In this talk, we will describe a new interactive procedure for segmenting 3D data sets using a mixture of ideas from control and image processing. More precisely, using a Lyapunov control design, a balance is established between the influence of a data-driven gradient flow and the human’s input over time. Automatic segmentation is thus smoothly coupled with interactivity. An application of the mathematical methods to orthopedic segmentation is shown, demonstrating the expected transient and steady state behavior of the implicit segmentation function and auxiliary observer.