Function, Types & mechanism of prosthetic knee joint

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

Classification
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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