Design Concepts for an Endoskeletal Child-Size Hand
Maurice LeBlanc, MSME, CP
Yoshio Setoguchi, MD
William D. Bowen, MSMSE
Cameron Miner, MSME
Frank Burkholder
Steven Chen
Introduction
This article describes the development of an endoskeletal, body-powered, child-size, mechanical hand that offers improvements in function and appearance.
Currently, five body-powered prehensors are available for young limb-deficient children: Three are hook and non-hand prehensors (Hosmer Dorrance 10Xa, TRS Adept F IIIb and CAPP Ia), and two are mechanical hands (NYU Childa and Hugh Steeper 2.0c) (see Figure 1)
. The hooks and non-hands are relatively efficient to use but are not as acceptable in appearance to parents. The hands are more acceptable in appearance but inefficient in ease of use (1). Some families request electric hands to satisfy both the need for cosmesis and for function. Present mechanical hands are so inefficient that grip is poor; they do not provide a satisfactory alternative to the electric hand and are seldom used.
An efficient mechanical hand is needed so children can get more grip force and families can have a less expensive option to electric hands. There is a gap in the arsenal of prehensors currently available to children with upper-limb deficiencies. Also, parents want both mechanical and electric hands to be smooth in appearance and soft in touch (2).
The problem addressed by this project is especially important for child-size mechanical hands because parents as well as children are involved and because a child hand should not be simply a scaled-down adult hand.
Concept
The authors attempted to address the problem of the child mechanical hand by exploring a new type of endoskeletal design. The hand would have an internal mechanism encased in foam. The foam would have a skin on the outside, thereby foregoing the need for a cosmetic glove since the mechanism would be encased and protected from water and sand. Self-skinning foams are used today in other applications and are attractive and durable. Many prosthetic feet in current use are similar in endoskeletal design where the keel is encased in a foam with an integral skin on the outside.
Potential advantages of such an endoskeletal design include the following:
- The hand would be soft and smooth.
- The mechanism would be entirely encased.
- There would be no glove. (Present gloves contribute significantly to the low efficiency of hands because the hand must work against the glove.)
- The hand would be light in weight.
- The hand would be inexpensive to make.
An endoskeletal hand is straightforward in concept but presents a challenge in design and development to produce one that not only is smooth and soft but that also improves efficiency.
Endoskeletal prostheses are not new in upper-limb prosthetics (3,4). Present versions have an inner glove over the mechanism and an outer glove for appearance. However, they are not very soft and are inefficient because the amputee must work against two gloves.
The authors also propose another change from present mechanical hands: activating the four fingers of the child endoskeletal hand and having the thumb stationary and able to be prepositioned. The four-finger grasp is more normal for young children than the "three-jaw-chuck" grasp for adults. In addition, the authors propose designing the fingers so the metacarpal, proximal interphangeal and distal interphangeal joints are active and give an "adaptive" grasp (i.e., the grasp will conform to some extent around the object). In theory, more surface area will be in contact with objects, and less force will be needed for grasp.
Work to Date
Two concepts have been pursued for the design of the endoskeletal mechanism. The first option involves forming the mechanism from a sheet of polypropylene plastic that is necked down at the joints to make "living hinges." The second possibility involves making fingers from TeflonŽ plastic tubes that are notched at the joints to create joint hinges. Each of these concepts will be discussed below, followed by a discussion of the self-skinning foam.
Polypropylene Sheet Mechanism
The polypropylene plastic concept is similar to the design of the toy Robot Handd (see Figure 2)
and the Collins Hande (see Figure 3)
. Polypropylene sheet is cut in the shape of a skeletal hand, and the material is necked down to .010-inch to .015-inch where joints are designed. Then cables are attached to the fingers and connected to the shoulder harness cable so standard body control motions will operate the hand (see Figure 4)
. Several different types of this design were modeled (see Figure 5)
; these designs varied in thickness of the living hinge, the shape of the cutout and how the fingers are activated.
Teflon Tube Mechanism
The Teflon design involves using plastic tubes as fingers and notching the tubes where joints are desired. The tubes bend at the notches to create joints in those locations. The tube fingers then can be incorporated into an internal palm to complete the endoskeletal structure (see Figure 6)
. Several different types of this design were modeled (see Figure 7)
; these designs varied in the material of the tubes, the shape of the notches and how the cables are attached for activation.
