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Lower-Limb Pediatric Prosthetics: General Considerations and Philosophy

Donald R. Cummings, CP
Susan L. Kapp, CP

Introduction

Although many prosthetic principles used in treating adults apply to the treatment of children as well, the child with a lower-limb deficiency presents the prosthetist with a unique range of considerations, both practical and philosophical. Most techniques used with adult amputees must be downsized, sequenced in degree of complexity, modified or completely altered to match the ever-changing needs of children.

History

Over the last three decades, the prosthetic management of limb-deficient children in the United States and Canada has developed into a subspecialty with a well-documented history.

During the 1940s two young orthopedic surgeons, Thomas Aitken, MD and Charles Frantz, MD, founded and directed what was probably the first formal pediatric amputee center in the United States-Mary Free Bed Rehabilitation Center in Michigan. Through their efforts, and those of UCLA, NYU and a growing number of prosthetic clinics throughout the country, the unique needs of pediatric and juvenile amputees were first identified and addressed.

By 1958, a nationally organized program included 13 clinics formed to focus on the problems of child amputees. In June 1961, the first clinic chiefs meeting was held in Washington, marking the foundation of what is today the Association of Children's Prosthetic-Orthotic Clinics (ACPOC). Another major outcome of this meeting was the development of a regular communication vehicle, the inter-Clinic Information Bulletin (ICIB), first published four months later. Today ICJB is known as the Journal of the Association of Children's Prosthetic-Orthotic Clinics (ACPOC). It continues to serve as an essential multidisciplinary forum to address topics of concern to all members of the more than 80 clinic teams in the United States and Canada who work with limb-deficient children (1). ICIB and JACPOC articles dealing with prosthetic management of children with limb deficiencies contain most of the foundation of the concepts-many still under debate-that will be discussed in this article.

Causes of Amputations Among Children

Lower-limb amputations among children may be divided into three broad categories: emergency amputations, congenital limb deficiencies and elective amputations.

Emergency Amputations Injuries leading to emergency amputations affect children of all ages and have a wide variety of causes, including motor vehicle accidents, lawn mower or power tool injuries, thermal or electrical burns, recreational accidents, gunshots and explosion wounds. Approximately 70 percent of acquired pediatric amputations are related to such trauma (2). Amputation surgery in children, as opposed to adults, differs in four ways:

• Preservation of all possible epiphyses. Whenever possible, the surgeon will try to preserve major epiphyses to allow continued longitudinal growth. For example, since approximately 70 percent of femoral length is contributed by the distal femoral epiphyses, a knee disarticulation is preferable to an above-knee amputation for a growing child (3).
• Consideration of osseous overgrowth (exostosis). The most common surgical complication among traumatic juvenile amputees is osseous overgrowth of the transected bone (4). This problem manifests itself as a spike-like prominence of new growth protruding from the distal end of the transected bone. The affected residual limb is often tender and inflamed, and prosthetic wear can grow increasingly uncomfortable. Although modifications to the socket or a new distal pad may relieve pressure temporarily, surgical revision becomes necessary when the overlying skin begins to erode.
  
  Terminal overgrowth among children is very common in below-knee amputations and involves the fibula more often than the tibia (3). The phenomenon is best explained by high osteogenic activity of the child's periosteum-perhaps further stimulated by weightbearing within the prosthesis-resulting in a cartilaginous spike that slowly ossifies. Terminal overgrowth is apparently not related to epiphyses growth since it cannot be prevented by epiphysiodesis (5).
  
  The problem of osseous overgrowth appears to be highest among children under age 10 with acquired amputations. It will generally cease when the child reaches skeletal maturity, but during the growing years multiple surgical revisions may be required. Surgical techniques, such as capping the transected bone with an autogenous bone graft or polyethylene endoprosthesis, hold promise for avoiding terminal overgrowth but lack long-term results (6).
• Exuberant healing processes. A surgeon can preserve as much length as possible in a child using skin grafts, skin traction or closure under tension, knowing that in most cases healing will be prompt. This is due to the plastic, growing state of the child's tissues, which generally exhibits maximum physiological tolerance (7). Such techniques would likely result in a painful, fragile or ulcerated residual limb if performed on an adult.
• Disarticulation vs. transections. In light of these considerations, disarticulation through a joint has at least five advantages over bone transection for the pediatric population:
• Epiphyseal growth is preserved.
• Terminal overgrowth is avoided.
• A longer lever arm is maintained.
• Suspension and rotational control in the prosthesis are enhanced.
• The residual limb is tolerant of distal weightbearing.

