Prosthetic Management for Children with Knee Disarticulations


The advantages of end-bearing, prosthetic directional control, proprioception, and active hip use are felt to be applicable to the child with knee-disarticulation conversion in congenital limb deficiency. Techniques have been developed to fit these patients with prostheses utilizing the principles of end-bearing with distal control and suspension.


Knee disarticulation is the site of conversion for a number of severe congenital limb deficiencies. In a juvenile amputee-clinic population this level of residual limb frequently presents problems to the prosthetist with a prescription for fitting.

In most cases the configuration of the residual limb at this level in children allows a fitting utilizing the principles of the standard above-knee prosthesis. However, the biomechanical principles of distal stump end-bearing and distal limb control of the knee-disarticulation prosthesis, as used for the adult, offer several practical and theoretical advantages over the above-knee fitting. Significantly decreased time for rehabilitation and gait training has been noted 5 . The primary use of an end-bearing prosthesis offers increased proprioception and increased directional control of the prosthesis. Active use of the hip joint and thigh musculature adds to the functional capacity and endurance of the patient 2 .

The Juvenile Amputee Clinic at Newington Children's Hospital feels that these advantages should be exploited in the child amputee, as well as in the adult. A clinical study is in progress to determine whether this impression is validated by objective evidence.

Despite its advantages, implementation of end-bearing and distal control has been found to be difficult in the child. In the young child the bulbous flare of the femoral condyles is less prominent than that seen in the adult knee disarticulation, so that the intrinsic suspension from prosthesis shape alone is not as effective. In addition, growth, particularly circumferential growth, occurs rapidly, precluding the use of the intimate fit required for stability in the conventional knee-disarticulation prosthesis 3 . The length of the disarticulated limb can present a problem since the special prosthetic components used in the adult prosthesis, particularly the knee joints, are not commercially available in the smaller sizes necessary for the child.

In an attempt to implement the desired end-bearing and distal limb control, a number of modifications in fabrication and fitting have been tried, and the present techniques have evolved.

Biomechanical Principles

In the above-knee prosthesis the main support 1 during stance phase is at the ischial tuberosity 3 '7. In the knee disarticulation, the main support is at the end of the stump 3 '7. In any prosthesis the main support point is also the point about which rotational forces occur. In the above-knee prosthesis this rotation occurs distally and is counteracted by lateral thigh support 3 . In the knee disarticulation the rotational forces act on the medial-proximal and lateral-distal aspects of the stump so that rotation is counteracted largely by proximal medial support 3 . It can be seen that the biomechanical principles involved in the two types of prostheses are quite different and require careful consideration in fabrication. Both prostheses must be designed to insure that the forces inherent in each are transmitted properly.

The mechanics of suspension also differ. During the swing phase the forces acting on either prosthesis tend to pull the socket off the stump 3 . It is necessary that the socket be held to the stump with the least amount of pistoning motion 1 . In the above-knee prosthesis, suction or some form of belt suspension is used. In the knee-disarticulation prosthesis, suspension is provided by a socket closely fitted over the flares of the distal femoral condyles. This fit is accomplished in the adult where the condyles are prominent by providing a removable window above the medial condyle.

In children, distal suspension is not as easily implemented. Although some children have a bulbous end, most do not (Figure 1. ). The problem may be solved by adding a Silesian belt, but this device hampers, to a certain extent, directional control of the prosthesis. Two methods for providing intrinsic suspension are now being used successfully. Where the femoral condyles are fairly prominent, a flexible-walled inner socket is adapted for the prosthesis. Where the condyles are smaller, an inserted wedge suspension over the medial condyle is more successful. The details of these two methods are outlined below.

Technical Aspects

In general, we use the standard measurement, casting, and plaster-mold-modification procedures recommended for the standard knee-disarticulation prosthesis 3 ; but some additions and variations are employed to adapt to the special needs of the child.

