Types of Knee Prosthesis for Leg Amputations

There are many prosthetic leg options to choose from, and the best fit will be dictated by an individual&#;s level of injury, physical factors, complications, as well as their lifestyle and vocational or recreational goals.

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Every amputation is different, and it is critical that persons who have sustained an amputation work with a certified prosthetist and physiotherapist to determine which kind of prosthesis will be the best option for them.

In this article, we will highlight prosthetic leg options and physiotherapy for through-knee or above knee amputation.

Table of Contents:

What is a Prosthetic Leg?

Prosthetic legs allow an individual who has sustained an amputation to walk with a more normal and energy efficient gait pattern and will often allow them to walk without the need for assistive devices such as crutches or walkers. In addition to enabling a more normal gait pattern, they allow the user to accommodate for sitting/standing, stairs/ramps as well as uneven terrain and different walking speeds.

A prosthetic leg includes several components including the socket, which is molded to the individual&#;s residual limb, the suspension system, which is how the prosthesis stays attached to the individual, and the prosthetic leg itself, which includes different options for knee and ankle joints.

Individuals can be fitted for a prosthesis once the wound from the amputation is stable and well-healed, which can be within a few weeks after the surgery. This may be delayed if they have complications related to the amputation.

After an above-knee or through-knee amputation, the residual limb shrinks, as the individual loses muscle mass in their thigh. Users will need to be fitted for multiple sockets over time to accommodate for the decreasing size of their residual limb.

Common Causes and Complications of Amputations

Amputation refers to the removal of a limb due to injury, disease, or surgery. It can be utilized as a surgical procedure to manage discomfort or a disease condition in the affected limb. Individuals can also undergo traumatic amputations resulting from motor vehicle accidents or workplace accidents.

There are several complications related to amputation such as infection, phantom limb pain (residual limb pain), slow wound healing, vascular issues and necrosis, neuromas, edema, and skin breakdown. Phantom pain is a common occurrence after an amputation and is perceived in the limb that has been removed from the body. It is typically characterized as a sharp or burning pain that occurs due to a miscommunication of nerve signals from the brain to the spinal cord.

Types of Knee Prostheses

Prosthetic knees allow a person who has sustained an amputation at or above the knee to regain use of the knee and ankle joints and participate more easily in their daily activities.

Prosthetic knees are divided into two categories, mechanical or computerized. Mechanical knees are further divided into single-axis, multi-axis, and polycentric knees. Each prosthesis has its own unique features which are chosen based on the user&#;s requirements.

Mechanical Knees

Single-Axis Knees Mechanism and Indications

This is the simplest type of prosthetic knee joint that allows for rotation around a single axis during flexion and extension of the knee. Individuals who use this type of knee require good muscle strength, as this type of prosthesis can be difficult to control. These are sometimes recommended when there are limited economic resources, as it is the most cost-effective option.

Advantages:

• This is a very simplistic design, durable, as well as light weight, which can be helpful for new amputees, who are just learning to walk.
• Most cost effective

Disadvantages: 

• Due to this simplistic design, the user has to generate significant muscle power to keep the knee stable while standing and walking.
• Creates an unnatural gait pattern due to the simplicity of the single-axis design
• Can be difficult to control the locking and unlocking during gait, putting the user at higher risk for falls and injuries.

Polycentric Knees Mechanism and Indications

Unlike a single-axis knee, a polycentric knee allows multiple points of rotation around several axes. Polycentric joints can have either 4 or 7 bars, meaning that you can either have 4 points of rotation or 7 points of rotation. This type of joint provides good stability when there is involuntary flexion during the heel strike phase of walking because the center of rotation is located more proximally and posterior when the knee is fully extended. This knee is typically recommended for active people, or people who are more likely to walk independently, without a gait aid.

Advantages:

• Can be very stable in stance phase
• Allows the user to sit down with a bent knee
• Due to the multiple axes and the center of rotation, the prosthetic length &#;shortens&#; at the start of the swing phase of walking to allow for foot clearance
• Lighter than computerized knees
• Less expensive than computerized knees

      Disadvantages: 

• Much heavier than a single-axis joint prosthesis
• Greater energy requirement of muscles during walking to maintain gait efficiency and balance.

