Control Mechanism of
Prosthetic Knee Joints
BY – ROHAN GUPTA
Rohan Gupta, MPO 1st Year, 3rd Batch
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What are Prosthetic Knees?
• Prosthetic knees are designed to mimic the bending (flexion) and swinging
(extension) of the anatomical knee joint as a patient walks. From advanced
computer controlled components to simple locking joints, the prosthetic
knee works together with the prosthetic foot and socket to achieve a smooth gait
pattern.
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Introduction
• 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.
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What does Mechanism means?
• “A system of parts working together in a machine or an object”
• In our field, the Prosthetic Knee joint is a Machine or an object which is working
with the help of different parts working together inside the prosthetic knee joint.
• “A mechanism is a device that transforms input forces and movement into a
desired set of output forces and movement. Mechanisms generally consist of
moving components.”
• In case of Prosthetic Knee, the movement of different parts inside the joint
results in or mimics the natural knee movements like flexion and extension.
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1. Single-Axis Knees – Control 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.
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Advantages
• Very simplistic,
• Durable,
• Very light,
• Economical.
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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.
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Additional Component Benefit
• 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.
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Schematic
diagram of
Single Axis Knee
Joint.

2. Polycentric Knees – Control 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.
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• 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.
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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
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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.
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3. Manual Locking Knees – Control Mechanism
• The manual locking knee is the most stable knee used in prosthetics.
• The manual locking knee is locked stiff at the knee when in use.
• The knee is locked during gait and the patient releases the lock mechanism in
order to sit down.
• Manual locking knees are primarily used with patients who have very short
residual limbs and/or poor hip strength and are unable to control the knee.
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• The knee will not bend until a release mechanism is operated to free the knee
lock (e.g. when sitting). This system makes the knee extremely safe.
• These types of knee are best suited to users with weak musculature or balance
issues. Occasionally they are used locked in the early stages of rehabilitation
with a view to unlocking them as the user progresses through therapy and
becomes more confident and able.
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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.
• Can be made very lightweight.
• Usually used by amputees who are weak, unstable or unwell.
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Disadvantages
• The patient will need circumduct or hip hitch to allow for foot clearance when
the knee is locked during gait.
• May be difficult to unlock when weight is being taken through the knee or if the
user has insufficient hand control or strength.
• Stiff legged gait requires higher energy input.
• Asymmetrical gait.
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Schematic
diagram of
Manual Locking
Knee Joint.

4. Mechanical Friction Control Knees – Control
Mechanism
• Mechanical friction knees are lightweight and relatively inexpensive due to their
simplicity.
• The friction (resistance to bending) in the knee joint is typically adjusted by
tightening a screw or bolt. The same amount of friction is applied to the knee
regardless of whether it is flexing or extending.
• A spring is often used to overcome some of the friction when the knee extends
(straighten). This speeds up the knee in the swing phase of gait.
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Advantages
• Simplicity
• Durable & robust
• Can be made lighter and smaller.
• May be used over one axis of rotation (monocentric knees) or multiple axes
(polycentric knees).
• Inexpensive
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Disadvantages
• Friction reduces with wear & may need frequent adjustment.
• Constant friction design does not adapt to different walking speeds.
• May become noisy when wearing.
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5. Constant Friction Control Knees – Control
Mechanism
• This system uses constant pressure against a rotating surface to resist knee
flexion.
• The amount of pressure is set by the Prosthetist and should not be altered by the
user.
• External spring(s) may be used to assist the knee to straighten faster than it
bends.
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Advanatages
• Simple
• Lightweight
• Dependable
• Can be found in both monocentric (single axis) and polycentric (multi axial)
knees.
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Disadvantages
• Can only be adjusted to one walking speed. Other speeds will have
compromised gait.
• Requires accurate adjustment by trained practitioner.
• Will require periodic adjustment to remain effective
• Knee becomes less stable as the knee wears (monocentric design).
• Becomes less efficient the more the knee bends.
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6. Variable Friction Control Knees – Control
Mechanism
• Variable friction utilizes a fluid control system (hydraulic or pneumatic) to
control the amount resistance to knee bend.
• As knee bend increases, resistance increases and vice versa.
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Advantages
• Progressive resistance as knee bends.
• Can be found in both monocentric (single axis) and polycentric (multi axial)
knees.
• More natural gait.
• Can be adjusted to allow for a range of cadences.
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Disadvantages
• Can be expensive.
• Heavier and more complex than other systems.
• Requires accurate adjustment by trained practitioner.
• May require periodic adjustment.
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7. Weight Activated Stance Control Knees –
Control Mechanism
• These knees are also called "safety knees“
• 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.
• Knees with a weight activated stance control feature a built-in braking or locking
mechanism which is activated by the user’s bodyweight.
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• When the mechanism is engaged it either exerts pressure on the knee
components to restrict flexion (monocentric design) or alters the geometry to
make the knee less prone to flexion (polycentric design).
