Northwestern University Feinberg School of Medicine

Northwestern University Prosthetics-Orthotics Center

The Shape & Roll Prosthetic Foot for Use in Low-Income Countries

Principal Investigator: Dudley Childress, PhD

Project Director: Andrew Hansen, PhD 

Co-Investigators: Margrit Meier, PhD, CPO; Steven Steer, MS

Funded by: National Institute on Disability and Rehabilitation Research (NIDRR) sub-contract with the Center for International Rehabilitation (CIR)

Status: Completed

Purpose

The Shape&Roll Prosthetic Foot is a low-cost prosthetic component intended for use in low-income countries. The goal of our work is a prosthetic foot with excellent performance in walking that is extremely durable, cosmetically pleasing and inexpensive. In addition we have developed simple tooling for the manufacture of the foot and its cosmetic cover. The process for manufacturing and fitting the foot will be made available at no cost to all interested parties.

Methodology

Our examination of roll-over shapes of prosthetic and able-bodied ankle-foot systems suggests that "high-performance" prosthetic feet conform to effective rockers that are similar to those taken by biologic ankle-foot systems during walking (Hansen et al., 2000; Hansen et al., 2004). Our study of a sample of the prosthetic feet from low-income countries has shown that their roll-over shapes are not similar to those of the able-bodied ankle-foot system (Sam et al., 2004A). The physiologic ankle-foot roll-over shape seems to be an appropriate goal for the design of a prosthetic foot since this component frequently replaces the function of the missing foot and ankle. For this reason, the Shape&Roll Prosthetic Foot was designed to mimic the roll-over shape of the physiologic ankle foot system.

The Shape&Roll foot is manufactured using simple, low-cost technologies (e.g. compression molding of co-polymer plastic, sawing, and drilling) and can be easily customized to the height, weight, and foot length of the user. A compression mold was created that makes use of a type two lever (see Figure 1). Once built, no additional tools are needed to operate the compression mold and the mold itself is the tool for removing the mandrel. Since wood is a thermal insulator, the plastic cools down slowly preventing internal stresses and warpage. The molding surfaces are end grain for added strength in compression during molding and tension during extraction. Common bar soap works as a parting agent between the mold and the copolymer plastic material used to make the foot.

The compression-molding device is used to create a "blank" foot that can be customized for right and left feet of various sizes. Figure 2 outlines the remainder of the process for creating a Shape&Roll prosthetic foot core. We have determined that two molding configurations can be used to create eight sizes of prosthetic feet. With a smaller molding surface and inner mandrel, feet can be made ranging between 22 and 25 cm. Using a larger mold and mandrel, feet can be made from sizes 26 to 29 cm. Molds have been created and tested for both sets of sizes and the complete line of feet is currently in production.

Lever compression mold-making machine
Figure 1: Drawing of a lever compression-molding machine used to manufacture Shape&Roll prosthetic foot "blanks".
Sheets of plastic used to make the Shape&Roll Foot
Shape&Roll foot before and after post-manufacturing processing

Figure 2: Pictures used to show the steps in creating a Shape&Roll prosthetic foot core. [A] Pieces of copolymer polypropylene-polyethylene plastic are melted in an oven. [B] These pieces are compressed in a lever-molding device to create a "foot blank" (see Figure 1). [C] The appropriate size and side of the intended foot are drawn on the "blank" using templates and lines are drawn on the forefoot depending on the patient's height. A circle is drawn using a template to indicate the location of the foot bolthole. [D] All cuts are made and an aluminum insert (not shown) is pressed inside the foot to strengthen its point of attachment. Cuts in the forefoot close to create an appropriate shape during walking [Figure taken from Sam et al., 2004B].

Our initial investigations of the Shape and Roll foot have included (1) Fatigue testing in our laboratory**, (2) Clinical testing in our laboratory**, (3) Clinical testing in El Salvador**, and (4) Roll-over shape testing in our laboratory.

(** These documents are in the Adobe PDF format. You will need the Adobe Reader, which is available for FREE, to view these documents.)

Roll-over shape measurements in our laboratory verify that the Shape&Roll foot conforms to an effective rocker that closely matches that of the physiologic ankle-foot roll-over shape (see Figure 3).

Ankle-foot rollover shapeShape&Roll foot with rollover shape superimposed

Figure 3: [Left] Drawing of an ankle-foot system and superimposed average ankle-foot roll-over shape from 24 able-bodied subjects. [Right] Shape&Roll Foot with its roll-over shape superimposed.

Related Publications

Hansen, A.H., Childress, D.S., Knox, E.H. (2000) Prosthetic Foot Roll-over Shapes with Implications for Alignment of Trans-tibial Prostheses. Prosthetics and Orthotics International, Vol. 24, No. 3, 205-215.

Hansen, A., Childress, D., Knox, E. (2004) Roll-over Shapes of Human Locomotor Systems: Effects of Walking Speed. Clinical Biomechanics, Vol. 19, No. 4, 407-414.

Sam, M., Hansen, A., Childress, D. (2004A) Characterization of Prosthetic Feet Used in Low-Income Countries. Prosthetics and Orthotics International, Vol. 28, 132-140.

Sam, M., Childress, D., Hansen, A., Meier, M., Lambla, S., Grahn, E., Rolock, J. (2004B) The Shape&Roll Prosthetic Foot (Part I): Design and Development of Appropriate Technology for Low-Income Countries. Medicine Conflict and Survival, Vol. 20, No. 4, 294-306.

Meier, M., Sam, M., Hansen, A., Childress, D. (2004) The Shape&Roll Prosthetic Foot (Part II): Field Testing in El Salvador. Medicine Conflict and Survival, Vol. 20, No. 4, 307-325.

Childress, D., Sam, M., Hansen, A., Meier, M., Lambla, S., Grahn, E., Rolock, J., Knox, E. (2004) The Shape&Roll Prosthetic Foot. 11th World Congress of the International Society for Prosthetics and Orthotics, presented by Childress in Hong Kong, China, August 1-6. Meier, M., Sam, M., Hansen, A., Childress, D. (2004) The Shape&Roll Prosthetic Foot: Field Testing in El Salvador. 11th World Congress of the International Society for Prosthetics and Orthotics, presented by Meier in Hong Kong, China, August 1-6.