Prosthetics - Knee Flashcards
What two principal requirements do knee prosthetics have?
stability in stance phase to support weight
flexion in swing to allow clearance
How can a knee prosthetic acquire stability
free knee - achieved from knee axis position and residual muscles
locks - manual or semi-automatic
brakes - friction or hydraulic
polycentric knee
What are manual and semi-automatic knee locks?
manual - amputee an engage the lock for stability (over rough ground) or can walk with the knee unlocked in which case knee stability is achieved using residual muscles
semi-auto - the knee locks automatically on extension (e.g. when the patients stands up) but can be unlocked manually for sitting down
How do knee brakes function?
when weight is applied to the prosthesis the contact surfaces are pushed together and friction locks the knee
during swing phase, the spring keeps the surfaces apart, allowing flexion
How do hydraulic knee brakes work?
weight activated - when a patient applies weight to a prosthesis, a mechanical linkage closes a valve in the hydraulic cylinder circuit, stopping the flow of fluid and effectively locking the piston
Name a polycentric knee mechanism
4 bar linkage
How does a 4 bar linkage system work?
the instantaneous centre of rotation of the knee joint moves as the ankle of flexion changes. with the knee straight (heel strike) the knee centre is high in the thigh and posterior to the GRF, enhancing stability.
What requirements are there of the knee prosthesis during swing phase?
allow knee to flex for toe clearance
control heel rise immediately after toe off
allow forward acceleration of the shank to ensure full knee extension at next heel strike
control knee extension so that terminal impact is minimised
What two methods of swing phase control are available
friction control
hydraulic or pneumatic
What are the advantages of friction control in swing phase?
relatively lightweight
reliable
adjustable
What are the disadvantages of friction control in swing phase?
cadence-specific - only works at one walking speed
What are the advantages of hydraulic and pneumatic control in swing phase?
effective over a range of walking speeds
can be controlled by microprocessor
What are the disadvantages of hydraulic and pneumatic control in swing phase
can be heavy
may require maintenance
what is the function of a microprocessor in pneumatic and hydraulic control?
the electronics sense an increase in cadence and the motor closes the valve to stiffen the system
Describe the Ossur power knee
microprocessor control and motorised active knee extension
