The EB1 Foot

Why Focus on Prosthetic Feet?

While all prosthetic components are subject to harsh conditions in the countries that POF serves, artificial feet are the most vulnerable. This is in part because artificial feet have to withstand the greatest physical force, but also because they are more exposed to the environmental elements than other components. Not surprisingly, the foot has historically been the component that wears out first, often causing the artificial limb to become unusable for the amputee. It is therefore very important that functional, durable, and affordable feet can be manufactured and re-supplied locally.

In the mid-1990s, the Prosthetics Outreach Foundation (POF), in collaboration with the Vietnamese Institute of Orthopedic Rehabilitation Sciences (IORS) and the Ha Tay Orthopedic Technology and Rehabilitation Center (OTRC), began an ambitious effort to design and manufacture of an artificial foot that could be made in Vietnam. The foot had to meet the following criteria:

• Durability of 3 years

• Functionally and cosmetically acceptable to amputees

• Manufactured using locally available materials and equipment

• Affordably priced for the local economy (price target: less than USD 5.00)

The Development Process

Analysis of existing components

In 1996, Mr. Pham Minh, a senior mechanical engineer with over 20 years experience in rehabilitation, was assigned as the IORS manager for the project. He interviewed many of the workshop technicians and amputees to document that current status of foot components available in Vietnam. Particular attention was paid to identifying durability issues (materials, adhesion) and patient preferences (function, cosmetics).

Selection of locally available materials and manufacturing equipment Next we located and evaluated sources of local materials that were appropriate and affordable to manufacture the foot component. A vulcanized latex rubber compound was identified as a material with proven durability. Experienced local rubber manufacturing staff was recruited to assist with equipping and renovating the production facility in the small town of Ba Vi located 50km west of Hanoi. Sources of local hardwood, metals and woven reinforcing materials were also found. Definition of foot function and cosmesis (appearance) The team's goal for foot function was to design a simple foot with a reinforced but compliant toe section that would provide a smooth and comfortable gait pattern for the amputee. A single durometer vulcanized rubber was used to mold the foot. The durometer of the rubber was chosen to compromise for a softer heel and reinforced to produce a stiffer but compliant toe section of the foot. The rigid wood keel provides many important design features of this foot: Easily shaped, affordable and locally available With the addition of a primer and mechanical features, adhesion to the vulcanized rubber is excellent A stable structural connection to the prosthesis; the ankle surface and the bolt hole have been reinforced with a steel plate and an aluminum bushing, respectively

Watch! The EB1 Foot Fabrication Process

Design of foot testing equipment

To test the foot for function and durability, two test machines were designed and manufactured in Hanoi. A hydraulic static force machine allowed the engineering staff to measure the force-deflection characteristics of each prototype foot design and to compare this with a patient's evaluation. A simple deflection machine that cycled at 1 hertz (1 cycle /sec) is used to test for materials and adhesion durability. Failure analysis of laboratory and field-tested feet is also vital to improving our design method. As part of the self-reliance program, it was also decided that all production equipment would be manufactured in Vietnam.

Mold and manufacturing development

The foot molds were made from cast aluminum in 2 halves. Achieving the higher quality of surface on the molds was a major challenge for the staff at BOTC. Persisting that the molds be made locally was an important step in the self-reliance plan.

Seven foot molds from size 16cm- 25 cm have been produced. As part of the self-reliance program, it was decided that all production equipment would be manufactured in Vietnam.

To process and mold the rubber two additional machines were required, a compounding roller, made in Hanoi, and a heated hydraulic mold press, made at OTRC.

Prototype testing

A series of prototype feet were molded, tested, and fitted on patients to determine the features that patients preferred and the designs that proved to be the most durable.

The final design has been documented with drawings, material specifications and manufacturing procedures to assure that the molded feet are produced to a consistent standard.

Continuing Improvements — The New EB2 Foot

POF and OTRC continue monitoring the performance of the EB1 foot. Following an independent evaluation of vulcanized rubber feet conducted by ISPO and an evaluation of feet being replaced at POF's Hanoi clinic the following durability issues have been identified:

First, in some cases where amputees living and working in damp conditions the wood core has softened and strength has been compromised. POF and OTRC are currently redesigning a new foot core to be molded in a water-impervious plastic.

Second, ISPO evaluation of vulcanized feet found that wear of the rubber foot sole was a common issue. Foot sole wear of the EB1 foot is the #1 reason for eventual foot failure. To address this problem, POF and OTRC will continue to research improved core shapes and internal toepad materials.

To date over 3,000 of the new EB1 feet have been fitted on amputees in Vietnam.