Choosing a foot component for a new prosthetic limb these days is no easy business. Once a simple choice among a handful of distinctly different designs, an informed selection of a specific ankle-foot mechanism today has become a complex matter requiring knowledge of, and experience with, a steadily growing spectrum now numbering more than 100 foot designs.
Without question, “innovation motivation” has taken hold in the once-staid prosthetics specialty as well-heeled U.S. and global manufacturers strive to “out-tech” each other to create the latest, greatest prosthetic leg. When that technology can be translated optimally to a particular individual’s anatomy, lifestyle and personal aspirations, there’s no telling how far the process will take us.
Nevertheless, selecting the “best” foot from the expanding list of contenders can be quite a challenge as time-honored favorites are regularly surpassed in technology, performance, and patient acceptance. It is the prosthetist’s role to remain current on the latest proven products and thereby help the prescribing physician, patient, caregivers, and others involved in the rehabilitation effort understand the benefits and drawbacks of the various feet under consideration.
The ankle-foot component is a critical determinant of prosthetic success. The closer it matches the abilities, environment and activity desires of the amputee, the better the outcome.
The Health Care Financing Administration’s system of functional levels governing Medicare reimbursement for lower-limb prosthetics provides a convenient framework for categorizing the various ankle-foot options by performance and patient type.
Level 1 – Household Ambulators
Amputees in this category tend to be older patients who have undergone amputation due to vascular insufficiency. They generally require safe, basic function and lightweight for moving relatively short distances. The SACH (solid ankle, cushion heel) foot is generally the foot of choice for this type of patient, although a single-axis foot may be appropriate for higher-level amputees.
The SACH foot simulates plantarflexion at heel strike by compression of an elastic heel wedge and provides forefoot dorsiflexion by means of a flexible toe section. The SACH foot’s simple construction (no moving parts), lightweight, low cost and minimal maintenance requirement make it the common choice for Level 1 patients; enhanced versions are frequently selected for Level 2, and occasionally even Level 3 patients as well. Reflecting its simplicity and comparative low cost, the SACH foot is frequently selected for preparatory (temporary) prostheses, regardless of functional level.
Note: The SACH foot generally offers less knee stability than articulating foot designs. Consequently, ankle-foot components with moving joints are generally preferred for above-knee applications.
The single-axis foot, the predominant prosthetic foot design until the early 1960s, was originally developed during the Civil War. This most basic of the articulating foot designs provides plantarflexiondorsiflexion movement about an “ankle” axis, limited and cushioned by bumpers. Single-axis feet are typically lightweight, low-cost and light-duty, although certain models incorporating dynamic response characteristics are rated as high as Functional Level 3. Because articulating feet increase knee stability in early stance phase, they are often preferred for above-knee amputation levels.
Level 2 – Limited Community Ambulators
Amputees whose functional potential fits in this category can benefit from more durable SACH foot models, non-articulating elastic keel feet, certain multiaxial designs, and feet incorporating lower-level energy-storing characteristics.
Non-articulating elastic keel feet provide function similar to a SACH foot but are a bit more flexible, allowing the forefoot to adjust to varied walking conditions and conform to uneven surfaces.
Multiaxial ankles are well-suited to community ambulators in that they provide triplanar accommodation of uneven terrain – inversion-eversion, internal and external transverse rotation, and dorsiflexion-plantarflexion.
The multiaxial mechanism may be a distinct ankle component mated to a separate prosthetic foot or an integral part of the foot. Originally suited primarily for only the strongest, most active patients, multiaxial components have evolved into less-complex and lighter designs that are now appropriate for less-capable individuals as well. Multiaxial feet are particularly appreciated by amputees who enjoy outdoor activities, notably hikers and golfers. They also lend themselves well to the needs of bilateral amputees.
Level 3 – Active Community Ambulators
Amputees within this classification have access to many advanced design features, which typically incorporate dynamic assist at toe-off, helping propel the leg into swing phase and reducing energy expenditure.
These energy-storing-and-release or dynamic response feet address a frequent complaint of lower limb prosthesis-wearers: the “dead leg” feeling experienced with each toe-off as residual limb musculature must provide total propulsion of the prosthesis. In a dynamic response foot, the flexible keel functions as a spring, which deforms during weight-bearing, storing energy, then releases that energy during late-stance phase, providing forward propulsion.
Early dynamic response feet were created mostly for amputee athletes, but steady improvements in design, weight reduction, reliability and cost have brought these components within the realm of moderately active, “everyday” amputees.
Most of the early dynamic response models lacked an ankle component; however, improvements in multiaxial design have made systems combining articulating and dynamic response characteristics not only possible but practical. These advanced designs are providing performance, comfort and stability never before available to most patients.
Level 4 – High Activity — Child, Active Adult, Athlete
True Level 4 applications are typically high-tech, high-impact and high cost. They are also the proving ground where the everyday systems of the future are developed. Relatively few amputees will qualify for reimbursement of ankle-foot components in this category, but the principles and features incorporated into these highly sophisticated systems have a way of appearing in feet appropriate for patients of lower functional level in future years.
The first lightweight, high-strength Flex-Foot designs, for example, were built to serve the needs of premier amputee athletes. Once proven, those advanced concepts of the 1980s were subsequently refined and incorporated into products more suited to the needs of Level 3 and sometimes even Level 2 patients.
The classic Flex-Foot J-shaped foot-shank composite is still provided primarily to younger, vigorous patients; however, the underlying carbon-fiber construction is being built into more-traditional foot models that can provide a more fulfilling lifestyle for older, less-active individuals.
So how do we arrive at a particular foot recommendation for a given patient? We start with a thorough assessment of the patient’s age, physical condition including amputation level, mental capabilities, lifestyle including vocational requirements, and the desires and expectations of the patient and his or her family. We assess ambulation potential, including stability and balance, predicted cadence, weight and overall fitness, then factor in financial resources and family support.
Foot selection typically entails tradeoffs involving performance, durability, weight and cost. While active patients and amputee athletes garner the lion’s share of the media and marketing attention, the far greater numbers of lower-limb amputees occupy the opposite end of the ability spectrum: typically older, dysvascular people who have neither the energy nor the desire to walk more than a block or two. For these patients low weight, and often low cost, become overriding factors.