For these reasons, disarticulations are the most common lower-limb elective amputations among children. Disarticulations are performed, if at all possible, when amputation becomes necessary due to trauma and for conversion of congenital anomalies such as longitudinal deficiency of the fibula and Proximal Femoral Focal Deficiency (PFFD).

This doesn't mean that a higher-level disarticulation is preferable to a lower-level transection. For example, if a Syme's amputation cannot be performed, then by all means the surgeon should attempt to retain BK function even though growth will be affected and later revisions for terminal overgrowth may be necessary (3).

Congenital Deficiencies

Most pediatric amputee clinics treat children under age 15. About 60 percent of these children have congenital limb deficiencies; another 10 percent have congenital anomalies that are treated as, or require, amputation (2). Approximately 40 percent of the children with congenital limb deficiencies will have multiple limb involvement, most commonly, combined upper and lower extremity deficits (8).

Some congenital amputations are clearly the result of constriction band syndrome (Streeter's Dysplasia), where amniotic bands have resulted in complete or nearly complete antenatal amputation. Others may possibly be familial, as in some cases of longitudinal deficiency of the radius or ulna.

Most congenital limb deficiencies, however, are probably due to an injury or developmental failure of the limb during the first six weeks of pregnancy. The cause may be anoxia, drugs, irradiation, chemicals, certain viral infections or an accident during the early part of the pregnancy. In the majority of cases, the cause is simply not identifiable (8).

Some anomalies classified as transverse deficiencies are homologues of acquired amputations and are easily identifiable as such. For example, transverse deficiency of the leg (upper-third) would receive prosthetic evaluation and fitting in essentially the same manner as an acquired BK amputation.

Initial fitting would, of course, begin much earlier, and components would be appropriate for a pediatric fitting. Basic prosthetic principles, however, still apply. For example, if ligamentous laxity at the knee is present, then the knee joint should be protected with a supracondylar socket or with joints and a thigh corset. Although distal edema is generally not a common problem for patients with congenital BK deficiencies, distal end pads are still indicated to prevent excessive pressure, for general comfort and to provide a means of accommodating growth.

Congenital transverse deficiencies presenting themselves as homologues of Syme's or partial foot amputations are also fitted with basic prosthetic techniques and componentry based upon thorough evaluation of the patient. For children with congenital anomalies other than the transverse "amputation," surgical conversion of the limb to allow eventual prosthetic fitting often provides the most positive long-term outcome.

Non-Standard Prostheses

Before proceeding to a discussion of elective surgical conversion of congenital anomalies, it is important to note that occasionally nonstandard prostheses are fitted to a non-amputated limb. It seems convenient to discuss these devices as fittings for congenital anomalies, even though no amputation-congenital or acquired-exists.

McCollough et al. (1963) identified four indications that apply to fitting non-standard prostheses for congenital limb deficiencies below the knee (9):

• when the parents and/or patient refuse surgical conversion but a prosthetic device will facilitate ambulation (see Figure 1 ).
• when surgical conversion is delayed in hope that maturational changes will improve or clarify the surgical outcome or where surgery may create as many problems as it solves.
• during the early periods of observation of longitudinal deficiencies (fibular or tibial) or during surgical correction of these deformities, which can take several years.
• in cases of longitudinal absence of the tibia or fibula when there are also bilateral upper-limb deficiencies, making it essential for the patient to use his or her feet for activities of daily living.

Since a non-standard prosthesis is not actually a replacement for an absent limb, it could, in many ways, be defined as an orthosis or a hybrid between prosthesis and orthosis (see Figure 2 ). In general, these systems provide sufficient stability and accommodation for leg-length discrepancy to allow the patient to become ambulatory. Such systems are as varied as the anomalies for which they are prescribed, and each must be designed on a case-by-case basis to meet each patient's unique needs.