Manual Locking Knees Mechanism and Indications

This type of joint locks while the user is weight-bearing on it. They will need to manually disengage the lock to be able to sit down.

      Advantages:

• Very stable
• Allows for an automatic lock in weight-bearing, with the additional option to manually lock the knee. This is especially important for people with less muscle control and balance, or who need more security while walking on uneven terrain.

      Disadvantages:

• The user will need to swing the leg out or hike the hip to allow for adequate foot clearance during gait. This is an unnatural gait pattern, which can lead to further issues in the future in surrounding muscles and joints.
• Need to manually unlock the knee to sit with a bent knee

Weight Activated Stance Control (Safety) Knees Mechanism and Indications

This knee provides a constant friction force while weight is on the limb. This helps to prevent it from buckling when standing on that leg, while still allowing it to swing freely when unweighted.

      Advantages:

• This is a very stable knee that is commonly prescribed when individuals first start using a prosthesis, and in older individuals and the less active community.
• This is also for people who can exert some control over their knee, but who fatigue quickly after a few steps, or people who forget that they cannot put weight onto a bent knee while using their prosthesis.

      Disadvantages:

• With this prosthesis you are unable to get into a sitting position without taking all your weight off the leg.
• The individual has to take all of the weight off their affected side to allow it to swing resulting in a less efficient gait pattern
• Due to the constant friction in the knee, the individual will walk slower, with smaller steps.

Pneumatic or Hydraulic Knees Mechanism and Indications

Pneumatic/hydraulic components (pistons with cylinders containing air or fluid) can be added to either single-axis or polycentric mechanical knees, as well as computerized knees. This is to help increase mobility and control with the leg and allow the user to vary speeds. When walking faster it will limit the air flow and fluid to reduce the flexion of the knee to allow a faster walking pace. The opposite will happen with a slower gait. Typically, hydraulic knees work well for more active individuals.

      Advantages:

• Allows the individual to walk more comfortably and with a more natural gait pattern.
• This type of knee also allows the individual to walk up and down the stairs with a reciprocal gait pattern, due to the resistance provided during flexion while there is weight on the leg.
• This is both less expensive and lighter than computerized knees.

Disadvantages:

• Hydraulic knees are often more expensive and heavier than pneumatic knees, they also need more maintenance.
• Cost is higher than mechanical knees.

Computerized Knees

Mechanism and Indications

These knees have microprocessors to allow feedback from within the knee or foot joint. Information from the sensors adjusts the range and speed of knee flexion and extension, according to the user&#;s requirements. Knee extension is powered, and resistance is provided through knee flexion, allowing the individual to more easily get in and out of a sitting position, and navigate stairs using a reciprocal gait pattern.  It also allows for a symmetrical weight distribution and a natural gait pattern.

      Advantages:

• These are much more advanced, which allows the gait to be more natural as well as allowing the leg to adjust to different speeds. Some offer stumble control, which will automatically put the leg into a position to prevent you from falling. This type of prosthesis allows the user to adjust to changes in speed, environment changes and specific situations. These also have different programmable modes for when you are doing different activities (i.e. cycling)
• More efficient gait pattern with less energy expenditure during gait
• Allows the user to navigate stairs using a reciprocal gait pattern

       Disadvantages:

• These are very expensive and need to be charged regularly.
• Heavier than the other options and can also be damaged by different environmental conditions.
• Can be more difficult to cover or wear certain types of clothing overtop
• Learning curve when first starting to use this type of device
• Regular maintenance required

Physiotherapy After Leg Amputation

The nature of rehabilitation after a leg amputation at or above the knee will depend on the client&#;s goals and physical presentation but will often include:

• Gait assessment and prosthesis training
• Gait aid training
• Transfer training
• Static and dynamic balance training
• Strengthening the hip muscles of the residual limb, as well the muscles of the sound leg, and core.
• Desensitization techniques / mirror box therapy
• Manual therapy &#; to normalize muscle tension and joint mobility
• Edema management
• Education on activity modification and return to recreational activities
• Collaboration with the prosthetist on adjustments to the prosthesis

Conclusions

When an individual has undergone an amputation at or above the knee, learning to use a prosthetic leg is often a crucial step in the rehabilitation process. A well-fitting prosthesis can help individuals regain mobility and return to their daily activities. This allows the freedom and independence to get back to doing the activities that are important to them.

