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TECHNICAL FORUM--A High-Performance, Variable Suspension, Transradial (Below-Elbow) Prosthesis

Robert (Bob) Radocy, MS
William D. Beiswenger, CPO

ABSTRACT

The Variable Suspension Prosthesis (VSP) is a transradial (below-elbow) design that combines the proven effectiveness of supracondylar (modified muenster) suspension with new silicone socket technology.

The prosthesis is unique and versatile. It can be worn either as a supracondylar socket with a sock or without a sock using a silicone suspension-type sleeve with an internal forearm interlock. The combination of the supracondylar socket and silicone sleeve provides superior suspension for improved performance in rigorous activities like windsurfing and weight lifting. The prosthesis' ability to be functionally and securely worn with a three-ply sock provides more traditional comfort and added convenience to the user for regular daily activities.

Design and fabrication of the prosthesis follow standard procedures with certain modifications recommended to ensure proper fit and function.

The VSP is applicable to most transradial amputees and is especially valuable to active prosthetic wearers. Field testing indicates the VSP is sound technology and a viable prosthetic alternative for the profession.

Introduction

In 1986, Radocy and Brown (1) described an alternative design for a high-performance, transradial (below-elbow) prosthesis. This design was unique in that it employed a partially lined, supracondylar socket with an unusually large brim and a socket-to-forearm alignment that was pre-extended, not pre-flexed. This prosthesis provided suspension and protection for the olecranon and elbow condyles while allowing for the transfer of distal prosthetic loads through the wide brim to the back of the humerus. The pre extended socket allowed for a wide range of high-performance, gross motor activities, such as weight lifting, archery. windsurfing and nordic skiing. The prosthesis proved to be a robust design, functional and versatile for its day.

Prosthetic designs have evolved rapidly since 1986. Driven by the increasing consumer demand for high-performance prostheses, the profession has developed elastomer suspension sockets and suspension sleeves. These designs can be applied to people missing portions of either the upper limb or lower limb.

Ross (2) in a 1990 presentation to members of the Association of Children's Prosthetic-Orthotic Clinics conveyed positive preliminary experiences regarding the application of 3S socket designs to upper-extremity amputees. Kristinsson (3) explained the ICEROSS suspension option, an alternative to the custom-fabricated 35. He also provided an excellent bibliography outlining the development of these various elastomer suspension systems for lower-extremity amputees. Radocy (4), a transradial amputee, compared for the first time the traditional supracondylar socket with the 3S and ICEROSS systems, describing their advantages and disadvantages.

Radocy's experiences with each of these systems led to the start of the following prosthetic project, which has resulted in the development of a variable suspension transradial prosthesis (VSP). The VSP may be worn as a traditional supracondylar socket yet can be designed to be compatible with 3S or ICEROSS suspension technologies.

This project was a cooperative effort by the authors. Radocy developed the design criteria and specifications and provided personal guidance and input on the limb. Beiswenger provided the professional assessments, casting and fabrication expertise. The goal was to design and fabricate a lightweight, high-performance transradial prosthesis that would take advantage of all of the combined benefits of the supracondylar, 3S and ICEROSS systems, thereby reducing or eliminating any disadvantages exhibited by any individual system.

Measurement and Fabrication

Measurements and impressions of the patient's left transradially amputated arm were taken in the usual manner. Two impressions were needed. The first impression was taken of the arm using a supracondylar-type casting technique. The second impression for the silicone liner was taken incorporating a cutoff strip because the impression had to be taken above the elbow to approximately mid-humerus.

The initial supracondylar impression was modified using supracondylar techniques of modification described by Otto Bock Orthopedics and by John Billock, CPO (5,6). The second cast was modified using the Fillauer 3S charts.

An initial silicone liner was fit to the residual limb and found to be too loose based upon the Fillauer modifications; therefore, a tighter silicone liner needed to be fabricated. During the same session, a clear evaluation socket was fit to ensure proper range of motion, function and reliefs for bony prominences.

