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An Endoskeletal Pediatric Knee Disarticulation Prosthesis with Rigid Cosmetic Cover

Over the past four years we have seen a relatively high number of patients at our facility who have been diagnosed with congenital deficiency of the tibia. This is a rare longitudinal deficiency in which the fibula is usually intact with total aplasia or marked dysplasia of the tibia. The involved limb is short and usually bowed. The foot is rigid with marked supination, foreshortening of the first metatarsal, and associated medial ray defects. A knee flexion contracture is present with a skin dimple on the proximal tibia.

The incidence of congenital deficiency of the tibia is one in one million live births in the United States. A familial incidence has been reported. The first case was reported in 1841. Centralization of the fibula underneath the femur was first described by Albert and popularized by Brown in the mid 1960s.

Kalamchi1 has classified tibial deficiencies into three types: Type I: Total absence of the tibia, Type II: Distal tibial aplasia, and Type III: Dysplasia of distal tibia with diastasis of tibiofibular syndesmosis.

Type I is characterized by total absence of the tibia with a marked knee flexion contracture greater than 45 with no active quadriceps function. Marked inversion and adduction of the foot is present with occasional medial ray deficiencies. There is proximal migration of the fibular head. The distal femur is markedly hypoplastic with a definite but variable reduction in width of the distal metaphysis and retardation of ossification of femoral epiphysis.

Treatment consists of early amputation at the knee disarticulation level. Femoral-Fibular fusion is considered if the femur is markedly hypoplastic.

Type II is characterized by distal tibial aplasia. The proximal tibia is present at birth and with further growth will develop an adequate articulating tibial plateau. There is active quadriceps function and a knee flexion contracture of 25 to 45. Distal femur development is generally normal. Proximal fibular migration is also less severe. Treatment consists of amputation of the foot and fusion of the fibula to the proximal tibial remnant to achieve greater length, stability, and better alignment. Knee function is generally excellent.

Type III is characterized by dysplasia of the distal tibia. The knee joint is normally formed and quadriceps are well developed. Hypoplasia of the tibia and shortening are usually limited to the distal third of tibia. The talus is usually subluxated proximally with varying degrees of diastasis and the foot in varying degrees of varus. Treatment consists of a Syme's or modified Boyd amputation.

Traditional prosthetic treatment for the pediatric knee disarticulation patient has consisted of an exoskeletal prosthesis with outside knee joints. This design allows for equalization of the mechanical and anatomical knee joints, as well as a durable prosthesis requiring relatively little maintenance. However alignment adjustability and cosmesis are compromised. Alignment changes are difficult and require major refabrication. This prosthesis does not provide knee friction or an extension assist. It also increases the already wide medial-lateral dimension of the knee joint.

Due to the increased availability of pediatric endoskeletal componentry in recent years, the difficulties of alignment and growth adjustability have been minimized. Durability of the cosmetic cover remains a major disadvantage of the endoskeletal system. In an effort to address this problem, an endoskeletal system was developed incorporating available pediatric endoskeletal componentry with a custom fabricated rigid shin fairing. The advantages of this system include modularity, friction control and an extension assist, knee rotation, alignment adjustability at the foot, little terminal impact noise, and a durable exoskeletal cover. This system does require more maintenance, however, the child is generally seen three to four times a year for growth adjustments at which time cleaning and lubrication of the knee unit can take place.

Componentry consists of a Daw TK-40C pediatric polycentric knee unit, Daw TFC-ECK-40C Rubber knee cap, Daw TSC-KD-L steel socket adaptor, Otto Bock 2841-1 tube adaptor, Otto Bock 2840-1 foot adaptor, and Syme foot. Weight limit of the DAW TK-40C is 35 kg (80 lbs). Weight limit of Otto Bock tube and foot adaptor is 45 kg (99 lbs). DAW has a tube adaptor and foot adaptor available however it does not allow alignment adjustability at the foot. A Syme style foot is necessary for distal stabilization of the shin fairing. The minimum clearance needed for componentry from distal end of socket to floor is 15.2 cm (6") plus foot clearance. This is generally available by two years of age. At minimal clearance levels the cosmetic appearance must be considered, as a large calf and ankle will result due to the size of the knee unit.

Static and dynamic alignment is obtained in the same manner as similar endoskeletal systems. Time consuming squaring and resquaring of outside knee joints is avoided. When alignment is complete, the socket attachment plate is riveted to socket. The rubber knee cap is attached to knee unit and parting agent applied to the top of knee cap. Plastic is applied to the distal end of socket, blending in the socket adaptor and knee cap. When this is complete, the knee joint is flexed and the posterior distal portion of the socket where the posterior linkages of the knee unit contact the socket is marked. Notches are then cut into socket until socket fully flexes, leaving extra clearance for finish lamination. The socket is then finish laminated.

To fabricate the shin fairing, a plaster or rigid foam blank is poured and shaped 6 mm (1/a") smaller in circumference than the patient's measurements and tracing of sound side limb. The length of the shin fairing should extend proximally about 6 mm (1/4") superior to the proximal medial and lateral edge of the knee cap. Anteriorly, it should extend to approximately the level of the knee cap screw. Posteriorly, it should be just low enough to allow full socket flexion. Distally, the shin fairing should extend to the keel of the foot. The proximal edge of the foot should be feathered to keep the transition line minimal. An A-P and M-L measurement of the proximal edge of foot will help in shaping the distal portion of the shin fairing. 6 mm (1/4") are subtracted from) these measurements to allow for plastic thickness.

