A method and a computer system for executing the method is provided for pre-operatively determining a degree of deformity of an anatomical structure joining a first (100) and second (150) limb, the method comprising the steps:a) providing data representing the pose of a first axis (A) of the first limb in a first reference system in which the first limb is fixedb) providing data representing the pose of a second axis (B) of the second limb in a second reference system in which the second limb is fixedc) providing force orientation data representing a relative pose between the first and second reference system with the first and second limb in extension, and with an external force being applied to eliminate or reduce the deformity of the anatomical structure andd) determining a relative angular orientation (α) between the first axis of the first limb and the second axis of the second limb based on the force orientation data, said relative angular orientation being referred to as force relative angular orientation.
