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Use of a Powered Versus a Passive Prosthetic System for a Person with Bilateral Amputations during Level-Ground Walking

Wolf, Erik J. PhD; Pruziner, Alison L. DPT

Journal of Prosthetics and Orthotics: July 2014 - Volume 26 - Issue 3 - p 166–170
doi: 10.1097/JPO.0000000000000029
Case Report

ABSTRACT Persons who have had bilateral lower-limb amputations, especially transfemoral amputation, must use biomechanical compensations to ambulate. Compensations during gait produce abnormal loads on the body and may reduce efficiency. Recent prosthetic advances have attempted to mimic the lost musculature by adding power to knee and ankle systems. A patient with bilateral amputations (right transtibial, left transfemoral) was evaluated 10 months after injury. The patient initially presented wearing a passive prosthetic system and was then fit with a powered prosthetic system and allowed to acclimate for 1 month. Kinematic and kinetic data were collected while the patient walked overground wearing both systems. The patient showed greater symmetry of step length, decreased vertical ground reaction force, and increased limb transition work using the powered system. Biomechanical variables showed some improvements when using a powered prosthetic system, which may indicate increased mechanical efficiency and decreased lower-limb loading.

ERIK J. WOLF, PHD, and ALISON L. PRUZINER, DPT, are affiliated with the Walter Reed National Military Medical Center and the DoD-VA Extremity and Amputation Center of Excellence, Bethesda, Maryland.

Disclosure: The authors declare no conflict of interest.

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.

Correspondence to: Erik J. Wolf, PhD, DoD/VA Extremity Trauma and Amputation Center of Excellence (EACE), Walter Reed National Military Medical Center, 8901 Wisconsin Ave, America Building, Room B320, Bethesda, MD 20889; email:

Compensating for lower-limb trauma, including amputation, results in asymmetries during gait.1 These asymmetries, for example, stance times and limb loading, have been reported to contribute to secondary injuries such as osteoarthritis, joint degeneration, and low-back pain.2–5 Service Members who have experienced traumatic injuries, including amputation, to both lower limbs may be placed at greater risk for secondary injury owing to additional compensations they must make to ambulate, specifically additional movements and powers required from the remaining intact joints, pelvis, and trunk.

Efforts to replace the knee function with mechanical devices have been met with varying degrees of success. Early prosthetic designs ranged from the use of a simple hinge to the application of friction wheels to control the extension rate of the knee during the swing phase of gait. With the advent of stronger and lighter materials, mechanical innovation, and advances in electronic technology, such as microprocessor control, prosthetic knees can dynamically adapt the prosthetic limb during overground walking, slope and stair negotiation, and other functional tasks. The literature contains a number of biomechanical studies that through the years can be shown to correlate improved walking biomechanics and metabolic efficiency with technological advances in prosthetic knee design.6–17 Specifically, studies comparing microprocessor knees with mechanical knees have shown decreased hip work and power generation from the affected limb12 when using the microprocessor knee, decreased metabolic energy expenditure when using the microprocessor knee,15 and improved user satisfaction in walking speed, walking distance, and energy cost when using the microprocessor knee.11 Decreased frequency of falls and stumbles and increased user satisfaction have been demonstrated with the use of microprocessor-controlled prosthetic knee units compared with mechanical devices.18,19 Research investigating the use of powered prosthetic knee devices has shown improved symmetry in hip moments during a sitting task and reduced reliance on the intact ankle, as defined by peak ankle power generation, during stair ascent.20,21

Recent advancements are also being made in prosthetic foot technology. Powers et al.22 showed that the Flex-Foot (Flex-Walk, Laguna Hills, CA, USA) was able to significantly reduce the intact limb’s initial vertical ground reaction force peak to a value comparable with peaks observed in the uninjured population at walking velocities comparable with this uninjured population. These positive results were in contrast to an increase in intact limb vertical ground reaction force when the users were wearing the other prosthetic feet studied (SACH, Carbon Copy II, Seattle, and Quantum).22 Increased impact on the intact limb may lead to the increased prevalence of secondary joint disease such as osteoarthritis in individuals with lower-limb loss.2,23–26 Passive prosthetic ankles have demonstrated the ability to provide energy absorption, storage, and some return; however, they are not able to provide the net positive work normally done by the plantarflexors during the stance phase of gait.27–32 Recent advances in prosthetic ankle technology demonstrate the potential to restore normative function during level-ground walking, reducing loading on the intact limb and joints and improving metabolic demand.33

