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Prehensor Grip for Children: A Survey of the Literature

Julie Shaperman, MSPH, OTR
Maurice LeBlanc, MSME, CP

ABSTRACT

Introduction

A three-phase study was conducted on ways to increase grip in pediatric body-powered prehensors. The study was part of a project for the Rehabilitation Engineering Research Center on technology for children with orthopedic disabilities. The first phase studied existing prosthetic systems and quantified inefficiencies in prehensors, cable control systems and harnesses. The second phase measured children's strength to find out how much arm and shoulder force they can produce for operation of a body-powered prosthetic system. The third phase defined how much grip a young child needs in a prehensor.

These findings helped establish design criteria for an improved body-powered hand for young children. The studies from phases one and two are the subject of another publication (1). This article describes the literature survey for phase three to define minimum grip forces young children need in a prehensor. This literature survey on grip strength is one part of a larger study and should be viewed within that context.

Survey of the Literature on Grip

The literature review's objective was to find out the minimum amount of grip that 2- to 4-year-old children need to perform play and self-care activities. The researchers assumed that additional amounts of grip strength, beyond the minimum, would be helpful and desirable, but it was necessary to first identify the minimum to provide a baseline. Five questions guided the literature survey on grip force:

• How much grip force do normal 2to 4-year-old children possess?
• How much grip force must prosthetic prehensors provide to hold the objects children use for play and other daily activities?
• How much grip force is reported in manufacturers' specifications for externally powered prehensors, and how much is measured in the laboratory on young children's voluntary-opening (VO) body-powered prehensors?
• How much grip force do young children achieve when using voluntary closing (VC) prehensors?
• How important are factors that enhance grip even though they are not included in grip force measures (e.g., friction and resilience of prehensor surfaces, prehensor opening span, depth and shape)?

Grip Force in Normal Hands

Very few studies were found that reported hand grip strength of 2- to 4-year-olds. Brown et al. (2) measured hand grip strength using a commercially purchased dynamometer. Twenty-five children ages 2 to 4 years used their four fingers and their thumbs to squeeze a stirrup-shaped handle. The grasp pattern appears similar to cylindrical grip. Brown reported an average of three maximum squeezes (see Figure 1 ). Forsberg et al. (3) measured palmar pinch between the thumb and index finger of 150 children ages 2 to 4 years using a specially constructed apparatus that resembled a pinch gauge and recorded force at each surface using strain-gauge transducers. The grasp pattern appears similar to a two-point palmar pinch. (It is important to note that grip strengths shown in Figure 1 are maxima and are unlikely to be required every time a child holds an object.)

Grip Force Required for Holding Objects Used in Play and Other Daily Activities

Gottlieb (4) assembled objects used by children of each age and used a strain-gauge instrumented hook to determine the amount of prehension force necessary to "just hold" and to "use" these objects. Grip forces could be adjusted in 0.5-lb. increments. "Just hold" prehension forces were lower than "use" prehension forces. Gottlieb reported that 2-year-olds can hold most objects if they have at least 2 lbs. of grip force; 3and 4-year-olds require 4 lbs. of grip force for their activities.

Carroll (5) reported that Dorrance VO hooks used regularly by 2- to 4year-olds had an average of 1.25 to 2.25 lbs. of grip force depending upon the child's level of amputation. She reported that children were successfully wearing and able to use their prostheses while performing four test tasks but gave no indication if this amount of grip force was adequate for the majority of daily activities.

Grip Force of Prehensors as Measured in the Laboratory and Reported by Manufacturers

Grip forces of three pediatric body-powered prehensors were measured in the laboratory using a pinch gauge. (6). b The Dorrance lox hook was measured with two rubber bands; the CAPP Terminal Device IC had a regular spring, and Steeper Mechanical Hand #2 was measured as shipped. Figure 2 shows results of grip force measures of these four body-powered prehensors and grip force specifications provided by manufacturers of three externally powered prehensors (7).

It also shows that the Dorrance hook with two rubber bands has 3 lbs. of grip force, but it is unlikely that a 2- to 4year-old child can operate a hook with this much rubber band loading. Figure 2 also shows that externally powered prehensors provide a range of grip forces from 4.5 to 10 lbs. No report was found indicating that children need 10 lbs. of grip to perform most of their activities. Therapists who observe patients using the VASI externally powered hands have stated that 4.5 lbs. of grip force is very adequate for most activities (8).

Grip Forces with Body-Powered Voluntary-Closing (VC) Prehensors

Occupational therapists in four clinics (9), where VC body-powered prehensors are often prescribed agreed to measure the maximum amount of grip their patients could achieve. The children used Adept F and VC prehensors or Steeper Hands modified for VC operation. Grip force was measured with a pinch gauge and recorded along with the child's age and prehensor type (see Figure 3 ). Two-year-olds achieved 1 to 2.3 lbs. of grip force while some 3-year-olds achieved 3 to 5 lbs. of grip force. Therapists noted that the Adept F prehensor has some flexibility in the "fingers," so it was difficult to accurately measure its grip force.

Additional Factors that Enhance Grip

Factors such as friction on gripping surfaces, resilience and compressibility of prehensor surfaces, prehensor opening span, depth and shape are not reflected in grip force measures, but they may influence security of grip on objects.

