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Clinical Evaluation of a New Design Prosthetic Prehensor

Susan Procter, O.T.R.
Maurice LeBlanc, M.S.M.E., C.P.

Introduction

This project resulted from previous work involving exploration of new designs of prosthetic prehensors which are neither hook nor hand. From three designs which were generated, the one herein was selected as the best to be clinically evaluated.

Three prototypes were made and were tested with three unilateral adult below-elbow amputees. Results indicated that the amputees preferred their standard hooks but welcomed the fact that work had been undertaken to investigate innovative ideas for possible improvement in prehensors.

Background

This project resulted from a survey of arm amputees who said that they do not mind having a hook or tool as a prosthetic prehensor, but they would like it to be nice looking (1). The intent was to develop a prehensor which offers the functional advantages of a hook but looks better. The idea included exploring the concept of a prehensor which is neither hook nor hand but unique for its own sake (2,3).

Much has been said about different kinds of prehensors and their use (4-7). The most popular one in the USA is the hook, which represents about 70 percent of prehensors provided (8). The most popular (largest selling) hook is the Hosmer-Dorrance 5XA (Figure 1) .

Of the three new designs which were developed and assessed by amputees, prosthetists, occupational therapists and lay people, the design described below was judged to have the most potential and best to test with amputees (9).

The new design prehensor has a fixed "palm" and a rotating "thumb" so that it can grasp much as a normal hand either against the forefinger or against the palm. In the normally closed or relaxed position against the forefinger, it functions as a voluntary opening (VO) prehensor. In the fully open position with the thumb rotated around against the palm, it functions as a voluntary closing (VC) prehensor. That is, it is both VO and VC, with a constant force spring giving about two pounds of prehension in the VO position and a prehension force proportional to cable pull in the VC position (Figure 2 , Figure 3 , Figure 4 , Figure 5 , and Figure 6 ).

Method

Three prototypes were made by Hosmer-Dorrance Corporation for the project. They were fitted to three unilateral adult below-elbow amputees by using their amputees' existing prostheses and replacing hooks with the new prehensors and duplicate shoulder harnesses for cable control. The three subjects were all male, experienced users. One was an aircraft mechanic aged 50, one a teacher aged 49 and one an accountant aged 40.

A clinical evaluation was conducted according to a defined protocol. The procedures included:

1. Pre- and post-test attitudinal surveys.
2. Rating the performance of each subject's hook use with a battery of ten functional tasks before fitting with the new prehensor. Grasping and holding were measured on a 1-5 scale, and time of task was recorded. The tasks were:
• unwrapping a stick of gum
• carrying a tray
• cutting with a knife and fork
• cutting paper with a scissor
• holding and discarding playing cards
• putting a dollar bill in a wallet
• holding a soda can
• sweeping with a push broom
• putting on a slip-over sweater
• pruning a tree with loppers
3. Fitting of the new prehensor and a half-hour to one-hour training session with it until the subjects felt they had mastered control.
4. Field testing of the new prehensor for two weeks full-time use.
5. Rating the performance of each subject's use of the new prehensor with the same battery of ten functional tasks after fitting and the two-week trial period.

Findings

As expected, there were advantages and disadvantages found in trial use of the new design prehensor. These findings by the amputee subjects are summarized below:

Appearance: In general, the new prehensor was preferred in appearance over the standard hook. It drew much the same public interest, but the subjects felt it did not look as ``vicious" or threatening as the hook. Also, there was some visual interest and novelty in the new prehensor with the thumb rotating from one grasp position to the other. The new prehensor was painted a slate blue color. One subject liked the idea of having a color choice, but the other two wanted the prehensor to match the prosthetic forearm.

Function: In general, the standard hook was preferred in function over the new prehensor. The amputee subjects liked the new prehensor for (1) pushing upright objects around with the cupped palm, (2) lifting large objects with both hands, (3) not scratching surfaces as easily as the hook, (4) putting on clothes with sleeves, (5) operating drinking fountain handles, (6) lifting and carrying children, (7) changing diapers and (8) having two different kinds of grasp.

