There is more to the prosthetic foot than meets the eye. Its first iteration appeared in the 1500s, albeit it had a fixed ankle position. The first modern prosthetic foot—capable of flexion and heel strike—didn’t arrive until the 1950s. This was when manufacturers started taking inspiration from the anatomical functions of the human foot to create better, more natural prosthetic feet.
The anatomy of the human foot
The foot consists of 26 bones, which are held together by an intricate network of ligaments. Meanwhile, over 20 different muscles that extend to the lower leg are responsible for moving and supporting the foot. This complicated yet well-designed system is what allows us to store energy, maintain balance, adapt to various terrains, and absorb shock. This is what modern prosthetic feet aim to replicate.
Different designs of prosthetic feet
Throughout the years, prosthetic technology has achieved several milestones in improving the design as well as the function of prosthetic feet. Here we'll take a look at some of the pivotal designs of the prosthetic foot.
Solid Ankle Cushion Heel (S.A.C.H.) foot
The S.A.C.H. foot was introduced in the 1950s. It has no moving parts, but areas made from rubber provide areas that bend, flex, or deform under load.
When walking, the soft heel absorbs the impact that occurs when the foot first hits the ground, while the foot's solid body allows the user to roll over the foot onto the toes.
S.A.C.H. feet are still being used today because they are light and inexpensive solutions for post-op prosthetic legs or as training feet. However, they do not store energy. This means someone using a S.A.C.H. foot may not be as energy-efficient, and they're unable to adapt to uneven terrain as a multi-axis foot. In addition, some prosthetists believe that this type of prosthetic foot should never be provided to diabetic amputees, partly because a S.A.C.H. foot doesn't save the sound side from excess loading.
The single-axial mechanical ankle
A more improved version of the S.A.C.H. foot came with a mono-axial mechanical ankle. It replicates the ability of the ankle to move in flexion. However, this foot is unable to offer shock absorption nor surface adaptability, similar to the S.A.C.H. foot.
A single-axis foot is typically not recommended for below-knee amputees, but it is highly recommended for new above-knee amputees. The motion of the ankle can indirectly stabilize the knee and prevent abrupt knee flexion.
The multi-axial mechanical ankle
The multi-axial mechanical ankle foot is an improved version of the mono-axial one; it was designed to replicate the eversion and inversion movement of the foot. With this development, prosthetic users were finally able to walk on uneven surfaces.
By the end of the 20th century, prosthetic technology utilized modern composite materials to create lighter prosthetic feet that were also capable of storing energy. Prosthetic feet made from carbon fiber and fiberglass elastic technology were able to fulfill all the primary purposes of an anatomical foot—energy retention, impact absorption, and surface adaptation—as well as pronation and supination, heel strike dampening, and smooth rollover.
Pneumatics and hydraulics
The use of pneumatics allowed prosthetic manufacturers to create a prosthetic foot that moved naturally. By utilizing a pneumatic cylinder system coupled with composite springs, prosthetic legs were able to adjust and control its shock absorption capabilities when the heel first hits the ground.
Meanwhile, prosthetic legs that utilize a hydraulic system offer slow and adjustable front to back movements for a more controlled and natural walking pattern. This feature is useful when walking up or down inclines.
The electronic or microprocessor foot builds on the previous iterations of the prosthetic foot and allows prosthetic users to mimic walking naturally, like faster adaptation to uneven terrain and slopes. For this to work, a microprocessor-controlled hydraulic system is combined with a multi-axial composite system.
Although prosthetic feet have evolved throughout the years, choosing a prosthetic foot comes down to lifestyle and preference, especially since prosthetic feet can be quite expensive.
We hope that this article has shed some light on the different prosthetic feet available today and what each type is capable of doing. This way, you can choose a prosthetic foot that suits you and your lifestyle.Which prosthetic foot are you currently wearing? Which prosthetic foot would you consider for your next prosthesis? Share your experiences in the comments section below.