• Friction of the prehensor grasping surfaces can enhance grip without requiring increased operating force (4,1011). To some extent, the friction of a hand prehensor's cosmetic glove enhances grip. The friction of the Kraton(r) pads of CAPP Terminal Device and the neoprene lining of Dorrance hooks also serve this function.
• Resilience of materials in prehensor grasping surfaces and object surfaces allows their shapes to conform and lessens reliance on grip force. When an object is held, the flesh of the finger/palmar areas of the human hand is deformed to conform to the object's shape and lessen the amount of muscle force needed to hold the object securely(l2).
• The amount of surface contact area is another important factor that enhances security of grip (4,12). Therefore, the wide surface contact area of the CAPP Terminal Device should provide greater enhancement of grip than the narrow hook fingers of the Dorrance hook. The small surface contact area between fingertips and thumb tip of most prosthetic hands and the objects they hold fails to take advantage of this feature. Prehensor design often cannot take full advantage of this feature because wide surface contact areas may obstruct the wearer's visibility of the objects held in the prehensor, and good vision of objects held is important to good function.
• Geometry of the prehensor (e.g., span and depth of opening, and shape of the prehensor's holding area) as well as configuration of the object held influence the amount of grip force needed for secure hold (4,12). If the object shape and size fit the grasping area of the prehensor, there is more contact and less chance that it will slip or drop out.

It is difficult to mathematically calculate the effects of specific amounts of friction, resilience and surface contact area in reducing the amount of grip force needed to hold different types of objects in various prehensors. Clinicians and researchers acknowledge that increasing the above-mentioned factors lessens dependency on grip force alone for secure hold on objects (11).Yet clinicians and researchers still rely mainly on measured grip force as an indicator of good grip.

Discussion

Findings from all of the studies described in this article were reviewed to find an answer to the original question: How much grip must a prehensor provide for a child at the age of 2, 3 or 4 years? To answer this question, it was necessary first to determine the minimum acceptable level of grip force that a prehensor must provide. The minimum establishes a baseline for evaluating the data in the studies described above. The minimum also is important to incorporate into design requirements for a pediatric prehensor.

Gottlieb (4) and Carroll (5) reported minimum levels of grip force needed to perform age-appropriate activities, but Carroll's patients were constrained by limited strength for operating VC prehensors while Gottlieb measured grip force requirements independent of problems with operating force. Also, Carroll's patients had less grip force than Gottlieb identified as the minimum required for performance of activities at ages 2 to 4 years. Since limitations in operating force had already been incorporated into the design specifications for the prehensor under consideration in the authors' study, the Gottlieb data appeared more relevant to the goal of identifying grip force needs independent of operating force abilities.

Next, the Gottlieb data (2 lbs. for age 2 and 4 lbs. at ages 3 to 4) were compared with findings on children wearing VC prehensors (9) and shown in Figure 3. Those children achieved less grip force at age 2 and equal or greater grip force at ages 3 and 4 than the amounts recommended by Gottlieb.

The basis for the work to improve grip in pediatric body-powered prehensors is that very young children currently wearing and using them have insufficient grip for functional activities. The findings of both the study on grip strength of children wearing VO prehensors (5) and VC (9) prehensors showed that very young children are getting less grip with these prehensors than Gottlieb (4) reported as the minimum required for activities at this age.

Based on these comparisons, the Gottlieb data were selected as minimums of grip force that should be provided by a body-powered prehensor for children between 2 and 4 years of age. Grip forces beyond these minimums were considered highly desirable. Also, grip force requirements can be lowered through the use of friction, resilience and effective prehensor geometry.

Conclusion

Grip force requirements for a 2- to 4year-old child's prosthetic hand were explored through a survey of literature. Studies provided data on normal hand grip as well as grip force of pediatric prehensors. Findings of a study by Gottlieb (4) showed that activities of 2- to 4-year-old children require minimum grip force of 2 lbs. for 2-year-olds and 4 lbs. for 3- to 4-year-olds. These findings are independent of limitations in operating force that have been identified previously. These minimum grip forces are being incorporated into design specification for a pediatric body-powered hand.

Acknowledgments

This work is supported in part by Grant #H133E0015 from the National Institute for Disability and Rehabilitation Research (NIDRR), U.S. Department of Education, Rancho Los Amigos Medical Center with Donald MeNeal, PhD, and Mark Hoffer. MD, as co-principal investigators of the Rehabilitation Engineering Research Center on Orthopedic Technology for Children. Opinions expressed in this article are those of the authors and should not be construed to represent the opinions of NIDRR.

JULIE SHAPERMAN, MSPH, OTR, works in the Rehabilitation Engineering Research Center on technology for children with orthopedic disabilities at Rancho Los Amigos Medical Center She formerly was affiliated with the Child Amputee Prosthetics Project at UCLA and Shriner's Hospital for Crippled Children, Los Angeles.

MAURICE LEBLANC, MSME, CP is director of research of the Rehabilitation Engineering Center at Packard Children's Hospital at Stanford and is a technical consultant on the project described in this article.

References:

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