Prosthetic Feet

Original Editor - Tarina van der Stockt Top Contributors - Tarina van der Stockt  

Prosthetic feet can be made from wood, rubber, urethane, titanium, graphite and carbon fibre. They can be lightweight, energy-storing, or dynamic and some can allow adjustability of heel height. All prosthetic feet should provide passive plantar flexion in early stance, neutral position in mid stance and toe hyperextension in late stance

Among different designs of feet it is worth mentioning those most common in use.

Non Articulated Feet[1]

Generally known as SACH (Solid Ankle Cushion Heel) feet and consist of a rigid feet without ankle articulation, where the heel absorbs the shock and the forefoot simulates the dorsal flexion of the foot. There are feet with different degrees of heel amortization offered in the market.
In spite of having a very simple design, SACH feet respond to functional necessities in all the phases of the gait.
Can be used for regular prosthetic treatment of the majority of amputees and/or the initial fitting before proceeding to more sophisticated foot design.

A dynamic SACH foot is when a flexibly (dynamic) keel is inserted inside the foot. This allows the forefoot to flex when load is applied. When the load is released the keel returns to the shape it was originally in. Additional shock absorber. Gives the individual a bit of a push with progression to toe load.

  • Advantage: Waterproof, low cost, durable, traditionally for low activity individuals
  • Disadvantage: SACH- rigid keel that cannot bend. The keel is in the dynamic SACH cannot be customized. Heel height is fixed, single bolt attachment failure, quickly worn out by active users. Debate over prescribing for low activity users especially with vascular problems as the rigid keel will increase weight bearing on the sound side due to poor push off.
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Articulated Feet [2]

They can be with certain range of liberty in only sagittal plane to simulate movements of planter- and dorsiflexion, or in sagittal and frontal planes to simulate planter- and dorsiflexion as well as inversion and eversion. There are feet with different degrees of amortization of all those movements.

The advantage of imitating the anatomical movements of the ankle is not only for cosmetic reasons but also for a functional one: after the heel contact, the immediate forefoot contact occurs, promoting the ground reaction forces that secure the prosthetic knee in extension. Able to mimic normal foot movement better.

Can be used for regular prosthetic treatment of the majority of amputees, in particular for TF amputees.

Single-Axis Foot:

  • Mechanism: Allows plantar- and dorsiflexion to allow knee stability. The quicker the whole sole of the foot makes contact with the ground, the more stable the prosthetic knee is.
  • Advantages: Patients with high TF amputations will benefit from this because it reduces the effort to control the prosthetic knee during the heel contact to standing phase.
  • Disadvantages: It will make the prosthesis heavier. Requires more frequent servicing than a non-articulated foot. Appropriate mostly for prosthetic users that need stability around the prosthetic knee

Multi-Axis Foot:

  • Mechanism: Simulate planter- and dorsiflexion as well as side to side
  • Advantages: Absorbs some of stresses of walking and reduce the pressure in the socket, thus protecting the skin and the prosthesis from wear and tear. Heel height can be changed. Waterproof. Able to adjust to individual. Improved function when compared to the SACH or single axis. For medium to high activity individuals.
  • Disadvantages: Heavier and more expensive than SACH or single axis. Poor shock absorption and energy return.
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Feet with Energy Return / Dynamic Response Feet [2]

  • Mechanism: The basic element of these feet is the design of the keel that simulates a spring molded carbon fiber plates. This design has better energy response during the toe-off phase (imitating the natural impulse of the foot) by means of the shape and the material of the keel. The foot store and release energy when the individual is walking by absorbing the energy from the keel in the "roll-over" phase of walking. This creates a push off action.
  • Advantages: Can be used for regular prosthetic treatment of the majority of amputees. In particular for those with intermediate activity level, meaning they vary their walking speed, walk longer distances, and change direction quickly. "Push off" gait is possible leading to a more symmetric gait. Some feet have a split-toe feature that allows the foot to mimic inversion and eversion. Because of the dynamic response of the foot it usually encourages individuals to progress from moderate to higher activity levels. It can decrease the impact and stress on the sound leg during gait.
  • Additional Components:
    • A shock absorber and torque adapter could be added to the prosthetic foot that will decrease the stress on the residual limb by absorbing the impact and any rotation due to environment.
    • It is also possible to get prosthetic feet with an adjustable ankle to allow the individual to wear different height shoes
  • Disadvantages: Cost. Stiff keel for individuals with low activity levels.
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(Views without cosmetic cover)

Microprocessor Feet [2]

  • Mechanism: These ankle/foot components are controlled by a small computer and sensors. The "computer" processes information from the prosthesis, prosthetic user's limb and the environment and then adjusts the speed and range of motion of the ankle depending on the action required. "Current MPC ankles use a variety of sensors, including ankle angle sensors, accelerometers, gyroscopes and torque sensors. The microprocessors in these systems then take the input signals and make decisions as to how to position the ankle, how to set the damping resistance in the ankle, and how to drive an ankle motor during stance phase"[2]
  • Advantages: This prosthesis is able to respond to the environment or different situations by changing the speed or ROM. The ankle alignment can also adjust for different situations, like a shoe with a heel, or when the individual carries a back pack or walks up an incline, this will improve the individual's mobility and balance in that situation. It also provides an active push-off or propulsion as well as active dorsiflexion. Depending on the manufacture these feet can be connected to a mobile device or computer to change settings to individualize certain actions of the foot in different scenarios.
  • Disadvantages: It does not communicate directly with the person, but rather adapts through sensing movement or the environment. It is powered by a battery, that needs to be charged. Electronic parts that may fail/break. Additional weight. More expensive than other prosthetic feet.
[3]

Motor Powered Ankles In addition to the microprocessor a motor provides power to actively push off and dorsiflex.

This website is a great resource to see the different types of prosthetic feet available as well as a list of manufacturers and the names of the feet in each category.

References

  1. NZALS Peke Waihanga, Aotearoa. Feet and Ankles. https://www.nzals.co.nz/products/categories/feet-and-ankles
  2. 2.0 2.1 2.2 2.3 Fact Sheet. Prosthetic Feet. Amputee Coalition. Updated August 2016. https://www.amputee-coalition.org/resources/prosthetic-feet/
  3. DelBiancoPA. Patient Education: Prosthetic Feet Available from: https://youtu.be/1DP_YWgMbDc