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Spectron 12 Cable for Upper-Limb Prostheses

Lawrence B. Carlson, D.Eng.
Bob Radocy, MS.
Peter D. Marschall, M.B.A.

Introduction

In spite of a significant amount of research and development in externally-powered prostheses, it is estimated that up to 90 percent of the upper-limb amputees who wear prostheses still wear "conventional" bodypowered prostheses (1). One may speculate on the reasons, but one important conclusion is that research efforts need to be applied to body-powered prostheses as well as to more sophisticated systems if we are to serve our amputee population.

Perhaps reflecting the statistic above, there has been a resurgence of interest in body-powered prosthetic research. Both the Veterans Administration (VA) and National Institute on Disability and Rehabilitation Research (NIDRR) now fund research programs in this area. Recent publications describe methodology to quantitatively measure the force and excursion requirements of prostheses (2), new methods for actuating prostheses (3,4), new shapes for prosthetic prehensors (5,6) and general improvements to the systems (7).

This paper describes the laboratory and clinical testing of an alternative cable material for actuating upper-limb prostheses.

Prosthetic Control Cable

A body-powered prosthesis usually employs a harness for two purposes: suspension and control. Through relative body motion, the amputee is able to generate tension in a control cable. By routing the cable through a housing, the tension can be transmitted to the prosthesis, where it can generate the desired effect.

Current control cables date back at least to the period following World War II, when aircraft technology was applied to artificial limbs. The result was rotary swaged stainless steel cable, often referred to as Bowden cable. It is commonly available in three sizes: 3/64-inch, 1/16-inch and 3/32-inch. End fittings are crimped on using a special tool in a vise.

While steel cable has served well, it has some drawbacks. It requires special tools to fabricate cable assemblies, so that amputees usually cannot replace their own cables. Its main disadvantage is its stiffness, which means that it cannot follow sharp radius bends. The ability to follow tight bends with high efficiency would open up many opportunities for new ways to actuate bodypowered systems, especially if it could be easily replaced.

Spectron? 12 Cable

Spectron? 12 cable (12) is manufactured from Spectra? fiber (13). Spectra is an ultrahigh molecular weight (UHMW) extended chain polyethylene fiber, characterized by a high degree of orientation and minimum chain folding. Conventional polyethylene fibers, in contrast, are of a relatively low molecular weight, have moderate orientation and extensive chain folding (Figure 1) .

As a result, Spectra fibers exhibit high tensile strength and toughness, have high abrasion resistance and good ultra-violet resistance combined with low specific gravity. These characteristics and Spectra's natural lubricity resulted in the selection of the material for evaluation as an alternative to standard rotary swaged stainless steel cable for operating body powered prehensors.

Spectra fibers in stockinette form (Spectralon (14)) have been used in fabricating laminated prostheses. The material has also had previous success in sail cloth, marine rope and cable, as a ballistic fabric, in boat hull laminates and in various pieces of sports equipment (8).

Laboratory and clinical research studies were conducted to ascertain whether Spectron 12 was a viable alternative for prosthetic cable.

Methods and Materials

The laboratory testing of Spectron 12 involved two areas: maximum tension and cable fatigue. Tension measurements were conducted utilizing an Instron tensile testing machine. The cable was wrapped around specially designed mandrels before clamping to minimize stresses at the attachments, ensuring that failure occurred between the grips. All cables were tested alone and with the end fittings which allow attachment into the prostheses.

Cable fatigue analysis involved cycling each of the described materials in a conventional dual-control above-elbow prosthesis mounted vertically in a test stand. The prosthesis consisted of a Hosmer E-400 elbow and a Hosmer Dorrance 5X split hook prehensor at two different load, or resistance, levels (3 hook tension bands and 7 bands). Tension was applied to the control cable by a rodless pneumatic cylinder. One cycle consisted of flexing the elbow fully against the flexion stop, opening the hook fully, closing the hook and allowing the elbow to extend fully.

Comparative data were developed between 3/64-inch, 1/16-inch and 3/32-inch diameter stainless steel cables and 1/16-inch diameter Spectron 12. In the fatigue tests, the Spectron 12 cable was tested with and without a standard Teflon housing liner. Four tests were performed for each configuration.

The clinical testing of Spectron 12 lasted six months and was accomplished with the assistance of 16 facilities. These included professional prosthetic companies, hospitals and rehabilitation facilities. The facilities chose patients who had previous experience wearing body-powered prostheses and substituted Spectron 12 for the cable material they normally used. Specialized fittings were designed and required for using Specton 12, because traditional swaging cannot be used to attach Spectron 12 into the standard prosthetic fittings. These fittings were of a two-piece design developed to capture a knot of Spectron 12 inside the components (Figure 2) . The knot forms a "plug" under compression, which jams itself into an internal taper in the fitting, preventing the Spectron 12 from pulling free. Complete installation instructions, illustrating the knots and assemblies, were provided to each facility.

