The block (1) comprises a core (4) made of flexible and elastic material which is dimensioned and has elastic properties selected such that it is slightly compressed when the block is positioned between the spinal apophyses (2a,3a) of a pair of adjacent vertebrae (2,3). These properties allow further compression, return to a neutral position, and slight extension, as the spine flexes naturally. Two rigid inserts (5) of wear-resistant material are inset into the upper and lower faces of the core. The inserts have a V-shaped configuration to engage against the surfaces of the vertebral apophyses, and may be secured to the apophyses by associated straps (6). The core is made from silicone and is enclosed in an envelope of e.g. polyester.