This study was designed to evaluate whether the use of a powered prosthetic knee (PK) resulted in improved biomechanical performance in two functional tasks, the SU and the STS, when compared with a prosthetic knee (C-Leg) that provides damping during stance and swing phase but does not generate positive power output. Moments at the hip and knee have been shown to be greater for STS than for walking or stair climbing [23,24]. Asymmetry in vertical ground reaction forces has been shown during STS performed by populations with unilateral lower limb disabilities, such as amputation, hemiparesis and joint replacements [25–27]. Subjects with transfemoral amputation, specifically, have been shown to load their amputated limb with no more than 5% of their body weight until the very end of the STS task [28]. Subjects using the PK generated more knee power than with the C-Leg on their affected side during STS in addition to exhibiting more symmetrical knee power. Peak vertical GRFs were decreased for the intact limb for subjects using the PK when compared to the C-Leg. These results support the second hypothesis that subjects would demonstrate improved limb loading with the PK versus the C-Leg. The results for the STS task also showed clear differences between the intact and amputated limbs, suggesting that subjects continued to favor the intact limb. In spite of the power generating capacity of the PK and significantly greater knee power symmetry compared to the C-Leg there was not a significant reduction in intact knee power generation. This indicates that users still heavily favor their intact limb in performing as STS task and may remain at risk for overuse injuries. The results from the SU task illustrated no significant differences between either the affected limbs or the intact limb when comparing devices. Peak knee power, ankle power, and peak
vertical GRFs were only different when comparing the intact and amputated limbs in a SU task, which is to be expected based on previous literature [11,13]. Subjects had difficulty performing the SU task with either knee. When using the C-Leg subjects were forced to either lock the knee in extension or use the free swing mode, when flexion and extension resistance are at a minimum. In either case subjects using the C-Leg relied on their intact limb to ascend a single step when leading with the prosthetic limb. Subjects using the PK also had difficulty and chose to either ascend with the knee in full extension, similar to ascent with the C-Leg, or to activate the knee into stair mode. Activation of stair mode proved difficult for a single step-up, as the mode is designed for the intact limb to lead the stair ascent. For this single step, the subject was forced to ‘‘trick’’ the prosthesis into believing the intact limb had stepped onto the first step. Subjects using the PK and the C-Leg showed greater trunk displacement than controls during sit-to-stand, specifically in the mediolateral direction towards their intact limb. More extensive experimentation and analysis could determine if the momentum provided by such strategies, rather than actual power and force generation, affected these kinetic differences. One limitation of this study was that subjects were required to perform the sit-to-stand task with their hands placed on their hips. This may not be representative of real world scenarios where many opportunities exist for using the upper extremities for assistance. A study of 100 healthy adults showed significant differences in vertical GRFs between STS performed with and without upper body assistance [29]. A study involving STS with assistance should provide different results however these results provide a worst case scenario for those with transfemoral amputations performing an STS task. A second limitation was the function of the PK during the SU task. As discussed previously, participants attempted to ‘‘trick’’ the knee into stair climbing mode to utilize the power generation for ascending the single step. This limitation is relevant however because persons with transfemoral amputations often encounter single steps (up or down) during activities of daily living. Results for this study might be different for a different group of those with transfemoral amputation. Those with more functional limitations or strength deficits due to age or injury to the contralateral limb might show more reliance on the active extension from a powered prosthesis.
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Fig. 3. Representative subject data showing vertical ground reaction force, knee power, and hip power during sit-to-stand task using the PK and the C-Leg. Dashed lines indicate the intact limb; dotted lines indicate the affected limb; solid lines indicate a trial performed by a control subject.
Future research in the development of prosthetic systems that can address and minimize these differences between intact and amputated limb kinematics and kinetics demonstrated by this study is essential. The performance of the PK relative to the C-Leg
during a STS and SU tasks illustrated few differences between knee units and no differences with respect to decreasing the impact on the intact limb. Future development of the device should consider SU tasks and methods of further increasing the power generated by
Fig. 4. Trunk displacement (m) for the sit-to-stand task. Trunk data was extracted from a single trial as a sample of one test subject and one control subject. Dashed (blue) lines indicate a trial performed with the PK. Dotted (red) lines indicate a trial performed with the C-Leg. Solid (black) lines indicate a trial performed by a control subject. (For interpretation of references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Range of the trunk displacement (cm) in the mediolateral and vertical directions during a sit-to-stand task.
