Variety Village Electromechanical Hand for Amputees under Two Years of Age

MARTIN MIFSUD, E. ENG. T.,* ISHAN AL-TEMEN, P ENG.,WILLIAM SAUTER, CPO(C),SHEILA HUBBARD, DIP. P and OT, B.Sc (PT),MORRIS MILNER, PH.D., P ENG., CCE, ANDSTEPHEN NAUMANN, PH.D., P ENG.**


Provision of upper-limb prostheses to child amputees during the last 16 years has begun to address the demonstrated need for suitable myoprosthetic systems. In 1971, William Sauter at the Ontario Crippled Children's Centre (now The Hugh MacMillan Medical Centre), fitted the first child-sized electric hand (Northern Electric hand) to an 8-year-old.1 Later, this hand was produced and marketed by Variety Village as the VV 105 hand. Rolf Sorbye of Orebro, Sweden, used this hand and the Otto Bock size 6 1/4 inch hand to show that children of preschool age could operate myoelectric controls and electric hands successfully.2/ In 1976, Systemteknik of Stockholm made available a hand appropriately sized for preschool aged children. The VV2-6 hand was released in 1984 from the Variety Village Electrolimb Production Centre (now Variety Ability Systems Incorporated). The weight of the terminal device was reduced while appropriate size for the young age group was maintained.3

We have noted that families of very young children have been reluctant to accept hook-type devices for aesthetic reasons. Attempts to provide cable-controlled hands have been disappointing because children do not have the strength to operate them. Our experience with myoelectric systems has shown that children can develop good prosthetic skills and full-time functional wearing patterns. This depends on the type, quality, size, and weight of the myoprosthetic components and carefully considered special fitting techniques. In addition, strong support from family members is important if the child is to accept and integrate the prosthesis into the body image and daily life style.4

A new myoprosthetic component, an electromechanical hand designed for children from approximately 15 to 36 months of age, follows the philosophy of providing children as young as possible with the necessary systems.

Design Considerations

The Powered Upper Extremity Prosthetic Research and Development Programme of The Hugh MacMillan Medical Centre is complemented by a Prosthetic Service Delivery Programme that has fitted more than 1200 upper-limb prostheses to amputees ranging in age from 18 months to retirement. Significant experience and insight have thus been gained. Close proximity of the clinical service to the design process promotes ongoing interactions between engineering and clinical staff and communication with children and their families. This aids in understanding users' needs and expectations while examining the developmental feasibility in terms of a practical outcomes Minimal design requirements for a hand for children up to 3 years of age were:

  1. Optimal appearance: Special consideration was given to hand size and shape to augment the ultimate acceptance of the prosthesis through enhanced amputee self-image and early perceptions of the family;
  2. Minimal forearm load: Emphasis was placed on minimizing the effects of forearm load, a common problem experienced by young amputees that results in fatigue and potentially less active use of the prosthesis. Therefore, materials and production methods were carefully considered early in the design process to minimize hand weight;
  3. Reliability: This was considered critical to minimize amputee inconvenience and frustration, and included the ability of the mechanism to produce high torques (i.e., adequate for grasping objects) and fast speed (i.e., comparable to existing terminal devices) with a small gear-train assembly that would function for at least a quarter million cycles. Strength and durability of the body and finger components had to be achieved to withstand the rigors of children's life styles without inhibiting them unduly;
  4. Provision of power for extended periods: Optimal use of the electrical power provided from a small rechargeable battery source was considered important so as to provide function for a day's use. This required losses in the electronics and the hand mechanisms to be minimized;
  5. Systems compatibility and a modular design approach: Essential to the service delivery program, this dictated the need for the hand to operate with commonly used control systems to allow alternative components to be substituted by the clinic when customizing the prosthesis. Compatible wrist units had to be available in several sizes to accommodate amputees with wrist disarticulations and higher levels of limb loss. These options would also eliminate the need for forearm replacement when growth adjustments require the next size hand. Modular design for all major components of the hand (i.e., motor gearbox, power bridge, energy saver, hand body and fingers and wrist) was required to minimize replacement time by the prosthetist and periods during which the child would otherwise have to go without the prosthesis; and
  6. Competitive costs: Few off-the-shelf components can be used in these designs, thus components should be fabricated, whenever possible, with standard production methods to be cost effective. The field of powered prostheses is small and specialized, precluding large scale production techniques. It was only through the generous support of Variety Club Tent 28, which has significantly promoted developmental projects such as this and the technology transfer process, that development was made possible.

Specifications

The new VVO-3 hand matches the operation and features of the VV2-6 hand with a 20 percent reduction in size and weight (Fig. 1 ). Unlike other designs, the VVO-3 hand's size and shape had to be predefined. The mechanism and electronics were then designed to work within the constraints identified. Once a prototype was fabricated, the appearance was optimized by an industrial designer and prosthetist before patterns were made. Patterns were sent to Centri Gummifabrik AB of Sweden, where a new glove was produced. The glove was sized for the hand, protecting it from the child's environment while enhancing the hand's appearance. Gloves are available in #30 Caucasian and #64 Negroid colors. Other colors are available on special order.

Weight of the VVO-3 with wrist unit (130 gm, 4.6 oz) compares favorably to the weight of the VV2-6 (168 gm, 6 oz). Lightness was achieved with injection-molded parts. The hand body is made of Acetal with 20 percent glass that provides good dimensional control of the finished product. The hand cover, which protects the motor, power-bridge assembly, fingers, and thumb, is made of super tough Zytel ST801 because of its excellent impact resistance. Acetal and Zytel are also used in the VV2-6 hand and have proven reliable. Fingers and thumb provide a three-point pinch suitable for young children.

