Seven certified prosthetist (CP) subjects were recruited from the Shirley Ryan AbilityLab via word-of-mouth, and seven high-activity (K3-K4) subjects with below-knee amputation (BKA) were recruited through the facility's prosthetics and orthotics clinic. Further details for the BKA subjects are presented in the Appendix. The study consisted of a complete 4'×'4 block and an incomplete 3'×'3 block, in which multiple prosthetist subjects were present for each BKA subject visit, and each prosthetist subject observed at least two BKA subjects. The split experimental blocks were chosen out of necessity, due to the difficulty in scheduling many prosthetists to be available simultaneously. Prior to the study, prosthetist subjects completed a questionnaire designed to elucidate the importance of prosthesis stiffness in clinical decision-making. This study was approved by the Northwestern University Institutional Review Board, and the study was carried out in accordance with the regulations and guidelines of the Northwestern University Institutional Review Board. Written informed consent was obtained by all BKA subjects and prosthetist subjects prior to participating in the experiment, as was written informed consent to publish their images.
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A quasi-passive ankle prosthesis capable of continuous stiffness variation was used to modify ankle stiffness between trials18. This experimental ankle'foot consists of a rigid footplate (23.5 cm or 25.3 cm based on subject height) with attached crepe shoe material, and a rotating ankle joint. To change ankle stiffness, a small motor in the keel repositions the fulcrum of a propped cantilever spring embedded in the structure18,19,20. Notably, the torque'angle relationship is linear, and plantarflexion stiffness is 33% of dorsiflexion stiffness19. The experimenter commanded stiffness changes to the prosthesis wirelessly. To mask auditory clues of the magnitude and direction of stiffness changes, the motor controller separated the movement of the moving fulcrum into multiple jogs. A small degree of accuracy may be lost in simulating the mechanics of prosthetic feet with a rigid foot and dynamic ankle, but this approach reduces the complicated mechanical behavior of prosthetic feet to a single controllable, reproducible, and reportable variable: the angular stiffness of the ankle joint.
A prosthetist on the research team (not included as a subject in the study) attached and aligned the variable-stiffness ankle'foot to the BKA subject's customary socket. After familiarization with the prosthesis set to a stiffness based on a previously found weight-normalized mean preferred stiffness19, the subject practiced walking on the prosthesis set to stiffness levels covering a range of 66'150% of the starting stiffness. During practice, the experimenter indicated the directionality of the changes to stiffness, to educate subjects on the relationship between changes to stiffness and the associated sensations. Subjects typically walked on each stiffness level for a couple minutes, until they were comfortable and ready to try a new stiffness. After the subject practiced walking at a range of stiffnesses, the subject provided verbal feedback regarding their preferred stiffness, which then served as the reference stiffness for the experiment. This process gave the subject approximately 30 min of time walking on the prosthesis.
Before the trials began, prosthetist subjects observed the BKA subject walking at several stiffness levels, spanning the range of levels to be tested. The experimenter described the direction of the stiffness changes, and indicated the highest and lowest stiffness. Prosthetist subjects were instructed not to communicate with one another. In two of the familiarization sessions, the range of stiffness levels to be tested was shifted upwards based on the prosthetists' request. Prosthetist subjects were informed that alignment could not be changed either before the experiment began or between trials.
For all trials, prosthetist subjects sat with a sagittal view of a 10 m walkway (Fig. 1). Trials consisted of the BKA subject walking down and back along the walkway, at a self-selected pace, with stiffness held constant throughout the trial. Stiffness was only changed between trials, while the BKA subject was standing still. A researcher walked on the outside of the subject, providing safety through a gait belt.
Figure 1Several CP subjects assessing a BKA subject wearing the Variable-Stiffness Prosthetic Ankle'Foot.
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The first part of the experiment consisted of 39 trials for each BKA subject, with 13 stiffness levels tested three times each. The stiffness levels were logarithmically spaced around the reference stiffness, with a 7% difference between levels, and the tested range spanning 66'150% of the reference stiffness. The order of stimuli was random and determined prior to the experiment, and all subjects were informed that their responses did not affect the subsequent stimuli.
