Springlite Foot: A Composite Material Engineer's Approach to Design
JOHN MERLETTE, BS, ME
Engineering and manufacturing expertise applicable to military aircraft has been turned to the problems of foot prostheses. Advanced composite materials and analysis techniques lend themselves to foot design. The design goal was to achieve a prosthesis that is simple, durable, light weight, and closely mimicks the natural foot in performance. Composite materials, such as carbon and fiberglass filaments in an epoxy resin matrix, have a high strength-to-weight ratio. Finite element analysis computer programs from the aircraft industry are valuable in creating designs which minimize material weight for the loads anticipated and optimizing local resistances to force.
The design process may be summarized in eight steps.
First, one must decide whether the new foot should be exo- or endoskeletal. Composite materials are very strong and stiff and thus do not need to be in a heavy exoskeletal shell. The narrow endoskeletal shank approach offers greater design freedom, permitting the introduction of torsional and bending flexibility not possible with an exoskeletal model. Closed cell foam can be used to finish the prosthesis for a more realistic surface texture and for use in water. A rectangular cross-section to the shank minimizes fabrication cost and permits some torsional bending and axial flexing.
Second, joining sites must be selected. Traditionally, the joint is at the ankle, via a vertical tubular shank joined to a separate foot. Although the natural foot flexes at the ankle, a prosthesis does not require such a joint. Composites allow making the foot in one piece without joints or local stiff zones.
Third, the shank, foot, and heel are made in one piece, with a thin composite laminate. Consequently, the heel portion must be prevented from deflecting uncontrollably when the user walks. Holes could be drilled to rivet or bolt the foot and heel rigidly, but this would violate a basic rule of carbon composite construction, namely avoid penetrations into the composite, especially in fatigue applications. Composites delaminate and fracture near holes, known as initiated cracks. By bonding the pieces in place with an elastomer, the parts can move and flex, within controlled limits, to impart a life-like action.
Fourth, a light, stiff composite combined with a tough elastomeric rubber will simulate the musculoskeletal and joint ligament systems in the anatomic foot. The elastomer that fastens the foot to the heel can be extended to the toe between the composite layers, thus allowing the toes to flex at the metatarsophalangeal joint line. By adding plies of composite to the heel and upper foot segment at the toe area, toe resistance can be increased according to the user's body weight or activity level.
Fifth, hybridizing dissimilar materials is also appropriate for the shank. Rubber sandwiched between thin anterior and posterior composite laminations provides exceptional ankle and torsional bending for a light child or frail geriatric patient. Composite layers are added as body weight increases. For heavy or very active individuals, solid composites are used.
Sixth, between the heel and foot laminates the rubber has a cavity molded into the posterior end of the rubber at its apex. Inserting and removing a separate, closely fitting rod allows one to shift the fulcrum of the cantilevered heel, increasing or decreasing the active heel length, thus altering heel resistance. The amputee can do this quickly at any time to accommodate various activities and footwear.
Seventh is the problem of attaching and aligning the foot. A clamping type of fitting was created that bolts directly to any of the available alignment tools and to several above-knee frames. Using lightweight aircraft aluminum and clamping to the upper shank of the prosthesis, the simple installation saves the prosthetist time and allows active amputees to change to a specially prepared prosthesis, for skiing or weight lifting, or for unusual footwear such as cowboy boots, without having to remove the socket from the residual limb.
Finally, one cannot clamp directly to the composite shank. A close union of shank to fitting is needed. A precision machined sleeve of tough nylon bonded to the solid shank acts as an interface. If adhered to a shank with the elastomer in its core, however, the bond could fail with a loud creaking noise. Highly elastomeric urethane is cast onto the shank to provide good fit that will not debond under high stress.
Clinical trials throughout the United States since 1988 have involved more than 200 patients, including three youngsters and eleven teenagers. A prospective interdisciplinary study at the University of Michigan has yielded encouraging preliminary results.
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