Abstract: A fusion cage adapted for promoting fusion of bone grafts or bone substitutes packed in the fusion cage with the adjoining vertebrae, wherein the fusion cage has a shape of cylindrical closed U of which upper and lower ends are opened such that an internal cavity is defined by a plane rear wall, both sidewalls, and a rounded front wall. An upper and lower ends of each sidewall are roundly swollen so as to secure enough contact with an upper and lower surfaces of the adjoined vertebrae.