Powered prosthetic knees and ankles have shown the potential to mitigate abnormal biomechanics, specifically at the contralateral limb, during gait by providing power to lost joints, including reduced ankle power generation during stair ascent, reduced intact leading limb transition work (LTW) during overground gait, and reduced peak vertical ground reaction force and peak knee adduction moment during overground gait.21,33,34 This reduction in loading may help to reduce long-term degenerative physical changes and preserve function. Application of powered prostheses currently available on the market has been restricted to replacing a single joint, for example, a powered ankle for a transtibial amputation (TTA) or a powered knee for a transfemoral amputation (TFA). Combining powered prostheses (feet and knees) for those with advanced injuries could show an increased benefit compared with prescribing them separately. The purpose of this case study was to examine the walking biomechanics of a patient with bilateral lower-limb amputation who had been prescribed both passive and powered prosthetic systems.

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The patient was 21 years old and had sustained bilateral lower-limb amputations (right TTA, left TFA) 10 months earlier caused by an improvised explosive device. The patient was originally seen for a clinical gait evaluation on referral from his primary physical therapist and prosthetist preceding prescription of a powered prosthetic system, which was to include a Power Knee 2 (Össur, Reykjavik, Iceland) prosthetic knee and bilateral BiOM (iWalk, Bedford, MA, USA) prosthetic feet. The patient presented in a passive prosthetic system consisting of a Total Knee (Össur) prosthetic knee and bilateral Elite Blade (Endolite, Basingstoke, England) prosthetic feet. The patient was comfortable performing community ambulation in his passive prosthetic system.

The patient’s height and mass with the passive system were 174 cm and 69.5 kg, respectively. For the initial clinical gait evaluation, the patient was asked to walk at his self-selected walking speed and at a forced overground walking speed (1.4 m/second) while using his passive prosthetic system, as defined by the standard procedure for clinical gait collections at the institution. An auditory feedback was used to notify the patient when he was walking as close to 1.4 m/second as possible. Sixty retro-reflective markers were placed on specific anatomical landmarks, as defined by a modified Cleveland Clinic marker set,35 on the patient to allow for the identification of anatomical segments (foot, shank, thigh, etc) during overground walking. The modifications include additional markers on the feet at the hallux and fifth metatarsal and additional markers on the trunk on the sternal notch, C7 spinous process, T10 process, and xiphoid process. Biomechanics data, specifically kinematics and kinetics, were collected with a 27-camera motion capture system (Vicon Inc, Oxford, UK) at a frame rate of 120 Hz and six force platforms (AMTI, Waterford, MA, USA) at a frame rate of 1200 Hz. The patient was asked to walk overground across the gait laboratory 10–15 times in each condition. Five “clean” foot strikes for each limb were collected during walking; a clean foot strike is defined as one where the foot falls completely within the boundary of any of the force platforms.

After the initial evaluation, the patient was prescribed the powered prosthetic system and allowed to acclimate to the system for 1 month before a follow-up clinical gait evaluation was conducted. The patient’s height and mass with the powered system were 179.5 cm and 75 kg, respectively. The patient conducted the same overground walking testing with the powered prosthetic system that he had performed with the passive prosthetic system. Data were analyzed using Visual 3D (C-motion Inc, Germantown, MD, USA). Kinematic and ground reaction force data were filtered with a fourth-order, zero-lag Butterworth filter at 6 and 50 Hz, respectively. Biomechanical variables of interest included stance time, double-limb support time, step length, step width, peak vertical ground reaction force, individual LTW during double support, and peak ankle power generation. Symmetry values of these variables were also calculated as a simple ratio between the right and left limbs, with a value of 1.0 indicating perfect symmetry. Individual LTW is described by Equation 136:



where F denotes the ground reaction force vector and v denotes the velocity vector of the center of mass of the body. The result is the calculation provides a value of leading and trailing work for each limb. For example, for a clean right foot strike, the leading limb work is calculated from foot strike until contralateral toe-off and the trailing limb work is calculated from contralateral heelstrike to ipsilateral toe-off.

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The 1.4 m/second walking trials were used to directly compare the temporospatial and kinetics of the patient in both conditions and are shown in Table 1 and Figure 1. Results from this clinical gait evaluation comparing a passive prosthetic system with a powered prosthetic system in a Service member with bilateral amputations (right TTA, left TFA) showed temporospatial and kinetic differences. The patient’s step width decreased, stance time increased, and step length increased when transitioning to a powered system compared with the passive system. Double support time decreased when transitioning from the transtibial side and increased when transitioning from the transfemoral side. Symmetry improved in all of the temporospatial measures.

