side walls 61 62 or 63 can comprise an inclined sliding surface to allow for easy clipping or snapping in of such pins or rods 100 into the corresponding cavities the grooves 51 52 53 are formed as rounded semi spherical recesses in a section this means that the recesses provided by the grooves 51 52 53 have a hollow cylindrical shape to accommodate rod shaped elements all three grooves 51 52 and 53 have different sizes so that the corresponding reception cavities 71 72 and 73 have three different sizes in other words each reception cavities 71 72 or 73 is adapted to accept a different fixation element i e a rod screw pin or wire having a different diameter one preferred embodiment of the first clamping assembly 20 has grooves to accept fixation elements having a diameter of 12 mm 8 mm and 5 mm respectively a different embodiment may have a sequence of diameters of 8 mm 6 mm and 4 mm respectively the second clamping assembly 30 according to the embodiment of fig1 also comprises two jaw portions 11 and 12 and these comprise three grooves 51 52 53 these grooves 51 52 53 also comprise a sequence of different sizes in the embodiment shown the inner jaws portion 12 have an identical structure as have the outer jaws 11 especially in view.