The amputee subjects disliked the new prehensor for (1) performing fine tasks, (2) being heavy (13 oz.), (3) having to be more careful with it around heat and chemicals, (4) having less VO prehension force, (5) handling small items, (6) having no curved hook function for carrying a suitcase or attache case, (7) stabilizing eating utensils and tools, (8) typing, (9) grasping objects in the VC mode and (11) holding an automobile steering wheel.

Control: The control motions for VO use of the new prehensor are identical as for the standard hook. It uses the same harness.

Amputee subjects liked the smooth action of the rotating thumb on the new prehensor, which has a constant tension spring. That is, the spring force remained constant regardless of the size of the opening.

There was substantial difficulty in using the VC grasp function of the new prehensor for several reasons: (1) it was at the end of the cable excursion; (2) the mating surfaces of the thumb and palm were not optimum; and (3) the angle was not good for visual feedback of the grasp.

Consequently, the VC feature of the new prehensor was not used much. In essence, the new prehensor was evaluated as a VO device not taking advantage of the dual VOVC potential.

Conclusions

• The new prehensor was preferred in appearance, and the standard hook was preferred for function by the three amputee subjects.
• Given the choice between the two, function would not be sacrificed for appearance. The amputee subjects were unanimous in their overall preference to retain their standard hooks.
• The amputee subjects felt that the concept of having VO and VC grasp in one prehensor is desirable and should be pursued.
• Because the VC grasp feature did not work well and was not used much, the new prehensor was evaluated mostly as a VO device. As such, it did not compete well with the standard hook, largely because the tips of the new prehensor were much bigger than the narrow hook fingers and were less functional for fine tasks, handling small objects and for paperwork.
• There was an appreciation by the amputee subjects that people were thinking about new ideas and different designs toward possible improvement in prehensors, implying that there is a need and that amputees are not completely satisfied with current hook and hand options.

Recommendations

From this clinical experience, it is the opinion of the authors that:

• The concept of using a rotating thumb prehensor with both VO and VC grasp should be further explored and evaluated.
• Other designs of prehensors which are neither hook nor hand should be explored and evaluated.

Acknowledgements

This work was supported in part by (a) Field-Initiated Grant No. 133MH70021 from the National Institute on Disability and Rehabilitation Research, U.S. Dept. of Education, (b) The Morris Stulsaft Foundation of San Francisco, and (c) NASA Ames Research Center, Technology Utilization Office.

Susan Procter is a consultant occupational therapist 328-2449.

Maurice LeBlanc is director of research at the Rehabilitation Engineering Center at the Children's Hospital at Stanford, 520 Sand Hill Rd., Palo Alto, California 94304, telephone (415) 324-9991 and fax (415)

References:

1. LeBlanc M. Innovation and improvement of body-powered arm prostheses: a first step. Clinical Prosthetics and Orthotics Winter 1985;13-16.
2. LeBlanc M, Parker D, Nelson C. Do we need a prosthetic prehensor which is neither hook nor hand? In: Proceedings of RESNA tenth annual conference, June 19-23, 1987;204-206.
3. LeBlanc M, Parker D, Nelson C. New designs for prosthetic prehensors. In: Proceedings of the ninth international symposium on external control of the human extremities, August 31-September 5, 1987;475-481.
4. American Academy of Orthopaedic Surgeons. Atlas of Limb Prosthetics. CV Mosby Publisher, 1981;122-132.
5. Billock J. Upper-limb prosthetic terminal devices: hands versus hooks. Clinical Prosthetics and Orthotics 1986;1O(2):57-65.
6. Fishman S, Berger N. The choice of terminal devices. Artificial Limbs 1955 ;2(2) :66-77.
7. Muilenburg A, LeBlanc M. Body-powered upper-limb components. In: Comprehensive management of the upper-limb amputee. Springer-Verlag Publisher, 1988.
8. LeBlanc M. Use of prosthetic prehensors. Prosthetics and Orthotics International December 1988:152-154.
9. Meeks D, LeBlanc M. Preliminary assessment of three new designs of prosthetic prehensors for upper-limb amputees. Prosthetics and Orthotics International April 1988:41-45.