The effects of common footwear on stance-phase mechanical properties of the prosthetic foot-shoe system

Novel testing system to determine shoe mechanical properties

BACKGROUND

Shoes play an important role in ankle foot orthosis (AFO) function and alignment. Despite this, shoe mechanical testing systems are rarely colocated with gait analysis systems, limiting their availability and use during AFO-related studies.

OBJECTIVE

The purpose of this study was to evaluate a novel mechanical testing system used to measure shoe heel stiffness and change in height with loading using equipment available in most gait analysis laboratories. The novel testing system will allow for shoe assessment during AFO studies at little additional cost.

STUDY DESIGN

Shoes were tested to determine initial stiffness, terminal stiffness, and total stiffness, and whether these measures changed with repeated compressions (early vs. late).

TECHNIQUE

The novel testing system consists of a baseplate for counterweights, uprights that support a low-friction hinge, and a lever arm with a heel-shaped indenter to apply force to the shoe. Minimal detectable change values were calculated using the standard error of measurement. Intraclass correlation coefficients were calculated in SPSS using a (2, k) model.

RESULTS

No significant differences in mean values, or interactions, were observed between rounds of testing and early and late compressions (P > .05). Intraclass correlation coefficient values were greater than 0.98, and minimal detectable change values were less than 20% of the average for each measure.

CONCLUSIONS

The novel mechanical testing system, combined with pre-existing gait analysis equipment, can be used to reliably assess shoe stiffness and change in height.

Comparing forefoot and heel stiffnesses across commercial prosthetic feet manufactured for individuals with varying body weights and foot sizes

BACKGROUND

Despite the effects of prosthetic foot mechanical properties on gait of people with lower limb amputation, scant forefoot and heel stiffness data exist to help guide prosthetic foot prescription.

OBJECTIVE

To measure forefoot and heel linear stiffness properties across commonly prescribed commercial prosthetic foot models and to describe variations in stiffness across feet targeted for users with different body weights and foot sizes.

STUDY DESIGN

Mechanical testing of five types of commercial prosthetic feet across nine user body weight and foot size combinations.

METHODS

Linear forefoot and heel stiffness (force vs. displacement) data were collected for 41 prosthetic feet. Quasistatic testing was conducted at -15 and +20 degrees to isolate loading of the heel and forefoot, respectively.

RESULTS

Overall, there was a significant relationship between user body weight and both forefoot and heel stiffness, when adjusted for foot size and type ( P < 0.001). However, there were a substantial number of inconsistencies across foot type within example user body weight and foot sizes combination. Furthermore, the relative order of forefoot stiffness across foot type differed from the relative order of heel stiffness across foot type.

CONCLUSIONS

The inconsistencies and differences in relative order of forefoot and heel stiffness across commercial foot type suggest the importance of publishing objective stiffness and other mechanical properties of prosthetic feet. These data can aid clinicians in better matching mechanical properties of prosthetic feet with the functional goals and abilities of prosthesis users.

Angulation vs translation of transtibial prosthetic socket: their difference analyzed by socket reaction moments

BACKGROUND

Previous studies investigated the effects of alignment changes in transtibial prostheses on socket reaction moments. However, the effects of angular and translational alignment changes with equal displacement between the foot and the socket were not directly compared.

RESEARCH QUESTIONS

What are the different effects of angular and translational alignment changes in transtibial prostheses?

METHODS

Ten individuals with transtibial prostheses participated in the measurement of temporo-spatial parameters, socket reaction moments, and their timings under nine alignment conditions (3° flexion/extension, anterior/posterior translation, 6° adduction/abduction, medial/lateral translation, and baseline). The displacement of the prosthetic feet was set to be equal between the angular and translational changes.

RESULTS

No significant changes in walking speed were found. Similar effects were observed in the magnitudes, but not in timing, of the moments under angular and translational changes in the sagittal plane (p < 0.01 for the differences in peak extension moment among anterior translation, baseline, and extension conditions, and in peak flexion moment among anterior translation, baseline, and extension conditions). In the coronal plane, similar effects were found in the magnitudes of the moments in the early stance (p < 0.01 at 5 %, 20 %, and 75 % stance). A significant difference in magnitude was observed in the late stance (p < 0.01 between adduction and medial translation conditions).

