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, 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. A sac for fusing spinal bone and a kit comprising the sac are also provided.