Abstract: An expandable interfusion cage comprising a cage body and a spacer. The cage body includes a seat part which is pierced by an orifice and a branch part which defines therein an inside space and has a plurality of elongate branches integrally formed at their proximal ends with the seat part. An opening is defined between two adjoining branches to communicate with the inside space. The spacer is movably assembled in the inside space of the cage body to expand the cage body radially outward. Inward projections are formed at distal ends, respectively, of the branches to project radially inward toward an axis of the cage body. The inside space of the cage body has substantially a circular or polygonal sectional shape. The spacer is engaged with the inward projections of the branches while expanding the cage body.