SIGNIFICANCE

The timing of the socket reaction moment may be different in the sagittal plane, while the magnitudes of the socket reaction moment in the late stance may be different in the coronal plane between the angular and translational alignment changes.

Emulating the Effective Ankle Stiffness of Commercial Prosthetic Feet Using a Robotic Prosthetic Foot Emulator

Prosthetic foot selection for individuals with lower limb amputation relies primarily on clinician judgment. The prosthesis user rarely has an opportunity to provide experiential input into the decision by trying different feet. A prosthetic foot emulator (PFE) is a robotic prosthetic foot that could facilitate prosthesis users' ability to trial feet with different mechanical characteristics. Here, we introduce a procedure by which a robotic PFE is configured to emulate the sagittal plane effective ankle stiffness of a range of commercial prosthetic forefeet. Mechanical testing was used to collect data on five types of commercial prosthetic feet across a range of foot sizes and intended user body weights. Emulated forefoot profiles were parameterized using Bezier curve fitting on ankle torque-angle data. Mechanical testing was repeated with the PFE, across a subset of emulated foot conditions, to assess the accuracy of the emulation. Linear mixed-effects regression and Bland-Altman Limits of Agreement analyses were used to compare emulated and commercial ankle torque-angle data. Effective ankle stiffness of the emulated feet was significantly associated with the corresponding commercial prosthetic feet (p<.001). On average, the emulated forefeet reproduced the effective ankle stiffness of corresponding commercial feet within 1%. Furthermore, differences were independent of prosthetic foot type, foot size, or user body weight. These findings suggest a PFE could be an effective tool for emulating commercial prosthetic feet, enabling prosthesis users to quickly trial different feet and provide experiential input as part of a prosthetic foot prescription.

Prosthetic forefoot and heel stiffness across consecutive foot stiffness categories and sizes

Prosthetic foot stiffness plays a key role in the functional mobility of lower limb prosthesis users. However, limited objective data exists to guide selection of the optimal prosthetic foot stiffness category for a given individual. Clinicians often must rely solely on manufacturer recommendations, which are typically based on the intended user’s weight and general activity level. Availability of comparable forefoot and heel stiffness data would allow for a better understanding of differences between different commercial prosthetic feet, and also between feet of different stiffness categories and foot sizes. Therefore, this study compared forefoot and heel linear stiffness properties across manufacturer-designated stiffness categories and foot sizes. Mechanical testing was completed for five types of commercial prosthetic feet across a range of stiffness categories and three foot-sizes. Data were collected for 56 prosthetic feet, in total. Testing at two discrete angles was conducted to isolate loading of the heel and forefoot components, respectively. Each prosthetic foot was loaded for six cycles while force and displacement data were collected. Forefoot and heel measured stiffness were both significantly associated with stiffness category (p = .001). There was no evidence that the relationships between stiffness category and measured stiffness differed by foot size (stiffness category by size interaction p = .80). However, there were inconsistencies between the expected and measured stiffness changes across stiffness categories (i.e., magnitude of stiffness changes varied substantially between consecutive stiffness categories of the same feet). While statistical results support that, on average, measured stiffness is positively correlated with stiffness category, force-displacement data suggest substantial variation in measured stiffness across consecutive categories. Published objective mechanical property data for commercial prosthetic feet would likely therefore be helpful to clinicians during prescription.

Effects of shear force reduction during mechanical testing and day-to-day variation on stiffness of commercial prosthetic feet: a technical note

BACKGROUND

Mechanical testing is the principal method used to quantify properties of commercial prosthetic feet in a controlled and standardized manner. To test feet in a mechanical testing machine without overconstraining the system, tangential shear forces must be minimized. However, there is scant published information comparing techniques for reducing shear forces during mechanical testing. Furthermore, there are no data on variability in linear stiffness across testing sessions.

OBJECTIVES

To compare techniques for reducing shear forces during mechanical testing of prosthetic feet and to evaluate variation in linear stiffness across testing sessions.