When shaping has been completed, a Durr-Plex "test fairing" may be drape formed with the seam posterior and a parting agent applied for easy removal from mold. The test fairing is then removed from the mold, trimmed out, and the distal 5.0 cm (2") of seam ground away to allow the insertion of fairing into foot. With the prosthesis fully assembled, the test fairing is applied to the prosthesis. Be sure the distal end of fairing extends to the keel of foot. The proximal edge of fairing is trimmed as previously described. Leave 3 mm (1/8") of clearance between the proximal medial and lateral wings of the shin fairing and the rubber knee cap. The shin fairing is marked just proximal to the posterior pylon clamping screw of the knee unit (Fig. 1 ). This is where a crepe wedge will be placed to stabilize proximal shin fairing. The shin fairing is removed and placed back on the mold. The mark on the fairing is then transferred to the mold. Fairing is removed and a piece of solder or stiff wire is wrapped around the mold where marked. The wire is removed taking care not to distort the shape. The inside edge of wire is traced onto a piece of crepe 1.5 cm (5/8") thick. A hole 3.2 cm (1.25") in diameter is drilled into crepe. The location of the hole is determined by the relationship of the proximal pylon to the inner walls of the shin fairing. Crepe is trimmed to proper size. A posterior radius is cut into the crepe to allow placement onto knee unit (Fig. 2 ). A crepe wedge is placed on knee unit just proximal to posterior pylon clamping screw. A heavy duty staple across the radius of the crepe wedge will hold it is place nicely. The shin fairing is placed back on prosthesis. The crepe wedge is trimmed as necessary to achieve a tight fit to the inner wall of fairing.

With the prosthesis completely assembled, the knee should be flexed as far as possible. The proximal posterior edge of shin is trimmed until full knee flexion is obtained with no interference from the shin fairing. The knee joint is then flexed to 90 and the prosthesis placed on table in a kneeling position. In this position the knee cap should rest on table, not the proximal edge of fairing. The proximal edge of fairing should be trimmed if necessary. Slight flattening of proximal anterior fairing is also an option.

The definitive shin fairing is now ready for fabrication. If significant alterations were made to fairing, it should be poured with plaster. Shin fairing may be either thermoplastic or laminated. The recommended thermoplastic is 3 mm (Vs") pigmented copolymer with the seam posterior. Due to the limited availability of copolymer colors, lamination of the shin fairing may be required to more closely match the skin tone of the patient. The recommended lay up for lamination is two Nyglass, one carbon, two Nyglass, two nylon. 3 mm (Vg") Pelite or Aliplast may be formed to mold before pulling or laminating to reduce noise reverberation in shin fairing. If this is done, remember to reduce circumference of distal fairing so it will fit into foot.

When the definitive shin fairing is completed, horsehide is used to cover the posterior proximal opening of the shin fairing. This will keep dirt and foreign objects, as well as the child's fingers, out of the knee unit. The leather is glued to the inside edge of the shin fairing and extends to the top of the knee cap. Care should be taken not to stretch the leather tightly across the medial-lateral sides as the knee unit will need room to protrude slightly during knee flexion. Speedy rivets are applied to proximal medial and lateral corners to anchor leather to shin fairing. The leather can be pigmented to match the color of prosthesis.

The prosthesis can now be completely assembled. Screws and bolts should be Loc-titened and torqued. The top of the knee cap is glued to the socket and the posterior medial and lateral extensions of the knee cap to the knee unit before bolting down. The inside of foot is glued to shin fairing. The foot bolt hole in bottom of foot should be plugged with silicone caulking material. The finished prosthesis is now ready for delivery (Fig. 3 , 4 ).

Some problems we have had with this system and have since solved include:

Problem: Posterior leather cover coming unglued.

Solution: Speedy rivet leather in place.

Problem: Aluminum socket attachment plate has broken.

Solution: We now use steel attachment plate.

Problem: Cosmetic knee cap has torn off attachment screw.

Solution: Glue knee cap to socket and knee unit.

Problem: A blow to the shin can reverberate through fairing.

Solution: Line fairing with Pelite or Aliplast to absorb noise.

We have fit 22 prostheses using this endoskeletal system with an exoskeletal shin fairing with excellent results. Both the parents and the patients have been pleased with the cosmesis and durability of the prostheses. We have also used the exoskeletal shin fairing with the Otto Bock 3C1 Active Line system with minor modifications to the procedure described in this paper.

This system is a good alternative to outside knee joints or an endoskeletal system with a foam cover. It is our system of choice. It requires a relatively compliant patient who will be seen a minimum of twice per year. Length adjustments are easily made by removing foot bolt and taking off shin fairing and foot at one time. The pylon is lengthened, knee unit blown out with compressed air and lubricated, and prosthesis reassembled. If a large length adjustment is made, the shin fairing may be removed from the foot, slid up, and reglued to the top one third of foot.

This procedure takes about the same amount of time as it takes to fabricate a foam cover. The advantage of this system is that when the child's next prosthesis is made, the shin fairing may be poured with plaster, lengthened, and matched to the child's new foot size. Fabrication time in this situation is much less than starting from scratch since a Durr-Plex test fairing is usually not necessary. Test fairings should be saved so that a good assortment of sizes and shapes are available for future patients.

There are other facilities who are doing a thin lamination over a foam cover to provide durability. The advantage of the shin fairing over that procedure is its ability to be lengthened as well as avoiding having to reshape the foam cover on the child's next prosthesis.

In the future we hope to have an assortment of prefabricated shin fairings available for purchase. This would provide for a very fast and easy way to finish and deliver the prosthesis.

Scottish Rite Children's Medical Center, 11101 Johnson Ferry Road, Atlanta, GA 30342

References:

  1. Kalamchi A: Congenital Lower Limb Deficiencies. New York, NY, Springer Verlag. 1989.