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The Carbon Copy II - From Concept to Application

Robert Arbogast, B.S.I.E.
C. Joseph Arbogast

In 1961, the Ohio Willow Wood Company introduced the first Solid Ankle, Cushion Heel (S.A.C.H.) foot manufactured in the United States. The designer, W.E. Arbogast II, had no way of knowing that from this small beginning, his initial efforts would someday result in the production and distribution of a state-of-the-art prosthetic foot that would gain worldwide acceptance. Today's design of the Carbon Copy II can only be a result of the dogged determination instilled in those who followed him to find "the better way."

This "better way" design began in 1974. New resources were finally beginning to filter down from the space program to industrial programs that required better mechanical properties accompanied by a corresponding reduction in weight. The emphasis on weight reduction was no doubt due to the high cost (about $1,000 per pound or $2,200 per kilogram¹) of placing matter in orbit. But before these "new technology" materials could be used effectively in the prosthetic and orthotic industry, criteria reflecting the benefits needed to be defined. The utilization of these advanced materials in meeting these criteria could not involve simply a material replacement. The cost of these advanced materials was too excessive to substantiate the increased cost of the end product without a substantial increase in benefits to the amputee. If these new materials could not result in improved benefits, then possibly the existing S.A.C.H. foot should still be the preferred solution in most cases.

It was decided to take an approach that would result in the most gain to the broadest range of people. No weight or functional advantages would be added to serve only a select group of amputees. Such attention to special groups would be addressed only if it would result in no distinct disadvantage to amputees as a whole. This "new technology" prosthetic foot would be as light as feasibly possible, without sacrificing strength. The age-old philosophy that a large mass at the distal end of the prosthesis is both necessary and beneficial was not followed. Rather, the intention was to manufacture a prosthetic foot with negligible weight and allow the prosthetist, if he so chooses, to add mass where be deems necessary, rather than constrain him with high fixed masses in areas he may deem undesirable.

lt was also decided to utilize qualities of certain new fiber reinforced composites to provide some form of assistance in initiating heel rise at toe-off and in propelling the prosthesis forward. Having manufactured prosthetic feet for 13 years before initiating this program, input from prosthetic facilities was readily available. Several prosthetists, and one in particular whose opinion has come to be valued very highly, explained that their patients experienced a much more normal gait pattern for a .limited time after they installed prosthetic feet that featured a "stiff" forefoot.² They were, in effect, describing one of the key factors responsible for the Carbon Copy II's success. The S.A.C.H foot in question incorporated a relatively high modulus material (able to bend with substantial resistance, but not resulting in any permanent deformation or fracture) used in place of conventional toe belting. The net effect was that the plastic coated belted material acted like a string, storing the kinetic energy obtained after midstance and prior to toe-off. This potential energy, when released gradually in the last few degrees prior to toe-off, caused heel rise and slight propulsion forward. Secondly, and possibly more importantly, it allowed the amputee's center of gravity to maintain a more constant vertical height. As the amputee propelled himself forward from mid stance to toe-off, be did not "fall off" the end of the keel of the prosthetic foot. In fact, the actual fulcrum or pivot point of the distal end of the prosthesis was moving forward with the amputee. When this more constant height of the center of gravity relative to the ground is experienced, the amputee is not continually expending energy to raise his body weight, only to lose this energy when he lowers his body weight shortly thereafter. Unfortunately, this high advantageous effect was soon lost due to the inability of the componentry to maintain the mechanical resistance to deflection, i.e., the materials' fatigue life was extremely short. Should a resource exist that had increased fatigue life, that was equal or less in physical size, that was equal to or less in weight, and that had a comparable modulus (stiffness), the fatigue problem would be solved. A high performance carbon fiber-epoxy laminate was a natural solution. Substitution of this material would not only result in a several fold increase in fatigue life,³ but also in a reduction in weight.

This thin laminate would, however, amplify a problem existing in conventional S.A.C.H. feet. The thin material normally used to provide toe resistance had the tendency to "knife" its way through the flexible urethane foam, eventually causing a tear in the foam. If the carbon plates could be surrounded with a material which demonstrated extreme resistance to abrasion, this failure could be minimized. Kevlar?, a DuPont product, demonstrated this very characteristic.⁴ A custom knitted Kevlar? sock was designed to be placed over the carbon "deflection plates" prior to surrounding it with the urethane shell. This "protection sock" also bonded very well to flexible urethane foam, subsequently acting as a reinforcement fiber for the urethane.

