A two-step parameter-free approach for non-rigid medical image registration is presented. Displacements of boundary structures are computed in the rst step and then incorporated as hard constraints for elastic image deformation in the second step. In comparison to traditional non-parametric methods, no driving forces have to be computed from image data. The approach guarantees the exact correspondence of certain structures in the images and does not depend on parameters of the deformation model such as elastic constants. Numerical examples with synthetic and real images are presented.