Abstract: A method for determining the position of a point F1 of a femur with respect to a point T1 of a tibia such that the distance between F1 and T1 is substantially invariant. The method uses a three-dimensional locating system including triplets of emitting elements especially including a pointer that can be located by the system, and includes the steps of linking a first triplet to the first organ; pointing with the pointer at the position of point T1 and locating this position with respect to the position of the first triplet; pointing with the pointer at the positions of an assembly of second points that are positioned within an area of the femur where the invariant point is likely to be found; calculating the distances separating point from each of points Fi; moving the tibia with respect to the femur add calculating the variations in the distances; and selecting point F1 for which the distance is invariant.