From the way you describe the problem my first inclination is that this is a nonlinear problem. Also, you probably need to include friction or some adhesion between the 'tube' and the bone material to transfer forces.
There is little details available in order for us to help you. Would it be possible to have an image on the part that you are working on as well as a representation as to how the force is applied? Also is the bone included in the analysis?