STUDY DESIGN

Repeated measures.

TECHNIQUE

Force-displacement data were collected at two pylon progression angles, one for the forefoot and one for the heel, and compared across three conditions: roller plate (RoPl), low-friction interface on the shoe (SB), and no method for reducing shear forces (NoSB). Data were collected for a range of commercial prosthetic foot models and sizes. Select data were collected over multiple days to assess variation over test sessions.

RESULTS

Differences in stiffness between RoPl and SB test conditions ranged from -0.9% to +2.6% across foot models. By contrast, differences between RoPl and no method for reducing shear conditions ranged from -2.9% to +14.6%. Differences in linear stiffness between test sessions ranged from -2.2% to +3.6%.

CONCLUSIONS

Methods for reducing shear force in this study demonstrated roughly equivalent effects. Thus, a low-friction interface may be used as a less expensive and less complex method for reducing shear force in prosthetic foot testing. In addition, mechanical testing results were relatively consistent across multiple test sessions, lending confidence to test consistency.

Biomechanical evaluation over level ground walking of user-specific prosthetic feet designed using the lower leg trajectory error framework

The walking pattern and comfort of a person with lower limb amputation are determined by the prosthetic foot’s diverse set of mechanical characteristics. However, most design methodologies are iterative and focus on individual parameters, preventing a holistic design of prosthetic feet for a user’s body size and walking preferences. Here we refined and evaluated the lower leg trajectory error (LLTE) framework, a novel quantitative and predictive design methodology that optimizes the mechanical function of a user’s prosthesis to encourage gait dynamics that match their body size and desired walking pattern. Five people with unilateral below-knee amputation walked over-ground at self-selected speeds using an LLTE-optimized foot made of Nylon 6/6, their daily-use foot, and a standardized commercial energy storage and return (ESR) foot. Using the LLTE feet, target able-bodied kinematics and kinetics were replicated to within 5.2% and 13.9%, respectively, 13.5% closer than with the commercial ESR foot. Additionally, energy return and center of mass propulsion work were 46% and 34% greater compared to the other two prostheses, which could lead to reduced walking effort. Similarly, peak limb loading and flexion moment on the intact leg were reduced by an average of 13.1%, lowering risk of long-term injuries. LLTE-feet were preferred over the commercial ESR foot across all users and preferred over the daily-use feet by two participants. These results suggest that the LLTE framework could be used to design customized, high performance ESR prostheses using low-cost Nylon 6/6 material. More studies with large sample size are warranted for further verification.

Effects of women’s footwear on the mechanical function of heel-height accommodating prosthetic feet

The loaded mechanical function of transtibial prostheses that result from the clinical assembly, tuning, and alignment of modular prosthetic components can directly influence an end user’s biomechanics and overall mobility. Footwear is known to affect prosthesis mechanical properties, and while the options of footwear are limited for most commercial feet due to their fixed geometry, there exists a selection of commercial prosthetic feet that can accommodate a moderate rise in heel height. These feet are particularly relevant to women prosthesis users who often desire to don footwear spanning a range of heel heights. The aim of this study was to assess the effects of adding women’s footwear (flat, trainer, 5.08 cm heel) on the mechanical properties (deformation and energy efficiency) of four models of heel-height accommodating prosthetic feet. Properties were measured through loading-unloading at simulated initial contact, midstance and terminal stance orientations with a universal materials test system, and statistically compared to a barefoot condition. Results suggest that the addition of footwear can alter the level of foot deformation under load, which may be a function of the shoe and alignment. Moreover, while each foot displayed different amounts of energy storage and return, the addition of footwear yielded similar levels of energy efficiency across foot models. Overall, prosthesis users who don shoes of varying heel heights onto adjustable prosthetic feet and their treating clinicians should be aware of the potential changes in mechanical function that could affect the user experience.