the amputated limb and further decreasing load on the intact limb and rehabilitation strategies to maximize function when using the device. Disclaimer The views expressed in this presentation are those of the authors and do not reflect the official policy or position of the Department of the Army, Department of the Navy, Department of Defense, or United States Government. Acknowledgements This research was supported by the Military Amputee Research Program, W81XWH-06-2-0073, and the DOD Defense Health Programs’ Center for Rehabilitation Sciences Research, NF90UG. The authors would like to acknowledge the contributions of Barri Schnall, Giovanni Ortega, Brian Baum, Dave Beachler, and Johanna Bell. Conflict of interest statement Authors have no financial or personal relationships with any persons or organizations that could inappropriately influence this work. References [1] Ehde DM, Smith DG, Czerniecki JM, Campbell KM, Malchow DM, Robinson LR. Back pain as a secondary disability in persons with lower limb amputations. Archives of Physical Medicine and Rehabilitation 2001;82(6):731–4. [2] Burke MJ, Roman V, Wright V. Bone and joint changes in lower limb amputees. Annals of the Rheumatic Diseases 1978;37(3):252–4. [3] Nolan L, Andrzej W, Krzysztof D. Adjustments in gait symmetry with walking speed in trans-femoral and trans-tibial amputees. Gait and Posture 2003;17: 142–151. [4] Norvell DC, Czerniecki JM, Reiber GE, Maynard C, Pecoraro JA, Weiss NS. The prevalence of knee pain and symptomatic knee osteoarthritis among veteran traumatic amputees and nonamputees. Archives of Physical Medicine and Rehabilitation 2005;86(3):487–93.
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Contents lists available at SciVerse ScienceDirect
Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Comparison of the Power Knee and C-Leg during step-up and sit-to-stand tasks Erik. J. Wolf a,*, Vanessa Q. Everding a,1, Alison A. Linberg a,2, Joseph M. Czerniecki b,c,3, COL Jeffrey M. Gambel a,4 a
Walter Reed National Military Medical Center, Department of Orthopaedics and Rehabilitation, Bethesda, MD, United States Seattle Veterans Administration Medical Center, Seattle, WA, United States c Department of Rehabilitation Medicine, University of Washington, Seattle, WA, United States b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 9 July 2012 Received in revised form 2 January 2013 Accepted 3 January 2013
For U.S. military service members with transfemoral amputations there are different prosthetic knee systems available that function differently. For example the C-Leg1 (C-Leg, Otto Bock Healthcare, GmbH, Duderstadt, Germany) is a passive microprocessor knee, and the Power KneeTM (PK, Ossur, Reykjavı´k, 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. Published by Elsevier B.V.
Keywords: Transfemoral Amputation Prosthesis Kinetics Biomechanics Power
1. 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.
* Corresponding author at: Walter Reed National Military Medical Center, America Building #19, Room B320, 8901 Rockville Pike, Bethesda, MD 20889, United States. Tel.: +1 301 400 2082. E-mail addresses: [email protected], [email protected] (Erik. J. Wolf), [email protected] (V.Q. Everding), [email protected] (A.A. Linberg), [email protected] (J.M. Czerniecki), [email protected] (C.J.M. Gambel). 1 CAREN Operator, Walter Reed National Military Medical Center, America Building #19, Room B328, United States. 2 DoD-VA Extremity Trauma and Amputation Center of Excellence (EACE), Walter Reed National Military Medical Center, America Building #19, Room B315, 8901 Rockville Pike, Bethesda, MD 20889, United States. 3 Regional Amputation Center, VA Puget Sound Health Care System, 1660 S. Columbian Way, Seattle, WA 98108, United States. 4 Department of Orthopaedics and Rehabilitation, Walter Reed National Military Medical Center, America Building #19, Room 1604, 8901 Rockville Pike, Bethesda, MD 20889, United States. 0966-6362/$ – see front matter . Published by Elsevier B.V. http://dx.doi.org/10.1016/j.gaitpost.2013.01.007
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 KneeTM (PK, Ossur, Reykjavı´k, 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 [2–9]. 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 CLeg has also outperformed other microprocessor knees, offering
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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. 1.1. General and specific aims 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 CLeg; 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. 2. Methods Approval to conduct this study was granted by the Institutional Review Board. Ten service members with unilateral transfemoral amputations and 10 non-injured controls were recruited to participate in this study. Inclusion criteria for subjects included a comfortable total surface bearing suction seal socket as a part of an existing C-Leg prosthetic system; independence as a community ambulator without an assistive device other than a prosthesis; and no contralateral limb injuries or co-morbidities that significantly affected gait, joint range of motion, or limb muscle activity. A crossover study design was used to evaluate differences between knees. Following informed consent, the first five subjects were organized into the PK user group (Group A). The second five were organized into the C-Leg user group (Group B). Group A subjects were fit with a custom PK prosthetic system and given sixweeks of training which taught users to navigate inclines/declines