A New Toddler Hand: Design and Preliminary Evaluation

Samuel Landsberger, ScD Julie Shaperman, MSPH, OTR Andrew Lin, BS Vincente Vargas, BSME Richard Fife, CP Yoshio Setoguchy, MD Donald McNeal, PHD


Summary

A better body-powered hand for children with upper-extremity limb deficiencies would fill a need long expressed by parents and clinicians. Our objective is to develop an improved hand and activation system for toddlers between the ages of one and four. The primary design goals are: (i) acceptable cosmesis (hand-like appearance), (ii) useful grasp function for play activities, and (iii) minimal requirements for harness force and motion inputs from the child.

We have explored several hand design strategies in parallel with methods of harnessing and, as needed, amplifying a child's energy to power the gripper. A simple hand called "Easy Feed" performs satisfactorily in grip tests and should accord to a toddler's cognitive and physical development. We are investigating coverings for the structure alongside design of the hand's mechanical linkage. Force and power augmentation modules are in development to assist children whose strength is inadequate to operate the hand for extended periods of use.

The Problem

Toddlers from one to four years of age requiring a hand prosthesis have few options. Most child prehensors (gripping devices) are miniatures of those designed for adults and so focus primarily on a grasp pattern of fine-tip pinch with inadequate grip force and opening size to be functional for the child's play activities. This is to be expected, because children's activities and commensurate gripping needs do not simply scale down from those of the adult world in proportion to body dimensions. A viable option is provided by electrical hands but many parents and health agencies cannot afford these. The powered devices also do not offer direct kinesthetic feedback of the forces or opening motion of the hand.

Objectives and Constraints

The hand should accommodate large (3 inch diameter) objects and offer secure grasp to enable the toddler to engage in developmenlally appropriate activities, but without requiring greater harness input force or motion than existing models. The design must be safe, simple,rugged, lightweight and low cost. The hand must be easy to maintain, as well as a functional and attractive device that children will want to use and parents will accept.

Methodology

Our methodology is to balance a basic investigation of the mechanics of various grasp and energy input strategies with practical development of testable models which may be refined through successive trials. The design process has comprised the following four elements.

  1. Films of normal toddlers in a local preschool and of toddlers with prostheses documented how they typically hold and use objects. The observations, coupled with literature on child development and prosthesis performance tests have helped both to determine desired grasp patterns and to develop a list of objects for use in testing prehensor function.
  2. Designs for the new hand structure, its covering and power assist devices are created, developed into hardware models, and refined through testing.
  3. A laboratory test of gripper performance is in development to simulate real-world use and so help predict the functionality of a new design before patient testing.
  4. Clinical trials of designs which test well in the laboratory are evaluated by patients at the Child Amputee Prosthetics Program of Shriners Hospital, Los Angeles. Designs are refined through this feedback process.
  5. Clinical trials are planned at other centers at a later stage of hand development.

Evolution of the Easy Feed Hand

Figure 1.
The primary grasp pattern required by toddlers for the non-dominant hand is, by our observation, a full-hand grip similar to cylindrical and spherical grasp patterns. Youngsters lack sophisticated coordination and do not generally use care in positioning objects in either their sound hands or a prehensor. Their strategy is to envelop the object as much as possible with the fingers rather than squeezing with a pincher-like grip. Development has therefore focused on hand geometries and activating mechanisms to provide "form closure grasp" wherein objects are restrained by secure capture rather than high pinch forces. Capture grip requires less energy and cognitive skill than the palmar pinch type grip of most conventional prehensors.

A concept termed "Easy Feed" has been selected as the primary design from among a host of possible configurations. Designed to evolve with growth of the child, the hand may be used initially as a cable-less prehensor into which the child (or her parents) can directly feed objects. As the toddler develops more strength and cognitive skill, the hand converts to cable-actuation and functions in either voluntary-opening or voluntary-closing mode, i.e. spring-loaded to close or open, respectively. The hand comprises an endoskeletal structure fitted with compliant foam and a cosmetic glove covering. The shape approximates the hand of a two-year-old. The Easy Feed has a large grasp opening of 3" span and provides an adjustable pinch force. The four fingers opposing the thumb pivot as a group at their connection to the base of the hand to further increase grip opening and compliance.

The primary design feature is a kinematic structure with sufficient articulation at the carpal metacarpal and distal interphalangeal joints to permit objects to be inserted into a captive, yet compliant grip. This is accomplished by two features of the hand kinematics. First, the thumb pivot is placed so that an insertion force opens the thumb, allowing the object to enter the hand. Second, all finger joints can passively flex inward. The result is a self-actuating geometry wherein a child can push an object against the fingers to insert it into the hand, without need for a cable. Since the natural prosthesis use pattern for toddlers is to feed objects into the prehensor with their sound hand, the concept of the Easy Feed Hand is developmentally appropriate.