At the end of each trial, the BKA subject and prosthetist subjects indicated the directionality of the change they would like to make to prosthetic ankle'foot stiffness. Prosthetist subjects were given a score sheet on a clipboard, and after each trial they drew a small arrow to indicate whether they would recommend stiffness to be increased or decreased. Prosthetist subjects were encouraged (but not required) to write comments after each trial, describing what visual cues prompted them to select their answer. The BKA subjects indicated their preference by pointing to either an up-facing or down-facing arrow on a sheet placed on the experimenter's desk, shielded from the view of the prosthetists. Since plantarflexion and dorsiflexion stiffness could not be independently varied, prosthetists were instructed to base their responses primarily on mid- and late-stance (dorsiflexion stiffness) in the event of competing preferences. Each trial took approximately 15 s, with an additional'~'15 s between trials for the stiffness to be changed and for prosthetists to write comments.
To obtain a better estimate of the BKA subject's consistency of preference, the subject completed an additional 14'23 trials after the 39 trials were completed. These trials were drawn from seven logarithmically-spaced levels, with half the spacing of the previous trials. The stimuli were centered around an estimate of the subject's preferred stiffness data, based on the first 39 trials. During this portion, the prosthetist subjects were not in the experiment room; all but one of the prosthetists had completed the day's experiment, with the remaining prosthetist (randomly chosen prior to the experiment) briefly waiting in a separate room prior to participation in Part 2.
In this portion of the experiment, the prosthetist subject and BKA subject were instructed to communicate to find their mutually preferred stiffness. They were encouraged to work collaboratively, as if they were fitting the BKA subject with a new prosthesis in the clinic.
To simulate the prosthetist as the final decision-maker regarding prosthesis purchasing and fitting, the prosthetist subject communicated directly to the experimenter the changes that the pair wished to be made. The prosthetist subject could indicate both the direction and magnitude of change to be made, with 'large,' 'moderate,' 'small,' and 'tiny' changes translating to changes of 28%, 18%, 7%, and 3.5% respectively. The specific magnitudes of the changes were unknown to the subjects. Trials ended when the BKA subject and prosthetist subject arrived at the ankle stiffness they considered optimal. Four trials were completed for all subjects except BKA #1 and BKA #2, who only completed three trials due to time constraints. The starting stiffness levels were block randomized to start with either the lowest or highest tested stiffness levels, with the third and fourth starting levels equal to 33% or 66% of the range.
Our statistical analysis was directed by several assumptions about preference variability and decision-making21,22. We assume both the percept of walking at a specific stiffness level and the internal reference of preferred stiffness are subject to Gaussian noise, and that participants follow a consistent decision rule: participants will report preferring stiffness to be increased if they perceive the present stimulus to be of lower stiffness than their preferred stiffness, and vice versa.
To obtain each participant's preferred stiffness, their raw preference data were fit with a psychometric function (cumulative normal) using maximum likelihood estimation22 (lapse rate set to 1%). The Point of Subjective Equality (PSE) signifies the preference (i.e. the stiffness at which participants were equally likely to prefer stiffness to be increased as decreased). The steepness of the cumulative normal is inversely proportional to the standard deviation of the underlying probability density function, which describes the difference between stimulus levels and their internal notion of an ideal stimulus. Normalizing this standard deviation by the preference, we describe the consistency of each participant by the coefficient of variation (CV). For an intuitive interpretation of the CV: a CV of 0.10 indicates that if the stiffness is 10% higher than their preference, the subject will respond they would prefer stiffness be decreased 84% of the time.
The additional 14'23 trials the BKA subjects completed around their preference allowed adequate estimation of their CV. To improve the estimation of the CP subjects' consistency, each CP's raw data was normalized by their PSE from each session, and pooled across sessions. A new cumulative normal was fit to this pooled data. While the spacing of the stimuli was logarithmic, the fitted curves are non-logarithmic to aid in interpretability.
To determine if prosthetist subjects preferred a higher or lower stiffness than BKA subjects, prosthetist preferences were normalized by the BKA preferences, and then averaged. For example: CP #2's preferred stiffness values for BKA subjects #1'4 were normalized to the respective BKA subject's preferred stiffness; we then describe CP #2's overall normalized preference as the mean of these four normalized stiffnesses. The seven prosthetist subjects' normalized preferences were then compared against one with a two-sided, one-sample t-test. BKA-specific effects on the difference between BKA and prosthetist preferences were investigated with an imbalanced one-way ANOVA, in which there were 2'4 observations for each of the seven BKA subjects (random factor). Differences in consistency (CV) between BKA subjects and prosthetist subjects were tested with a Welch's unequal variances t-test. Finally, to see if either the BKA subject or prosthetist subject had a stronger impact on the mutual preference, the mean logarithmic distances between mutual preference and the individual preferences (\(\left|\mathrm{log}\left(Mutual\right)- \mathrm{log}\left(CP\right)\right|\) and \(\left|\mathrm{log}\left(Mutual\right)- \mathrm{log}\left(BKA\right)\right|\)) were compared in a paired t-test.