Most drive elements are housed within a single metal casing to ensure precise gear alignments. The assembly is supported within the hand body and can be replaced easily, if required. A miniature motor drives the mechanism and, while fastened to the housing, can also be replaced readily. The gear train has several stages of spur gearing and one planetary gearing arrangement. An anti-rollback mechanism maintains pinch at any finger position, even when power is removed from the motor; this feature can be bypassed manually by rolling down the glove and inserting a small screwdriver to engage the main drive spindle for manual finger and thumb opening and closing. Because of the gear train's complexity and precision, service, if required, should be performed by the manufacturer.

Pinch force was designed to approximate that of the VV2-6 hand to suit older children who may be fitted with the VVO-3. If pinch is excessive for the very young user, one may place a resistor in series with the motor. A 20 ohm, 1/2 watt resistor reduces pinch force by approximately half.

The power bridge and energy saving circuits are identical to those in the VV2-6 hand. These operate from the outputs of the control system and consume no current unless activated. Circuits require less than 35 milliamperes when initially activated and then less than 5 milliamperes after the time taken for the fingers to travel to the open or closed position. The energy saving circuit module, located in the wrist unit, can be used in both left and right hands. Power bridge modules differ in physical layout only and are accessible under the hand cover. Hands are provided with a 4-pin Otto Bock connector (9#53). Specifications appear in Table 1 .

Testing

Laboratory testing of the VVO-3 hand has exceeded our expectations. The hand was exposed to the same accelerated cycle testing performed with the VV2-6 hand which cycled a quarter million times before failure. In comparison, the VVO-3 hand cycled more than a half million times. The hand was fitted to three children between the ages of 2 and 3 years with excellent results. The parents of one child previously provided with the VV26 hand were very appreciative of the VVO-3 hand's smaller size.

Based on field experience with the VV2-6 hand, we expect the VVO-3 to perform as reliably, if not better. Since its release in Spring 1984, 175 VVO-3 units have been sold and only twelve returned, largely for minor problems. These included minor gear wear; sand and water exposure; and wires disconnected or broken. Because a few energy saving circuits were physically damaged, a plastic protective cover has been incorporated within the wrist unit. We expect-service to be required at two-year intervals.

Fitting Considerations

Depending on the user, the hand may be acquired with options such as a regular power bridge, an energy saving power bridge, or without a power bridge. Wrist units are manufactured in two diameters (33 and 40 mm) and three lengths (9, 17, and 28 mm) to allow for optimal forearm matching and interchangeability. The 9 mm long wrist unit, intended for children with wrist disarticulation, does not provide pronation/supination because of size constraints. The other wrist units have an adjustable friction collar for passive pronation/supination.

Electrical and wrist unit options should be selected to coordinate with the control system and the size and length of the prosthesis. For example, the hand may be obtained with an energy saving power bridge which extends the daily operating time of a single battery pack significantly.6 A long wrist unit of either diameter, which holds and protects the energy saving circuit, would accompany this. This combination would not suit a child with a long belowelbow amputation limb who should have the hand with the power bridge only and a shorter wrist unit. Generally, a power bridge should be employed when myoelectric controls are used. Switch-operated systems can power the motor directly.

Conclusions

In direct response to requests from parents, we have been able to lower the age for fitting children with prostheses, which has resulted in the need for small systems. The VVO-3 hand helps fulfill this need because of its reduced size, lighter weight, and cosmetic appeal.

Fitting children younger than two years of age is now, more than ever, realistic and advantageous in terms of facilitating the child's acceptance of the prosthesis. The combination of myoelectric control systems and electrically powered hands provides a self-contained prosthesis that is beginning to achieve or promise maximum esthetic restoration with appropriate function and reliability. We hope our current experience with seven children, 1 1/2 years old, who have unilateral transverse terminal forearm deficiency, will help determine whether children fitted with the new systems do become better functional users because of earlier experience.

The VVO-3 hand is now available from Variety Ability Systems Incorporated. It has a one year warranty on parts and labor. Assistance to prosthetists and users provided by clinical and technical teams helps resolve problems arising from early myoelectric fittings.

Acknowledgments

The work reported here was supported generously by Variety Club Tent 28. We also thank the Powered Upper Extremity Prosthetic Service Programme for the benefit of its members' clinical acumen.

*Rehabilitation Engineering Department, The Hugh MacMillan Medical Centre, 350 Rumsey Road, Toronto, Ontario M4G 1R8

**Department of Rehabilitation Medicine and Institute of Biomedical Engineering, University of Toronto

References:

  1. Sauter W: Prosthetic Research: Special Fittings. In 1973-73 Annual Report of the Ontario Crippled Children's Centre, Rehabilitation Engineering. Pages 11-15.
  2. Sorbye R: Myoelectric Controlled Hand Prostheses in Children: Clinical Considerations. In Proceedings of the 2nd European Conference of Rehabilitation International, Brighton, England. Pages 334-342.
  3. Al-Temen 1, Literowich W Mifsud M, Milner M: On a New Electromechanical Hand for Young Children. Inter-Clinic Information Bulletin 19:6-8, 1984.
  4. Scotland TR, Galway HR: A Long-Term Review of Children with Congenital and Acquired Upper Limb Deficiency. Journal of Bone and Joint Surgery 65-B:346-349, 1983.
  5. Mifsud M, Milner M: A Model of a Powered Upper Extremity Prosthetic Research Development Program. In Proceedings of the IEEE/Engineering in Medicine and Biology Society, 8th Annual Conference. Vol. 3. Pages 1889-1893.
  6. Mifsud M, Literowich W Milner M: Energy-saving Power Bridge for Children's Artificial Hands. Medical and Biological Engineering and Computing 23:479-481, 1985.