A surgical aid for joints which allows the positioning of mono-compartment or three-compartment prosthetic components so that the isometry of the capsular-ligament tissues of the knee is maintained during the entire range of motion. Such surgical aid comprises: a first element (10) intended to be positioned in contact with a first bone of a joint； a second element (20), intended to be positioned in contact with a second bone of the joint； the first and second element (10,20) being movable relatively to one another along at least a first direction (X)； a displacement sensor (30) predisposed for measuring a relative displacement between the first and second element (10,20)； an actuator/motor (50) predisposed for exerting a force that tends to move the first element (10) and second element (20) away from one another； a pressure sensor (40) which measures the force that is exerted between the first and second bone； a processing module (60) arranged to control the actuator/motor (50) so as to calculate the variation of the slope arising from the relationship between distance and force； the processing module (60) receives in input the signal emitted by the displacement sensor (30), the force sensor (10) and the pressure sensor (40).