Experimentation was implemented using a full length carbon deflection plate that extended from the toe posteriorly to nearly the tip of the heel. Surprisingly, it was found that the storage of energy in the deflection plate at heel contact resulted in two distinct disadvantages as compared to a conventional heel wedge. First, .when contact of the posterior portion of heel is initiated, a negative moment about the knee axis is generated. If the knee is not designed with enough stability to offset this moment, the knee will be forced to buckle. This restraint limits the flexiblity of the prosthetist in making future alignment changes. Second, when walking barefoot, the collapsing or compression of the flexible urethane heel wedge permits a large relatively evenly distributed load base while weight is being transferred to the prosthesis. Conversely, the use of a laminated spring capable of storing sufficient energy to decelerate the vertical downward forces in a normal gait pattern resulted in a concentrated load over a much smaller area. This situation creates a significant amount of instability upon initial heel contact, due to the smaller area of heel contact and more uneven load pattern. More simply stated, the laminate more closely mimics the situation at heel contact in shoes exhibiting a very hard heel rather than in casual shoes with a soft rubber sole. It was also resolved that some method of responding to an amputee's need for a varying amount of resistance at different walking velocities would be desirable. Much research was initiated and subsequently substantiated by others that two basic levels of resistance for any one amputee was desirable. It was apparent that, during a normal walking gait pattern, the forces experienced by the deflection plates vary only a small amount relative to the velocity, as long as a walking gait pattern is maintained. However, when the velocity is increased so that the amputee enters a jogging or running pattern, or while descending stairs step over step, the strain on the deflection plates increases dramatically by approximately a factor of two. This data confirms the need for two distinct modes of resistance, which was accomplished by the inclusion of a secondary deflection plate. This secondary deflection has only minimal effect during the normal walking cycle, exhibiting resistance to deflection only when higher loads are experienced by the prosthetic foot. In this application it could be viewed as a "helper spring." It does, however, serve one more very important function. At mid-stance, it creates a very stable anterior base, resisting bending until a load sufficient to cause the principle deflection plate to distort is applied.

The next deficiency in conventional prosthetic feet that new materials might successfully address was in the keel. Conventional wooden keels, which have been used almost exclusively until recently by nearly all manufacturers, are susceptible to deformation in the area beneath the head of the foot bolt. This compression of the wood fibers, which is drastically amplified in the presence of moisture, ultimately results in a loss of torque in the foot bolt. Should this loss of torque occur, the alternating loads applied during the normal gait cycle will ultimately result in failure of the bolt due to fatigue. A thermoplastic nylon-Kevlar? continuous fiber reinforced composite was chosen for this application. The advantages of this compression molded substitute prove to be many. Due to the increase in mechanical properties of the composite material, the breakage of the foot bolt is substantially reduced. Deterioration of the keel due to low moisture absorbency of the composite when compared to wood also makes this material an attractive substitute.⁵ Another positive aspect of the nylon-Kevlar? composite is the high impact resistance coupled with the tendency for the Kevlar? to dampen vibration associated with impact rather than transmit it, as do most other composite materials.⁵ This permits the keel to act as a shock absorber between the walking surface and the remainder of this prosthesis.

Last, the cosmesis of the prosthetic foot needed to be addressed. It was decided that lifelike molds would be used. Lifelike, that is, to a point short of compromising advantageous properties. Although lifelike reproductions (Figure 1) require a whole new approach to molding prosthetic feet, the appearance seemed well worth the added cost and effort. Past manufacturing experience indicated that a flat bottom on the prosthetic foot was most desirable. The intent was to have contact at the outer edge of the foot, resulting in a larger, more stable base. A space between the great toe and the others was sacrificed due to possible reduction in life of the carbon plates and the substantial added expense of contouring the carbon plates around this space. However, with the advent of computer controlled water-jet cutting machines, the inclusion of this space may be a possibility.

Once the criteria were established, testing and evaluation of the various types of materials for all the necessary components began. Initially, various high performance composites, including both long and continuous fiber reinforced thermoplastics and thermosets were examined and tested for their suitability as deflection plates. Hundreds of layups were prepared with various numbers of plys of fiber reinforced, preimpregnated materials oriented in various directions and bonded together by many different matrices. These principal and auxiliary deflection plates were flexed hundreds of millions of cycles. It must also be noted that maximum as well as minimum standards were established in regards to the ultimate number of cycles the principal deflection should experience before failure. This procedure was by no means performed in order to establish a limit for planned obsolescence. Rather, it was initiated to arrive at the minimum amount of carbon-epoxy laminate that was necessary fo fully accomplish the purpose. Various other designs which contained larger amounts of this laminate successfully met and often greatly exceeded these minimum standards. However, since the density of this carbon fiber-epoxy composite is higher than .any other major component of the Carbon Copy II,⁶ a weight penalty to all wearers in order to accommodate a few would have resulted. As previously mentioned, it was deemed inappropriate to force all wearers to make the sacrifice in order to accommodate a few. However, it was resolved to fulfill the needs of special cases on an individual, special order basis.