The physiotherapists at Propel Physiotherapy are trained in the rehabilitation of individuals who have sustained an amputation. They will work with you to explore your individual needs and rehabilitation goals and create an individualized treatment plan. If you have any questions about rehabilitation after an amputation, please reach out to us at or 416-621- to schedule a complimentary consultation with one of our clinicians.

Hydraulic & Pneumatic Knees

Hydraulic and pneumatic knees utilize air or fluid to adapt their behavior to different walking speeds as the wearer swings the leg forward and backward. Additionally, some designs allow the user to lock the knee for stability while standing for longer periods of time, or when walking in unfamiliar or unsafe environments.

Both pneumatic (air) and hydraulic (fluid) systems mimic the muscles of the natural knee by responding to varying walking speeds. Since they require fluid, hydraulic knees tend to be heavier than pneumatic knees, but they can also provide a smoother walking pattern across a greater range of speeds. Both hydraulic and pneumatic knees require frequent routine maintenance.

Hydraulic and pneumatic knees are best suited to moderately active people who want to vary their walking speeds. Your prosthetist will discuss the benefits and considerations and will help determine the best option for you.

Prosthetic Knees

Prosthetic knees can be divided into Mechanical or Computerized. Mechanical knees can then be subdivided into single-axis and multiaxis/polycentric knees. All prosthetic knee require some sort of stability mechanism, this can be manual or a weight-activated locking system. They also require a way to control the flexion and extension motion, this can be done by friction or a hydraulic/pneumatic control. Different prosthetic knee technology will be mentioned below, manufacturers use a combination of these technologies together to create different types of prosthetic knees. There are many varieties of prosthetic knee on the market and this page only covers the concepts and not specific brands. When assessing a patient for the first time or when they upgrade their prosthesis it is important to go to ask the prosthesist and or do a web search to read more about the knee/prosthesis and watch the available videos. Each knee has different features that will influence the gait of the individual as  

Single-Axis Knees[1][2]

Mechanism: This is a simple hinge type knee.

During the flexion/extension these articulations execute a simple rotation around the knee axis. They are of simple design and their easy alignment responds to the rules of mechanics. There are exoskeletal and endoskeletal knees, both versions can have manual or automatic blocking of the flexion to be used in users with poor muscle power. The knees without blocking can be used for regular prosthetic fitting of amputees with adequate muscle control and/or in situations of limited economic resources.

Advantages: Very simplistic, durable, very light, and economical.

Disadvantages: Due to the simplicity the individual has to use their own muscle power in the limb to keep the knee stable with heel contact and standing.

Additional components: A manual lock can be added to give more stability in standing. A constant friction control can also be added which will prevent the leg from swinging through very quickly.

 

Polycentric Knees[1][2]

Mechanism: This knee has multiple axes of rotation. Polycentric knees can be four bar knees (4 axes of rotation) or seven bar knees (seven axes of rotation). Knees of the most frequent use are of 4 axes (or 4 bars). Without giving importance to the number of axes, the knees of poly-axial design have one thing in common - the Instant Centre of Rotation (ICR) is situated much higher and posterior than the mechanical axes when the knee is in extension. To localize the ICR of a polycentric knee, we need to extend virtually the center lines of the lateral bars towards proximal - the intersection of those lines will indicate the ICR. This causes a high level of stability in the knee against involuntary flexion during the heel strike. Standard polycentric knees have a single walking speed but when a manufacturer includes pneumatic or hydraulic features the patient will be able to vary their walking speed.

