Roto-molded plastic prosthetic components

Introduction

Living in more remote locations with a lower limb amputation is a challenge. A suitable prosthesis requires a combination of properties. It has to be cheap, strong, simple and waterproof. Experience has shown that an exoskeleton design is preferable in these conditions.

Mobility Equipment for Needs of Disabled Trust (MEND) is in the process of developing a low cost, simple prosthetic solution that stands up to the demands seen mainly in low income countries. MEND in New Zealand has been experimenting with designs from plastic (high density polyethylene) drainpipe. The technique used has been called "RotoMolded" and MEND has also used this technique in making low-cost Roto-Molded wheelchairs.

Currently MEND and it's partners in Nepal (Green Pasture Hospital and Rehabilitation Centre) and India (Handicapped International) are testing prototypes of components of this design.

The technique

Molds are first prepared locally using sheet metal or aluminum. A weighted quantity of polyethylene power is then put into the molds and heated at a constant temperature of 270 degrees centigrade. The mold is rotated slowly, making sure the melted plastic is evenly distributed. By adding increasing amounts of powder the wall thickness can be regulated
The product is formed by the melted plastic running over the INTERIOR surface of the mold and is therefore hollow.

Costs

1.	A mold for a complete. upper and lower plastic limb uses approximately $15USD of sheet metal 1.6mm thick (or 5" / 6" diameter steel pipe with 1.6mm wall) and takes about 5 hours per mold to make using simple tools such as a welder, bender and press.

2.

Each prostheses consumes about 1.2 kg of plastic powder (giving a 3mm wall thickness) and costs about $2.50 USD per kg for first grade quality, or $1.50 USD per kg for second grade quality, which is adequately strong. A metal tube insert at the knee joint and nylon bushings cost $1 USD. Total manufacture cost is around $4 USD including gas heating.

Cooking

1.	The metal mold with powder inserted is cooked for about 25 minutes in a gas-fired oven, or using several gas burners underneath.

2.	Metal bearing tubes at the knee joint are inserted into the mold BEFORE molding and become an integral part of the finished product. This allows for replacement of knee axis bushings.

3.	Any shape or size prosthesis can be designed as long as it can be released from the metal mold

Colours

Skin colour can be matched perfectly to include freckles and spots by addition of some black or brown powder.

Fitting To reduce the travel cost for amputees it may be possible to eliminate the plaster casting process. This maybe accomplished by using a heat resistant sock over which an extra thin plastic is molded. This thin liner could then be welded to the outer roto-molded section of the prosthesis. Iif this can be successfully accomplished it would speed up the total fabrication time and reduce the overall costs.

Testing

The first prototypes of the plastic limbs are being tested in Green Pastures Hospital, Pokhara, Nepal and by Handicap International, Delhi, India. The advantages seen to date are the low weight of the material, the exoskeletal design and the possibilities of different colors. There has been positive feedback from one wearer who remarked, very positively, on the cosmetic value of the prosthesis.

Overall it seems the material and technique is strong and rigid enough for use in a transfemoral (above-knee) prosthesis. Some of the design improvements suggested include increasing the number of degrees available for flexion in the knee, the addition of a locking system for the knee and improving the elastic strap used for the extension assist. Improvements are still needed in the welding and molding processes. If any of our readers are interested in testing this system, MEND is interested in developing this technique further with other organizations. Please contact Rob Buchanan at the following:

Mobility Equipment for Needs of Disabled mist (MEND) P.O. Box 94, Keri Keri, Northland, New Zealand email address - mend@xtra.co.nz