Biomechanical characterization of the foot-ground interaction among Service members with unilateral transtibial limb loss performing unconstrained drop-landings: Effects of drop height and added mass

There exist limited data to guide the development of methodologies for evaluating impact resilience of prosthetic ankle-foot systems, particularly regarding human-device interaction in ecologically valid scenarios. The purpose of this study was to biomechanically characterize foot-ground interactions during drop-landings among Service members with and without unilateral transtibial limb loss. Seven males with, and seven males without, unilateral transtibial limb loss completed six drop-landing conditions consisting of all combinations of three heights (20 cm, 40 cm, 60 cm) and two loads (with and without a 22.2 kg weighted vest). Peak ground reaction forces (GRF), vertical GRF loading rate and impulse, as well as ankle-foot, knee, and hip joint negative (absorption) powers and work were compared across groups (i.e., contralateral side and prosthetic side vs. uninjured controls) by height and load conditions. Loading occurred primarily in the vertical direction, and increased with increasing drop height and/or with added load. Vertical GRFs were overall ~ 15% smaller on the prosthetic side (vs. controls) with similar loading rates across limbs/groups. From the most challenging condition (i.e., 60 cm with 22 kg load), ankle-foot absorption energies on the prosthetic side were 64.6 (7.2) J; corresponding values were 187.4 (8.9) J for the contralateral limb and 161.2 (6.7) J among uninjured controls. Better understanding biomechanical responses to drop-landings in ecological scenarios will help inform future iterations of mechanical testing methodologies for evaluating impact resilience of prosthetic ankle-foot systems (enhancing prescription criteria and return-to-activity considerations) as well as identifying and mitigating risk factors for long-term secondary complications within the contralateral limb (e.g., joint degeneration).

Reducing stiffness of shock-absorbing pylon amplifies prosthesis energy loss and redistributes joint mechanical work during walking

BACKGROUND

A shock-absorbing pylon (SAP) is a modular prosthetic component designed to attenuate impact forces, which unlike traditional pylons that are rigid, can compress to absorb, return, or dissipate energy. Previous studies found that walking with a SAP improved lower-limb prosthesis users' comfort and residual limb pain. While longitudinal stiffness of a SAP has been shown to affect gait kinematics, kinetics, and work done by the entire lower limb, the energetic contributions from the prosthesis and the intact joints have not been examined. The purpose of this study was to determine the effects of SAP stiffness and walking speed on the mechanical work contributions of the prosthesis (i.e., all components distal to socket), knee, and hip in individuals with a transtibial amputation.

METHODS

Twelve participants with unilateral transtibial amputation walked overground at their customary (1.22 ± 0.18 ms^-1) and fast speeds (1.53 ± 0.29 ms^-1) under four different levels of SAP stiffness. Power and mechanical work profiles of the leg joints and components distal to the socket were quantified. The effects of SAP stiffness and walking speed on positive and negative work were analyzed using two-factor (stiffness and speed) repeated-measure ANOVAs (α = 0.05).

RESULTS

Faster walking significantly increased mechanical work from the SAP-integrated prosthesis (p < 0.001). Reducing SAP stiffness increased the magnitude of prosthesis negative work (energy absorption) during early stance (p = 0.045) by as much as 0.027 Jkg^-1, without affecting the positive work (energy return) during late stance (p = 0.159), suggesting a damping effect. This energy loss was partially offset by an increase in residual hip positive work (as much as 0.012 Jkg^-1) during late stance (p = 0.045). Reducing SAP stiffness also reduced the magnitude of negative work on the contralateral sound limb during early stance by 11-17% (p = 0.001).

CONCLUSIONS

Reducing SAP stiffness and faster walking amplified the prostheses damping effect, which redistributed the mechanical work, both in magnitude and timing, within the residual joints and sound limb. With its capacity to absorb and dissipate energy, future studies are warranted to determine whether SAPs can provide additional user benefit for locomotor tasks that require greater attenuation of impact forces (e.g., load carriage) or energy dissipation (e.g., downhill walking).

Focusing research efforts on the unique needs of women prosthesis users

Introduction

Women with lower limb loss represent a relevant and growing patient cohort with unique rehabilitation needs. These needs are emphasized in a growing body of literature and the most recent Veteran Affairs/Department of Defense clinical practice guidelines.

