Introduction to Below Knee Prosthetics
An amputation, especially at any level below the knee, does not usually present a particularly disabling condition. With the use of modern prostheses and treatment methods, below knee amputees who are free from complicating medical problems can do most of the things they could do prior to amputation.
Studies suggest that there are 1.5 amputees per 1000 persons. Therefore, the current number of amputees in South Africa amounts to approximately 60 000.
Below Knee Prosthetic requirements
In order for a patient to be able to wear a below knee prosthesis, at least 40% of the following criteria should be met so as to obtain a positive outcome.
The residual limb (stump) should be well-formed and round at the distal end.
The residual limb (stump) should be well healed with no open wounds.
Good muscle tone should be evident
No swelling should be evident
There should be full movement of the hip and knee.
Weight should not exceed 10kgs beyond normal weight.
The ability to walk on crutches and achieve good balance is essential.
The residual limb (stump) should be able to sufficiently carry body weight.
Patients should display high levels of motivation.
The prosthetic should be affordable to the patient.
There are more below knee (trans tibial) amputees than any other type. Surgeons preserve the knee joint whenever it is possible and practical to do so and attempt to fashion the residual limb (stump) at the lowest practical level. Very short below knee stumps make fitting extremely difficult whereas very long below knee stumps limit the choice of prosthetic feet.
The Harmony® system controls the volume of the residual limb and prevents volume fluctuations. As opposed to a specific weight bearing socket, a total surface weight bearing socket is used.
At a glance, the Harmony® system:
Monitors residual limb volume
Enhances blood circulation to the residual limb
Ensures an excellent fit
Relieves strain on the joints and spinal column
The Harmony® system creates an elevated vacuum environment between the socket and the liner. Using the patented Harmony® pump, each step removes air from the socket to enable a precise socket fit at all times. The superior fit helps prevent residual limb/socket pseudo arthrosis and improves proprioception.
The Harmony® system helps prevent fluctuations in volume
The Harmony® system’s elevated vacuum helps regulate volume changes in the residual limb to achieve a consistent fit at all times. In using a total surface weight bearing socket, pressure is relieved on load-bearing areas. Beyond pulling air from the socket, the pump also provides shock absorption and rotation.
The Harmony® system consists of a pump, polyurethane liner and gel-coated sleeve.
Below Knee Prosthesis Process
The below knee prosthesis process involves:
Design of prosthesis
Design of prosthesis
Most prostheses that cater for amputations between the knee and ankle consist of three major parts, which include:
The shank or shin; and
The most common socket used is a form of Patellar-Tendon-Bearing (PTB) design whereby a person’s entire weight is carried through the residual limb (stump). The PTB socket completely encloses the stump and usually contains a soft liner to provide a cushioning effect. However, a hard socket may also be worn, which has a cooling effect.
All trans tibial (below knee) prosthetic sockets that are manufactured at our facility are what we call ‘total contact sockets’, which are cast under vacuum to ensure maximum mobility and comfort. Within this casting system various designs are utilised depending on the nature of the residual limb. With over three decades as a specialist in the field, Roger Wolfson utilises the most appropriate design based upon the shape and size of the residual limb.
The prosthetic may be held in place using a number of methods, including:
- A cuff above the knee cap
- By the shape of the brim of the socket
- A silicone sleeve that sticks to the skin and attaches to the prosthetic socket either by vacuum seal, pin or belt.
- Joints inside steels that go up into the thigh are often utilised in the case of a bony residual limb that tends to be short, enabling the patient to reduce the amount of weight placed on the residual limb.
Sockets are manufactured using acrylic resin, and other sockets such as the Iceross system, which works on the suction socket system, are available for trans tibial amputees.
Many artificial foot designs are currently available. Each design presents its own set of advantages and disadvantages. These designs can be divided into two classes, as follows:
- Non-Articulated Feet(Doesn’t contain any movable parts or joints)
- Articulated Feet(Contain movable parts or joints)
- Articulated feet require higher maintenance and are slightly heavier than most non-articulated feet, due to the addition of moving joints with in the foot.
Single Axis foot
- The single axis foot (one joint) enables ankle action that is controlled by two rubber bumpers, either of which can be altered in order to allow for greater or less motion, as required. It is often used to assist in keeping the knee stable.
Multi Axis Foot
- The multi axis foot is often recommended for people who need to walk on uneven surfaces as it enables a certain level of motion across all three axes of the ankle. The multi axis foot is slightly heavier than other types of feet and requires a higher level of maintenance.
SACH (Soft Ankle Cushioned Heel) foot
The simplest type of non-articulated foot is the SACH (Soft Ankle Cushioned Heel). The heel is rigid and the ankle action is provided by the soft rubber heel which compresses under load during the early part of the stance phase of walking. The rubber heel wedges are available in three densities of soft, medium and hard. SACH feet do not have joints and are usually manufactured using Pedilen Foam. The Pedilen Foam SACH Foot is marketed under the brands Dynamic Foot and Dynamic Plus Foot.
