Patella_tendon

Main Forum
1

why the patella tendon was modeled as a rigid linkage between the patella and tibia? is this assumption based on the literature?
Many thanks

2

The patella tendon is technically a ligament, which transfers the full force of the strongest muscle group in our body, the quadriceps, between the patella and the tibia. For this reason, it is quite thick. It is also short, meaning that it is quite stiff. Of course, nothing in this world is completely stiff, but we make the assumption of stiffness of bones and other elements to make the simulation faster and easier. It might be possible to model the elasticity of the patella ligament by using force-dependent kinematics (FDK) but I do not think the effect would justify the added computational cost.
You can probably look up values for the stiffness of the patella ligament in the anatomical literature. You can then compare it with the forces transferred through the ligament in your simulations and quickly estimate the extent of elastic deformation. This would allow you to assess whether the question deserves closer investigation.

3

many thanks for your answers.
In the Anybody, the definition of Patella tendon is below
AnyKinPLine Patella = {
// These are the nodes that the measure refers to. The connection is regarded
// as being stiff.
AnyRefNode &Origin_patella_tendon = …Seg.Patella.Origin_patella_tendon;
AnyRefNode &Insertion_patella_tendon = …Seg.Shank.Insertion_patella_tendon;

//AnyRefNode &Insertion_patella_tendon = ...Seg.Shank.Insertion_patella_tendon;

AnyDrawPLine Patella_tendon = 
{
  Thickness = 0.001;
  RGB = {1,0,0};
};

};
AnyKinEqSimpleDriver PatellaMovement = {
AnyKinPLine &Jnt = .KinematicMeasures.Patella;

// Scale patella tendon length based on the over all leg length
AnyVar lengthScale = (vnorm(..Seg.Thigh.HipJoint.sRel-..Seg.Thigh.KneeJoint.sRel) + vnorm(..Seg.Shank.KneeJoint.sRel - ..Seg.Shank.AnkleJoint.sRel))
            / (vnorm(..Seg.Thigh.StdPar.HipJoint-..Seg.Thigh.StdPar.KneeJoint) + vnorm(..Seg.Shank.StdPar.AnkleJoint-..Seg.Shank.StdPar.KneeJoint));


  

       
AnyVar TendonLength =  lengthScale *..ModelParameters.PatellarLigamentLength; 



DriverPos = {TendonLength};
DriverVel = {0.0};
Reaction.Type = {On};  // The muscles must do the work

};

does it mean the distance between the patella and tibia was a constant during the simulation? By this way, the computational cost was reduced without considering the physiological character of elongation and strain for the patella tendon.

4

Hi,

Yes, the distance between a point on patella and a point on tibia remains the same. But patella rotates wrt. the femoral condyles.

And yes, this is what John said - the computational effort was reduced by this simplification. If this simplification is unacceptable for your application - you may change this behaviour.

Kind regards,
Pavel