saddle 10a 10b being necessary the same applies to the saddles 17 and 29 on account of the inclination of the walls 19a and 19b in the case of saddle 17 and on account of the lateral position of the projection 31 in the case of saddle 29 fig6 in fact shows a saddle 29a identical to the saddle 29 in fig5 the eye 36 being positioned in the direction of the fifth lumbar vertebra l5 and its projection 31 being positioned in the direction of the tubercles 40 in contrast the second saddle 29b positioned to the other side of the tubercles 40 has its projection 31 positioned on the other side of the body 32 compared to the projection 31 of saddles 29 and 29a and in the direction of the tubercles 40 for this reason the inclinations of the axes of the holes 33 and of the corresponding screws 42 are opposed which allows them to be driven into the intended areas of the sacral wings it will be noted that in contrast saddles 9 and 23 are symmetrical relative to the median plane of their u shaped body and can therefore be used alternately on one side or the other of the line of tubercles 40 the positioning of a saddle according to the invention is carried out in the following manner the path of the first screw such as 38 passing through the eye is prepared it extends from the outer part of the foot of the s1 articulation and is directed obliquely towards the line.