Px knees provide 3 function
- Support during stance phase
- Smooth and controlled swing phase
- Unrestricted flexion for sitting, kneeling, stooping and related activities
Knee design classifications
Outside hinge knee (most voluntary control) Single axis Stance control Polycentric Manual locking (least voluntary control)
Polycentric hydraulic knee
Stance flexion
Swing phase hydraulic control
Geometric stance stability
Allows for variable cadence
Outside hinge knee and single axis knee
Simple hinge mechanism
Mechanically simple=popular and frequently use
Stance stability= alignment stability (involuntary control) amputee muscle contractions (voluntary control)
Disadvantage- lacks mechanical stability
Weight-activated stance control knee
When weight is applied, breaking mechanism mechanically prevents the knee from flexing or buckling
- ‘brake’ engagement and prevent flexion can be adjusted depending on amputee’s weight, activity level, and stance control needs
- braking mechanism is usually only effective to maximum range of 15-20 degrees flexion
Weight-activated stance control
Used for weak or debilitated amputees who cannot rely on more complicated and demanding means of stance control
Disadvantage- increased maintenance, delayed initiation of swing phase if the stance control “brake” is set for a high degreee of stance stability
Polycentric axis
4/5 bar linkage that provides more than one point of rotation
Changing instantaneous center of rotation b/w px thigh and shank
- advantage of varying mechanical stability throughout the gait cycle with enhanced stability during heel strike and decreased stability at toe off, allowing for easier initiation of swing
- inherent shortening of the shank during flexion, which improves foot clearance in swing phase and allowing enhanced sitting cosmetics for very long RL
THREE categories of amputees using polycentric knee
- KD- high instant center of rotation is advantageous knee will swing under the thigh when the amputees sits, allowing the appearance of equal thigh and shank lengths compared to the sound limb
- Short AK (FEMUR < 50%) will benefit from this unit because thy also can take advantage of high instantaneous center of rotation and increased zone of stability provided by the polycentric unit
- Individuals with weak hip extensors
Disadvantage of polycentric
Increased weight and build due to numerous linkage mechanism and moving parts
Manual locking knee
Unit automatically locks in extension but can be unlocked by voluntary action.
Stable in stance =lack of knee flexion =increased energy expenditure and gait deviations
Indicated for- weak, unstable, debilitated amputees or unstable situations- uneven terrain