and stairs, transfer from a sitting to a standing position, and walk on level ground. The training was carried out by a physical therapist that had expertise in use of the PK. Subjects in Group B began in their existing C-Leg prosthetic system and received six weeks of C-Leg specific training with a physical therapist on the same basic tasks as Group A. This six-week period allowed participants to become familiar with the prosthetic systems and allowed for observation of the impact of the tested knee technologies on mobility. At the end of the six-week period, data were collected from each subject, in their assigned prosthetic system. Participants from Group A were returned to their C-Leg prosthetic system and participants from Group B were fit with custom PK fitting prosthetic knee systems. Training specific to each knee resumed for both groups for another six weeks. Data were again collected at the end of these six weeks from each subject in their assigned prosthetic system. Four platforms, each with a height of 20 cm, were used for the SU task (Fig. 1). Step Platforms 1, 3, and 4 were arranged around Step Platform 2, which was placed on an instrumented force platform, to provide an even surface for the subject. Step Platforms 1, 3, and 4 were not instrumented. All subjects were instructed to begin with one foot on Force Plate 1 and the other foot on the floor beside it. They were then asked to initiate a single upward step from Force Plate 1 onto Step Platform 2. The trailing foot followed onto Step Platforms 1 or 3, depending on the side of the trailing foot. The subject was asked to continue with a step onto Step Platform 4. The motion was completed when both of the subject’s feet were on Step Platform 4. This motion was repeated five times initiating with the right leg, and five times initiating
Fig. 1. Example of platform configuration for a right step up. The black footprints indicate foot placement; L indicates a left foot placement and R indicates a right.
with the left leg. Rest intervals were taken as needed on an individual basis to avoid fatigue effects. For the STS task, subjects were instructed to sit on a stable, backless, armless stool placed adjacent to two force plates. The height of the seat was adjusted to match the height of the center of rotation of the intact knee such that a 908 angle was formed between the thigh and the intact shank. Subjects were instructed to place one foot on each force plate and then to rise to a standing position with their hands placed on their hips. Fig. 2 displays a snapshot of (a) a test subject and (b) a control subject performing the STS task. Subjects were asked to repeat this task eight times. Rest intervals were taken as needed on an individual basis to avoid fatigue effects. A 23 camera Vicon motion capture system (Vicon, Oxford, UK) with two instrumented force plates (AMTI Corp, Watertown, MA) was used to capture lower body and trunk kinematics along with ground reaction forces for both tasks. Kinematic and kinetic data were simultaneously collected at 120 Hz and 1200 Hz respectively using Vicon Nexus software (Vicon, Oxford, UK) and processed using Visual 3D (C-Motion, Germantown, MD). For the SU task, the capture interval began when the initiating foot left Force Plate 1 and ended when that same foot left Force Plate 2. For the STS task, the capture interval began with the initiation of standing and ended with its completion. Initiation was defined by forward motion of the trunk, and completion was defined by the first instance of fully upright posture [18]. Kinematic and kinetic data were filtered with a bi-directionally passed, 2nd order, Butterworth filter at 6 Hz and 50 Hz, respectively. Average peak sagittal joint powers (hip, knee, and ankle), as well as vertical GRFs were compared for the amputated and intact limbs of the PK and C-Leg groups. Peak data were extracted between the times when the subject broke contact with the stool until the prosthetic knee reached full extension. This end point was used as a time when the subject had completed the standing action and had begun to assume a standing posture. Joint powers were examined to provide clinical significance and to directly assess the design intentions of the powered prosthesis. Symmetry in these measures, between the amputated and intact limbs of the PK and C-Leg groups, was calculated using the symmetry index (Eq. (1)) [19]. This provided a symmetry index ranging from 200 to 200 where a value of zero represents perfect symmetry. SI ¼
intact affected 100 0:5 ðintact þ affectedÞ
(1)
A one-factor repeated measures analysis of variance (ANOVA) was performed to determine if there were limb (intact vs. amputated), knee device (PK vs. C-Leg), and/ or limb by device interaction effects. Where an interaction effect was significant, a post hoc paired samples t-test was used to determine whether there were differences between knee devices within each limb. Paired samples t-tests were
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Fig. 2. Image of a test (a) and control (b) subject performing the sit-to-stand task. The test subject is wearing his C-Leg in this example. The dashed line connects the markers on the spine (C7, T4, Left and Right Posterior Iliac Spine, and Super-sacral) to illustrate the orientation of the trunk. Other markers used to define the full body model were removed from the control subject prior to the photo. (For interpretation of references to colour in the text, the reader is referred to the web version of this article.)
