Knee Disarticulation Socket Design for Juvenile Amputees
JOSEPH H. ZETTL, C.P. HANS BRUNNER ROBERT L. ROMANO, M.D.
This article describes a segmental prosthetic socket design for knee disarticulation in a growing child. The design utilizes two separate socket sections, distal and proximal, joined together by external uprights of the knee hinges. This design allows portions of the residual limb to be unencumbered by the socket, reduces prosthesis weight and some bulk, and most of all allows for ease of length adjustment for the growing child. Suspension of the prosthesis is inherent in the socket design and is simplified by elimination of the conventional waist belt and forked suspension strap.
The classical knee-disarticulation amputation in a growing child preserves the distal femoral epiphysis, thus leaving the growth center undisturbed. Amputation at the supracondylar level, particularly in a very young child, may result in an extremely short femur and should be done only when necessary because of tumor or other such factors. Condylar shaving or trimming to allow fitting with a closed socket is not advocated or attempted. This practice may also cause damage to the epiphysis, and it eliminates some of the rotational control that the condyles provide in the socket.
The knee-disarticulation amputation provides a long lever arm which, in conjunction with the presence of the condyles, provides rotational stability, socket control, and proprioception, as well as a maximum weight-bearing area for distal contact. Thus, any attempt to eliminate the condyles eliminates some of the advantages of the knee-disarticulation amputation.
The knee-disarticulation prosthesis is often bulky because of the outside location of the knee joints. The appearance is further compromised by the necessity of lowering the effective knee center in comparison with the sound side. Lack of an effective knee-friction system or swing-phase control compounds these prosthetic problems further.
With a growing child frequent prosthetic replacements are anticipated, not only because of growth but also because of wear and tear to the prosthesis itself. In most instances the prosthesis breaks down in the knee joint because of hard wear that is accelerated by lack of an effective friction or swing-phase control. While some circumferential growth of the thigh can be accommodated by grinding out the socket to enlarge it, growth in length usually requires socket replacement.
Current Socket Designs
Various types of socket design are applicable during the growth period of a child. Best results are obtained when retardation of growth on the amputated side allows fitting of a knee-disarticulation prosthesis with suction suspension ( Figure 1 ). In this case the condyles are not prominent enough to interfere with donning and doffing the prosthesis, and the resulting design allows partial ischial and partial distal residual-limb weight-bearing. In addition, this situation allows installation of a knee-shin assembly with conventional friction or swing-phase control, thus providing equal knee centers and improved function and cosmesis ( Figure 2 ).
Prominent femoral condyles require windowing of the socket to an extent that will permit application and removal of the prosthesis. One patient had completed much of his growth at the time of amputation, necessitating the use of outside hinges, a stump sock, and auxiliary waist-belt suspension ( Figure 3 ). Suspension over the supracondylar area also provides some socket control. Obviously the windowed socket section should be replaced with a hinged trap door for improved socket control; but when given a choice, patients seem to prefer a simple strap in place of the hinged door, since it reduces weight, bulk, and noise, and because it provides a cooler stump-socket environment.
It is also possible to use the principle of a closed, double-walled socket with a flexible inner wall that expands as the condyles pass through. This approach, however, prohibits any future growth adjustments on the inside of the socket, and frequent replacements are required. Also, the expansion of the flexible inner socket is limited; and in the case of prominent femoral condyles a rather large, bulky socket design may be required.
For these reasons we attempted to use a flexible but removable insert constructed initially of flexible resin, then of Kemblo, followed by Cordo and finally Pelite ( Figure 4 and Figure 5 ). While in all instances this approach provides limited socket adjustments, the resulting socket hulk was uncosmetic in appearance and therefore was unacceptable ( Figure 6 ).
Segmental Socket Design
The segmental socket design consists of two separate socket parts, the socket brim and the distal socket section ( Figure 7 ). The parts are joined by the external uprights of the knee hinges. Close proximal and distal fit provides stump-socket stability. Socket suspension is achieved by means of a circular strap over the supracondylar flares, thus eliminating use of a waist belt and forked suspension strap ( Figure 8 ). The distal socket portion is lined with Pelite to allow socket expansion for donning and doffing, thus providing inherent socket suspension ( Figure 9 ). The socket is easily adjusted for ischial, distal, or combination weight-bearing, and in case of growth, for repeated length adjustments. Because of the size of the condyles, the resulting socket of this prosthesis is still bulky ( Figure 10 and Figure 11 ). Since the residual limb is not totally confined in the socket, the prosthesis is lighter and creates a cooler residual-limb environment ( Figure 12 ). It is possible to replace only the proximal socket brim in case circumferential growth can no longer be accommodated. An adjustable socket brim is being considered to further simplify this change.
Patient acceptance has been satisfactory in our limited experience with this system, but we hope for a more positive response with increased experience.
Editor's note: In subsequent correspondence with Mr. Zettl, it was agreed that the proposed socket design might usefully be combined with U.S. Manufacturing Company's new, small polycentric knee unit, which, because of its endoskeletal design, would also permit length adjustments in the shank.