Place a new, high-performance prosthetic foot next to a foot designed 10 years ago and you won’t believe your eyes! What was once bulky and rigid has evolved into something sleek and flexible. In fact, the modern prosthetic foot resembles a contemporary sculpture. And for people who rely on it to move them through life, it’s not an overstatement to describe it as a work of art.
Over the past decade, options in prosthetic feet have expanded from a mere two or three basic styles to an array of 40 or more different models. Virtually every aspect of prosthetic foot design has changed, starting with the materials.
Fabricated from high-tech plastics and carbon fiber composites, this new breed of prosthesis is both durable and flexible...two qualities once thought to be mutually exclusive. The heaviness and bulk of older prosthetic feet have been eclipsed by contemporary designs that are light in weight and sleek in profile. Mimicking the function of the human foot, today’s dynamic feet actually store and return energy. This feature makes walking easier and less tiresome, while increasing the user’s comfort and control.
A prosthetic foot is often actually a combination of foot, ankle and pylon. It may be an exposed metal or composite structure or it may have a cosmetic covering that gives it the appearance of a human ankle and foot. Most feet are attached directly to the pylon, which is the portion of the prosthesis that replaces the amputee’s lower leg. Some feet have the added feature of being adjustable. This increases the user’s stability on inclines, stairs, rough terrain and uneven surfaces.
Selecting the right foot for YOU
So how can you select just the right foot for your lifestyle? Several factors must be considered: your activity level, occupational requirements, foot size, weight and amputation level. With so many choices available, it is useful to place prosthetic feet in broad categories and examine one category at a time, starting with basic cushioning feet.
Most basic prosthetic feet come in two types: SAFE ( Solid Ankle Flexible Endoskeleton) and SACH (Solid Ankle Cushioned Heel). These designs consist of crepe neoprene or urethane foam molded over a wooden keel and shaped to closely resemble a human foot. These feet offer cushioning and energy absorption but they do not store and release energy and are not considered dynamic. Because they have no moving parts, basic feet are virtually maintenance-free. SAFE and SACH feet are used most often on a patient’s first or temporary prosthesis and are eventually replaced by a more dynamic type of foot. Basic prosthetic feet may be single-axis or multi-axis in their design. "Axis" refers to the natural motions of the human foot and ankle, which occur on three different planes. First, there is abduction and adduction, which are the clockwise and counterclockwise rotations of the leg while the foot is in contact with the ground. Inversion and eversion refer to the motions of the ankle and leg inward and outward during ambulation. Dorisflexion and plantarflexion describe the up and down movements at the ankle that enable the leg to move forward over the foot, pushing the forefoot to the ground. Prosthetic feet that function on two or three axes provide increased surface contact for the foot, improving the user’s stability.
The second tier of prosthetic feet, known as dynamic-response feet, actually store and release energy as the user ambulates. Dynamic-response feet are very much like sophisticated springs that cushion when the heel strikes and use the absorbed energy to push the foot forward into mid-stance and then into toe-off. The spring action at toe-off propels the prosthesis through the swing phase of the gait, and the pattern then repeats. The comfort and responsiveness of a dynamic foot can be an important part of enabling a prosthetic user to advance from a minimal activity level to a moderate activity level. Whether walking on an even surface, on stairs or inclines, or running, the gait pattern of the prosthetic foot replicates that of the human foot - heel strike, mid-stance, toe-off, swing. The dynamic-response foot provides good "energy return," meaning that a high percentage of the energy expended when the user steps forward is stored within the foot system and then returned as momentum for each successive step.
People with more-active life styles require these more-responsive feet. They are light-weight and comfortable, and usually operate on multiple axes. The result is increased surface contact for the foot, which means greater stability and control for the user. Many dynamic -response feet feature a split-toe design that further increases stability by mimicking the inversion/eversion axis of the human ankle and foot. Again, inversion and eversion are the movements of the ankle inward and outward during ambulation, while the sole of the foot maintains solid surface contact. Cassie Chambless is a 10-year-old below-knee amputee who likes the added stability of a multi-axis foot. "Last year, I played in 85 softball games," she says. "I feel steadier than ever with my new foot."