Advantages: It is very versatile in terms of stability and get be adjusted to be extremely stable when the patient goes into stance phase, but in the same time allow an easy swing and allows sitting down with a bent knee. Due to the multiple axes and the ICR, the prosthetic length "shortens" at the start of toe-off and will allow for foot clearance. It is suitable for patients with the potential to be independent with the prosthesis in their home and community as well as the more active person[4].

Disadvantages: It is heavier than a single axis knee. More parts that need servicing.   Most polycentric knees do not have stance flexion resistance and therefore cannot yield during sitting, ramps, or stairs. A person with a knee that is not controlled by a microprocessor, needs to actively generate a knee extension moment in the stance phase to prevent the knee from buckling and cause the person to fall down.

Manual Locking Knee[1][2]

For more Five-axis Pneumatic Knee Jointinformation, please contact us. We will provide professional answers.

Advantages: This will allows for automatic locking of the knee with weight bearing, but the patient can choose to manually lock the knee. This is especially for people who need extra security to keep the knee from buckling in standing or with heel contact or when walking on uneven terrain. The indication for this type of knee is usually for K1 ambulators or debilitated individuals who cannot voluntarily control their prosthetic knee.

Disadvantages: The patient will need circumduct or hip hitch to allow for foot clearance when the knee is locked during gait.

Weight-Activated Stance Control [1][2]

These knees are also called "safety knees"

Mechanism: There is a constant friction system in the knee, which means it will apply a braking force as the patient puts weight on the prosthesis, to prevent the knee from buckling. The rest of the time the knee will swing freely, until the weight is applied to it.

Advantages: Very stable knee. Prescribed for first time prosthetic users who need the stability especially in the older or less active population but are still able to exert some control over the knee. Or a person who fatigues quickly after just a few steps. This is especially valuable for the patient who forgets that they should not put their weight on a partially bent knee, the friction in the knee will brake if this happens and prevent the knee from collapsing into flexion.

Disadvantages: When sitting down the patient will have to take the weight off the leg to allow it to bend, this means that they will not be able to use the prosthetic side in the sitting motion. The patient will also need to take the weight off the leg before the knee will bend, this means that the normal knee flexion at toe off will not happen. Due to the friction in the knee the patient will also walk slower and take smaller steps.

Pneumatic or Hydraulic Knees[1][2]

Mechanism: This is when a pneumatic or hydraulic component (piston within cylinders containing air or fluid) is added to single-axis or poly-axial knee. These components control the swinging action of the prosthetic knee and allows the individual to vary speeds. When speed is increased the valve in the cylinder closes and gradually limits the air flow/ fluid to limit flexion of the knee, when knee flexion is reduced the individual can walk at a faster pace. The opposite will happen with slower walking, the fluid or air will be able to more easier, allowing for more knee flexion and slower gait. Pneumatic control will compress the air as the knee is flexed and then stores the energy and releases it with knee extension. Spring coils can be added for more control during gait. Pneumatics or hydraulics are also added to computerized prosthetic knees. Hydraulics are better for very active individuals.

Advantages: The individual will be able to walk more comfortably at different walking speeds with a more natural gait. Less expensive and lighter than computerized knees. Weight can be kept on the prosthetic leg when sitting down and the knee will assist the individual to sit down. The resistance in the knee will allow the individual to climb down step over step when walking down stairs, when weight is kept on the leg before and during the motion.

Disadvantages: Hydraulic knees are heavier, need more maintenance, and have a higher initial cost when compared to pneumatics. Overall cost is more than earlier mentioned knees.

 

Computerized Knees

 

Mechanism: These knees have a microprocessor that receives feedback from sensors located inside the knee joint and/ or the foot. The data from the sensors are used to adjust the knee flexion and extension range and speed to match what the individual requires at that moment in time. It can be explained as an "enhanced hydraulic system" where the computer is controlling the opening and closing of the valves to allow the flow of hydraulic fluid within the unit. A typical microprocessor knee has a hydraulic actuator and a powered knee has a motor actuator. In a motor-powered knee, knee extension is 'powered' for standing up from sitting and controlled resistance is provided when sitting down. It provides active flexion and extension during gait. Symmetrical weight distribution and natural gait[1][2][13].