Flex Foot (Solid Ankle Flexible Endo-Skeletal Foot)
The Solid Ankle Flexible Endo-Skeletal Foot enables the same action as the SACH Foot plus the ability for the sole to conform to slightly irregular surfaces; which provides for easier walking over uneven terrain. This type of foot enables easier walking due to its flexibility and is sometimes referred to as the Flexible Keel Foot.
The heel height of the shoe is an extremely important factor in the alignment of the prosthesis. Hence, when shoes are changed, it is important that the effective heel height remain the same. The effective heel height is obtained by subtracting the thickness of the sole from the apparent heel height. The heels of the shoes should also be replaced frequently in order to prevent alignment changes as a result of general wear and tear. A badly worn shoe will also increase the wear on a prosthetic foot. A prosthetic should not be worn without shoes, since, temporary misalignment will cause excessive stress on the stump and knee joint and the wear on the foot will result in permanent misalignment.
In general, the earlier a prosthesis is fitted, the better. One of the most trying challenges facing the amputee as well as those involved in the treatment process, is oedema (the accumulation of fluids) and swelling of the stump. In all cases, oedema will be present to a certain extent, which makes the fitting of the prosthesis difficult. However, certain measures can be taken to reduce the amount of fluid build-up when oedema is present. This involves the use of a rigid dressing. Once the rigid dressing has been removed, and when the prosthesis is not being worn; elastic bandages are used to prevent fluid build-up.
Wrapping of the stump
The wrapping of the stump with elastic bandages results in numerous benefits, which include:
The elimination of swelling
Protecting the stump while waiting for a prosthesis
When the leg is not yet stable and is required to be placed into the socket each morning
When the leg is either unable to fit into the socket or there is an abrasion present when not wearing a prosthesis
Alleviation of stump paid during the night
Guidelines for wrapping of the stump
For the average adult, one to two elastic bandages of approximately four inches or 10 centimetres wide is the norm.
It is critically important that the bandages should neither be too tight nor too loose.
The bandages should contain no wrinkles and should be wound high in the leg.
Bandages should not be worn if the stump displays any open wounds.
Should no open wounds be present, the stump should be bandaged constantly. However, the bandages should be changed every four to six hours. Bandages should never be kept in place for longer than twelve hours without re-bandaging. Should throbbing occur, the bandages should be removed and re-wrapped.
Special elastic Shrinker Socks can be used as an alternative to elastic bandages. While not considered as effective by some as a correctly applied bandage, a Shrinker Sock is considered to be far more effective than a poorly-applied bandage.
While the below knee amputee possesses normal knee function, training in the use of a prosthetic is necessary in order to achieve optimum gait and comfort. Early training is provided by the prosthetist during fitting trials.
Physical therapists usually provide additional training as required. Initially, the new prosthesis should be worn for short periods only. However, wearing time should gradually be increased on a daily basis, with longer wearing periods being achieved over time depending on individual circumstances.
Most prosthetic socks are woven from virgin wool. However, synthetic yarn socks are also available. Prosthetic socks come in two different thicknesses, 3 ply and 5ply; and are worn on the stump inside the socket of the prosthesis.
A prosthetic sock should be stretched on firmly and evenly so that no wrinkles occur and the wearer should avoid placing the seam over any bony areas. Prosthetic socks should be changed daily in order to prevent skin irritations and dermatitis.
Prosthetic socks should be donned sequentially in order to gradually obtain the desired thickness. Should the overall thickness of the socks worn exceed 15 ply at any given time, this usually indicates a loss of muscle mass and that a new socket may be required.
The overall thickness of the Prosthetic socks should be adjusted when necessary and usually require attention when fluid changes occur in the stump, which can happen in the following circumstances:
Excess heat/high temperatures
Increased fluid intake
Failure to bandage the leg during the night
Allowing the leg to hang down without the use of the prosthetic
Changes in daily activities
Add an extra sock when:
There is excess weight/pressure under the patella
There is excess weight/pressure under the amputated tibia
The stump feels loose under pressure
Remove a sock when:
The stump is not deep enough inside the socket
There is excess pressure at the tibia head
The stump should be bathed daily, preferably in the evening, since any resulting irritation may cause swelling preventing the leg from being able to fit into the socket. Shaving of the stump could also cause irritation.
Prosthetic socks should be hand-washed with warm water and detergent free cleansers and laid flat to dry.
The socket should be wiped daily using a damp cloth and mild soap or alcohol.
Start with the bandage held in place on the inside of the thigh just above the knee and unroll the bandage so that it is laid diagonally down on the outer side of the stump, while maintaining approximately two-thirds of the maximum stretch of the bandage.
Bring the bandage over the inner end of the stump and diagonally up the outer side of the stump.
Bring the bandage under the back of the knee, continue over the upper part of the kneecap and down under the back of the knee.
Bring the bandage diagonally down the back of the stump and around over the stump.
Continue up the back of the stump to the starting point on the inside of the thigh and repeat the sequence in such a way that the entire stump is covered once the end of the bandage has been reached.
The end of the bandage is then held in place using the special clips provided. It is important that the tightest part of the bandage be at the end of the stump.
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