suitable buttresses 14 and 16 have a body 22 and 24 defining a bore 34 see fig6 into which an end 18 of rod 12 may be inserted bore 34 has an opening 36 at the opposite end of the body 22 to the abutment surface 26 the bore 34 is a blind bore with bore end 46 so that the end 18 of rod 12 inserted into the bore 34 does not protrude from the abutment end of the body 22 as shown in fig5 the bore 34 is preferably elliptical or oval shaped in transverse section when the rod 12 is substantially circular in transverse section bore 34 has a first radius 37 which is greater than the radius of rod 12 so that buttress 14 can easily be moved axially along rod 12 when rod 12 is inserted in bore 34 second radius 39 is approximately equal to or slightly less than the radius of rod 12 the elliptical shape of bore 34 in conjunction with rod 12 may be of compatible configuration to that used in the isola spinal implant system isola is a trade mark of acromed corporation although it may equally be designed to be compatible with other spinal implant hardware systems body 22 also includes a threaded lateral bore 38 which extends from a side 40 of body 22 and intersects with bore 34 a grubscrew 42 as shown in fig1 may be inserted into threaded bore 38 and screwed until it bears against rod 12 and presses it against surface 44 until grubscrew 42.