Advantages: Microprocessor knee lowers the amount and effort that an individual needs for walking. More natural gait. The knee is able to quickly adapt to accommodate different walking speeds, changing environment, or for specific situations. Some knees also have a stumble recovery to prevent the individual from falling. Can use a mobile device or computer to adjust settings. Can have different walking/ activity modes. Once learned, it takes less cognitive effort. Allows descending stairs step over step. The individual will be able to use the prosthetic side when sitting down or standing up. Some knees offer stumble recovery. If the knee is able to adjust automatically according to the load the patient is carrying (like adding a hiking pack or carrying a child), then it will reduced the perceived exertion of the person and reduce the adaptive long stance on the sound limb[14].

Disadvantages: Cost - very expensive. The battery needs to charge. Weight is more than other knees. Cosmesis may be difficult. Specific foot selection. Can be damaged by environmental conditions (water, heat, cold, etc), kneeling etc. Initial steep learning curve and commitment to gait re-education. Regular servicing[1][2][13].

This website is a great resource to see the different types of prosthetic knees available as well as a list of manufacturers and the names of the knees in each category.

 

Prosthetic Knee Systems

OF ALL PROSTHETIC COMPONENTS, THE KNEE SYSTEM IS ARGUABLY THE MOST COMPLEX. IT MUST PROVIDE RELIABLE SUPPORT WHEN STANDING, ALLOW SMOOTH, CONTROLLED MOTION WHEN WALKING, AND PERMIT UNRESTRICTED MOVEMENT FOR SITTING, BENDING AND KNEELING.

Prosthetic knees have evolved greatly over time, from the simple pendulum of the s to those regulated by rubber bands and springs or pneumatic or hydraulic components. Now, some knee units have advanced motion control modulated through microprocessors.

For the transfemoral (above-knee, including hip and knee disarticulation) amputee, successful function depends on selecting the correct knee to fit the person&#;s age, health, activity level and lifestyle. The latest or advanced knee is not necessarily the best choice for everyone. For some amputees, safety and stability are more important than functional performance. Active amputees, on the other hand, prefer a knee that will give them a higher level of function even if it requires greater control.

Given the wide variety of choices and consumer needs, prosthetists and rehabilitation specialists can help amputees choose the best prosthetic knees for their individual requirements. They can also teach amputees how to use their new knees properly, which is critical for avoiding discomfort, stumbling and falling.

A key way to evaluate an individual&#;s prosthetic needs is to observe his or her walking cycle, which can be divided into two parts: the &#;stance phase&#; (when the leg is on the ground supporting the body) and the &#;swing phase&#; (when the leg is off the ground, also referred to as &#;extension&#;). The happy medium between these two extremes (stance, or stability, versus ease of swing, or flexion) is different for each individual.

Although over 100 individual knee mechanisms are commercially available, they can be divided into two major classifications: mechanical and computerized. Mechanical knees can be further separated into two groups: single-axis knees and polycentric, or multiaxis, knees. All knee units, regardless of their level of complexity, require additional mechanisms for stability (manual or weight-activated locking systems) and additional mechanisms for control of motion (constant or variable friction and &#;fluid&#; pneumatic or hydraulic control).

Single-Axis Vs. Polycentric Knees

The single-axis knee, essentially a simple hinge, is generally considered the &#;workhorse&#; of the basic knee classes due to its relative simplicity, which makes it the most economical, most durable, and lightest option available.

Single-axis knees do have limitations, however. By virtue of their simplicity, the 09/10/control; amputees must use their own muscle power to keep them stable when standing.

To compensate for this, the single-axis knee often incorporates a constant friction control and a manual lock. The friction keeps the leg from swinging forward too quickly as it swings through to the next step.