Prosthetics Challenges

Women with limb loss experience greater dissatisfaction with prosthetic fit, appearance, and types of footwear they can use. There is a lack of prosthetics solutions to accommodate the desire of women to wear different footwear varieties, including high heels. The choice to wear a variety of footwear is important to attire and hence community participation. Despite these recognized challenges, women are still underserved in prosthetics research, which limits available information to guide the rehabilitation process.

Research Initiatives

This narrative review describes considerations of lower limb prosthesis prescription and use by women, and examples of current research to address these topics. Research efforts are beginning to explore factors that contribute to prosthetics prescription for women, and design creative prosthetics solutions to expand the range of available footwear options. Research is still needed to characterize the types of footwear women with limb loss prefer to use, and the effects of prosthesis designs, footwear, and lower limb loss on women mobility outcomes and community participation.

Conclusions

Through targeted research initiatives, scientists and clinicians can be responsive to the specific needs of women to provide evidence-based guidelines for prosthetics prescription and improve the patient-centered care after limb loss.

Using a Simple Walking Model to Optimize Transfemoral Prostheses for Prosthetic Limb Stability-A Preliminary Study

The interaction between the prescribed prosthetic knee and foot is critical to the safety of transfemoral prosthesis users primarily during the stance phase of the gait, when knee buckling can result in a fall. Nonetheless, there is still a need for standardized approaches to quantify the effects of prosthetic component interactions and associated mechanical function on user gait biomechanics. A numerical model was defined to simulate sagittal plane prosthetic limb stance based on a single inverted pendulum and predict effects of prosthetic knee alignment and foot stiffness on knee moment to identify optimal solutions. Model validation against laboratory gait data suggests it is appropriate to preliminary simulate prosthetic gait during single-limb support, when prosthetic knee stability may be most at risk given reliance on the prosthetic limb and proximal anatomy, but only for knees with flexion smaller than 4°. Model predictions identify a solution space containing those combinations of knee alignment and foot stiffness (via roll-over shape radius) guaranteeing knee stability in early and mid- single-limb support, whilst facilitating knee break at the end of it. Specifically, a posterior to in-line knee alignment should be combined with low to medium ankle-foot stiffness, whereas anterior knee alignments and rigid feet should likely be avoided. Clinicians can use these solution spaces to optimize transfemoral prostheses including knees with little to no change in stance flexion, ensuring the safety of users. Model prediction can further inform in-vivo investigations on commercial device interactions, providing evidence for future Clinical Practice Guidelines on transfemoral prostheses design.

Physical sciences

In the original edition of Prosthetics and Orthotics International, Dr Sidney Fishman identified what he anticipated as foundational educational needs for the emerging field of clinical prosthetics and orthotics. Within the broader construct of the physical sciences, this included mathematics, physics, chemistry, biomechanics, and material sciences. The clinical application of these disciplines to expanding the collective understanding within the field is described, including the biomechanics of able-bodied and prosthetic gait, the material science of socket construction, the physics of suspension and load distribution, and the engineering of prosthetic components to mimic human biomechanics. Additional applications of the physical sciences to upper limb prosthetics and lower limb orthotics are also described. In contemplating the continued growth and maturation of the field in the years to come, mechatronics and statistics are suggested as future areas where clinical proficiency will be required.

Characterizing adaptations of prosthetic feet in the frontal plane

BACKGROUND

Energy-storage and return feet incorporate various design features including split toes. As a potential improvement, an energy-storage and return foot with a dedicated ankle joint was recently introduced allowing for easily accessible inversion/eversion movement. However, the adaptability of energy-storage and return feet to uneven ground and the effects on biomechanical and clinical parameters have not been investigated in detail.

OBJECTIVES

To investigate the design-related ability of prosthetic feet to adapt to cross slopes and derive a theoretical model.

STUDY DESIGN

Mechanical testing and characterization.

METHODS

Mechanical adaptation to cross slopes was investigated for six prosthetic feet measured by a motion capture system. A theoretical model linking the measured data with adaptations is proposed.

RESULTS

The type and degree of adaptation depends on the foot design, for example, stiffness, split toe or continuous carbon forefoot, and additional ankle joint. The model used shows high correlations with the measured data for all feet.

