Prosthetic Knees

Original Editor - Tarina van der Stockt Top Contributors - Tarina van der Stockt, Kim Jackson, Jess Bell, Stacy Schiurring, Tony Lowe and Simisola Ajeyalemi  

Introduction

Prosthetic knees fall into two broad categories: mechanical and computerised. This page introduces the concepts underlying each to support clinical reasoning when assessing a patient or reviewing their prosthesis. There are many prosthetic knees on the market, and this page covers the underlying concepts rather than specific brands or models.

Every prosthetic knee has to solve two problems. The first is stability: preventing the knee from buckling under load during stance. This is achieved either mechanically, through a manual lock or a weight-activated stance-control (braking) mechanism, or through computerised control. The second problem to solve is controlling speed and range of flexion and extension during swing. This is achieved through friction or through hydraulic/pneumatic control.

Mechanical knees are further divided by their axis design into single-axis and polycentric (multiaxis) types. The sections below describe the technologies used to address stability and swing control in turn. In practice, most knees on the market combine several of these features rather than representing a single "pure" type.

When assessing a patient for the first time, or when they are being fitted with a new prosthesis, it is worth discussing the specific knee model with the prosthetist and searching online for manufacturer information and demonstration videos. Each knee's particular features will influence the person's gait and shape the focus of their rehabilitation. This is particularly relevant to activities such as sitting down and standing up, negotiating ramps and stairs, walking on uneven terrain, and walking at different speeds.

Mechanical Knees

Single-Axis Knees

Mechanism: This is a simple hinge-type knee. During flexion/extension, these articulations execute a simple rotation around the knee axis. They have a simple design, and their easy alignment responds to the rules of mechanics. Single-axis knee units are most often built as part of a modern endoskeletal (modular) prosthesis, though older exoskeletal (rigid-shell) designs also exist and remain relevant in some lower-resource settings. Either construction can incorporate manual or automatic locking of the knee, for use in individuals with poor muscle power. Knees without a locking mechanism can be used for regular prosthetic fitting of individuals with adequate muscle control and/or in situations of limited economic resources.

Advantages: Single-axis knees are very simple, durable, light, and economical.

Disadvantages: Due to their simplicity, people have to use their own muscle power to keep the knee stable during 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 prevents the leg from swinging through very quickly.

Polycentric Knees

Mechanism: A polycentric knee has multiple axes of rotation rather than a single hinge point. Most designs use a four-bar linkage, meaning the knee unit has four fixed pivot points, or axes of rotation, though designs with more bars also exist. These four axes do not move, but as the linkage articulates through flexion and extension, a separate, virtual point called the instant centre of rotation (ICR) shifts continuously, tracing the knee's effective centre of rotation at any given moment. In extension, the ICR sits well above and posterior to the knee's mechanical axis. It can be located by extending the centre lines of the anterior and posterior linkage bars proximally; the point where they intersect is the ICR. This ICR position is what gives the knee its stability. At heel strike, the ground reaction force line falls in front of (i.e. anterior to) the ICR rather than behind it. This creates a passive extension moment, which tends to straighten the knee rather than let it collapse. This is generated by the geometry of the linkage itself rather than by any muscular effort from the individual. This passive extension moment is the main source of a polycentric knee's intrinsic stability during early stance. A standard polycentric knee provides an optimal single walking speed. Adding pneumatic or hydraulic features allows the individual to vary their walking speed.

Advantages: Polycentric knees are versatile. They can be adjusted to be extremely stable during stance phase, but still facilitate an easy swing and allow a person to sit down with a bent knee. Due to the multiple axes and the ICR, the prosthetic length "shortens" at the start of toe-off and allows for foot clearance. Polycentric knees are suitable for people who have the potential to be independent with a prosthesis in their home and community, as well as for more active people.

Disadvantages: Polycentric knees are heavier than single-axis knees and have 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 non-microprocessor knee needs to actively generate a knee extension moment during stance to prevent the knee from buckling, which would cause a fall. Hybrid designs that combine a polycentric linkage with hydraulic or pneumatic stance-flexion control do exist and continue to be developed, allowing controlled yield under load without a locking mechanism.

Manual Locking Knee

Mechanism: This style of knee locks automatically once it reaches full extension and remains locked throughout stance and swing, providing a very stable but stiff-legged gait. The individual unlocks it manually, usually by pulling a lever or cord, in order to bend the knee for sitting. This is an important feature for people who need extra security to keep the knee from buckling in stance or with heel contact or when walking on uneven terrain. Manual locking knees are usually indicated for K1 ambulators (i.e. people with the potential to use a prosthesis for transfers or for walking on level surfaces at a fixed pace, typically limited to the home) or people who cannot voluntarily control their prosthetic knee.