The data requested from each test facility included the age and sex of the subjects, cable materials previously used, the answers to three questions and comments/observations. The three questions were designed to rate Spectron 12 against the cable material(s) previously used in the following three areas: overall installation, operation efficiency and durability (life). Responses could range as follows: significantly worse, slightly worse, same, slightly better, significantly better. Comments and observations could be contributed at each question level and/or for the overall subject's experience. A facility questionnaire was also included to summarize subject experiences (see Appendix).

Results

Cable strength comparisons are listed in Table 1 and illustrated in Figure 3 . As would be expected, the strength of the steel cable increases with size, ranging from 314 lb. (1399 N.) for 3/64-inch cable to 1122 lb. (4993 N.) for the 3/32-inch size. The most commonly used size (1/16-inch) steel cable had a tensile capacity of 538 lb. (2394 N.), while the same size Spectron 12 cable is slightly less, at 419 lb. (1862 N.)

With fittings attached, however, the tensile capacities of all cables were significantly less, and no longer proportional to cable size. As seen in Figure 3, all strengths fell within a range from 149 lb. (664 N.) for Spectron 12 to 225 lb. (999 N.) for 3/32-inch steel.

As can be seen in Table 2 and Figure 4 , there is no apparent correlation of fatigue life with cable size. At three bands of resistance, the 3/64-inch and 3/32-inch diameter steel cables exhibited mean lives of about 15,500 and 13,300 cycles, respectively. The 1/16-inch diameter steel cable lasted less, 7,700 cycles, while the same size Spectron 12 lasted approximately 25,000 cycles.

At seven bands of resistance, fatigue lives declined in all materials. The two smaller steel cables both lasted a little more than 4,000 cycles, while Spectron 12 only lasted an average of 2,000 cycles before failure. The 3/32-inch diameter steel cable lasted 12,200 cycles, which was close to its value at three bands.

The Teflon housing liner with Spectron 12 cable had the effect of increasing the fatigue life by 40 percent at three bands and more than tripling the life to 6,500 cycles at seven bands, which was 55 percent higher than the mean life 1/16-inch steel cable.

A significant fact is that all of the cable assemblies tested failed at the lift tab, which is where the cable makes the tightest bend. This led to the speculation that perhaps cable life could be extended considerably if the sharp bend at the lift tab could be eliminated.

A simple lift pulley was designed, patterned after the Lift Lock described by Carlson and Childress (9) but without the locking feature. As shown in Figure 5, the lift pulley consists of two halves which are free to rotate with respect to each other and an internal pulley which can rotate freely on ball bearings. The cable path is always tangent to the pulley, regardless of elbow angle.

As seen in Table 3 and Figure 6 , there is an increase in fatigue life for both steel and Spectron 12 cables at both load levels. At three bands, the lives for steel and Spectron 12 increased by factors of 19 and 23, respectively. At seven bands, the lives increased by factors of 10 and 100.

Clinical research results were as follows. Nine of the 16 facilities submitted data. Twenty-six subjects were evaluated with an average age of 13.2 years. Seventy-seven percent of the participants were male; twenty-three percent were female. Materials used in comparison to Spectron 12 were standard 1/16-inch diameter stainless steel cable, with and without the use of Teflon liners and nylon or Dacron line. In one instance 3/32-inch diameter stainless steel cable had been used previously. Questionnaire results are shown in Figure 7 and Figure 9 .

Comments and observations included the following:

• Fittings were too large to use effectively with small children;
• Possible trouble with water contamination;
• Excellent for use "inside" CAPP II prehensor;
• Lasts much longer, much stronger, better overall;
• Adults don't feel secure with Spectron 12;
• Concern about burning/melting; and
• Need simpler fittings to reduce/ease installation time.

Discussion

At 149 lb., the mean tensile strength of the Spectron 12 cable assembly was slightly lower than that of steel. However, published values show that the maximum harness tension that an amputee can generate is less than 70 lb. (321 N.) (10 J). Our own laboratory tests (11) on four amputees yielded maximum cable tensions on the order of 90 lb. (400 N.), with an average value of 81 lb. (360 N.). Therefore, all of the cables tested should be adequate for prosthetic use.

When used in a Teflon liner in a conventional system using a lift tab, Spectron 12's fatigue life is longer than that of 1/16-inch diameter steel: almost five times longer at three bands and 44 percent longer at seven bands. The use of the lift pulley can apparently increase fatigue life for any cable used at least an order of magnitude.

Overall, the clinical survey results were encouraging. Question 2, dealing with ease of installation (Figure 7) , suggested that the new system was minimally harder to install. This can be partially attributed to inexperience with a new technology compared to one that has been around for more than 40 years. Several comments indicated that installation became easier with practice.

Most responses rated operation and efficiency the same (Figure 8) . One response indicated the Spectron 12 to be significantly worse, and 11 responses rated it better.

Results dealing with life (Figure 9) were mixed. Those fitting children scored Spectron 12's life significantly better, while those fitting adults reported the life to be substantially worse. This may be due to higher loads and harsher service in adult prostheses. However, the small sample of six adults is probably too small to yield conclusive results.