Some examples of dynamic-response feet well suited for community ambulators include Ossur’s Sure-Flex and K2 Sensation. The Genesis II and the Seattle Lite also fit well in this category. Energy return percentages for this level of dynamic foot may reach 90 percent or greater.
Dynamic-response feet sometimes include cushioning bumpers at forefoot, mid-stance and heel, and bushings at the ankle to help control alignment. These extra features play an important role in dampening and absorbing shock to the residual limb and help create a more fluid gait pattern. Bumpers and bushings are usually customized by the prosthetist to accommodate each individual’s activity level and body weight. College Park’s TruStep foot utilizes multiple bumpers, pads and bushings; the Ossur Masterstep integrates various replaceable bumpers with an adjustable heel height. Patients need to remember that feet with multiple moving parts usually require more maintenance. Also, due to the need to have access to moving parts, it is difficult to design an effective cosmetic covering for this type of foot.
The final tier of prosthetic feet are those considered to be high-performance. While young athletes seek to gain a competitive advantage with these feet, it’s important for older adults to realize that they, too, can gain dramatic improvements in mobility. Although these are two very different kinds of gain, each is of equal importance to the individual user. With energy return reaching 95 percent or greater, high-performance feet are for any amputee who wants to be very active and very comfortable.
Included in the high-performance category are Ossur’s Re-Flex VSP and Modular III and Springlite’s Advantage DP. Ohio Willow Wood has recently introduced the Pathfinder foot, an innovative new design that includes a pneumatic heel spring.
At the top end of the high-performance category are feet designed specifically for running. These are considered to be high impact and are used primarily by athletes. The Flex-Sprint III and the Springlite Sprinter are examples of prosthetic running feet. In fact, below-knee amputee Roderick Green competed in the 2000 Paralympics running on a customized Springlite Sprinter.
As the amputee population has grown, so has the demand for prosthetic designs that accommodate specific activities. Examples of this are feet with adjustable heels. Typically, there are three reasons individuals may want to try an adjustable foot. First, they may wish to adjust their heel height to accommodate different shoes; second, they might spend much time walking or standing on uneven surfaces like golf courses and other outdoor terrain; and third, they may engage in recreational activities that require a lot of footwork like tennis, bowling, or basketball. Adjustable feet give users much greater control over their dorisflexion-plantarflexion. Some are user adjustable, while others require a prosthetist to modify them. A few examples of this type of foot are the SAFE ADJ, the Century XXII Total Foot and Ossur’s Masterstep.
Another example of a specialized design is the Leisure Activity Ankle from Ortho Enterprises. This device allows users to adjust the angle of their feet from the standard right angle to a pointed toe angle, which would be useful for swimming, snorkeling and scuba diving. Maria Greenfield is a bilateral lower-extremity amputee who is enjoying this adaptive design. "When I learned to scuba dive, I got these articulating ankles. I just turn a little key at the ankle, and I can point my foot down and put my swim fins on so I can kick and propel myself through the water," she says. "I’ve asked for something like this for years. Before, I always had to rely on my husband to pull me forward. It is an awesome thing to be able to do it myself!"
Putting your best foot forward is an important goal for all amputees, not just those who are young or athletic. Today’s array of dynamic-response feet literally offers something positive for every lower-extremity user, irrespective of age or activity level. The correct foot makes walking easier, requiring less energy from users while increasing their comfort and stability. Patients who are interested in specific feet should discuss the pros and cons of each one with their prosthetist and together they can make the best choice. Fortunately, some manufacturers allow a trial period. That means users can actually try out and compare feet before deciding which one is right for them. So don’t wait any longer to step into a new foot and see where it can take you.
Since this article was written, two new feet have been introduced: Springlite’s Luxon™ Max, a high load bearing foot for toughness, resiliency, vibration dampening and impact resistance, and Ossur’s Talux™, offering natural stride, ideal gait, and secure mobility on uneven terrain.
For more information on these and other prosthetic feet visit www.scottsabolich.com where you will find many links to related Web sites.
About the Author:
Scott Sabolich, CP, is owner and clinical director of Scott Sabolich Prosthetic & Research Center in Oklahoma City, Oklahoma, and was the lead prosthetist of the National Institutes of Health study on Diabetic Neuropathic Feet.