Disadvantages: People using a manual locking knee need to circumduct or hip hitch to allow for foot clearance when the knee is locked during gait.

Weight-Activated Stance Control Knee

Weight-activated stance control knees are also called "safety knees".

Mechanism: These knees have a built-in braking mechanism that is activated by the person's body weight. As the person loads the prosthesis, a friction brake engages and resists knee flexion, preventing the knee from buckling. When the weight is taken off, a spring releases the brake and the knee swings freely. The brake must be adjusted carefully so that it releases at the right point in the gait cycle: the knee only frees up once part of the person's weight has transferred to the other leg, and if the setting is wrong, there is still a risk of buckling or stumbling.

Advantages: Weight-activated stance control knees are very stable. They are often prescribed for first-time prosthetic users who need that stability, particularly in older or less active people who can still exert some control over the knee, and those who fatigue quickly after just a few steps. They are especially valuable for someone who forgets that they should not put their weight on a partially bent knee. If this happens, the brake engages and prevents the knee from collapsing into flexion.

Disadvantages: The person has to take the weight off the leg to allow the knee to bend. This has two consequences. They cannot use the prosthetic side to lower themselves when sitting down, and normal knee flexion at toe-off does not occur. Due to the friction in the knee, people with this type of knee walk more slowly and take smaller steps.

Pneumatic or Hydraulic Knees

Mechanism: A pneumatic or hydraulic swing-control unit (i.e. a piston moving within a cylinder of air or fluid) can be added to a single-axis or polycentric knee. These units principally control the swing phase of gait, regulating the speed of knee flexion and extension between toe-off and heel strike. Resistance varies automatically with the speed of movement: the faster the knee flexes, the harder the air or fluid is forced through a restricted opening in the cylinder, and the greater the resistance. This restricts how far the knee flexes at faster cadences and allows freer flexion at slower ones, so a single unit works across a range of walking speeds. Resistance also varies with knee angle, with less resistance early in flexion and more towards the end of the range to limit heel rise and cushion the impact at full extension. Pneumatic units use compressed air, which compresses under load and acts as a spring. Hydraulic fluid does not compress, so hydraulic units generate larger damping forces. They are also heavier than pneumatic units.

Advantages: These components allow people to walk more comfortably across a range of walking speeds with a more comfortable gait, so they suit active walkers who vary their pace. Units that provide resistance to flexion while the leg is loaded also allow the person to descend step over step down stairs and slopes.

Disadvantages: Hydraulic knees are heavier, need more maintenance, and have a higher initial cost when compared to pneumatics. Piston seals wear in both types, so regular servicing is required. These knees cost more than mechanical knees, but are cheaper than computerised knees.

Computerised Knees

Mechanism: Computerised 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 person requires at that moment in time. It is essentially 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, while 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 is intended to provide active flexion and extension during gait, producing more symmetrical weight distribution and a more natural gait pattern.

Advantages: Microprocessor knees lower the effort required for walking and produce a more natural gait. The knee is able to quickly adapt to different walking speeds, terrain and situations, giving better control on uneven surfaces, ramps and stairs (allows for a step-over-step gain pattern). Many units offer stumble recovery, more than one operating mode, and settings can be adjusted using a mobile device or computer. Once learned, walking takes less cognitive effort. The microprocessor knee also supports weight bearing during sit-to-stand.

Evidence for microprocessor knees: Systematic reviews and cohort studies published since 2021 report that microprocessor knees (MPKs) improve balance confidence, functional mobility and patient satisfaction compared with non-microprocessor knees, across community and limited community ambulators (i.e. Medicare Functional Classification Levels K2–K4). The 2024 US VA/DoD Clinical Practice Guideline for Rehabilitation of Individuals with Lower Limb Amputation (published January 2025) suggests prescribing microprocessor knee units over non-microprocessor units for prosthetic ambulators to reduce falls, optimise functional mobility, and improve satisfaction. Most of this evidence relates to passive (hydraulic) MPKs rather than motor-powered knees; direct comparative evidence between passive MPKs and powered knees is still limited and largely confined to small biomechanical studies.

Disadvantages: These are the most expensive and heaviest knee units, and they need frequent charging. They weigh more than other knees, full-length cosmetic covers may not fit well, and foot options can be limited. Kneeling can damage the unit, as can extremes of heat and cold. There is a steep initial learning curve, and established prosthetic users need to commit to gait re-education. This knee unit also requires regular servicing.

Additional Resources

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.