A surgical implant for insertion between a pair of adjacent vertebrae has first and second telescopic members (10 and 12) provided with vertebrae-engaging flanges (14 and 34). The members (10 and 12) have mutually engageable walls (24 and 42) provided with teeth (26 and 44) or surface roughening thereon. Fixing screws (50) pass through slots (30) in the member (10) and engage in internally screw-threaded apertures (46) on the member (14) to hold the walls (24 and 42) in mutual engagement.