performed on the symmetry measures. Due to the exploratory nature of this study, and the small number of subjects, results were considered statistically significant for p < 0.1, which is standard for exploratory studies [20–22]. Time to complete the STS task was determined from initiation and completion events marked at the first forward motion of the trunk and the final extension of the trunk/hips to reach a neutral standing posture. These definitions were selected to provide a consistent outcome between participants. This measure is important because kinetics could be affected when comparing subjects who stand at different rates. Statistical significance for time measurements were determined by a paired samples t-test performed between PK and C-Leg conditions. Differences from controls were evaluated using an independent samples t-test with a Bonferroni correction for multiple statistical tests. Results of these tests were considered statistically significant for p < 0.033.
3. Results Demographic data for all subjects are presented in Table 1. Eight subjects and 10 controls successfully completed the testing. Three subjects withdrew from the study due to unforeseen personal time conflicts. Data from these subjects were not included due to the crossover design of the study. With approval from the IRB, an additional subject was recruited. 3.1. Step-up task When subjects were asked to perform the SU task, no significant interactions between limb and device were observed. Comparing between limbs, peak sagittal knee powers, ankle powers, and vertical GRFs (Table 2) were greater for the intact limb versus the amputated limb across PK and C-Leg groups (pKnee < 0.001, pAnkle = 0.003, pGRF = 0.047). No significant differences were seen at the hip with regard to peak power generation.
3.2. Sit-to-stand task Fig. 3 shows representative data from one subject performing the sit-to-stand task. There was an interaction between limb and knee device for both knee power (ppower = 0.011) and vertical GRFs (pGRF = 0.057) during the STS task. The post hoc analysis revealed that for the amputated limb, generated peak knee power was greater for the PK versus the C-Leg (ppower = 0.003). The intact limb peak vertical GRFs were lower in the PK versus the C-Leg (pGRF = 0.078). Peak sagittal hip power, knee power, and vertical GRFs (Table 2) were again greater for the intact limb versus the amputated limb across PK and C-Leg groups (pHip < 0.001, pKnee < 0.001, pGRF < 0.001). Observation during testing revealed that test subjects moved their trunk toward their intact limb during the STS task. An additional analysis of the trunk was conducted to quantify this anomaly using the same statistical methods for assessing the time to complete STS. Using dotted (red) lines to connect the markers on the spine, Fig. 2 illustrates this difference in the STS task just after subjects broke contact with the bench. Figs. 4 and 5 present additional sit-to-stand data, showing the three-dimensional position of the trunk, and range of the mediolateral (ML) and vertical (VT) displacement of the trunk, respectively. Ranges of motion in the anteroposterior (AP) and VT directions were not significantly different between controls (AP = 12.4 1.8 cm; VT = 2.2 1.2 cm) and subjects using either the PK (AP = 12.3 2.8 cm; VT = 3.5 1.4 cm) or the C-Leg (AP = 12.4 2.2 cm; VT = 3.6 1.4 cm), while the range of motion
Table 1 Subject demographics (mean SD). Subjects
Height (m)
Weight (kg)
Age (yrs)
Time since amputation (yrs)
Controls (n = 10) TFA using Cleg (n = 8) TFA using PK (n = 8)
1.83 0.08 1.79 0.06 1.79 0.06
86.3 11.1 80.0 11.8 82.3 11.2
27.4 7.8 28.4 6.6 28.4 6.6
NA 1.8 1.5 1.7 1.5
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Table 2 Kinetic data for step-up task and sit-to-stand (mean SD). Peak ground reaction force is body weight normalized and therefore unitless. Step up
Peak vertical ground reaction force
Peak ankle power (W/kg)
Peak knee power (W/kg)
Peak hip power (W/kg)
Control Power knee intact C-Leg intact Power knee affected C-Leg affected Power knee symmetry C-Leg symmetry
1.02 0.03 0.94 0.32a 1.19 0.21b 0.46 0.31a 0.47 0.19b 81.6 53.8 90.1 33.4
0.69 0.25 0.76 0.44a 0.75 0.27b 0.22 0.25a 0.18 0.09b 119.9 71.6 119.2 46.9
2.11 0.56 2.09 0.98a 2.37 1.02b 0.24 0.23a 0.11 0.07b 156.2 41.1 179.3 17.5
2.08 1.39 2.69 3.59 2.62 1.58 1.65 1.51 1.75 0.64 32.1 52.7 30.8 53.8
Sit to stand