Elective Amputations

Elective amputation among children necessitated by disease or trauma is usually planned to preserve as much of the patient's skeletal growth potential as possible (10). Other factors special to childhood amputation, following trauma or disease, include the following:

Skin grafts. Grafted skin that might not do well in an adult will often provide an excellent result in a child, thereby allowing surgeons to salvage limbs that might otherwise be revised to higher-level amputations. Prosthetists treating children may often be presented with residual limbs covered at least partially by skin grafts. These usually thicken and tolerate weightbearing remarkably well over time.

Partial foot amputations. Mid-foot amputations, often frowned upon in adult treatment centers, can provide excellent function among children. Levels such as the Chopart's amputation generally provide good function and preserve tissues even if later revision becomes necessary.

Burns. Tissue damaged by burns will generally heal better in a child, allowing surgeons to preserve more of the limb. Prosthetic fitting precautions should be the same as for adults, but the child's exuberant healing processes will usually result ultimately in a much more pressure- and shear-tolerant residual limb.

Revisions. It is not uncommon for diaphyseal amputations among children to require revisions. This usually is due to subperiosteal terminal bone overgrowth and may require several revisions during the child's growing years.

Disarticulations. Among adults, disarticulations often result in long amputations with a bulbous distal end that may be difficult to fit cosmetically. In children, since the affected bones do not continue to grow normally following an amputation, the residual limb will have all the benefits of length, suspension and distal weightbearing, but will have a far more cosmetic result when the child reaches adulthood.

Occasionally Syme's amputations done in infancy may have to be revised. Most often this is because of migration of the heel pad posteriorly as the child grows. This migration occurs because the heel pad, under tension from the tendo achilles, separates from the distal end of the limb. This is not always a problem. In fact, the child's ability to actively contract and draw the heel pad up anteriorly or posteriorly (depending on the muscles attached) may sometimes be used to enhance suspension (8).

Elective Amputations for Congenital Deficiencies

Each patient with a congenital deficiency will provide the clinic team with a unique set of circumstances and clinical considerations, so an in-depth discussion is beyond the scope of this article. In general, longitudinal absence of the fibula (complete) will require a Syme's amputation. Longitudinal absence of the tibia (complete) will generally require knee disarticulation, but if the proximal tibia is present, below-knee function may be preserved through transfer of the fibula into the proximal tibia along with ablation of the foot.

Anomalies such as PFFD present multiple surgical and prosthetic options and merit a separate discussion. Generally, severe forms of PFFD may be converted to provide AK function through Syme's amputation and knee fusion. Though still controversial, BK function may be simulated through the Van Nes rotationplasty, which involves rotating the foot by 180 degrees so ankle motion can control the prosthesis (see Figure 3 ). Other options include non-standard prostheses or shoe-lifts with no surgical conversion.

As a basic principle, no form of amputation should be performed until all parties involved agree to the decision. Each case must, of course, be considered individually. The key joint to be preserved must be identified and then surgical procedures can be performed to correct leg lengths, increase stability or correct malrotation. Amputation may be performed when recognized as the best option, and prosthetic fitting can then proceed.

Aitkin and Pellicore (1981) described several biomechanical losses frequently associated with congenital lower-limb anomalies: inequality of leg lengths, malrotation, inadequacy of proximal musculature and instability of proximal joints.

Although recent techniques such as the Ilizarov method of lengthening and correcting deformities hold promise, for many congenital anomalies carefully planned amputation offers the greatest functional gain. Prosthetic fitting solves the problem of length discrepancy and can compensate for malrotation. External joints attached to a corset or belt can help compensate for unstable anatomic joints, and often the prosthesis can improve cosmesis.

General Philosophy and Considerations in Pediatric Prosthetics

Physical Considerations

Staging. Unlike the adult amputee who is aging and decelerating, the child is changing, growing and dynamic; hence, prosthetic designs should be staged based upon the child's developmental readiness. For example, a prosthetic component that may be too complex for the child today may be exactly what he or she needs two years from now.

Age at Fitting. The child with congenital limb absence or early amputation is considered ready for lower extremity prosthetic fitting when he or she begins pulling up to stand. This usually occurs between nine and 12 months of age. Independent ambulation will begin between 15 and 22 months. Initially, all children walk with a wide-based gait with hips and knees flexed. Normal heel-to-toe gait patterns do not usually begin until age five.

