Sizing And Prehension Forces For Dorrance Voluntary Opening Devices

Leila Carroll, OTR

At a meeting of the Subcommittee on Children's Prosthetics Problems held in Chicago in January, 1960, New York University-Child Prosthetic Studies presented a preliminary report of findings from the Normative Survey, based on 216 cases. In this report, data on terminal device prehension force were analyzed, and it was found that 88% of the prostheses provided a force of from one to four pounds, and that the prehension force provided was only generally related to age levels. The fact that 30% of the twelve to fifteen year age group were provided with only one or two pounds of prehension force was a point of great interest. This pinch force is obviously inadequate for this age group.

Paragraph seven of the summary of New York University's report indicated that size criteria for terminal device prescription required clarification, and that additional study was indicated to determine the prehension force needs of children.

Critical Analysis

This report stimulated the Occupational Therapy Department at Mary Free Bed Hospital, charged with the responsibility of training children, to critically analyze the potential prehension force for child amputees. The study has been pursued for one year and naturally included consideration of the relative sizes of terminal devices according to the size of the patient.

One hundred upper extremity cases were selected at random from the total case load at the Hospital. These children were not required to meet any specific standards in skill of operation and function. The prosthetic types and terminal devices included in the sample were:


Each prosthesis was examined at the time of the test to ensure that it was in good mechanical condition and the hook was compared to the sound hand for determining adequacy of sizing. In the bilateral cases, the patient and his prostheses were compared to another child of comparable height and weight.

Test Items

The test items for the determination of prehension forces included both unilateral and bilateral functions which varied according to the age of the child. Each test required the same number and variety of approaches.

The items used were: (1) forms of various sizes and shapes (triangles, squares, circles, bolts and dowels) (Fig. 1 ); (2) a screw type jar and lid or nested barrels depending upon the child's age; (3) assembly and dismantling of a simple tinker toy construction (Fig. 2 and Fig. 3 ), or extension cord and plug, depending again on the patient's age; (4) a food service tray with five to ten pounds of weight distributed evently, again depending on the age of the child (this item was not used with children under four years of age). These items tested static prehension only. The patient was either able to hold the objects and perform or they slipped out of the hook because of insufficient prehension force.

The fifth test item was a paper cup containing water to test the child's ability to control the prehension force. Two cup sizes were used and the size of the hook determined the choice of a large or small cup. If the patient could control the prehension when the cup was grasped in the middle, the cup would neither slip out of the hook nor crush.

All test items were performed initially with the prehension forces the patients normally used. Performance was graded as follows:

Good-Successful on first trial without assistance
Fair-Successful after several trials or minimal assistance
Poor-Only partially successful with assistance
0-Cannot perform

The prehension force was then gradually increased and the testing repeated until the patient could no longer control the prehension force adequately. Opinions of both the patient and the therapist concerning the various forces were recorded. The desired prehension force was then determined by using the test results (the force with which most tasks could be accomplished without excessive effort) with consideration given to the desires of the patient.

These "optimal" prehension forces were then measured, utilizing the pinch force gauge and procedures provided by New York University-Child Prosthetic Studies2 (Fig. 4 ).


It was noted that more patients were fitted adequately in regard to size of terminal device than in relation to prehension force. In most cases (with the exception of the toddler group), the initial prehension forces were found to be inadequate for performance of one or more of the test items. In approximately 60 cases, the prehension force was increased. In no case was a patient found to be wearing an excessive number of rubber bands requiring a decrease in prehension force.

A chart (Table 1 ) which gives the average height and weight and terminal device size for five age groupings from two to twenty years has been prepared. The chart also gives the average pinch force recommended for below-elbow, above-elbow and shoulder-disarticulation amputees in each of the age groups.

Leila Carroll is Director, Occupational Therapy, Mary Free Bed Guild Children's Hospital and Orthopedic Center, Grand Rapids, Michigan

1. Preliminary Analysis, Normative Survey Data New York University-Child Prosthetic Studies January 1960. 
2. Memorandum #10. Procedures for Determining Terminal Device Pinch Force Using the Measurement Gauge, New York University-Child Prosthetic Studies, September 22, 1961.