The CAPP Two-Way Shoulder Joint

Yoshio Setoguchi, M.D. Carl Sumida, C.P.O. Julie Shaperman, M.A., O.T.R.

This work was supported by Grant #MC-R-060004-10, Bureau of Community Health Services, Health Services Administration, Department of Health, Education and Welfare.


A new two-way shoulder joint has been designed and developed at the Child Amputee Prosthetics Project at UCLA. This article describes the features of the new shoulder unit and the concept of the design. Results of preliminary clinical testing indicate that because of its reliable friction and good appearance, the CAPP Two-Way Shoulder Joint should be a valuable new component for children wearing shoulder-disarticulation prostheses.


A new two-way shoulder joint has been designed and developed at the UCLA Child Amputee Prosthetics Project by Carl Sumida, C.P.O. It has a number of unique features:

  1. It offers a pleasing cosmetic contour which is as close as possible to the natural shoulder profile.

  2. It is a completely self-contained unit with its own cover. It requires no shaping at the shoulder, and the prosthetist needs only to attach it to the socket, and add the humeral extension to bridge the space to the elbow.

  3. It has smooth, reliable friction in both the flexion-extension and abduction-adduction planes with a wide range of friction adjustments for both motions (Fig. 1 ).

Clinical experience with children wearing shoulder-disarticulation-type prostheses led to the design of this new shoulder joint. These children need a shoulder joint which is mechanically reliable, which requires minimal maintenance, which gives them good appearance at the shoulder, and which offers smooth, reliable friction in all planes. Previously we had designed a one-way shoulder joint which provided exceptionally smooth, reliable friction (Fig. 2 ). It was based upon the same principle as the CAPP Adjustable Friction Wrist Unit (Available from the Hosmer/Dorrance Corporation), which controls friction by having a ring clamp compress a wide surface area. The amount of friction is adjustable in small increments by turning an Allen screw. The CAPP One-Way Shoulder Joint, though especially well liked by CAPP patients, was not widely applied because most centers prefer a joint which moves in the abduction-adduction plane as well as in the flexion-extension plane. Also, the one-way joint, which was used in a "canted" position, tended to give a wide shoulder contour (Fig. 3 ).

The CAPP Two-Way Shoulder Joint presented several problems in design:

  1. Narrow Shoulder Contour. Since the acromion is still present on the shoulder-disarticulation stump, any joint added at the shoulder will increase width beyond that of the normal shoulder. This new joint was designed to add the least width possible; and, by making this joint a finished, self-contained unit, the narrow contour is assured on the finished prosthesis.

  2. Friction Mechanisms in a Narrow Area. Because considerable force is reflected from the whole prosthesis to the shoulder area, the shoulder joint must have a very strong, reliable friction mechanism. Yet, the joint's narrowness at the shoulder left minimal space for any friction mechanism, especially one which utilizes the principle of maximum surface area for friction in two planes. Therefore, a different method was designed to utilize the principle of maximum surface area for friction control. The abduction friction mechanism consists of two hinged friction arms which clamp around the entire surface of a Delrin rod. The flexion-extension friction mechanism uses a tapered wedge to increase the surface area in the narrow space allowed for friction control. Both can be adjusted with an Allen wrench (Fig. 4 ).

Clinical Testing

Twelve CAPP Two-Way Shoulder Joints were produced at CAPP for clinical testing. All of the joints have been fitted or have been reserved for patients. They have been used in a variety of clinical applications. Patients from 2 years to 17 years of age have been fitted with this joint. Three have bilateral shoulder disarticulations, two have unilateral shoulder disarticulations, and one has a unilateral phocomelia and wears a shoulder-disarticulation type of prosthesis. They were all fitted in conjunction with commercially available components (some electric and some body-powered), except for the two-year-old, who wore the CAPP Infant Modular Shoulder Disarticulation Prosthesis and a CAPP Terminal Device (Fig. 5 ).

Results of the Clinical Testing

  1. All patients fitted found the shoulder joint very acceptable in appearance. It provided a highly cosmetic profile at the shoulder, and it retained its cosmetic contour through the entire range of motion, assuring the amputee a pleasing appearance during active use of the prosthesis (Figs. 6 and 7 ). Very symmetrical shoulder contours were demonstrated by unilaterally deficient children wearing a prosthesis with the CAPP Two-Way Shoulder Joint. Their clothing fit well over it, and it was not apparent that they had a prosthesis that extended to the shoulder (Figs. 8 and 9 ). It was not possible to fit young children with this joint and a conventional locking elbow, however, because the total humeral section was too long.

