The invention relates to an implant for placing between the sacrum of a patient and the fifth lumbar vertebra. It comprises a connection bar (22), fastener means (24, 26) for fastening each end of the connection bar to the sacrum, and a spacer (20) having a first end for co-operating with the spinous process of the fifth lumbar vertebra, and a second end for co-operating with said connection bar, the implant being characterized in that said connection bar (22) has two ends (38, 40) presenting a common first geometrical axis, and a middle portion (36) connected to said ends, said middle portion lying on a second geometrical axis parallel to the first geometrical axis but offset relative thereto, second end of the spacer co-operating with said middle portion.