Measurement and Casting

  1. Measurements and casts are taken with the patients in a standing position. A weight-bearing stand cushioned with foam supports the residual limb.
  2. The distance from the end of the stump to the floor is measured and recorded. Where femoral shortening is present, the distance from medial tibial plateau to floor is measured on the sound side and recorded for tibial length. Calf and ankle circumferences of the sound side are measured.
  3. The medial-to-lateral width across the femoral condyles is measured with full weight-bearing on the stump. In those residual limbs which are not bulbous (Figure 1. ), the medial-to-lateral width and the circumference measurements are taken at a point just proximal to the femoral condyles and are recorded (these measurements are used in the fabrication of the medial wedge).
  4. Two cast socks are placed over the stump to allow for the circumferential growth expected in child amputees. This practice will allow the wearing of a heavier wool stump sock in the finished prosthesis, and later substitution of thinner socks as the patient grows.
  5. Starting at the midpoint over the femoral condyles, the stump is marked at 50-mm (2-in.) intervals. Circumference measurements are taken at each point and recorded.
  6. If the patella is present, it is marked for relief. The patient is asked to activate both flexor and extensor muscles, and the movement of the patella proximally and distally is marked to allow sufficient relief for this excursion.
  7. The distal third of the residual limb is wrapped with elasticized plaster bandage. The patient is asked to stand with his weight on the distal end of the stump until the plaster sets. In the nonbulbous limb the plaster is hand formed over the medial condyle to allow for placement of the medial wedge.
  8. The remainder of the stump is then wrapped with plaster, and the proximal portion is hand formed into a quadrilateral socket. The ischial seat is placed about 12 mm (1/2 in.) below the tuberosity in the child to allow for linear growth. In the adult knee-disarticulation prosthesis 25 mm (1 in.) is recommended 3 .
  9. After the plaster has set, the cast is split and removed, the socks are removed, and the cast is poured.

Plaster Mold Modifications

Proximal modifications to the plaster mold are similar to those employed in the. standard above-knee prosthesis. The Scarpa's triangle is accentuated, and the ischial seat is shaped in conventional fashion. In the knee-disarticulation socket, however, the following modifications are made.

  1. Because of pressure that will be applied by the proximal medial wall, this area must be formed with a generous flare at its proximal margin to accept the soft tissues without skin irritation, and avoid formation of a medial roll.
  2. On the lateral wall, from a point just proximal to the lateral condyle, 35 mm to 50 mm (1 xji in. to 2 in.) from the end of the stump, plaster is removed to a depth of 3 mm to 6 mm ('/g in. to '/i in.). This relief extends to a point approximately two-thirds of the distance from the end of the stump to the greater trochanter. This modification provides the necessary lateral stability to prevent axial rotation of the socket.
  3. Proximal to this relief, 6 mm to 11 mm ('/4 in. to x/i in.) is removed on the lateral proximal portion of the cast.
  4. Plaster is added anteriorly and posteriorly to regain the original measured circumference at each marked point.
  5. For the nonbulbous stump where the medial wedge suspension is to be used, plaster is removed proximal to the medial condyle, and this area is shaped to accommodate the wedge (Figure 2. ). The circumferential measurement in this area is again retained by adding plaster anteriorly and posteriorly as needed to allow for tissue displacement.

Socket Fabrication

Depending on the age of the child and the configuration of the residual limb, one of three sockets is used for the child with a knee disarticulation. For the very young child with a nonbulbous residual limb, a straight socket with Silesian-belt suspension is usually employed. The belt suspension is used up to the age of 4 or 5. It renders the prosthesis easy to don and doff and provides the additional suspension stability necessary for this age group.

In an older child where the distal femoral condyles are not particularly prominent, the wedge suspension of a PE LITE insert will usually provide adequate intrinsic suspension and control. For the patient with a more bulbous distal end, an expandable-inner-wall socket may be used 4,6 . This arrangement is preferred to the removable window, since the solid walls add to the strength and durability of the prosthesis, and adds to cosmesis. In addition, it is quite easy to apply and remove.

