Health

What are the parts of an upper-limb prosthesis and what is it composed of?

The upper-limb prosthesis has various components that assist the child in recreational activities and activities of daily living. The various components that may be used for the infant or child depend greatly on the level of the deficiency as well as the child’s age and comprehension. A basic upper-limb prosthesis is composed of a socket that fits onto the residual limb, a suspension system that keeps the prosthesis on the limb, a prosthetic shoulder or elbow joint if necessary that helps position the prosthesis and a terminal device or hand which manipulates objects.

The prosthetic shoulder, elbow and terminal device are operated by the child typically with two types of control systems. The body-powered control system works through one or two cables that are attached to a suspension system such as a shoulder harness and rings.

This system is referred to as body-powered because movement of the prosthesis depends on certain motions of the child to manipulate the prosthetic limb. When the child moves his shoulder or shoulder blades, tension is placed on the cable, which causes a joint to move or terminal device to open or close. The child can control which part of the prosthesis is being controlled by locking various parts of the prosthesis manually.

For example, with an above-elbow amputation, the child may move the upper arm out to the side, which causes the prosthetic elbow to bend. Then the child pushes his arm into his body, depressing a button that causes the elbow to lock in this position. Then any movement in the shoulder that follows operates the terminal device.The other type of control system is myoelectric. This is a prosthesis with sensors inside the socket that detect muscle activity in the forearm created by the child’s voluntary movement. The sensors detect this electrical activity and amplify the signal, which in turn is used to open or close the terminal device. Occasionally, outside electrical activity is used to generate the impulse, but this is less common.

The socket and suspension system that is used really depends on the size and shape of the residual upper limb. Children with amputations near the shoulder or who have complete deficiency of the upper limb may not use a prosthesis. Instead, only a shoulder cap may be used in order to preserve the normal contour of a shoulder when shirts or other garments are worn.

In contrast, a child with a deficiency that involves the forearm will have a normally functioning elbow and shoulder. Therefore, the socket can be fitted over the residual limb and the suspension system can wrap around the upper back and shoulders. Those with partial-hand amputations may elect to use a prosthesis for cosmetic reasons but may not need one for functional purposes. Amputations through the wrist (wrist disarticulations) may present a challenge.

This type of residual limb can fit readily into a socket but because of the length of the residual limb, the prosthesis may end up being longer than the normal side and may not be cosmetically appealing. Fortunately there are thinner wrist components that can be adjusted to simulate wrist motion and are not too long, thereby creating a symmetrical length.

The terminal device is located at the end of an upper-limb prosthesis and essentially functions as the “hand” of the prosthesis. One of the first terminal devices used in infants with limb deficiencies is a simple mitt. This type of prosthesis may be shaped like a hand and encourages the infant to integrate the prosthesis into play and may offer him the opportunity to hold objects with both hands. There are numerous other types of terminal devices that can be used depending on the activity level and age of the child. A complete description of terminal devices for children is found at the following link. There are also many other specialized terminal devices that allow children to climb, play sports and play a musical instrument with their upper limbs.

What are the various parts of a lower-limb prosthesis? What advantages do they offer?
In general, lower-limb prosthesis involve various components, including the socket, suspension system, skeleton or frame, joints (hip, knee and ankle), as well as some type of foot. The type, durability and expense of these components vary depending on the type of residual limb as well as the age and activity level of the child. For example, some types of sockets incorporate a brace, also known as an orthosis, into the socket so that contact is maintained between the prosthesis and the residual limb. This type of socket is sometimes used in children with unusual residual limbs or hip disarticulations. In patients with transfemoral amputations, an ischial containment or quadrilateral socket is often used.

The ischium is another term for the bones of the pelvis that we sit on. In an ischial containment socket there is better contact between the residual limb and the prosthesis because the ischium is held within the socket together with the residual limb. This allows a better distribution of the weight from the body onto the lower limb as well as providing better stability and muscle function. The quadrilateral socket is named because it has four sides making up its walls. Each socket type has certain advantages that enable a transfemoral amputee to walk more effectively and comfortably. It varies considerably which type of child with an above-knee amputation or deficiency uses which type of socket.