Conclusion

Laboratory and clinical research data with Spectron 12 cable suggest that this material should be employed further in upper limb body-powered prosthetics. Basic concerns regarding safety and efficiency have been preliminarily satisfied. Durability in adult populations needs to be further investigated in the field. Durability with child populations has been demonstrated.

Although the fittings are relatively small and easy to use, several comments suggested that they should be smaller and simpler. Our experience has shown that while it is very easy to tie the knot and assemble the fitting, it is somewhat difficult to achieve exactly the right length of the cable assembly. Once tied, it is difficult to adjust cable length.

Therefore, a simpler fitting was designed that makes it easier to fabricate cable assemblies at the desired length (Figure 10) . The Spectra is laced through the two holes, then tied in a simple fishing knot. Laboratory testing of this fitting has shown it to be of adequate tensile capacity and resistant to untying when subjected to repeated cycles of tension and relaxation. In addition, it has the advantage of being one piece instead of two.

Since most handless persons in the United States prefer to use body-powered prostheses, Spectron 12 offers the potential of improving their performance and usage by increasing the efficiency of operation of their prosthetic equipment.

Acknowledgements

The clinical research portion of this study was made possible by the cooperation of the following facilities:

• Bardach-Schoene Co., Chicago, Ill.;
• Children's Hospital at Stanford University, Palo Alto, Calif.;
• Elizabethtown Hospital and Rehabilitation Center, Elizabethtown, Pa.;
• J.F. Rowley Co., Cincinnati, Ohio;
• Prosthetics by Nelson, Buffalo, N.Y.;
• Rehabilitation Institute of Chicago, Chicago, Ill.
• Scheck and Siress Co., Oak Park, Ill.; Shriner's Hospitals at Tampa, Fla. and Springfield, Mass
• Trautman's-Minneapolis Artificial Limb Co., Minneapolis, Minn.; and
• UCLA/CAPP, Los Angeles, Calif.

This research was sponsored by the National Institute on Disability and Rehabilitation Research, Project Number 133GH70186, Grant Number G008720116.

Lawrence E. Carlson, D. Eng., is an associate professor of mechanical engineering at the University of Colorado at Boulder, Campus Box 427, Boulder, CO 80309. He is the recipient of a Distinguished Mary E. Switzer Research Fellowship for 1990-91 from the National Institute on Disability and Rehabilitation Research.

Bob Radocy, MS., is president, chief designer and director of TRS, Inc., 1280 28th St., Boulder, CO 80303, which markets various models of body-powered prehensors.

Peter Marschall, M.B.A. lives in Manlius, N.Y.

References:

1. Childress, DS. Northwestern University, Personal communication, 1987.
2. Carlson, LE and MP Long. Quantitative Evaluation of Body-Powered Prostheses, Modeling and Control Issues in Biomechanical Systems, American Society of Mechanical Engineers 1988;(1 1)12;1-16.
3. LeBlanc, MA. Innovation and Improvement of Body-Powered Arm Prostheses: A First Step, Clinical Prosthetics & Orthotics 1985 ;9(1):13-16.
4. LeBlanc, MA. Evaluation of Cable vs. Hydraulic Transmission of Forces for Body-Powered Arm Prostheses, Proceedings of the 8th Annual Conference of the Rehabilitation Engineering Society of North America, June 24-28, Memphis Tenn. 1985;71-73.
5. LeBlanc, MA. New Designs for Prosthetic Prehensors, Proceedings of Ninth International Symposium on Advances in External Control of Human Extremities, August 31-September 5, 1987.
6. LeBlanc, MA. Use of Prosthetic Prehensors, Prosthetics and Orthotics International 1988; 12(3): 152-154.
7. Radocy, B and LE Carlson. Research to Improve Body-Powered Prosthetics, Proceedings of 15th Annual Meeting of the American Academy of Orthotists and Prosthetists, February 1989, Orlando, Fla.
8. Allied-Signal, Inc., P.O. Box 31, Petersburg, Va. 23804, Spectra sales literature, 1990.
9. Carlson, LE and DS Childress. The lift lock: a device to increase the lifting ability of dualcontrol prostheses, Bulletin of Prosthetics Research 1975;10-23:158-168.
10. Anderson, MH. Harness and Control Systems, Chapter IX in Manual of Upper Extremity Prosthetics, 2nd Edition, Edited by WR Santschi, Department of Engineering, University of California at Los Angeles 1958;155-t88.
11. Radocy, B. The Biomechanical Control of GRIP Terminal Devices. 1982;12 unpublished.
12. Spectron 12 is the trade name of cable and rope products manufactured by Samson Ocean Systems, Inc., Ferndale, Wash.
13. Spectra is the trade name of UHMW polyethylene fiber manufactured by Allied-Signal, Inc., Petersburg, Va.
14. Spectralon is the trade name of cloth woven from Spectra and distributed by Comfort Products, Inc., Croydon, Pa.