  2. Friction in the abduction plane has been smooth, adjustable in fine increments, and reliable. It rarely needed adjustment. However, friction in the flexion-extension plane needed adjustment more often than was anticipated. Therefore, this friction control was redesigned.

  3. There were a number of functional benefits from the CAPP Two-Way Shoulder Joint. It was possible to select and balance the friction adjustments of the two planes of shoulder motion, as well as the elbow turntable and wrist unit, so that children could grasp the terminal device and pull it to any desired spot for an activity. The friction joints of the prosthesis would "give" to allow the movement, and then they held that position during use. Children were largely unaware of the shoulder positioning; and when they were asked whether they positioned the shoulder and what activities they positioned it for, they replied that it was rarely or never moved. Yet, from observing them during play, it was apparent that shoulder positioning was frequent and that the children had incorporated the positioning of the total prosthesis, by holding and moving the terminal device, into their automatic use patterns. It was only when the friction required readjustment that the children became aware of how much they had come to rely upon the stability and friction of the shoulder joint.

Some individual use patterns were of interest. The 2-year-old found the abduction motion especially useful because she was interested in play activities which used objects which were large in relation to her small body size. She did not select individual joint positions of any friction component; rather, she used the pattern described above and placed her terminal device where she wanted it for the activity. She developed automatic use patterns which made her prosthesis function appear very natural and symmetrical with the sound arm. When she received an activated elbow lock and forearm lift, she had difficulty noting any functional benefit from this added operation. She had learned to substitute for elbow flexion by using shoulder abduction and elbow turntable motion, and this was more convenient since it did not require elbow locking or cable action (Fig. 10 ).

The 17-year-old girl with unilateral phocomelia found that she could select exactly the amount of friction that would allow her to move the prosthesis with her phocomelic hand, and that the friction was maintained at that level. Since she could move the prosthesis into flexion and abduction without using her sound hand, her use of the prosthesis appeared very natural. Since her hand was inside the humeral segment, she could not fully abduct the shoulder joint. However, this limited abduction was useful to her for dressing, carrying books, and for desk-top activities (Fig. 11 ).

One bilateral shoulder-disarticulation patient, who had previously used single-axis shoulder joints, found the abduction motion an advantage in table-top activities. The table-chair height relationship now was not as critical, since he could compensate for a slightly high table by abducting his arms to allow his elbows to clear the table. Abduction also increased his work area for other activities.

Current Status of the CAPP Two-Way Shoulder Joint

The results of the clinical-testing period appear very promising. Before producing more shoulder units for wider testing, Mr. Sumida has made some improvements in those features which encountered problems.

  1. The total length of the humeral section was shortened 2.8 cm (1 1/8 in.) to allow the joint to be fitted to very young children in combination with commercially available elbow units. Since the clinical testing showed that very young children enjoyed the greatest functional benefits from the two-way joints, it will be important to fit the joint to young children. Shortening the humeral section necessitated rearrangement of the abduction friction-control arms in the shorter space. However, the same principle for friction control was retained in the new design, since it has proved extremely reliable.

  2. Friction control in the flexion-extension plane was redesigned. The new design returns to a ring clamp for maximum friction surface area. This design has several advantages. Its principle of friction control has proved successful in other CAPP-designed units, and its use simplified manufacturing and assembly of the friction-adjustment mechanism for flexion, as compared to the one made for the first prototype. Fortunately, the redesigned friction mechanism uses the same ring clamp as the one used on the CAPP One-Way Shoulder Joint. This makes it possible to fit the shoulder as either a one-way or a two-way joint.

Currently, the molds for the humeral section of the improved shoulder joints have been completed and are ready for castings to be made. After this, the joints need to be assembled and finished. Since the principle of the joint, its cosmesis, and the abduction friction-control mechanism have already had some preliminary testing, it is anticipated that the CAPP Two-Way Shoulder Joint should be ready for wider testing in the not-too-distant future.

Yoshio Setoguchi is Medical Director, Child Amputee Prosthetics Project, University of California at Los Angeles, Los Angeles, California

Carl Sumida is Research Prosthetist, Child Amputee Prosthetics Project, University of California at Los Angeles, Los Angeles, California

Julie Shaperman is Research Occupational Therapist, Child Amputee Prosthetics Project, University of California at Los Angeles, Los Angeles, California