This disclosure relates to methods and systems to assist in surgery of a joint. The system determines a mechanical property of one or more ligaments associated with the joint based on measurement data indicative of a movement of the bones relative to each other under multiple mechanical loads. The system then determines a predicted characteristic of the joint after the surgery based on a spatial parameter of the surgery and based on the mechanical property of the one or more ligaments and generates an output signal indicative of the predicted characteristic to assist the surgery. Since the predicted characteristic of the joint is determined based on the mechanical property of the ligaments, the prediction is more accurate than other methods that rely on only bone geometries to predict a surgery outcome.