Peak vertical ground reaction force
Peak ankle power (W/kg)
Peak knee power (W/kg)
Peak hip power (W/kg)
Control Power knee intact C-Leg intact Power knee affected C-Leg affected Power knee symmetry C-Leg symmetry
0.50 0.04 0.81 0.11a,d 0.87 0.08b,d 0.23 0.05a 0.21 0.06b 109.0 22.3 117.5 0.04
0.16 0.04 0.13 0.11 0.12 0.07 0.12 0.06 0.07 0.04 1.5 72.5e 62.3 74.6e
1.31 0.29 2.22 0.67a 2.49 0.73b 0.56 0.28a,c 0.09 0.08b,c 116.6 37.4e 187.7 9.4e
0.97 0.4 2.53 0.62a 2.61 0.87b 0.54 0.32 0.76 0.69b 132.75 30.5 125.1 52.0
Statistical differences (p < 0.1) are denoted by a PK intact versus PK affected. b C-Leg intact versus C-Leg affected. c PK affected versus C-Leg affected. d PK intact versus C-Leg intact. e PK Symmetry versus C-Leg symmetry.
in the ML direction was significantly different between controls and subjects using both knees. Increased motion in all three planes, particularly the mediolateral, contributed to increased trunk displacement for the PK and C-Leg conditions. There was no significant difference in time to complete the STS task between the subjects using either the PK (1.96 0.27 s) or C-Leg (1.92 0.40 s), and the controls (1.71 0.18 s). This result confirms that the kinetic results are comparable between subjects and conditions. 4. Discussion This study was designed to evaluate whether the use of a powered prosthetic knee (PK) resulted in improved biomechanical performance in two functional tasks, the SU and the STS, when compared with a prosthetic knee (C-Leg) that provides damping during stance and swing phase but does not generate positive power output. Moments at the hip and knee have been shown to be greater for STS than for walking or stair climbing [23,24]. Asymmetry in vertical ground reaction forces has been shown during STS performed by populations with unilateral lower limb disabilities, such as amputation, hemiparesis and joint replacements [25–27]. Subjects with transfemoral amputation, specifically, have been shown to load their amputated limb with no more than 5% of their body weight until the very end of the STS task [28]. Subjects using the PK generated more knee power than with the C-Leg on their affected side during STS in addition to exhibiting more symmetrical knee power. Peak vertical GRFs were decreased for the intact limb for subjects using the PK when compared to the C-Leg. These results support the second hypothesis that subjects would demonstrate improved limb loading with the PK versus the C-Leg. The results for the STS task also showed clear differences between the intact and amputated limbs, suggesting that subjects continued to favor the intact limb. In spite of the power generating capacity of the PK and significantly greater knee power symmetry compared to the C-Leg there was not a significant reduction in intact knee power generation. This indicates that users still heavily favor their intact limb in performing as STS task and may remain at risk for overuse injuries. The results from the SU task illustrated no significant differences between either the affected limbs or the intact limb when comparing devices. Peak knee power, ankle power, and peak
vertical GRFs were only different when comparing the intact and amputated limbs in a SU task, which is to be expected based on previous literature [11,13]. Subjects had difficulty performing the SU task with either knee. When using the C-Leg subjects were forced to either lock the knee in extension or use the free swing mode, when flexion and extension resistance are at a minimum. In either case subjects using the C-Leg relied on their intact limb to ascend a single step when leading with the prosthetic limb. Subjects using the PK also had difficulty and chose to either ascend with the knee in full extension, similar to ascent with the C-Leg, or to activate the knee into stair mode. Activation of stair mode proved difficult for a single step-up, as the mode is designed for the intact limb to lead the stair ascent. For this single step, the subject was forced to ‘‘trick’’ the prosthesis into believing the intact limb had stepped onto the first step. Subjects using the PK and the C-Leg showed greater trunk displacement than controls during sit-to-stand, specifically in the mediolateral direction towards their intact limb. More extensive experimentation and analysis could determine if the momentum provided by such strategies, rather than actual power and force