Polycentric knees, also referred to as &#;fourbar&#; knees, are more complex in design and have multiple axes of rotation. Their versatility is the primary reason for their popularity. They can be set up to be very stable during early stance phase, yet easy to bend to initiate the swing phase or to sit down. Another popular feature of the knee&#;s design is that the leg&#;s overall length shortens when a step is initiated, reducing the risk of stumbling.

Polycentric knees are suitable for a wide range of amputees. Various versions are ideal for amputees who can&#;t walk securely with other knees, have knee disarticulation or bilateral leg amputations, or have long residual limbs.

A standard polycentric knee has a simple mechanical swing control that provides an optimal single walking speed; however, many polycentric knees incorporate fluid (pneumatic or hydraulic) swing control to permit variable walking speeds. The most common limitation of the polycentric design is that the range of motion about the knee may be restricted to some degree, though usually not enough to pose a significant problem. Polycentric knees are also heavier and contain parts that may need to be serviced or replaced more often than those of other types of prosthetic knees.

STABILITY OPTIONS

Manual Vs. Weight-Activated Locking Systems

Some amputees need or desire the security of a knee joint that locks in extension to prevent buckling. One option is the manual locking knee, which incorporates an automatic lock that can be unlocked voluntarily. This is the most stable knee available. Walking is possible with the lock either engaged or disengaged, although the locked knee requires excessive energy to use and produces a stiff, awkward gait. The manual locking knee is appropriate for weak or unstable patients as well as more active individuals who frequently walk on unstable terrain.

Another option is the weight-activated stance-control knee. This knee is very stable and is often prescribed for a first prosthesis. When weight is placed on the prosthesis, the knee will not bend until the weight is displaced. This system functions as a constant-friction knee during leg swing but is held in extension by a braking mechanism as weight is applied during stance phase. This knee is a common choice for older or less active amputees.

MOTION CONTROL OPTIONS

Constant Friction Vs. Variable Friction

All knee systems require some degree of swing control to maintain a consistent gait. In many cases, this control is provided by mechanical friction at the axis of rotation and is adjusted to match the normal cadence of the opposite leg. Constant-friction knee units are simple, lightweight and dependable. Their main disadvantage is that the knee is adjusted for a single walking speed at any given time.

Variable friction provides increased resistance as the knee bends from full extension. This provides &#;cadence response,&#; allowing variable walking speeds; however, this system requires frequent adjustment and replacement of moving parts and is considered less advanced than fluid control knee systems.

Fluid Control Systems: Pneumatic Vs. Hydraulic

Advanced swing control for prosthetic knees uses fluid dynamics to provide variable resistance, enabling amputees to walk comfortably at different speeds.

These units consist of pistons inside cylinders containing air (pneumatic) or fluid (hydraulic). Pneumatic control compresses air as the knee is flexed, stores the energy, then returns the energy as the knee moves into extension. Gait control can be further enhanced with the addition of a spring coil. Pneumatic systems are generally considered to provide superior swing control to friction knees but to be less effective than hydraulic systems.

For active amputees, hydraulic systems provide the closest thing to normal knee function. Hydraulic systems use a liquid medium (usually silicone oil) instead of air to respond to a wide range of walking speeds. Although hydraulic knees provide a smoother gait, they are heavier, require more maintenance, and a higher initial cost.

Microprocessor Knees

Microprocessor knees are a relatively new development in prosthetic technology. Several such new knees are now available or in development.

Onboard sensors detect movement and timing and then adjust a fluid /air control cylinder accordingly. These microprocessor-controlled knees lower the amount of effort amputees must use to control their timing, resulting in a more natural gait.

In spite of all of the amazing inventions and constant tweaks and improvements, the perfect prosthetic knee has yet to be invented; otherwise, there wouldn&#;t be over 100 different designs on the market. As advanced as the technology seems today compared to the earliest designs of the s, one can only imagine the developments that will eventually result as researchers further explore the potential of mechanical, hydraulic, computerized and &#;bionic,&#; or neuroprosthetic, technology.

 

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