This invention consists in an implant to be placed between the lumbar spinous and lumbar-sacral apophysis in order to keep them separated and enable a normal flexion and extension range. The implant is a cuneiform body which frontal end is rounded and higher than the rear end thus forming an open angle towards the frontal part of the implant. It has two supporting surfaces, opposite to the spinous apophysis and having in its most rear end two sharp edges which are opposite to the spine and located transversally to the longitudinal axis of said faces, and wherein said supporting surfaces are confined by two rhomboidal surfaces which major axis is perpendicular to the longitudinal axis of the implant.