Instrument for locating the cutting plane on the head of a long bone in a surgicalprocedure in which the head of the bone is replaced with a joint prosthesis component,which includes an axial reference shaft which can be arranged parallel to theaxis of the bone, a cutting guide which can be moved relative to the axial referenceshaft, the cutting guide having a reference block for location against the bonerelative to the edge of the bone at one side and at least one arm which is shaped sothat it can extend from the cutting guide over the face of the head, in which thearm can be located positively with respect to features on the head so that the locationof the reference block along the axis of the bone can be set with reference to featureson the head of the bone which are contacted by means of the arm.