The first prosthesis for a toddler with a knee-disarticulation or AK amputation will generally be non-articulated or include a locked knee (see Figure 4 ). By age three or four, the child may be able to adapt to an unlocked knee. Children with bilateral above-knee amputations may require manually locking knees until well beyond age six (see Figure 5 ).

In light of the many changes that occur in walking ability and gait patterns during the first five years of life, each new prosthesis may differ greatly in design, alignment and componentry from the previous one (see Figure 6 ).

Growth. Children grow both longitudinally and circumferentially. Bony alignment is changing also. For example, a newborn child's knee will generally exhibit genu varum. This condition usually straightens out by the first or second year, moves into genu-valgum by the third year, then resolves spontaneously thereafter (11).

The prosthesis must accommodate growth and other physiological changes. Clinically proven methods to allow for growth and increase the useful lifespan of the prosthesis may include:

• Socket liners. Liners are easily modified, provide added protection and are a convenient way to allow for circumferential growth.
• "Slip" or "triple-wall" sockets. As early as 1964, child amputee clinics were proposing methods to decrease the frequency of socket replacement due to growth (12). The "Slip" socket is simply a removable inner layer of the socket that can be pulled out when the socket becomes tight. It usually consists of a laminated inner socket separated from the next layer by a PVA or PVC sheet. Although removal of the inner socket will provide more room circumferentially, it does little to account for longitudinal growth.
• More socks. If the prosthesis is fitted over a five-ply sock initially, growth can be accommodated by simply decreasing sock thickness. Again, this does little to compensate for changes in length.
• Distal pads. Distal pads 1/2-inch thick or more allow for some longitudinal growth as well as for terminal bone overgrowth. As the child begins to grow out of the prosthesis, the pad can be replaced with a thinner pad. Pour-in-place pads, such as RTV silicone with a foaming agent, are an excellent way of dealing with terminal overgrowth of BK amputations. When the child begins to complain of distal discomfort, a new pad can be poured, automatically relieving changes in the bony overgrowth.
• Flexible sockets with ISNY frames (13). Flexible sockets have been used successfully in many pediatric centers. Surlyn? ,polyethylene or other thin-walled flexible sockets hold great promise for dealing with growth in child amputees (see Figure 7 ). Among their many potential advantages:
  • Clear sockets allow for visual evaluation of socket fit.
  • Minor socket changes can be made simply by heating and stretching areas of the socket.
  • New sockets can be made over the original cast with modifications for growth.
• Frequent follow-up. Because frequent adjustments for growth will be necessary, children should be seen every three to four months. At each visit, the patient's limb should be examined for signs of pressure over bony areas, and overall length of the prosthesis should be checked against the sound side. Modifications made most frequently include a relief for bony prominences and lengthening of the prosthesis. A new prosthesis will probably be necessary for a growing child every 18 months on average; actual useful lifespan of the prosthesis depends primarily on the child's rate of skeletal growth.
• Modular systems. Component interchangeability and alignment adjustability are major advantages of endoskeletal prostheses for growing children. Traditionally, such systems required soft protective coverings, which children quickly destroyed. Recent methods of combining thermoplastic or laminated sockets with "discontinuous" cosmeses have enhanced endoskeletal durability. In some cases sprayed-on "skins" or flexible laminations can improve durability of soft covers.
• Growth-oriented suspension system. Because the child is growing both longitudinally and circumferentially, suspension systems should incorporate adjustability for growth. Self-suspending sockets that fit intimately over the child's bony anatomy can be provided at a very early age but require readjustments as the child grows. They will be successful only when regular follow-up is feasible.
  
  Other suspension options such as neoprene sleeves, silicone suction sockets, supracondylar cuffs, internal suspension pads or even waist belts are commonly used when rapid growth is anticipated. These latter suspensions can be adjusted easily to accommodate growth without significant alterations to the socket itself.
• Growth-oriented modifications/alignment. Alignment, suspension or socket modifications should always be planned to allow for growth. Epiphyses are open and cartilaginous, and thus more susceptible to damage. Ligaments are generally more lax and may require protection through conservative alignment, socket design or suspension systems. As a general rule, socket contours and alignment of children's prostheses should be more forgiving and less severe than with adults. For example, with adults, BK alignment should produce a consistent varus moment at midstance through relative inset of the foot. With young children, this force may be undesirable and can be reduced by aligning the foot with less inset. In the child's prosthetic socket, a deep patellar tendon protuberance is best avoided since the patellar tendon is still growing and its attachment points are not completely ossified (14).
• Activity Level. Children's blood supply, healing potential and general tissue metabolism are maximal, so the amputated limb will heal more quickly, withstand stresses well and recover from damage more easily than will comparable amputations in adults (10). These factors, combined with the extremely active nature of most children, mean that prostheses for children will be subjected to diverse and high degrees of stress. Practical guidelines to enhance the clinical usefulness of children's prostheses include:

