A Simple Flexor Assist For Below-Elbow Prostheses

Earl E. VanDerwerker, Jr., M.D. Josef Rosenberger, C.P.O.

In "amputations" of congenital origin, the most frequently occurring is the upper limb terminal transverse partial hemimelia. The prosthetic classification of most of the children with this type of congenital skeletal deficiency is short or very short below-elbow. A preliminary report (Interim Report, Normative Survey Study, New York University-Child Prosthetic Studies, June 1961) by New York University-Child Prosthetic Studies of 629 children included in the Normative Survey indicates that twenty to twenty-five per cent of the total sample fall into this particular prosthetic classification. The short or very short below-elbow type "amputations" are most frequently present as unilateral involvements although they do occur occasionally in conjunction with deficiencies of one or more additional limbs.

The typical method of fitting these patients, and the one we follow at the Newington Hospital, involves the use of a split socket and step-up hinges. Depending on the type of step-up joints used, flexion of the forearm (with terminal device) is thus increased 1 1/2 or two times that of the stump socket. The price paid for this increased flexion range, however, is a proportionate increase in stump effort to flex the forearm or to maintain "live lift" in the flexed position. This drawback has been manageable with a hook terminal device, although at times we have had to use a lift loop assist.

Now, however, as more children and teen-agers with short and very short below-elbow stumps are being fitted with the APRL-Sierra No. 1 or the APRL No. 4 Hand, the problem has been accentuated. These hands weigh considerably more than their hook counterparts. The No. 1 Hand, for example, weighs approximately 170 grams as compared to approximately 60 grams for a Dorrance 10x hook. Moreover, this added weight is acting through a forearm lever arm of some eight to ten inches. Thus, in some cases, the added weight of the hand has made operation of the prosthesis cumbersome and detracted from the effectiveness of its use.


To meet this situation, we have devised an elbow flexor assist that is light, simple, effective, relatively inexpensive and has been worn up to a year and a half with little if any servicing.

The mechanism (Figure 1 , Figure 2 , and Figure 3 ) consists of:

A: A flexible anchor pin - The solid tip of the pin fits into a hole drilled in the base of the wrist unit, while the end of the coil receives the tip of the cylinder (tubing). The coil provides flexibility to the system during flexio and extension of the socket forearm shell

B: A cylinder (tubing) which houses a piston and spring.

C: A piston spring to develop the tension for flexion assistance.

D: A piston with a pivoted base plate for attachment to the distal-anterior socket.


In installing the "flexor assist", the lengths of cylinder and piston are adjusted so that the piston spring is under tension when the socket and shell are in the extended position (Figure 2). Thus the release of this tension during forearm flexion provides the required assistance. Spring tension also helps the stump maintain the forearm in a flexed position, as for example, in supporting a coat draped over the arm. The extent of the assistance provided can be varied by changing either the amount of spring compression or the strength of the spring.

In fitting children in our clinic, we have used the mechanism in only one size, involving tubing of 5/16" I.D. However, it should be perfectly feasible to make the item in three sizes to cover the range of possible users from small children to adults. We estimate the cost of an individual unit, including materials and labor, to be approximately 18 dollars.

To date we have fitted 12 patients (six to twelve years of age) with the "flexor assist" and all have expressed satisfaction with the improved use of both hand and hook.

(Editor's Note: Because of the favorable results reported by Dr. VanDerwerker and Mr. Rosenberger, New York University-Child Prosthetic Studies will seek further information concerning the application of the "flexor assist".)

Newington Hospital for Crippled Children Newington, Connecticut