Table 1

Table 1

Figure 1

Figure 1

Peak vertical ground reaction force was reduced on the left (TFA) side when using the powered system, whereas there was no change on the right (TTA) side. Trailing limb and leading LTW increased in the powered system compared with the passive system.

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Reduction in asymmetrical gait, for example, stance time and limb loading, is critical for those with bilateral lower-limb amputations who wish to ambulate, especially for those injured at a young age. Service members experiencing bilateral lower-limb amputations are typically young and active and interested in pursuing upright ambulation as their primary means of mobility. The addition of powered prosthetic components, namely, knees and feet, in recent years provides an opportunity to normalize gait biomechanics; for example, limb loading, step length, and ankle power generation, improving economy of gait and reducing the risk of long-term injuries secondary to amputation.20,21,33,34

The greatest limitation to the results presented in this case study is that they are limited to only one subject. Lower-limb powered prosthetic devices are becoming more commonly used in the military and veteran population, likely helping drive continued development and research demonstrating their clinical benefits. These powered devices are not typically marketed or prescribed for use by individuals with bilateral lower-limb amputations, so the opportunity to collect data with similar patients is rare. Another limitation was that the patient’s height when wearing the powered system was greater than while wearing the passive system (179.5 vs. 174 cm, respectively). As discussed below, this height difference affected the results. However, the patient was tested in the prosthetic system that he used for everyday ambulation and therefore was most comfortable and functional.

Step width in the powered system was decreased, indicating a narrower base of support during overground gait. The step length of the patient increased, most likely because of the increase in overall height, and became more symmetrical when wearing the powered system. Increases in symmetry may be due to an increased effective foot length resulting from the powered prosthesis. Persons using prosthetic feet often experience “drop-off,” including decreased step length and increased contralateral limb loading, because of suboptimal rollover characteristics of the prosthetic foot.37,38 By providing power and active plantarflexion, the powered ankle increases the effective length of the affected foot, resulting in greater step length and reduction in the vertical ground reaction force at heelstrike. The more anatomical rollover shape may also explain the reduction in peak vertical ground reaction force on the limb with TFA. The prosthetic knee is “locked” in full extension just before heelstrike and the addition of active plantarflexion to the limb with TTA is allowing for a smoother rollover and softer landing.22,39

Both left and right trailing LTW increased with the powered system. This result is expected because of the active power provided by the prosthetic feet. However, the leading LTW had a greater and more negative magnitude when comparing the passive system with the powered system. These results are inconsistent with data reported by Herr and Grabowski,33 which showed a decrease in the intact leading LTW when subjects used a powered ankle on their trailing leg. Components of the force and velocity vectors were examined to explain this result and showed that, although the peak vertical ground reaction force either remained identical between systems or decreased, the peak anteroposterior ground reaction force, the braking component, increased in the powered system (−0.16 vs. −0.22 N/kg on the right side and −0.17 vs. −0.20 N/kg on the left side). In addition, although the average walking speeds were similar between conditions, the peak velocity of the center of mass was higher in the powered system at heelstrike (1.46 vs. 1.55 m/second after right heelstrike and 1.44 vs. 1.54 m/second after left heelstrike). Increases in anteroposterior ground reaction force and center of mass velocity most likely contribute to the increase in leading LTW and likely resulted because of the increased height of the patient, resulting increased step length, and increased propulsion from the powered ankle. The confounding result of increased leading LTW limits the benefit that the patient is receiving from the active propulsion provided by the powered ankles. This result might be remedied by adjusting the overall height of the prosthetic components. In addition, more experience ambulating in the powered system combined with gait training specific to the powered devices could result in a more mechanically efficient gait.

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Biomechanical variables showed some improvements for a patient using a powered prosthetic system versus a passive system. Increases in ankle power generation and trailing LTW could indicate increased efficiency and less work required from the patient. Improved symmetry of step length and reduced peak vertical ground reaction force on the TFA side could be an implication for reduction of the development of secondary injuries in the future. More research is required to determine if those with more extensive lower-limb injuries can benefit from powered prostheses.

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We thank the staff in the Military Advanced Training Center for their service and dedication and the injured and uninjured Service Members for their service to our country.

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work; prosthesis; biomechanics; gait

© 2014 by the American Academy of Orthotists and Prosthetists.