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.