The test socket mold was enlarged to allow for the size of the silicone liner. A small Fillauer shuttle lock was incorporated into the fabricated socket, and the forearm was foamed in a pre-extended position.

The prosthesis then was re-fit using a three-ply cotton terry sock for standard supracondylar suspension. The prosthetic socket also was fit with the 3S liner and a nylon sheath over the liner to ease the donning of the prosthesis. Range of motion and alignment were checked to ensure the patient was satisfied before final structural lamination. During the socket lamination and final exoskeletal lamination, carbon fibers were incorporated to provide a lightweight yet strong prosthesis.

This same process has been used with an ICEROSS silicone liner using the same casting and fitting techniques.

Results

Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , and Figure 7 illustrate the definitive prosthesis. The prosthesis is light, 1.5 lbs. (681 g), yet strong due to its carbon-fiber, acrylic-laminated matrix. The partial liner provides excellent cushioned protection for the condyles and olecranon. Additionally, to offer a comfortably fitting socket, the partial liner's material is compliant, providing a margin of contact "fit tolerance" within the socket, which is not available in a rigid or semi-flexible design.

The pre-extended design allows for unrestricted activity at full arm's length with a locked elbow, but it prohibits movement of a prehensor close to the face. This can be an acceptable trade-off for a unilateral amputee. The large brim of the prosthesis provides for the effective transfer of distal loads to the back of the humerus, allowing the amputee to comfortably tolerate heavy lifting. The modified Northwestern Figure of 9 rapidly adjustable harness provides a lightweight, efficient means of capturing body power to create the cable excursion required to activate a prehensor.

The convenience of a traditional supracondylar fit with the optional security of a silicone suspension socket has been achieved in this design with no compromise in weight, appearance or performance.

The prosthesis is worn daily with a three-ply, fully lined cotton terry sock. The partial liner and supracondylar fit prove convenient and comfortable for most all general activities. The 3S liner is donned for special activities such as weight lifting, skiing and windsurfing. A nylon stockinette is worn as an interface to allow for easier donning/doffing of the prosthesis. The 3S or any comparable silicone suspension system provides a more intimate fit and the security required for high-performance, gross motor activities.

The residual limb-to-socket rotation, which can occur during certain activities and result in a loss of suspension in a traditional supracondylar socket, is reduced or eliminated with a silicone suspension sleeve. Once the activity is completed, the amputee has the option of removing the silicone suspension socket, donning a fresh sock and wearing the same prosthesis for the balance of the day's activities.

Conclusion

The VSP appears to be a viable alternative for short- to mid-length, transradial amputees who demand high-performance prosthetic function. The success of this design is based upon the suspension options offered, which greatly increase the prosthesis' versatility. Improved versatility yields expanded function and performance. The ability to be more competitive and more capable bilaterally can now be even more enhanced due to these advanced prosthetic technologies.

ROBERT (BOB) RADOCY MS, is prosthetic component designer and president of TRS Inc. in Boulder, Cob.

WILLIAM D. BEISWENGER, CPO is president of Abilities Unlimited in Colorado Springs, Cob.

References:

1. Radocy R, Brown RD. Technical note: an alternative design for a high-performance below-elbow prosthesis. Orth and Pros, 1986;40:3:43-7.
2. Ross JB. Preliminary experiences in applying 3S prostheses to upper-extremity amputees. Presentation Annual Meeting of the Association of Children's Prosthetic-Orthotic Clinics, 1990, Portland, Ore.
3. Kristinsson 0. The ICEROSS concept: a discussion of a philosophy. Pros Orth Internat April 1993;17:1:49-55.
4. Radocy B. A comparison of prosthetic sockets on a transradial amputee. Proceedings 19th Annual Meeting & Scientific Symposium American Academy of Orthotists and Prosthetists, March 1993, Las Vegas, Nev., p. 37.
5. Myobock. Otto Bock Orthopedic Industry Inc. 1980:84-109.
6. Billock JN. The Northwestern University supracondylar suspension technique for below-elbow amputations. Selected readings: a review of orthotics and prosthetics. 1980:229-35.