Various types of suspension devices are available to keep the prosthesis held onto the residual limb. One type of suspension system uses one of two types of belts, either known as a Silesian belt or Total Elastic Suspension (TES) belt.

The Silesian belt anchors to the socket on its outer aspect, then wraps around the child’s pelvis and opposite hip and then reattaches back to the anterior socket. The TES belt works differently. Made from elastic neoprene, it is worn around the waist; because of its ability to retain body heat, it can be quite warm in hotter climates or during the summer months. Fortunately, both of these suspension devices can be adjusted to accommodate the growth of children. Neither of these two belts should interfere with normal function, though, and parents should ensure that it does not become entangled with objects as the child plays.

For the older child with a stable lower-limb deficiency or amputation, a suction suspension system can also be tried. There are basically two types of suction devices, the first a traditional suction suspension.

With this type of suspension, the residual limb is essentially pulled into the socket using a donning sock. The donning sock is then pulled out through a valve hole leaving the residual limb snugly in the socket. After the valve hole is sealed, a negative pressure system is created and the limb is held in the prosthesis via a vacuum phenomena. With this system, there is better contact with the prosthesis and less motion occurs while the limb is in the socket. Excessive motion can be uncomfortable and lead to skin breakdown and blisters.

If a child does not have the strength or dexterity to pull the sock through the valve hole, another type of suction device known as roll-on silicon suction suspension can be tried. This system uses a silicon sleeve rolled onto the residual lower limb. The outer portion of the silicon sleeve is lubricated and placed into the socket. Once all of the air is displaced out of the socket, the valve hole is then covered, maintaining suspension via a vacuum system. The disadvantage of this system is that the silicon can be hot and sweaty on the residual limb.

Additionally, the residual limb must be of a consistent size, not changing much in volume due to swelling. These suction systems can be used in conjunction with some of the belt systems described above for added security.Since the child with a limb deficiency is continually growing, there may be times when the limb will not fit adequately into the socket and the prosthesis will need to be replaced.

However, one way to prolong the fit of the socket on a rapidly growing limb is to use a flexible socket made of plastic material that can be heated and remolded so as to improve fit. Additionally, slip sockets may be used. A slip socket has one or more thin inner layers or liners that can be peeled out of the socket to accommodate the expanding limb. A child also may use socks to help improve the fit and longevity of a socket. These socks are made of a thin cloth material that can come in different plies (thicknesses) and when the residual limb grows, the sock ply can be reduced allowing for better fit.

When the child gets down to a single ply it may be time to replace the socket. What types of prostheses are used for children with above-knee amputations or limb deficiencies?

Various types of knees are available to children with above-knee amputations or limb deficiency. However, the type of knee used in the prosthesis depends on the age of the child and his activity level. One of the first knees used in a prosthesis is what is known as a manual locking knee. This simple type of knee is usually appropriate for the toddler who is taking his first steps or a bilateral above-knee amputee who needs more stability when walking. The manual locking knee is a hinge type knee joint that remains locked while the child learns how to stand and walk; parents can unlock the knee by pushing a button or pulling a lever, allowing it to bend when the child wants to sit or crawl.

When a child reaches the age of three, a true mobile knee joint can be used in the prosthesis. Typically, this knee is either a constant friction (single axis) knee or a stance control (weight-activated friction) knee (Figure 34). The constant friction knee works like a hinge and has a built-in mechanism that slows the swinging of the prosthesis as the child walks.

The stance control knee is typically used for amputees with very short residual limbs who may need more stability when they walk. This type of knee has a braking mechanism that prevents the knee joint from buckling when it bends too far. In this way the child has a built-in anti-stumble mechanism. The disadvantage of both of these knees is that they do not allow the toddler or child to increase walking speeds, so a different knee component may be needed as the child learns to walk fast and to run.Another type of knee joint is a polycentric knee.

The most common type of this knee has four bars that simulate the knee joint as it rotates during the normal walking motion and therefore more closely mimics a true knee joint’s motion. This type of knee joint is often used in children with knee disarticulations or with very long residual limbs so that the axis of rotation more closely simulates a normal knee.

