Function, Types & mechanism of prosthetic knee joint

Function of knee joint
Main function:
Support during stance.
Shorten the leg during swing to allow clearance.
Allowing sitting.
Adaptation to variable cadence.
Impact absorption during weight acceptance.
Stop the extension during swing.

On the basis of biomechanical performance prosthetic knees are grouped into :

  1. Constant friction
  2. Stance Control
  3. Polycentric knees
  4. Manual Locking
  5. Hydraulic & pneumatic
  6. Hybrids

Constant Friction Joints (“single axis” knees)

  • Simple axis connecting the thigh and shank segments.
  • needs strong hip extensors to prevent the knee from buckling.
  • ground reaction kept in front of the knee from initial contact through midstance to maintain extended knee.
  • ground reaction behind the knee at terminal stance to allow knee flexion.
  • relatively inexpensive, simple to manufacture.
  • friction prevents shank from swinging forward too fast.
  • best for level ground walking at constant speed.

Stance Control (weight-activated)

  • force of the body weight engages a brake that keeps the knee from buckling (friction lock)
  • spring loaded brake bushing binds when loaded during stance but released during swing
  • amount of friction is adjustable
  • cannot support full body weight in extreme flexion
  • device must also be fully unloaded before sitting down
  • very popular for elderly patient with poor hip control
  • bilateral amputee virtually impossible

Polycentric Knees

  • 4 bar linkage, instantaneous center of rotation.
  • posterior (and proximal) when extended (for stability at heel strike)
  • anterior (and more distal) as soon as knee starts to flex (to facilitate unlocking)
  • Posterior (and distal) as knee flexes more (to shorten leg and improve clearance)
  • excellent stance phase stability, especially at heel strike
  • allows load bearing during flexion
  • also used for knee disarticulation (because of lack of space)

Manual Locking prostheses

  • ultimate stability but seldom required
  • produces uncosmetic and energy-consuming gait pattern
  • useful for the manual labourer who needs stability
  • remote release cable requires a free hand to release it prior to sitting

Hydraulic & pneumatic Knees

  • fluid (silicone oil) or gas filled piston allows amputee to vary speed & cadence voluntarily
  • swing and stance control types
  • piston is attached to a pivot in the thigh section of the prosthesis behind the knee bolt
  • cylinder is attached to pivot in shank
  • amount of resistance required provided automatically for a given walking speed
  • mimics loading response stance flexion for impact absorptiondoes not interfere with normal flexion and extensionfluid filled devices stronger
  • produce the most normal looking gait
  • relatively heavy and expensive
  • most popular with vary amputees

Hybrid Devices

  • combine some of the properties of the above groups
  • e.g. titanium polycentric device with mini hydraulic unit for swing phase control (Otto Bock)
  • “bouncy” knees which control knee flexion during stance (Blatchford)
  • “intelligent ” knees with microprocessors
  • Allow going down the stairs and climbing down a slope.


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