CONCLUSIONS

The ability of prosthetic feet to adapt to uneven ground is design-dependent. The split-toe feet adapted better to cross slopes than those with continuous carbon forefeet. Joints enhance this further by allowing for additional inversion and eversion. The influence on biomechanical and clinical parameters should be assessed in future studies.

CLINICAL RELEVANCE

Knowing foot-specific ability to adapt to uneven ground may help in selecting an appropriate prosthetic foot for persons with a lower limb amputation. Faster and more comprehensive adaptations to uneven ground may lower the need for compensations and therefore increase user safety.

Functional joint center of prosthetic feet during level ground and incline walking

Energy storage and returning prosthetic feet do not provide a well-defined articulation point compared to the human ankle. Calculation of user relevant parameters, such as ankle power, requires such a joint center point when using traditional mechanical models. However, shortcomings of current calculation methods result in some errors. The aim of this case study was to compare conventional ankle joint calculations to a functional joint center (FJC) using data collected on a roll-over test machine and in a motion lab during dissimilar walking tasks. Three prosthetic feet were evaluated on a roll-over test machine. Then, two trans-tibial amputees were each fitted with the same three prosthetic feet matching their weight and activity category. Kinematic data were collected during walking on level ground, as well as up and down a slope. The FJC during the stance phase of gait was calculated for each test method and compared with outcomes using conventional methods. The location of the FJC was generally anterior and inferior to the estimated anatomical joint position. Importantly, the FJC location varied for the different prosthetic feet and was task dependent as per the three gait conditions. This was reflected in different ankle angles and moments of FJC calculations compared to conventional methods for level ground walking. Differences in the calculated FJC between conditions represented the variations in prosthetic foot deformation, and explained how this parameter is influenced by the prosthetic's stiffness. For level ground walking, calculated FJC location between human subject testing and machine evaluation were strongly correlated. Both stiffness and task dependent demands of the prosthetic foot should be considered during testing. The FJC of elastic ankles can serve as a parameter for characterization and differentiation between various prosthetic foot designs and be an important parameter for prosthetic foot designers to consider. As the position of the FJC is dependent on the design and task, it is a more informative measure of the prosthetic foot's response to the user's needs. Furthermore, prosthetists could use this metric in clinical practice to better appreciate amputee feedback and perception. FJC provides an alternative center during calculation of ankle power using standard methods.

A more compliant prosthetic foot better accommodates added load while walking among Servicemembers with transtibial limb loss

Selecting an optimal prosthetic foot is particularly challenging for highly active individuals with limb loss, such as military personnel, who need to seamlessly perform a variety of demanding activities/tasks (often with and without external loads) while minimizing risk of musculoskeletal injuries over the longer term. Here, we expand on prior work by comparing biomechanical and functional outcomes in two prosthetic feet with the largest differences in mechanical response to added load (i.e., consistently "Compliant" and "Stiff" forefoot properties). In each foot, fourteen male Servicemembers with unilateral transtibial limb loss (from trauma) completed instrumented gait analyses in all combinations of two loading conditions (with and without 22 kg weighted vest) and two walking speeds (1.34 and 1.52 m/s), as well as the Prosthesis Evaluation Questionnaire. With the Stiff foot, sound limb peak loading was 2% smaller (p = 0.043) in the loaded versus unloaded condition, but similar between loading conditions in the Compliant foot (note, the Stiff foot was associated with larger loads, overall). Independent of load or walking speed, the Compliant (versus Stiff) foot provided 67.9% larger (p < 0.001) prosthetic push-off, 17.7% larger (p = 0.01) roll-over shape radii, and was subjectively favored by 10 participants. A more Compliant versus Stiff prosthetic foot therefore appears to better accommodate walking with and without added load, and reinforce the notion that mechanical properties of prosthetic feet should be considered for near-term performance and longer-term (joint) health.

Prosthetic push-off power in trans-tibial amputee level ground walking: A systematic review

OBJECTIVE

Unilateral trans-tibial amputation signifies a challenge to locomotion. Prosthetic ankle-foot units are developed to mimic the missing biological system which adapts push-off power to walking speed in some new prosthetic ankle-foot designs. The first systematic review including the two factors aims to investigate push-off power differences among Solid Ankle Cushion Heel (SACH), Energy Storage And Return (ESAR) and Powered ankle-foot units (PWR) and their relation to walking speed.