The prosthetist subjects' written comments were separated into nine groups corresponding to the most common comments (for example, 'drop-off effect' and 'loss of anterior support' are both categorized under 'drop-off effect'). The few comments that did not fit in these categories were excluded. Comments were then organized by their distance from the prosthetist's preferred stiffness for the corresponding patient. For example, if CP #2 preferred a stiffness of 500 Nm/rad for BKA #3 and they wrote 'prolonged heel contact' during a trial of stiffness 600 Nm/rad, then that comment would be logged at 1.2 (600/500).
A prosthetic foot, or foot prosthesis, is an artificial replacement for part or all of your natural foot. Prosthetics substitute for body parts that you don't have or that don't work as they should. A prosthetic foot is primarily a mobility aid. It can enable you to walk on two feet when you couldn't otherwise.
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You might use a prosthetic foot if you've had a lower extremity or foot amputation. Common reasons for lower extremity amputations include tumors, trauma, infection and peripheral arterial disease. Diabetes-related foot conditions are the most common reason for foot amputations, specifically.
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Different types of prosthetic feet offer different levels of functionality. Some are more rigid, and others move in different ways. Some offer more stability, and others more flexibility. Some accommodate more active lifestyles. Your prosthetist will help you select the right prosthesis for your needs and goals.
Types include:
The SACH foot is the simplest type of prosthetic foot, and often the first type you'll use. For most people, it's not a permanent solution but a temporary one that you'll wear until your permanent foot is ready. It has a rigid keel (center), with foam molded in the shape of a foot around it, and a rigid ankle.
The keel is the weight-bearing center of the foot that mimics the function of your foot's arch. It provides support and shock absorption when you walk. A flexible keel stores and transfers energy as you walk by bending a little when you put weight on it. A rigid keel doesn't do this, so walking is a bit clunkier.
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An elastic keel or flexible keel foot is similar to a SACH foot, except that the keel gives a little when you walk. This helps the foot accommodate uneven terrain a little better. Everything else in the foot is rigid, making it very stable and easy to control. This foot can be helpful for limited walking if your leg is weak.
Axial feet have mechanical ankle joints that mimic the movement of a natural ankle. There are two types: single-axis and multi-axis. A single-axis ankle moves up and down to help the foot accommodate an incline or decline. A multi-axis ankle also moves side to side, which helps with uneven surfaces.
Axial feet allow limited walkers to walk in more places. They're heavier than the simpler models, but they're durable and offer good stability, especially for those with weak or unstable knees. The moveable ankles help to absorb some of the stress of walking that might otherwise transfer to your leg.
A hydraulic ankle-foot uses compressed fluid, such as water or oil, to provide a smooth, rocking motion at the ankle, heel and ball of the foot. This imitates the movement of a natural foot in different walking phases. Hydraulics provide good shock absorption and reduce pressure on the prosthesis socket.
A dynamic response foot provides more flexibility for more physically active users. It's a contoured foot made with strong but flexible materials, like carbon fiber and foam, that spring back when you push off it. This helps to recycle some of the force you generate by walking, so it takes less energy to walk.
Dynamic response feet are also called energy-storage-and-return (ESAR) feet. They walk for longer distances with more comfort and with a more natural gait than simpler models. They can change speed or direction with ease, making them versatile for many uses. Most sports feet are ESAR models.
A microprocessor foot has computer chips and sensors that detect how you're using the foot and what the terrain is like and make automatic adjustments. This improves overall mobility and relieves stress and strain when walking. Because it uses advanced technology, it's sometimes called a bionic foot.
Microprocessor feet are battery-powered and require charging at night. They're more expensive and less durable than other models, and you can't let them get too wet or dirty. They're also heavier, although some models use their electric power to help propel the foot, doing some of the work for you.
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You might use an alternate prosthetic foot for certain sports or activities. Some of these include:
If you have a partial foot amputation, you might benefit from a partial foot prosthesis. Options include:
Contact us to discuss your requirements of types of prosthetic knees. Our experienced sales team can help you identify the options that best suit your needs.