Weinstein Lovell and Winters Pediatric Orthopaedics 7e

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CHAPTER 30  |  The Child with a Limb Deficiency

Full pelite insert

Insert split to allow passage of bulbous distal end Outer plastazote build up

Anterior view

A C FIGURE 30-50.  Common socket designs for Syme amputation prosthesis. A : Obturator design is often needed if the distal end of the residual limb is large and bulbous or the medial malleolus is prominent. It is the least cosmetic. B : The removable or segmented liner consists of a complete separate foam liner, which has a split in the side to allow the distal end of the limb to pass. Once the patient applies the liner, he or she slides the limb covered by the liner into the prosthesis. The patient must have the manual dexterity and strength to use this suspension, which will eliminate some patients with hand anomalies from using it. Limbs with a small heel do best with this system. C : The bladder design has a built-up silicone sleeve, which the patient slides the limb past. This socket fits more loosely and does not stabilize the heel pad as well as the other designs. B

(Fig. 30.50C). All of the above designs maintain total contact, and the proximal brim is at the level of the patellar tendon. This ensures that the biomechanical forces are adequately spread up to a load-bearing landmark to increase comfort and function. Prosthetic Knees.  It has been estimated that more than 100 prosthetic knees are commercially available, and the num- ber is growing each year (223). Although most are for adults, recently there are a number of new knees available for chil- dren. The prosthetic knee is composed of the knee mechanism or frame and may contain a control unit. The control unit consists of a pneumatic, hydraulic, or mechanical system, or some combination of these three. The control unit responds to changes in cadence and dampens sudden, abrupt changes. The faster a hydraulic or pneumatic unit is compressed, the faster the energy is released, and this helps to regulate the lower shank of the prosthesis. The prosthetic knee unit can be fur- ther subdivided into single axis and polycentric types.

The maturation of gait from infant to adult carries with it the need for sound practice in selecting the appropriate knee, on the basis of amputation level, functional level, and body size. In general, the single-axis internal knee without any control unit is the first knee to be used on the child, because of its light weight, short lever arm, and simplicity. In the single-axis knee joint, the lower shank rotates around a single point in relation to the socket. A polycentric knee was introduced in 1998 for the infant and toddler, and it may be used if space permits. Internal ­polycentric knees move around a center of rotation that varies with the flexion angle of the lower shank (213). The four-bar linkage knee is the most common polycentric knee and the most widely used by prosthetists (224) (Fig. 30-51). The inherent sta- bility during stance, the fluid knee-flexion movement, and the mechanical design to give more ground clearance during flexion increase patient and practitioner confidence in the unit (225). Changes in design and technology have now widened the boundaries and age distinctions for the prescribing of specific