Laboratory cycle testing of the nylon-Kevlar? thermoplastic keel was effected during the final cycle testing of the carbon plates. The mechanical characteristics of the composite were so far in excess of the wood it replaced that the testing was actually pleasurable. In all cases - including compression strength, effect of moisture, specific density, resistance to compression set, and fatigue resistance - the composite excelled. Probably most significant was the 400% increase in impact resistance exhibited by the composite keel when .compared to the maple keel used by most S.A.C.H. foot manufacturers. Resistance to compression set and creep was far greater than unfilled thermoplastics such as nylon and DuPont's Delrin?.⁷ Any foot bolt breakage that might occur can now be attributed almost entirely to the opposite side of the interface, the shin, as bolt torque can be maintained over long periods.

After testing the basic components comprising the Carbon Copy II, it was thought that every major obstacle had been successfully overcome. However, problems with existing light-density, flexible urethanes were encountered. The requirements demanded of this material had not yet been addressed by the plastics industry. The flexible urethane shell would undoubtedly encounter tensile forces, while already in compression in the transverse direction. This situation was encountered during the gait cycle from mid-stance to toe-off. These tensile forces would be present in the urethane foam in the area directly beneath the primary deflection plate. Surprisingly enough, no low density flexible urethane foam had been developed to resist failure in tension while already being loaded in compression in the opposite plane. All urethane of this nature was designed to resist failure primarily in the compression mode for applications such as seat cushions and shock absorbers. A minimal skin on the outer surface, which was resistant to abrasion and as resistant to compression set as possible, was also required. Several more months were spent with a very accommodating major chemical company in successfully developing and testing an adequate material. Even though the failure rate of the Carbon Copy II is less than 3%, nearly all failures are attributed to failure of this component. Research is continuing in order to eliminate or at least reduce even this small problem area.

The final task of putting all the pieces together (Figure 2 and Figure 3 ) was the most pleasant part of the project. After nearly ten years of on .again off-again reseach and an investment of over $400,000 in totally private funds, the end was finally in sight. Subsequent laboratory cycle testing (Figure 4 and Figure 5 ) and clinical testing went much better than anticipated. A prosthetic foot containing components similar to those in the Carbon Copy II, called the Carbon Copy I and manufactured for Mauch Laboratories for use in the 3-C ankle, was being fully tested in a modified version. All techniques and components which would be used in the Carbon Copy II were proven to be without fault. And the bottom line, patient acceptance, was much better than anticipated.

The final test, marketability, was rapidly approaching. Should the Carbon Copy II fail here, all efforts thus far would have been in vain. Was the prosthetic arena ready to accept a highly superior product at a cost of up to six times what it had previously been paying? In June, 1986 the answers began to be apparent. Attention seemed to come to the Carbon Copy II almost spontaneously. It was featured in several technical publications as a result of the application of so many different materials, i.e., long fiber reinforced thermoplastics, high performance fiber reinforced thermosets, Kevlar? in its natural state and also used as a reinforcing fiber within a matrix, specially formulated low density flexible urethane, and low density rigid foam urethane. The Carbon Copy II was the medical division winner of "The Better Way" Design Award of the Year contest sponsored by Plastic World magazine. Even now, over two years after its introduction, it continues to be mentioned in technical trade journals such as Advanced Composites (August, 1988) and to be displayed by materials manufacturers such as Polymer Composites and E.I. DuPont at trade shows such as the National Plastics Exposition (June, 1988). With the demand still rising at a steady pace as a result of remarkable patient acceptance, and with constant subtle improvements still being made, it appears that most of the goals have been accomplished and that Carbon Copy 11 will be around for quite some time.

Robert E. Arbogast, B.S.I.E., is President of The Ohio Willow Wood Company, P.O. Box 192, Mount Sterling, Ohio 43143.

C. Joseph Arbogast is Vice-President of The Ohio Willow Wood Company in Mount Sterling, Ohio.

References:

1. Schwartz, M., Composite Materials Handbook, McGraw Hill Book Company, 1984, p.2.1.
2. Francis, R., C.P., Personal Communication, June 1976, President, Tidewater Prosthetic Center, Norfolk, Virginia.
3. Talreja, Ramesh, Fatigue of Composite Structures, Technomic Publishing Company, 1987, pp. 26-38.
4. Preston, J., "Aramid Fibers," Encyclopedia of Composite Materials and Components, 1983, pp. 97-125.
5. Smith, W.S., 'Environmental Effects on Aramid Composites," Kevlar Composites, 1984, pp. 94-119.
6. Margolis, J., Advanced Thermoset Composites, Van Nostrand Reinhold Company, 1986, pp. 95-109.
7. Youngs, A., 'Long Fiber Thermoplastic Composites," Advanced Composites, The Latest Developments, ASM, 1986, pp.253-256.