generation, affected these kinetic differences. One limitation of this study was that subjects were required to perform the sit-to-stand task with their hands placed on their hips. This may not be representative of real world scenarios where many opportunities exist for using the upper extremities for assistance. A study of 100 healthy adults showed significant differences in vertical GRFs between STS performed with and without upper body assistance [29]. A study involving STS with assistance should provide different results however these results provide a worst case scenario for those with transfemoral amputations performing an STS task. A second limitation was the function of the PK during the SU task. As discussed previously, participants attempted to ‘‘trick’’ the knee into stair climbing mode to utilize the power generation for ascending the single step. This limitation is relevant however because persons with transfemoral amputations often encounter single steps (up or down) during activities of daily living. Results for this study might be different for a different group of those with transfemoral amputation. Those with more functional limitations or strength deficits due to age or injury to the contralateral limb might show more reliance on the active extension from a powered prosthesis.
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Fig. 3. Representative subject data showing vertical ground reaction force, knee power, and hip power during sit-to-stand task using the PK and the C-Leg. Dashed lines indicate the intact limb; dotted lines indicate the affected limb; solid lines indicate a trial performed by a control subject.
Future research in the development of prosthetic systems that can address and minimize these differences between intact and amputated limb kinematics and kinetics demonstrated by this study is essential. The performance of the PK relative to the C-Leg
during a STS and SU tasks illustrated few differences between knee units and no differences with respect to decreasing the impact on the intact limb. Future development of the device should consider SU tasks and methods of further increasing the power generated by
Fig. 4. Trunk displacement (m) for the sit-to-stand task. Trunk data was extracted from a single trial as a sample of one test subject and one control subject. Dashed (blue) lines indicate a trial performed with the PK. Dotted (red) lines indicate a trial performed with the C-Leg. Solid (black) lines indicate a trial performed by a control subject. (For interpretation of references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 5. Range of the trunk displacement (cm) in the mediolateral and vertical directions during a sit-to-stand task.
the amputated limb and further decreasing load on the intact limb and rehabilitation strategies to maximize function when using the device. Disclaimer The views expressed in this presentation are those of the authors and do not reflect the official policy or position of the Department of the Army, Department of the Navy, Department of Defense, or United States Government. Acknowledgements This research was supported by the Military Amputee Research Program, W81XWH-06-2-0073, and the DOD Defense Health Programs’ Center for Rehabilitation Sciences Research, NF90UG. The authors would like to acknowledge the contributions of Barri Schnall, Giovanni Ortega, Brian Baum, Dave Beachler, and Johanna Bell. Conflict of interest statement Authors have no financial or personal relationships with any persons or organizations that could inappropriately influence this work. References [1] Ehde DM, Smith DG, Czerniecki JM, Campbell KM, Malchow DM, Robinson LR. Back pain as a secondary disability in persons with lower limb amputations. Archives of Physical Medicine and Rehabilitation 2001;82(6):731–4. [2] Burke MJ, Roman V, Wright V. Bone and joint changes in lower limb amputees. Annals of the Rheumatic Diseases 1978;37(3):252–4. [3] Nolan L, Andrzej W, Krzysztof D. Adjustments in gait symmetry with walking speed in trans-femoral and trans-tibial amputees. Gait and Posture 2003;17: 142–151. [4] Norvell DC, Czerniecki JM, Reiber GE, Maynard C, Pecoraro JA, Weiss NS. The prevalence of knee pain and symptomatic knee osteoarthritis among veteran traumatic amputees and nonamputees. Archives of Physical Medicine and Rehabilitation 2005;86(3):487–93.
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