A partial endoprosthesis device (11) for preserving the motion of a vertebral joint for implant into a spinal segment (10) comprising an articular portion (13) being elongated along a longitudinal axis (14) with a first face (15) and a second face (17) opposite to the first face (15), the first face comprising a central protrusion (18) having a height (H), with respect to the face, and a surface (16) of an osteointegrable material. The protrusion is configured so that, by implanting the device (11) with the articular portion (13) inserted between an upper articular facet (3') of a lower vertebra (10') and a corresponding lower articular facet (3'') of upper vertebra (10") adjacent to the lower vertebra, and with the first face (15) in contact with a first articular facet selected between the upper articular facet (3') and the lower articular facet (3''), and with the second face (17) in contact with a second articular facet respectively with the lower articular facet (3'') and with theupper articular facet (3'), it is obtained that the articular portion (13), with the first face (15), pushes against the first articular facet and finally becomes integral by osteointegration with the lower articular facet (3'') and, with the second face (17), slidingly engages with the lower articular facet (3').