Foam as Soft Tissue and Skin
Figures 8 and 9 show preliminary attempts at making the foam covering to encase the endoskeletal mechanisms. Fabrication of the covering requires making a mold of the hand and injecting the self-skinning foam into the mold. The right combination of soft foam and firm skin is difficult to accomplish since soft foam tends to produce a thin skin and vice versa. Thus, developing a foam that feels relatively soft but has a skin thick enough to be durable requires compromise. Another possibility is to fabricate the foam and skin separately. Experimentation was conducted with different molding techniques and silicone and polyurethane foam materials.
Testing of Models
One-finger models of each of the two designs were made along with a model of a standard linkage mechanism for comparison. The models were tested through a complete range of finger motion measuring the force and excursion required for movement. With these measurements, it was possible to calculate the work in versus the work out and arrive at an efficiency in effort of use. The data are summarized in Table A
.
These efficiencies were calculated the same way as in previous studies for other prehensors (1,5) and therefore can be compared. The work in is the force multiplied by excursion to open or close voluntary-opening (VO) or voluntary-closing (VC) hands, respectively. The "work out" is the force multiplied by the excursion a VO hand returns in grasp or a VC hand requires for opening against the grasping force. The work or energy efficiency is the work out divided by the work in. (The force out divided by the force in is not an efficiency but rather a ratio of forces that depends in part on mechanical advantage and can be varied.)
Discussion
In comparison, the efficiency of the NYU Child Hand with glove is 27 percent, and the efficiency of the Steeper 2.0 Hand with glove is 25 percent (1). These low efficiencies of the mechanical hands currently available make it difficult for young limb-deficient children to achieve grip force sufficient to be useful in activities.
The higher efficiencies of the polypropylene sheet and Teflon tube mechanisms are encouraging. These endoskeletal mechanisms will have to be encased in foam, which will lower the overall operating efficiency much the same way as cosmetic gloves contribute to the overall low efficiency of present hands. However, with good design it may be possible to minimize the foam at the joints and therefore minimize the effort to operate the fingers. Such design, coupled with the lack of need for a separate cosmetic glove, increases the likelihood this approach could be successful.
The two approaches, the Teflon tube and the polypropylene sheet, are similar in their reliance on plastic material to create the hinge, making a simple and inexpensive mechanism. They are dissimilar in that the tube design creates a path or housing for the cable whereas the sheet design incorporates a cable path that must be controlled in some other way.
Conclusion
The authors' attempts at designing and developing an endoskeletal, body-powered hand have potential for improving hand prosthesis efficiency as well as smoothness and softness of the device, which parents desire for their children. It may be possible to produce a mechanical hand that will offer a feasible alternative to present prosthetic hands.
Work on harnessing and control of body-powered hands for young children will be continued by Don McNeal, PhD; Yoshio Setoguchi, MD; Sam Landsberger, ScD; and Julie Shaperman, MS, OTR, at Rancho Los Amigos Medical Center. The work described in this article on endoskeletal construction of child hands will be continued by Maurice LeBlanc, MSME, CP, and others at Packard Children's Hospital. The direction of this work will be to 1) test the stability of the tube and sheet mechanisms, 2) encase the mechanisms in self-skinning foam, and 3) test the efficiency of the endoskeletal hands with foam covering.
Acknowledgements
This work was supported in part by Grant #H133E0015 from the National Institute for Disability and Rehabilitation Research (NIDRR), U.S. Department of Education, to the Los Amigos Research and Education Institute at Rancho Los Amigos Medical Center with Mark Hoffer, MD, as principal investigator and Donald McNeal, PhD, as co-principal investigator. Opinions expressed in this article are those of the authors and should not be construed to represent the opinions or policies of NIDRR. This work also was supported in part by the Ternesvary Fund at Packard Children's Hospital's Rehabilitation Engineering Center. Appreciation is expressed to Julie Shaperman, MS, OTR, for her ongoing input of ideas on the project and review of the article.
References:
- LeBlanc M, Setoguchi Y, Shaperman J, Carlson L. Mechanical work efficiencies of body-powered prehensors for young children. Children's Pros Orth Clinics Winter 1992;27:3.
- Meeting of Project Advisory Group on May 19, 1993. ACPOC Annual Meeting, St. Petersburg, Fla.
- Cosmesis and modular limb prosthesis. Report of workshop held March 3-7, 1971.
- LeBlanc M, Bechtol C. Endoskeletal upper-extremity prosthesis. UCLA, December 31, 1968, final research report.
- Corin JD, Holley TM, Hasler RA, Ashman RB. Mechanical comparison of terminal devices. Clin Pros Orth Fall 1978;11:4.
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