Polycentric Knee

Polycentric knee has moving center of rotation , i.e. it rotates around more than one through a four or more bar linkage system.

Polycentric knee design allows combination of rotational and translational motion.

Polycentric knee has ICOR. The lines extending from anterior and posterior links will intersect at a point , that point will be the ICOR .

A proximal and posterior location of ICOR promotes knee stability during stance.

Stance phase stability also make it an option for individuals who have short residual limb or weakness of hip extensors.

As the knee unit flexes during swing phase , the polycentric axis of motion leads to relative shortening of distal prosthesis ( shin and foot components ) which enhances toe clearance throughout swing phase.

The locus of rotation of the four-bar knee varies as the flexion angle changes since the links intersect in different locations throughout the range of motion.

The the locus is parabolic.

It is especially helpful for individuals with long residual stump or knee disarticulation because changing center of rotation allows the shin to tuck under thigh when sitting , resulting in a more natural & cosmetic appearance of thigh and shin lengths. 

Disadvantage - Due to multiple mechanical joints it's durability is low . Hence not given in case of child amputee.

4 Bar Linkage

Characterised by four elements thigh, shin and two links.

The intersection of anterior and posterior knee links determines the Instantaneous Center of Rotation ( ICOR )

Offset - Position of ICOR w.r.t. TKA line.

              Offset can be anterior , posterior  

               or zero offset when ICOR lie on 

               the line.

Stance Phase Stability 

(i) α stability

α = distance from ICOR to TKA line 

α is +ve when ICOR is posterior to TKA line .

α is -ve when ICOR is anterior to TKA line 

• Positive alpha is the stable condition.

In single axis knee  α = 0 , at full extension. As knee begins to flex alpha become  negative and progressively   unstable as flexion continues. In polycentric knee alpha is positive. As the  knee begins to flex the value become  smaller but it will remain positive. (ii)   β stability    β = perpendicular distance between ICOR  & Knee Center. The ICOR is superior to mechanical knee  level. So the patient gains mechanical  advantage over single axis knee.