The system comprises at least a support (5) to be attached to a patient by fixing means (4), said support having an outer thread (7), a first nut (9) with an inner thread (11 ) cooperating with said outer thread (7) and an anchor having a spherical bottom end (10) being placed between said support and said nut, a spherical head (15) said spherical bottom end and head being linked by a shaft, a second nut (16) having an inner thread (17) and intended to be screwed on an external device having a corresponding thread (31 ), such as a platform (20) or a surgical device (30), whereby the tightening of at least one of the nuts (9,16) on the support and/or on the external device allows to block the position of the anchor.