@Rouben Rostamian Thank you so much for helping me.
That's amazing how much effort you guys put on my issue.
Still, I do not understand why that quadric equation looks like a cyclinder when it is not. I got it by using the LinearFit of an equation [1,x,z,x^2,y^2] on a matrix of points taken from an stl file. This file contained the coordinates of the surface points of a femur.
Femoral condyles are like quadric parabolids. They are intersecting the tibia plateau (the plane) creating an ellipse. The centre of this ellipse is where the contact forces are acting. But! When I use the equation given by the LinearFit in x and z, I cannot find the right point.
In fact, if I use your mw.mw file taking the coords xc yc and zc it is not where I would expect it to be.
I'm bit confused at the moment. Is that only a problem of visualization or it means that my parabolid is not a parabolid at all? Why is that a cyclinder directed with y? It shouldn't be!
Thank you again guys for your kind support.