• Maximize prosthetic performance. Although available choices of componentry for children (particularly under age 10) are often limited, whenever possible try to use components (such as energy-storing feet) that will maximize performance. All aspects of prosthetic design for children should be geared toward a physically active, athletic lifestyle (seeFigure 8 ).

• Protect from injury. Because children are normally extremely active, prosthetic design should reflect a concern for preventing injury to the remaining joints. This doesn't mean that every child with a BKA should be fit with joints and a corset. It does mean that if there is doubt about the ligamentous stability of a child's knee, it is better to err on the side of protection. Socket liners, distal pads, higher trimlines, and joints and a thigh lacer (when necessary) are all examples of protective measures that may prevent a knee injury and avoid the lifetime mobility impairment that could result, even though specific indications are difficult to document.
• Reinforce the prosthesis. Older children and adolescents with below-knee or Syme's amputations are often involved in sports such as football, basketball, skiing or swimming. Since extremely high stresses are applied to the prosthesis during these activities, all areas susceptible to breakage should be reinforced. High-strength materials such as carbon acrylic or graphite should be used whenever possible (see Figure 9 ).
• Minimize weight. To facilitate a high activity level, overall weight of the prosthesis should be as light as possible without sacrificing necessary strength. Again, state-of-the-art materials such as graphite, acrylics, thermoplastics and titanium should be used when applicable.

As always, it is up to the prosthetist and the clinic team to determine the balance between the ideal and what is truly practical for each individual patient.

Psycho-Social Considerations

From a psychological and social aspect, the child with an amputation is quite different from the adult. First, while great differences exist among individuals, in general children are less responsible and more mentally and emotionally immature than the ideal adult. The prosthetist must take this into account when designing the prosthesis and in the treatment plan. For example, removable components will probably get lost. Instructions will often be forgotten or ignored. Fitting problems will often not be reported by the child unless they become severe. The success of the prosthesis will depend on good design, appropriate training of the child and his parents, and regular follow-up.

Secondly, according to their age and maturity, children are dependent on adults. Whether the amputation was congenital or acquired, the child's reaction to it depends to a great extent on how his or her parents have dealt with the difference (15). Prosthetic training, monitoring skin tolerance, sock wear and donning/doffing of the prosthesis will require adult supervision, so parental education is extremely important.

Thirdly, because the child has not decided on a vocation and is also malleable physically, socially and emotionally, rehabilitation goals following amputation differ from those of an adult. As the child grows and matures, goals of training and prosthetic fittings will change as the child changes. Unlike the adult amputee whose vocation is often already chosen, the child can choose vocations that minimize the impact of amputation.

Finally, since the child is dependent on a family group, pressures he or she feels in these areas are very different from adults. Again, the child's parents and family situation are primary determinants of his or her adjustment to limb absence.

Conclusion

Individual prosthetic treatment is the hallmark of optimal pediatric care (see Figure 10 ). Each child's physical condition, aspirations and life circumstances are reflected in the prosthetic prescription and treatment plan. Prosthetic success is not guaranteed by good technique and componentry alone, but rather by the harmony between prosthetic management and the ability of the clinic team to help both the parent and child anticipate and deal with the effects of constant growth and change that characterize childhood.

DONALD R. CUMMINGS, CP, is director of prosthetics for the Texas Scottish Rite Hospital for Children, 2222 Welborn St. Dallas, TX 75219.

SUSAN L. KAPP, CP, is assistant professor and acting director of the prosthetic & orthotic program at the University of Texas Southwestern Medical Center, 6011 Harry Hines Blvd., Suite V. 5100, Dallas, TX 75235-9091.

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