In 2001, Chris Moon was one of only a handful of people to complete the 300-mile Death Valley 300 run.
Many amputees often use one prosthesis for normal, everyday walking and another for more strenuous or athletic activities. Such activities include anything from fishing, horseback riding, basketball, volleyball and track and field to “extreme” sports such as skiing, snowboarding, parachuting and the Ironman triathlon. With the rapid growth of interest in amputee sports during the past decade, there has been a corresponding increase in the variety of prosthetic feet available.
Functional limitations such as the inability to run and excessive muscular fatigue are the main reasons given by those amputees who choose not to participate in sports or active recreation. The level of mobility is largely determined by the biomechanical characteristics of the prosthesis. Another factor that strongly influences comfort and function is the stiffness of the foot-ankle system (specifically, the heel and forefoot keel). Selection of an appropriate stiffness is primarily based on an amputee's body weight, choice of activities, and the intensity level of those activities, but other factors such as residual limb length, sensitivity and sense of stability may also be considered.
The most difficult challenge facing prosthetic feet manufacturers lies in designing a foot that is not only light, flexible and resilient for comfort, but also strong and durable enough to withstand higher levels of activity and various weight loads, particularly in the case of high-activity feet. Critical performance characteristics assessed in the development of prosthetic feet include:
• Energy efficiency and energy return
• Ability to change directions quickly and easily
• Balance, whether moving side to side, backwards, or up and down inclines
• Comfort and natural feel while walking on uneven terrain
• Jumping, acceleration, and controlled stopping.
Because lower-extremity amputees obviously have less in the way of natural shock absorbers, they are particularly vulnerable to the adverse effects of the repetitive, high-magnitude forces experienced by the human foot even during normal walking. This is even truer of higher-level activity, such as running and jumping. Running is the most difficult skill for amputees to master, as the foot is required to withstand the pressure of nearly three times normal body weight while in motion.
Prosthetic feet undergo a wide variety of fatigue, static (gradual pressure load) and impact tests to establish the parameters of their performance in all types of environments. Current developments in prosthetic foot design are intended to reduce the impact and energy cost of movement and increase the fatigue strength of the prosthetic material.
The LuXon Journey is Otto Bock's most recent development in high-activity feet. The Journey is considered most appropriate for moderate- to high-level amputees who participate in activities such as golfing, bowling, biking and hiking. Engineered to be maintenancefree, it features the Motion-Sphere joint, an integrated unit surrounded by LuXon urethane material for easy rollover and controlled multiaxial movement.
Ossur's Low Profile LP Vari-Flex offers superior flexibility and energy return in a design that is comfortable to wear and easy to cosmetically cover and align. As the name indicates, the LP Vari-Flex is a low-profile option to the popular Vari-Flex for amputees who cannot use a J-shaped foot due to build height, but who would benefit from an energy-storing foot that supports them during the day.
Endolite is currently developing two new foot products. Because the devices are still subject to change, names and pictures were not yet released at press time. Both are designed for the more active amputee. Endolite recently completed the first run of trials with exciting results. A second series of extended trials is planned to be conducted by August of this year. The first foot is scheduled to be released in late 2004.
The second device, planned to be released in mid-2005, is an evolution of a prototype (the Mercury Sports Foot) specifically designed for marathon runner Chris Moon. In 2001, Moon was one of only a handful of people in the world to complete the 300-mile Death Valley 300 run, from the lowest point of the USA continuously on foot to the highest point (Mount Whitney) and back again. The prototype performed extremely well, helping Moon complete the race in just over six days in temperatures ranging from 20 degrees to over 130 degrees Fahrenheit.
Choosing the appropriate components for your prosthesis is critical to your comfort and level of activity, whether you plan to trek across Death Valley or stroll to your mailbox. The foot may well be the most difficult choice you will have to make, not only because it is such an important component, but simply because there are so many to choose from. Ask your prosthetist for his or her opinion as to what would be best suited to your specific needs. Your prosthetist has access to brochures and other information for feet of all levels of activity.