The geometry is also "self-energizing" in that a direct outward pull of the object causes the thumb to close more securely, making the hand into a type of one-way valve. Passive springs on the digits act as a safety valve to release grip when a reasonable withdrawal force is applied to the captive prey. The compliant hand thus relies on object capture through self-energizing grasp geometry as well as squeeze force to provide stable hold. This should minimize the child's need to precisely position objects in the hand.

Figure 2.

Coverings

The purpose of a covering is threefold: to provide good cosmesis, to improve the grip friction, and to protect the inner workings of the hand. We are striving to develop an acceptable glove to cover the Easy Feed hand. It should minimally constrain operating efficiency and range of hand opening. Tight-fitting PVC gloves severely restrict motion and increase the forces needed to operate the hand, yet are also expensive, easily stained and may tear in a week's use. Unfortunately, they are not easy to improve without severely compromising properties such as cosmesis or durability. Alternatives under study include vinyl gloves with pleat geometries, new silicone compounds, and Spandex type fabrics coated by a special "New Skin" material. The current goal is to find a solution that is cosmetically acceptable, mechanically flexible and durable enough to permit patients to test the Easy Feed hand over a period of several weeks. Further cosmetic refinement will be made after proof-of-concept clinical testing is complete.

Power Augmentation

Previous studies have confirmed and quantified clinical observation that a significant percentage of children with limb deficiencies also have weakened musculature. We endeavor to develop "helper packs" to permit operation of the hand when a child's own strength is insufficient. The helper packs are intended as augmentative tools to be used until the child develops enough strength for independent hand control at a level that is comfortable for full day use. Two concepts have been developed. The first resembles the principle of power steering, employing a battery-powered amplification module that works in conjunction with the control cable. The child uses his or her own force and motion, as available, to pull on the harness cable while a motor supplements this in a preset proportion. The second helper module is a non-powered force amplifier employing a disk with two pulley diameters to exchange some cable excursion for higher pull force. This enables the child to use a prehensor fitted with a stronger spring to achieve better grip, but increases the amount of harness motion needed for full opening. Since most voluntary-opening prehensors available will accept objects pushed into them once the are partially opened, the force amplifier should then help with a number of grippers. The Easy Feed hand is specially designed to enable the child to insert an object with minimal thumb opening. The force module attaches unobtrusively to the prosthesis forearm like a wristwatch. Both the power and force modules preserve direct, if scaled, proprioceptive feedback to the child of forces and motions applied to the hand.

Laboratory Test of Grip Performance

A test has been developed to measure grip performance of new hand prototypes in the laboratory to assure baseline levels of functional effectiveness prior to clinical evaluation. A validated test will also offer means of evaluating new designs and comparing those with existing children's prehensors, and indicate directions for improvement in grip function. The laboratory test employs a variety of toddler toys, representing a range of sizes and shapes, and where possible incorporates these with grasp patterns recorded in the videotapes of children at play. Grip is quantified for each object by applying graduated forces to the objects in specific directions, corresponding approximately to conditions of actual use, and recording the force required to dislodge the toy. The test equipment and procedures are standardized to improve the reliability of measures. The utility of this assessment tool will depend on its correlation to clinical trials.

Results

The Easy-Feed concept has been applied to modify a child's prehensor, the CAPP I. Four children at Shriner's Hospital Los Angeles below the age of two have tested the "CAPP with lips" and demonstrated the viability of pushing an object into a receptive but initially closed device, without need for a cable. Prototype Easy Feed hands have proven satisfactory in the laboratory grip test and await clinical trial in the Spring of 1998 once their covering is ready. The grip test has also been applied to the CAPP #1 and Steeper 2" hand. Progress has been made towards a hybrid fabric/elastomer covering that is stretchable, reasonably cosmetic, durable and easily replaceable by a caregiver. Children have successfully tested both the power and force amplifier module. A smaller size power-assist is needed for convenient installation on the prosthesis.

Future

Our primary emphasis will be on refinement of the Easy Feed hand design both with and without cable activation. The covering appearance will be improved. As resources permit, the power module will be miniaturized and a clutch module refined to improve capture in the cable-activated hand. Clinical testing will accompany and guide these developments. As the development progresses, we plan to expand clinical testing to as many centers as possible.

Acknowledgements

This work is supported by Grant #H133E50006 from the National Institute for Disability and Rehabilitation Research (NIDRR), US Department of Education for a RERC on Technology for Children.