The CAPP Pre-Flexed Wrist Unit



A constant-friction wrist-rotation unit with built-in flexion has been designed, developed, and tested at the UCLA Child Amputee Prosthetics Project (CAPP). It is lightweight, is simple to fabricate, and provides good function for the high-level, bilateral, upper-limb amputee. The CAPP Pre-Flexed Wrist Unit is ready for production and should be available soon.


The CAPP Pre-Flexed Wrist, designed by Carl Sumida, C.P.O., gives the high-level, bilateral, upper-limb amputee a lightweight, reliable, constant-friction wrist unit which holds the terminal device in 30 degree of flexion.

Rationale for Built-In Wrist Flexion

Bilateral above-elbow and shoulder-disarticulation amputees need wrist flexion to reach the body midline to dress themselves and perform personal hygiene activities. Initially, patients at the CAPP Clinic wore prostheses with wrist-flexion units. These units permitted the patient to preposition the terminal device in the desired positions of flexion and rotation. Since wrist-flexion units are metal, they add terminal weight to the prosthesis--a significant consideration for the young, high-level amputee. Also, patients needed extensive positioning to get all of their friction components properly arranged before performing an activity, thus requiring considerable amounts of energy, time, and thought.

In 1962 an adult patient demonstrated to the CAPP staff that he could keep his hooks in the flexed position at all times, and that this position was entirely functional for all activities.

Other patients at CAPP then repeated this test, using various amounts of wrist flexion, and found that 25 degrees to 30 degrees of flexion is the most functional position for all activities. Since that time, patients at CAPP with high-level, bilateral amputations have worn prostheses with built-in wrist flexion. That is, the standard constant-friction wrist-rotation unit is laminated into the forearm in 25 degree to 30 degree of wrist flexion.

It is interesting that during 18 years of experience with built-in wrist flexion, patients have continued to demonstrate that they can perform all activities--those that require and those that do not require wrist flexion with equal ease if the wrist is permanently flexed 30 degrees. A report of 16 cases was published in 19701.

Advantages and Disadvantages of Built-in Wrist Flexion

There are some important reasons to consider fitting patients with prostheses that have built-in wrist flexion as an alternative to commercially available wrist-flexion units. Some advantages of built-in wrist flexion are:

  1. Less terminal weight is added to the prosthesis. The built-in, pre-flexed wrist unit can be a lightweight, reliable, constant-friction wrist-rotation unit. For the past 10 years, the CAPP Nylon Body Wrist Unit has been used and has proven almost ideal for this purpose.
  2. No positioning is needed to flex the wrist for activities. Bilateral amputees use so much energy positioning the components of the prosthesis that any decrease in prepositioning is a real advantage. The built-in wrist flexion also simplified rotational positioning, since it decreased the margin for error that results from combined action of many friction components.

Experience has also revealed some disadvantages to lamination of wrist-rotation units into the forearm in a flexed position:

  1. Lamination procedures require considerable care. An error in the angle of the wrist unit has a significant effect on the position of the terminal device and thus affects function.
  2. Resulting amounts of flexion change slightly from one prosthesis to the next because it is not possible to control the wrist-unit position precisely during the lamination procedure. Although amputees adapt to these differences, accommodation should not be necessary.
  3. Appearance is objectionable to some amputees, since this procedure results in a curve in the forearm.


The design of the CAPP Pre-Flexed Wrist Unit was planned to eliminate the disadvantages and to preserve the advantages listed above. The pre-flexed wrist unit gives the amputee a straight forearm, and the amount of flexion remains constant from one prosthesis to the next. Fabrication does not require exacting positioning techniques.

The design of the CAPP Pre-Flexed Wrist Unit is based on the design of the CAPP Adjustable Friction Wrist unit designed by Carl Sumida. That wrist unit is now marketed as the CAPP Nylon Body Wrist Unit by the Hosmer-Dorrance Corporation2. Both wrist units incorporate the same ring clamp surrounding the threaded portion of the wrist unit, which holds the stud of the terminal device. The only difference between the two wrist units is that the pre-flexed wrist unit has the threaded receptacle for the terminal device, and the ring clamp, tilted 30 degree in relation to the outer circumference of the wrist-unit body (Figure 1 ).


Five CAPP Pre-Flexed Wrist Units have been tested by four CAPP patients. Table 1 contains a description of the patients by age, limb deficiency, and prosthesis type, and the results of fitting the pre-flexed wrist unit. During this test there was no attempt to validate the benefit of pre-flexion for function, since that has been well established in the past. From this limited test, it appears that the CAPP Pre-Flexed Wrist Unit will simplify fabrication and improve appearance in comparison with the standard wrist-rotation unit positioned in flexion. The patient who had previously worn a prosthesis with built-in wrist flexion commented on the improved appearance. The prosthetist who fabricated two of the prostheses reported that it was much easier to align the pre-flexed wrist unit on the forearm than it was to align the standard wrist unit in flexion. Since forearms for the two other patients were made in "central fabrication," no comparison on ease of fabrication could be made (Figure 2 and Figure 3 ).


The CAPP Pre-Flexed Wrist Unit is now ready to be produced commercially, and it should be available for purchase in the near future. Although it is needed by only a small number of patients, this wrist unit provides them with a significant benefit.


  1. Clarke, S., C. Kral, and J. Shaperman, Built-in wrist flexion for children's prostheses. Inter-Clin Inform Bull, 9:5:1--7, February 1970.
  2. Hosmer-Dorrance Corp., CAPP Nylon Body Wrist Unit. Ninth edition catalog, Nos. 1414, 1413, 1412, page 14. Hosmer-Dorrance Corp., P.O. Box 37, Campbell, CA 95008.