Unfortunately only a few types of polycentric knees exist for children.As a child begins to walk with varying speeds and starts to run, a hydraulic or pneumatic knee joint may be used. This type of knee uses either pressurized fluid or air that allows the prosthetic limb to change speeds depending on the walking speed. These units are therefore referred to as cadence (speed) responsive.

Occasionally polycentric knee joints described above can be combined with a cadence-responsive knee (hydraulic or pneumatic), to make the prosthetic knee function as close to a real knee as possible. Pneumatic knees are unaffected by changes in temperature but quicker children can occasionally overpower the pneumatic pressure control, occasionally resulting in decreased function.

In contrast, hydraulic units can be affected by extreme changes in temperature. However, they may allow for a greater range of walking and running speeds. The disadvantages of a hydraulic knee are that it may be heavier and more expensive. Newer hydraulic knees are much lighter, but there is a limited selection for children compared to adults.

What are the types of prostheses used in children with below-knee limb deficiencies?
Children with below-knee deficiencies may benefit from a suspension device known as a supracondylar cuff. This device is also referred to as a Patellar Tendon-Bearing Supracondylar Cuff since most of the weight bearing of the residual limb occurs through the patellar tendon as well as through the inner knee. The patellar tendon, a soft tissue structure that runs from the knee cap (patella) and inserts onto the upper shin, is able to withstand a significant amount of weight.

This prosthesis is held onto the lower limb via cuffs that extend above the knee. There are two bony prominences above the knee joint called condyles which project out. The cuff of this socket essentially hangs onto these condyles, keeping the prosthesis suspended onto lower limb, hence its name. Occasionally added suspension is provided by using a Silesian or TES belt as previously described. A thigh corset may also be used to provide additional stability and support.

Some children may also use a liner over their residual limb to help improve the contact between the limb and the prosthesis. Though the liner may also help control limb edema, this should not be its primary function. Occasionally the liner may also have a pin placed into the end, which then locks into an opening in the prosthesis, serving to improve suspension from the deficient limb. When the prosthesis needs to be removed, a button near the locking mechanism is pushed, releasing the pin from inside and freeing the limb from the prosthesis.

This type of suspension device is called a shuttle/pin suspension system and is well accepted by older children because of its comfort, stability, ease of application and overall performance. The other advantage of using a liner is that it can often be doubled up to accommodate a smaller limb not quite ready to accommodate a prosthesis. Later as the child grows, one of the liners can be removed enabling the prosthesis to fit the child for a longer period of time.

Another component of a prosthesis is called the skeleton. The skeleton is the actual material that the prosthesis is composed of: wood, plastic or metal. The skeleton can either have an exoskeleton or endoskeleton (metal) design. An exoskeleton design usually has a hard outer cover that appears like the other limb and is generally very durable and resistant to stains. It can be used to improve the appearance of a prosthesis especially an initial one. An endoskeleton design, also called a modular design, has a different appearance. It has a plastic socket attached to a metallic pylon, which is attached to a knee and/or foot.

Often this type of skeleton is used as a trial or temporary prosthesis. A soft synthetic covering may be placed over the modular component or endoskeleton once permanent adjustments have been made. This serves to improve the appearance as well as protect other children from the hard components when they play. Unfortunately, this material may become easily stained by an active child and may not be suitable for a child with poor control of bowel or bladder. The synthetic cosmetic covering may also hinder the functioning of the underlying components such as the knee joint. Therefore, some parents and children elect not to use any covering at all.

What are the types of prostheses used in partial-foot or Syme’s amputation?
A vast array of feet and ankles are available for a lower-limb deficient child. Children missing a few toes or part of the foot may only require a soft toe filler in their shoes for comfort. However, a child with a partially amputated foot may develop an abnormal posture to the foot called equinus. An equinus foot is pointed down and the child may develop problems when he starts to try to walk on it. As the child tries to push off on an equinus foot, the end of the foot may become sensitive and may even develop skin breakdown. For this reason, the rehabilitation team should work with the family and child to provide appropriate stretching exercises and splints to prevent this deformity.

