Computer-based simulations support surgeons in preoperative planning of osteotomy and assessing the improvement of the forearm motion. To this end, an in-silico model of patient-specific forearm kinematics is required. In this paper we introduce a motion model of the forearm which is based on a patient's joint morphology, the form and shape of the joints. The morphology of the articulations is represented by 3-dimensional splines. In this way the gliding motion of the articulations is expressed analytically in a closed-form. Our algorithm was designed to work with available clinical planning data and requires minimal user interaction. This allows an integration in computer-aided planning systems that are operated by surgeons. The accuracy of the simulation results is verified via cadaver experiments.