A surgical instrument (1) for knee balancing, comprising:a static tibial element (2) including a base and a tibial paddle (21)；a femoral element (3) including a femoral paddle (31) and a femoral arm；a movable element (4) including a guide (40) movable in translation on the base according to a longitudinal axis (AL), wherein the femoral element is pivotally mounted on said movable element according to an anterior-posterior axis (AP), and a transverse arm；a system (5) for setting the mobility of said guide in order to provide a distraction function；a reversible blocking system coupled between the transverse arm and the femoral arm to pivotally immobilize the femoral element with respect to the movable element in a reversible manner in order to set the pivot angle of the femoral paddle with respect to the tibial paddle at different angle values.
