An adaptor is secured to a first object. The adaptor includes an anchoring part and an adjustment part. The anchoring part includes a distally facing anchoring surface and a proximally facing first control surface. The adjustment part has a distally facing second control surface positioned to abut against the first control surface. The first control surface or the second control surface or both is/are helical. Thereby a relative z position of the adjustment part with respect to the anchoring part is defined by the relative orientation of the adjustment part with respect to the common axis while the second control surface abuts against the first control surface. This is used to adjust the z position of the adjustment part relative to the first object, in connection with the orientation of the adjustment part being defined and possibly fixed by the function of the adaptor/the adjustment part