Structural Considerations of the Prosthetic Foot

The structural considerations of the prosthetic foot may not seem like a very exciting topic of conversation...unless you are at the amputee rehab center with people whose whole lifestyle depends on that specific understanding.

Not many people spend a lot of time considering the complicated anatomy of the feet and ankles they walk on. Understanding all the axis of movement a foot and ankle has, is critical in the consideration of designing the ideal prosthetic foot, however. When you look at the skeletal rendition of the foot and ankle in your anatomy and physiology book you may wonder why the foot has all those little bitty bones in it and how the deceptively simple structure of the ankle can support the weight and activity associated with human activities of daily living. (Reverend Ellis, William Webb, Advameg Inc. The Gale Group, Inc.)

Or, you may not think about it at all...unless you need to find the perfect prosthetic foot to support your own weight and the activities of your lifestyle.

In the past ten years or so the range of options when choosing a prosthetic device has grown to include upwards of fifty different models. The different dynamics of force and motion, flexion and rotation, and energy absorption and release required for different activities require a different design to optimize the experience of the user so a "one style fits all" simply does not work.

The human foot is the most mechanically and functionally complicated part of the muscle-skeletal system. It supports, propels, balances and bears great amounts of weight. It is functionally a three part anatomical structure consisting of the fore-foot, the mid-foot and the hind-foot. When all is well with the feet, the tendons, ligaments, muscles, and bones all work together in such an intricate way that subtle adjustments are constantly being made to maintain perfect balance and comfort even when standing still. When the body is in motion the foot is continually flexing and extending, rotating inwardly and outwardly, and sustaining stresses of inversion and eversion, heel and toe flexion, shock absorption, multi-axial rotation and load-compression.

In order to fashion a prosthetic foot that performs all these functions in a fully useful and nearly flawless manner, the hardware and electronics begin to use up weight and space limitations. A near perfect prosthesis should be aesthetically appealing as well as functionally sound. Women need different shapes of leg and shoe appliances. Athletes need significant flexion and cushioning. Business men need a firm stability and discretely unnoticeable reliability.

Options in prosthetic feet have gone from a solid foot connected to an un-yielding shank to a dynamic response foot that stores and releases the walking energy to give a more normal "push off" sense to the wearer (Military In-Step, A Publication of the Amputee Coalition of America in Partnership with the U.S. Army Amputee Patient Care Program).

The article on prosthetic feet on the ACA publications site (http://www.amputee-coalition.org/easyread/military-instep/feet-ez.html) details these types of feet:

1. Solid Ankle and Cushioned Heel

2. Elastic Keel - (a keel is the foot-shaped section, forward of the ankle flexion point)

3. Single-axis and Multi-axis feet

4. Dynamic-response foot

Prosthetics companies will have variations on these four basic types that help accommodate a specific activity. A good table of prosthetic feet and their possible usages complete with advantages and disadvantages can be found at http://emedicine.medscape.com/article/317358-overview .

More than a thousand amputees have come home from Iraq and Afghanistan, 3 of those have lost all four limbs, 21 have lost three limbs, 258 have lost two limbs and 832 have lost one limb, there are more than 2000 babies born each year with a "limb difference"; there are 1.7 million Americans living with limb loss, 26% of them lost limbs due to cancer related amputations. http://www.amputee-coalition.org/fact_sheets/amp_stats_cause.html

Amputees initially just want to be able to stand and walk with reliable stability. Joining the land of the living again after an amputation and the recovery period often leads to a desire to re-visit old sports activities or engage in more vigorous activities with friends and family. Finding the perfectly structured foot prosthesis is the gateway to a more inclusive and holistic lifestyle.

References:

http://www.faqs.org/sports-science/A-Ba-and-timeline/Ankle-Anatomy-and-Physiology.html

http://www.oandp.org/jpo/library/1995_04_114.asp

http://www.faqs.org/sports-science/Dr-Fo/Foot-Anatomy-and-Physiology.html

http://www.eorthopod.com/content/knee-anatomy

http://www.ottobock.com/cps/rde/xchg/ob_us_en/hs.xsl/28202.html