Knee Mechanisms

1. Weight Activated Stance Controlled Knee

Principle - when load ( i.e. body wt. ) is applied, a braking mechanism mechanically prevents knee from buckling.

• Brake mechanism is usually only effective to a maximum range of 15-20° flexion.
• Can be utilised both in endoskeleton & exoskeleton
• During swing phase this knee unit act as single axis knee. 
• If initial contact is made when the knee is not completely extended, as in uneven terrain, the braking mechanism provides additional mechanical stability to keep knee from rapid buckling.
• Constant friction.

Indications - 

1. Individuals who have recently undergone amputation.

2. Short stump amputees.

3. Weaker hip extensor amputees.

Disadvantage - 

1. Increased maintenance.

2. Delayed initiation of swing phase if stance control is set for a high degree of stance stability.

2. Manual Locking Knee 

• This knee unit automatically locks in extension but can be unlocked voluntarily ( there's a pull switch ).
• Due to locking , this mechanism is the most stable and requires least voluntary control.
• Both in endo and exo.
• It is basically a single axis knee with addition of a locking pin mechanism
• The prosthesis is often fit to be slightly shorter than the sound side limb to facilitate toe clearance during swing of prosthesis.
• Also called Knee Joint of Last Resort.

Indications -

1. Weak and unstable amputees.

2. Amputees in unstable condition such as uneven terrain when hiking or hunting or activities like fishing while standing in a boat.

3. Friction Based Knee Units

• Knee swing is controlled by some form of mechanical friction usually applied at axis of rotation & this is most commonly used system for controlling swing phase.
• The friction is adjusted to the patient's normal cadence so that the pendulum action of shank will correspond to that of opposite limb.

Disadvantage - 

Friction can be set for only one cadence & any variation in cadence by amputee results in prosthetic knee and shank that will not flex & extend with the same timing as natural leg.

Type - 

(i) Fluid Based Friction

a) Pneumatic

b) Hydraulic

(ii) Sliding Friction

a) Constant Friction

b) Variable Friction

Pneumatic Control Knee Units

• Pneumatic control of swing of prosthetic shank is provided by pneumatic cylinder attached to knee & placed within the upper shank.
• This mechanism consist of a piston rod that is attached to thigh section of prosthesis behind the knee bolt.
• Pneumatic control is more responsible to variable walking speeds & is more advanced form of swing control.
• Because air is more compressible it acts as an extension aid within pneumatic unit. E.g. pushing the piston of syringe when the tip is closed, air is compressed. And after releasing the force piston will come back.
• Pneumatic knee unit has varying cadence capability.

How does it works ?

When knee is flexed > piston move down into cylinder > piston forces the air to move upward and sides of cylinder through a port at the top of cylinder and back into central cylinder.

Resistance to knee swing velocity can be adjusted by adjusting the opening size of port by using an adjustment knob.

The knob can be turned clockwise or anticlockwise to increase or decrease this port opening.

Decreasing the opening provides greater resistance of air & therefore greater swing control. 

Setting this opening too small would make swing too stiff & prevent required knee flexion.

Disadvantages - 

1. Increased maintenance.

2. Increased weight and expense. But these are simpler , lighter and less expensive than hydraulic control knee units.

Hydraulic Control Knee Units

• It also utilizes cylinder and piston rod but the medium is liquid.
• Liquid provides resistance to motion depending upon it's viscosiy & temperature. ( Silicone oil is used mostly ) .
• Hydraulic control achieves nearly normal knee action over a wide cadence range. These are cadence responsive.
• Flow of hydraulic fluid through narrow channels provides functional resistance which increases with speed of compression. Thus varying control is caused by flow of hydraulic fluid.

Cadence Resposive - forward progression of prosthetic shin changes as gait speed changes.

Indications - 

Teenagers

Active females and adult males.

Disadvantages - 

Increased weight

Increased maintenance

Increased expense.

Types - 

(i) Hydra Cadence Hydraulic System

It is entire knee , shank , ankle & foot system that is hydraulically linked at knee and ankle.

It allows free Plantarflexion at heel strike & provides Dorsiflexion after 20° of knee flexion in swing for improved ground clearance.

Heel height is adjustable through hydraulic mechanism of ankle and knee.

It is quite heavy & expensive & less durable & reliable during recent years.

(ii) Mauch Swing-N-Stance ( S-N-S ) 

The most advanced system of hydraulic control & the only system that includes hydraulic stance phase control.