A method for fusing spinal bone is provided. The method comprises placing a sac between two or more adjacent sections of the spine to be fused, inserting a cage into the sac, and filling the sac with bone tissue. The surfaces of the sac abutting the sections of the spine comprise porous material for allowing bone to grow between the spine and the sac. Surfaces of the sac not abutting the sections of the spine are nonporous to bodily fluids and thereby prevent premature deterioration of the bone tissue inside the sac. The cage maintains the sac in an expanded state upon filling. A kit comprising the sac and cage is also provided. In other embodiments of the invention, the method comprises placing an inner sac inside the inventive sac and filling the sacs with bone tissue for fusion of spinal bone.
