First, you need a Geometrical model of femur and tibia, for that you can process 3D CT scan data of your subject, then you need to define material model for bones, Soft tissue, ligaments (see database of Biodigital); you also need ligament attachments.

Once you are done with all that you need to define Boundary conditions and loadings on the system. Contact modeling is also a big issue.

I suggest you read work of Cheung et al, Tao et al. and Johnson et al

• As Zeeshan explained, you can find femor and tibia 3d model by CT scan, but you should simplify them to the basic shapes as like as cylinders and spheres and the other basic shapes, using this method the model become simpler but it's not high accurate. Whenever you need to get more accurate, you can use more basic shapes to show the final results.