DATA SOURCES

A literature search was undertaken in the Web of Science, PubMed, IEEE xplore, and Google Scholar databases. The search term included: ampu* AND prosth* AND ankle-power AND push-off AND walking.

STUDY APPRAISAL AND SYNTHESIS METHODS

Studies were included if they met the following criteria: unilateral trans-tibial amputees, lower limb prosthesis, reported analysis of ankle power during walking. Data extracted from the included studies were clinical population, type of the prosthetic ankle-foot units (SACH, ESAR, PWR), walking speed, and peak ankle power. Linear regression was used to determine whether the push-off power of different prosthetic ankle-foot units varied regarding walking speed. Push-off power of the different prosthetic ankle-foot units were compared using one-way between subjects' ANOVAs with post hoc analysis, separately for slower and faster walking speeds.

RESULTS

474 publications were retrieved, 28 of which were eligible for inclusion. Correlations between walking speed and peak push-off power were found for ESAR (r = 0.568, p = 0.006) and PWR (r = 0.820, p = 0.000) but not for SACH (r = 0.267, p = 0.522). ESAR and PWR demonstrated significant differences in push-off power for slower and faster walking speeds (ESAR (p = 0.01) and PWR (p = 0.02)).

CONCLUSION

Push-off power can be used as a selection criterion to differentiate ankle-foot units for prosthetic users and their bandwidth of walking speeds.

The effect of coronal prosthetic alignment changes on socket reaction moments, spatiotemporal parameters, and perception of alignment during gait in individuals with transtibial amputation

Introduction

The alignment of a prosthesis is clinically determined based on observations by clinicians and the subjective perception of amputees during gait. However, this process has been reported to be unreliable. Socket reaction moment has been reported to be significantly impacted by alignment changes, but the impact of these alignment changes on other gait parameters is unclear. The aim of this study was to investigate the effects of coronal alignment changes of a transtibial prosthesis on socket reaction moment, spatiotemporal parameters, and perceptions of alignment during gait in amputees.

Methods

Nine individuals with transtibial amputation participated in this study. Socket reaction moment and spatiotemporal parameters (step time, step length, step width, single limb support time, cadence, and gait speed) were measured under nine coronal alignment conditions (angulation: ±3°, ±6°, translation: ±5 mm, ±10 mm, and baseline) using a three-dimensional motion capture system (Vicon) and an embedded load-cell system (Europa™). In addition, subjective perceptions of alignment were examined.

Results

Coronal alignment changes of the transtibial prostheses demonstrated significant changes in socket reaction moment; however, no significant changes were found with spatiotemporal parameters or the amputee’s perception.

Conclusion

Measurement of socket reaction moment, along with the embedded load-cell system, may be a better metric for tuning the coronal alignment of transtibial prostheses compared to spatiotemporal parameters and amputee’s perceptions.

Does Decreasing Below-Knee Prosthesis Pylon Longitudinal Stiffness Increase Prosthetic Limb Collision and Push-Off Work During Gait?

Investigations have begun to connect leg prosthesis mechanical properties and user outcomes to optimize prosthesis designs for maximizing mobility. To date, parametric studies have focused on prosthetic foot properties, but not explicitly longitudinal stiffness that is uniquely modified through shock-absorbing pylons. The linear spring function of these devices might affect work performed on the body center-of-mass during walking. This study observed the effects of different levels of pylon stiffness on individual limb work of unilateral below-knee prosthesis users walking at customary and fast speeds. Longitudinal stiffness reductions were associated with minimal increase in prosthetic limb collision and push-off work, but inconsistent changes in sound limb work. These small and variable changes in limb work did not suggest an improvement in mechanical economy due to reductions in stiffness. Fast walking generated greater overall center-of-mass work demands across stiffness conditions. Results indicate limb work asymmetry as the prosthetic limb experienced on average 61% and 36% of collision and push-off work, respectively, relative to the sound limb. A series spring model to estimate residuum and pylon stiffness effects on prosthesis energy storage suggested that minimal changes to limb work may be due to influences of the residual limb which dominate the system response.