• While weight is directed through the knee and heel the brake is active. De-
weighting the knee releases the brake mechanism.
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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.
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• Can be found in monocentric (single axis) style knees using either friction,
hydraulic or pneumatic systems.
• Featured in all polycentric knees with more than four centers of rotation.
• Enhanced stability at heel strike.
• Often an excellent choice for a primary amputee’s first prosthesis or for less
active amputees.
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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.
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• Requires accurate adjustment by a knowledgeable practitioner to ensure the lock
is effective.
• May require frequent adjustment (a poorly adjusted lock can be ineffective).
• User’s weight must be lifted off the prosthesis to unlock the knee (e.g. when
sitting).
• Steeper learning curve.
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8. Pneumatic (Air) Control Knees – Control
Mechanism
• Prosthetic knee joints can be classified by the way that the flexion and extension
(bending and straightening) of the knee joint is regulated.
• This is known as the control mechanism.
• Pistons move through the control medium as the knee bends and extends.
• As the pistons move, control valves provide varying degrees of resistance
depending on the angle of the knee.
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• These systems allow a different stiffness during different phases of gait.
• The result allows the user to walk more comfortably at different speeds.
• Pneumatic (air) controlled knees have inner chambers which house a sliding
piston.
• The piston seals against the side of the chamber much like a bicycle pump.
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• As the piston moves, air is compressed by it, regulating the pressure against the
bending and straightening moments of the knee.
• The pressure differential allows the user to walk more comfortably at different
speeds.
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Advantages
• May be used over one axis of rotation (monocentric knees) or multiple axes
(polycentric knees).
• Efficient use of air compression lowers energy use.
• Provides better swing control than constant friction systems.
• Lighter than comparable hydraulic units.
• Generally lighter and less expensive that hydraulic knees.
• Allows variable walking speeds.
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• 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.
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Disadvantages
• Piston seals are prone to wear.
• Less effective than hydraulic systems.
• Can produce heat when actively worked for long periods.
• Slightly heavier and more expensive than mechanical friction knees.
• Complex.
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9. Hydraulic (Fluid) Control Knees – Control
Mechanism
• Prosthetic knee joints can be classified by the way that the flexion and extension
(bending and straightening) of the knee joint is regulated.
• Hydraulic (fluid) controlled knees have fluid filled inner chambers which house
a sliding piston.
• The piston seals against the side of the chamber much like a bicycle pump.
• As the piston moves, liquid (usually silicone oil) is transferred from one chamber
to another
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• The flow of fluid is regulated by control valves which can be adjusted to
moderate the bending and straightening resistance of the knee.
• The movement of fluid allows the user to walk more comfortably at different
speeds.
• As the pistons move, control valves provide varying degrees of resistance
depending on the angle of the knee.
• These systems allow a different stiffness during different phases of gait.
• The result allows the user to walk more comfortably at different speeds.
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Advantages
• May be used over one axis of rotation (monocentric knees) or multiple axes
(polycentric knees).
• Accurately mimics anatomical knee function.
• Provides better swing control and stability than constant friction or pneumatic
systems.
• Generally lighter and less expensive that microprocessor knees.
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• Allows more variation walking speeds than pneumatic units due to the fine
control afforded by the valves.
• Weight can be kept on the prosthetic leg when sitting down and the knee will
assist the individual to sit down.
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Disadvantages
• Piston seals are prone to wear.
• Can produce heat when actively worked for long periods.
• Heavier and more expensive than mechanical friction knees.
• Require more accuracy when being adjusted.
• More expensive than pneumatic or mechanical friction systems.
• Complex.
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10. Computerised or Microprocessor Control
Knees – Control 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.
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• 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.
• Computerised knees feature an on-board micro-processor which receives
feedback from sensors inside the joint and foot.
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• The micro-processor uses the received data to constantly adjust the knee in real time
to match the user’s walking characteristics.
• In this respect they are type of enhanced hydraulic system where the computer
operates the control valves in the knee to restrict or allow the flow of hydraulic fluid.
• Micro-processor knees can adjust to different walking speeds, terrain and situations
and may also exhibit ‘stumble recovery’ features.
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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.
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• 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
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• Can have different walking/ activity modes.
• Once learned, it takes less cognitive effort.
• Superior control on uneven surfaces& ramps.
• The ability to descend stairs step over step.
• Supports weight bearing in stand to sit transition.
• Powers down safely and gracefully.
• Ability to adjust to different cadences.
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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.
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• Initial steep learning curve and commitment to gait re-education.
• Regular servicing.
• Does not pair well with full length cosmetic covers.
• Sensitive to environmental conditions (dust, vibration, salt or fresh water,
chemicals, excess heat or cold, strong magnetic fields).
• For established amputees: requires significant commitment to gait re-education.
• May require trained & certified practitioner to make adjustments.
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THANK YOU
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