Hello Elisabeth I expect you can share the knee geometry. Please archive the workbench project and attach the .wbpz file to your reply. You can also insert some screen snapshots into your reply.
There are many techniques to manipulate the geometry in SpaceClaim. One example is you can copy all the faces of a solid and turn that into a surface model. You might simplify some of the faces of the bones to make manipulation easier. Another technique is to split the bone in half using a Plane. The Shell feature may work on that new face.