Technical Note: Friction-Free Cable System: Alternative Cable System for Transhumeral-Level Conventional Prosthesis
Waldo Esparza, CP
Joseph J. Ivko Sr.
Abstract
An improved cable guide assembly applicable to conventional transhumeral prosthetic devices has been designed. When subjected to varying rotational and directional cable stress, this assembly has the ability to automatically align itself, minimizing cable stress. The assembly is particularly applicable to prosthetic devices and, besides extending cable life significantly, the occasional changing of the cables is facilitated and can be easily accomplished by the partient.
Introduction
A friction-free cable system has the ability to align itself along the operative axis of the cable, thereby minimizing stress placed on the cable and significantly increasing its life span (1).
Cable guides commonly used on machinery, scientific instruments, conventional prosthetic devices and other apparatus generally consist of tubular housings through which the cable moves (2). When the cable moves back and forth in a rectilinear path, such cable guides normally are satisfactory. However, when a cable is subjected to angular movement relative to the tubular axis of the housing, friction stress is placed on the cable, substantially reducing its useful life (3).
Such stress-/abrasion-induced cable failure is a common problem with cables used to activate prosthetic devices worn by unilateral and bilateral upper-extremity amputees. These devices subject the cable to a variety of stresses. Because cables are difficult to reinstall, the user of such a prosthetic device is constantly subjected to inopportune breakdowns of the cable system. Lining the interior surface of a cable sheath or housing with TeflonŽ is only a short-term solution (4). When the Teflon liner wears out, the cable wears directly on the steel housing, causing the cable to abrade quickly and necessitating frequent cable replacement. When conventional cable sheaths are replaced by cable guide assemblies that automatically align themselves with directional stresses, the life expectancy of the assembly can be increased from six weeks to one year or longer.
Methods
The friction-free cable system is primarily applicable for transhumeral and higher-level prostheses. The system consists of three self-aligning guides made of self-lubricating ultra-high molecular weight (UHMW) plastic. The guides are mounted to a stainless steel block via a universal joint referred to as a "dumbbell." Each of the three guides has an alignment at the base to match the three different placements at the prosthesis (see Figure 1)
. The guides will accept conventional prosthetic cable.
Analysis
The friction-free cable system performs quietly and more smoothly with heavy-duty cable; it is easy to use. The guides are aligned posteriorly and medially at the humeral section (see Figure 2)
. Elbow flexion capabilities (see Figure 3)
are improved since the angle of lift has been increased.
This design was found to be effective for reducing the amount of cable travel and friction loss without affecting cosmesis. In addition, the simplicity of the system allows the patient to replace or repair a broken cable himself.
The forearm will require modification (see Figure 4)
to use this system. A slot 5/8-inch wide by 2 inches in length is needed at the proximal base of the wrist attachment to allow full pronation and supination of the terminal device. The anterior border of this opening should be scived.
The system may be used for new prostheses or retrofitted to existing devices.
Testing
Operating (and opening) a terminal device using six rubber bands and employing cable housing and a Teflon liner requires 55 to 60 lbs of excursion of the scapula. The friction-free cable system, on the other hand, requires only 20 to 25 lbs of excursion force.
References:
- Bowker JH, Michael JW. Atlas of limb prosthetics, surgical prosthetics and rehabilitation principles, 2nd ed. St. Louis: Mosby Year Book, 1972;347.
- Santachi WR. Manual of upper-extremity prostheses, 2nd ed. Los Angeles: Department of Engineering, University of California, 1959.
- Carlson LE, Veatch BD, Frey DD, et al. Efficiency of prosthetic cable housing. JPO 1995;7:3:96-9.
- Sammons F. The use of low-friction housing liner in upper-extremity prosthese. Bull Pros Res 1983;10:4:77-81
|