The prosthesis, therefore, may consist of an Ankle Foot Orthosis combined with some type of modified shoe.

This type of partial-foot prosthesis seems to be better tolerated than using a large amount of filler in the shoe.A child with a Syme’s amputation, missing fibula or other lower-limb deficiencies just at or above the ankle may be able to bear some weight through the residual limb, but there will obviously be some leg-length discrepancy. To account for this difference and improve comfort, a Syme’s prosthesis may be tried. This type of prosthesis simulates the lower leg but has a foot attached to it. The residual limb slides into the back part of the prosthesis and may be suspended using a supracondylar cuff. The drawback of this type of prosthesis is its appearance since the end of the prosthesis near the foot needs to bulge out in order to accommodate the ankle bones. These bones may get sensitive if they do not fit appropriately in the prosthesis.

What are the types of prosthetic feet used in lower-limb deficient children?

The type of foot used in a prosthesis can be quite variable and is dependent upon the activity level of the child, cost and maintenance. One must realize that the foot is a vital component of the prosthesis since its structure, composition and position can aid in knee stability as well as ease of walking.One of the most basic types of prosthetic feet available is known as a Solid Ankle Cushioned Heel (SACH) foot. This foot does not have a mobile ankle joint but does have a heel that comes in variable firmnesses. The firmness of the heel can affect the stability of the knee joint above as well as how fast weight is transferred onto the foot during walking.

For example, a softer heel leads to the knee being more stable and is used more commonly in lighter individuals, whereas a firmer heel is used in heavier patients to slow the amount of weight being transferred onto the foot. Unfortunately, the SACH foot is not a very dynamic foot in that it does not have much resiliency. It does not provide much push off or spring into the prosthetic leg when the limb is leaving the ground during the walking cycle. Other types of basic prosthetic feet include the single-axis foot, multiple-axis foot, Stationary Attachment Flexible Endoskeleton (SAFE) foot and Stored Energy (STEN) foot.

Each of these has advantages and disadvantages, but in general all are somewhat heavy and are used for children at a lower activity level. However, they can accommodate different types of uneven ground and do not need a lot of maintenance. In contrast to these types of feet, a dynamic or energy-storing foot stores and release energy when the patient with a prosthesis is walking. As the prosthetic limb accepts the weight of the individual, this type of foot compresses slightly. Then when the limb is advanced and takes the next step, energy is released like a spring helping to propel the prosthetic limb forward. This type of feet may look like a foot or may appear somewhat unusual. Although these feet are lighter, more expensive and require more maintenance than a non-dynamic foot, they are best used in very active or athletic children. However, the choice of which foot is to be used should be left to the individual on a case-by-case basis.

How often will a prosthesis need to be adjusted for a growing child with a limb deficiency or amputation?
As a child grows, the prosthesis will need to be adjusted every 3-6 months, and the entire prosthesis will usually need to be replaced every 1-2 years. Therefore, it is important to be in contact with a physician or prosthetist 3-4 times a year so that the prosthesis can be adjusted appropriately. If a facility is too far away, there is a simple way to determine if the prosthesis is now too short and the lower legs are unequal.

While the child stands facing you, place your hands on the child’s hips. If your hands appear to be level while resting on the child’s hips, then the limbs are probably of equal height.Problems with a socket can be more difficult to diagnose and treat. A tip for discerning a socket problem may be the fact that the child no longer wants to wear the prosthesis or verbalizes some discomfort while wearing it. The parent may notice redness on the residual limb. Minor blisters, abrasions or occasional pimples are not cause for too much concern. Children can be quite tolerant of pain until there is a significant problem. One way to test that the limb is fitting uniformly in the socket is to put some crayon or lipstick on the socket and then have the child don the prosthesis. Uneven pressure will show as crayon located unevenly on the residual limb.

However, this test may not always identify the problem; therefore it is prudent to check periodically with the prosthetist to ensure that the socket fits properly and comfortably.