Amputee perception of prosthetic ankle stiffness during locomotion

Background

Prosthetic feet are spring-like, and their stiffness critically affects the wearer’s stability, comfort, and energetic cost of walking. Despite the importance of stiffness in ambulation, the prescription process often entails testing a limited number of prostheses, which may result in patients receiving a foot with suboptimal mechanics. To understand the resolution with which prostheses should be individually optimized, we sought to characterize below-knee prosthesis users’ psychophysical sensitivity to prosthesis stiffness.

Methods

We used a novel variable-stiffness ankle prosthesis to measure the repeatability of user-selected preferred stiffness, and implemented a psychophysical experiment to characterize the just noticeable difference of stiffness during locomotion.

Results

All eight subjects with below-knee amputation exhibited high repeatability in selecting their Preferred Stiffness (mean coefficient of variation: 14.2 ± 1.7%) and were able to correctly identify a 7.7 ± 1.3% change in ankle stiffness (with 75% accuracy).

Conclusions

This high sensitivity suggests prosthetic foot stiffness should be tuned with a high degree of precision on an individual basis. These results also highlight the need for a pairing of new robotic prescription tools and mechanical characterizations of prosthetic feet.

Choosing appropriate prosthetic ankle work to reduce the metabolic cost of individuals with transtibial amputation

Powered ankle prostheses have been designed to reduce the energetic burden that individuals with transtibial amputation experience during ambulation. There is an open question regarding how much power the prosthesis should provide, and whether approximating biological ankle kinetics is optimal to reduce the metabolic cost of users. We tested 10 individuals with transtibial amputation walking on a treadmill wearing the BiOM powered ankle prosthesis programmed with 6 different power settings (0-100%), including a prosthetist-chosen setting, chosen to approximate biological ankle kinetics. We measured subjects' metabolic cost of transport (COT) and the BiOM's net ankle work during each condition. Across participants, power settings greater than 50% resulted in lower COT than 0% or 25%. The relationship between power setting, COT, and net ankle work varied considerably between subjects, possibly due to individual adaptation and exploitation of the BiOM's reflexive controller. For all subjects, the best tested power setting was higher than the prosthetist-chosen setting, resulting in a statistically significant and meaningful difference in COT between the best tested and prosthetist-chosen power settings. The results of this study demonstrate that individuals with transtibial amputation may benefit from prescribed prosthetic ankle push-off work that exceeds biological norms.

Considering passive mechanical properties and patient user motor performance in lower limb prosthesis design optimization to enhance rehabilitation outcomes

Background

Selection of prosthesis mechanical characteristics to restore function of persons with lower-limb loss can be framed as an optimization problem to satisfy a given performance objective. However, the choice of a particular objective is critical, and considering only device and generalizable outcomes across users without accounting for inherent motor performance likely restricts a given patient from fully realizing the benefits of a prosthetic intervention.

Objectives

This review presents methods for optimizing passive below-knee prosthesis designs to maximize rehabilitation outcomes and how considerations on patient motor performance may enhance these outcomes.

Major Findings

Available literature supports that considering patient-specific variables pertaining to motor performance permits a multidimensional landscape relating device characteristics and user function, which may yield more accurate predictions of rehabilitation outcomes for individual patients. Moreover, the addition of targeted physical therapeutic interventions that encourage user self-organization may further improve these outcomes. We note the potential of existing paradigms to address these additional dimensions, and we encourage investigators to consider the many different performance objectives available for prosthesis optimization.

Conclusions

By considering user motor performance in combination with prosthesis mechanical characteristics, a staged optimization approach can be formulated which acknowledges that device modifications may only improve outcomes to a certain extent and user self-organization is a critical component to complete rehabilitation. An iterative process that can be integrated within existing rehabilitative practices accounts for changes in patient status through combined targeted prosthetic solutions and physical therapeutic techniques, and embodies the concept of personalized intervention for patients with lower limb-loss.