Soliworks can be used to design the knee prosthesis in CAD, if you are using a regular shape. Otherwise I would recommend using programmes like Rhino that allow you to intersect irregular geometries (say, femoral condyles) with regular geometries (say a sphere) and achieve the correct shape you are looking for.
one way is take CT/MRI images and convert them to 3D model. Then perfect shape and couture will come . Softwares like invesalius, 3D slicer, Mimics can be used for this purpose.
If we use Cad software like Creo, inventer, Rhino for contour generation unless we have the mathematical equation govern that contour we will not get perfect shape. so my opinion is take CT/MRI images of the required part and convert them to 3D model