Medial Wedge Socket Fabrication Detail

  1. A cone is made from 5-mm PE LITE, heated, and pulled over the distal third of the cast.
  2. A PE LITE cap is fashioned over the distal end.
  3. The medial aspect of the PE LITE liner is filled with PE LITE from the medial condyle to the proximal border of the liner to provide a straight medial contour (Figure 3. ).
  4. The socket is then laminated over the mold as with any above-knee socket.
  5. This socket is pulled from the cast while the plastic is still warm by forcing compressed air into the distal end.

Expandable-Inner-Wall Socket

  1. The distal end of the cast is capped with PE LITE (5-mm thick). This ca] will become the distal pad for the finished socket.
  2. Three layers of nylon stockinette are applied to the cast. The distal anc proximal aspects of the cast only are laminated with Laminae resin.
  3. The stockinette is laminated with elastomer or similar material, from thi midcondyles to the point proximally where the circumference is equal to th circumference over the condyles.
  4. The anterior medial and posterior surfaces in this area are built up with wa: to the desired contour of the outer socket. No wax is applied over the lateral aspec of the inner wall. This is important to preserve lateral stability of the stump in thi socket (Figure 4. ).
  5. Three layers of stockinette are added and laminated. The wax is melted out and the cast is pulled from the socket. Removal can be accomplished most easily b; forcing compressed air into the distal end.

Alignment Principles

In the conventional above-knee prosthesis, the socket, except for very shor stumps, is placed in a slightly exaggerated position of adduction with the media wall vertical and the lateral wall definitely sloped inward 2,7 . This is to allow th' abductor musculature of the hip to contract isometrically in stance phase unde normal length and tension'. The same principle of alignment is utilized in thi knee-disarticulation prosthesis, but without any exaggeration of the adduction 3 . Ii extension the femur falls into its normal anatomical position, which allows idea adduction of the femoral shaft.

In an above-knee prosthesis the socket and stump are placed in some flexion ii relation to the alignment of the prosthetic leg, to improve stability, to enhance hi extensor muscle function, and to eliminate lordosis'. In the knee-disarticulatio: prosthesis this initial flexion is neither needed nor desired 3 .

Bench Alignment 3

  1. In the medial-lateral plane a line through the center of the distal socke bisects the heel (Figure 5. ).
  2. In the anterior-posterior plane a line bisecting the proximal socket fall through the knee and through the center of the attachment bolt of the SACH fooi The TKA line is very nearly neutral. The posterior offset of the knee hinge is onl minimal, since inherent stability is adequate in the end-bearing prosthesis (Figure 6. ).

Dynamic Alignment

Minor changes are sometimes necessary during the preliminary dynami alignment. In the very young child and in new amputees, increased knee stability is sometimes needed; but, as the patient becomes accustomed to the prosthesis, this extra stability can usually be eliminated, and initial bench alignment is retained in the finished prosthesis.

Swing Control and Length of Residual Limb

The length of the knee-disarticulation stump can present a problem in attaining a suitable knee unit for swing control. The hydraulically controlled four-bar-linkage knee joint utilized in the adult knee-disarticulation prosthesis is not presently commercially available in smaller sizes. Friction control is available in external knee joints in the adult sizes, but it is not presently available in the smaller units. Outside knee joints without friction control tend to be noisy, and bending and breaking occur frequently.

Fortunately, in most children with knee disarticulations resulting from congenital limb deficiency, the femoral length is either shorter than that on the normal side, or becomes proportionately so with growth (Figure 1. ). If this shortening is anticipated, external knee joints without friction can be used on smaller children with the expectation that eventual proportionate shortening will allow the interposition of a conventional single-axis knee as the child grows (Figure 5. and Figure 6. ). In one case where shortening of the femur had not occurred, and where it was not expected, a distal femoral epiphysiodesis was performed at the appropriate time to provide the necessary six to seven centimeters necessary to interpose a conventional single-axis knee. This shortened femoral segment did not seem to interfere in any



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