An image processing system (IPS) includes an input port (IN) for receiving a surface image of an outer layer (OL) of a current biomechanical assembly (TOR). The surface image is acquired by a surface imaging apparatus (DSC) along at least one imaging direction. The assembly includes relative to the imaging direction behind the outer layer, an inner wall (RC) element coupled from behind to the outer layer. The image processing system further includes a wall estimator (WE) configured to provide an estimate for geometrical data of the inner wall element (RC), the estimate being based on image information as per the surface image.