One embodiment includes at least a fixed element designed to be secured to a vertebra or sacrum, at least a mobile linking element and at least an intermediate element that articulates the mobile element relative to the fixed element. The intermediate element is received in an internal volume of the mobile element and is deformable, so as to be introduced by impingement into the inner volume. The fixed element is received at least partly in an internal volume of the intermediate element and has a position for use wherein said fixed element has three degrees of freedom in rotation, but is linked in translation, relative to the intermediate element.